Having been doing non-standard work for so long, the tolerance fit between bearings and shafts, as well as the tolerance fit between bearings and holes, has always been achieved through small clearance fit, and it is easy to assemble and disassemble. However, local parts still require a certain degree of fitting accuracy.

Fit tolerance refers to the sum of the tolerances of the holes and shafts that make up the fit. It allows the variation from clearance to interference.

The size and position of the tolerance zone for holes and shafts constitute the fit tolerance. The size of the hole and shaft fit tolerance represents the fit accuracy of the hole and shaft. The size and position of the tolerance zone for hole and shaft fit represent the fit accuracy and fit properties of the hole and shaft.

Selection of tolerance levels

The tolerance level of the shaft or bearing seat hole that fits with the bearing is related to the bearing accuracy. The tolerance level of the shaft that fits with P0 level precision bearings is generally IT6, and the bearing seat hole is generally IT7. For occasions with high requirements for rotational accuracy and smoothness of operation (such as electric motors), the shaft should be IT5 and the bearing seat hole should be IT6.

Selection of tolerance zones

The equivalent radial load P is divided into several cases, such as "light", "normal", and "heavy" loads, and its relationship with the rated dynamic load C of the bearing is: light load P ≤ 0.06C normal load 0.06C ＜ P ≤ 0.12C heavy load 0.12C ＜ P

1) Axis tolerance zone

The tolerance zone of the shaft for installing radial bearings and angular contact bearings shall refer to the corresponding tolerance zone table. In most cases, when the shaft rotates and the radial load direction remains unchanged, that is, when the bearing inner ring rotates relative to the load direction, a transition or interference fit should generally be chosen. When the shaft is stationary and the radial load direction remains unchanged, that is, when the bearing inner ring is stationary relative to the load direction, transitional or small clearance fits can be selected (too large clearance is not allowed).

2) Shell hole tolerance zone

Refer to the corresponding tolerance zone table for the shell hole tolerance zone of installing radial bearings and angular contact bearings. When selecting, attention should be paid to avoiding clearance fits for outer rings that swing or rotate in the direction of the load. The size of the equivalent radial load also affects the fit selection of the outer ring.

3) Selection of bearing seat structure form

Unless there is a special need, the bearing seat of rolling bearings generally adopts an integral structure. Split type bearing seats are only used when there are difficulties in assembly or when the advantages of convenient assembly become the main consideration. However, they cannot be applied to tight or more precise fits, such as K7 and tighter fits than K7, and for seat holes with a tolerance level of IT6 or more precise, split type bearing seats cannot be used.

Fit tolerance standards for bearings and shafts

① When the inner diameter tolerance zone of the bearing forms a fit with the shaft tolerance zone, the tolerance code of the transition fit in the general basic hole system will become an over fit, such as k5, k6, m5, m6, n6, etc., but the over fit amount is not large; When the bearing inner diameter tolerance is combined with h5, h6, g5, g6, etc. to form a fit, it is no longer a clearance and becomes an over fit.

② The outer diameter tolerance zone of bearings is a special tolerance zone due to its different tolerance values from general reference axes. In most cases, the outer ring is fixed when installed in the outer shell hole, and some bearing components require structural adjustment, so their fit should not be too tight. They often fit with H6, H7, J6, J7, Js6, Js7, etc.

Attachment: In general, the axis is marked as 0 to+0. If it is not frequently disassembled, it is+0. 005 to+0. The interference fit of 01 is sufficient, and if frequent disassembly and assembly is required, a transitional fit is sufficient. We also need to consider the thermal expansion of the shaft material itself during rotation, so the larger the bearing, the better it is -0. The clearance fit between 005 and 0 should not exceed a maximum of 0. 01 clearance fit. Another issue is dynamic interference and static clearance.

Bearing fits are generally transitional fits, but in special cases, interference fits are optional, but they are rare. Because the bearing and shaft fit is the inner race and shaft fit of the bearing, using a base hole system. Originally, the bearing should be completely zero aligned, and we can also believe this in practical use. However, in order to prevent the inner race from rolling and damaging the surface of the shaft when the inner race of the bearing fits with the minimum limit size of the shaft, our bearing inner races all have 0 to several μ The lower deviation tolerance is used to ensure that the inner ring does not rotate, so the bearing usually chooses a transition fit, and even if a transition fit is selected, it cannot exceed the interference of 3 threads.

The fitting accuracy level is generally selected as level 6, and sometimes it also depends on the material and processing technology. In theory, level 7 is a bit low, but for a level 5 fitting, grinding is necessary. I usually choose K6 or K7 for the matching hole between the bearing inner ring and the shaft.

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