Bearings used in machine tool spindles are divided into two categories: roller bearings and plain bearings. From the perspective of rotation accuracy, both types of bearings can meet the requirements, but compared to their performance indicators, plain bearings are better than plain bearings. The advantages of bearings are as follows: (1) Bearings can work stably under large conditions. speed and load changes. (2) Bearings can operate without clearance or even under preload (with some interference). (3) The friction coefficient of bearings is low, which helps reduce heat generation. (4) The bearings are easy to lubricate and grease can be used. Grease cannot be replaced until it has been repaired. If oil is used for lubrication, the amount of oil required per unit time is also much less than that of plain bearings. The disadvantages of bearings are: (1) The number of rolling elements is limited, so the radial stiffness of the bearing changes during rotation, which is one of the causes of vibration. (2) Bearing damping is low. (3) The radial size of the bearing is larger than that of the sliding shaft.
Under normal circumstances, machine tools should try to use bearings, especially most vertical spindles. Bearings can be lubricated with grease to prevent oil leakage. Plain bearings are only used on machine tools with low surface roughness and horizontal spindles, such as cylindrical grinders, surface grinders and high-precision lathes, etc. Bearing accuracy and stiffness directly affect spindle rotation accuracy, which are explained separately below.
Although bearings have a series of advantages and have been widely used in mechanical equipment, plain bearings need to be used in situations such as high speed, high precision, heavy load and structural requirements. Indeed, plain bearings have certain characteristics than rolling bearings. cannot replace. Its main advantages are: simple structure, convenient manufacturing, assembly and disassembly; good shock resistance and vibration absorption performance, smooth operation, high rotation precision and long service life. Its main disadvantages are: complex maintenance and high requirements for lubrication conditions. When the bearing is in a state of limit lubrication, friction and wear are significant.
Plain bearings are mainly composed of bearing seats and bushings. They are mainly divided into radial plain bearings (also called radial plain bearings), thrust plain bearings and radial thrust plain bearings according to the direction of the load, they are divided into liquid lubricated plain bearings; bearings according to the lubrication state, gas lubricated bearings, mixed lubricated bearings, solid lubricated bearings and non-lubricated bearings according to the rolling method, they are divided into lubricated mold bearings, direct contact bearings and others; bearing plain bearings. The backing pad is the main part of the sliding bearing. When designing the bearing, in addition to selecting the appropriate backing pad material, the structure of the backing pad should also be designed reasonably, otherwise the operating performance of the sliding bearing will be affected.
Due to their own structural factors, ordinary plain bearings are prone to low oil film rigidity, short precision maintenance period, difficult maintenance, high processing surface roughness and marks. vibration evident during production. Practice has proven that when the spindle plain bearing is selected as the liquid friction plain bearing, it has higher precision and rigidity, and the processing quality and productivity can be improved.
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