Bearings rarely die of old age. What kills them early is nearly always something done to them: the wrong lubricant, the wrong quantity, dirt in the housing, or a mounting error. SKF's damage analysis puts numbers on it.
- 36% of premature bearing failures trace to lubrication1
- 34% to fatigue1
- 16% to mounting and handling1
- 14% to contamination1
Read that split carefully, because it describes bearings that failed early rather than bearings in general. SKF also teaches that roughly 90% of bearings outlive the machine they are fitted to, which is the more encouraging half of the same finding.
Which means the most valuable half hour you can spend on a bearing is usually before it goes in.
Grease Covers Most Cases
Grease is oil held in a soap thickener that releases it slowly. It stays where you put it, helps seal the bearing against contamination, and needs no circulating system. Something like nine out of ten rolling bearings run on it, and for good reason.
Its limits are speed and heat. As speed rises, churning grease generates heat faster than the bearing can shed it. Above a certain speed factor, grease stops being viable and oil takes over.
Oil Is for Speed, Heat, and Contamination
Oil carries heat out of the bearing rather than trapping it, and a circulating system carries debris out with it to a filter. That is why high-speed spindles, gearboxes, and anything running hot use oil, whether by bath, mist, or air-oil injection.
The cost is complexity. Oil needs seals, a reservoir, and a way to deal with what leaks. If a bearing can run on grease, it usually should.
The Mistakes that Actually Cause Failures
- Overgreasing. More is not better. A bearing packed full churns, overheats, and can blow out its own seals. Most bearings want the free space roughly one-third to one-half full.
- Mixing incompatible thickeners. A lithium grease and a polyurea grease can react into something with the consistency of soup and none of the lubricating properties of either. If you are switching, purge rather than top up.
- Ignoring viscosity. The grade has to suit the operating temperature and speed. A viscosity chosen for a cold start will be too thin once the machine is at temperature.
- Relubricating on the calendar rather than the duty. A bearing at 80 degrees Celsius needs attention far more often than the same bearing at 40.
Spindle Bearings Are Their Own Problem
High-speed machine tool and grinding spindles run at speed factors where ordinary rules stop applying. They are typically supplied greased for life with a precisely metered charge, or fed by air-oil, and they are unforgiving of improvisation. Regreasing a sealed precision spindle bearing in the field is usually a way of shortening its life.
If you are rebuilding a spindle, match the original arrangement, preload and lubrication specification exactly. Send us the spindle designation and we will confirm what it was built with.