Bearing is an important part of the spindle assembly, and its type, structure, configuration, accuracy, installation, adjustment, lubrication and cooling directly affect the performance of the spindle assembly. The rotation accuracy of the spindle is largely determined by the bearing. Excellent micro bearing small series to introduce the design of CNC machine tool bearing selection skills.
Spindle rolling bearing
(1) Type of rolling bearing
The rolling bearing has small friction resistance, can be pre-tightened, simple lubrication and maintenance, and can work stably in a certain speed range and load variation range. Rolling bearings are produced by professional chemical plants, easy to buy and maintain. However, compared with plain bearings, rolling bearings have large noise, limited number of rolling elements, varying stiffness, slightly poor vibration resistance and great restrictions on speed. Rolling bearings are divided into ball bearings and cylindrical roller bearings according to the structure of rolling elements. Ceramic bearing refers to the bearing rolling body is made of ceramic material, while the inner and outer rings are still made of bearing steel. The reason why the rolling body is selected is mainly because it has the following characteristics: one is light weight, the second is small thermal expansion coefficient, and the third is large elastic modulus. Therefore, it has the characteristics of small centrifugal force, small dynamic friction force, stable preload force, small elastic deformation and high stiffness.
(2) Reasonable arrangement of main shaft bearings
1) Main shaft bearing selection and configuration form
Machine tool spindle has front, back 2 supports and front, middle and back 3 supports, the former is more common. The configuration form of two-support spindle bearings, including the selection, combination and arrangement of spindle bearings, is mainly based on the requirements of the spindle components involved in speed, bearing capacity, stiffness and accuracy.
The following general principles should be followed when determining the configuration of the two-supported spindle bearing:
Adapt to the requirements of stiffness and bearing capacity: the selection of the main shaft bearing configuration form should first meet the required stiffness and bearing capacity.
(2) Adapt to the speed requirements: the speed allowed by different types, specifications and accuracy grades of bearings is different. Under the same conditions, the point contact is higher than the line contact; Cylindrical rollers are taller than conical rollers.
(3) The requirements of adaptation accuracy: the configuration form of the thrust bearing under axial force of the spindle assembly directly affects the axial position accuracy of the spindle.
2) Three-support spindle assembly
Due to the structural design of some machine tools, the length of the headstock is longer, and the span between the two supports of the main shaft is much larger than the span, at this time, the intermediate support should be considered to improve the stiffness and vibration resistance of the main shaft assembly.
3) Application design of spindle rolling bearing
① Stiffness and bearing capacity of spindle rolling bearing
The stiffness of rolling bearings mainly depends on the contact stiffness between rolling element and raceway. The stiffness of sports bearing is related to bearing type, load and preload, and is nonlinear with the increase of load.
② Lubrication of main shaft bearing
Lubrication has a great influence on the working performance of the spindle assembly and the life of the bearing. Improper lubrication may cause heat increase, reduce spindle working accuracy, and accelerate bearing wear. Lubricants for rolling bearings can be divided into two categories, namely grease lubricants and liquid lubricants.
③ Sealing of spindle rolling bearing
The role of the rolling bearing seal device is to prevent coolant, chips, dust, debris, etc. from entering the bearing, and keep the lubricant in the bearing without leakage, so as to give full play to the performance of the bearing and ensure the service life of the bearing. The requirements for the spindle assembly sealing device are: suitable for the spindle speed, suitable for the working environment, tight structure. The main shaft bearing seal is divided into contact type and non-contact type. The former can be divided into radial sealing ring sealing box felt ring sealing, generally suitable for low speed spindle. The non-contact seal can be divided into clearance seal and curved sealing box gasket seal. In order to ensure the sealing effect, the radial clearance between the rotating part and the fixed part should be less than 0.2-0.3mm, and there should be oil return control at the sealing place to prevent oil leakage.