WEAR TESTING AND MEASUREMENT TECHNIQUES FOR

POLYMER COMPOSITE GEARS

N. A. Wright and S. N. Kukureka

Wear, 250, 1567, (2001)

Presented at 13th International Wear of Materials Conference, Vancouver, 2001.

INTRODUCTIONINTRODUCTION

Polymers and composites widely used for machine elements including gears, cams and pulleys

Polymers used for power transmission and motion transmission

Small components – often highly stressed

Typical applications • Automated teller machines (ATM) • Computer printers, photocopiers • Small drives and gear mechanisms

IntroductionIntroduction

IntroductionIntroduction

Advantages of polymer gears

low mass and inertia

cost reduction if moulded

design freedom

no lubrication

corrosion resistance

noise reduction

IntroductionIntroduction

Disadvantages temperature effects

Polymer – metal contact better heat dissipation

Polymer – polymer contact cheaper if complex geometry

Typically conformal contacts polymers against steel

Non-conformal contacts polymers against polymers

Gear geometry

Gear contact is concentrated and non-conformal throughout

Gears have an involute profile and a constant velocity ratio

The sliding velocity, contact radius, and contact load all vary over the tooth face

Motion is a combination of rolling and sliding

IntroductionIntroduction

Rolling and sliding in gears

IntroductionIntroduction

IntroductionIntroduction

Gear action Initial contact – tip of driven gear near root of driver

Between initial contact and pitch point – a mixture of rolling and sliding

Rolling always same direction on both teeth

Contact length less on driving than on driven gear flank since pitch point nearer to root than tip

Therefore gear teeth must also slide to move contact to pitch point

Rolling and sliding in gears

IntroductionIntroduction

IntroductionIntroduction

At the pitch point pure rolling momentarily

Direction of friction force now changes on both teeth

On driving gear direction of friction is always away from pitch line

On driven gear direction of friction is always towards the pitch line

Therefore friction and rolling are - opposed for half the action - in the same direction for half the action

Sliding motion greatest at start and end of tooth engagement

Rolling and sliding in gears

IntroductionIntroduction

TEST METHODSTEST METHODS FOR GEARSFOR GEARS

Test methods for gearsTest methods for gears

Direct gear testing • Electronic measurement • Weight loss • Co-ordinate measurement

Gear contact simulation • Twin-disc test

Gear test rig Test methods for gearsTest methods for gears

Electronic measurement • solely at pitch line • combines

• wear • viscoelasticity • hub movement • moisture absorption • thermal expansion

Test methods for gearsTest methods for gears

Weight loss • moisture loss at running temperatures • differences in absorbed water for

filled polymers • dry weighing tedious

Test methods for gearsTest methods for gears

Alternative - control gears

Co-ordinate measurement • tooth profiles measured • qualitative information • individual teeth scanned • compared with involute profiles at

various roll angles

Test methods for gearsTest methods for gears

Datum axes for co-ordinate measurement

Test methods for gearsTest methods for gears

Orientation of co-ordinate measuring scan

Test methods for gearsTest methods for gears

Orientation of the tangential co-ordinate measuring scan

Test methods for gearsTest methods for gears

Wear measurement by co-ordinate measuring data

Test methods for gearsTest methods for gears

Gear contact simulation

Test methodsTest methods

twin-disc testing

by

Twin-disc machine Test methodsTest methods

Test methodsTest methods

Twin-disc tests

• separate rolling from sliding

• two discs loaded and run together

• if discs at same velocity then pure rolling

• velocity difference allows controlled rolling and sliding

• slip ratio = (Sliding velocity) / (Average rolling velocity)

Test methodsTest methods

Discs run at varying normal loads, speeds and slip ratios

Wear and coefficient of friction measured continuously

Wear measured by on-line displacement monitoring weighing samples

Heat dissipation conduction (40%), convection (40%), radiation (20%) superimposed local flash temperatures

MATERIALSMATERIALS

Materials

MaterialsMaterials

Polyamide 66 • + 30% short glass fibres • + 30% short glass fibres • + 30% short carbon fibres • + 30% short carbon fibres + 15% PTFE • + 30% long glass fibres

+ 15% PTFE

SPECIMENSSPECIMENS

MATERIALS

INJECTION MOULDING MACHINE

GEARS DISCS

SpecimensSpecimens

Specifications of gear specimens

Standard involute profile • Teeth • Pitch circle diameter • Module • Addendum height mm • Face width

SpecimensSpecimens

30 mm 17 mm 2

2 mm 17

Geometry of disc specimens

SpecimensSpecimens

EXPERIMENTALEXPERIMENTAL METHODSMETHODS

Testing parameters for gear specimens

Torque 10 Nm

Speed 1000 rpm

Experimental methodsExperimental methods

RESULTS RESULTS AND AND

DISCUSSIONDISCUSSION

Corrected weight loss against cycles

Results and discussionResults and discussion

Weight rates compared with pin-on-disc tests

Results and discussionResults and discussion

Glass Fibre Reinforced Gear Teeth after Testing

Results and discussionResults and discussion

Results and discussionResults and discussion

Geometric change by co-ordinate measurement

• great differences in worn profiles

• load sharing complicates load changes

• wear rates for carbon fibre polyamide change with roll angle and change tooth profile

• changing tooth profiles poor for gear performance

Results and discussionResults and discussion

Overall wear from co-ordinate measurement

(Average measured wear rate over all roll angles) x (length of involute in contact) x (tooth face width) = (total wear volume / tooth)

Hence total wear for all teeth

More accurate since measured over rangeof roll angles

Weight loss and co-ordinate measuring methods

Results and discussionResults and discussion

Co-ordinate measuring and twin-disc methods

Results and discussionResults and discussion

CONCLUSIONSCONCLUSIONS

Conclusions

ConclusionsConclusions

1. Measuring wear of polymer gears is difficult

2. Recording displacement at the pitch line and weight loss measurements both have problems

3. Co-ordinate measurement provide data on wear rates as a function of roll angle, and hence load, sliding speed and slip ratio

4. Geometry can be separated from materials effects by twin-disc tests

5. No correlation with pin-on-disc tests in simple sliding

6. Some correlation between twin-disc tests and gears

7. Materials effects important including fibres and matrix crystallinity