Lubricant functions; grease and solid lubricant use

Main functions of lubricants

1. Reduces friction between moving surfaces by forming a thin film that separates them.
2. Reduces wear and tear, which increases the life of machine parts.
3. Acts as a coolant: carries away heat produced by friction.
4. Acts as a seal: prevents leakage of gases, e.g. between piston rings and cylinder.
5. Prevents corrosion by coating metal surfaces from air and moisture.
6. Cleans by washing away dirt, metal particles and carbon deposits.
7. Saves energy: smoother motion means less power loss and higher efficiency; also reduces noise and vibration.

Where grease is used

Grease is a semi-solid lubricant (oil thickened with metal soap). It is used where oil cannot be retained or supplied regularly: slow-speed, heavily loaded or intermittently working bearings, open gears, chassis and axles, and machines where leakage and dust must be avoided (semi-solid grease stays in place and seals).

Where solid lubricants are used

Solid lubricants are solids with a layered structure that slide easily over each other, such as graphite, molybdenum disulfide (MoS₂), talc and soapstone. They are used where oils and greases fail:

• at very high temperatures (graphite up to about 450 °C in air) where oil burns or evaporates;
• under extremely heavy loads and very slow speeds where an oil film would be squeezed out;
• in vacuum or at very low temperature;
• where oil would contaminate the product (food, textile, paper machines) or collect dust;
• as an additive or dry film on parts that cannot be lubricated often.