There are many kinds and forms of rolling bearings and a wide range of sizes. In order to select the Selection appropriate bearing to achieve the design purpose,
it is necessary to make a comprehensive analysis from the aspects of mechanical service Selection conditions, performance requirements of the bearing, specifications and parameters around the bearing, market and economy.
Since the shaft diameter is generally determined first and then the bearing is selected, the bearing type shall be preliminarily determined based on the bearing inner diameter and Selection considering the conditions such as bearing installation space and configuration mode;
Secondly, the size of bearing is determined by analyzing and Selection comparing “bearing life required by machinery” and “bearing life calculated according to bearing load”; After that, it is necessary to determine the internal structure of bearing such as accuracy grade, internal clearance, cage and lubricant.
The general steps for selection are:
1. Determination of bearing type and configuration. (reference factors include: bearing installation space; bearing load size, direction and nature; rotating speed; rotation Selection accuracy; noise and friction torque; rigidity; relative inclination of inner ring and outer ring; installation and disassembly; bearing configuration; market and economy).
2. Determination of bearing size. (mechanical required bearing life; equivalent dynamic load; equivalent static load; speed; main dimensions Selection of bearing; basic rated dynamic load; basic rated static load; allowable axial load).
3. Determination of accuracy level. Selection (rotation accuracy; noise and friction torque; speed; bearing accuracy).
(size and nature of load; temperature during operation; material, size and accuracy of shaft and shell; fit; Selection temperature difference between inner ring and outer ring; speed; preload; bearing accuracy; internal clearance of bearing).
5. Material and type of cage. (speed; noise).
6. Special environmental countermeasures. (medium Selection conditions, such as temperature, seawater, vacuum, medicine, dust, gas, magnetic field, etc.; special materials; special heat treatment; special surface treatment; lubricant).
7. Lubrication method, lubricant and sealing device. (service temperature; speed; lubrication mode; lubricant; sealing device; limit speed of bearing; grease life).
8. Installation and disassembly methods and installation related dimensions.
9. Final determination of specifications and parameters of bearing and its surroundings. Finally, the bearing with satisfactory performance is obtained.
During the heat treatment of bearing parts, there are thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, which is complex and changeable.
Because it can change with the changes of heating temperature, heating speed, cooling mode, cooling speed, part shape and size, heat treatment deformation is inevitable. Understanding and mastering its change law can put the deformation of bearing parts (such as the ellipse of ferrule, size increase, etc.)
in a controllable range, which is conducive to production. Of course, mechanical collision during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
During the heat treatment of bearing parts, if they are heated in oxidizing medium, the surface will be oxidized, reducing the mass fraction of carbon on the surface of parts and causing surface decarburization.
If the depth of surface decarburization layer exceeds the allowance of post-processing, the parts will be scrapped.
The depth of surface decarburization layer can be measured by metallographic method and microhardness method. The measurement method of microhardness distribution curve of surface layer shall prevail and can be used as arbitration criterion.
3. Quenching crack
The crack formed by internal stress during quenching and cooling of bearing parts is called quenching crack. The reasons for this kind of crack are as follows: because the quenching heating temperature is too high or the cooling is too urgent, the structural stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel;
The original defects on the working surface (such as surface micro cracks or scratches) or internal defects of steel (such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage cavity residue, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation;
The tempering of parts after quenching is insufficient or not tempered in time; Excessive cold punching stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of oil groove, etc.
In a word, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main cause of quenching crack.