Fit Tolerance

Fit tolerance refers to the sum of hole and shaft tolerances that comprise a fit. It is the amount of change that allows clearance or interference. Fit tolerances are made up of the tolerance zone size and the tolerance zone position of holes and shafts. The fit tolerance of hole and shaft indicates the fit accuracy of hole and shaft. The size and position of hole and shaft fit tolerance zone indicate the fit accuracy and fit nature of hole and shaft.

Clearance fit:：Fit tolerance = maximum clearance - minimum clearance

Interference fit：Fit tolerance = maximum interference - minimum interference

Transition fit:：Fit tolerance = maximum clearance + maximum interference

Tolerance zone

Choice of tolerance class

The tolerance class of the shaft or bearing housing hole matched with the bearing is related to the bearing accuracy. The tolerance class of the shaft matched with the precision bearing of Grade P0 is generally IT6, and the bearing’s seat hole is generally IT7.In the case of high requirements for rotation accuracy and running stability (such as motor, etc.), the shaft should be SELECTED as IT5 and the bearing seat hole as IT6.

Choice of tolerance zone

The equivalent radial load P is divided into "light", "normal" and "heavy" loads, etc., and its relationship with the rated dynamic load C of the bearing is: light load P≤ 0.06c normal load 0.06c

The standard of tolerance 

(1) When the bearing inner diameter tolerance zone and the shaft tolerance zone form a fit, the tolerance code of the transition fit in the general base hole system will be changed into the interference fit, such as K5, K6, M5, M6, N6, etc., but the interference amount is not large; When the bearing’s bore tolerance zone is fitted with H5, H6, G5, G6, etc., it is no longer clearance but transition fit.(2) Because the tolerance value is different from the general reference shaft, the bearing’s outer diameter tolerance zone is also a special tolerance zone, in most cases, the outer ring is fixed to the shell hole ,some bearing components are required to adjust the structure, its fit should not be too tight, often with H6, H7, J6, J7, Js6, Js7, and so on.

In general, the shaft is generally marked 0 ~ +0.005 if it is not often removed, is +0.005 ~ +0.01 interference fit can be, if the shaft is often to be disassembly, the transition fit can be used. We also take into account the shaft material itself in the rotation of the thermal expansion, so as the larger bearing, it is best to be-0.005 ~ 0 clearance fit, but the maximum is not more than 0.01 clearances fit. Also notice, the moving ring is to be interference; the static ring is to be clearance.

How to choose the tolerance fit

Such as Φ 12 hole and shaft fit, if the base hole system is chosen, the hole should be processed first, after the hole tolerance is H0 ~ H18, minimum diameter of 12 holes, the maximum is 12 + tolerance, so that the hole is well done, then reprocessing Φ 12 axes, according to needs, transition fit can be chosen, and clearance fit or interference fit. If choose Φ 12 h7 hole, namely Φ 12 (+ 0.018/0), the interference fit, shaft can use Φ 12 p6, namely Φ 12 (0.029/0.018), and their tolerance matching Φ 12 h7p6 = Φ 12 (0, 0.029), the maximum size of the holes 0 = 0.018 - axis of the minimum size of 0.018, the smallest size hole - 0.029 = 0 - axis the maximum size of 0.029 .

If the base shaft system is chosen, the shaft should be processed first. The shaft tolerance is H0 ~ H18, the maximum diameter of the shaft is 12, and the minimum is 12+ tolerance (the tolerance is negative value).after this shaft processing is well done, reprocessing Φ 12 holes, according to need ,transition fit, clearance fit or interference fit can be chosen. If Φ 12 h6 shaft is chosen, namely Φ 12 (0, 0.011), the interference fit, the holes can be used Φ 12 p7, namely Φ 12 (- / - 0.011 to 0.029), and their tolerance matching Φ 12 p7h6 = Φ 12 (0, 0.029), the holes - 0 = the maximum size of the smallest size - 0.011-0.011 - shaft, hole - 0.029 = the maximum size of the smallest size - 0.029 - axis of 0.

For the same match to the hole axis diameter Φ 12, the same fit tolerance Φ 12 (0, 0.029), when base hole and basis shaft are chosen, tolerance of hole and shaft are not the same. The advantage of the basic hole system is that the hole is more difficult to be processed than the shaft. We can first process the hole and then take different shafts to cooperate with it. The transition interference clearance can be processed at will. But sometimes we have to adopt the base shaft system, such as bearing outer ring and bearing housing fit, or other shaft can be used directly without machining, then we will use the base shaft system. Both the base hole system and the base shaft system are measures taken to reduce production costs and improve efficiency.

Choice of tolerance class

The tolerance class of the shaft or bearing housing hole matched with the bearing is related to the bearing accuracy. The tolerance class of the shaft matched with the precision bearing of Grade P0 is generally IT6, and the bearing seat hole is generally IT7.In the case of high requirements for rotation accuracy and running stability (such as motor, etc.), the shaft should be SELECTED as IT5 and the bearing seat hole as IT6.

Choice of tolerance zone

The equivalent radial load P is divided into "light", "normal" and "heavy" loads, etc., and its relationship with the rated dynamic load C of the bearing is as follows: light load P≤ 0.07c normal load 0.07C & LT;P≤ 0.15C heavy load 0.15C<P

1) tolerance zone of the Shaft

For axial bearings and an gular contact bearings, refer to the corresponding tolerance zone table. In most cases, the transition or interference fit should be chosen when the bearing inner ring is rotating relative to the direction of load. In the case that the bearing inner ring is static relative to the direction of load, transition or small clearance fit can be selected (too large clearance is not allowed).

2) Tolerance zone of the Housing hole

For mounting the housing bore tolerance zone of centrist-bearing and angular contact bearing, refer to the corresponding tolerance zone table. When selecting, pay attention to avoid the clearance fit for the outer ring that swings or rotates the load direction. The size of the equivalent radial load also affects the selection of the outer ring fit

3) Selection of structural form of bearing housing

Rolling bearing of the bearing seat unless you have special need, the integral structure more commonly, split bearing just have trouble in the assembly, or on the advantages of convenient assembly become main consideration when using, but it cannot be applied to tight fit or with more precision, such as K7 and then K7 tighter fit, and like a tolerance level for IT6 or more precision of hole, are not allowed to adopt split housings. Tolerance is the possible range of shaft or hole size, and fit tolerance is the possible range of clearance after shaft and hole fit. For example, axis tolerances & OSlash; Tolerance of 12F6-0.016 / -0.027 holes & OSlash;12H7 0.018/0 Matching tolerance & Oslash;0.045=0.018-(-0.027). 0.016=0-(-0.016). The deviation of the base hole system refers to H. The deviation of any IT0 to IT18 is ok, and the deviation of the shaft is ok.H0 / a0, for example, H1 / d4, H7 / a0, H1 / f4, H5 / e4, H3 / f4, and so on are the base hole system, tolerance H is characterized by lower deviation is 0, for example Φ 12 H7 0.018/0 the basic shaft system refers to the axis of tolerance to choose H, tolerance grade, arbitrary IT0 to IT18, hole tolerance of any A to Z can be, and tolerance grade any IT0 to IT18 can be, too. Such as A1 / h0, B2 / h5, C3 / h6, D4 / h7, E5 / h9, F1 / h8/21 dated, and so on are the basic shaft system. Tolerance is characterized by the deviation of 0 h, for example Φ 12 h7 0/0.018 if the hole of tolerance to choose h, shaft tolerance h, then can be considered as base hole system, can also be regarded as the basic shaft system.