Plastic Gear Materials David Sheridan Celanese, Auburn Hills, Michigan © 2013 Celanese Gear-006 AM 10/13 The Plastic Gear Development Team Project Engineer Molder Tool Builder Manufacturing Engineer Gear Engineer Purchasing Material Supplier Plastics Engineer © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials Quality Control Engineer 2 Plastic Gear Development ► Identify Application – Voice of the Customer (VOC) ► Define Operating Requirements ‒ Ratio ‒ Prime mover ‒ Torque and speed ‒ Inertia, natural freq. ‒ Load(s) ‒ Torque and speed ‒ Special conditions ‒ Inertia, natural freq. ‒ Precision ‒ Efficiency ‒ Lubrication ‒ Environment ‒ Temperature ‒ Chemical exposure ‒ Moisture exposure ‒ Duty cycle ‒ Test requirements ‒ Life ‒ Other ‒ Physical limits ► Anticipate Future Applications © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 3 Plastic Gear Development Select Materials Preliminary Gear Sizing ► Suit operating environment ‒ Temperature range ► Select materials ► Select preliminary gear geometry ‒ Number of teeth ‒ Dimensional behavior ‒ Property behavior ‒ Size (pitch or module) ‒ Chemical environment ‒ Face width ‒ Dimensional behavior ‒ Property behavior ► Nominal ambient conditions ► Appropriate property mix ‒ Fatigue ► Simple load analysis ‒ K-Factor ‒ Stiffness ‒ Unit load ‒ Impact ‒ Creep ► Interaction with other components ‒ Friction ‒ Wear For more information see AGMA 920-A01, Materials for Plastic Gears © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 4 Preliminary Plastic Gear Sizing © 2013 Celanese Gear-006 AM 10/13 Preliminary Sizing Process Power Ratio Speed Inputs Specifications Analyze Final Design Advanced Analysis © 2013 Celanese Gear-006 AM 10/13 K-Factor / Unit Load Y Plastic Gear Materials Results OK? N 6 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 7 K-Factor ► Measure of surface stress intensity ‒ Surface durability ► Derived from Hertzian contact stress equation Sc ‒ ‒ ► K EG,P 0.70 1 1 cos EG E P sin K = Modulus of gear, pinion = Pressure angle Sc © 2013 Celanese Gear-006 AM 10/13 Wear Plastic Gear Materials 8 K-Factor Calculation K ext Ft D p, Pinion 1 1 f NG /NP Ft = Tangential load NG = Number of teeth, gear Dp = Operating pitch diameter NP = Number of teeth, pinion f = Net face width ► Independent of material properties ► Inversely proportional to face width © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 9 K-Factor Celcon® M90 vs. Celcon® M90 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 10 K-Factor Celcon® GC25A vs. Celcon® GC25A © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 11 Unit Load (UL) ► Measure of bending stress intensity ‒ Tooth strength ► Derived from Lewis Formula ► UL Tooth breakage © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 12 Unit Load Calculation F Pd sb fY Ft Pd UL f Lewis Formula Ft = Tangential load F = Load on cantilever Pd = Diametral pitch Y = Lewis geometric factor f = Face width ►Independent of material ►Directly proportional to diametral pitch © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 13 Unit Load Celcon® M90 vs. Celcon® M90 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 14 Unit Load Celcon® GC25A vs. Celcon® GC25A © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 15 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 16 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 17 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 18 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 19 Effects of Loading Rate and Temperature Modulus Stress Increased Loading Rate Crystalline Tg Amorphous Tg Increased Temperature Tm Temperature Strain © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 20 Creep and Fatigue vs. Temperature Log Stress Log Modulus Increasing Temperature Log Cycles Log Time © 2013 Celanese Gear-006 AM 10/13 Increasing Temperature Plastic Gear Materials 21 Temperature Limits for Plastic Gears 250 Temperature, °C 220 220 200 170 150 150 150 PBT PA 6/6 170 130 115 100 Acetal GF Acetal © 2013 Celanese Gear-006 AM 10/13 GF PBT GF PA 6/6 GF PPS Plastic Gear Materials GF LCP 22 Elastic Modulus Preferred Alternate ► DMA curves ► Tensile data ► Parameters ‒ Temperature ► Parameters ‒ Temperature ‒ Moisture ‒ Moisture ‒ Chemical exposure ‒ Chemical exposure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 23 Dynamic Mechanical Analysis (DMA) Tensile Design Apply Harmonic Displacement Displacement 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 time Phase Shift (δ) Test Specimen Load 7 time Measure Load © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 24 Shear Modulus vs. Temperature for Acetal Copolymer 3000 Shear Modulus (MPa) 2500 2000 Glass Reinforced Grade 1500 Standard Grade 1000 500 0 -80 Impact Modified Grade -60 -40 -20 0 20 40 60 80 100 120 140 160 180 Temperature (°C) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 25 Shear Modulus vs. Temperature for Acetal Copolymer 10000 S hear M odulus (M P a) Glass Reinforced Grade 1000 Standard Grade 100 10 -80 Impact Modified Grade -60 -40 -20 0 20 40 60 80 100 120 140 160 180 Temperature (°C) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 26 Typical Acetal Copolymer DMA Curves Normalized at 23°C Norm aliz ed M od ulu s 10 1 Glass Reinforced Grade Impact Modified Grade 0.1 0.01 -60 -40 -20 0 20 40 60 80 100 120 140 160 Temperature (°C) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 27 Typical Acetal Copolymer Tensile Data ISO Method 100 -40°C 90 80 70 23°C Stress, MPa 60 40 50 40 80°C 30 100°C 20 120°C 10 0 0 2 4 6 8 10 12 14 Strain, % © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 28 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 29 Bending Fatigue Strength Preferred Alternate ► Gear tooth bending ► ASTM D671 fatigue stress vs. life cycles (S-N curves) ► Parameters ► Add temperature correction ‒ Temperature ‒ Moisture ‒ Chemical exposure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 30 Tooth Root Bending Fatigue Strength Unfilled Acetal Copolymer Greased vs. Air Temperature Bending Strength (MPa) 100 20°C 60°C 80°C 100°C 10 1.00E+05 1.00E+06 1.00E+07 1.00E+08 Cycles © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 31 Tooth Root Bending Fatigue Strength Hostaform® HS15™ Acetal Copolymer vs. Low MI Acetal Homopolymer Gear Performance Oil Lubricated at 40°C 100 Torque (in-lbf) 90 Hostaform® HS15™ POM 80 Low MI Acetal Homopolymer 70 60 50 100 1,000 10,000 Thousand Cycles to Failure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 32 Early Plastic Gear Fatigue Data © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 33 Acetal Copolymer Fatigue Data by ASTM D671 100 S tress (M P a) Glass Reinforced (Cross Flow) Standard Grade 10 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 Cycles to Failure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 34 Acetal Copolymer Fatigue Data by ASTM D671 10000 S tress (psi) Glass Reinforced (Cross Flow) Standard Grade 1000 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 Cycles to Failure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 35 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%) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 36 Typical Acetal Copolymer Tensile Data ISO Method 40 -40 35 Stress, MPa 30 23 25 40 20 15 80 10 100 120 5 0 0 0.2 0.4 0.6 0.8 1 Strain, % © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 37 Wear Rate (Dry and Greased Gears) Preferred Alternate ► Hertzian contact stress vs. life cycles ► ASTM D3702 ring on disk (thrust (S-N curves) ► Failure criteria inconsistent washer) test ‒ Tooth failure ‒ Weight loss ‒ Increase in backlash ‒ Transmission error ► Parameters ‒ Temperature ‒ Moisture ‒ Chemical exposure © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 38 Permissible Contact Stress Acetal Copolymer Below 60°C Contact Stress (MPa) 100 Wear Modified Grades Standard Grades 10 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 Cycles © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 39 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. © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 40 Optimum Surface Roughness ► POM vs. steel ‒ Adhesive and deformative friction sum Coefficient of friction µ µdef Optimum ( 1-2 µm ) µadh Surface roughness ( Steel ) in µm © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 41 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. © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 42 Celanese Tribological Grades Celcon® and Hostaform® 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 © 2013 Celanese Gear-006 AM 10/13 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 Plastic Gear Materials 43 Ticona Kelsterbach NEW rheometer/tribo meter © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 44 Celanese Tribological Test Methods ► Capabilities span a wide range of speeds and loads ‒ Ability to measure wear, friction, and noise generation Different Tribology Tests 1000 Celanese Kelsterbach NEW rheometer/tribomete r sliding speed in m/min 100 Celanese USA ASTM D3702 Ticona 10 1 Celanese Kelsterbach stick slip (noise) test Celanese USA ASTM D1894 slow speed N F 0.1 0.001 © 2013 Celanese Gear-006 AM 10/13 0.01 0.1 1 load pressure in MPa Plastic Gear Materials 10 100 45 Optimizing Sliding Partners Sliding partners Plastics Metal Steel Wear Friction coefficient Squeaking Semi-crystalline Amorphous (PC, PMMA) (PBT, PA) Aluminum Brass POM C 9021 against itself AW, BSX, SW BSX, SW EWX, TF, FSK AW, TF, FSK K, SOEK EWX RM, M140L1, G BSX, EWX AW, SW FCT1 BSX, EWX SW, TF, FSK BSX, SW AW, TF, FSK SOEK AW, SW, TF EWX BSX, EWX, FSK SW SOEK, G AW, BSX, TF FSK EWX, SW AW, BSX FSK SW BSX, EWX TF, FSK AW, BSX, SW BSX, SW, TF SOEK, TF, FSK AW BSX, SW AW SOEK, TF, FSK BSX, SW AW FSK BSX, SW TF, FSK BSX, SW SOEK, FSK AW, TF © 2013 Celanese Gear-006 AM 10/13 BSX, SW TF, FSK Plastic Gear Materials BSX, SW, TF AW BSX, SW AW, TF 46 Example: Celanese Stick-Slip Test FN: normal force = 5 - 30N (load varies, up to ~200 psi) A: surface area a: acceleration FR: friction force FH: static friction force vr: sliding speed = 1-10mm/s (0.2 - 2 ft/min) Test time: 45 minutes FN Checked with: C&E – matrix Gage R&R Control chart a, FR, FH A vr © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 47 Celanese Stick-Slip Test Data Set ► POM grades against C 9021 POM using stick-slip test ‒ Low speed – high load Hostaform C 9021 vs. Hostaform grades T= 23°C / friction, wear + stick slip after 45min C 9021 LW90EWX wear in mm C 9021FCT-1 C 9021 SW C 9021AW C 9021 K C 9021 G no stick slip S 9243 LW90BSX S 9244 LW90TX C 9021 TF C 9021 M stick slip < 0 dB C 9021 GV1/20XGM stick slip > 0 dB C 9021 + 2% silicone oil C 9021TF5 + 2% silicone oil coefficient of friction © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 48 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 49 Temperature Model 1, 2 1 i 17100 k 2 6 . 3k 3 a 0.136 P 1, 2 z 2 5 bk z1, 2 vm A k2 experimental value bk face width temperatu re P power coefficient of friction i gear ratio = z2 v pitch line velocity m module z1 z 2 number of teeth on gear z1 number of teeth on pinion © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 1, 2 index k3 experimental value A area 50 Coefficient of Friction for Material Combinations in Temperature Model POM PBT PA steel POM 0.28 0.18 0.18 0.2 PBT 0.18 PA 0.18 steel 0.2 0.2 0.2 0.2 = 0.09 for one-time lubrication at initial assembly = 0.07 for oil mist lubrication = 0.04 for continuous lubrication POM = Acetal Copolymer PBT = Polyester PA = 6/6 Nylon © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 51 Temperature Model Experimental Value, k2, for Temperature Model POM PBT PA steel POM 2.5 2.5 PBT 2.5 1.0 PA steel 1.0 2.4 1.0 1.0 Experimental Value, , for Temperature Model = 0.4 POM = 0.4 PBT = 0.75 PA Experimental Value, k3, for Temperature Model k3 = k3 = 0 0.04 - 0.13 k3 = 0.172 © 2013 Celanese Gear-006 AM 10/13 Open gears with free air access Partially enclosed gear box in which air cannot circulate freely Totally enclosed gear box Plastic Gear Materials 52 Dimensional Variation ► Temperature ‒ Thermal expansion coefficient ► Moisture ‒ Starting approximation ‒ % absorbed / 4 = % dim change © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 53 Coefficient of Thermal Expansion Material in/in/°F-10 -5 cm/cm/°C-10 Nylon – GR 1.3 2.3 Acetal – GR 2.2 4.0 Nylon 4.5 8.1 Acetal 4.8 8.5 -5 (On laboratory test bars, test similar parts for better prediction.) GR = Glass Reinforced © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 54 Additional Mechanical Data ► For overload condition ‒ Tensile strength ‒ Shear strength ‒ Creep ► For shock loads (impact) ‒ Izod ‒ Charpy © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 55 Additional Thermal Data ► Deflection Temperature Under Load (DTUL, HDT) ► UL Relative Thermal Index (RTI) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 56 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 57 Deflection Temperature Under Load (DTUL) ISO Method A B C AST Method 264 psi 66 psi none Stress 1.8 MPa (264 psi) 0.45 MPa (66 psi) 8 MPa (1,160 psi) Apparent 930 MPa 230 MPa 4,100 MPA Modulus 135,000 psi 34,000 psi 600,000 psi © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 58 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 59 UL Relative Thermal Index Property Ratio, % 100 50 % 10 0.01 0.1 1 10 100 1000 10000 100000 Time (hours) © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 60 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 © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 61 Lubricant Caveats ► Oils ‒ Check compatibility ‒ Check appearance ‒ Check dimension change ‒ No EP oils ► Greases ‒ See above ‒ Use caution with filled materials ► Surface finish critical © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 62 Questions? Thank you! © 2013 Celanese Gear-006 AM 10/13 Disclaimer Contact Information This 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. Americas Copyright © 2013 Celanese or its affiliates. All rights reserved. 8040 Dixie Highway, Florence, KY 41042 USA Product Information Service t: +1-800-833-4882 t: +1-859-372-3244 Customer Service t: +1-800-526-4960 t: +1-859-372-3214 e: info-engineeredmaterials-am@celanese.com Europe Am Unisys-Park 1, 65843 Sulzbach, Germany Product Information Service t: +(00)-800-86427-531 t: +49-(0)-69-45009-1011 e: info-engineeredmaterials-eu@celanese.com Asia 4560 Jinke Road, Zhang Jiang Hi Tech Park Shanghai 201203 PRC Customer Service t: +86 21 3861 9266 f: +86 21 3861 9599 e: info-engineeredmaterials-asia@celanese.com David Sheridan 248-340-7485 D.Sheridan@celanese.com © 2013 Celanese Gear-006 AM 10/13 Plastic Gear Materials 64