Define the Gear Duty Before Comparing Materials
Start with the required torque, speed range, life target and allowable backlash. Record running loads separately from startup, stall and other peaks, then use the checklist below to complete the geometry, duty cycle, environment and lubrication requirements.
Define the operating environment around the gear mesh. Ambient temperature alone may not represent the tooth temperature because speed, load, friction, and lubrication can generate local heat. Include nearby heat sources, cooling, chemicals, dust, washdown, humidity, expected noise, maintenance limits, and whether the gear must run dry, with initial lubrication, or with continuous lubrication.
- Gear type, module or diametral pitch, tooth count, face width and ratio
- Input and output torque, speed range, rotation direction, start-stop frequency and duty cycle
- Running torque and startup, stall, jam, reversal and braking peaks; target cycle count
- Shaft support, alignment, housing stiffness and allowable backlash
- Ambient and local temperature, chemicals, contamination and moisture
- Lubrication method, maintenance interval, noise and efficiency target
Identify the Governing Failure Mode
Gear teeth can fail at the root from repeated bending, at the flank from wear or surface damage, or through deformation that changes the mesh. Creep, thermal expansion, molded shrinkage, bore movement, hub stress, shaft misalignment, and housing movement can alter backlash and tooth contact even when the material does not fracture.
Do not collapse these risks into one request for a 'stronger' or 'more wear-resistant' grade. A higher modulus may reduce tooth deflection but does not automatically improve impact tolerance, weld-line performance, noise, or mating-part wear. Likewise, a lower friction result does not by itself establish fatigue life or dimensional accuracy. Rank each failure mode and define how it will be measured.
Gear failure evidence map
Evaluate the Complete Contact Pair
The same POM grade can behave differently against hardened steel, softer metal, another POM, PA, or a coated surface. Record the driver and driven materials, surface hardness and finish, tooth geometry, contact pattern, alignment, lubrication, pressure, speed, and expected debris or contamination. Wear on the mating gear may be as important as wear on the POM gear.
Review breakaway and running friction, adhesive or abrasive wear, noise, vibration, frictional heat, lubricant compatibility, and run-in behavior separately. Plastic-on-plastic contact and dry running require particular care because heat dissipation and adhesive wear can govern the result. Use grade-specific tribology data only when the mating material, pressure, speed, temperature, surface and test method are relevant to the application.
- Driver and driven material, hardness, finish and tooth contact pattern
- Dry, initially lubricated or continuously lubricated operating condition
- Pressure, sliding and rolling speed, duty, temperature and heat removal
- Wear of both gears, noise, efficiency, debris and lubricant condition
Gear and hub illustrations: check concentricity in compound gears, contact and axial loading in helical assemblies, and press-fit stress in hubbed gears.
- Compound spur gear
- Helical gear assembly
- Hubbed gear
Design Material Selection and Precision Molding Together
A gear that meets material calculations can still fail through poor roundness, concentricity, tooth spacing, profile, runout, bore fit, or backlash. Review gate position, flow direction, weld lines, wall transitions, rim and web thickness, hub and rib design, cooling balance, ejection, and cavity variation. These features affect molded shrinkage, residual stress, tooth accuracy, and the relationship between the bore and pitch circle.
TDS shrinkage is not a finished-gear tolerance. Establish the intended grade and molding window, then measure cavity-marked gears after a defined conditioning time. Check critical dimensions and functional mesh together. Machined prototypes can support early geometry or assembly work, but an injection-molded prototype is needed to reproduce molded skin, shrinkage, orientation, weld lines, and process variation.
Shortlist Grades by Dominant Risk
When using the ETM 100P gear-endurance case, first compare its load, motion and lubrication conditions with the project. For dry sliding, stick-slip or tooth wear, compare EPTL402 and EMS102/EMS162. For load deformation, compare glass-fiber-reinforced POM stiffness, orientation and mating-gear wear.
Use POM as a starting point for low moisture-related dimensional change. Include PA when a different toughness or temperature balance is required; record dry and conditioned states separately and compare grades at equal torque, temperature and backlash limits.
- Use standard POM as the baseline for molding, fatigue and precision
- Wear-modified POM when tooth and counterpart wear govern service life
- Low-friction POM when running force, stick-slip, noise or heat governs
- Reinforced or alternative polymers only after orientation and trade-off review
Validate the Gear Train Under Representative Conditions
Use calculation and simulation to screen geometry and material candidates, but release the gear through testing. Mold traceable prototypes in the intended tool or a representative cavity and record material, lot, process, dimensions and conditioning. Test the complete gear train with the intended shafts, bearings, housing, mating gears, lubrication, alignment, load spectrum, speed, reversals, temperature and environment.
A useful test plan measures torque or efficiency, temperature near the mesh, noise or vibration, backlash, dimensional change, tooth-root damage, flank wear, mating-part wear, lubricant or debris condition, and failure cycles. Include startup, stall, shock, and endurance conditions where applicable. After the test, inspect the failure location and compare it with the original risk ranking before approving a grade or changing the design.
Separate acceptance criteria from failure observations
In the ETM 100P massage-chair armrest gear trial, the report recorded 1 sample meeting the 27,500-cycle project requirement under 20 kg on each side. Later testing recorded small-gear wear and slipping, while the second-stage gears remained functional at the documented checks.
The customer subsequently adopted ETM 100P for production and placed repeat orders.
The test results apply only to that historical sample and test conditions. For a comparable test, define load, motion, mating components, lubrication, environment, sample quantity, inspection items and failure criteria. Record each gear stage separately so the findings can inform material and design reviews.