Friction, moisture and wear: choosing the right technopolymer for gears

When designing technopolymer gears, the choice between glass-fiber reinforced PA6, POM-C and PK (polyketone) is never interchangeable. Each material answers to different tribological and environmental logics. A selection error translates into early wear, loss of kinematic precision or, in the worst cases, brittle tooth fracture.

The critical problem is almost never tensile strength, often abundant relative to nominal loads. It's the combination of friction coefficient, hygroscopicity and dimensional stability under cyclic load. A PA6 gear that absorbs moisture in a humid environment can change its meshing clearance enough to cause interference. A POM-C one subjected to impulsive shocks can fail by brittleness. A PK one might be more balanced, but at a cost that the application has to justify.

Glass-fiber PA6, POM-C and PK: where they truly diverge

These three materials don't differ by grade of quality: each governs a precise application territory, with minimal overlap.

Glass-fiber PA6: high mechanical strength, with tribological variables to manage

Glass-fiber reinforced PA6 is the material with the highest mechanical strength among those in the catalog. Glass-fiber reinforcement increases stiffness, elastic modulus and dimensional stability, as well as the tooth's bending strength. It's the natural choice when transmitted torques are high and peripheral speed is contained.

After the first run-in cycles, the glass fiber emerges on the tooth surface and can increase friction at the interface compared with standard PA6. For this reason reinforced PA6 performs best in systems at moderate speeds or with lubrication available. Some formulations include molybdenum disulfide as an internal lubricating additive: they reduce friction and improve performance under sliding conditions. If your system runs at high speed and dry, consider this option before ruling out the material.

PA6 has good resistance to oils, greases, organic solvents and alkalis: in most industrial environments it works without problems. In the presence of strong acids it's preferable to turn to a different material.

Recommended applications: transmission systems with high loads and lubrication available, conveyors, pulleys, anti-wear guides, components subject to fatigue in environments that aren't critical for moisture. In metal replacement applications it's valued for light weight, corrosion resistance and an excellent cost/performance ratio.

POM-C: smoothness and dimensional stability, sensitive to impulsive stress

POM-C occupies a completely different territory. Its highly crystalline structure gives it an intrinsically low friction coefficient and a dimensional stability that no polyamide can match. With variations below 0.2% even in humid environments, the meshing clearance calculated at the design stage stays the real one in service. This makes it the preferable choice for precision gears, actuators and compact gearmotors where positioning repeatability is a requirement.

The most relevant limit in transmission applications is mechanical. POM-C has a resilience, that is, the energy absorbed before fracture, significantly lower than PA6 and PK. In the presence of impulsive loads, torque peaks or abrupt starts, the tooth can fail by brittle fracture before the failure is announced by visible deformation. It's one of the most insidious failure modes because it isn't progressive.

POM-C minimizes the "stick-slip" effect, that is, the difference between static and dynamic friction that generates micro-oscillations. It's a particularly useful feature where noise and motion smoothness are critical.

Recommended applications: compact gearmotors, precision actuators, automation in humid environments, medical devices. To be evaluated with care in the presence of high impulsive loads or temperatures above 90°C.

PK (polyketone): stable tribological behavior and high chemical resistance

Polyketone (PK) is a semi-crystalline technopolymer that combines good mechanical strength, low friction coefficient and dimensional stability. It absorbs very little moisture: the dry-material dimensions stay valid in service too, simplifying tolerance calculations. It works well up to about 100°C and has good chemical resistance to hydrocarbons, oils and solvents.

The dry friction coefficient of PK is 0.30. The relevant figure isn't the number itself, but its stability over time: this tribological characteristic is an intrinsic property of the polymer chain, not an additive that wears out. PK keeps its sliding performance for the entire service life of the component. In practice, you can run dry without having to plan re-lubrication interventions, provided the peripheral speeds stay within the limits foreseen for the material.

Chemical resistance is excellent across a wide range of solvents, acids and bases, superior to both PA6 and POM-C. Resilience is high, comparable to PA6: the risk of brittle fracture under impulsive load is reduced.

PK has a higher raw-material cost than the other two and a still-limited availability of semi-finished products. Its adoption is justified in high-speed applications, in chemically aggressive environments, or where you want to eliminate lubrication-related maintenance entirely.

Recommended applications: high-speed gears running dry, transmissions in chemically critical environments, high-reliability industrial automation systems, applications where maintenance is difficult or costly.

How to choose the material: three questions to orient yourself

Before consulting the catalogs, three questions guide the choice almost deterministically.

Is the load mainly static, with lubrication available? If the load is high and lubrication is present, glass-fiber PA6 is generally the most efficient choice for the cost/mechanical-strength ratio. It offers good machinability and a contained cost in producing wheels and anti-wear parts.

Is the priority kinematic precision and quietness, with continuous operation and low torques? If the load is moderate and shocks are absent, POM-C generally offers the best dimensional stability and the lowest noise.

Does the application run at high speed and dry, in a chemically aggressive environment, or require maximum service life without maintenance? In these cases PK is the technically most coherent choice. The cost difference relative to the other materials should be assessed in the context of the overall TCO (total cost of ownership).