Interference fit: the actual size of the hole is always smaller than the actual size of the shaft. There is interference when the hole is assembled with the shaft (including the minimum interference is zero). At this point, the tolerance zone of the hole is below the shaft. For example, C. When the shaft with interference fit is assembled with the hole, it needs a certain external force or the part with hole is heated and expanded before the shaft can be pressed into the hole. The tolerance of interference (static) fit parts should be H7/N6 ~ H8/S7;
Transition fit: There may be clearance fit or interference fit when the hole is assembled with the shaft. At this time, the tolerance zone of the hole and the tolerance zone of the shaft overlap each other. The tolerance of transition connector shall be H7/k6~H8/k7.
Fit refers to the relationship between holes with the same basic size and the tolerance zone of the shaft. Determine the compactness of the combination. The algebraic difference obtained by subtracting the size of the mating shaft from the size of the hole is positive clearance, negative clearance is interference, and sometimes interference is negative clearance.
Relationship between combined holes with the same basic size and shaft tolerance zone. Determine the compactness of the combination. The algebraic difference obtained by subtracting the size of the mating shaft from the size of the hole is positive clearance, negative clearance is interference, and sometimes interference is negative clearance. According to the relationship between the hole and the shaft tolerance zone, that is, the characteristics of clearance, interference and their changes, the fit can be divided into three situations:
① Clearance fit. The tolerance zone of the hole is above the tolerance zone of the shaft, and there is clearance fit (including the minimum clearance is equal to zero). The function of clearance is to store lubricating oil and compensate various errors, and its size affects the relative motion between the hole and the shaft. Clearance fit is mainly used for the movable connection between hole and shaft, such as the connection between sliding bearing and shaft.
② Interference fit. The tolerance zone of the hole is lower than that of the shaft, and it is an interference fit (including the minimum interference is equal to zero). In interference fit, because the size of the shaft is larger than the size of the hole, it is necessary to use pressure or thermal expansion and cold contraction to assemble. Interference fit is mainly used for the fastening connection between the hole and the shaft, such as the connection between the gear ring and the hub of a large gear.
③ Transitional cooperation. The tolerance zones of holes and shafts overlap each other, and there may be gaps or interference fits (gaps and interference are generally small). Transition fit is mainly used for positioning connection that requires good coaxiality and coaxiality between the hole and the shaft and is easy to disassemble, such as the connection between the inner diameter of the rolling bearing and the shaft.
The allowable deviation of clearance or interference in fit is called fit tolerance. It is equal to the sum of the tolerances of the holes and shafts that cooperate with each other, indicating the allowable variation range of the fit tightness.
Fit tolerance: the allowable deviation of clearance or interference. For clearance fit, the fit tolerance is equal to the algebraic difference between the maximum clearance and the minimum clearance; For interference fit, the fit tolerance is equal to the algebraic difference between the minimum interference and the maximum excess; For transitional fit, the fit tolerance is equal to the algebraic difference between the maximum clearance and the maximum allowance. Fit tolerance is equal to the sum of hole tolerance and shaft tolerance, which indicates the allowable variation range of fit tightness.