Fan bearing running hot: what is really too hot, and where to look first
A housing that feels hot at 60 to 70 degrees C can be perfectly healthy. What matters is the trend, the level and the cause, and the four families of cause.
What counts as too hot
A bearing housing feels hot to the hand from about 60 to 70 °C (140 to 160 °F). A correctly lubricated bearing is perfectly happy at that level, so touch alone is a poor judge. Use an infrared thermometer, take readings at the same spot on a regular schedule, and look for sustained change or an elevated level rather than a single high number.
Reading the temperature
- A housing at about 80 °C that climbs by 10% or more over a period is a warning, even though 80 °C is acceptable in itself.
- A housing running at 100 °C with no obvious application reason (hot gas, hot process side) has almost certainly got a problem that needs investigating.
- Measure on the housing near the bearing, not on the guard, and note the ambient and the process temperature in the same log.
Four families of cause
| Family of cause | Typical findings | Read next |
|---|---|---|
| Load: too high, too low, misaligned or belt-driven | Raceway damage in a load zone, smeared rollers, outer ring turning in the housing | Hot from load |
| Mounting: fits, drive-up, seal design, no room for the shaft to grow | Polished or fretted seats, preloaded bearing, discoloured seal lip or counter-face | Fits and seals; Locating and floating bearings |
| Lubrication and cooling: wrong grease or oil, too much or too little, no heat removal | Churned or dry grease, blocked feed lines, missing oil ring, hot gas reaching the bearing | Lubrication and cooling |
| Not the bearing at all: drive system, hot gas leaking along the shaft | Belts, couplings, pulleys slipping or vibrating; hot casing | Belt drive; Lubrication and cooling |
Two things not to do
Do not cool an overheated housing with an external blower. The housing shrinks around the outer ring, which can grip it so it cannot slide, load the bearing axially and make the overheating worse. Cool the shaft instead (cooling disc) or improve the lubrication and sealing. And do not keep adding grease to a bearing that only seems hot: over-greasing causes churning heat, damages seals and often is the cause of the temperature in the first place.
Before anything else
A bearing that runs hot after a repair, a belt change or cleaning of the impeller is first a question of alignment and balance. Re-check both before looking at the bearing itself.