This is the most common cage substitution in bearing purchasing, and the one most likely to be made silently. Both cages work in ordinary duty; they diverge sharply at temperature and in certain chemical environments, and the failure is sudden rather than gradual.
| Criterion | Polyamide (TN9/TVP) | Brass (M) |
|---|---|---|
| Temperature | 120 °C maximum | 250 °C |
| Noise | Lowest | Moderate |
| Mass | Very low | High |
| Strength at speed | Good to high | Very high |
| Ammonia / refrigerant | Attacked | Unaffected |
| Water | Slight absorption | Corrosion possible |
| Cost | Low | High — 2–4× the cage cost |
| Typical use | Appliance motors, fans | Large, hot, fast bearings |
Polyamide 66 has a glass transition around 60–80 °C and a practical ceiling near 120 °C. Below the ceiling it is strong and elastic; above it, the pockets deform under the ball load and the cage stops spacing the elements. There is no gradual warning in the grease or in the noise — the bearing goes from normal to failed within hours of crossing the line, often during a fault condition such as a blocked cooling fan.
Polyamide is attacked by ammonia, by several refrigerants and by some extreme-pressure additives at temperature. In refrigeration and in plants that clean with ammonia-based detergents, a polyamide cage is a known short-life item — and the failure is often misdiagnosed as a lubricant problem because the grease looks fine.
Above about 120 °C continuous. The material softens rather than degrading slowly, so there is little warning.
Yes — machined brass is the standard for very high speed in large bearings. Its limitation is mass, not strength.
The bearing is running above 120 °C, usually because of grease starvation, over-preload or a cooling failure. The cage is reporting the temperature, not causing it.
No, but brass tolerates high-temperature greases that polyamide cannot survive, so the two are normally specified together.
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