The short answer
Acetal is the default for small precision gears and bushings: low friction, almost no moisture uptake and good fatigue life. Nylon 66 or 6 is tougher, handles shock and abrasive dirt better and runs quieter, but it grows as it absorbs water, so it needs more clearance. Run unlike materials together (acetal driving nylon, or plastic on hardened steel). Use UHMW-PE for slow, wet or abrasive sliding, and step up to filled PPS or PEEK above about 100°C (212°F).
Most plastic gear and bushing failures come from heat, moisture or the wrong mating surface rather than a weak resin. Choose the material after you know the load, speed, temperature, whether the part runs wet or dry, and what it runs against. If a part is already failing, start with the checklist in plastic parts wearing out. This page is about choosing the resin in the first place.
The usual candidates
| Resin | Wear | Friction | Moisture growth | Max continuous temp (approx.) | Strength (tensile) | Cost tier |
|---|---|---|---|---|---|---|
| Acetal homopolymer | Excellent | Low | Very low | ~80°C (176°F) | High (~68–75 MPa) | Moderate |
| Acetal copolymer | Excellent | Low | Very low | ~80°C (176°F) | Good (~60–65 MPa) | Moderate |
| Nylon 66 / nylon 6, unfilled | Very good; best of these against abrasive dirt | Low to moderate; lower in oil-filled grades | High: ~2.5–3.5% water at 50% RH | ~80–100°C (176–212°F), PA66 at the top | High dry (~70–90 MPa), lower conditioned | Moderate |
| PBT, unfilled | Good | Low | Very low | ~100–120°C (212–248°F) in dry air | Moderate (~50–60 MPa) | Moderate |
| UHMW-PE | Outstanding against abrasion | Very low | None | ~80–90°C (176–194°F) | Low; creeps under steady load | Low to moderate resin; parts are machined |
| PPS, carbon/PTFE bearing grade | Very good | Low | Negligible | ~200–220°C (392–428°F) | High | High |
| PEEK, carbon/PTFE/graphite bearing grade | Excellent | Low | Very low | ~240–260°C (464–500°F) | Very high | Very high |
Typical values for general-purpose grades; individual grades vary. Continuous temperature is a long-term figure in air with light load. A loaded gear tooth runs hotter than the air around it, so check the data sheet for your grade and leave margin.
Acetal: homopolymer or copolymer
Acetal (POM) has low friction against steel, absorbs about 0.2–0.25% water in 24 hours and holds tight tolerances in humid or wet service. Homopolymer is about 10–15% stiffer and stronger, with better fatigue and creep resistance, so it is the usual choice for loaded gears. Copolymer is easier to mold, more stable in the barrel and better in hot water and alkaline cleaners, which suits pump, plumbing and food equipment parts. See acetal homopolymer vs. copolymer and acetal.
Acetal shrinks a lot (about 1.2–2.4% for copolymer and 1.7–2.2% for homopolymer), so the tooth form of a molded gear depends on gating and packing as much as on the steel.
Nylon 6 and 66: tough, but plan for moisture
Nylon takes shock loads and gritty conditions better than acetal and damps noise. Its weakness is water. Unfilled nylon holds about 2.5–3.5% moisture at 50% relative humidity and 8–10% when saturated. Unfilled nylon 66 grows about 0.6% at 50% RH and about 2.6% saturated, and its stiffness drops sharply as it conditions. A bushing sized on dry parts can close up on its shaft in humid service. See nylon and moisture and acetal vs. nylon.
Bearing grades help. Oil-filled (internally lubricated) nylon lowers friction where the part cannot be greased. MoS₂-filled nylon gives a harder, finer-grained surface and lower wear against steel, and it is common in cast nylon stock for machined gears and wear pads. Glass fiber adds strength and heat resistance but is abrasive to the mating part. See nylon.
PBT, UHMW-PE, PPS and PEEK
PBT has low friction and very low moisture uptake, so it holds size in humid air. It is less tough than nylon and should stay out of hot water and steam, where it hydrolyzes.
UHMW-PE has outstanding abrasion resistance, very low friction and no moisture growth. It is soft, creeps under steady load and standard grades cannot be injection molded, so it is machined from sheet or rod into wear strips, chain guides, liners and slow bushings. See HDPE vs. UHMW.
PPS and PEEK are for heat. Bearing grades with carbon fiber, PTFE and graphite keep their shape and wear rate at temperatures where acetal and nylon soften, and they shrug off hot water and most chemicals. They cost far more, so use them only where the temperature calls for it. See high-temperature plastics.
What it runs against matters as much as the resin
- Avoid same-material pairs under load. Acetal on acetal is the classic case: wear rises sharply above a critical load, and squeak and stick-slip are common. Acetal driving a nylon gear usually runs with lower friction and less wear than either material against itself.
- Plastic on hardened steel is the lowest-wear pairing for most bushings. Steel carries frictional heat away, and a smooth, hard shaft gives the plastic little to abrade. A rough or soft shaft wears the plastic quickly.
- Glass-filled against glass-filled wears both parts, and glass-filled plastic can score soft metals such as aluminum or brass.
- Check the lubricant. Some oils and greases stress-crack certain plastics.
Load, speed and heat
Bushings and thrust washers are rated by PV, bearing pressure times surface speed. Above a grade's limiting PV, frictional heat builds faster than the part can lose it, and the surface smears or melts. Data sheets give limiting PV or a wear factor from thrust-washer tests (ASTM D3702 is common). Compare grades tested the same way, because results depend on the mating surface, speed and test method.
Design to continuous use temperature, not heat deflection temperature, which is a short-term bend test. Plastics also expand far more than steel with heat, so a clearance that is right on a cold part can close up when the bearing warms.
Filler options
PTFE lowers friction and squeak (it is a PFAS, so check restrictions). Silicone helps at start-up. MoS₂ and graphite are solid lubricants. Aramid fiber resists wear and is gentle on the mating surface. Carbon fiber adds stiffness and pulls heat from the wear zone. Glass adds strength cheaply but is abrasive. The trade-offs of each are in wear and friction fillers.
Molded or machined, and noise
Molded gears are cheapest at volume. Gate at or near the hub (a diaphragm or several balanced gates) so shrinkage is even around the gear, keep weld lines off the teeth, and expect to correct the tooth geometry after first articles. Glass-filled grades shrink differently along and across the flow, which can push a gear out of round. Machining from rod or sheet suits prototypes, large gears, low volumes and anything in UHMW-PE.
Plastic gears run quieter than steel, and nylon damps more than acetal. A mixed acetal and nylon pair is a common way to cut noise and wear together, and lubricated grades reduce squeak.
What to send us for a quote
- The grade you run now, or the part drawing
- Load, speed and duty cycle, and the mating material
- Operating temperature, and whether the part runs wet, oily or dry
- Molded or machined, and the filler or lubricant you need
- Quantity for this order and per year, and ship-to city
Common questions
What is the best plastic for gears?
For most small precision gears, acetal: low friction, very low moisture uptake and good fatigue life. Nylon is better for shock loads, abrasive dirt and quiet running if the design allows for moisture growth, and filled PPS or PEEK take over above about 100°C (212°F).
Is acetal or nylon better for gears?
Acetal holds its size in humid or wet service and has lower friction. Nylon is tougher, handles abrasion and shock better and runs quieter, but grows as it absorbs water. Many gear trains use both, with acetal meshing against nylon.
Can acetal gears run against acetal gears?
At light loads, yes, but wear rises sharply above a critical load and same-material pairs tend to squeak. A mixed pair such as acetal driving nylon, or acetal against hardened steel, usually wears less.
What plastic is best for bushings?
Acetal or oil-filled nylon for general duty, PTFE-filled acetal or nylon where friction must be low, UHMW-PE for slow, wet or abrasive sliding, and carbon- or PTFE-filled PPS or PEEK above about 100°C (212°F).
Why do nylon bushings seize?
Usually because the nylon absorbed moisture and grew, closing the running clearance. Unfilled nylon 66 grows about 0.6% at 50% relative humidity and about 2.6% when saturated, so size the bore for the conditioned part or switch to acetal or PBT.
Should plastic gears be molded or machined?
Molding is the economical route at volume, but the tooth form depends on shrinkage, gating and packing. Machining from rod or sheet suits prototypes, low volumes, large gears and any part in UHMW-PE, which standard grades cannot be injection molded.
What is PV for a plastic bearing?
PV is bearing pressure multiplied by surface speed. Each bearing grade has a limiting PV above which frictional heat softens or melts the surface, so compare PV and wear data from the same test method.
Tell us about the gear or bushing
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province