Keywords: CNC machining, bearings and bores，tolerance representation and management, reliable design, uncertainty, cylindrical roller bearings
Bearing is an important component in contemporary mechanical equipment. Its main function is to support the mechanical rotating body, reduce the friction coefficient during its movement, and ensure its rotation accuracy.
Best company provides non-standard services. Customized tolerances for bearings and shafts, and tolerances for bearings and holes, have always been achieved with a small clearance fit. Some local parts require a certain degree of fit.
Fit tolerance refers to the sum of the tolerances of the holes and shafts that make up the fit. It is the amount of allowable change from gap to interference.
The tolerance zone size and location of the hole and shaft constitute the fit tolerance. The size of the hole and shaft fit tolerance indicates the accuracy of the hole and shaft fit. The size and position of the tolerance zone of the hole and shaft fit indicate the accuracy and properties of the hole and shaft fit.
BEST PARTS Control on Tolerance
Before you know the strict control of tolerance control on the bearing, you must first understand the choice of tolerance level. Generally speaking, the tolerance grade of the shaft or bearing hole matched with the bearing is related to the bearing accuracy. Alexandria University research also shows: The geometric dimensions and tolerances of the bearing housings especially affect the working clearances, and then the fatigue life of rolling bearings.
For shafts matched with P0 precision bearings, the tolerance level is generally IT6, and the bearing housing hole is generally IT7. Where high accuracy is required for rotation accuracy and smooth running (such as electric motors), the shaft should be IT5 and the bearing housing hole should be IT6.
Concept of Tolerance Zone
The equivalent radial load P is divided into "light", "normal" and "heavy" loads.
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
Refer to the corresponding tolerance zone table for the tolerance zone of shafts with radial bearings and angular contact bearings.
For most occasions, when 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 stationary shaft and the radial load direction are not changed, that is, when the bearing inner ring is stationary relative to the load direction, a transition or a small clearance fit can be selected (too large clearance is not allowed).
2) shell hole tolerance zone
Refer to the corresponding tolerance zone table for tolerances of housing holes for installing radial and angular contact bearings.
When selecting, pay attention to avoid the clearance fit for the outer ring that swings or rotates in the load direction. The equivalent radial load also affects the fit of the outer ring.
3) selection of bearing seat structure
Unless there is a special need for the bearing housing of rolling bearings, generally adopt a monolithic structure.
Split bearing housing is only used when it is difficult to assemble or the advantage of convenience in assembly becomes the main consideration, but it cannot be used for tight fit or more precise fit, such as K7 and tighter fit than K7, For seat holes with tolerance class IT6 or higher, split bearing seats are not allowed.
Tolerance standard of bearing and shaft
① When the bearing inner diameter tolerance zone and the shaft tolerance zone are matched
In the general base hole system, the tolerance code that originally belonged to the transition fit will become a win-win fit, such as k5, k6, m5, m6, n6, etc., but the win-win amount is not large; when the bearing inner diameter tolerance is replaced with h5, h6, g5, When a g6 and other components form a fit, it is not a gap but an overwhelming fit.
② The bearing outer diameter tolerance band is different from the general reference shaft due to the tolerance value
It is also a special tolerance zone. In most cases, the outer ring is fixed in the hole of the housing. Some bearing components require structural adjustments. The fit should not be too tight. They are often used with H6, H7, J6, J7, Js6, Js7. And so on.
Attachment: In general, the shaft is generally marked with 0 ~ + 0. 005 If it is not often disassembled, it is an interference fit of +0. 005 ~ + 0.01. If it is often disassembled, it is a transition fit.
We also need to consider the thermal expansion of the shaft material when it is rotating, so the larger the bearing, the better it is -0.05 ~ 0 clearance fit, and the maximum should not exceed 0.01 clearance fit. The other is the interference of the moving coil and the gap of the static coil.
Bearing fits are generally transition fits, but interference fits can be selected in special cases, but rarely. Because the matching of the bearing and the shaft is the matching of the bearing's inner ring and the shaft, the base hole system is used. The bearing should be completely zero, and we can think of it in actual use.
Our engineers, but in order to prevent the inner ring from rolling when the inner limit of the bearing is matched with the shaft, and damage the surface of the shaft, our inner ring of the bearing has a tolerance of 0 to several μ to ensure that the inner ring does not rotate Therefore, it is generally sufficient to select a transition fit for the bearing. Even if the transition fit is selected, the interference amount of 3 wires cannot be exceeded.
The coordination accuracy level is generally chosen to be 6, and sometimes it depends on the material and processing technology. Theoretically, the 7 level is a little bit bottomed. If the 5 level is matched, the running-in is required.