Backlash and pairings in technopolymer gears
Tooth backlash isn't a machining defect. It's a design parameter, and in technopolymers it must be calculated with more care than in steel: thermal expansion and moisture absorption change tooth dimensions in service, not just in the workshop.
Tooth backlash: definition and function
Tooth backlash (the gap between tooth flanks measured on the pitch circle) is the difference between the space width and the tooth thickness. In the ideal transmission backlash is zero, but in reality it's always present and necessary. It serves to compensate for pitch errors, elastic deformation, thermal expansion and, in the case of technopolymers, moisture absorption.
Insufficient backlash leads to seizing or accelerated wear from interference. Excessive backlash increases impact noise and the angular play of the transmission. There's no universal value: it must be calculated for each material-temperature pair.
The problem of thermal expansion
The thermal expansion coefficient of POM-C is about 110 × 10⁻⁶ /K, nearly 10 times that of steel. On a gear with module 2 and 30 teeth (pitch diameter 60 mm), a temperature change of 40°C produces a diameter change of about 0.26 mm.
The assembly clearance must therefore account not only for the dimensional tolerance of the profile, but also for the thermal swing expected in service.
If the pinion is POM-C and the wheel is steel, the differential expansion is at its maximum. In these configurations, calculate the initial clearance under the most unfavorable temperature conditions before deciding the profile tolerances.
The same reasoning applies to PA6: moisture absorption produces an additional dimensional change on top of thermal expansion alone. PA6, POM-C and PK answer to different application logics. PA6 offers the highest mechanical strength among the three and an excellent cost/performance ratio; in humid environments it requires including the dimensional change from absorption in the clearance calculation, a parameter managed at the design stage. POM-C is preferable when dimensional stability is the priority and loads are moderate. PK is the choice for chemically aggressive environments or when you want to minimize maintenance.
Practical measurement of backlash
The simplest method to check backlash on an assembled pair is the lead (or wax) method: place a deformable lead wire between the teeth during a full rotation, then measure the squashed thickness with a micrometer.
For small gears (module below 2), use a dial indicator with one gear locked and the other oscillating.
Watch out for fixed center distances
When the center distance is imposed by the structure (for example, holes already present in sheet metal) and isn't adjustable, the backlash depends exclusively on the combination of tolerances on the profile and on the diameters.
In this case specify the involute-profile tolerances according to the ISO 1328 classes and check the minimum backlash under the most unfavorable temperature and moisture conditions.
In our experience, most seizures on technopolymer pairs with a fixed center distance occur in the first hours of hot operation: the backlash had been checked cold, but not under the real operating conditions.
If your application involves high thermal swings, mixed materials or non-adjustable center distances, write to the technical office: we calculate the most suitable backlash configuration for you.