Liquid lubricants and their classification

Liquid lubricants (lubricating oils) are liquids placed between moving surfaces to form a fluid film that reduces friction. They are the most widely used lubricants because they flow easily, carry away heat and are cheap.

Classification

Liquid lubricants
 ├── Animal and vegetable oils
 ├── Mineral (petroleum) oils
 ├── Synthetic oils
 └── Blended oils

1. Animal and vegetable oils: castor oil, olive oil, palm oil, tallow oil, whale oil. They have good oiliness (stick well to metal) but oxidise, become gummy and decompose at high temperature. Used in light machinery and as additives.
2. Mineral oils: obtained by fractional distillation of petroleum. They are cheap, stable and available in many grades, but have poor oiliness. Used in engines, turbines, gears.
3. Synthetic oils: silicones, polyglycols, organic esters, chlorinated hydrocarbons. They keep their viscosity over a wide temperature range and are used in jet engines and extreme temperatures.
4. Blended oils: mineral oil with additives (oiliness agents, antioxidants, pour-point depressors, viscosity-index improvers) to give special properties.