Grease and oil are not a preference. Grease is limited by its ability to reach the raceway and by its life at temperature; oil is limited by plumbing and leakage. Two numbers — the speed factor and the heat balance — decide which limits bind first.
| Criterion | Grease | Oil |
|---|---|---|
| Speed factor (ndm) | Up to ~350,000 | To 1,000,000 and beyond |
| Heat removal | Poor — housing only | Good — the oil carries heat away |
| Sealing | Simple: the grease stays put | Requires seals and a drain |
| Maintenance | Relubrication or sealed for life | Level checks, filtration, changes |
| Life | Finite, halves per 15 °C | Indefinite with filtration |
| Cost of system | Low | High — pump, filter, cooler |
| Cleanliness | Clean | Leak risk |
| Typical use | Motors, conveyors, fans | Gearboxes, turbines, spindles |
Multiply bore diameter in millimetres by speed in rpm. A 50 mm bearing at 6,000 rpm gives ndm = 300,000, which is inside the grease range but near the top; the same bearing at 10,000 rpm gives 500,000, where grease can no longer be relied on to reach the raceway and oil (or oil-air) is required. The threshold is not absolute — it depends on grease type and fill — but 350,000 is the practical line for standard grease.
A bearing generating more heat than the housing can dissipate will keep heating until the grease fails. Oil solves this because the flow carries heat out of the housing to a cooler. That is why a gearbox running at 90 °C bath temperature is perfectly normal while the same temperature in a greased bearing means imminent failure — the oil system is removing heat that a greased housing cannot.
No. A sealed bearing is greased for life; oil cannot reach the raceway and the seal is not designed for liquid oil circulation.
30–50% of the free space for slow duty, less at high speed. Overfilling causes churning and overheating.
Yes, substantially — up to and beyond 1,000,000 ndm with jet or oil-air systems, against about 350,000 for grease.
Only if the housing has a sump, a drain and suitable seals. Converting the other way — oil to grease — is usually easier but loses heat removal.
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