Overheating & lubricant breakdown
Colour is a temperature record. Urgency
Not separate ISO modes, but the overheating, seizure, cage and alignment problems that surround them.
What overheating & lubricant breakdown looks like
- Rings or rollers tinted straw, brown, purple or blue
- Grease dark, hard, burnt or runny; seals hard or distorted
- In severe cases the steel has softened and lost hardness
Rotec AI schematic illustration. It shows the typical pattern and is not a photograph of a real bearing.
How overheating & lubricant breakdown develops
Heat builds when friction or lack of lubricant raises temperature faster than the machine can shed it. Past a point the steel tempers and the lubricant oxidises, so the bearing runs hotter still, a runaway that ends in seizure. Straw and brown tints mean it saw well above normal running temperature; blue means very hot indeed.
How fast it progresses: A runaway. Temperature that keeps climbing should be treated as an emergency.
Causes of overheating & lubricant breakdown
Lubrication
- Too little, too much (cannot purge) or wrong lubricant, or the wrong delivery system
Environment & sealing
- Seals too tight, stacked, misaligned, over-speed or dry
Design & seats
- Clearance lost in operation: wrong class, hot shaft against cool housing, tight fits, over-driven tapered seat, pinched housing
Operating conditions
- Overload after a design change, misaligned units, angular contact bearing fitted backwards, imbalance, preload
How to confirm it is overheating & lubricant breakdown
- Measure shaft and housing temperatures near the bearing and compare with the last known-good reading
- Check clearance after mounting against the specification
- Check grease amount and condition, and seal drag
How to fix and prevent overheating & lubricant breakdown
- Use the right clearance class (C3 or C4) for the fits and temperature gradient
- Right lubricant and quantity; consider oil circulation or cooling for heat-prone duty
- Reduce seal drag and fix alignment
- Review loads after any redesign (a belt drive instead of a coupling, higher speed)
Easily confused with
- Seizure (end-stage failure): Seizure is the locked end state; overheating is the colour and lubricant evidence on the way.
- Adhesive wear (smearing & skidding): Smearing adds transferred metal; overheating is mostly colour and burnt lubricant.
Worth knowing
What the heat does to the steel
Colours run from straw through brown and purple to blue as the temperature rises. Above roughly 200 degrees C (about 400 degrees F) the steel starts to anneal and loses hardness and capacity, which makes the bearing fail early even after it cools. Tight fits that use up the clearance at operating temperature produce a heavy wear path in the bottom of the raceway and can crack the inner ring.
Sealed motor bearings often die of old grease, not fatigue
Normal bearing temperature in an electric motor is 70 to 110 °C. In a worked handbook example, the calculated fatigue life of sealed motor bearings was far above the 30,000 h target, but standard grease lasted only about 20,000 h. The same bearings filled with grease made for motors (suffix GJN or WT) met the target. Vertical shafts halve the grease life.
Other related modes
Questions about overheating & lubricant breakdown
What causes overheating & lubricant breakdown in a bearing?
Too little, too much (cannot purge) or wrong lubricant, or the wrong delivery system. Seals too tight, stacked, misaligned, over-speed or dry. Clearance lost in operation: wrong class, hot shaft against cool housing, tight fits, over-driven tapered seat, pinched housing.
How do you fix or prevent overheating & lubricant breakdown?
Use the right clearance class (C3 or C4) for the fits and temperature gradient. Right lubricant and quantity; consider oil circulation or cooling for heat-prone duty. Reduce seal drag and fix alignment.
What does overheating & lubricant breakdown look like?
Rings or rollers tinted straw, brown, purple or blue. Grease dark, hard, burnt or runny; seals hard or distorted.