Linear Motion - Stork...

Selection of Linear Precision Drives

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Linear Motion – Overview

Main Selection Criteria

Comparison of Common Linear Drive Mechanisms

Screw Driven Solutions

Application Examples

Linear Motion - Overview

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Linear Motion - Overview

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Direct Drive Linear Motors Rotary to Linear Conversion

Rack & Pinion

Timing belts

Ball screws

Lead Screws


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Linear Motion – Main selection criteria

Electro-mechanical design: balancing the trade-offs

PrecisionResolution, Repeatability, Accuracy

ThroughputSpeed, Duty Cycle, ….

LifeWear, Contamination Resistance, Maintenance,…

Cost


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Comparison of Linear Drive Mechanisms

Direct Drive Linear Motor

Very high speed

Very high precision

No backlash

Fast move and settle time

Lower force density compared to othertechnologies

Requires linear feedback for properoperation

More difficult to use in vertical applications

Can be expensive

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Rack and Pinion

Very good for long travel applications

High speed

Requires precise alignment

Can be noisy

Difficult to remove backlash

Typically lower precision

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Timing Belt and Pulley

High speed applications

Long life

Maintenance free

Belt tensioning required

Care must be taken in designing pulleybearing

Specific errors (pulley out of round precision,lead error caused by pitch diameterprecision,…)

Form factor may be an issue

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High precision application withmoderate speed

Applications requiring goodrepeatibility

Applications requiring high forcedensity

When non-backdriving isrequired (lead screw)

Precision alignment required (ball screws)

Right lubricant, coating, nut materials, …required

Specific potential issues to be addressed:critical speed, noise,…

Ball screws require maintenance

Ball Screws & Lead Screws


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Technology Highlights

Limits of the technology

Components & Systems Selection


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Screw driven solutions

Rotary Motor & GearboxPrecision Lead Screw and Nut

Linear guide

Controller/Driver

Power Supply

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Screws – A few definitions

Lead: linear displacement for one revolution Pitch: distance between to consecutive threads

Lead = Pitch x nb of starts

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Lead Screw versus Ball screwFeature Leadscrews Ballscrews

Loads light or moderate moderate or heavy

Efficiency 20% up to 85% above 95%

Lead Accuracy 0.1µm/mm-0.6µm/mm 0.1µm/mm or less (C10 to C7 grade)

Repeatability 1-2 microns (compensated backlash) varies with C grade

Backdriving sometimes always

Noise quiet operation noisy-metal to metal recirculating balls

Lubrication solutions without grease available necessary

Maintenance none or very little required

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Lead Screw versus Ball screwOther criteria

Size Lead screw/nut assembly are much more compact for the same screw diameter

Lead (travel/revolution) Small leads feasible only with lead screws (down to 0,3mm)

Alignment Ball screws are highly sensitive to alignment

Environmental Ballscrews are highly sensitive to dirty environments.

Life Depends on the application

Customization Lead screws are easier to customize than ball screws

Cost Cost of leadscrew is less than a ground ball screw and a little less than rolledballscrew

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Lead Screws – useful formulas

Torque to hold the load

Torque to move the load

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Lead Screw efficiency

Efficiency depends onlead angle andcoefficient of friction

Back drive efficiency islower than forward driveefficiency

A lead screw is non-backdrivable if backwardefficiency is zero (orforward efficiency lowerthan approx. 50%)

Lead angle

Lead angle

Ball screws


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Technology Overview




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Limits of Lead Screw Driven Solutions

(*): with backlash compensation and constant preload

Criteria

Precision Open loop : repeatability down to 5-6µm (*)Closed loop: sub-micron accuracy achievable

Speed/Throughput Compromise between speed and force

Expected life From a few thousand cycles to a few million cyclesDepends on a lot of different application parameters

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Precision limiting factors in open loop:

Motor resolution (steppers)

Backlash, Lost motion, hysteresis

Lead accuracy

« One revolution errors »

Linear resolution (mm): lead (mm) /number of steps per revolution

Backlash = clearance between nut and screw (can be compensated)Material deflection may add additional error (cannot be compensated)

Cumulative error defined in µm/mm

Screw straightness and warious alignment issues contribute tocreating an error with a period of one revolution

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Backlash = clearance betweenthe nut and the screw

Lost motion = backlash + stiffnessLost motion = backlash + stiffness

Stiffness of anti-backlash nutswith « soft » material

Axial preload

Radial preload

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Backlack compensationBacklack compensation

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Stiffness: several different plastics available

StiffnessStiffness

Metal nuts:- Stiff- Lubrication required (maintenance)- Shorter life than plastic nuts

Thermoplastic nuts:- Longer life- No maintenance required- Various materials available

depending on stiffness andapplication conditions (PV,temperature, environment…..)

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Lead accuracy and « one revolution » errorsLead accuracy and « one revolution » errors

y = 2,252x + 1,423-2

0

2

4

6

8

10

12

0 0,5 1 1,5 2 2,5 3 3,5

Erro

r (um

)

Displacement (mm)

Error vs. Displacement

Série2

2 4 6 8 10

Lead accuracy:0,6 µm/mm

One revolution error: 4 µm

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Potential issues limitating the velocity:

Nut Material PV (pressure x velocity)

Screw Critical speed

Noise

Motor Torque/speed characteristics

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Limits of Lead Screw Driven SolutionsRPM

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screw supported at regular intervals

Overcoming the critical speed issue: the RGS slidesOvercoming the critical speed issue: the RGS slides

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Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motors

Size 17 (42mm) stepper & DC motors (*)with 1,27mm lead screw

(*) Without gearbox

533N

120N

9,5mm/s 44 mm/s

:4,5

X4,6

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Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motorsSize 17 (42mm) stepper & DC motors (*)

with 3,17mm lead screw

(*) Without gearbox

265N

80N

30mm/s 108 mm/s

:3,3

X3,6

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Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motorsSize 17 (42mm) stepper & DC motors (*)

with 6,35 mm lead screw

(*) Without gearbox

102N

40N

50mm/s 216 mm/s

:2,5

X4,2Ball screw

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Life is increasing with load derating

Lubrication (dry, grease, teflon coating?)

Alignment issues

Environmental issues

Anti-backlash nuts with wearcompensation increase life

Life expectancy

020406080

100120

1 100 10,000 1,000,000 100,000,000

number of cycles

perc

ent % percent load vs

number of cycles

Main factors impacting life:


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Technology Overview





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Select the base technology: force, speed, accuracy, life,..

High speed, high accuracy: direct drive linear motors

Speed:• > 1m/s : linear motor, belt & pulley, rack & pinion• 0.5 – 1m/s: ball screw,…• < 0,5 m/s: lead screw,…

Duty cycle, efficiency:• > 50%: usually ball screws• <50% lead screws,…

Application specific parameters:• Accuracy• Force density, form factor• Back drive• Environmental issues,….

Select the motor technology (rotary to linear conversion): DC motor (brush, brushless), stepper motor

Select the guide technolgy: ball guides, friction guides,…

Components selectionComponents selection


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Separate Components or Integrated Systems?Separate Components or Integrated Systems?Rotary Motor Precision Lead Screw and NutDriver Linear guide


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