1. According to the physical form of lubricants
（1) Gas Lubrication
Utilizing certain inert gases such as air, steam, or helium as lubricants allows the friction surfaces to be separated by high-pressure gases. Applications include navigation gyroscopes, thrust bearings in vertical turbines of heavy machinery, rotating supports for large telescopes, and bearings for high-speed grinding heads. A significant advantage of gas lubrication is its extremely low friction coefficient, nearly approaching zero. The viscosity of the gas is unaffected by temperature, resulting in bearings with minimal resistance and high precision.
（2) Liquid Lubricant
The reducer, gear housing, and precision oil film bearings of rolling mill machines are all lubricated with different viscosity and performance-grade liquid lubricants. The liquid lubricants include mineral oils, synthetic oils, and emulsified oils. Water can also serve as a lubricant and coolant for the vulcanized babbitt bearings of the initial rolling stand.
（3) Semi-Solid Lubricant
Lubricating grease is a semi-solid material that falls between a fluid and a solid, existing in a plastic or paste-like state. It encompasses a variety of types, including mineral, synthetic, and animal or vegetable-based lubricants. It is widely used on various types of rolling bearings and vertical-mounted flat guide rails.
（4) Utilizing solid lubricants with special lubricating properties, such as graphite, molybdenum disulfide, and tungsten disulfide, to replace oils and greases for isolating frictional contact surfaces, forms an excellent solid lubricating film. This achieves effective lubrication by nourishing friction and reducing wear.
2. Categorized according to the distribution status of the lubricating film between the friction surfaces
（1) Full Film Lubrication
Lubricant is present between the friction surfaces, forming a complete lubricating film that completely isolates the friction areas. During the movement of the frictional partners, friction occurs internally between the molecules within the lubricating film, rather than as direct external friction between the friction surfaces. This condition is known as full-film lubrication. It represents an ideal lubrication state.
（2) Non-full-membrane lubrication
Friction surfaces can be damaged due to roughness, excessive loads, or changes in speed, leading to the destruction of the lubricating film. A portion of this is referred to as dry friction, and the condition is known as non-full-film lubrication. Generally, this state occurs frequently in equipment due to changes in motion speed (start-up, braking, reversal), variations in loading characteristics (sudden increases, impacts, localized concentrations, variable loads, etc.), and poor lubrication. We should strive to minimize and avoid this condition.
