Oil is used for two reasons: speed, and heat. Above a speed factor of roughly 350,000 grease cannot be relied on to reach the raceway, and in any application where the bearing generates more heat than the housing can dissipate, oil is the only lubricant that can also carry that heat away. The penalty is plumbing, filtration and the risk of leakage.
Bath: the bearing dips into an oil sump, level at the centre of the lowest rolling element — higher than that and the bearing churns. Circulation: a pump feeds oil through the housing and returns it through a filter and cooler. Jet: oil is sprayed at the raceway at pressure, used above about 500,000 ndm. Oil-air (oil mist): a measured quantity of oil is carried in a slow air stream to each bearing, standard in machine tool spindles.
The oil must be thick enough at operating temperature to keep the surfaces apart, and thin enough not to churn. That is expressed as the viscosity ratio κ — the actual viscosity at operating temperature divided by the viscosity the bearing needs. Aim for κ between 1 and 4. Below 1 the film breaks; far above 4 the bearing runs hot from shear.
In a spindle at 20,000 rpm the bearing generates more heat than convection can remove. The oil flow is sized by heat balance, not by lubrication: the film needs very little oil, but the temperature needs litres. That is why spindle oil systems are rated in litres per minute and why oil-air is used where a full flow would churn.
| Speed factor | 350,000 to over 1,000,000 depending on method |
|---|---|
| Bath level | Centre of the lowest rolling element |
| Circulation flow | 2–5 litres per minute per bearing, typical |
| Filtration | 10 µm or better for high-speed duty |
| Viscosity | Chosen so the film parameter stays above 1 |
| Heat removal | The main advantage over grease |
| Typical use | Gearboxes, turbines, spindles, high-speed rotors |
| Method | Speed (ndm) | Heat removal | Complexity | Typical use |
|---|---|---|---|---|
| Oil bath | to 350,000 | Moderate | Low | Gearboxes, pumps |
| Circulation | to 500,000 | Good | Medium | Turbines, large motors |
| Oil jet | 500,000–1,000,000 | Excellent | High | High-speed rotors |
| Oil-air / mist | above 1,000,000 | Good | High | Machine tool spindles |
| Splash (gear oil) | to 300,000 | Moderate | Lowest | Reducers, gearmotors |
At the centre of the lowest rolling element when the machine is stopped. Higher causes churning and foaming; lower starves the bearing.
Only if the housing is designed for it — oil needs a sump, a seal and a drain. A sealed bearing cannot be oil lubricated at all.
Yes, on condition or on interval. Oxidation, water ingress and particle count decide; a gearbox may run years, a hot circulating system months.
A metered quantity of oil — a few cubic millimetres per cycle — carried to the bearing in a slow air stream. It lubricates and slightly pressurises the housing to keep contamination out.
Send the designation, the three dimensions and the quantity — we reply with price, brand options and lead time within 30 minutes on working days.