Why Nylon Is Chosen for Precision Gearing
Nylon (polyamide, PA6/PA66) is one of the few plastics that regularly replaces metal in load-bearing, meshing gear applications. Unlike general-purpose plastics, engineering-grade nylon maintains dimensional stability under moderate heat and cyclic loading, which is the baseline requirement for any gear that must hold tooth profile accuracy over its service life. On a recent spur gear project — a 30-tooth PA66 gear machined for a conveyor drive — the deciding factor over POM or steel was the combination of self-lubrication and noise reduction, not material cost.
Machining Tolerances That Actually Matter
Nylon’s higher thermal expansion coefficient (roughly 8–10x that of steel) is the single factor most commonly underestimated by buyers sourcing “metal-equivalent” tolerances in plastic. On precision CNC-machined gears, we typically hold bore tolerance at ±0.02–0.05 mm and outer diameter at ±0.03 mm, but tooth-to-tooth pitch accuracy should be verified at operating temperature, not just at room temperature on the inspection bench.
| Feature | Achievable Tolerance | Design Note |
|---|---|---|
| Bore diameter | ±0.02–0.05 mm | Moisture absorption can swell bore by 0.1–0.3% |
| Tooth profile (module) | DIN 6–8 typical | Tighter grades need post-machining inspection |
| Outer diameter | ±0.03–0.05 mm | Hold fixture with minimal clamping stress |
DFM Considerations Before You Release Drawings
Wall Thickness & Hub Design
Uneven wall thickness around the hub causes differential shrinkage and warping during machining, especially when parts are cut from extruded or cast nylon rod. Keep hub-to-rim thickness variation within 20% where the design allows.
Moisture Conditioning
PA6/PA66 absorbs ambient moisture, which changes both dimensions and mechanical properties over time. For gears requiring tight backlash control, specify moisture-conditioned (not as-machined “dry”) stock, or budget for conditioning in the fit allowance.
Material Grade Selection
Glass-filled nylon (PA66-GF30) raises stiffness and reduces creep but increases mating-gear wear if paired with another plastic gear — pair glass-filled nylon against metal, not against unfilled plastic, to avoid accelerated abrasive wear.
Typical Applications
Frequently Asked Questions
How does nylon gear life compare to steel under continuous load?
Nylon gears generally handle lighter-to-moderate continuous loads well due to their self-lubricating nature, but steel still outperforms in high-torque, high-temperature, or continuous heavy-duty cycles. For mixed-load applications, a nylon-steel gear pair is often the best compromise — the nylon gear wears (and is replaced) while protecting the steel gear and reducing noise.
Can nylon gears hold the same tolerances as metal gears?
Not directly to the same class without compensation. Nylon’s thermal expansion and moisture absorption mean a drawing-correct dimension at 20°C and 0% RH may shift once the gear is in service. Tolerances should be specified with the operating environment in mind, not just as-machined values.
Is glass-filled nylon always the better choice for strength?
No. Glass-filled nylon increases stiffness and reduces creep, but it is more abrasive against a mating gear and can increase wear on a soft counterpart. Unfilled nylon is often preferable when the mating gear is also plastic or when quiet operation matters more than peak load capacity.
Does nylon gear performance change with humidity?
Yes. PA6/PA66 absorbs moisture from the air, which can change dimensions by a few tenths of a percent and slightly reduce stiffness while improving impact resistance. For precision applications, specify moisture-conditioned stock or account for this shift in your fit tolerances.
What inspection should I request for precision nylon gears?
At minimum, request tooth-to-tooth and total composite error measurement on a gear tester, plus bore/OD dimensional reports. For critical applications, ask the supplier to measure at or near operating temperature, since room-temperature-only inspection can miss thermal-expansion-related deviations.
Need Precision Nylon Gears Machined to Your Drawing?
PFTWORLD manufactures precision CNC-machined nylon and metal gears with full dimensional inspection reports. Share your drawing for a DFM review and tolerance recommendation.
Post time: Oct-23-2026
