Bearing Lubrication
Lubrication is the single most important factor in how long a bearing lasts. The lubricant does far more than "make things slippery": it separates the rolling elements from the raceways with a thin film, reduces friction and wear, carries away heat, protects against corrosion, and helps keep contaminants out. Get the lubrication right and most bearings reach their design life; get it wrong and they fail early.
Grease or oil?
Most bearings are lubricated with grease; a minority use oil. The choice depends mainly on speed, temperature and how heat is managed.
| Grease | Oil | |
|---|---|---|
| Best for | The majority of bearings — easy to retain and seal in | High speed or high temperature; where heat must be carried away |
| Advantages | Stays in place, helps seal against dirt, simple, low maintenance | Removes heat, can be filtered and circulated, suits very high speed |
| Watch-outs | Limited heat removal; needs correct quantity and re-greasing | Needs a sealed/circulating system; more complex |
What grease is made of
Grease is a base oil held in a thickener (often a metallic soap), with additives for oxidation resistance, corrosion protection and load capacity. Its stiffness is graded by the NLGI consistency number — NLGI 2 is the typical general-purpose grade for bearings. The base oil viscosity matters as much as the thickener: it must suit the speed and temperature of the application.
Choosing the right lubricant
Match the lubricant to the duty by considering:
- Speed — higher speed needs a lower base-oil viscosity; very high speed may need oil.
- Temperature — both the operating range and any extremes; greases have a usable temperature band.
- Load — heavy or shock loads may need extreme-pressure (EP) additives.
- Environment — water, dust or chemicals call for a grease that resists wash-out and corrosion.
How much — and why more is not better
Quantity matters. As a general guide, the bearing and a portion of the surrounding free space are filled with grease — but the housing should not be packed full, especially at higher speeds.
Relubrication
Grease does not last forever — it ages, loses oil and picks up contamination. Re-greasing intervals shorten with higher speed, higher temperature and dirtier or wetter environments. Many smaller bearings are "sealed for life" and never re-greased; larger and harder-working bearings are re-greased on a planned schedule. When re-greasing, add the right amount of the correct grease and, where possible, let old grease purge out.
Common lubrication mistakes
- The wrong grease type or base-oil viscosity for the speed and temperature.
- Too little or too much lubricant.
- Mixing incompatible greases (different thickeners can react and break down).
- Missed or over-long re-greasing intervals.
- Contaminating the grease with dirt or water during application.
Each of these shows up later as heat, noise or premature wear — see the grease condition guide and the Failure Diagnosis Library.
Frequently asked questions
Should I use grease or oil for my bearing?
Grease suits most bearings and is simpler to retain and seal. Oil is used for high speed or high temperature, or where heat needs to be carried away by a circulating system.
Can I mix two different greases?
Avoid it unless they are known to be compatible. Different thickeners can react, soften or harden the grease and destroy its performance. If changing grease, purge the old one out.
How often should bearings be re-greased?
It depends on speed, temperature and environment — faster, hotter and dirtier conditions mean shorter intervals. Many small sealed bearings are never re-greased; larger ones follow a planned schedule.
Part of the Bearings Encyclopedia. For diagnosis of lubrication-related problems, see the Failure Diagnosis Library.