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Supporting selection of imported bearings: The shaft is supported radially and axially by two bearings. At this time, the inlet bearing on one side is called the fixed side bearing. It bears both radial and axial loads and plays the role of fixing the relative axial displacement between the shaft and the bearing housing. The other side is called the free side, which only bears the radial load and can move relatively in the axial direction, so as to solve the problem of the telescopic part of the shaft and the spacing error of the installed bearing caused by the temperature change. For fixed side bearings, it is necessary to select bearings that can move axially with the rolling surface (such as cylindrical roller bearings) or with the assembly surface (such as centripetal ball bearings). On a relatively short shaft, if there is no difference between the fixed side and the free side, use a bearing with only one-way fixed axial movement (such as centripetal thrust ball bearing)
The cracks formed by internal stress during quenching and cooling of bearing parts are called quenching cracks. The causes of this kind of crack are: because the quenching heating temperature transition 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 surface of the task (such as micro cracks or scratches on the surface) or the external defects of steel (such as slag inclusion, major non-metallic impurities, white spots, shrinkage residual, etc.) form stress concentration during quenching; Significant surface decarburization and carbide segregation;
Tempering of parts after quenching is lacking or not tempered in time; Excessive cold punching stress, forging folding, deep turning tool marks, sharp edges and corners of oil groove, etc. formed in the later process. In short, the cause of forming quenching crack can be one or more of the above factors, and the existence of internal stress is an important cause of forming quenching crack. The quenching crack is deep and slender, the fracture is straight, and the fracture surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; The shape on the bearing steel ball can be S-shaped, T-shaped or ring-shaped. The microstructure characteristic of quenching crack is that there is no decarburization on both sides of the crack, which is significantly different from forging crack and data crack.
The microstructure overheating after quenching can be observed from the rough mouth of imported bearing parts, but the microstructure must be inspected to determine the overheating level. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is quenched superheated structure. The cause of formation can be one-sided overheating caused by too high quenching heating temperature transition or too long heating and holding time; It can also be due to the large banded carbide in the original structure, the needle like coarseness of martensite is formed in the low-carbon region between the two bands, and the formed part is overheated. The retained austenite in the superheated structure increases and the dimensional stability decreases. Because the quenched structure is overheated and the steel crystal is coarse, the toughness and impact resistance of the parts will be reduced, and the service life of the bearing will also be reduced. Overheating will even form quenching cracks.
In the thermal solution process of imported bearing parts, if they are heated in oxidizing medium, the surface will be oxidized, increasing the mass fraction of carbon on the surface of the parts and forming surface decarburization. If the depth of decarburization layer exceeds the allowance of post-processing, the parts will be scrapped. Metallographic method and microhardness method can be used to determine the depth of surface decarburization layer. The measurement method of microhardness dispersion curve of outer layer shall prevail and can be used as arbitration criterion.
During the thermal solution of imported bearing parts, there are thermal stress and structural stress. This internal stress can be superimposed or partially eliminated, which is complex and changeable. Because it can change with the change of heating temperature, heating speed, cooling method, cooling speed, part shape and size, the thermal solution deformation is inevitable. Being familiar with and controlling its change rules can make the deformation of bearing parts (such as the ellipse of ferrule, size increase, etc.) in a controllable scale, which is conducive to consumption. Of course, mechanical collision in the process of thermal solution will also deform the parts, but this deformation can be increased and prevented by improved operation.