Tolerance fits between bearings and shafts, and between bearings and holes

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Having been in the field of custom manufacturing for so long, I’ve found that for the tolerance fits between bearings and shafts, as well as between bearings and holes, a slight clearance fit is sufficient to ensure proper functionality, while also making installation and removal easier. However, local components still require a certain degree of fit precision. Fit tolerance refers to the sum of the tolerances of the hole and shaft that make up a fit. It is the allowable variation from clearance to interference. The size of the tolerance zones for holes and shafts, as well as their positional relationships, constitute the fit tolerance. The magnitude of the fit tolerance between the hole and the shaft indicates the precision of their fit. The size and position of the fit tolerance zones for holes and shafts indicate the precision and nature of the fit between them. The selection of the tolerance grade is related to the bearing accuracy, as well as the tolerance grades of the shaft or bearing housing hole in the bearing fit. For shafts that are used in conjunction with bearings of P0 precision, the tolerance grade of the shaft is generally IT6, while the tolerance grade of the bearing housing hole is generally IT7. In applications with high requirements for rotational accuracy and smooth operation (such as electric motors), an axis grade of IT5 and a bearing housing bore grade of IT6 should be selected. The selection of tolerance zones is based on classifying the equivalent radial load P into categories such as “light,” “normal,” and “heavy” loads. The relationship between these loads and the bearing’s rated dynamic load C is as follows: light load – P ≤ 0.06C; normal load – 0.06C < P ≤ 0.12C; heavy load – 0.12C < P. 1) Tolerance zones for shafts: For shafts on which radial bearings or angular contact bearings are installed, the corresponding tolerance zone tables should be referred to. In most cases, where the shaft rotates while the direction of the radial load remains unchanged – that is, when the inner ring of the bearing rotates relative to the direction of the load – a transition or interference fit should generally be chosen. When the axis is stationary and the direction of the radial load remains unchanged, that is, when the inner ring of the bearing is stationary with respect to the load direction, a transition or slight clearance fit can be chosen (a large clearance is not allowed). 2) Tolerance zones for housing holes: The tolerance zones for housing holes used to install radial bearings and angular contact bearings are referred to in the corresponding tolerance tables. When making the selection, be careful: for outer rings that may swing or rotate in relation to the load direction, avoid clearance fit. The magnitude of the equivalent radial load also affects the selection of the fit for the outer ring. 3) Selection of the bearing housing structure: For rolling bearings, integral housing structures are generally used unless there is a specific need. Split-type bearing housings are employed only when assembly difficulties arise, or when the convenience of assembly becomes the primary consideration; however, they cannot be used in cases of tight fits or highly precise fits. For example, split-type bearing housings should not be used for fits of type K7 or tighter, nor for housing bores with a tolerance grade of IT6 or higher. Tolerance standards for the fit between bearings and shafts ① When the tolerance zone of the bearing’s inner diameter combines with that of the shaft, tolerance codes that were considered intermediate fits in the standard hole system become tight fits, such as k5, k6, m5, m6, n6, etc.; however, the degree of tightness is not great ； When the bearing inner diameter tolerance class is combined with h5, h6, g5, g6, etc. to form a fit, it is no longer a clearance fit but rather an interference fit. ②The tolerance zone for the outer diameter of bearings is also a special type of tolerance zone, as the tolerance values differ from those of ordinary reference shafts. In most cases, the outer ring is fixed in the hole of the housing; however, due to structural requirements of certain bearing components, it is necessary to allow some flexibility in the fit, and such fits are typically H6, H7, J6, J7, Js6, Js7, etc. Appendix: Under normal circumstances, the axis is generally marked 0 to +0. 005 If it is not removed frequently, it is +0. 005～+0. A clearance fit of 01 is sufficient; if frequent disassembly and assembly are required, a transition fit will do. We also need to take into account the thermal expansion of the shaft material itself during rotation; therefore, the larger the bearing, the better it is if it has a value of –0. A clearance fit of 005～0, with the maximum not exceeding 0. Clearance fit of 01. Another one is the dynamic coil interference and the static coil clearance. Bearing fits are generally transitional fits, but in special cases an interference fit can be used, though this is rare. Since the fit between the bearing and the shaft is that of the bearing’s inner ring with the shaft, the base hole system is used. Ideally, the bearing should fit perfectly flush; in practical use, this can indeed be assumed. However, to prevent the inner ring from rolling when it is at its minimum allowable fit size with the shaft, thereby damaging the surface of the shaft, the inner rings of bearings are given a clearance tolerance of 0 to a few microns to ensure that they do not rotate. Therefore, a transitional fit is generally sufficient for bearings, and even in the case of a transitional fit, the amount of interference should not exceed 3 microns. For the accuracy grade of fit, grade 6 is generally chosen; sometimes it also depends on the material and the manufacturing process. Theoretically, grade 7 is a bit too low, and grinding is required for a fit of grade 5. Generally, the following is chosen: for the fit between the inner ring of the bearing and the shaft, a K6 bearing is used; for the fit between the outer ring of the bearing and the hole, a K6 or K7 bearing is selected.