1. Classification according to the physical form of lubricants

(1) Gas lubrication

Using certain inert gases such as air, steam or helium as lubricants allows the friction surfaces to be separated by high-pressure gases. For example, inertial gyroscopes used in navigation; thrust bearings of vertical turbines in heavy machinery; rotating supports of large astronomical telescopes; bearings of high-speed grinding heads, etc. can all be lubricated by gas. The advantage of gas lubrication is that the friction coefficient is small, close to zero. The viscosity of gas is not affected by temperature, so gas-lubricated bearings have low resistance and high precision.

(2) Liquid lubrication

The reducer, gear seat, precision oil film bearings, etc. of the rolling mill 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 bakelite bearings in blooming mills.

(3) Semi-solid lubrication

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

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Solid lubricants with lubricating properties, such as graphite, molybdenum disulfide, tungsten disulfide, etc., are used to replace lubricating oil and grease to isolate the friction contact surface and form a good solid lubricating film to achieve good lubrication that supports friction and reduces wear.

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

(1) Full film lubrication

There is lubricant between the friction surfaces, and a complete lubricating film can be formed to separate 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 direct contact between the friction surfaces. This state is called full film lubrication. This is an ideal lubrication state.

(2) Non-full film lubrication

Due to roughness and unevenness of the friction surface or the influence of factors such as excessive load and speed changes, the lubricating film is destroyed, and part of it is dry friction. This state is called non-full film lubrication. Generally due to changes in movement speed (starting, braking, reversal), changes in load characteristics (sudden load, impact, local concentration, variable load, etc.) and poor lubrication, the equipment often appears in this state, and its wear is relatively rapid. We should strive to reduce and avoid this state.