51359867 Mechanical Actuators

8/3/2019 51359867 Mechanical Actuators

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Mechanical actuators


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ACTUATION SYSTEM:

The actuation systems are the elements ofthe control system and they are responsible

for transforming the output of a

microprocessor into a controlling action on

a machine or device.

Actuators produce physical changes such

as linear and angular displacement.


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There are four types of actuators.

1. Mechanical actuators.

2. Electrical actuators.

3. Hydraulic actuators.

4. Pneumatic actuators.


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Example:

In a CNC milling machine, there may be an

electrical signal output from the CNC

controller to move the milling table in the xdirection for a certain length. There you

need an actuation system


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MECHANICAL ACTUATION SYSTEMS

Mechanical systems :- Mechanisms are deviceswhich can be considered to be motion converters

in that they transform motion from one form to

other form.

They might, for example, transform linear motioninto rotational motion, or motion in one direction

into a motion in a direction at right angles, or

perhaps a linear motion into rotary motion, as in

the internal combustion engine the reciprocatingmotion of the pistons is converted into rotary

motion of the crank and hence the drive shaft.


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Mechanical elements can include the use of

gears, rack-and-pinion, chains, belt drives; etcrack-and-pinion can be used to convert rotation

motion. Parallel shaft gears might be used to

reduce a shaft speed.

Bevel gears might be used for the transmissionof rotary motion through 90. A toothed belt or

chain drive might be used to transform rotary

motion about one axis to motion about another .

Cams and linkages can be used to obtainmotions which are prescribed to vary in a

particular manner.


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Many of the actions which previously were obtained byuse of mechanisms are, however, often nowadays are

obtained by the use of microprocessor systems. For example rotating shafts were previously used for

machines in order to give a timed sequence. Such asopening a valve to let water into the drum, switching aheater on/off, etc.

Modern washing machines use a microprocessor basedsystem with the microprocessor programmed to switch onoutputs in the required sequence.

Mechanisms still, however, have a role in mechatronicssystems. For example, the mechatronic system in use in

an automatic camera for adjusting the aperture for correctexposures involves a mechanism for adjusting the size ofthe diaphragm


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While electronics might now be used often for

many functions that previously were fulfilled by

mechanisms, mechanisms might still be used toprovide such functions as:

1. Force amplification, e.g. that given by levers.

2 Change of speed, e.g. that given by gears.

3Transfer of rotation about one axis to rotation

about another,

e.g. a timing belt.

4. Particular types of motion, e.g. that given by aquick-return mechanism.


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Gears

Rotary motion can be transferred from oneshaft to another by a pair of rolling cylinders

however; there is a possibility of slip. The

transfer of the motion between the two

cylinders depends on the frictional forcesbetween the two surfaces in contact. Slip

can be prevented by the addition of meshing

teeth to the two cylinders and the result isthen a pair of meshed gear wheels.


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Gears can be used for the transmission of

rotary motion between parallel shaftsF

ig.(a) and for shafts which have axes inclined

to one anotherFig. (b). the term bevel gear

is used when the lines of the shafts

intersect, as illustrated in Fig. (b). When twogears are in mesh, the larger gear wheel is

often called the spur or crown wheel and the

smaller one the pinion.


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Gears for use with parallel shafts may have axialteeth with the teeth cut along axial lines parallel tothe axis of the shaft Fig. (a). Such gears are thentermed spur gears.

Alternatively they may have helical teeth with theteeth being cut on a helix Fig. (b) and are thentermed helical gears.

Helical gears have the advantage that there is agradual engagement of any individual tooth andconsequently there is a smoother drive andgenerally prolonged life of the gears.

however, the inclination of the teeth to the axis of

the shaft results in an axial force component onthe shaft bearing. This can be overcome by usingdouble helical teeth.(fig.c)


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Gear trains

Gear trains are mechanisms that are verywidely used to transfer and transform

rotational motion. They are used when a

change in speed or torque of a rotating

device is needed.

For example, the car gearbox enables the

driver to match the speed and torque

requirements of the terrain with the enginepower available.


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Gear trains The term gear train is used to describe a

series of intermeshed gear wheels. The term simple gear train is used for a

system where each shaft carries only one

gear wheel, as inF

ig. For such a gear train, the overall gear ratio

is the ratio of the angular velocities at the

input and output shafts and is thus A/ B


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Consider a simple gear train consisting ofwheels A, B and C, as in Fig. with A having9 teeth and C having 27 teeth.

Then, as the angular velocity of a wheel isinversely proportional to the number of teeth

on the wheel, the gear ratio is 27/9 = 3. The effect of wheel B is purely to change the

direction of rotation of the output wheelcompared with what it would have been with

just the two wheels A and C intermeshed.The intermediate wheel, B, is termed theidler wheel.


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The term compound gear train is used to

describe a gear train when two wheels aremounted on a common shaft.

Figure (a) and (b) shows two examples of

such a compound gear train. The gear

train in Fig. (b) enables the input and

output shafts to be in line.


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When two gear wheels are mounted on

the same shaft they have the same

angular velocity. Thus, for both of thecompound gear trains in Fig. B

The overall gear ratio G is thus


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Ratchet and Pawl Ratchets can be used to lock a mechanism when it is holding

a load. Figure shows a ratchet and pawl. The mechanism

consists of a wheel, called a ratchet, with saw-shaped teethwhich engage with an arm called a pawl.

The arm is pivoted and can move back and forth to engagethe wheel. The shape of the teeth is such that rotation canoccur in only one direction.

Rotation of the ratchet wheel in a clockwise direction isprevented by the pawl and can only take place when the pawlis lifted.

The pawl is normally spring loaded to ensure that itautomatically engages with the ratchet teeth. Thus a winch

used to wind up a cable on a drum may have a ratchet andpawl to prevent the cable unwinding from the drum when thehandle is released.


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The rack and pinion

Another form of gear is the rack and pinion(Figure). This transforms either linear

motion to rotational motion or rotational

motion to linear motion.


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Belt drives Belt drives are essentially just a pair of rolling cylinders, as

described in Fig. with the motion of one cylinder being

transferred to the other by a belt. Belt drives use the friction that develops between the

pulleys attached to the shafts and the belt around the arc ofcontact in order to transmit a torque.

Since the transfer relies on frictional forces then slip can

occur. The transmitted torque is due to the differences intension that occur in the belt during operation.

This difference results in a tight side and a slack side for thebelt. If the tension on the tight side is T1, and that on theslack side T2. then with pulley A in Fig. as the driver: Torque

on A = (T1 T2)ra Where rais the radius of pulley A. For thedriven pulley B we have:Torque on B = (T1 T2)rb


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Where rb is the radius of pulley B. Since the

power transmitted is the product of the

torque and the angular velocity, and sincethe angular velocity is v/ra for pulley A and

v/rb for pulley B, where v is the belt speed,

then for either pulley we have: Power = (T1 - T2)v


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The belt drive shown in Fig. gives the driven

wheel rotating in the same direction as the

driver wheel. Figure a & b shows two typesof reversing drives. With both forms of

drive, both sides of the belt come into

contact with the wheels and so V-belts ortiming belts cannot be used.

a. Open belt drive

b. Crossed belt drive


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Types of belts

The four main types of belts (Fig.) are:

1. Flat: The belt has a rectangular cross-section. Such a drive has an efficiency of

about 98% and produces little noise.

They can transmit power over long

distances between pulley centres crowned

pulleys are used to keep the belts from

running off the pullets.


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2Round: The belt has a circular cross-section and is

used with grooved pulleys.

3 V-belts: V-belts are used with grooved pulleys andare less efficient than flat belts but a number of

them can be used on a single wheel and so give a

multiple drive.

4. Timing: Timing belts require toothed wheels,having teeth which fit into the grooves on the

wheels. The timing belt, unlike the other belts,

does not stretch or slip and consequently

transmits power at a constant angular velocityratio. The teeth make it possible for the belt to be

run at slow or fast speeds.


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Chains Slip can be prevented by the use of chains which

lock into teeth on the rotating cylinders to give theequivalent of a pair of intermeshing gear wheels.

A chain drive has the same relationship for gear

ratio as a simple gear train. The drive mechanism

used with a bicycle is an example of a chain drive. Chains enable a number of shafts to be driven by a

single wheel and so give a multiple drive. They are

not as quiet as timing belts but can be used for

larger torques.


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Bearings

Whenever there is relative motion of one

surface in contact with another, either by

rotating or sliding, the resulting frictional

forces generate heat which wastes energy

and results in wear. The function of a bearing is to guide with

minimum friction and maximum accuracy

the movement of one part relative toanother.


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Of particular importance is the need to

give suitable support to rotating shafts, i.e.

support radial loads.

The term thrust bearing is used for

bearings that are designed to withstand

forces along the axis of a shaft when therelative motion is primarily rotation.


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Plain journal bearings

Journal bearings are used to support rotating

shafts which are loaded in a radial direction. Theterm journal is used for a shaft.

The bearing basically consists of an insert of

some suitable material which is fitted between

the shaft and the support (Fig. ).

Rotation of the shaft results in its surface sliding

over that of the bearing surface.


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The insert may be a white metal, aluminumalloy, copper alloy, bronze or a polymer such as

nylon or PTFE. The insert provides lower friction and less wear

than if the shaft just rotated in a hole in thesupport.

The bearing may be a dry rubbing bearing orlubricated. Plastics such as nylon and PTFE aregenerally used without lubrication, the coefficientof friction with such materials being exceptionallylow.

A widely used bearing material is sinteredbronze, This is Bronze with a porous structurewhich allows it to be impregnated with oil and sothe bearing has a `built in' lubricant.


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Hydrodynamic : The hydrodynamic journal

bearing consists of the shaft rotating

continuously in oil in such a way that itrides on oil and is not supported by metal

(Fig.). The load is carried by the pressure

generated in the oil as a result of the shaftrotating.


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Hydrostatic A problem with hydrodynamic lubrication is

that the shaft only rides on oil when it isrotating and when at rest there is metal to-

metal contact.

To avoid excessive wear at start-up andwhen there is only a low load, oil is

pumped into the load-bearing area at a

high-enough pressure to lift the shaft Off

the metal when at rest.


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3. Solid-film

This is a coating of a solid material such

as graphite or molybdenum disulphide.

4. Boundary layer

This is a thin layer of lubricant which

adheres to the surface of the bearing.


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Ball and roller bearings

With this type of bearing, the main load is

transferred from the rotating shaft to its supportby rolling contact rather than sliding contact. Arolling element bearing consists of four mainelements:

an inner race, an outer race, the rolling elementof either balls or rollers, and a cage to keep therolling elements a part (Fig.). The inner andouter races contain hardened tracks in which therolling elements roll.


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There are a number of forms of ball bearings:

1. Deep-groove: This is good at withstanding radial

loads but is only moderately good for axialloads. It is a versatile bearing which can be used

with a wide range of load and speed

2. Filling-slot: This is able to withstand higher

radial loads than the deep groove equivalent but

cannot be used when there are axial loads.

3. Angular contact: This is good for both radial and

axial loads and is better for axial loads than thedeep-groove equivalent.


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4.Double-row

Double-row ball bearings are made in a number oftypes and are able to withstand higher radial loads

than their single-row equivalents. The figureshows a double-row deep-groove ball bearing,there being double-row versions of each of theabove singlerow types.

5.Self-aligning Single-row bearings can withstand a small amountof shaft misalignment but where there can besevere misalignment a self-aligning bearing isused. This is able to withstand only moderate

radial loads and is fairly poor for axial loads.6. Thrust grooved race

These are designed to withstand axial loads butare not suitable for radial loads.


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There are also a number of forms of roller bearing, thefollowing being common examples:

1. Straight roller

This is better for radial loads than the equivalent ballbearing but is not generally suitable for axial loads. Theywill carry a greater load than ball bearings of the samesize because of their greater contact area. However,they are not tolerant of misalignment.

2. Taper rollers

This is good for radial loads and good in one direction foraxial loads.

3. Needle rollers

This has a roller with a high length/diameter ratio andtends to be used in situations where there is insufficientspace for the equivalent ball or roller bearing.


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Selection of bearings

In general, dry sliding bearings tend to be only used for smalldiameter shafts with low load and low speed situations, ball

and roller bearings, i.e. bearings involving rolling, with amuch wider range of diameter shafts and higher load andhigher speed, and hydrodynamic bearings for the high loadswith large diameter shafts.

Figure shows a chart indicating the selection of bearings

based on their load-shaft speed characteristics for a numberof different diameter shafts.

Thus suppose we want a bearing for a 25 mm diameter shaftrotating at 10 rev/s and carrying a radial load of10 000 N.This is beyond the limit for a dry sliding bearing and is a point

on the graph below the line for rolling bearings for such adiameter and speed, hence rolling bearings can be used.


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Failure of ball and roller bearings generallyoccurs as a result of fatigue. With fatigue failuresthere is always a scatter of values at which

failure of an individual item occurs. The life of a bearing is thus defined as the

number of millions of shaft revolutions that 90%of the bearings are expected to exceed before

failing. This life L10 depends on the applied load F. For

ball bearings the relationship is:

where C is a constant for a particular form ofbearing. For roller bearings:


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Manufacturers often tabulate data for bearings interms of the number of hours of life at a

particular speed given in units of rev/min. The life in hours = 10 6/(3600 x n/60) x Lo in

millions of revs = (16 667/n) x Lo in millions ofrevs; n is the number of revolutions per minute.

For example, a particular ball bearing may berated as 3000 h at 500 rev/min for a radialloading of10 kN. This gives L0, as 90 millionrevs and hence C as 44.8 kN.

Thus with a load of, say, 20 kN at 400 rev/minthen the life we can expect is 11.2 millionrevolutions or468 h. If this is not long enoughwe need to select a ball bearing with a higherrating.


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Thank you

51359867 Mechanical Actuators

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