More bearings fail from lubrication than from fatigue. The lubricant separates the rolling elements from the raceway by a film a fraction of a micrometre thick, and when that film breaks the bearing is destroyed in minutes. Choosing the lubricant is therefore a design decision, not a purchasing detail — and it is the one most often left to whoever answers the phone.
There are three ways to lubricate a bearing: grease (a thickener holding oil), oil (supplied continuously or as a bath), and solid lubricants such as PTFE or molybdenum disulphide for vacuum and extreme temperature. Almost every industrial bearing is grease lubricated, because grease stays put and needs no plumbing.
Two numbers govern the choice. The first is the speed factor ndm — bore diameter in mm times rotational speed in rpm — which decides whether grease can reach the raceway at all. The second is temperature, because grease life halves for every 15 °C above about 70 °C. A grease rated for 120 °C running at 100 °C does not last twice as long as one at 115 °C; it lasts four times as long.
The most common mistakes we see are overfilling (which churns the grease and overheats it), using a high-temperature grease in a low-temperature application (wrong base oil, poor film at start-up), and mixing two greases with incompatible thickeners (which liquefies both).
| Method | Speed factor (ndm) | Temperature | Life | Typical use |
|---|---|---|---|---|
| Grease | up to ~350,000 | −40 to +120 °C (standard) | Greased for life or relubricated | Most industrial bearings |
| High-temperature grease | up to ~200,000 | to 250–280 °C (PFPE) | Shorter, check base oil | Fans, kilns, hot pumps |
| Oil bath / circulation | up to ~500,000 | to 120 °C (mineral) | Continuous with filtration | Gearboxes, turbines |
| Oil jet / oil-air | above 1,000,000 | to 150 °C | Continuous | Spindles, high-speed rotors |
| Solid (PTFE, MoS2) | low | −200 to +350 °C | Finite, no relubrication | Vacuum, cleanroom, extreme heat |
| Sealed for life | as grease | as grease | Matches bearing life | Motors, conveyors, appliances |
What the thickener does, how much to put in, and how long it lasts.
Read →When grease cannot reach the raceway, or when heat has to be carried away.
Read →For vacuum, extreme temperature and cleanrooms where oil is forbidden.
Read →What to use when the seal survives but the grease does not.
Read →For food, beverage and pharmaceutical lines where leakage is a risk.
Read →The claim is a calculation, not a guarantee — here is the calculation.
Read →For a sealed bearing, 25–35% of the free space. For an open bearing in a housing, 30–50% for slow duty and less for high speed. Overfilling is more damaging than underfilling at speed, because the grease churns and overheats.
It depends on speed, temperature and grease type; the practical rule is to relubricate when the calculated grease life falls below the maintenance interval. Doubling the interval is safe; halving it is not always harmless, because each purge pushes out old grease and can push contamination in.
Avoid it. Different thickeners — lithium and polyurea, for example — can be incompatible and soften into a liquid. If you must change grease, purge the housing completely.
None. It is greased for life. The only maintenance decision is when to replace it, based on the calculated grease life at the actual operating temperature.
Tell us the application — speed, load, temperature and environment — and we will name the seal, clearance, class and cage that fit it.