Ask ten gear engineers which plastic to use and most will name the same two materials: POM (acetal, also sold as Delrin) and nylon (polyamide, PA6 or PA66). Both are proven, both are economical, and both can run for millions of cycles — yet they behave very differently in service. Choosing the wrong one rarely causes an immediate failure. Instead it shows up months later as noise, backlash growth, or cracked teeth. This article compares the two materials on the properties that actually decide gear life, and ends with simple selection rules by application.
Why the material decision matters so much
A plastic gear is not a metal gear in a cheaper material. The polymer decides how much the tooth deflects under load, how the mesh friction heats the contact, how the part reacts to humidity, and how much the dimensions move between the molding machine and the customer's assembly line. Two gears with identical drawings, molded in POM and in PA66, will measure differently a week later and age differently in the field. Material selection is therefore part of the gear design, not an afterthought for purchasing.
POM (acetal): the precision default
POM is the material most engineers reach for first in small precision gears, and for good reasons:
- Dimensional stability. POM absorbs very little water (around 0.2% at saturation in standard grades), so a gear that measures correctly at inspection stays correct in a humid warehouse or a bathroom appliance.
- Low friction and good wear behaviour. Its naturally slippery surface suits unlubricated or lightly greased meshes, and POM-on-POM or POM-on-steel pairs run quietly.
- Stiffness and fatigue strength. POM teeth deflect less under load and hold up well in the repeated bending of high-cycle actuator duty.
- Clean molding. It fills thin tooth profiles well and reproduces mold detail faithfully, which matters at small modules.
Its limits are impact and temperature. Standard POM homopolymer and copolymer grades serve continuously up to roughly 90–100 °C, and a sharp blow on a cold tooth is less forgiving than nylon. For very high torque in a tiny envelope, or for gears that must survive abuse loads, look at nylon.
Nylon (PA6 / PA66): the toughness option
Nylon trades some of POM's precision behaviour for toughness:
- Impact and abuse tolerance. Nylon teeth absorb shocks and overload peaks that would chip a POM tooth. This is why it is common in larger actuator and power-transmission gears.
- Strength, especially glass-filled. PA66 with 30% glass fibre reaches stiffness and strength levels POM cannot match, at the cost of more abrasive behaviour against the mating gear.
- Higher temperature capability. PA66 grades tolerate hotter environments than standard POM, which matters near motors and in automotive engine-bay-adjacent applications.
- Lower material cost in larger parts, where the gear volume starts to dominate the piece price.
The well-known catch is moisture. Nylon absorbs water from the air — PA6 can take up several percent at saturation — and it swells as it does. A PA gear's outside diameter, tooth thickness and bore all grow slightly after molding, and backlash shrinks. Experienced designers allow for this: they specify dimensions in the conditioned state, choose PA66 over PA6 when stability matters, or design the mesh with enough backlash margin. Ignoring moisture is the single most common cause of "the samples were fine, the production batch binds" problems.
Side-by-side comparison
| Property | POM (acetal) | Nylon (PA6 / PA66) |
|---|---|---|
| Dimensional stability | Excellent, almost unaffected by humidity | Moves with moisture uptake, needs design allowance |
| Friction / noise | Very low friction, quiet meshes | Good, usually slightly noisier; grease helps |
| Impact strength | Moderate | High, best of the two for shock loads |
| Stiffness (unfilled) | Higher | Lower when dry, drops further when conditioned |
| Glass-filled option | Available, less common | Very common, big strength gain, abrasive to mating gear |
| Continuous service temperature | ≈ 90–100 °C (standard grades) | ≈ 100–120 °C for PA66 grades |
| Typical gear sizes | Micro and small-module precision gears | Small to larger, higher-torque gears |
Figures are typical ranges for standard commercial grades; always confirm against the datasheet of the exact grade being quoted.
Selection rules by application
Choose POM when…
- The gear is small (micro and small-module work, such as the gears in our micro gearboxes and gear components) and accuracy of mesh matters more than brute strength.
- The product lives in humidity — smart bathroom devices, kitchen appliances, outdoor locks — and dimensional drift is not acceptable.
- Quiet running on little or no lubrication is a selling point, as in door-lock actuators and office equipment.
- The duty is high-cycle, moderate-load: metering, positioning, small actuators.
Choose nylon when…
- Torque and shock loads are high relative to gear size, for example automotive actuators that must survive stall and abuse cases (see our automotive applications).
- The environment runs hot, close to a motor or in a vehicle.
- A glass-filled grade is needed for strength or for a structural gear housing — and the mating pinion can be specified in a compatible, wear-resistant material.
- Some dimensional movement is acceptable, or the design can absorb it with backlash margin.
And sometimes, use both
A POM gear meshing with a nylon gear is a legitimate, widely used combination: the pair can run quieter than either material against itself, and wear concentrates predictably on the cheaper, easier-to-replace element. Mixed-material pairs are one of the options our engineers check during the design review described in our capabilities overview.
What to send your supplier
Material choice goes faster when the enquiry includes the working conditions, not just the drawing: continuous and peak torque, speed, expected cycles, ambient temperature and humidity, lubrication (none, grease, oil), and the noise target. With that information, a gear manufacturer can confirm the material, flag a moisture or temperature risk, and quote tooling for the right shrinkage rate — POM and PA shrink differently, so the mold is cut for one specific material from the start.
Still weighing the two for a live project? Send us the drawing and the duty details; we will come back with a material recommendation together with the tooling and unit-price quotation.