Plastic Gear Materials

Plastic Gear MaterialsDavid SheridanCelanese, Auburn Hills, Michigan

© 2013 Celanese Gear-006 AM 10/13

The Plastic Gear Development Team

Molder

MaterialSupplier

Plastics Engineer

Quality ControlEngineer

ManufacturingEngineer

Gear Engineer

Project Engineer

Purchasing

ToolBuilder

© 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 2

Plastic Gear Development

► Identify Application – Voice of the Customer (VOC)►Define Operating Requirements

‒ Prime mover ‒ Torque and speed‒ Inertia, natural freq.

‒ Load(s)‒ Torque and speed‒ Special conditions‒ Inertia, natural freq.

‒ Duty cycle

‒ Life

‒ Physical limits

© 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials

‒ Ratio‒ Precision‒ Efficiency‒ Lubrication‒ Environment

‒ Temperature‒ Chemical exposure‒ Moisture exposure

‒ Test requirements ‒ Other

►Anticipate Future Applications

3

Plastic Gear Development

Select Materials Preliminary Gear Sizing

► Suit operating environment‒ Temperature range

‒ Dimensional behavior‒ Property behavior

‒ Chemical environment‒ Dimensional behavior‒ Property behavior

► Appropriate property mix‒ Fatigue

‒ Stiffness

‒ Impact

‒ Creep

► Interaction with other components‒ Friction

‒ Wear

► Select materials► Select preliminary gear geometry

‒ Number of teeth

‒ Size (pitch or module)

‒ Face width

► Nominal ambient conditions► Simple load analysis

‒ K-Factor

‒ Unit load

For more information see AGMA 920-A01, Materials for Plastic Gears

© 2013 Celanese Gear-006 AM 10/13 4Plastic Gear Materials

Preliminary Plastic Gear Sizing


Inputs

Analyze

Results OK?

N

K-Factor / Unit Load

Power Ratio Speed

YAdvancedAnalysis

FinalDesign

Specifications

Preliminary Sizing Process


Preliminary Sizing Process

Plastic Design Feasibility►K-Factor (surface stress)►Unit load (bending stress)

Assumptions / Limitations►External, parallel-axis, single stage gear set►Load well distributed, perfect shaft alignment►No center distance variation considered►Load shared by > 1 pair of teeth►No tooth errors►Lubricated environment


►Measure of surface stress intensity‒ Surface durability

►Derived from Hertzian contact stress equation

‒ EG,P = Modulus of gear, pinion‒ = Pressure angle

► K Sc Wear

K-Factor


K

E1

E1

0.70S

PG

c

sincos

Ft = Tangential load NG = Number of teeth, gear

Dp = Operating pitch diameter NP = Number of teeth, pinion

f = Net face width

K-Factor Calculation

► Independent of material properties► Inversely proportional to face width

PGPinionp,

text NNfD

FK/11


K-Factor Celcon® M90 vs. Celcon® M90


K-Factor Celcon® GC25A vs. Celcon® GC25A


Unit Load (UL)

►Measure of bending stress intensity‒ Tooth strength

►Derived from Lewis Formula

► UL Tooth breakage


Unit Load Calculation

fPFUL dt

►Independent of material►Directly proportional to diametral pitch

YfPFs d

b Lewis Formula

Ft = Tangential load

F = Load on cantilever

Pd = Diametral pitch

Y = Lewis geometric factor

f = Face width


Unit LoadCelcon® M90 vs. Celcon® M90


Unit Load Celcon® GC25A vs. Celcon® GC25A


Advanced Ticona Analysis Support

►Additional materials, dry running

►Full design optimization‒ Profile modification‒ Tooth tip relief‒ Tooth strength balance

►Multi-stage gear trains and housings

►More complex gear types and arrangements

►Detailed geometry for specifications


Plastic Gear Data

Early Plastic Gear Data


► Material properties mixed with gear geometry‒ Limited acceptance by gear designers

‒ Did not fit with standard gear design

► Limited temperature and environmental data‒ Property changes not included in design

► Limited understanding of failures‒ Dry gears wear

‒ Greased gears show wear and bending

‒ Other mechanisms not defined

Plastic Gear Data

Present Plastic Gear Data


► Developing properties in terms of temperature rather than geometry

► Operating temperature prediction possible► Developing improved testing methods► Much improved geometry design

Future Plastic Gear Data► Better load analysis data► Improved temperature and

environmental effects prediction► Improved failure

mode understanding

Plastic Properties

Strength and Modulus Vary


► Temperature► Moisture► Chemical exposure, lubricants

Dimensions Change► Temperature

‒ Thermal expansion > metals (x10)

► Moisture► Chemical exposure, lubricants► Shrinkage, stress relief

Effects of Loading Rate and Temperature

IncreasedLoading Rate

Stress

Strain

IncreasedTemperature

Modulus

Temperature

CrystallineTg

Tg

Amorphous

Tm


LogModulus

Log Time

IncreasingTemperature

IncreasingTemperature

LogStress

Log Cycles

Creep and Fatigue vs. Temperature


Temperature Limits for Plastic Gears

115

130

150 150

170 170

220 220

100

150

200

250

Acetal GF Acetal PBT PA 6/6 GF PBT GF PA 6/6 GF PPS GF LCP

Tem

pera

ture

, °C


Elastic Modulus

Preferred Alternate

23

► DMA curves► Parameters

‒ Temperature

‒ Moisture

‒ Chemical exposure

► Tensile data► Parameters

‒ Temperature

‒ Moisture



Dynamic Mechanical Analysis (DMA)Tensile Design


ApplyHarmonic Displacement

Measure Load

0 1 2 3 4 5 6 7

Displacement

Load

time

PhaseShift

0 1 2 3 4 5 6 7

TestSpecimen

time

(δ)

0

500

1000

1500

2000

2500

3000

-80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Temperature (°C)

Shea

r Mod

ulus

(MPa

)Shear Modulus vs. Temperature for Acetal Copolymer


Standard Grade

Glass Reinforced Grade

Impact Modified Grade

10

100

1000

10000

-80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Temperature (°C)

Shea

r Mod

ulus

(MPa

)Shear Modulus vs. Temperature for Acetal Copolymer


Standard Grade



0.01

0.1

1

10

-60 -40 -20 0 20 40 60 80 100 120 140 160

Temperature (°C)

Norm

aliz

ed M

odul

usTypical Acetal Copolymer DMA Curves Normalized at 23°C




0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Strain, %

Stre

ss, M

PaTypical Acetal Copolymer Tensile DataISO Method


120°C

100°C

80°C

40

23°C

-40°C

Poisson’s Ratio

►Varies with environmental conditions►Has minimal effect on calculations► If specific data is available, use it►Otherwise use 0.35 for all plastics


Bending Fatigue Strength

Preferred Alternate

30

► Gear tooth bending stress vs. life cycles (S-N curves)

► Parameters ‒ Temperature

‒ Moisture


► ASTM D671 fatigue► Add temperature correction


Tooth Root Bending Fatigue StrengthUnfilled Acetal Copolymer Greased vs. Air Temperature


10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

Cycles

Ben

ding

Str

engt

h (M

Pa)

60°C

20°C

80°C

100°C

Tooth Root Bending Fatigue StrengthHostaform® HS15™ Acetal Copolymervs. Low MI Acetal Homopolymer


Gear PerformanceOil Lubricated at 40°C

50

60

70

80

90

100

100 1,000 10,000

Thousand Cycles to Failure

Torq

ue (i

n-lb

f)

Low MI Acetal Homopolymer

Hostaform® HS15™ POM

Early Plastic Gear Fatigue Data


Acetal Copolymer Fatigue Data by ASTM D671


10

100

1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Cycles to Failure

Stre

ss (M

Pa) Glass Reinforced (Cross Flow)

Standard Grade

1000

10000

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Cycles to Failure

Stre

ss (p

si)

Acetal Copolymer Fatigue Databy ASTM D671


Glass Reinforced (Cross Flow)

Standard Grade

Fatigue Temperature Correction

►Use tensile strength curves‒ Parameters

‒ Temperature‒ Moisture‒ Chemical exposure

►Unreinforced‒ Strength at yield not important, strain too high

‒ Strength at a particular strain (pick 1% strain)

►Reinforced ‒ Strength at a particular strain (try 1%)


0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8 1

Strain, %

Stre

ss, M

Pa

Typical Acetal Copolymer Tensile DataISO Method


-40

23

40

80

100120

Wear Rate (Dry and Greased Gears)

Preferred Alternate

38

► Hertzian contact stress vs. life cycles (S-N curves)

► Failure criteria inconsistent ‒ Tooth failure

‒ Weight loss

‒ Increase in backlash

‒ Transmission error

► Parameters‒ Temperature

‒ Moisture


► ASTM D3702 ring on disk (thrust washer) test


Permissible Contact StressAcetal Copolymer Below 60°C


10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09

Cycles

Con

tact

Str

ess

(MPa

)

Wear Modified Grades

Standard Grades

Tribology When Running Dry Without Lubricants

Performance of system governed by


► Speed, load, motion type (sliding, rolling…), etc.► Similar vs. dissimilar materials

‒ Polarity, surface energy, & adhesion of counterparts

► Surface roughness & asperity deformation‒ Optimum differs with polarities

► Incorporated lubricants, e.g. PTFE, Si, etc.

►POM vs. steel‒ Adhesive and deformative friction

µdef

µadh

µsum

Coe

ffici

ent o

f fric

tion

Surface roughness ( Steel ) in µm

Optimum Surface Roughness


Optimum ( 1-2 µm )

Tribological Mechanisms of Lubricants and Fillers

A Few Examples:


Mineral / Chalk► Chalk hardens the POM-matrix and reduces its

wear; in abrasive systems it may increase wear if it stays in the sliding area.

PTFE► PTFE is incorporated in POM as a micro powder.

It builds up a lubricating film in between the sliding partners. As PTFE has a low coefficient of friction in contact with many materials it reduces the friction in the system.

Silicone Oil► Silicone oil is incorporated as liquid droplets.

These droplets continue to come to the sliding surface as the covering surface layer abrades.

Celanese Tribological GradesCelcon® and Hostaform®

43

Polytetrafluorethylene PTFE► Hostaform C 9021 TF / TF5 (Celcon

LW90F2)► Hostaform C 27021 TF

UHMW-PE► Hostaform C 9021 G► Hostaform C 2521 G

Wax► Hostaform LW90EWX► Hostaform LW15EWX► Hostaform C 13021 RM► Hostaform C 9021 FCT1► Celcon M140L1

Molybdenum Disulfide► Hostaform C 9021 M

Chalk► Hostaform C 9021 K (Celcon LW90)► Hostaform C 13031 K

Silicone Oil► Hostaform C 9021 SOEK (Celcon

LW90SC)► Celcon LW90S2► Hostaform LW90BSX► Celcon LW25S2

PTFE & Silicone Oil► Celcon LW90FS-K

Special PE► Hostaform C 9021 AW (Celcon M90AW) ► Hostaform C 9021 SW (Celcon M90SW)

Glass Fiber and Glass Beads► Hostaform C 9021 GV 1/30 GT► Hostaform C 9021 GV 3/30 TF2► Hostaform C 9021 GV 1/20 XGM



Ticona Kelsterbach

NEW rheometer/tribo

meter

Celanese Tribological Test Methods

►Capabilities span a wide range of speeds and loads‒ Ability to measure wear, friction, and noise generation


0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100

slid

ing

spee

d in

m/m

in

load pressure in MPa

Different Tribology Tests

Celanese USAASTM D3702

Ticona

FN

Celanese Kelsterbachstick slip (noise) testCelanese USA

ASTM D1894 slow speed

CelaneseKelsterbach

NEW rheometer/tribomete

r

Optimizing Sliding Partners

C 9021

BSX, SW AW, TF, FSKEWX

EWXSWAW, BSX, TFFSK

BSX, SWTF, FSK

AW, BSX, SW EWX, TF, FSKK, SOEKRM, M140L1, G

AW, SW, TFBSX, EWX, FSKSOEK, G

BSX, SWAWSOEK, TF, FSK

BSX, EWXSW, TF, FSK

SWBSX, EWXTF, FSK

BSX, SWTF, FSK

BSX, EWXAW, SWFCT1

EWX, SWAW, BSXFSK

BSX, SWAWFSK

BSX, SW, TFAW

BSX, SW, TFAW

BSX, SWAW,TF

BSX, SWAW, TF, FSKSOEK

AW, BSX, SWSOEK, TF, FSK

BSX, SWSOEK, FSKAW, TF

Sliding partners

POM

against itself

Plastics

Semi-crystalline(PBT, PA)

Steel AluminumBrass

Wear

Frictioncoefficient

Squeaking

Metal

Amorphous(PC, PMMA)


Example: Celanese Stick-Slip Test


vr

a, FR, FH

FN

A

Checked with:C&E – matrix Gage R&R Control chart

FN: normal force = 5 - 30N (load varies, up to ~200 psi)

A: surface areaa: accelerationFR: friction forceFH: static friction forcevr: sliding speed = 1-10mm/s

(0.2 - 2 ft/min)Test time: 45 minutes

Hostaform C 9021 vs. Hostaform gradesT= 23°C / friction, wear + stick slip after 45min

C 9021 + 2% silicone oil

C 9021 TF

S 9244

LW90TX

S 9243C 9021 SW

C 9021TF5 + 2% silicone oil

C 9021FCT-1

C 9021 K

C 9021 G

C 9021 M

LW90BSX

C 9021AW

LW90EWX

C 9021 GV1/20XGM

C 9021

coefficient of friction

wea

r in

mm

no stickslip

stick slip < 0 dB

stick slip > 0 dB

Celanese Stick-Slip Test Data Set

►POM grades against C 9021 POM using stick-slip test‒ Low speed – high load


Temperature Model

►The gear designer must predict the gear tooth operating temperature to properly select the properties

►The following model provides an approximation

►Testing is required to verify temperature


pinionon teeth ofnumber gearon teeth ofnumber

=ratiogear

frictionoft coefficienpower

retemperatu

1

2

1

2

zz

zzi

P

3.617100

51136.0 3

2,1

2

22,1 2,1

Ak

vmzbk

ziP

ka

areavaluealexperiment

indexmodule

velocitylinepitch widthface

valuealexperiment

3

2,1

2

Ak

mv

bk

k

Temperature Model


Coefficient of Friction for Material Combinations in Temperature Model


= 0.09 for one-time lubrication at initial assembly = 0.07 for oil mist lubrication = 0.04 for continuous lubrication

POM PBT PA steelPOM 0.28 0.18 0.18 0.2PBT 0.18 0.2PA 0.18 0.2steel 0.2 0.2

POM = Acetal CopolymerPBT = PolyesterPA = 6/6 Nylon

Temperature Model


POM PBT PA steelPOM 2.5 2.5 1.0PBT 2.5PA 2.4 1.0steel 1.0 1.0

Experimental Value, k2, for Temperature Model

k3 = 0 Open gears with free air accessk3 = 0.04 - 0.13 Partially enclosed gear box in which air

cannot circulate freelyk3 = 0.172 Totally enclosed gear box

Experimental Value, k3, for Temperature Model

Experimental Value, , for Temperature Model

= 0.4 POM = 0.4 PBT = 0.75 PA

Dimensional Variation

►Temperature‒ Thermal expansion coefficient

►Moisture‒ Starting approximation

‒ % absorbed / 4 = % dim change


Coefficient of Thermal Expansion

(On laboratory test bars, test similar parts for better prediction.)

GR = Glass Reinforced

Material in/in/°F-10-5 cm/cm/°C-10-5

Nylon – GR 1.3 2.3

Acetal – GR 2.2 4.0

Nylon 4.5 8.1

Acetal 4.8 8.5


Additional Mechanical Data

►For overload condition‒ Tensile strength

‒ Shear strength

‒ Creep

►For shock loads (impact)‒ Izod

‒ Charpy


Additional Thermal Data

►Deflection Temperature Under Load (DTUL, HDT)►UL Relative Thermal Index (RTI)


Deflection Temperature Under Load (DTUL)

►The method‒ Sample mounted in test chamber in 3-point bending

‒ Specific stress applied

‒ Temperature increased at specific rate

‒ Continue until a specific deflection (strain) is reached

►Report temperature►True Result – the temperature at which a particular modulus is

reached►That modulus is a combination of creep, relaxation,

and bending


Deflection Temperature Under Load (DTUL)

ISO Method A B CAST Method 264 psi 66 psi noneStress 1.8 MPa

(264 psi)0.45 MPa(66 psi)

8 MPa(1,160 psi)

Apparent 930 MPa 230 MPa 4,100 MPAModulus 135,000 psi 34,000 psi 600,000 psi


UL Relative Thermal Index (RTI)

►Property specific‒ Mechanical without impact

‒ Mechanical with impact

‒ Electrical

►Heat aging temperature at which the sample will lose half of the initial property when stored at that temperature for 100,000 hours under NO LOAD

►NOT continuous-use temperature


UL Relative Thermal Index

10

100

0.01 0.1 1 10 100 1000 10000 100000

Time (hours)

Prop

erty

Rat

io, % 50 %


Material Caveats

►Switching materials – same cavity‒ Good results luck

►Highly modified materials and lubrication‒ If some is good, more is…

►Running internally lubricated materials in oil

►Check lubricant compatibility


Lubricant Caveats

►Oils‒ Check compatibility

‒ Check appearance

‒ Check dimension change

‒ No EP oils

►Greases‒ See above

‒ Use caution with filled materials

►Surface finish critical


Questions?Thank you!


DisclaimerThis publication was printed on 1 October 2013 based on Celanese’s present state of knowledge, and Celanese undertakes no obligation to update it. Because conditions of product use are outside Celanese’s control, Celanese makes no warranties, express or implied, and assumes no liability in connection with any use of this information. Nothing herein is intended as a license to operate under or a recommendation to infringe any patents.

Copyright © 2013 Celanese or its affiliates. All rights reserved.


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64

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Plastic Gear Materials