Heat, grease life and bearing life: what a 20 °C drop is worth
A common rule of thumb says grease life halves for every 15 °C rise. A drop of 21 °C then gives about 2.6 times the grease life. The same rule is not a measure of bearing life.
The rule
Grease ages faster when it runs hot. As a rule of thumb, the life of grease halves for each 15 °C rise in operating temperature, and doubles for each 15 °C fall. It is approximate. The true curve depends on the grease, the bearing type and the speed.
What a fall in temperature is worth
| Fall in temperature | Grease life factor (2^(ΔT/15)) |
|---|---|
| 5 °C | about 1.3 times |
| 10 °C | about 1.6 times |
| 15 °C | 2 times |
| 21 °C (57 °C to 36 °C) | about 2.6 times |
| 30 °C | 4 times |
Why heat hurts
- Oxidation of the grease speeds up with temperature.
- The oil bleeds out faster, so the thickener is left with less oil.
- A hot grease is softer: its consistency falls, and it can leak from the housing.
- A thin oil film in the contact reduces the bearing's rating life.
Bearing life is a different calculation
One author estimated about five times longer bearing life after a bolt fix that cooled a pump bearing by 21 °C. Treat that as an estimate. The rating life in ISO 281 depends on load, speed, the viscosity ratio and cleanliness. Temperature enters through viscosity, so a cooler bearing with the same oil has a thicker film and a better ratio, but the gain is not a fixed multiple.
How to use it
- Measure bearing temperature before and after a repair and write both down.
- Convert the fall into a grease life factor and re-plan the greasing interval, with care.
- Look for the cause of heat: looseness, misalignment, over-greasing and too little grease all make bearings run hotter.