(1) The bearing fit is generally a transition fit,
The fit between bearing and shaft is the fit between the inner ring of bearing and shaft. The base hole Tolerance Grade system is used. Originally, the bearing should be completely zero. We can also think so in actual use.
However, in order to prevent the inner ring rolling when the inner ring of bearing is matched with the small Tolerance Grade limit size of shaft and damage the surface of shaft, our bearing inner rings have 0 to several μ
The lower deviation tolerance to ensure that the inner ring does not rotate, so the bearing generally Tolerance Grade selects the transition fit. Even if the transition fit is selected, it cannot exceed the interference of 3 wires. However, in special cases, interference fit is optional, but rarely.
Generally, the matching accuracy level is level 6. Sometimes it depends on the material and processing Tolerance Grade technology. Theoretically, level 7 is a little low, and grinding is required for level 5 matching.
The general selection is: K6 for the matching of bearing inner ring and Tolerance Grade shaft, K6 or K7 for the matching of bearing outer ring and hole
(2) Fit tolerance standard of bearing and shaft
① When the bearing inner diameter tolerance zone forms a fit with the shaft tolerance zone, the tolerance code of the original transition fit in the general base hole system will become an over win fit, such as K5, K6, M5, M6, N6, etc., but the over win amount is small; When the inner diameter tolerance of the bearing forms a fit with H5, H6, G5, G6, etc., it is no longer a clearance but an over win fit.
② The bearing outer diameter tolerance zone is also a special tolerance zone because the tolerance value is different from the general reference shaft. In most cases, the outer ring is fixed in the shell hole, and some bearing components need to be adjusted according to the structural requirements. Its fit should not be too tight, and it often fits with H6, H7, J6, J7, js6, js7, etc.
In general, the shaft is generally marked with 0 ~ + 0. 005 if it is not Tolerance Grade often disassembled, it is + 0. 005～＋0。 The interference fit of 01 is OK. If it needs to be disassembled frequently, it is a transition fit.
We should also consider the thermal expansion of the shaft material itself during rotation, so the larger the Tolerance Grade bearing, it is – 0. The clearance fit of 005 ~ 0 shall not exceed 0. 01 clearance fit. Another is the dynamic coil interference and static coil clearance.
Fit tolerance refers to the sum of hole and shaft tolerances constituting the fit. It is the variation of allowable clearance or interference.
The tolerance zone size and position of the hole and shaft constitute the fit tolerance. The size of hole and shaft fit tolerance indicates the fit accuracy of hole and shaft. The size and position of the hole and shaft fit tolerance zone represent the fit accuracy and fit nature of the hole and shaft. Size of fit tolerance = size of tolerance zone; Fit tolerance zone size and location = fit nature.
The tolerance grade of shaft or bearing seat hole matched with bearing is related to bearing accuracy.
The tolerance grade of shaft matched with P0 precision bearing is generally IT6, and the bearing seat hole is generally it7. For occasions with high requirements for rotation accuracy and operation stability (such as motor), the shaft shall be it5 and the bearing seat hole shall be IT6.
Selection of tolerance zone
The equivalent radial load p is divided into “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) Shaft tolerance zone
For the tolerance zone of the shaft installed with radial bearing and angular contact bearing, 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, transition or interference fit should be selected.
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, transition or small clearance fit can be selected (too large clearance is not allowed).
(2) Housing hole tolerance zone
The tolerance zone of housing hole for installing radial bearing and angular contact bearing shall refer to the corresponding tolerance zone table. When selecting, pay attention to avoid clearance fit for the outer ring swinging or rotating in the load direction. The size of equivalent radial load also affects the fit selection of outer ring.
(3) Selection of bearing pedestal structure
Unless there are special needs, the bearing seat of rolling bearing generally adopts the integral structure.
The split bearing seat is only used when it is difficult to assemble or when the advantage of convenient assembly becomes the main consideration, but it cannot be applied to tight fit or more precise fit, such as K7 and tighter fit than K7, or seat hole with tolerance grade IT6 or more precise, Split bearing pedestal shall not be used.