Friction is the worst enemy of machinery. The rubbing of one component against another wears out the metal, increases power consumption, and generates excessive heat. Lubricants are substances applied to the bearing surfaces to reduce friction and minimize its negative effects.
In the past oils from animals and plants were the main lubricants. This changed when petroleum oil was discovered. At first petroleum oils were used as fuel for lamps but was soon discovered that it can be made into high-quality lubricant. Unlike its animal and vegetable oils that preceded which were quick to degrade, petroleum oil is more resistant against degeneration resulting from oxidation and heat.
Today more materials are being used as lubricants. There are synthetic oils, industrial grease, solid lubricants and anti-friction paints. There are even modern devices that use gases as lubricants.
A quality that is essential to lubricants is wetting power. This allows the lubricant to spread easily and penetrate between bearing surfaces. It must also have a good hold to prevent being removed by the rubbing motion.
Perhaps the most important quality of lubricants is viscosity. This quality makes it stick together and resist being squeezed out of the lubrication point. Too much viscosity, however, can negatively affect machine performance. Its thickness can interfere with the movement of the mechanical components of the machine. There needs to be a right balance between viscosity and fluidity. As a general rule, heavy loads require more viscosity, while high speeds require more fluidity.