Pneumatic Grippers

HandlingMachiningAssemblyOrganisation

PneumaticsElectronicsMechanicsSensoricsSoftware

ChineseEnglishFrenchGermanRussianSpanish

Blue Digeston Automation

053 435

HesseGrippers andtheir applicationsincluding vacuum devices

328 mm

160 mm

120 mm 42 mm8 mm

47

,3 m

m8

8,5

mm

15

8,5

mm

19

5 m

m2

25

mm

Ha

nd

ling

Pn

eu

ma

ticsE

ng

lishB

lue

Dig

est

He

sseG

ripp

ers a

nd

the

ir ap

plica

tion

s

��

�

Hesse

Grippers and their applications

Grippers and

their applicationsincluding vacuum devices

Blue Digest

on Automation

Handling

Pneumatics

Stefan Hesse

Blue Digest on Automation

© 2004 by Festo AG & Co. KG

Ruiter Straße 82

D-73734 Esslingen

Federal Republic of Germany

Tel. 0711 347-0

Fax 0711 347-2155

All texts, representations, illustrations and drawings included in this book are

the intellectual property of Festo AG & Co. KG, and are protected by copyright

law. All rights reserved, including translation rights. No part of this publication

may be reproduced or transmitted in any form or by any means, electronic,

mechanical, photocopying or otherwise, without the prior written permission

of Festo AG & Co. KG.

It has been a long held dream by man to one day be free of the drudgery

of manual labour through the use of automatic devices. Needless to say, this

vision always depends on the technical components available at the time.

The automatic production lines of the twenties used by the English company

MORRIS MOTORS had to be mechanically controlled to a large extent, which

was not very successful. Not until the sixties did a new basic technology become

established: The NC machine and the industrial robot. Both are computer-aided

and therefore freely programmable as far as movement is concerned.

The robot is an important handling machine which roughly reproduces the

human arm. In order to be effective, it also requires mechanical hands, which

are generally referred to as grippers. These are also required on pick-and-place

devices and a wide range of other automatic systems. In principle, there are

two basic designs of grippers: Those, designed in the form of fingers and those

which do not ressemble fingers in any way. Thousands of individual patents can

be found, each of which claiming to be able to solve a gripping problem more

successfully than previously known. This demonstrates that the gripper has a

key role in automatic handling.

For the user, it is becoming increasingly difficult to take in the now wide range

of gripper technology. This is the reason why this brief introduction has been

published. Above all, it is intended to provide advice and ideas to practical

users, since the selection of the right gripper is by no means a trivial task. As

with other technologies, there is a risk of making the wrong decision. Nowadays,

however, most gripper tasks can be accomplished using standard grippers.

Therefore, special grippers are only developed for exceptional cases. A sound

basic knowledge of grippers and their use is always a good investment for the

future.

Stefan Hesse

Foreword

Table of contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1 Analysis of gripper functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 Gripper applications for component production and assembly . . . . . . . . 16

3 Grippers and hand axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Construction of gripping effectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5 Forces acting on grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6 Technical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

7 Application areas and gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

8 Checklist for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

9 Suction grippers – abhorred by nature . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

10 Suction cups for every application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

11 Suction cups in handling technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

12 List of illustrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

13 List of special terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

1 Analysis of gripper functions

Grippers further enhance the performance capability of automated handling

devices. These devices include not only industrial robots but also insertion

devices, manipulators and special feed devices, for example for automatic

machines, testing machines and batch-assembly systems. Grippers form the

link between all kinds of workpieces and the manipulating machine concerned.

Whilst man can pick up even complex workpieces easily and without hesitation,

gripping in the world of technology requires careful planning to obtain a desired

sequence – and a situation which has been defined must then be maintained

extremely accurately. This series of articles will deal with a number of aspects

which ensure that grippers produce the desired effect.

A general definition of grippers is given in the VDI Guideline 2860. According to

this, the distinguishing feature of a gripper is the temporary grasping, retaining

and subsequent releasing of objects of a particular geometrical shape. Grippers

act like hands in automated machinery. The word “gripper” essentially describes

the ever-growing family of accessories used in handling systems. It is not always

easy to select the right type of gripper. However, it is not always appreciated

that a correctly-formulated problem description can, in itself, be halfway towards

solving the problem.

A gripper application can be defined in terms of technological requirements,

workpiece parameters, the machine to be used for the handling operation and

environmental parameters.

The technological requirements may include the time available for gripping,

the gripping path, the gripper force curve and the number of workpieces to be

gripped simultaneously. The most important workpiece parameters are mass,

shape, dimensions with tolerances, the position of the centre of gravity,

stability, surface properties, material, strength and temperature. The data

required for the handling device comprise its positioning error, axial acceleration

and connection conditions. Environmental parameters consist of process forces,

the space available for gripping, set-down conditions and the dirt, humidity and

vibration present. It is difficult if a gripper is required to handle several different

workpieces in succession, since it is then necessary to satisfy several require-

ment profiles. It must also be borne in mind that it is not always possible to grip

a workpiece on all sides; for example, it may be necessary to avoid precision-

machined functional surfaces. A feed sequence may also impose limitations.

This can be seen clearly in this example: (Fig. 1-1).

9

1

Analysis

of gripper functions

Grippers are the

solution, but to which

problem?

Determining the

requirement profile

10

Fig. 1-1:

Division of a workpiece into

gripping zone (G), clamping

zone (C) and set-down zone

(S)

1 Workpiece

2 Magazine

3 Clamping device

4 Gripper

5 Magnetic gripper

Fig. 1-2:

How can a workpiece

be picked up?

a) External gripping

b) Internal gripping

c) Combination of internal

and external gripping

A workpiece is to be picked up and brought into a clamping position. The work-

piece cannot be gripped in the area of the set-down zone S, which fits into the

magazine, since this is covered. The available gripping zone thus gives rise to

the fact that the workpiece might slip away from the desired position. We must

therefore clarify for each workpiece the question of which zones can be used for

gripping. The situation will of course change if the user selects a different axial

position in the magazine or another workpiece magazine. In our example, the

set-down and clamping zones are identical. The gripping zone must be defined

positively. The example involves external gripping. Workpieces which are hollow

or have holes or recesses can also be gripped internally, as shown in Fig. 1-2.

This is an important factor in the selection of a gripper, since in the case

of internal gripping the holding force acts from the inside to the outside,

requiring a double-acting gripper. External gripping, on the other hand, usually

requires more space around the gripper.


12

3

S

4

5

G

G

A

a) b) c)

11

Fig. 1-3:

The right choice of

gripping point can affect

the positioning error during

assembly

a) Gripping a component

by its body

b) Gripping

by connecting wires

Is a controlled gripper

necessary at all?


Positioning accuracy is more important in assembly than in conveying.

Positioning errors may result from :

• the repetition accuracy of the handling device

• gripper positioning errors

• workpiece tolerances or

• errors in the basic workpiece to which the axis motions are matched.

An example is shown in Fig. 1-3.

The connecting wires of an electronic component are to be inserted into a

printed circuit board. If the component is gripped by its body, distortions such as

slightly bent connecting wires will seriously impair the success of the assembly

operation; the effect being greater as the distance “a” increases. If, on the other

hand, the workpiece is gripped by its wires at a distance “b” from the end of

these, the situation is considerably better. Consideration must also be given to

the intervals at which the components are to be fitted. Any gripper requires a

certain minimum space to operate.

Workpiece handling does not in itself create added value. Only in assembly

operations or when used to guide tools can an industrial robot add value to a

workpiece. It is therefore a welcome advantage if handling devices can be

simplified or even eliminated altogether. Example – a user wishes to feed work-

pieces to a clamping device (Fig. 1-4).

a)

a

b)

b

He envisages a controlled mechanical jaw-type gripper with internal gripping.

But why? A uncontrolled spring-loaded gripper is fully adequate. This can pick

the workpiece from the magazine and take it to the clamping point. The clam-

ping jaws then close and the plug gripper is retracted. Efforts should always be

made to find the simplest solution. There are, however, often parameters which

prevent this. The principle nonetheless remains – first simplify, then automate!

There are many workpieces which can withstand the necessary gripping force

without sustaining damage. But there are other workpieces which are for

example polished, thin-walled, soft, brittle or super-finished and which can be

damaged during gripping, especially by clamp-type grippers which impose a

point loading (See Fig. 1-5).

12

Fig. 1-4:

Feeding a clamping device

1 Clamping point

2 Simplest type of gripper

(mandrel or plug gripper)

3 Supply magazine

High point loadings can

damage workpieces


1

3

2

Point loading is the contact force per unit gripping area which results from

clamp gripping. Deformation occurs at the point of contact. Contact force should

not, however, be assumed to be the same as the closing force of the gripper.

V-shaped grippers, for example, spread forces. Gravity may also be a factor,

depending on the orientation of the gripper, as may the coefficient of friction µ.

Excessive point loadings may produce clamping marks on workpieces

and dents in hollow workpieces.

In the case of grippers with plain-bearing guides, oscillations of the overall

system may produces transient effects on the coefficients of static and sliding

friction. This means that during a motion (subject to vibration), contact force

may increase due to the fact that sliding friction becomes effective for a short

time, reducing the friction in the guides which diminishes the gripper force.

This phenomenon occurs hardly at all with roller-bearing jaw guides.

When it comes to the fine positioning of objects, the average person relies on

eye/hand coordination and manages without difficulty, for example, to thread

a needle. Mechanical gripping must be just as precise and trouble-free.

Problems can occur in 3 situations:

• When picking up a workpiece

• When aligning this in the gripper device

• When setting the workpiece down in the desired position.

13

Fig. 1-5:

Types of point loading

resulting from gripping

a) Area/area

b) Line/area

c) Point/area

d) Double line/area

Problems with accuracy


a)

c)

b)

d)

Gripping devices have only a limited working range. Workpieces outside or at

the limit of this range will not be picked up reliably. The answer is either to use

a wide-range gripper (which requires a longer gripping time) or to reduce the

workpiece placing error. This can often be achieved by simple means. Fig. 1-6

shows an example. In the old arrangement, the workpiece is positioned very

inaccurately at the pick-up point. An improvement is produced by using a tem-

plate with a centring effect.

Workpieces are not aligned to the gripping centre, since the gripper closes in an

arc and operates with workpieces of different diameters. The gripping centre,

also known as the tool centre point (TCP), is however the value entered in the

programming of the handling machine. Deviations of this kind may causes

problems with close-tolerance assembly operations. This type of problem is

shown in Fig. 1-7 [1]. This effect is not encountered with parallel-jaw grippers.

Problems may also be encountered in obtaining set-down at precise points.

This is subject to a total error comprising the handling machine positioning error,

the gripper error and the workpiece error [2]. Shape errors in particular may be

problematic with long workpieces and narrow gripper jaws, as shown in Fig. 1-8.

The remedy is to use wider gripper devices with a compliant covering on their

gripping surface.

14

Fig. 1-6:

Unambiguous pick-up points

ensure reliable gripping

a) Inaccurate workpiece

position

b) V-shaped template

centres workpiece

Fig. 1-7:

Gripper devices which close in

an arc may cause a shift of the

gripping centre

a) Scissors-type gripper

with 2 different work-

pieces 1 and 2

b) Parallel-jaw gripper


a) b)

a) b)

1

2

1

2

δx

The accuracy situation must therefore be studied thoroughly. A gripper with

V-shaped jaws will compensate for preceding placing errors and will align the

workpiece to the TCP. A magnetic gripper or suction cup cannot do this.

These types of grippers will retain the placing error and add errors of their own.

The process technology also has an effect – a clamping collet opens only a short

distance and is more difficult to feed than a wide-opening jaw-type chuck.

A gripping operation can be influenced by a number of parameters which

may (but need not necessarily) cause the gripper to “miss” the workpiece

in the worst-case scenario. We have discussed a number of reasons for this.

A thorough study of the gripper application will help to recognise problems in

good time [3].

[1] Volmer, J. (Hrsg.): Industrieroboter – Funktion und Gestaltung

(“Industrial Robots: Function and Design”),

published by Verlag Technik, Berlin and Munich 1992

[2] Hesse, S.: Montagemaschinen (“Mounting machines”),

published by Vogel-Buchverlag, Würzburg 1993

[3] Hesse, S.: Greifer-Praxis (“Praxis of grippers”),

published by Vogel-Buchverlag, Würzburg 1991

15

Fig. 1-8:

Centre deviation resulting

from workpiece form errors

Summary

Publications


δx

δx

δy

The choice of gripper type is always determined by the properties of the object

to be gripped and the purpose of the handling operation concerned. Classic

applications include production of components (feed) and assembly (handling

of components for assembly). New applications have, however, also emerged,

with different parameters from those in production. These applications include

packaging (requiring high speeds) and commissioning (with an undefined initial

object position). It is thus necessary to adapt grippers constantly for new and

important applications.

Standard grippers have reached a high level of development and can be used

for many applications, not just the classic applications. There are also gripper

systems for special applications. Rails and adapter plates can be used, for

example, to create double grippers, multi-position grippers and multi-workpiece

grippers. We will illustrate this with a few selected examples.

3-point grippers are the preferred type for handling cylindrical workpieces.

These grippers give both good centring and a high degree of reliability [1]. Care-

ful design of the gripper fingers can provide a certain measure of adaptability

to different workpiece dimensions. Fig. 2-1 shows an example, using hardened

gripper pins for the internal gripping of small workpieces. These pins can if

desired be repositioned in other bores provided in the gripper fingers. The result

is an enhanced gripper range, albeit with the need for manual resetting. The

gripper pins need not be used concentrically – it may be better to arrange them

at gripping points in accordance with the internal contours of the workpiece,

for example in order to grip a housing with a rectangular aperture (Fig. 2-1c).

It is also possible to grip into hole patterns extremely effectively using gripper

pins.

The minimum effective length of the pins should be 5 mm. This also applies to

mechanical gripper fingers.

16

2

Gripper applications

for component pro-

duction and assembly

Workpiece handling

with standard grippers

Fig. 2-1:

Using a 3-point gripper

a) Adjustable gripper pins

b) Concentric internal

gripping of a flange ring

c) Non-concentric internal

gripping of a housing

2 Gripper applications for component production and assembly

a) c)b)

For workpieces with a length of 200 mm and more, it is better to use multi-point

grippers. The best way to produce grippers of this kind is by combining two

standard grippers. This is shown in Fig. 2-2, taking the example of the gripping

of a sheet-metal profile. For this purpose, the grippers are mounted on an

adapter rail. The required gripping force per gripper is of course halved and the

problematic torque forces which may result from high-speed manipulation can

be absorbed more easily.

The centre of gravity of the workpiece should be placed at the exact midpoint

between the grippers. When single grippers are used, the centre of gravity

should be as close as possible to the gripping point.

Frequent use is made of modified gripper systems based on standard grippers.

One example of this is in-line arrays of grippers used to pick up workpieces

from pallets row by row or to set down rows of workpieces on pallets, in crates,

etc. In this case, too, the grippers are mounted on a rail and then act as multi-

workpiece grippers. This is illustrated in Fig. 2-3. Since the grippers act at a large

number of points simultaneously, the workpiece positions must be maintained

to close tolerances. If workpieces need to be picked up, for example from

a conveyor belt, it may be necessary to align these on the belt beforehand.

17

Fig. 2-2:

Multi-point gripper

for long workpieces

2 Gripper applications for component production and assembly2

The following problem may be encountered when feeding workpieces

to clamping points: at the moment of clamping, the flow of forces is a closed

kinematic chain, due to the fact that the gripper must also hold the workpiece.

This means that the clamping point forces the gripper and thus the handling

machine to adopt its position. In the long term, this may cause damage to robots

and grippers. Ways must therefore be found of compensating for this effect.

There are robots which in these circumstances switch to a “soft” action and

do not therefore attempt at all costs to maintain their position but rather yield.

Grippers can also be provided with compliant mountings (“floating” grippers),

and there are also grippers with an integrated or attached pressure device.

In this case, the gripper opens in the clamping device, at which time the work-

piece must be held against the clamping device by the pressure device, for

example a pneumatic ram. The clamping device then closes and the gripper

can withdraw.

18

Fig. 2-3:

Multi-workpiece gripper for

transfer of complete rows

of workpieces


There are many details which require consideration in the planning of an auto-

mated assembly line. The most important of these, however, are time and

accuracy. Time can be gained, for example, by using turret grippers.

It is perfectly possible to use standard grippers for this purpose. Fig. 2-4 shows

a mounting plate equipped with the necessary number of grippers. Assembly

tools can also be fitted in certain cases. The advantage for the user is that a

number of idle robot motions within the assembly sequence can be avoided,

thus shortening the sequence. The large diameter of the interference circle, on

the other hand, is a disadvantage. It must be determined as a first step whether

sufficient working space is available.

New technology, such as video recognition systems, has led to new demands

being placed on gripper systems. One example of this is the insertion of chocola-

tes into a blister pack. This also falls under the broad heading of “assembly”

work. Fig. 2-5 shows a solution in which several suction cups are used to pick-up

the rectangular chocolates from a conveyor belt.

The suction cups are fitted to single-acting standard cylinders which are

protected against torsion. Each suction cup is able to move forward indepen-

dently at high speed. Once the recognition system system has detected a work-

piece and determined the coordinates and the orientation of the workpiece

(longitudinal axis), the gripper adjusts its angle accordingly. The suction cup now

advances, picks up a workpiece from the moving belt and returns to its initial

position. Once all the suction cups have picked up workpieces, the robot swivels

to the packing conveyor belt and sets down all the workpieces simultaneously in

the nests of the blister packs. Since all workpieces are already correctly aligned

relative to the grippers, their alignment with the packaging is also correct.

19

Gripping in assembly

operations

Fig. 2-4:

Multi-workpiece gripper

for assembly operations


20

Fig. 2-5:

Multiple suction-cup grippers

for assembly operations

1 NC rotary axis

(robot hand axis)

2 Angle mounting plate

3 Standard cylinder with

non-rotating piston rod

4 Suction cup

5 Conveyor belt

6 Workpiece (confectionery)


1

2

3

4

56

Workpieces can be held within gripper fingers by (frictional) force. They can,

however, also be held simply by physical systems such as the shape of the

gripper or even by adhesion, e.g. adhesives. The theoretical possibilities are

shown in Fig. 2-6.

Clamping force is very frequently used to hold workpieces. We must, however,

consider the following: in order to hold the workpiece, the fingers must act on

the workpiece with a force FG of at least FG = m · g/µ (disregarding safety

margins and the effects of other forces for the moment). In the above, m is

the coefficient of friction and m the mass of the workpiece. In an assembly

operation, however, this force is not sufficient, since a joining force FS is also

required. The required gripping force is thus FG = (m · g + FS)/m.

If we assume a coefficient of friction of 0.1, the gripping force FG would be

10 times the total weight of the workpiece to be assembled and the joining

force. This may lead to deformation of or damage to the workpiece, particularly

if this is delicate. It is therefore desirable to use a positive-locking connection.

Fig. 2-7b shows how, with this type of gripping, the workpiece is now able to

rest on the gripper finger, allowing the clamping force to be kept relatively low.

21

Positive-locking or

force-locking grippers?

Fig. 2-6:

Methods of holding a work-

piece (example: ball bearing)

1 Enclosure

only without clamping

2 Partial enclosure

combined with clamping

force

3 Clamping force only

(force-locking connection)

4 Holding with suction

(force field)

5 Holding

with magnetic field

6 Holding with adhesive

layer, such as grease


1 2 3

4 5 6

m.g

A positive-locking connection may also be desirable for feed motions. When a

workpiece is lifted rapidly, it is subject not only to a weight force m · g but also

to an inertia force FT which is a function of vertical accelera-tion. If, however, the

gripper hand is turned through 90° before the workpiece is lifted, the previous

force-locking connection temporarily becomes a positive-locking connection for

this motion sequence. These examples show that the gripper is a component

where different influences come to bear and should therefore never be used

without taking into considera-tion all the various factors involved.

22

Fig. 2-7:

Gripping principle:

A positive-locking connecion

places much less stress on

workpieces during gripping

and holding

a) Gripping an egg

with the human hand [2]

b) Gripping a workpiece and

holding it during assembly

c) Attitude of gripper hand

during feed motion


m.ga)

b)

c)

Force locking Positive locking

Grippers are well-proven components and can also be used as workpiece

feeders to release a certain number of workpieces from a magazine. Generally,

workpieces are released one at a time – we therefore refer to this process as

“separation”. Fig. 2-8 shows an example of an application using a gate-type

feeder.

Sliding gates have been fitted in place of the gripper fingers. These should

be as short as possible, as is usually the case in gripping applications, to avoid

overload of the linear guides of the gripper jaws and reducing their service life.

This solution is worth considering only for the feed of small workpieces, since

other solutions are available for large heavy workpieces [3]. In order to reduce

the load acting on the feed slide, a stepped track has also been developed

which enables a proportion of the load due to workpiece build-up to be

supported by the step.

[1] Seegräber, L.: Greifsysteme für Montage, Handhabung und Industrieroboter

(“Gripper Systems for Assembly, Handling and Industrial Robots”),

published by expert Verlag, Renningen, 1993

[2] Bohmann, J.; Nönnig, R.: Ein Greifer für empfindliche Teile

(“Grippers for Sensitive Workpieces”),

article in magazine “Konstruktion” No. 45 (1993) pp. 95-97

[3] Hesse, S.: Atlas der modernen Handhabungstechnik

(“Handbook of Modern Handling Technology”),

published by Vieweg Verlag, Wiesbaden, 1995

23

Grippers

as separators

Fig. 2-8:

Gate feeder using

a parallel-jaw gripper

1 Parallel-jaw gripper

2 Sliding gate

Publications


1 2

Grippers are holding devices; this is their main function. In order to obtain

a given effect, we must provide grippers with the ability to move in three

dimensions. This is achieved by using motion axes. A robot application for

example with 6 axes is not particularly complicated, since the robot provides

the necessary motion capability. In applications where costs are critical but

ultra-high speed is not required, it is worth considering hand axes, which are

often available as flexible modules and can take the place of a robot.

This produces solutions which can be installed quickly at reasonable cost.

Depending on the application in question, a hand axis may be of interest for the

following motions. Rotation > 360°, swivelling < 360°, thrust motions (generally

with short strokes) and screwdriving motions, particularly for the insertion of

screws. The most typical motion is, however, swivelling, which is why gripper

manufacturers almost always offer compatible swivelling units. Fig. 3-1 shows

a two-axis module which can swivel between 0 and 270° and provide a linear

thrust stroke of up to 100 mm. The positions are finely adjustable, with a

cushioned approach. But what exactly can we do with this motion capability?

Let us first consider the term “degrees of freedom”. A workpiece can have

a maximum of 6 degrees of freedom, expressed as 3 linear motions on the

3-dimensional axes x, y and z and 3 rotary motions α1, α2 and α3 about the

axes x, y and z. Handling machines, by the way, can have more than 6 degrees

of freedom. We then speak of degrees of mechanical freedom or travel freedom.

Thrust motions (Fig. 3-2) are described as follows:

1 Vertical up-down

2 Forward/backward

3 Lateral left/right

24

3

Grippers

and hand axes

Fig. 3-1:

Three-point gripper combined

with a swivel/linear unit

Degrees of freedom

of the hand

3 Grippers and hand axes

while rotary motions, following aviation practice [1], are designated as follows:

α1 Pitch, tilt

α2 Roll, twist

α3 Yaw, turn.

As you will know, the closing motion of the gripper jaws is not considered as a

degree of freedom, since this motion has no influence on the motion path of the

gripper.

Particularly in applications involving the feeding of machine tools, it is desirable

for the machine to resume work as quickly as possible after the workpiece has

been changed. After this, the handling machine will generally have ample time to

set down workpieces and pick up new workpieces from a magazine. The double

gripper was developed for this type of application. Fig. 3-3 shows a common

design of double gripper.

25

Fig. 3-2:

The human hand can execute

motions with 6 degrees

of freedom

(according to Bejczy)

a) Biological

b) Technical

Double grippers

save process time

Fig. 3-3:

Double gripper designed

as a crown turret

a) Radial gripper

b) Axial gripper


1

2

3 G

a) b)

α1

α2

α3

a) b)

The crown turret is driven by a rotary cylinder with adjustable endposition

cushioning at both ends and fine adjustment of its end positions. An important

feature is the backlash compensation in the rack-and-pinion unit, since other-

wise considerable positioning errors could result at the gripping point. Double

grippers of this kind are often used with gantry robots. Whether gripping is

carried out radially or axially depends on the workpiece axis position in the

magazine. Long and thin workpieces are generally fed horizontally, with radial

gripping, while axial gripping is the more common method for short and thick

workpieces and also castings with or without flanges. The crown turret is

designed to accept standard grippers.

Gripper systems are also sometimes modified for a given application.

The shaft gripper in Fig. 3-4 is an example of this. In this case, standard grippers

are mounted on a swivel plate, and the motion is executed by a suspension-

mounted pneumatic cylinder. A fast but cushioned approach is provided to the

cylinder end positions. Set-screws are used to adjust the angle. This solution is

used, for example, in conjunction with line gantry robots to feed material to

machine tools and test machines.

26

Fig. 3-4:

Shaft gripper

1 Connecting flange

2 Cushioned stop

3 Set screw

4 Parallel-jaw gripper

5 Pneumatic cylinder


1

2

3

4

5

1

2

3

4

5

6

7 8

9

In series production applications, pneumatic components have proved ideal as

providers of motion. For example, a swivel/linear drive can be used to create a

complete handling module, as shown in Fig. 3-5. In this case, the gripping point

is not placed coaxially with the piston rod of the swivel/linear unit but deviates

from this. This creates an arc-shaped working area within which the positions to

be approached must lie. This is an extremely simple solution for normal inserti-

on tasks. The workpieces are fed stepwise on a cross-table to a fixed pick-up

point. If ultra-high speed was required, the answer would be to provide a second

gripper opposite the first one. This would allow pick-up from the magazine and

insertion of workpieces to be carried out in parallel.

Simple handling modules can also be quickly and easily assembled using

standard suction cups and semi-rotary actuators (Fig. 3-6). An additional short-

stroke axis would turn this combination into a pick-and-place device. A hollow

flange shaft can be used as a throughfeed for the vacuum line. The actuator can

operate at switching frequencies of up to 3 Hz.

27

Hand axes used

to assemble small

workpieces

Fig. 3-5:

Handling module

for assembly of small

workpieces

1 Swivel cylinder

2 Lifting cylinder

3 Adapter plate

4 Standard gripper

5 Transfer system

6 Workpiece for insertion

7 Receiver workpiece

8 Gripper finger

9 Magazine


It is often necessary to rotate or turn over workpieces between workstations,

for example, inverting parts such as receiver workpieces in an assembly process.

A simple in-line solution for inverting a workpiece is shown in Fig. 3-7.

A standard gripper executes a swivel motion of 180°. The gripper fingers in this

case are arranged like a mouth. The workpiece runs against the stops in the

open “mouth”; the gripper then closes and transfers the workpiece overhead

to the next conveyor belt.

28

Fig. 3-6:

Handling unit with suction

cup and semi-rotary actuator

Fig. 3-7:

Inverting workpieces

1 Gripper jaw

2 Standard gripper

3 Semi-rotary actuator

4 Workpiece

5 Conveyor belt


In the interests of fast processing time, machines with rotary indexing tables are

often equipped with double clamping devices. Machines of this kind are also

referred to as duplex machines, producing two finished workpieces during each

working cycle. It would be an attractive idea to construct a quadruple gripper,

able to remove two finished workpieces and at the same time bring two fresh

blanks. A gripper of this kind would, however, be bulky and difficult to use, parti-

cularly with large irregularly-shaped workpieces. This problem can be solved by

using a triple turret gripper. This is shown in Fig. 3-8. The free gripper G1 first

picks up a finished workpiece from the clamping point S1. A new blank is then

inserted into the vacant clamping point, while the vacated gripper G2 can pick-

up the second finished workpiece from the clamping point S2. The second blank

is then positioned by the gripper G3 [2]. If a single gripper were used, it would

be necessary for the robot to execute a number of idle strokes, making the

operating cycle longer.

29

Feeding double

stations

Fig. 3-8:

Triple gripper installed on a

special machine with double

stations

G Grippers

S Clamping points


Fig. 3-9 shows a particularly simple solution for feeding ferromagnetic sheet

metal. A suction cup is mounted directly on the hollow piston rod of a standard

cylinder. The suction cups reach through the gaps between the top rollers of a

roller conveyor and contact and pick up a sheet metal workpiece. The work-

pieces are secured by the permanently-magnetised upper rollers and are then

moved on to a normal roller conveyor. The stack of workpieces is progressively

raised by a lifting device. If the roller conveyor is inclined by a few degrees, the

sheet metal workpieces will move along the conveyor by gravity alone. Further

suggestions for the handling of sheet metal can be found in [3,4].

30

Feeding sheet metal

Fig. 3-9:

Picking up ferromagnetic

sheets from a stack using

a suction-cup/lifting module

1 Suction cup

2 Standard cylinder

with hollow piston rod

3 Frame

4 Magnetic rollers

5 Roller conveyor

6 Sheet metal stack

7 Lifting table


1

2

3

4

56

7

The excellent quality of pneumatic components has made them popular far

beyond the field of mechanical engineering alone. Pneumatic modules are

being used for manipulative movements in all kinds of new areas. The specimen

shaker shown in Fig. 3-10 is an example from the field of laboratory automation.

This shaker has been created using a standard cylinder, a swivel unit with a

hollow flange shaft and a standard gripper and adapter. It would also be

possible to create an array of shakers or provide other motion combinations.

The critical factor is to create a quick, inexpensive assembly, without the need

for a great deal of preparatory work.

[1] Siegert, H.-J.; Bocionek, S.: Robotik: Programmierung intelligenter Roboter

(“Robotics: Programming Intelligent Robots”),

published by Springer Verlag, Berlin, Heidelberg et alia. 1996

[2] Breuer, H.J.: Bestehende Fertigungsanlage

für Schwenklager mit zehn Industrierobotern automatisiert

(“Automating An Existing Production Line For Swivel Bearings Using

Ten Industrial Robots”), in the magazine “Werkstatt und Betrieb”

123(1990)12, pp. 929-932

[3] Hesse, S.: Blechteile automatisch handhaben

(“Automatic Handling Of Sheet-Metal Workpieces”),

in the magazine Bänder, Bleche, Rohre 37(1996)4, pp. 21-23

[4] Hesse, S.: Umformmaschinen (“Shaping Machines”),

published by Vogel Buchverlag, Würzburg 1995

31

Specimen shaker

Fig. 3-10:

Simple specimen shaker

made from standard

components

Publications


It requires many technical components and procedures, such as an industrial

robot, a controller, a program, a workpiece magazine, sensors and grippers to

enable a handling device actually to pick up an object automatically. At the end

of the chain, it is the gripper jaws or similar which provide the contact with the

workpiece. This contact is often only a minute point, and it is here that major

differences are encountered – a lump of iron is easy to pick up, a pickled herring

is not. We can see from this that gripper jaws must be matched to the work-

piece. So, how many different designs of gripper jaws are there?

“Gripper jaws” are separate components, generally shaped and interchangeable,

which provide a positive- or force-locking contact with a workpiece and hold

this in place. “Gripper fingers” are elastic or articulated force transmission

components which are positioned around a workpiece. Non-articulated compo-

nents are also in practice referred to as gripper fingers. The gripper jaws are

fitted to these, using either a fixed or movable connection.

Gripper jaws are generally produced by the user or machine manufacturer.

Certain companies help in this process by providing “neutral” universal jaws

which require machining to the negative contour of the workpiece but have

ready-made connection surfaces. There are also moulded jaws, in which the

workpiece contour is produced by being pressed into synthetic resin, vulcanised

rubber or molten metal.

A “securing” function comprises the temporary holding of an object in a defined

position and orientation, followed by release as an inversion of holding or a

cancellation of the securing function. In relation to mechanical grippers, we

speak of “clamping” and “unclamping”. The holding function is normally

provided by gripper jaws, which are thus described as a holding system, as

shown in Fig. 4-1. The function of a holding system is an essential feature

of all grippers. Holding can be achieved through mechanical clamping and also

through fluidic or magnetic force fields or the enmeshment of surfaces [1].

32

4

Construction

of gripping effectors

Main purpose of

grippers is to provide

a holding system

4 Construction of gripping effectors

It is essential when designing gripper jaws to know the points at which the

workpiece is to be gripped. Technical parameters naturally also have an certain

influence. Fig. 4-2 illustrates this with the example of a two-finger gripper. Area

contacts are generally preferable to line or point contacts.

Conditions are not always ideal. If we consider the case of “parallel flat sur-

faces”, we see that some workpieces are not in fact parallel at all; for example,

plastic mouldings may have slight moulding bevels. If the deviations from

parallel are small, it may be sufficient to fit the gripper jaws with a compliant

rubber covering. It is, however, sometimes better to provide pendulum jaws,

of which three types are shown in Fig. 4-3.

33

Fig. 4-1:

Some of the sub-systems

of a mechanical gripper

1 Adapter

2 Force generator

3 Force conversion

4 Force transmission,

with finger as transmission

components

5 Gripper jaw

as gripping component

Fig. 4-2:

The contour at the gripping

point of the workpiece

determines the jaw shape

used, 1, 2 or 3.


1 2 3 4 5

1 2 3

Holdingsystem

Interface

Actuation Kinematics

Ball-jointed pressure plates can compensate for angular errors on two planes.

Rubber coverings or specially-produced gripper cushions are often sufficiently

compliant for this purpose and have the further advantage that they increase

the coefficient of friction (µ approx. 0.5), meaning that a lower gripping force can

be used.

34

Fig. 4-3:

Gripper jaws

with compliant surfaces

1 Rubber or plastic covering

2 Pendulum jaw

3 Ball-jointed pressure plate


1 2 3

With some grippers, the tool centre point changes when workpieces with

different dimensions are gripped. Designs include vice-type grippers (one fixed

and one mobile finger) and scissor-tong grippers. In the former case, the

position changes must be allowed for in programming, while in the case

of scissor-tong grippers, compensation can be provided by specially-shaped

grippers. The gripping surfaces are arched, as shown in Fig. 4-4, and do not

have the usual simple V shape.

Dimensioning should be carried out in accordance with the following:

• The ratio between diameter D1 (largest workpiece) and D2

(smallest workpiece) should not be more than 2.5.

• The contact point angle α should be roughly 20 to 25°.

The following equations are used:

D =

A =

B = 0.5 · R

R1 = – 0.5 · D

R2 = + 0.5 · D

35

Dealing with differen-

ces in workpiece

dimensions

Fig. 4-4:

Jaw shape with centring effect

for scissor-tong grippers

TCP = Tool centre point

D = Workpiece diameter


TCP

A A

C1

B

R

DR2

R1

α

α

D1 + D2

2

0.5 · R

tan(α · 3.14/180)

0.5 · R

sin(α · 3.14/180)

0.5 · R

sin(α · 3.14/180)

This calculation also applies to angle grippers (gripper fingers with separate

pivot points C1 and C2). Radius R1 then becomes larger, while R2 becomes

smaller. The angle β between the lines TCP-C1 and TCP-C2 should lie in the range

0 < β < (2α – 40°).

The case is different when several workpieces need to be gripped simulta-

neously. Here, too, it is essential to achieve a certain degree of equilibrium.

The best solution, of course, is to divide up the degree of mobility using the

multiple clamp principle, i.e. with individual pressure points as shown in

Fig. 4-5.

To enable a larger range of dimensions to be handled, it is also possible to use

a stepped V-shaped jaw. Thus, using a jaw stroke of 50 mm in each case, it is

possible to grip workpieces with diameters ranging from 1 to 110 mm.

The grippers jaws used for this are shown in Fig. 4-6 [2]. We must, however,

accept displacements dx of the tool centre point. A typical application of this

kind of gripper would be gearbox assembly in an assembly cell, in which shafts,

bearings and gearwheels, all with different diameters, must be gripped in

succession.

36

Fig. 4-5:

Gripping several workpieces

simultaneously, using

a pressure distributor to

compensate for tolerances


F F

Sequence grippers are used to grip a defined number of different objects

in an unvarying sequence. The gripper jaws must accordingly have a number

of gripping points which match the gripping points of the workpieces in each

case. The easiest way to explain this is by using an example, such as the one

in Fig. 4-7, with 4 workpieces. The orientations and gripping points required

for the process concerned are defined. The gripper jaws must be provided with

a negative contour for each gripping point, which makes the jaws extremely

complex in shape. This is of course, not always possible, but gripper jaws have

been designed which allow 9 different workpieces for a gearbox assembly line

to be gripped without changing the gripper jaws. This case, by the way, would

be encountered only in assembly cells, in which one robot is required to assem-

ble as many different components as possible. On assembly lines for mass

production, robots and thus grippers are generally set up for one specific task.

37

Fig. 4-6:

Jaws of a parallel gripper

for 3 diameter ranges

Jaws for sequence

grippers


φ 3070 mm

...

φ 130 mm

...

φ 70100 mm

...

50 mm 50 mm

y

x

δx

Grippers can be designed for many special applications [3]. There are, for

example, grippers with rotary jaws, which can turn a workpiece through 90°.

One jaw is passive, while the other is equipped with a rotary actuator. Attempts

are, however, always made to use a normal basic gripper before developing a

special gripper. Fig. 4-8 shows some examples of special grippers.

The moulding jaw incorporates thin movable metal plates. When the gripper

closes, the plates are pressed against the workpiece, forming an impression

of it’s contour. The entire lamellar assembly is then clamped into place.

The impression is reversible, i.e the lamellae can be reset. Gripper jaws can

also be provided with serrated adjusters, allowing the gripping width, but not

the stroke, to be matched to the workpiece dimensions. The jaws can also be

used turned through 180°. In order to grip several workpieces simultaniously

38

Fig. 4-7:

Gripper jaws with specisally-

shaped multiple gripping

surfaces

Jaws for special

applications

Fig. 4-8:

Variants of jaws

for parallel grippers

1 Moulding jaw

with lamellar assembly

2 Width-adjustable jaw

3 Jaw of combination gripper


A

B

C

D

A

B

C

D

1 2 3

(combination grippers), jaws must be provided with an appropriate number of

identical indentations. Here, too, we need to consider the tolerance problem [4].

It is even possible to equip gripper fingers with mobile jaws. Fig. 4-9 shows an

example. As the gripper fingers close, they contact the workpiece and lift this

out of the V-shaped recess. There is no need to provide the handling device with

a lifting motion. This does, however, require jaws and workpieces with a suitably

smooth surface. The contact point must be below the centre of the workpiece.

The purpose of this design is thus to save the need for a motion axis.

39

Fig. 4-9:

Mobile gripper jaws lift the

workpiece out of the V-shaped

recess in the magazine

1 Gripper housing

2 Finger

3 Mobile jaw

4 Rotary axis

5 Support surface

6 Torsion spring

7 Workpiece


In selecting the shape of the gripper jaws, allowance must be made for the

type of approach of the gripper to the workpiece, since this may influence the

required stroke. The approach may be axial or radial – technical conditions will

generally dictate which. Fig. 4-10 shows 2 cases, taking the example of V-shaped

jaws for which different gripping strokes “c” are required for the same work-

piece. An opening safety margin “a” and clamping safety margin “b” are always

required; these compensate for tolerances and provide the necessary latitude.

[1] Cardaun, U.: Systematische Auswahl von Greifkonzepten

(“Systematic Selection of Gripper Concepts”).

Doctoral thesis, University of Hanover 1981

[2] Volmer, J. (Hrsg.): Industrieroboter – Funktion und Gestaltung

(“Industrial Robots: Function and Design”),

published by Verlag Technik, Berlin and Munich 1992

[3] Hesse, S. (Hrsg.): Industrieroboterperipherie

(“Industrial Robotics Peripherals”),

published by Hüthig Verlag, Heidelberg 1990

[4] Hesse, S.: Lexikon Handhabungseinrichtungen und Industrierobotik

(“Lexicon of Handling Devices and Industrial Robotics”),

published by expert Verlag, Renningen 1995

c c

a ab b

40

Taking the jaw stroke

into account

Fig. 4-10:

The type of approach affects

the required opening

a Opening safety margin

b Clamping safety margin

c Required jaw stroke

Publications


Axial approachRadial approach

The main purpose of a gripper is to hold objects securely for a certain period.

Grippers using the force-locking principle, on which we will concentrate in this

article, are required to generate holding forces to balance out all the steady-

state and dynamic forces and torque values which occur during a motion

sequence. The required gripping force is thus a major criterion for the selection

of the right type and size of gripper.

The required gripping force can be calculated approximately. This calculation

should not be neglected, but may not provide a final answer. In doubtful cases,

you should also carry out tests or recommend users to do so, since some of

the calculation variables are subjected to fluctuations or are only estimates.

If you go too far over to the “safe side”, this may be disadvantageous for the

user – a heavy gripper may necessitate a handling device which is one size

larger in terms of handling capacity or may reduce the working load of such a

device.

This law, formulated by Isaac Newton in 1687, states the following:

The actions of two bodies upon each other are always equal and directly

opposite, i.e. reaction is always equal and opposite to action.

This means that force and counter-force are in equilibrium. A simple experiment

to demonstrate this is shown in Fig. 5-1a. A rod is subjected to a tensile load. In

the first case, one end of the rod is clamped, while in the second case a person

pulls on each end. The tensile force in the rod in each case is not 400 N as

you might think but only 200 N. If we apply this to the parallel gripper shown in

Fig. 5-1b, this is subject to the same prin-ciple. It makes no difference whether

only one finger moves and applies a gripping force of 200 N or whether two

opposed fingers each generate 200 N. The two grippers shown are equivalent

in terms of force.

41

5

Forces acting

on grippers

Newton's Third Law

of Motion

5 Forces acting on grippers

Newton’s Third Law of Motion also applies to three-point grippers in somewhat

modified form, as we shall see. In this case, forces act in three directions.

If we confine our study of holding principles to the balance of forces with

mechanical grippers, we see that the gripping force is only a means to an end.

From the point of view of friction, the gripping force acts like a normal force.

The actual holding function is produced by friction forces FR which are created

in accordance with Coulomb's Law of Friction in the direction opposite to the

direction of motion and oppose the weight force G of the gripped object.

The simplest case is shown in Fig. 5-2.

This situation applies when the handling device is in a state of rest

(or moving very slowly). The following equation is obtained:

G = FG · µ · n

42

Fig. 5-1:

The law of interacting forces

a) The tensile force in the rod

is 200 N in both cases

b) Due to the principle

of action = reaction,

it makes no difference

with parallel grippers

whether the gripping

force FG is applied

by one finger or two

Friction forces create

a holding effect

Fig. 5-2:

Forces acting on gripped

object (state of rest)

1 Finger

2 Gripper jaw

3 Workpiece

µµ Coefficient of friction


200 N

200 N 200 N 200 N

FG FG FG

200 N 200 N

a)

b)

1

2

µ µ

3G

FG FG

FR FR

or expressed another way:

FG =

Variables in formula:

FG = Minimum required gripping force in N

G = Weight force of gripped object in N

g = Gravitational acceleration in m/s2

m = Workpiece mass in kg

n = Number of fingers or gripper jaws

µ = Coefficient of friction between gripper jaw and object.

As we can see, allowance is made in the formula for the number of fingers, since

of course a friction force FR is created at each gripper jaw. With 3 contact points,

n = 3. There are several possible variants for the 3-point solution. Fig. 5-3 shows

a “true” three-finger gripper and a solution based on a two-finger gripper. In this

latter case, the gripping force is split at the V-jaw into the contact forces FKi.

If, in the case of the two-finger gripper, a V-jaw angle of 120° is selected, the

same as the finger positioning of a three-finger gripper, the two grip-pers will

be the same from the point of view of holding forces. There will be differences

with other V-jaw angles. In these cases, we must refer to the contact angles,

which can be determined by the following formula if random V-jaw angles are

permissible:

FKi =

in which

i = 1, 2, 3

and

α1 = 180° – α23

α2 = 180° – α13

α3 = 180° – α12

43

Fig. 5-3:

Plan view

of 2 gripper situations

a) Two-finger gripper

with V-jaw

b) Three-finger gripper


m · G

µ · n

G = F .Σ µKi

α

α

G = F . . 3G µ

F = FG K1FG

FG

FG

FG

FK2

FK3

β

G · sinα1

µ · (sinα1 + sinα2 + sinα3)

The total of the 3 friction forces FR1 to FR3 (Fig. 5-4) must be at least large

enough to compensate for the weight force G produced by gravity. Knowledge of

contact forces is also required if we wish to check the gripping pressure per unit

area in the case of sensitive workpieces.

Fig. 5-5 shows the mathematical relationships governing gripping force during

an upward motion in the case of the commonly-used V-jaw grippers with

3- or 4-point object contact. A distinction can be made between 3 variants

of gripping:

• Pure positive-locking gripping

• Positive locking in combination with friction locking

• Pure friction-locking gripping.

44

Fig. 5-4:

Calculation of contact forces

for a gripper with a V-jaw on

one side


α12

α13F = FG K1

FK3

FK2

FR3

FR1

FR2

G

α23

45

Fig. 5-5:

Forces at the parallel-jaw

gripper with V-jaw for

workpieces

a Linear acceleration

g Gravitational acceleration

m Mass

S Safety factor

µµ Coefficient of friction


FG

FG

FK1

FK2

a

m.g

α1 α2

FGFG

FR2FR1

FK1 FK2

a

m.gα1 α2

FG

F =G

FR

FR

FK1

FK2

a

m.g90° α

FG

FG

FRFK

a

m.g

2α

Sketch Contact forces Gripper force upward

Pure positive-locking gripping

Positive locking with friction locking

Pure friction locking

FK1 =

FK2 =

m(g + a)sinα2

sin(α1 + α2)

m(g + a)sinα1

sin(α1 + α2)

FK1 =

FK2 =

m(g + a)

2cosα1

m(g + a)

2cosα2

FK1 = m(g + a)tanα2

FK2 = m(g + a)

2cosα2

FK = m(g + a)

4µ

FG = m(g + a) · S

FG = tanα · Sm(g + a)

2

FG = FK1 · S

FG = sinα · Sm(g + a)

2µ

Effective gripping depends strongly on the coefficient of friction. The following

can be taken as guide values:

• Workpieces with smooth surfaces, lightly oiled µ = 0.1

• Metal-to-metal contact µ = 0.5 to 0.2

• Gripper jaws with pointed-tooth surface µ = 0.3 to 0.4

• Jaws with non-slip covering or metal/rubber contact µ = 0.5 to 0.7.

The equations given in Fig. 5-5 incorporate allowances for certain factors which

we have not yet mentioned:

• The coefficient of friction m fluctuates quite widely, like a person’s blood

pressure. A safety factor S must therefore be included. In practice, the factor

used is between 1.5 and 4.

• The weight force G represents only part of the load. Allowance must also be

made for other forces, particularly the inertia forces resulting from the accele-

ration “a” of the robot arm. Process forces may also be involved, for example

during assembly, by inserting components.

It may be, in the case of a multi-axis handling machine, that the force situation

changes from time to time during a motion cycle. It may be possible to counter-

act certain forces which occur by using positive-locking gripping (lateral

motions), while other forces may call for a higher coefficient of friction. It is

therefore a question of identifying the motion phase requiring the highest

holding force and selecting a gripper on this basis. Fig. 5-6 shows a number

of typical motions and the forces operative during these.

46

Check the motion cycle

Fig. 5-6:

Force situations

during gripper motion

a) Rest state

b) Upward motion

c) Downward motion

d) Lateral motion

e) Inclined upward motion


a) G G

G

G

G

b) c)

d) e)

v=0

v

v v

v

FR

FG FG FG

FG FG

FG FG FG

FG FG

FR FR

FR FR

FR FR FR

FR FR

FB FB

FBFB

In the case of an upward motion, the weight force G and the inertia force FB can

be compensated by the friction forces FR. During the downward motion, the iner-

tia force acts in opposition to the weight force and makes the object effectively

“lighter”. During the braking phase, however, a deceleration force then becomes

operative. In the case of lateral motions, inertia force is absorbed by positive-

locking gripping, requiring no additional friction forces. The inertia force,

attempts, however, to force open the gripper jaw. With inclined motions, consi-

deration must be given to the horizontally- and vertically-acting components of

the inertia force. The position is of course somewhat different with V-jaws and

other types of handling devices. It is necessary to study the complete handling

operation.

The inertia forces can be calculated as a general principle as follows:

FB = m · a

or in the case of a rotary motion:

FB = m · r · ε

or in the case of a rotary motion:

a = Linear acceleration in m/s2

m = Mass of gripped object in kg

r = Distance from pivot point in m

ε = Angular acceleration in rad/s–1.

It would be possible to use the acceleration values specified by brochures as

being achieved by robots under normal operating conditions. This would not,

however, be quite correct. The critical values are not those of normal conditions

but of exceptional situations, particularly those following an EMERGENCY STOP

of the robot. We can, for example, take the following standard values for accele-

ration:

Acceleration in normal operation, linear: a = 5 m/s2,

EMERGENCY STOP acceleration, linear: aN = 10 m/s2,

Acceleration in normal operation, rotary: ε = 10 rad/s–2 and

EMERGENCY STOP acceleration, rotary: εN = 17 rad/s–2.

If more precise data is available from technical documentation, this should be

used.


Example: An industrial robot is to pick up an object at P1, transport this via P2

to P3 and set it down at P4 (Fig. 5-7). What gripping forces must the two-jaw

gripper provide?

(Let us assume: µ = 0.2; Safety factor S = 2).

Gripping force during lifting:

Gripping force during lateral stroke:

Gripping force during EMERGENCY STOP downwards

(after acceleration phase):

Grippers should in general be selected on the basis of EMERGENCY STOP

situations, since the requirement in these situations is that the workpiece

should continue to be held by the gripper and not ejected. The highest accelera-

tion values (in fact deceleration values) occur during an EMERGENCY STOP.

48

Fig. 5-7:

Example of handling task


P1

P2

P3

P4

Mass = 1 kg

FG = = = 74Nm(g + a) · S

µ · n

1 · (9.81 + 5) · 2

0.2 · 2

FG = + m · a = + 1 · 5 = 54Nm · g · S

µ · n

1 · 9.81 · 2

0.2 · 2

FG = = = 99Nm · (g + aN) · S

µ · n

1 · (9.81 + 10) · 2

0.2 · 2

In the cases discussed up to now, it has been assumed that the centre of gravity

of the workpiece is positioned precisely between the gripper jaws. It can of

course also lie elsewhere, but this should be avoided, since this may result in

workpieces twisting under heavy acceleration.

The following applies in case 1:

For cases 2 and 3 the following applies:

An associated problem is the ability of grippers to handle off-centre loads.

Gripper manufacturers generally state in graphs of load capacity, the forces due

to finger length resulting in a torque which tends to open the gripper jaws, or to

rotate the workpiece in cases where the gripper does not have V-jaws. The wider

the gripper jaws, the better the distribution of forces and the lower the gripping

forces required when the workpiece centre of gravity lies outside the gripper

jaws.

49

Problems with torque

Fig. 5-8:

Examples of torque created

as a result of gripped work-

pieces


FK1

a b

FK2

G

1

lc

FK2

FK1

G

2

lc

FK2

FK1

G

3

FK1 = and FK2 = G · b

a + b

G · a

a + b

FK1 = and FK2 = G(l + c)

l

–G · c

l

Why is it necessary to observe these limits?

Fig. 5-9 shows the forces which occur. In accordance with the law of levers

(force times force arm equals load times load arm), the gripping force FG pro-

duces tilting forces in the finger guide. These in turn produce increased friction.

Part of the generated gripping force is thus counteracted by friction forces in the

guide. Moreover, wherever friction occurs, wear will soon follow. The higher the

tilting moments in the guide, the higher the load. If the permissible limits are

exceeded, the gripper will not achieve either its specified service life or the

desired gripping force.

50

Fig. 5-9:

Eccentric forces acting

on a gripper finger

FN Normal forces

acting on guide

x Finger length

up to point of action

y Distance from point of

actIon to centre of gripper


FNx

FNx

FNy

FNy

FG

FG

FR

y

x

v

Users need to know whether or not their grippers develop a constant gripping

force throughout the stroke. This information is provided by a characteristic

curve of gripping force. Fig. 5-10 shows 2 transmission mechanisms as exaples.

As the angle lever (angle gripper) swivels, the gripping force varies throughout

the gripper stroke as a function of the cosine of the angle of rotation a. In the

second example, on the other hand, FG = constant. If the angle of rotation α of

an angle gripper is small, this effect can be ignored. Some lever mechanisms,

however, have a characteristic gripping force curve with very pronounced

variations. A typical case is a lever operated gripper, whose curve has a very

steep gradient, with high gripping force available only within a short range of

travel. These two examples demonstrate that not every gripper offers a constant

force throughout its travel.

51

Characteristic curve

for gripping force

Fig. 5-10:

Gripping force FG as a

function of gripper stroke h

a) Angle gripper

b) Parallel gripper

Q Tensile force

h Gripper stroke


The technical properties of grippers and their price form the basis for an

assessment of their suitability for a given application and a comparison with

other makers’ products. The more suitably a gripper range is matched to the

users’ average requirements, the easier it will be for users to select the “right”

gripper. There will of course always be special applications requiring special

grippers. But what in fact are the characteristics of a gripper?

Any use of grippers must start with a study of the planned application. This will

reveal what grippers are required to do, and what loads they must withstand. If

studies of this kind are carried out inadequately, over-hastily and incompletely,

the result is likely to be annoyance – if grippers are the wrong choice, they may

fail quickly and thus not provide the expected performance. Never try to talk

someone into using a particular gripper – the choice must be taken on the basis

of the degree to which the requirement profile and the gripper performance

coincide.

The properties of a gripper can be illustrated by some characteristic data.

The table of Fig. 6-1 gives a list of this data. The data can be sub-divided into

primary and secondary characteristic data. In examining the suitability of a

gripper, we will proceed step by step, first checking the primary characteristics

data and then using the secondary data to make a final selection. Of course,

not everything is always important in all cases. It is advisable to weight the

characteristic data as appropriate to a given application. For example, the

opening and closing times of a gripper are not important if the process

concerned is not time-critical, which is the case if the handling time of the

gripper is not in series with the main process time but runs parallel to this.

As regards the gripping force, only a low value will be required in order to fit

components with up to 3 connecting wires to printed circuit boards. Reliable

handling can be obtained even with grippers which close only by spring force.

If, however, the application also involves gripping heavier components (such as

elays), the gripping force will be important and must be checked.

In specific cases, it can also be advisable to consider further characteristic

values, for example the jaw changing time in applications where the gripper

jaws need to be changed frequently, perhaps even several times a day, due to

wear or the need to handle different products. A secondary characteristic value

may become a primary value, for example if a gripper is required to be suitable

for use in clean rooms. Even a consideration of primary data will eliminate a

number of possible candidates.

52

6

Technical properties

A few words about

characteristic data

6 Technical properties

Characteristic data for grippers

• Type designation

• Design

• Size

Primary characteristic data

• Operating principle

- mechanical

- fluidic

- magnetic

- adhesive

• Gripping force in N

• Gripping force pattern

(Gripping force diagram)

• Gripper stroke per jaw in mm

or opening angle in degrees

• Gripping width adjustment

• Load capacity max. in N

• Closing (gripping time) in s

• Opening (release time) in s

• Load limit values

- Forces

- Torques

- Finger length

• Number of gripper components

• Main dimensions in mm

• Dead weight in kg

53

Fig. 6-1:

Characteristic data

for grippers


Secondary characteristic data

• Performance/mass ratio

in N/grams

• Mass moment of inertia in kgcm2

• Operating pressure range in bar

• Maintenance cycles

• Design of bearings and guides

• Range of sizes

• Repetition accuracy in mm

• Operating temperature range

in degrees

• Mode of operation

- Single acting

- Double acting

• Working frequency max. in Hz

• Mounting position

• Energy type and consumption

• Retention of gripping force in case

of power supply failure

• Monitoring of gripping stroke

• Material specifications

• Service life

• Interface data

- mechanical

- fluidic

- electrical

• Environmental characteristics

- Clean-room class

- Exhaust air

- Abraided particles

Only idealised handling operations run perfectly smoothly. If we take a closer

look, we will see that it is always necessary to accommodate tolerances on all

axes. Theoretically this is true, the only question is the order of magnitude of

the resulting errors. Fig. 6-2 shows an exaggerated form of the general situation.

The workpiece feed is subject to errors, the robot motions are imprecise and the

target position, for example a basic workpiece to which others are to be fitted, is

also subject to tolerances.

Only an analysis of the tolerances will show whether the situation is critical

or not. Roughly one-third of all assembly applications are a question of inser-

ting pins into holes. Some of these operations can be regarded as precision

assembly, with clearances of only a few hundredths of a millimetre. In cases of

this kind, it may occur that the repetition accuracy of the gripper and industrial

robot used exceeds the permissible limit. The repetition accuracy of a gripper is

defined as the variation in the jaw end position during 100 successive gripping

operations (closing motions). This figure may, for example, be ± 0.02 mm in the

case of a parallel gripper.

In order to carry out close-tolerance assembly, all the following measures must

be adopted:

• Improvement of repetition accuracy, particularly that of the handling device

• Design of components to be assembled in such a way as to facilitate

assembly, particularly by providing guide chamfers

• Combination of grippers using joining mechanisms.

Joining mechanisms are devices placed ahead of the gripper which are designed

to compensate for angular and positioning errors between the connecting part

held by the gripper and the connecting axis defined by the basic assembly part.

A distinction is made between active (IRCC = instrumented remote centre com-

pliance) and passive joining mechanisms (RCC = remote centre compliance).

In the interests of simplicity, most are are of the RCC type. These can easily

54

Accuracy of gripping

Fig. 6-2:

General model of a

handling operation

1 Workpiece store

on suspended conveyor

2 Industrial robot

on a mobile unit

3 Assembly carriers

on a transfer line


1

3

2

compensate for position deviations of 2 mm with an orientation error of 2°, and

a permissible clearance between the two joining parts of as little as 0.01 mm.

In order for these devices to operate, guide chamfers are required on the basic

assembly part (bore) and/or the joining part to be added (pin), a factor which

should not be forgotten. Fig. 6-3 shows the principle of an RCC unit. Compliance

can be provided by using a special configuration of elastomer components or

leaf springs. The inner pair of joints correct angular errors, while the outer pair

of joints compensate for positioning errors. The apparent (remote) pivot point of

the joining part to be inserted lies at the tip of this. It is not necessary for users

to make their own joining mechanisms; these are commercially available in a

range of sizes.

The problem of fitting a pin into a hole is similar to the problem encountered

in feed motions of inserting a workpiece into a clamping device. There is little

fundamental difference between these two operations. We shall consider this

type of operation next.

A gripper and clamped workpiece represent a rigid structure. If this is used to

feed a clamping device, this may cause overload of the gripper if the clamping

device is required to close before the gripper is allowed to open. This is shown

in Fig. 6-4, which is based on the assumption that the axes are not perfectly

aligned. This is theoretically always the case.

The gripper and handling device are pressed in the direction of the clamping

axis, which is in effect an overload. There is a brief force-locking connection

between the handling device and the machine to which the workpiece is being

fed. A flexible flange plate on the gripper can help prevent damage

55

Fig. 6-3:

Joining mechanism

with combined lateral and

angular compensation

1 Lateral (position)

compensation

2 Angular compensation

3 Apparent pivot point

for angular compensation

4 Gripper support plate

5 Parallel gripper

6 Gripper jaws

7 Joining part

8 Basic assembly part

Compensating for axial

alignment error


1 24

5

6

7

83

This problem theoretically also occurs when a gripper picks-up a workpiece

from a pallet under conditions of axial misalignment. As the gripper tightens

on the workpiece, the robot arm is once again pushed out of position and conse-

quently uses the power of its actuators to attempt to regain its programmed

position. This may even lead to damage to the robot drive system. In this case,

too, a slightly flexible flange connection provides a remedy (Fig. 6-5). Moreover,

the value chosen for the clearance of the workpieces in the workpiece carriers

should not be too small in order to allow the workpieces to adapt to the gripper

position.

56

Fig. 6-4:

Situation in which an overload

of the gripper and handling

device can occur if no com-

pensation is provided.

a) Feed to a clamping device

b) Axial correction during

closing of clamping device

Fig. 6-5:

Picking-up a workpiece

from a magazine pallet

a) Axial misalignment x

during approach

to position

b) Compensation

during gripping operation


a)

b)

Force closure

a)

x

b)

The situations here are more than the grippers can cope with. Attention must be

paid to axial misalignment.

What is the answer? There are various possibilities:

• Depending on the type of clamping device (clamping collet, vertical insertion),

it may be possible to work in the sequence “Set down/open jaws/clamp in

machine/retract gripper” or it may be essential to work in the sequence

“Insert workpiece/clamp in machine/open jaws/retract gripper”. The former

case does not present difficulties, since there is no force-locking connection

between the clamping device and gripper.

• There are industrial robots which allow a “soft” switching action within certain

limits. The arm behaves compliantly and the robot does not attempt to

reestablish its old position. This is achieved by means of a larger area of

coincidence for the evaluation of the signals from the positional transducers

of the respective robot axes.

• It is possible to use hand-joint sensors to detect misalignment. The sensor

data is used to derive corrective motions for the robot arm.

• The simplest method is to use compliant intermediate plates (rubber, springs),

which provide adequate compensation at least for minor errors. Even gripper

jaws with compliant faces are often enough.

• It is also possible to arrange for the gripper to open in stages.

In the transitional phase, the gripping force is slightly reduced. Commercially-

available grippers with this type of function operate with spring fingers

(leaf springs) driven by a three-position cylinder.

• Grippers have also been produced with a definite floating mounting for use in

cases where large deviations can be expected between the actual position

and programmed setpoint position, for example when picking cartons from

shelves. Once the workpiece has been gripped and raised, the gripper travels

to the centre of the axis and is locked in this position. This requires a locking

device, which is integrated into the gripper.

• The sequence “Insert workpiece/open jaws/clamp in machine/retract

gripper” can, at least with small workpieces, be achieved by using a pressing

element as shown in Fig. 6-6. This is an additional facility complementing

the gripper function whereby the pressing element is clamped when the work-

piece is picked up. Once the clamping position is reached, the gripper opens.

The pressing element now acts on the workpiece and presses it against

the contact area of the clamping device, into which it is then clamped.

The gripper, having completed its task, is retracted without being subjected

to overload.


It can be seen from the number of remedies available that this is a problem

which needs to be taken seriously.

Fig. 6-7 shows a very simple device which can be used to assist assembly

operations. The gripper is mounted on a cone able to tilt by about 15°. This

makes the gripper flexible in the x, y and z directions in cases where the joining

part misses its destination and rests on the basic assembly part. In these cases,

the cone lifts slightly, creating some “breathing space” in the x-y plane.

The gripper is now able to deflect in the appropri-ate direction. It is, however,

necessary for the mating parts to have guide chamfers.

58

Fig. 6-6:

Gripper combined

with a pressing element

1 Pressure spring

2 Gripper

3 Pressure plate

4 Workpiece

5 Gripper jaws

Fig. 6-7:

Simple joining mechanism

for vertical assembly

1 Centring cone

2 Connector plate for gripper

3 Gripper

4 Gripper jaw

5 joining part

6 Basic assembly part

7 Arm of handling device


1

2

3

4 5

1

2

3

4

5

6

7

x

y

z

Grippers are final effectors, which is to say that they are positioned at the end

of a kinematic chain (free-arm robots) and thus have the greatest radius of

action of all the robot components. This in turn means that grippers are subject

to the greatest risk of collision. The more complex and delicate a gripper is,

the greater the chance of damage in the case of a collision. Collision-protection

devices (or shut-off devices) have thus been developed to prevent this. These

devices are fitted between the gripper and robot arm and complement the

gripper control system. The protective devices are triggered when an adjustable

load threshold is exceeded and generate a shut-off signal. In the case of the

device shown in Fig. 6-8, a pneumatically-pressurised chamber is used to keep

the device stiff. In the case of a collision, the cushion of compressed air is

depressurised and the mechanism becomes “soft”, i.e. slightly flexible.

There are also spring-loaded mechanisms in which the gripper disengages under

overload and recoils from an obstacle. These, however, offer little convenience

in the form of adjustment but are very simple in design. The deciding factor is

of course the application in question and the probability that unexpected ob-

stacles will be encountered. This will indicate whether it is necessary to protect

a gripper against collisions. If there are no obstacles anywhere near the gripper,

collision protection will certainly not be required.

59

Protection

against collision

Fig. 6-8:

Collision protection

with adjustable parameters

for a gripper, showing

reaction capability

a) Rotational

b) Vertical

c) Horizontal

F Triggering force

z Vertical impact path

αα Angle of deflection

resulting from collision

ββ Angle of tilt


β

F

F

α

a)

z

F

b) c)

Every gripper requires space to operate. The route to the gripping point requires

a gripper working area or feed channel which must be free of obstructions.

The minimum size of this area is governed by the contour of the gripper with

open jaws or with a workpiece if this projects beyond the edges of the gripper.

As the result of this, it may prove better to use a parallel-jaw gripper instead

of an angle gripper. This is illustrated in Fig. 6-9. It has been assumed that the

workpiece to be press-fitted into a basic assembly part needs to be picked up

by a gripper making a positive-locking connection in the insertion direction.

This can, however, also be achieved by using a parallel gripper, with the ad-

vantage that this will permit the storage locations on the magazine pallet to

be positioned more closely together. This increases storage capacity, which is

generally desirable.

If the gripper is kept as light as possible, this means a higher payload for the

handling device and minimum impairment of dynamic machine characteristics.

Fig. 6-10 shows the relationship between the nominal load of the handling

device and the tool load, with grippers often being regarded as tools. The nomi-

nal load specification refers to the interface between the robot arm and the

connector flange of the gripper.

If a handling device is operated with the maximum possible load, speed and

acceleration must be reduced. This may affect one motion axis or several.

The handling cycle will thus become slower. This is not a problem if the process

times are significantly longer than the cycle time for a handling operation.

It is therefore worth considering in appropriate cases whether an increased

handling-device load can be used.

60

Gripper working area

Fig. 6-9:

When working with three-

dimensional object configu-

rations, consideration must be

given to the clearance contour

of the gripper

a) Radial gripper

b) Parallel gripper

1 Clearance edge

2 Pitch circle of gripper jaws

3 Workpiece

4 Flat pallet

5 Open gripper jaws

x Distance between

storage locations

Load-bearing capacity

and dead weight


1

2

3 5

4

a)

x x

b)

The performance of a gripper can be expressed by the ratio of gripping force

in newtons to dead weight in grams. The performance index for a gripper with

a mass of 420 g and a gripping force of 300 N would thus be 0.71 newtons

per gram of dead weight. Values of over 1 indicate very good grippers. Most

commercially-available grippers, however, have in-dexes well below 1.

The service life of a gripper is an important selection criterion. Modern grippers

are expected to last for at least 10 million gripping cycles. This is achieved by

using high-quality materials and providing appropriate treatment of the contact

surfaces of active components and precise wear-resistant guides. It must be

ensured that the type and level of load specified for the gripper in question are

not exceeded. Grippers with additional seals must be used if coolant, casting

dust or grinding dust are present.

61

Fig. 6-10:

Specifications of load capacity

for industrial robots

Service life


Nominal load

Tool load Working load

Maximum load

Additional load

Maximum working load

The excellent flexibility of an industrial robot from both the mechanical and

control technology points of view and the speed of a pick-and-place device can

provide a practical benefit only if the selected gripper meets the requirements of

the application in question. The applications of a given gripper are not, however,

subject to any rigid definition – with a little imagination, modifications can

always be found to provide the optimum solution to a gripper application.

The aim of this article is to provide some suggestions for this.

One of the main uses of industrial robots and insertion devices is without doubt

machine-feed and assembly applications. Both these areas may involve require-

ments and customer wishes which go well beyond the “aver-age case”. It may

also be the case that a single gripper module is required to deal with objects of

widely-varying geometry. Each individual gripping task must be thought through

thoroughly before a recommendation is made.

It is almost impossible to specify particular types of grippers for particular

applications, since virtually every type of gripper can be made suitable for a

given application by selecting an appropriate size, jaws, peripheral devices,

magazining technique and gripping strategy. Fig. 7-1 nonetheless shows a

rough correlation between object features and gripper types. This correlation

relates to average situations and covers parallel grippers, radial grippers

(jaws opening 90°), angle grippers (opening angle per jaw 18°), 3-point grippers

and suction grippers. There are always wide variations within each gripper

type and special cases, such as combination suction grippers which can lift

sheet-metal workpieces weighing several tonnes. The angle-gripper principle

is used for large forging manipulators with load-bearing capacities of as much

as 250 tonnes.

62

7

Application areas

and gripper types

Application areas

of grippers

7 Application areas and gripper types

Gripper types

Gripped objects

Mass 0,2 ... 1 kg

1 ... 10 kg

10 ... 50 kg —

> 50 kg —

Dimension 20 ... 50 mm

50 ... 300 mm

300 ... 1000 mm —

> 1000 mm — —

Internal gripping — —

Surface Smooth

Rough —

Porous

Sensitive — —

Round parts Disc —

Short cylinder

Shaft/Rod — — —

Prisms Block —

Flat/short —

Flat/long — — —

Plastics —

Textiles — — — —

Foil — — — —

Glass

Pottery

The process of selecting grippers is often dominated by operating parameters

and special properties. Fig. 7-2, for example, shows a radial gripper whose large

swivel angle allows it to grip flanged sheet-metal workpieces when equipped

with gripper jaws shaped like spot-welding tongs. This is almost impossible to

achieve with other types of grippers.

63

Fig. 7-1:

Approximate correlation

between gripped objects

and gripper types

Ideal

Suitable

Suitable in certain cases

— Not applicable


The size of the workpiece to be handled need not necessarily dictate the size of

the gripper. If, for example, a gripper is required for large integrated circuits (ICs)

with 40 pins, this gripper will almost always need to be capable of executing

a powerful closing motion, since with large IC’s considerable force is required

to bend the numerous “legs”. As Fig. 7-3 shows, the reason for this is that the

gripper must adjust the legs to a precise spacing during its closing motion.

The IC legs are prebent to an angle of approximately 15°, allowing them

to conform to the correct spacing in the gripper.

Applications such as the feed of automatic machine tools and the removal of

processed workpieces may require 2 workpieces to be picked up simultaneously.

Special multiple grippers can be designed for this purpose, but it is sometimes

possible to use a slightly modified simple parallel gripper for this purpose. This

is shown in Fig. 7-4. It is, however, then necessary to use suitable gripper jaws

to suit the distance between the workpieces.

64

Fig. 7-2:

Radial gripper holding

a sheet-metal workpiece

Fig. 7-3:

ICs have splayed pins which

are aligned to the desired

spacing during the gripper

motion

a) IC with straight legs

b) Pins splayed

out at an angle

c) Gripper jaws


These examples demonstrate again that it is not possible to achieve a strict

correlation between gripper types and workpiece properties.

There are cases where the clamping point of a machine is not freely accessible

to the gripper due to the fact that passage is obstructed by tools, safety or test

equipment, etc., leaving only a certain “feed channel”. In cases of this kind, the

clearance contour of the gripper when holding a workpiece is a critical factor in

the selection process. Fig. 7-5 shows a solution in which a workpiece is gripped

parallel to the main axis of a 3-point gripper. The gripper has been fitted for this

purpose with specially adapted gripper jaws. The workpiece should be gripped

close to its centre of gravity to prevent unnecessary moments which would have

the effect of rotating the workpiece out of the jaws.

65

Fig. 7-4:

Example of twin-workpiece

gripper as a special use

of a parallel jaw gripper

Feed gripper

as special solution

Fig. 7-5:

Handling lengths of bar

material with a 3-point

gripper

1 Arm of a handling device

2 Three-finger gripper

3 Gripper jaw


1

2

3

clamped

open

A further gripper application is shown in Fig. 7-6. A rectangular workpiece is to

be placed precisely in a clamping device. This requires the workpiece to be

aligned on two axes. The 2-point gripper is, however, able only to align the work-

piece on the x-axis. The accuracy on the y-axis depends on how accurately the

workpiece is positioned at the pick-up point. If the workpiece has suitable geo-

metrical properties, these can be exploited (Fig. 7-6b) to produce an alignment

effect on the y-axis as well. It is also possible to provide the workpiece with a

suitable geometrical feature just for this purpose. This is a form of automation-

compatible design.

An alignment effect can also be obtained by clamping the workpiece on the

diagonal (Fig. 7-6c). If, however, the design of the clamping device means that

the corners must be left free, a 4-point gripper can be used. A suction gripper

would not be suitable due to the interrupted surface of the workpiece and the

gripper’s less accurate positioning (displacement of soft sealing lips, floating

during pick-up). 4-point grippers are commercially available. It is, however, also

possible to combine 2 parallel-jaw grippers to form a gripper system as shown

in Fig. 7-7. This type of arrangement is also known as a combination gripper.

66

Fig. 7-6:

Gripping a rectangular

workpiece

a) 2-point gripper

b) Exploitation

of geometrical features

c) Corner-to-corner

clamping

d) 4-point gripper


a)y

x

c)

b)

d)

Great progress has been achieved in recent years in the automation of assembly

work at all technological levels. For example, automatic machines have been

developed for the assembly of electronic components which allow cycle times

well below 1 second. This cannot be achieved by industrial robots, but these

play a valuable role in flexible assembly systems for short-run assembly work.

Assembly robots are without doubt an essential element of the “factory of the

future”.

One of the ways of achieving flexibility is to use automatic gripper changing

systems. The idea is to constantly interchange individual specialised grippers as

appropriate to technical requirements. In cases of this kind, an effector is more

than just a gripper and may also include further function groups (Fig. 7-8).

67

Fig. 7-7:

A 4-point gripper created by

combining 2-point grippers

Grippers for assembly

applications


The changer system provides a mechanical coupling and connections for signal

and power-supply lines, for example for compressed air. Each individual gripper

must be equipped with a lower changer system. Automatic changing (setting

one gripper down and picking up another) takes approx. 5 seconds. The purpose

of the joining mechanism is to provide automatic compensation for axial offset

and small anglar deviations. A collision protecting device can also be valuable,

particularly in cases where it is necessary to protect a complicated and costly

gripper from damage. If the gripper is overloaded, the collision protection device

disengages, triggering an emergency stop of the handling device.

For long-run assembly operations, other gripper systems can be considered,

such as assembly grippers. This term is commonly used to refer to all grippers

in assembly operations but should really only be applied to grippers inside

which an assembly operation can be carried out. These will of course be special

grippers built for a specific purpose or combination grippers. Fig. 7-9 shows an

example of these.

68

Fig. 7-8:

Gripper module

for flexible assembly

1 Industrial robot

connector flange

2 Upper part of

changer system

3 Joining part


1

2

3

Collision protection

Uncontrolled joiningmechanism

Gripper with lowerchanger system

The gripper system consists in this case of a suction gripper and parallel-jaw

gripper. These are independent and are activated individually. It is perfectly

possible to produce this combination gripper from standard components.

In accordance with the assembly sequence shown in Fig. 7-10, the joining part

is first picked up by the suction gripper. The basic assembly part is then gripped

by the parallel gripper. The joining process is carried out while the effector

travels to the set-down position. This operation can also be carried out during

set-down, by inserting the joining part into the basic assembly part. This method

can be used, for example, to place lenses in mounts. The advantage is that no

external assembly device is required.

69

Fig. 7-9:

Assembly gripper

1 Hollow piston rod

2 Connecting piece

3 Parallel gripper

4 Short-stroke cyl.

5 Suction cup

6 Gripper jaws

Fig. 7-10:

Sequence for assembly

within a gripper

a) Approach

to pick-up position

b) Gripping the joining part

c) Lifting the joining part

d) Approach to

2nd pick-up position

e) Gripping the basic

assembly part

f ) Joining process

g) Module assembled

h) Set-down of

completed module


1

2

3

4

56

a)

e)

b)

f )

c)

g)

d)

h)

Grippers are the direct interface between automation devices and the objects

to be gripped. The geometry of these objects can vary greatly. Operating con-

ditions, too, are certainly not constant and may be far from ideal. This makes it

difficult to select grippers. In individual cases, it may be that no standard gripper

is acceptable, making it necessary to develop a special gripper. The rule there-

fore is – check all the requirements of a given application and consider their

feasibility. The selection of grippers is a matter which needs to be taken very

seriously!

Up to the present time, no uniform guidelines have been developed for the

design and sizing of grippers. If a variety of workpieces need to be handled,

the selection of grippers will be determined chiefly by object and process para-

meters and other parameters under the user's control. There is at the moment

no universal algorithm to determine the structure of gripper systems and the

design of grippers. Programs are, however, avail-able for the calculation of

technical/physical parameters.

The relationships between the major technical/physical factors governing grip-

per applications are shown in Fig. 8-1. The critical aspect is not steady-state

conditions but the dynamic effects in a moving system. Furthermore, it is not

enough to consider random moments which occur at some point during a hand-

ling operation. Rather, it is important to determine the maximum values which

are encountered at various points in time within a motion sequence. There are

then two possibilities:

• Relaxation of requirements by changing motion and time parameters and/or

• Selection of a gripper on the basis of the maximum parameter values within

the motion sequence.

70

8

Checklist for grippers

Many interrelationships

between factors

8 Checklist for grippers

c

Avoid placing unnecessarily high requirements on the gripper technology,

since this will increase purchase and operating costs. Rather than have a robot

continually returning to a waiting position, it is better to reduce its speed

slightly.

With complicated workpiece shapes, the procedure must start somewhat earlier

with a search for suitable gripping surfaces on the object. This process is shown

in Fig. 8-2. We must be aware that any change in grip-ping points will have

effects on the dynamic behaviour of the gripper and must be recalculated for

a given gripper. If a workpiece has recesses or shoulders which can be used to

create a positive-locking connection with the gripper, these should be used.

This will allow the gripping force to be reduced.

Anordnung vonRoboter undMaschine

Bewegungsrichtung desGreifers zum Greifobjekt

Form und Anzahl derElemente, Auswahl desGreifers und der Kine-matik der Arbeits-elemente des GreifersLage der Teile am

BereitstellplatzGreifzone und Oberflächean der Griffstelle

Berechnung von Kräftenund Momenten, diewährend der Bewegungdes Teils im Raumauftreten

Lage der Teile inder Übernahme-einrichtung

Form und Grösseder Greifobjekte

Masse und Eigen-schaften derObjekte

Werkstoff undOberfläche desGreifobjekts

Bewegungsfolgein den Achsen

Geschwindigkeitund Beschleuni-gung des Robo-ters je Achse

Greiferflanschund Energieart

Auswahl des Typs desGreiferantriebs

KonstruktionGreiferanschluß

Kontrollrechnung undOptimierung derGreiferkonstruktion

Festlegen der Antriebs-parameter für Greifer-antrieb

Bestimmen desGreifergetriebes

Berechnung derFlächenpressung ander Griffstelle

Lage der Objekteim Greifer

Berechnung der auf denGreifer wirkendenKräfte bei Bewegungendes Roboters

71

Fig. 8-1:

Mutually influential factors

and basic variables relating

to the selection of grippers,

from the technical point of

view


Position and components in transfer device

Configuration of robot and machine

Position and components at pick-up point

Calculation of forces and moments which occur during 3D motion of object

Shape of size of object to be gripped

Mass and properties of objects

Material and surface of gripping object

Axis motion sequence

Direction of motion ofgripper towards object

Gripping zone and surface at gripping point

Determination of gripper gearing

Calculation of forces acting on gripper during robot motion

Selection of type of gripper drive

Shape and number of gripper components, choice of gripper and kinematics of gripper components

Position of objects in gripper

Calculation of contact pressure at gripping point

Definition of drive parameters for gripper drive

Design of gripper connection

Checking calculation and optimisation of gripper design

Speed and acceleration of robot per axis

Gripper flange and type of power source

A number of attempts are being made to produce programs which will allow

the automatic planning of handling systems, including grippers. The procedure

used here, too, is to identify all possible gripping surface pairs on the object to

be manipulated (relating to two-jaw grippers, i.e. parallel surfaces). The contact-

free regions (gripping zone) are then defined. We then determine all the grippers

able to work within the defined gripping zones. There must be adequate coinci-

dence between the object gripping surfaces and gripper surfaces (gripper jaws).

The decision as to whether a gripper is able to grip a workpiece by the pair of

gripping surfaces selected is then taken in a process which takes into account

various physical and other parameters. Once all possible types of grippers have

been identified, an optimisation process is started to identify the best gripper

(type and variant).

The most practical method would of course be a comprehensive simulation

system in which a gripper could execute all the required motion sequences on

the computer screen in the form of an animation. This would allow continuous

output of all major parameters, with automatic signalling whenever limit values

were exceeded. It would also allow cases to be identified in which even slight

changes in speed or acceleration would offer an advantage.

72

Fig. 8-2:

Generation of characteristic

gripper data, starting with

the gripper location


Berechnung:- Kraft- Moment- Trägheitsmoment

Überprüfen derGreiferdaten

Verschieben derGreifebene

Abmessungen vonWerkstückabsätzen

Oberflächen-beschaffenheit

Dokumentieren derErgebnisse

vorläufigerGreifort

Griff-Flächesuchen

Greifzoneermitteln

Greifervarianten undFeststellung der

EignungSurface properties

Calculation of - Force- Moment - Moment of inertia

Finding a grippingsurface on the object

Estimation of workpiece dimensions

Determination ofgripping zone

Provisional location of gripper

Documentation of results

Checking of gripper data

Relocation of gripper plane

Study of gripper variants and suitability of these

The first step towards finding the right gripper is to undertake a comprehensive

description of the task for which the gripper is to be used. This will often involve

the combination of several components in an assembly station and will require

severval grippers. The question then is whether to use a gripper changer system

or not. Gripper systems which can be matched to individual assembly operations

make cycle times shorter and thus result in faster flexible assembly systems.

The time required for gripper changing must, however, be short, and only an

automatic changer system can ensure this. The determining variables are the

range of workpiece variants and the batch sizes involved. As a rule of thumb,

automatic gripper changer systems will be economically viable if the number

of geometrically different component variants per batch is 5 or more or the

production time per batch is up to 2 hours.

The flow chart shown in Fig. 8-3 can be used to select an individual gripper.

Each activity can be correlated to typical questions. These can help in a

discussion with users to arrive at a binding list of technical characteristics.

Intuitive selection of a gripper on the basis of gripping force alone, as is

still done in many cases today, can easily lead to error and should be avoided

at all costs.

73

Step-by-step gripper

selection


Let us now consider the questions:

1. Are the object properties, especially mass, size, fragility and surface quality,

sufficiently well known or are further tests required?

2. Is there good access to the gripped object (gripper freedom “feed channel”)?

3. Has the gripper application (handling cycle) been defined in a detailed

and binding way or are changes likely?

4. Is a single gripper required to handle both blanks and ready-machined

workpieces (radical changes in shape during working cycle)?

5. Are all working conditions known (pressure, temperature, object condition,

cycle time, dust generation, oil mist, humidity, coefficient of friction,

mass, etc)?

74

Fig. 8-3:

The main steps

in the selection of grippers


ja

nein

nein

ja

1 bis 5

22 bis 26

6 bis 12

27 bis 32

33 bis 35

36 bis 39

40

13 bis 16

17 bis 21

Abklärung aller Bedingungen (Masse, Größe, Form) am Greifobjekt,der Bewegungssequenz und einschränkender Randbedingungen

von Prozeß und Umgebung

Zusammenstellung sonstiger wichtiger Anforderungen wie z.B.Genauigkeit, Anschlußbedingungen, Überlastschutz,

Greifpunktverlagerungen und Kontrollen

Festlegung des Greifprinzips: Einzel-, Mehrfachgreifer,Sondergreifer, Haltesystem, Kinematik bzw. Kraftfelder

Gestaltung der Greiferbacken, sensorische Ausstattung,Medienführung und Befestigung

Entspricht das Konzept den Anforderungengemäß Aufgabenstellung ?

Bewertung und Typauswahl (monetär sowie nichtmonetär)

Realisierung durch Zukauf, Fremdvergabe oder Eigenbau

Stopp

Start TypischeFragen

Ermitteln der notwendigen und auftretenden Kräfte sowie der dabeivom Werkstück zu ertragenden Belastungen

Kann die Belastungssituation in jederBewegungsrichtung beherrscht werden ?

Determination of all conditions (mass, size, shape) applying to the gripped object,the motion sequence and any limiting factors originating in the process or its environment

Start

Outsourcing, special external manufacture or in-house

Definition of gripper principle: Single, multible or special grippers,holding system, kinematics or force fields

no

Determination of necessary forces, forces occuring and loads to which workpiece is subject

Can the load be successfully handled in everydirection of motion?

List of other important requirements, such as accuracy, connection conditions, overload protection, deviation of gripping points and monitoring devices

yes

yes

Design of gripper jaws, sensor equipment, media throughfeeds and mounting

no Does the concept conform to the requirementsas expressed in the problem description?

Evaluation and selection of gripper model (financial and nonfinancial factors)

Stop

Preguntastípicas

1 to 5

6 to 12

13 to 16

17 to 21

22 to 26

33 to 35

27 to 32

36 to 39

40

6. Can the object be held by a force-locking connection, or is positive locking

or a combination of the two methods also possible?

7. What principle is to be used, clamping or adhesion

(fluidic, magnetic, frictional)?

8. Are the object gripping surfaces (and forbidden zones) specified

or can they still be changed?

9. Is the workpiece being gripped at its centre of gravity to avoid moments?

10. Would a gripper changer system or turret gripper be worthwhile

for the handling of several different workpieces?

11. Is a positive-locking connection provided in the direction

of highest acceleration?

12. Is the workpiece position at the pick-up point in the magazine the same

as at the machining point?

13. Is it necessary also to make allowance for process forces,

such as occur during assembly operations, etc.?

14. Are high-friction or patterned gripper jaws advisable?

15. Are the grippers and the motion devices to which they are connected

dimensioned for peak forces and moment (e.g. in an emergency stop

situation)?

16. Can the workpiece withstand the intended contact pressure or must larger

or additional contact areas be provided?

17. Is it possible to dangerous situations (uncontrolled release of workpiece)

to result from a high-speed emergency stop)?

18. Has a sufficient safety factor been incorporated into the calculated gripping

force required?

19. Must the gripping force be precisely limited to prevent damage

to the workpiece?

20. Is a device required in order to maintain gripping force

(double non-return valve)?

21. Is it advisable to fit a collision- and overload-protection unit?

22. Is the achievable accuracy sufficient in order to provide a reliable solution

for the task in question?

23. Can the clearance contour of the gripper when open (or closed

with a workpiece) cause a risk of collision in the gripper's environment?

24. What kind of centring effect (during pick-up and alignment) is the gripper

expected to provide?

25. What triggering method (e.g. directional control valve combination)

should be recommended to users of pneumatic grippers?

26. Is the gripper control system required to be event-triggered

(by the presence of the object to be gripped)?

27. Are the gripper fingers (lever arms) as short as possible?

28. Is it necessary to monitor the finger positions (open, closed) with sensors?

29. Particularly in the case of assembly operations, is it advisable to provide

assembly mechanisms and/or force sensors?

30. Are several sets of gripper jaws (with facility for quick changing)

to be provided?


31. Are adapter plates required for the mechanical connection of the gripper,

and are these commercially-available designs?

32. Are the gripper jaws required to compensate for parallelism errors

at the gripping surfaces of the object?

33. Is the achievable time sequence (for pick-up, transportation and release)

acceptable?

34. Will the gripper provide the expected service life under the operating

conditions in question?

35. Are all operating values for the gripper within permissible limits?

36. Will a standard gripper, possibly with special jaws, be suitable,

or will a special solution be required?

37. Are the conditions of supply and guarantee as expected?

38. Do auxiliary devices (pressure devices, gripper magazine) need to be

considered?

39. What level of cleaning, servicing and maintenance will be required?

40. Has a satisfactory gripper been found or should the problem be referred

to a gripper specialist?

The Festo gripper selection tool (GST) can be used to make a selection on the

basis of technical/physical parameters.

The input parameters are as follows:

• Distance between centre of gravity of object and gripper

• Mass of object and fingers

• Mass moment of inertia of gripper finger and distance to centre of gravity

• Gripping motion type (internal, external)

• Direction of acceleration and maximum acceleration value

• Coefficient of friction between finger and object

• Operating pressure in compressed air supply network

• Safety factor

• Eccentricity of centre of gravity of object.

The result of this is a recommended gripper size, with an indication of the

degree of utilisation of maximum capacity in percent. This provides a starting

point for optimisation. It is possible, for example, to move to the next smallest

size of gripper by lengthening the closing times (if the process permits this)

if this is the only parameter which exceeds the permissible limit. An additional

program is available for the determination of mass moments of inertia.

Programs of this kind do not of course provide all the answers but they relieve

users of time-consuming calculations and provide more time for an examination

of all the other parameters. Technical/physical parameters governing fit and

clearances have first priority in nay case in the gripper selection process.

76 8 Checklist for grippers

In the Middle Ages, vacuum was very much an unknown quantity. It was not

until Otto von Guericke of Magdeburg conducted his famous experiment with

hemispheres and teams of horses that a first insight was gained into this stran-

ge empty state which nature was said to abhor. People were, however, aware of

such things as the 45-centimetre scars seen on the skin of whales, caused by

the suction cups of giant octopuses. These cups can be 10 to 15 centimetres in

diameter and are located on the octopuses' tentacles, which are up to 15 metres

long. Today, suction cups are in widespread use in industry as an inexpensive

and simple automation tool. This is the first of three articles dealing with these

devices.

Vacuum technology operates with a flowing compressible medium – air. Vacuum

is said to be present in a space in which the air is diluted, resulting in a pressure

significantly lower than the surrounding (atmospheric) pressure. This principle,

applied to suction cups, means that there is a pressure difference between the

interior of the suction cups and the surrounding air. Atmospheric pressure

presses the lips of a suction cups against a workpiece. The suction cup is thus

a means to create the boundary of a pressure zone. Fluctuations in atmospheric

pressure mean variations in holding force. For every 100 metres increase in

altitude above sea level, atmospheric pressure falls by 12.5 mbar. This pressure

is 1013 mbar at sea level, falling to 763 mbar at an altitude of 2,000 metres

above sea level.

Suction cups are a popular and simple solution for repetitive gripping appli-

cations of the “pick up, move, set down” type, provided that the workpieces in

question have flat non-porous surfaces. A further advantage is that suction cups

can be used with non-magnetic materials such as glass, ceramics and wood.

We can make a general distinction between 2 types of suction-cup applications:

• Large suction area and small pressure difference

The advantage here is that the holding force can be built up quickly and that

there is little deformation of soft flexible workpieces. In the case of slightly

porous materials, air is not drawn through these.

• Small suction area and large pressure difference

This means high gripping forces as the suction cups used become smaller.

This allows the clearance radius of manipulators to be made smaller, which is

often a decisive factor when space is limited.

Fig. 9-1 shows the most important functions of a suction cup. Not all these

functions will be used in every application.

77

9

Suction grippers –

abhorred by nature

Air as a medium

9 Suction grippers – abhorred by nature

Suction cups are suitable for a large number of handling operations, such as

sorting, feeding, clamping, turning and stacking, and are used as grippers with

lifting devices, balancers, feed devices, stacking systems, packing machines and

production lines. Suction cups are particularly convenient when workpieces have

the following features:

• Awkward dimensions

• Susceptible to deformation

• Non-magnetic

• Surfaces sensitive to scratching (ground, polished, painted)

• Undulating but non-porous surfaces.

78

Fig. 9-1:

The most important functions

and properties in relation to

suction grippers

1 Vacuum line

2 Pressure switch

3 Angular freedom

4 Quick exhaust

5 Vertical freedom

6 Quick interchangeability

7 Generation of holding force

8 Workpiece contact sensor


1

2

3

4

5

6

7

8

The purpose of this is to define the vacuum in the suction chamber and the size

of the suction area in such a way that these compensate reliably for all the

forces occurring during manipulation operations. In the case of slow motions,

such as the movement of suction-held workpieces on a balancer, it is sufficient

to consider static forces. With high-speed motions, dynamic forces must also be

considered. Fig. 9-2 illustrates the relevant force conditions. The following

applies as a general principle:

F = (po – pu) · A · n3 · η · z ·

The terms used in the above are as follows:

A = Theoretical area of suction cup.

F = Working load; weight force of gripper object;

total load acting on suction bond.

n3 = Coefficient of deformation. Very soft lips (bell-shaped suction cups)

deform strongly as vacuum builds up, which may reduce the effective

suction area.

n3 = 0.9 to 0.6.

po = Atmospheric pressure; dependent on altitude above sea level.

pu = Pressure in seal suction chamber.

S = Safety factor to guard against detachment of workpiece.

A state of equilibrium alone is not sufficient – the gripped object

must be pressed against the suction cup with a certain force.

S = 2 to 3.

z = Number of suction cups.

η = Efficiency of system, including leakage allowance.

With high-speed motions, allowance must also be made for forces resulting from

weight, mass moment of inertia and centrifugal force. This results in different

lines of action for the overall force. Furthermore, the centre of gravity of the

gripped object may not coincide with the centre of the suction cup. Fig. 9-3

shows the resulting typical load cases and the calculation of the required

suction cup force FS.

79

Sizing suction cups

Fig. 9-2:

Force conditions with

vertically-moving suction cup


1

S

FS

F

pu

po

A

The force F is always the force resulting from all static and dynamic effects,

including allowance for superimposed motions.

80

Fig. 9-3:

Typical force situations

at suction cup


v

µ

FSS

F

6

FS

FS

v1

S α

µ

FS

FX

FZ F

v2

S

FS

F

v

r R

3

α

µ

FS

FX

FZ

v

S

F

r

4

rα

µ

FS

Fy

FZ

S

FR

5

Fs ≥ n1 · F

F Sum of all forces producing detachment and shift

FS Force produced by vacuum

n1 Safety coefficient against shift

Fs ≥ F(n1 · cosα + n2/µ · sinα)

Fs ≥ n1 · Fz + n2/µ · Fx

n2 Coefficient of protection against shift

µ Coefficient of friction (suction cup/workpiece)

Fs ≥ n1 · k1 · F

k1 = 1 + r/R

k1 Coefficient of eccentricity of line of force action

r Distance between force action and suction-cup axis

R External radius of suction cup

Fs ≥ n1 · k1 · Fz + n2/µ · Fx or

Fs ≥ F(n1 · k1 · cosα + n2/µ · sinα)

α Angle between force action and vertical

S Centre of gravity of gripped object

Fs ≥ n1 · k1 · Fz + n2/µ · k2 · Fx or

Fs ≥ F(n1 · k1 · cosα + n2/µ · k2 ·sinα)

k2 = 1 + r/R + Fz/Fy · µ

k2 Coefficient of eccentricity of line of force action

Fs ≥ n2 · F/µ

Special case of (2) with α = 90°

With a horizontal suction-cup axis, the holding force

remains less than 50% of the value for a vertical axis

In cases of lateral motion and where the suction surface is positioned vertically,

we must consider a further variable – the coefficient of friction µ. This can be

taken as µ = 0.5 for clean dry glass, stone and plastic, falling to µ = 0.1 to 0.4

with damp and oily surfaces. Other sources quote the following guide values:

Type Type of surface Coefficient of friction

of suction cup with peak-to-valley height

Ra = 0.05 µm Ra = 1.5 µm

Rigid Oil-free 0.85 —

Slightly Oil-free 0.45 0.65

deformable

Rigid/slightly Lubricated with 0.15 0.35

deformable drilling emulsion

Rigid Lubricated with coolant 0.05 0.25

Slightly Lubricated with coolant 0.025 0.15

deformable

Notwithstanding this, exercise caution; the coefficient of friction can fluctuate

widely, like a person’s blood pressure.

In the case of oiled or greased sheet metal, which often requires handling during

shaping processes, problems may be encountered due to the fact that the speci-

fied coefficient of friction between the steel and rubber no longer applies. The

coefficient will be considerably lower, since in most cases the lips of the suction

cup will not penetrate the oil film. In situations of this kind, it is the law of fluid

friction which applies and not Coulomb’s law of friction. Tests should always be

made in cases of this kind before a suction cup is selected.

To select a suction cup, proceed as follows:

1. Identify all the external forces acting on the suction cup.

These will include weight, inertia and centrifugal forces. Examples, assistance

and notes on this calculation can be found in [1] to [3].

2. Determine the suction force FS, to be generated by vacuum in accordance with

the force situation shown in Fig. 9-3. The load may vary during the handling

sequence, e.g. if the attitude of the suction surface changes from horizontal

to vertical. Calculation should always be carried out on the basis of the worst

load case.

3. Choose a vacuum operating pressure as appropriate to the vacuum generator.

Always try to choose an economic pressure. It is possible, for example, by

increasing the vacuum from pu = –0.6 bar to pu = –0.9 to boost force by a

factor of 1.5, but energy consumption will rise by a factor of 10. A vacuum

of pu = –0.7 bar will generally be used.

4. Calculate the suction cup size from the required suction area.

If a standard suction cup smaller than the necessary size has been chosen,

calculate the number of cups required. Distribution of the holding force

among several suction cups makes a handling system more reliable.


5. Define the suction time (evacuation time).

This is calculated from the volume to be evacuated (suction cups, lines)

and the performance curve for the vacuum generators, or in other words

the volume to be evacuated divided by the evacuation flow rate per unit time.

Suction cups used in handling systems operate with rough vacuum, which

ranges from 105 to 102 Pa. Other ranges include fine, high and ultra-high

vacuum. 70% vacuum is the value generally used with suction cups. This means

0.7 bar vacuum or 0.3 bar absolute pressure.

We will present 4 types of vacuum generation in this article (Fig. 9-4).

These are:

• Vacuum pumps and blowers

• Vacuum generators operating on the venturi principle (ejectors)

• Adhesive suction cups

• Pneumatic cylinders.

The use of vacuum pumps has the following advantages:

• Higher vacuum is possible

• Low operating costs

• Low noise level.

The disadvantages are the higher purchase cost and the cost of further

accessories, such as air reservoirs. Some companies, for example light-bulb

manufacturers, use not only a central compressed-air supply but also a central

vacuum supply. In these cases, decentral vacuum generators are not required.

82

How can the necessary

vacuum be achieved?

Fig. 9-4:

Methods of producing

a vacuum

a) Rotary pump or other

type of pump

b) Vacuum generator

c) Adhesive suction cup

d) Piston suction system


a)

c)

b)

d)

Vacuum blowers produce only a relatively low vacuum, as can be seen from the

comparison in Fig. 9-5. They do, however, have a large suction capacity, making

them useful in cases where it is necessary to compensate for the porosity of

workpieces.

Venturi-type vacuum generators have the following advantages:

• Very simple design, with no moving parts and low purchase cost

• No additional equipment required; fast response time

• Extremely reliable.

The disadvantages are the higher operating cost resulting from the consumption

of compressed air and the need for silencers. These vacuum generators must

be sized for peak load, since no reservoir is used. Suction air is generated in

the ejector as compressed air passes through the restricted cross-section at the

drive nozzle. This restriction produces an increase in flow velocity. Following this,

the air expands and exits via the receiver nozzle. If the exhaust-air duct is shut

off (Fig. 9-6b), an ejector pulse effect is produced.

The venture principle is named after Giovanni Battista Venturi (1746 to 1822), an

Italian physicist. Venturi's main work was concerned with hydrodynamics and

hydraulics and he invented the nozzle with flow restrictor which bears his name

today. This nozzle is also used as a measuring nozzle to determine flow rate in

accordance with the Bernoulli formula.

83

Fig. 9-5:

Comparison of performance

of typical vacuum generators


0

0 100 200 300 400 500 600 700 800 900 1000

-0,1

-0,2

-0,3

-0,4

-0,5

-0,6

-0,7

-0,8

-0,9

Vaku

umin

bar

Saugvermögen in l/min

Vakuumpumpe

Vakuumgebäse

Ejektor

Va

cuu

m i

n b

ar

Vacuum pump

Ejector

Vacuum blower

Suction capacity in l/min

Ejectors can be multi-stage and can be operated in parallel. In comparison with a

single-stage ejector, a multi-stage (multi-chamber) ejector is a series connection

of several vacuum generators (Fig. 9-7). Series circuits permit the rapid suction

of large quantities of air (shorter evacuation times). The advantage of this

arrangement is thus the high suction capacity.

Adhesive suction cups are merely pressed onto the workpiece to force out the

air they contain; a vacuum is then created by the resilience of the suction cup

material or by weight forces. It is virtually impossible to compensate for leakage

losses. The suction surfaces must be smooth and non-porous.

Piston suction systems are occasionally used with automatic assembly

machines. These systems produce vacuum and an ejector pulse in the same line

in alternation in synchronisation with the machine cycle. The piston stroke and

timing are stored in the form of a control cam.

It is even possible to generate vacuum using a solenoid, as with the patented

suction cup shown in Fig. 9-8. As the solenoid picks up, the volume of the

chamber below the suction cup increases, creating a vacuum which holds the

workpiece in place.

84

Fig. 9-6:

Mode of operation of venturi

nozzle, with shut-off valve to

produce ejector effect

a) Suction

b) Ejection

1 Shut-off valve

2 Receiver nozzle

3 Driver nozzle

4 Compressed air supply

5 Suction cup

6 Workpiece

Fig. 9-7:

Vacuum generators

in a series circuit

p Pressure

V Vacuum


a) b)

1 2 3

4

5

6

p

V

In order to make a suction cup work, it is necessary to connect up a number of

other components. Fig. 9-9 shows a typical circuit based on an ejector. A vacuum

switch is used to monitor the vacuum and detect whether the necessary vacuum

has been reached after the start of suction. Only then does the handling device

continue its motion sequence. Pressure data can also be used for “vacuum

management”, i.e. to switch off the vacuum generator for short period to save

energy. Vacuum switches are also used to generate an alarm in the case of an

undesired pressure drop, for example in the case of balancers and small hoists

where operators are working in the vicinity of the load.

The line diameter in the vacuum circuit should not be undersized, since this will

increase flow resistance, but should not be oversized that suction times become

too long. We see in nature that trees need to feed vital liquids to the very last

leaf tip, which has resulted in the evolution of appro-priate distribution systems.

85

Fig. 9-8:

Solenoid-actuated suction cup

Vacuum circuits

Fig. 9-9:

Example of a vacuum circuit

based on an ejector

1 Directional control valve

for compressed air supply

2 Ejector

3 DCV to control exhaust air

(switchover to ejection)

4 Silencer

5 Filter

6 Pressure switch for vacuum

7 Suction cup

8 Distributor


1

p

2 3 4

5

6 7

8

A similar task faces technicians who need to size the lines of a suction system.

They must consider the flow resistance in the tubing of the system. If we

imagine a tree fitted with suction cups, as shown in Fig. 9-10, we must size the

tubing in accordance with the factors shown. Each further branch should be

made smaller by a factor of 1.42.

[1] Automatisieren mit Vakuum (“Automation With Vacuum”; 4th edition),

published by FESTO Pneumatic Esslingen

[2] Greifer für die Handhabungstechnik (“Grippers For Handling Systems”;

brochure) by FESTO Pneumatic Esslingen 1996

[3] Vakuum-Greifer und -Saugdüsen einfach und schnell auswählen

(“Fast And Easy Selection Of Vacuum Grippers And Generators”; slide rule),

produced by FESTO Pneumatic Esslingen

86

Fig. 9-10:

Correct choice of tubing

diameter is important in the

distribution of suction air

1 Suction cup

2 Line

D Tubing diameter

Publications


D

0,71.D

0,5.D

0,35.D

1

2

Suction cups are the active components which create the contact between a

handling device and the workpiece to be handled. There are many different

workpieces and gripping applications, and an equally diverse range of suction-

cup variants, with differences of size, material, geometry, Shore hardness and

design. We will investigate some of these variants in the following.

Familiar suction-cup materials include perbunan (buna-N), silicone, polyrethane

and neoprene. Natural rubber is also used. In certain applications, there may be

a requirement for suction cups which do not mark workpieces, for example for

use with polished plate glass or polished metal workpieces. One method is to

use textile hoods under the suction cups or textile laminates. There are also

fluorine rubber suction cups which are non-marking. Shore hardness values (in

accordance with DIN 535051) lie in the range of 30° to 90°. The choice of suction

cups is greatly influenced by the intended application and the associated loads

presented by the workpiece and environment. Particularly important are pro-

perties such as resistance to abrasion, oil resistance (chemical resistance),

suitability for use with food and short- or long-term temperature resistance.

With standard quality rubber, elementary sulphur is often used in conjunction

with vulcanisation accelerators. It is possible for some free sulphur to remain

which then reacts with the workpiece. Only sulphur-free elastomers should

therefore be used to handle metals.

Workpiece temperature in general can vary between –50 and 250° C. Anything

over 70° C can be regarded as a special case and will usually require special

materials. At temperatures below zero, the hardness of suction cups may in-

crease, making the cups virtually rigid and preventing adequate adaptation to

the surface of the workpiece.

The elasticity of suction cups means that handling applications in general cannot

achieve positioning accuracies of better than ±1.0 mm. Additional technical

measures are therefore required if the positioning error is to be reduced further.

Normal suction-cup diameters range from 1 to 630 mm (flat suction cups).

87

10

Suction cups

for every application

Large number of

application parameters

10 Suction cups for every application

If we could bring the suction cups available from every manufacturer together

in one place, we would have a collection as garish as any eastern bazaar.

Let us, however, first consider the different shapes and designs of suction cups.

As shown in Fig. 10-1, these are as follows:

• Bellows suction cups

Suitable for slightly curved, inclined, easily deformable and uneven surfaces.

Provides a slight lifting motion; compensates for height differences; can have

up to 6.5 pleats. Small diameters of this type are Suitable for thin materials.

Bellows suction cups with a large number of pleats are sometimes fitted with

an internal or external support spring to provide additional rigidity.

• Flat suction cups

In general terms universal suction cups, Suitable for non-porous flat and

slightly curved surfaces; able to transmit high vertical forces.

• Deep suction cups

Good adaptation to round surfaces (Fig. 10-5) and profile sections.

Should not be used for flat surfaces, since rigidity is low and wear is rapid.

• Ribbed suction cups

Suitable for flat and unstable surfaces. The ribs across the mouth of the cups

prevent thin materials from being drawn into the cups and make these more

resistant to lateral deformation. This type of suction cup is also useful for

use with vertical surfaces, since the ribs provide increased friction when the

workpiece is in contact with these after being picked up. Since the lips of

the suction cups do not flex very much, virtually 100% of the effective suction

area is maintained. The more rigid design means on the one hand that suction

cups can be produced in larger sizes without a support plate but on the other

means that the suction cups cannot grip objects with any pronounced curve.

• Suction cups with cellular rubber seal

These provide a good seal with uneven and heavily-textured surfaces, such as

corrugated sheet metal, textured glass, concrete slabs, fireproof bricks, etc.

Not good for applications with vertical workpieces.

• Oval suction cups

Good for long, narrow or slightly-curved workpieces. Can be used for

“spiders” (large grippers with a large number of suction cups spread over

their area) in the automobile industry; typical features are metal baseplates

and narrow flexible sealing lips.

88

Suction-cup shapes

and designs


• Double-lip suction cups

A seal is provided by a combination of a sealing lip and sealing ring. This gives

great elasticity. When operated at close to maximum load, these suction cups

may alternate in a relatively uncontrolled way between their inner and outer

sealing lips.

• Self-adhering suction cups

These are not connected to an external vacuum source. A vacuum is created

by the suction cup itself as it presses against a workpiece. There is no com-

pensation for leakage losses. A hand-lever valve is used to break the vacuum.

As the cup is pressed against a workpiece, its volume is reduced, causing air

to be displaced. As the cup springs back into shape, a vacuum is created

under the suction cup. The level of this vacuum is, however, hard to define,

since the deformation force can vary widely.

• Lifting suction cups

Combination of a piston system and a suction cup. Once a workpiece has

adhered to the suction cup, it rises away from the workpiece stack. A lifting

suction cup can therefore make a separate lifting axis unnecessary. Strokes of

up to 50 mm are normally available. This type of suction cup is used to handle

cut cardboard, paper, foil pieces, thin sheet metal, packaging items, etc.

89

Fig. 10-1:

A small selection of the major

types of suction cup

1 Bellows suction cup

2 Flat suction cup

3 Deep suction cup

4 Ribbed suction cup

5 Profile suction cup

6 Suction cup with cellular

rubber seal

7 Lifting suction cup

8 Oval suction cup

with metal plate

9 Double suction cup

10 Double-lip suction cup

11 Self-adhering suction cup


1

5

9

2

6

10

3

7

11

4

8

Fig. 10-2 shows the results of a comparative study [1] of flat suction cups

without metal reinforcement (A), flat suction cups with metal plates and small

flexible sealing lips at the extreme edges (B), double-lip suction cups (C) and

bellows suction cups (D). The aim was to find the maximum trans-mittable

vertical and horizontal forces, the flexible vertical stroke produced by a

vertical force and the residual volumetric flow rate between the suction cup

and the workpiece. These involve relationships, in certain cases very complex

relationships, between the shape and material of the suction cup and the

surface properties of the workpiece.

Despite its relatively simple design, type A exhibits good results for all criteria.

Its comparatively large flexibility in the vertical direction as the vertical force

increases, makes it suitable for use in all but a few applications. Type B allows

very high vertical forces, since the vacuum chamber stays in shape even under

high vacuum thanks to spacers and the small narrow sealing lips. In the case of

type C, the double seal results in a very low residual volumetric flow rate. The

complex seal system, however, takes up more space, thus reducing the effective

diameter of the suction cup. Type D is characterised by the low maximum

vertical forces which it can transmit, its lack of geometrical stability under the

action of lateral forces, and its very large elastic vertical stroke. This rules this

type out for a variety of handling applications.

Criteria Transmittable Transmittable Flexible Residual vertical horizontal vertical volumetricforce force stroke flow rate

— —

90

Fig. 10-2:

Evaluation of various

types of suction cups

Very good

Ideal

Suitable

Suitable in certain cases

— Not applicable


Design

D

A

B

C

The ideal workpiece surface for suction cups is one which is perfectly flat and in

particular non-porous. In many cases, however, workpiece surfaces are not flat,

which means that suction cups must be compliant or adjustable on their vertical

axis and in their angular attitude. There are various ways of achieving this

(Fig. 10-3). In the simplest cases, bellows suction cups can be used. They offer

a certain degree of angular compliance. Multi-axis freedom of movement is

required mainly in cases involving large irregularly-shaped workpieces, such

as are typically encountered in the automobile industry.

Spring-loaded suction cups can also cushion the impact of workpiece contact

and compensate for height differences. The spring tension also offers the

advantage that the suction cup comes into contact with the workpiece before

the handling device stops. This reduces the time taken to build up the required

vacuum in the end position.

91

Freedom of movement

of suction cups

Fig. 10-3:

Freedom of movement

of suction cups

1 Bellows suction cup

2 Spring-loaded flat

suction cup

3 Angle adaptability

provided by

ball-and-socket head

4 Longitudinal freedom

of movement through

suction cup's own mass

5 Fixed angle setting

6 Longitudinal and angular

freedom of movement

through double joint

and longitudinal guide


1

4

2

5

3

6

Ball-and-socket brackets also reduce the bending forces which occur during

the handling of movable objects. Another method which has been suggested of

achieving flexibility is a matrix of individual suction cups which are able to

execute a large vertical motion (Fig. 10-4). The suction cups can then adapt to

a given surface as they are lowered onto an indi-vidual workpiece or a pile of

workpieces. When in position, all the rods are clamped into place, allowing the

handling device to store the workpiece shape temporarily and lift a layer of

individual workpieces.

Let us at this point once again mention deep suction cups (bell-shaped), which

have the adaptability to handle concave and convex workpieces very well, as

shown in Fig. 10-5.

92

Fig. 10-4:

Suction-cup array fitted to

rods allowing longitudinal

movement and used to pick up

workpieces of constantly

varying contours [2]

Fig. 10-5:

Deep suction cups can adapt

well to curved surfaces

a) Gripping a convex body

b) Gripping on a concave

surface


a) b)

A special type of vacuum gripper is shown in Fig. 10-6 which in a way combines

a bellows with the sleeve principle of a rolling diaphragm, resulting in a very

large freedom of movement. This is intended for use with objects whose geo-

metry, position and orientation vary continually within certain limits. Suction

cups with numerous pleats are often used in the food industry.

Non-rigid sheet workpieces have always been considered difficult to handle.

“Normal” grippers are defeated by foils, due to the “choking” effect which

occurs. Fig. 10-7 shows this. The foil material is drawn into the suction cup and

finally lies directly over the suction hole. This means that the gripped area is

very small. The workpiece cannot be held and falls away from the suction cup.

93

Fig. 10-6:

Vacuum gripper with very

large freedom of movement

of suction components

1 Suction cup

2 Workpiece

3 Magazine plate

Fig. 10-7:

Standard suction cups are not

very suitable for use with thin

foil material


1

2

3

1

2

3 4

The answer is to use numerous individual suction cups, each operating at a low

vacuum. Even better are low-pressure grippers whose active surface consists of

porous material which can let air through. Special plates are also available with

a large number of fine suction holes. Components of this kind can be combined

to form large units, as shown in Fig. 10-8. One disadvantage of course is that the

gripper is awkward due to its size.

Particularly with soft-lipped and bellows suction cups, it is often necessary to

take measures to ensure a correct gripping position. These measures can be as

follows:

• Precise alignment and stopping of workpiece while contact is established by

the gripper. The aim of this is to prevent slippage during pick-up.

• Fitting of internal positioning devices

• Attachment of external positioning aids (Fig. 10-9).

We accordingly use centring aids, insertion guides and support stops. In the

case of electronic components for fitting to PCBs, which require gripping from

above, it can improve accuracy to grip the components a second time after they

have been aligned mechanically.

94

Fig. 10-8:

Low-pressure gripper

equipped with porous plastic

or perforated plates

Ensuring a correct

gripping position


The needless discharge of suction air means a waste of energy and usually also

indicates a gripper malfunction. Attempts are therefore made to equip suction

cups in such a way that they activate vacuum generation only when they reach

the workpiece surface. There is a further problem: When several suction cups

are used in an array, it may occur that not all the suction cups are covered by

the workpiece, for example during the handling of packages of varying sizes in

positions which are not always precisely defined. Any suction cups which remain

uncovered must be deactivated in order to prevent the vacuum from collapsing.

The basic concept is illustrated in Fig. 10-10.

95

Fig. 10-9:

Positioning aids and stops for

use with vacuum grippers

a) Fine positioning

during gripper contact

b) Stop to reduce displace

ment during lateral motion

c) Alignment by spring-

loaded stops before

gripper contact

1 Centring mandrel

2 Spring

3 Suction cup

4 Workpiece

5 Magazine feed

6 Arm

7 Support

8 Vacuum connection

9 Roller conveyor

10 Tapered guide

11 Spring-loaded

guide wedge

Workpiece-controlled

activation of suction air

Fig. 10-10:

Automatic deactivation

of uncovered suction cups

1 Vacuum

2 Basic body

3 Airborne ball bearing

4 Suction cup

5 Workpiece


1

2 6

7

8

10

119

3 4 5

a) b)

c)

v

2

1

3

4

5

The flow of incoming air closes the uncovered suction cup. Flow-activated valves,

triggered by flow losses, can also be used with permeable surfaces such as

perforated plates to deactivate any uncovered suction device. Valves of this kind

are commercially available and are generally equipped with a filter to prevent

dirt particles from entering the vacuum circuit. In the case of the Festo vacuum

efficiency valve ISV-..., a spring-loaded “float” is used as a shut-off device.

Provided that a vacuum starts to build up under the suction cup, a ring seal

will open and release the required suction cross-section. Fig. 10-11 shows

an application in the form of a circuit diagram.

It also takes some organisation to activate a suction cup at the right moment.

Sensor valves are often used for this purpose, as are proximity sensors.

Fig. 10-12 shows a number of variants. Sensor valves are installed in the vacuum

line and act directly on this, while external sensors supply an electrical signal

which is used to activate directional control valves. With the solution shown

in Fig. 10-12b, the handling device travels towards a stack whose height is of

course constantly changing. Once the workpiece has been reached (topmost

sheet), the lowering motion is stopped and the vacuum is activated. In the case

of Fig. 10-12d, a “positive signal” triggers the vacuum by actuating a directional

control valve.

96

Fig. 10-11:

Circuit diagram for a suction

head fitted with vacuum

efficiency valves

1 Vacuum generator

2 Exhaust air

3 Distributor

4 Vacuum efficiency

valve with filter

5 Flat suction cup


1

2

3

4

5

Rapid ejection of workpieces from suction cups is just as important for fast

machining cycles as a fast pick-up. There are various ways of achieving this.

If a vacuum generator is used to produce the vacuum, it has become standard

practice to fill a small reservoir with compressed air during vacuum generation.

When the compressed-air supply to the generator is switched off, vacuum gene-

ration ceases and at the same time the compressed-air reservoir discharges

abruptly. This creates a positive pressure in the suction chamber, ejecting the

workpiece from the suction cup (Fig. 10-13).

If it is necessary to pressurise long supply lines from a vacuum pump to

the suction cup, the ejection process of course takes much longer. Here, too,

however, it is possible to provide a “short-circuit” to atmosphere. This is shown

in Fig. 10-14. As the suction cup contacts and holds the workpiece, the head

retracts. Once the air suction is switched off, it only requires a slight fall in

pressure for the spring to cause the head to advance. This exposes the bypass

97

Fig. 10-12:

Activating the vacuum

a) Integrated

electrical sensor

b) External electrical sensor

c) Vacuum sensor valve

in suction line

d) Integrated

inductive sensor

1 Vacuum connection

2 Electrical sensor

3 Sealing lip

4 Stack of workpieces

5 Limit switch

6 Flat suction cup

7 Sensor valve

8 Suction bore

9 Inductive sensor

10 Workpiece held

by vacuum

Ejector systems

Fig. 10-13:

Circuit diagram for a vacuum

generator with an ejector

pulse system

p Input pressure

V Vacuum


12

3

78

9 10

4

5

6

a)

c)

b)

d)

p

V

hole, which accelerates the creation of a pressure equilibrium. The speed

of response when triggered depends on the size of the bypass cross-section

in the valve or any other equivalent opening leading to atmosphere. These

openings should therefore be as large as possible.

It is of course also possible to switch straight from suction air to compressed air,

and this is done in practice. An example of this is shown in our last illustration,

Fig. 10-15. By the way, precise ejection at the desired point is particularly

important with fragile or very light workpieces, since these could otherwise

stick to the suction cup momentarily and then fall at a greater height from the

handling device during its return stroke and possibly suffer damage.

[1] Braun, D.: Industrieroboter - Auslegung von pneumatischen Flächengreifern

(“Robots industriales: Dimensionado de ventosas de sujeción

neumáticas”), publicado por Verlag T+V Rheinland, Cologne 1989

[2] Tella, R.; Birk, J; Kelley, R.: Una ventosa de vacío adaptada al contorno,

10º Simposio Internacional de Robots Industriales, Tagungsband,

Milán 1980

98

Fig. 10-14:

Vacuum head with “short-

circuit” hole to atmosphere

Fig. 10-15:

Circuit diagram for a suction

gripper with a vacuum

generator and compressed-

air ejector system

p Supply pressure

V Vacuum

Publications


p

V

The range of applications of standard and special suction cups is very wide,

covering everything from sanitary porcelain and strips, planks and panels to

foodstuffs. The processes involved generally form part of a medium or large

scale production operation. Suppliers of suction cups usually offer a complete

selection of individual units and modular systems. Apart from suction cups

themselves, systems of this kind include vacuum generators, valves, tubing

and piping, instrumentation, control equipment and flexible mounting systems.

We will consider some selected examples of applications below.

It is always surprising to see how suction-cup applications can be modified.

This will be demonstrated by bellows suction cups and the examples shown in

Fig. 11-1. It is, for example, possible to exploit the angular flexibility of suction

cups with inclined workpiece surfaces or alternatively to work with different

levels of vacuum to produce bowing of thin sheet-metal workpieces. It is also

possible to bring an inclined workpiece surface into a horizontal position as it

is picked up by providing leveling stops on the gripper. Conversely, workpieces

which are normally straight can be inclined if this is required in packing or

magazining operations.

The last example concerns a situation which is frequently encountered and

which we will now study more closely.

99

11

Suction cups in

handling technology

Exploiting the pro-

perties of suction cups

Fig. 11-1:

Some typical applications

of bellows suction cups

1 Picking up inclined

workpieces

2 Leveling of an inclined

formed part

3 Limiting suction force

4 Highly-flexible

suspension for undulating

workpieces in random

orientation

5 Picking up stepped

workpieces

6 Gripping formed parts

with undulating surface

7 Picking up flat workpieces

from magazine

8 Separating and picking up

flat workpieces

11 Suction cups in handling technology

1 2 3

4 5

6 7 8

p1 p1

p p1 2>

p2 p2

It is not easy to grip thin sheet-metal material, since sheets can stick to a stack

over their entire area and burred edges can become tangled. Plastic panels may

also stick together due to electrostatic charges. It is therefore necessary to

prevent two workpieces from being picked up at the same time. Fig. 11-2 shows

a number of ways of achieving this [1].

In Fig. 11-2a, the suction cups do not merely lift the panel but first generate

undulations along the length of the panel to detach any second panel which

may be adhering to the underside of the first. Only then is the panel lifted. Each

lifting cylinder must accordingly be controlled separately. “Peeling” effects can

also be produced by combining spring-loaded suc-tion cups with a non-spring-

loaded suction cup at the edge of the panel. In this case, the panel is lifted at

the edge while still being held down at other points by the spring force of the

suction cups. It is also possible to equip flat suction cups with a separator

insert, which is a fixed support within the suction chamber. When vacuum is

present, the panel picked up by the suction cup bends slightly around the sepa-

rator insert, due to the upward motion of the soft suction-cup lips. This effect

can be exploited with thin sheet metal up to approx. 3 mm.

When panels are picked up which are slightly porous, such as chipboard, a

“through-suction” effect may be encountered, also resulting in two panels being

lifted from a stack at the same time. The remedy here is to increase the suction

area (by using more suction cups) and reduce the vacuum level.

Problems may also be experienced with magnetic grippers when picking up

thin sheet-metal workpieces, since field lines may pass right through the first

workpiece and pick up a second workpiece as well. In order to solve this pro-

blem, combination grippers have been developed, as shown in Fig. 11-3. The

workpiece is first gripped by a suction cup and slightly lifted by this.

100

Feeding of thin panels

and sheet-metal

material

Fig. 11-2:

Picking up thin panels

with a suction cup

a) Undulating effect

b) Air nozzle to assist

separation

1 Suction cup

2 Blast nozzle

3 Indexing motion

4 Lifting device to raise stack


a) b)

1

2

3

4

The magnet is now activated, significantly increasing the holding force. This

higher force allows high-speed manipulations to be carried out.

Double lifting of small blanks can be rectified by using a second suction cup to

remove the second workpiece, which is held less firmly than the first workpiece

and can thus be “vacuumed” away and set down at another place. The first

gripper can swivel away from the second suction cup or else rotate, as shown

in Fig. 11-4. The two grippers rotate synchro-nously in opposition. The suction

forces are adjusted to different levels. A second workpiece is picked up along

with the first, carried along and then ejected into a set-down tray.

A further combination of physical effects is shown in Fig. 11-5. It can be used

only with ferromagnetic workpieces, since it utilizes “spreader magnets” at

the sides of the stack of metal sheets. The magnetic fields of these produce

repulsion forces within the stack which cause the top sheets to float (peel

away). This reduces the risk of two sheets being picked up at once and allows

the suction cup to make gentler contact with the workpiece. The number,

101

Fig. 11-3:

Combination gripper for hand-

ling thin ferritic sheet-metal

workpieces

1 Electromagnetic coil

2 Suction-cup lip made

of soft cellular rubber

V Vacuum

Fig. 11-4:

“Vacuuming” second work-

pieces away with a rotating

suction cup

1 Blanks magazine

2 Suction cup

for second workpiece

3 Set-down tray

for second workpieces

4 Rotary gripper

5 Feed device

for production machine


200 mm

1

V

2

1

2

3

4

5

arrangement and cross-section of these permanent magnets is governed by the

thickness and size of the sheet-metal workpieces. In the case of metal strips, it

is sufficient to have a spreader magnet positioned at the ends where the suction

cup is applied to the strip.

Here is another feed solution based on suction cups. In duplication systems,

the device shown in Fig. 11-6 is used to prevent double pick-ups. The magazine

outlet features a width restrictor which causes workpieces to become bowed as

they are removed from the magazine. Any second workpiece adhering to the first

is detached and remains in the magazine.

102

Fig. 11-5:

Workpieces removed from

stack using a suction cup

and spreader magnets

1 Suction cup

2 “Floating” sheet

at top of stack

3 Permanent magnet

4 Stack of sheet-metal

work pieces

Fig. 11-6:

Forced bowing of thin blanks

at magazine outlet prevents

double pick-ups

1 Vertical magazine

2 Toothed insert

3 Spring-loaded ratchet

4 Suction cup


1

2

3

4

2 mm

1

2

3

4

The bowing effect is used in many other adapter feed devices. The feed station

shown in Fig. 11-7 also features a suction cup which holds the sheet-metal work-

pieces at their centre. The dead weight of the workpieces causes these to sag.

After the sagging phase during lifting, the sheet-metal workpiece springs into

the pick-up roller slot. The rollers grip the metal workpiece and convey it out-

wards, whilst the suction cup simultaneously detaches from the workpiece and

returns to its “home” position.

103

Fig. 11-7:

Feeding station

for thin sheet metal

1 Lifting cylinder

2 Suction

3 Pick-up rollers

4 Contact point

5 Workpiece stack


1

2 3

4

5

The number of applications which fall under this heading is colossal. We can the-

refore do no more than show just a few examples, which may prove useful for

your own applications. As is well known, suction cups with soft lips do not ope-

rate very accurately. Furthermore, the workpiece in question may be displaced

when the suction cup “springs” into action if it is not specially guided. This is a

disadvantage, but one which does not become apparent in many applications,

since the workpiece is precisely centered by other technical means at its

destination, particularly in the case of feed operations. This can be seen in the

example of a tub filling and closing machine shown in Fig 11-8. A tub released

by a distributor device is picked up by vacuum and set down on a rotary table.

The suction cup passes through the workpiece carrier on the table in order

to do this. The tub aligns itself precisely on the table. It would in theory also

be possible to allow the workpieces to fall into their carriers by gravity, but this

would not provide sufficient reliability for automated operation. If workpieces

are allowed to move at random even for a short time, there can be no guarantee

of accurate movement. Even if the result is acceptable 99 times out of 100, this

is not good enough.

Fig. 11-9 shows the feeding of food containers on a filling line. Retaining brushes

on the magazine ensure that only one container is removed at a time.

The gripper arm carries a suction cup and is able to reach down between the

two conveyor belts and set down the deep-drawn foil container. Only when this

has been transported onwards can the arm swivel back into its pick-up position.

A rotary pneumatic cylinder can be used as a drive or, as shown here, a rack-

and-pinion gear unit with a linear cylinder.

104

Distribution

and feeding

with suction cups

Fig. 11-8:

Feed system

on a packing machine

1 Vertical magazine

2 Distributor

3 Suction cup

4 Rotary indexing table

5 Lifting cylinder

6 Tub (workpiece)

7 Production machine


1

2

3

4

56

7

Suction cups are also frequently used in combination with other grippers to

provide auxiliary functions. This is illustrated in the example, Fig. 11-10 showing

the stacking of spools of textile thread. These spools are gripped internally by a

mandrel gripper and placed on a pallet. The auxiliary function to be provided

by the handling device, is to insert a separator board between each layer. The

device picks up the separators from another stack via suction cups which are

advanced into their working position specifically for this operation. In this way, it

is possible to operate without a gripper-changing system. Combinations of this

kind can be produced relatively easily by using pneumatic cylinders with hollow

piston rods.

105

Fig. 11-9:

Feeding of food containers

1 Retaining brush

2 Stack of containers

3 Magazine rod

4 Spindle-driven lift ram

5 Limit switch

to monitor feed

6 Double conveyor belt

7 Lateral guide

8 Suction-air line

9 Gripper arm drive

10 Swivel arm

11 Suction cup

Fig. 11-10:

Multi-layer stacking

of textile thread spools,

using a combination gripper

1 Gripper connection

2 Lifting cylinder

3 Hollow piston rod

4 Separator

5 Mandrel gripper

6 Gripper thread spool

7 First stack layer

8 Transport pallet


11

1

2

3

4

5

6 7 8

9

10

1

2

3

4

5

6

7

8

Fig. 11-11 shows a glass bulb feeder system. The bulbs enter the feed conveyor

by gravity and are transferred to a pick-up position. These workpieces, which

are fragile and curved on all sides, can be reliably picked up via suction gripper

and fed into the machine in a 2-second cycle. If it is not possible to align light

workpieces sufficiently well by gravity, it may be necessary to provide an aid.

This can take the form of mechanically-operated V-shaped jaws at the pick-up

station. A mechanical radial gripper can provide a suitable basic module for a

device of this kind.

[1] Hesse, S.: Atlas der modernen Handhabungstechnik

(“Atlas Of Modern Handling Technology”),

published in German by Vieweg Verlag, Wiesbaden 1995

106

Fig. 11-11:

Feeding station

for glass bulbs

1 Swivel magazine

2 Handling device

with suction cup

3 Forked carrier in feed chain

4 Glass bulbs

5 Filling area

6 Magazine rail

Publications


1

2

3

4

5

6

78

Fig. 1-1: Division of a workpiece into gripping zone (G),

clamping zone (S) and set-down zone (A) . . . . . . . . . . . . . . . . . . . . 10

Fig. 1-2: How can a workpiece be picked up?. . . . . . . . . . . . . . . . . . . . . . . . . 10

Fig. 1-3: The right choice of gripping point can affect the positioning

error during assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Fig. 1-4: Feeding a clamping device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Fig. 1-5: Types of point loading resulting from gripping. . . . . . . . . . . . . . . . . 13

Fig. 1-6: Unambiguous pick-up points ensure reliable gripping . . . . . . . . . . 14

Fig. 1-7: Gripper devices which close in an arc may cause a shift

of the gripping centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Fig. 1-8: Centre deviation resulting from workpiece form errors . . . . . . . . . . 15

Fig. 2-1: Using a 3-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Fig. 2-2: Multi-point gripper for long workpieces . . . . . . . . . . . . . . . . . . . . . . 17

Fig. 2-3: Multi- workpiece gripper for transfer of complete rows

of workpieces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Fig. 2-4: Multi-workpiece gripper for assembly operations . . . . . . . . . . . . . . 19

Fig. 2-5: Multiple suction cup grippers for assembly operations. . . . . . . . . . 20

Fig. 2-6: Methods of holding a workpiece (example: ball bearing) . . . . . . . . 21

Fig. 2-7: Gripping principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Fig. 2-8: Gate feeder using a parallel jaw gripper. . . . . . . . . . . . . . . . . . . . . . 23

Fig. 3-1: 3-Point gripper combined with swivel/linear unit . . . . . . . . . . . . . . 24

Fig. 3-2: The human hand can execute motions with 6 degrees

of freedom (according to Bejczy) . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Fig. 3-3: Double gripper designed as crown turret . . . . . . . . . . . . . . . . . . . . . 25

Fig. 3-4: Shaft gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Fig. 3-5: Handling module for assembly of small workpieces . . . . . . . . . . . . 27

Fig. 3-6: Handling unit with suction cup and semi-rotary actuator . . . . . . . . 28

Fig. 3-7: Inverting workpieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Fig. 3-8: Tripple gripper installed on a special machine

with double stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Fig. 3-9: Picking up ferromagnetic sheets from a stack using

a suction cup/lifting module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Fig. 3-10: Simple specimen shaker made from standard components . . . . . . 31

Fig. 4-1: Some of the subsystems of a mechanical gripper . . . . . . . . . . . . . . 33

Fig. 4-2: The contour at the gripping point of the workpiece

determines the jaw shape used 1, 2 or 3 . . . . . . . . . . . . . . . . . . . . . 33

Fig. 4-3: Gripper jaws with compliant surfaces. . . . . . . . . . . . . . . . . . . . . . . . 34

Fig. 4-4: Jaw shape with centring effect for scissor tong grippers . . . . . . . . . 35

Fig. 4-5: Gripping several workpieces simultaneously

using a pressure distributor to compensate for tolerances . . . . . . . 36

Fig. 4-6: Jaws of a parallel gripper for 3 diameter ranges. . . . . . . . . . . . . . . . 37

Fig. 4-7: Gripper jaws with specially shaped multiple

gripping surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

107

12

List of illustrations

12 List of illustrations

Fig. 4-8: Variants of jaws for parallel grippers . . . . . . . . . . . . . . . . . . . . . . . . 38

Fig. 4-9: Mobile gripper jaws lift the workpiece out of the V-shaped

recess in the magazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Fig. 4-10: The type of approach affects the required opening . . . . . . . . . . . . . 40

Fig. 5-1: The law of interacting forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Fig. 5-2: Forces acting on gripped objects (state of rest) . . . . . . . . . . . . . . . . 42

Fig. 5-3: Plan view of 2 gripper situations . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Fig. 5-4: Calculation of contact forces for a gripper

with a V-jaw on one side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Fig. 5-5: Forces at the parallel jaw gripper with V-jaw for workpieces. . . . . . 45

Fig. 5-6: Force situations during gripper motion . . . . . . . . . . . . . . . . . . . . . . 46

Fig. 5-7: Example of a handling task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Fig. 5-8: Examples of torque created as a result

of a gripped workpiece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Fig. 5-9: Eccentric forces acting on a gripper finger . . . . . . . . . . . . . . . . . . . . 50

Fig. 5-10: Gripping force FG as a function of gripper stroke h . . . . . . . . . . . . . 51

Fig. 6-1: Characteristic data for grippers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Fig. 6-2: General model of a handling operation . . . . . . . . . . . . . . . . . . . . . . 54

Fig. 6-3: A joining mechanism with combined lateral

and angular compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Fig. 6-4: Situation, in which an overload of the gripper and

handling device can occur if no compensation is provided . . . . . . . 56

Fig. 6-5: Picking up a workpiece from a magazine pallet . . . . . . . . . . . . . . . . 56

Fig. 6-6: Gripper combined with a pressing element . . . . . . . . . . . . . . . . . . . 58

Fig. 6-7: Simple joining mechanism for vertical assembly . . . . . . . . . . . . . . . 58

Fig. 6-8: Collision protection with adjustable parameters

for a gripper showing reaction capability . . . . . . . . . . . . . . . . . . . . . 59

Fig. 6-9: When working with three-dimensional object configurations,

consideration must be given to the clearance contour

of the gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Fig. 6-10: Specifications of load capacity for industrial robots . . . . . . . . . . . . 61

Fig. 7-1: Approximate correlation between gripped objects

and gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Fig. 7-2: Radial gripper holding a sheet metal workpiece . . . . . . . . . . . . . . . 64

Fig. 7-3: IC’s have splayed pins, which are aligned to the desired

spacing during the gripper motion . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Fig. 7-4: Example of twin workpiece gripper as a special use

of a parallel jaw gripper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Fig. 7-5: Handling a length of bar material with a 3-point gripper . . . . . . . . . 65

Fig. 7-6: Gripping a rectangular workpiece . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Fig. 7-7: A 4-point gripper created by combining 2-point grippers . . . . . . . . 67

Fig. 7-8: Gripper module for flexible assembly . . . . . . . . . . . . . . . . . . . . . . . . 68

Fig. 7-9: Assembly gripper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Fig. 7-10: Sequence for assembly within a gripper . . . . . . . . . . . . . . . . . . . . . 69

108 12 List of illustrations

Fig. 8-1: Mutually influential factors and basic variables relating

to the selection of grippers from the technical point of view. . . . . . 71

Fig. 8-2: Generation of characteristic gripper data starting

with the gripper location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Fig. 8-3: The main steps in the selection of grippers . . . . . . . . . . . . . . . . . . . 74

Fig. 9-1: The most important functions and properties in relation

to suction grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Fig. 9-2: Force conditions with vertically moving suction cup . . . . . . . . . . . . 79

Fig. 9-3: Typical force situations at suction cup . . . . . . . . . . . . . . . . . . . . . . . 80

Fig. 9-4: Methods of producing a vacuum. . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Fig. 9-5: Comparison of performance of typical vacuum generators . . . . . . . 83

Fig. 9-6: Mode of operation of Venturi nozzle with shut-off valve

to produce ejector effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Fig. 9-7: Vacuum generators in a series circuit . . . . . . . . . . . . . . . . . . . . . . . . 84

Fig. 9-8: Solenoid actuated suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Fig. 9-9: Example of a vacuum circuit based on an ejector . . . . . . . . . . . . . . 85

Fig. 9-10: Correct choice of tubing diameter is important

in the distribution of suction air . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Fig. 10-1: A small selection of the major types of suction cups . . . . . . . . . . . . 89

Fig. 10-2: Evaluation of various types of suction cups . . . . . . . . . . . . . . . . . . . 90

Fig. 10-3: Freedom of movement of suction cups. . . . . . . . . . . . . . . . . . . . . . . 91

Fig. 10-4: Suction cup array fitted to rods allowing longitudinal

movement and used to pick up workpieces of constantly varying

contours. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Fig. 10-5: Deep suction cups can adapt well to curved surfaces . . . . . . . . . . . 92

Fig. 10-6: Vacuum gripper with very large freedom of movement

of suction components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Fig. 10-7: Standard suction cups are not very suitable for use

with thin foil material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Fig. 10-8: Low pressure gripper equipped with porous plastic

or perforated plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Fig. 10-9: Positioning aids and stops for use with vacuum grippers . . . . . . . . 95

Fig. 10-10: Automatic deactivation of uncovered suction cups . . . . . . . . . . . . . 95

Fig. 10-11: Circuit diagram for a suction head fitted with vacuum

efficiency valves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Fig. 10-12: Activating a vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Fig. 10-13: Circuit diagram for a vacuum generator with an ejector

pulse system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Fig. 10-14: Vacuum head with “short-circuit” hole to atmosphere . . . . . . . . . . 98

Fig. 10-15: Circuit diagram for a suction gripper with a vacuum

generator and compressed air ejector system . . . . . . . . . . . . . . . . . 98

10912 List of illustrations

Fig. 11-1: Some typical applications of bellows suction cups . . . . . . . . . . . . . 99

Fig. 11-2: Picking up thin panels with a suction cup . . . . . . . . . . . . . . . . . . . 100

Fig. 11-3: Combination gripper for handling thin ferritic sheet

metal workpieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Fig. 11-4: “Vacuuming” second workpieces away with a rotating

suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Fig. 11-5: Workpieces removed from stack using a suction cup

and spreader magnets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Fig. 11-6: Forced bowing of think blanks at magazine outlet

prevents double pick-ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Fig. 11-7: Feeding station for thin sheet metal . . . . . . . . . . . . . . . . . . . . . . . . 103

Fig. 11-8: Feed system on a packing machine . . . . . . . . . . . . . . . . . . . . . . . . 104

Fig. 11-9: Feeding of food containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Fig. 11-10: Multi-layer stacking of textile thread spools using

a combination gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Fig. 11-11: Feeding station for glass bulbs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

110 12 List of illustrations

2-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3-dimensional axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

A Accuracy of gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Adapter rail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Adhesive suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Alignment effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Angle gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36, 51

Angular compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Application area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Assembly gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Assembly mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Axial alignment error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Axis gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

B Ball-and-socket head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Bellows suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 99

C Centre deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Centring aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Changing system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Characteristic curve for gripping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Characteristic data for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Checklist for grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Clamping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Clamping marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Clamping safety margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Clamping zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Clearance contour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Coefficient of friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 80

Collision-protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59

Combination gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 66, 100, 105

Compensate for tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Compliance device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Contact force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 43

Contact sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Crown turret . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Cushioned stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

D Deceleration force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Deep suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 92

Degrees of freedom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Degrees of transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Diameter of the interference circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Distribution with suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Double gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Double-lip suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Duplex machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

111

13

List of special terms

13 List of special terms

E Eccentric force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Ejector system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

External gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

F Feed channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 65

Feed gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Feeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Feeding station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103, 106

Final effector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Force conditions with suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Force-locking connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Friction force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Friction locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

G Geometrical error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Gripped object . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Gripper finger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Gripper fingers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Gripper jaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 37

Gripper module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Gripper pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Gripper selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Gripper selection tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

Gripper system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66

Gripper types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Gripper working area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Gripping centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Gripping force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41, 43

Gripping point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 17

Gripping stroke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Gripping surface pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Gripping zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 72

Guide wedge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

H Hand axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 27

Hand-joint sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Handling module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Handling of sheet metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Handling operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

High point loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

I Inertia force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Interference circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Internal gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

IRCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

J Jaw shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Jaw-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

K Knee-lever gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

112 13 List of special terms

L Lateral compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Lifting suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Load-bearing capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Low-pressure gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

M Magazine pallet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Maximum load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Moulding jaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Multiple suction-cup gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Multi-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Multi-stage ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Multi-workpiece gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

N Nominal load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Non-slip covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Normal force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

O Opening safety margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Oval suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Overload of gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

P Parallel-jaw gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 23, 26

Peak-to-valley height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Pendulum jaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Pick-and-place device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Piston suction system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Placing error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Planned gripper application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Plastic covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Plug gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Point loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Position compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Positioning aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Positioning error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 14

Positive-locking connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Positive-locking gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Pressure device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Pressure distributor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Pressure per unit area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Pressure plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Pulse effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Q Quick exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

R Radial gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 62, 64

RCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

Repetition accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Ribbed suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Rubber covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Running freedom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

S Safety factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Scissors-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

11313 List of special terms

Securing function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Self-adhering suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Sensor, inductive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Sequence gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Service life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Set-down zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Shaft gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Short-stroke axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Shut-off device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Single-stage ejector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Specimen shaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Spreader magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Spring-loaded gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Standard gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Stepped track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Suction cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77, 96

Suction-cup array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Suction-cup shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Suction-cup/lifting module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Swivel/linear unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

T TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Third law of motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Three-finger gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Three-point gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Thruster device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Tool centre point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 35

Triple turret gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Turning workpieces over . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Turret gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Twin-workpiece gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Two-finger gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 43

Type of approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

U Universal jaws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Uses of grippers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

V Vacuum blower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Vacuum circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Vacuum efficiency valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Vacuum generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Vacuum management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Vacuum switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Vacuum pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Venture-type vacuum generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Venturi nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Vertical magazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Vice-type gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

V-jaw gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

W Wide-range gripper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

114 13 List of special terms

Pneumatic Grippers

Documents

Transcript of Pneumatic Grippers