1. Classification according to the substance form of the lubricant

(1) Gas lubrication

The use of certain inert gases such as air, steam or helium as lubricants allows the friction surfaces to be separated by high-pressure gases. Such as marine inertial gyroscopes; thrust bearings of vertical turbines in heavy machinery; rotary bearings of large astronomical telescopes; bearings of high-speed grinding heads can be lubricated with gas. The advantage of gas lubrication is that the friction coefficient is small and close to zero. The viscosity of the gas is not affected by temperature, so the gas lubricated bearings have small resistance and high accuracy.

(2) Liquid lubrication

Rolling mill reducers, gear holders, precision oil film bearings, etc. are all lubricated with liquid lubricants of different viscosities and properties. Liquid lubricants include mineral lubricants, synthetic lubricants, and emulsified oils. Water can also be used as a lubricant and coolant for the bakelite bearings of the preliminary rolling mill.

(3) Semi-solid lubrication

Grease is a semi-solid substance in a plastic state or a grease state between a fluid and a solid. It includes mineral grease, synthetic grease, animal and vegetable grease, and so on. For rolling bearings and vertically mounted flat guides.

(4)Use solid lubricants with lubricating properties, such as graphite, molybdenum disulfide, tungsten disulfide, etc., instead of lubricating oil and grease to isolate frictional contact surfaces to form a good solid lubricating film to achieve a good lubricating effect of nourishing friction and reducing wear.

2. Classification based on the distribution of lubricating film between friction surfaces

(1) Full film lubrication

There is a lubricant between the friction surfaces and can form a complete lubricating film, separating the friction surfaces. When the friction pair moves, the friction is the internal friction between the internal molecules of the lubricating film, rather than the external friction of the direct contact of the friction surface. This state is called full film lubrication. This is an ideal state of lubrication.

(2) Non-full-film lubrication

The friction surface is damaged due to rough and uneven surface or excessive load, speed change and other factors, which cause the lubrication film to be damaged. Part of it is dry friction. This state is called non-full film lubrication. Generally due to changes in motion speed (starting, braking, reversing), changes in load properties (burst, impact, local concentration, variable load, etc.) and poor lubrication, this state often occurs in the equipment, and its wear is relatively fast. We should strive to reduce and circumvent this situation.