Specifying pom vs pa66 wear parts is one of the most common material decisions in geared mechanisms, and getting it wrong shows up as dimensional creep, moisture-related failure or premature wear rather than an obvious mistake on the drawing. Both resins are semi-crystalline engineering plastics with sharp melting points and good chemical resistance, but they behave differently in service. This guide compares acetal and nylon for gears, bushings, wear pads and slides, and sets out a sourcing checklist for each.
Moisture: the difference that decides most pom vs pa66 wear parts
PA66 absorbs roughly 1.0-1.5% moisture in 24 hours and can reach 2.5-3% at equilibrium, swelling the part, softening it and shifting tolerances. POM absorbs only about 0.2-0.25%, so dimensions hold across humid shops, washdown areas and outdoor service. For any application where a fit must stay tight over years in a damp environment, acetal is almost always the safer choice; the trade-off appears later, because nylon toughens slightly when conditioned while acetal stays constant.
Strength, stiffness and temperature
Dry PA66 is the stronger material, with tensile strength near 75-85 MPa against 60-70 MPa for POM, and it keeps an edge on toughness and fatigue life under cyclic load. Temperature separates them too: PA66 serves continuously around 95-120 C and melts near 255 C, while POM softens above roughly 85-100 C and melts near 175 C. Glass-filled PA66 pushes heat deflection higher still, which is why under-hood and hot-equipment parts usually specify PA66-GF30 rather than unfilled grades.
Friction and wear behaviour
For acetal vs nylon wear performance, the numbers favour acetal in dry sliding. POM runs against steel at a coefficient of friction around 0.15-0.35 versus 0.25-0.40 for unfilled PA66, and its wear factor of roughly 1-3 sits an order of magnitude below nylon's 10-40, so POM parts generate less heat and wear more slowly. PV limits follow the same order, near 90-130 MPa·m/min for POM against 80-120 for PA66. Above about 1 m/s in dry sliding, both materials generate enough frictional heat to soften, so high-speed pairs need internally lubricated grades; POM plus PTFE drops the wear factor to 0.5-1.0 for boundary-lubrication duties.
Chemical exposure and the failure modes to avoid
Neither resin tolerates strong oxidising acids. Beyond that they diverge: POM handles fuels, oils and solvents well but swells and cracks in brake fluid and some glycol coolants, and ketones attack it. PA66 resists those fluids better but hydrolyses in hot water or steam above about 60 C, and phenols attack it. Matching the fluid list to the resin is therefore part of material selection, not an afterthought.
Choosing between a pom gear material and a pa66 bushing moisture environment
- Gears, cams, bushings, slides and clips in dry or lubricated service: POM for dimensional stability and low wear
- Impact-loaded or abrasive parts such as sheaves and rollers: PA66 for toughness, with dimensional drift accepted
- Hot or fuel-wet service above 100 C: PA66-GF30
- Hot water or steam contact: POM or a fluoropolymer rather than nylon
Moulding notes matter as much as service conditions. POM shrinks more, around 1.8-2.5% against 1.0-2.0% for PA66, so steel-safe cavities differ between the two. POM is difficult to bond or overmould, while PA66 bonds more readily, which decides assemblies that combine plastic with metal or two-shot grips.
Sourcing checklist
Ask suppliers for datasheets that state whether values are dry-as-moulded or conditioned, since PA66 numbers move with moisture while POM numbers barely do. For wear parts, request thrust-washer or block-on-ring data rather than marketing claims, and confirm the grade, filler and lubricant package on the certificate. Our comparison of POM vs PTFE for wear parts covers the low-friction end of the family, and nylon PA6 vs PA66 explains how the two nylons differ. Resin options are listed in the plastic materials category, with verified producers in the company directory.
