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Tolerance fit between bearing and shaft, bearing and hole Part 1

2022-08-02

We have been in this industry for so long, the tolerance fit between the bearing and the shaft, as well as the tolerance fit between the bearing and the hole, has always been able to achieve the function with a small clearance fit, and it is easy to assemble and disassemble. However, some parts still need to have a certain matching accuracy.
Fit tolerance is the sum of the hole and shaft tolerances that make up the fit. It is the amount of variation that allows clearance to interference.
The size of the tolerance zone and the position of the tolerance zone for the hole and shaft make up the fit tolerance. The size of the hole and shaft fit tolerance indicates the fit accuracy of the hole and the shaft. The size and position of the hole and shaft fit tolerance zone indicate the fit accuracy and fit nature of the hole and shaft.
 
 01 Selection of tolerance class
 
The tolerance class of the shaft or housing bore that fits the bearing is related to the bearing accuracy. For the shaft matched with the P0 grade precision bearing, the tolerance level is generally IT6, and the bearing seat hole is generally IT7. For occasions with high requirements on rotation accuracy and running stability (such as motors, etc.), the shaft should be selected as IT5, and the bearing seat hole should be IT6.
 
02 Selection of tolerance zone
The equivalent radial load P is divided into "light", "normal" and "heavy" loads. The relationship between it and 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) Shaft tolerance zone
For the tolerance zone of the shaft on which the radial bearing and angular contact bearing are mounted, refer to the corresponding tolerance zone table. For most occasions, the shaft rotates and the radial load direction does not change, that is, when the bearing inner ring rotates relative to the load direction, a transition or interference fit should generally be selected. When the shaft is stationary and the radial load direction is unchanged, that is, when the inner ring of the bearing is stationary relative to the load direction, transition or small clearance fit can be selected (too much clearance is not allowed).
 
(2) Shell hole tolerance zone
 
For the housing bore tolerance zone for radial and angular contact bearings, refer to the corresponding tolerance zone table. When selecting, pay attention to avoid clearance fits for outer rings that oscillate or rotate in the direction of load. The size of the equivalent radial load also affects the fit selection of the outer ring.
 
(3) Selection of bearing housing structure
 
Unless there is a special need, the bearing seat of the rolling bearing generally adopts the integral structure. The split bearing seat is only used when the assembly is difficult, or the advantage of convenient assembly is the main consideration, but it cannot be used for tight fit. Or a more precise fit, such as K7 and a tighter fit than K7, or a seat hole with a tolerance class of IT6 or more, shall not use a split housing.