Why Do Plastic Gears Fail? A Practical Guide for Engineers

Most plastic gear failures come down to one decision made too early: material selection. Here's what to look for — and what to avoid.

Plastic gears are lighter, quieter, and corrosion-resistant. But when they fail, the cause is almost always the same — the wrong material for the actual operating conditions.

Not the wrong design. The wrong material.

Here are the three failure modes that account for the majority of plastic gear problems in the field, and the materials that solve each one.

1. Creep — The Silent Killer

Creep is permanent deformation under continuous load over time. The gear looks fine. Then one day it doesn't mesh properly, backlash increases, and the system fails.

It happens because most plastics start to creep noticeably above 60–80°C. Standard POM-H and unreinforced Nylon are the most common victims — widely used, well-priced, but not built for sustained load at elevated temperature.

What works instead:

• PEEK — stable up to 250°C continuous, exceptional creep resistance

• Carbon-filled POM-C — meaningful upgrade from standard acetal at lower cost

• PPS — strong performer in high-temperature, high-load environments

If your gear runs continuously above 60°C, creep resistance is your first selection criterion. Tensile strength is not.

2. Wear — When the Tooth Profile Disappears

Every gear tooth contact generates friction. Friction generates heat. Heat accelerates wear. Left unchecked, the tooth profile degrades, noise increases, and dimensional tolerance is lost.

The solution isn't external lubrication — it's building lubrication into the material itself.

Self-lubricating options by application:

3. Chemical Attack — The Failure Nobody Expects

"Chemical resistance" is not one property. It depends on the specific chemical, concentration, temperature, and exposure time. A material rated for water immersion may swell and seize in hydraulic fluid.

Common traps:

• PA6 / PA66 absorbs moisture and swells — causes tight gears to bind in humid environments

• POM-C degrades in strong acids or alkalis

• Standard grades have no UV protection outdoors

Chemical-resistant alternatives:

• PVDF — resists concentrated acids, halogens, oxidizing agents

• PPS — broad solvent resistance up to 200°C

• PEEK — when you need chemical resistance and mechanical performance together

• PA12 — significantly lower moisture absorption than PA6/PA66

The One Thing Most Engineers Get Wrong

Tensile strength is the most quoted property in engineering plastic datasheets. It is also the least useful for predicting gear performance in real operating conditions.

A single pull test at room temperature tells you nothing about creep at 80°C, wear after 10 million cycles, or dimensional stability in a wet chemical environment.

The properties that actually predict gear performance:

• Creep modulus at operating temperature

• PV limit (pressure × velocity) for wear applications

• Chemical resistance at process temperature — not room temperature

• Moisture absorption for humid or wet environments

Quick Selection Reference

Working on a Gear Application?

MBH supplies PEEK, POM-C, PVDF, PPS, PA12 and filled grades in rod and sheet — direct from our extrusion facility. Custom diameters and modified grades available. Sample pieces for testing on request.

inquiry@marbach-ep.com WhatsApp: +86 159 6785 6316