Lubrication science
What a lubricant actually does: six jobs, not one
Quick answer
Reducing friction is only the first job. A lubricant also protects, cools, cleans, seals and sometimes transmits power.
Six jobs
| Job | What it means in practice | What goes wrong without it |
|---|---|---|
| Reduce friction | A film keeps moving surfaces apart so they slide or roll with little resistance. | More power drawn, heat, and surfaces that touch. |
| Reduce wear | The film, plus additives at the contact, stops metal removing metal. | Scoring, scuffing, pitting, shortened life. |
| Carry heat away | Oil takes heat from the contact and the bearing to the sump or cooler. | Overheating, thinner oil, faster ageing, discoloured steel. |
| Protect against corrosion | Oil and inhibitors coat the metal and keep water off it. | Rust, etching and fretting damage, especially at standstill. |
| Control contamination | Oil carries dirt and wear debris to a filter or sump. Grease also seals the housing. | Particles stay in the contact and dent the surfaces. |
| Transmit power | In hydraulic systems the fluid itself moves force and motion. | Slow, noisy, hot hydraulics and worn pumps. |
Why bearings care
A rolling bearing carries load on a film that is only a fraction of a micrometre thick in the contact. Every one of the six jobs affects whether that film survives. Most premature bearing failures trace back to one of them being neglected: wrong viscosity, too little lubricant, dirt or water.
The film is not always there
At low speed, high load or high temperature the film thins and the surfaces touch (boundary lubrication). Additives then do the protecting. The Stribeck curve topic shows where each regime sits.
Takeaway. Ask which of the six jobs is failing. The answer usually points straight at the cause: heat, dirt, water, viscosity or a missing film.
Put it to work in the tool