POM Gears and Sliding Parts: Acetal Design Rules for Low Wear

POM gears sit inside window regulators, appliance timers, pumps, printers, seat adjusters and textile machinery. Acetal earns that place because it is stiff, slides well, shrugs off fatigue and barely notices humidity. It is also easy to get wrong. When a POM gear fails early, the cause is more often pairing, geometry or moulding than the material itself.

This note covers gears first, then bushes, rollers and slide rails, and closes with how the OSATAL grades map onto each.

Why acetal for moving parts

POM is a highly crystalline polymer with low moisture absorption. For a gear, that second point is the big one. A nylon gear grows as it takes on water, which is manageable but has to be designed for (see our notes on drying nylon and PA66 vs PA6). An acetal gear holds its tooth geometry far better through a monsoon season in Jakarta or Ho Chi Minh City.

Copolymer acetal can be used continuously in air up to about 100 °C, and gears and bearings are among its textbook applications. Acetal is not best in every condition, though. In dry running, some nylons resist wear better; in wet conditions, acetal usually has the edge.

Pairing: the first design decision

Gear wear depends on the pair, not on one material. UK researchers testing polymer gears on a rig found that running acetal against nylon behaved very differently from same-material pairs, with a low wear rate when acetal was the driver. Earlier work by the same group showed that acetal gear life is wear-limited at low torque, and limited by the maximum permissible surface temperature at high load.

If your drive runs hot, the question is less about the material's rated temperature and more about how heat leaves the mesh. Duty cycle, speed, tooth load and the thermal path through shafts and housings all count.

For the nylon side of a POM/PA pair, look at the PA6 and PA66 grades in OSAMIDE.

Gear geometry and moulding

Tooth and rim

• Use a generous root fillet. A full-radius root lowers the stress concentration where teeth crack.
• Set backlash for the hottest operating condition, not room temperature. POM expands more than steel, and meshing gears warm up.
• Keep rim, web and hub thicknesses close. One processing guide suggests holding variations within 15% of the nominal wall.

Web, hub and gate

• Prefer a solid, symmetric web to spokes. Spokes create weld lines and uneven shrinkage, which show up as runout.
• Core out thick hubs rather than leaving a solid boss.
• Gate centrally (diaphragm) or with several symmetric gates on the web. A single edge gate tends to produce an oval gear.
• Zero draft is possible on gear teeth and bearing bores, because POM releases well.

Shrinkage, mould and melt

POM shrinks a lot, typically 2 to 3.5%, because it is so crystalline. Higher mould temperatures within the usual range (about 50 to 105 °C) reduce post-moulding shrinkage, which is why precision gear moulders run warm tools. Typical melt temperatures are 190 to 210 °C for copolymer and 180 to 230 °C for homopolymer.

Two processing rules prevent most trouble. First, do not overheat or hold the melt in the barrel for long; acetal decomposes and releases formaldehyde. Second, never let PVC or halogenated FR plastics into the same barrel. Even traces can trigger rapid decomposition, so purge thoroughly and keep these materials apart on the shop floor.

Bushes, rollers and slide rails

Bushes

A plain POM bush works well against a hardened, smooth shaft. Allow running clearance for thermal expansion, and remember that a bush pressed into a housing closes up at the bore. Keep the load and sliding speed within the limits on the grade's TDS, and use a lubricated grade where you cannot add grease.

Rollers

Rollers carry point loads and often live outdoors or near windows. Use a UV-stabilised grade for anything that sees daylight. If the roller rides on a pin, treat the bore exactly like a bush.

Slide rails

Long, thin rails warp if cooling is uneven, so wall uniformity and tool cooling matter more than usual. A lubricated grade keeps friction steady against aluminium extrusions and painted steel. Check heat build-up at the highest sliding speed.

Mapping to OSATAL grades

OSATAL is our polyacetal family: low friction, wear resistant, chemically resistant and low in moisture uptake.

The rule of thumb: unfilled and lubricated for anything that meshes or slides; glass-filled for stiffness, creep resistance and dimensional stability in parts that carry load but do not rub. If a glass-filled gear is unavoidable, test the counterface wear as well as the gear.

All OSATAL grades can be supplied in custom colours and with UV or other specified properties on request. Our lab supports development with MFI, tensile and flexural testing on a UTM, Izod impact, HDT and Vicat, DSC and moulded test specimens. A gear rig on your own pair remains the real qualifier, and we supply material for those trials. See the wider range in the material finder and typical parts on our applications page.

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