The fit tolerance refers to the sum of the tolerances between the components of a fit, including the hole, shaft, and bearing. It represents the permissible variation from clearance to interference.The fit tolerance of the hole and shaft consists of the size and position tolerance zones, indicating the precision of the fit between the hole and shaft. The size and position of the tolerance zones also denote the precision and nature of the fit between the hole and shaft.
Selection of Tolerance Grade
The tolerance grade of the shaft or bearing seat hole, in conjunction with the bearing precision, is essential. For shafts matched with P0 precision bearings, the tolerance grade is generally IT6, while for bearing seat holes, it's typically IT7. For applications requiring high rotational precision and smooth operation (e.g., electric motors), a shaft grade of IT5 and a bearing seat hole grade of IT6 are recommended.
Selection of Tolerance Zones
When the equivalent radial load P is categorized as "light," "normal," or "heavy," based on its relation to the rated dynamic load C, the following applies: light load P ≤ 0.06C, normal load 0.06C < P ≤ 0.12C, heavy load 0.12C < P.
1. Shaft Tolerance Zones
Refer to the corresponding tolerance zone table for the installation of radial and angular contact bearings on shafts. Generally, for rotating shafts with constant radial load direction, a transitional or interference fit is preferred. For stationary shafts with constant radial load direction, a transitional or small clearance fit is suitable, avoiding excessive clearance.
2. Housing Hole Tolerance Zones
Refer to the corresponding tolerance zone table for the installation of radial and angular contact bearings in housing holes. When selecting fits, avoid clearance fits for outer rings subjected to swinging or rotating loads. The magnitude of the equivalent radial load also influences the fit selection for outer rings.
3. Selection of Bearing Seat Structure
For rolling bearings, integral bearing seats are commonly used, except when split bearing seats are necessary due to assembly constraints or convenience. Split bearing seats are not suitable for tight or precise fits, such as fits tighter than K7 or fits with tolerance grades of IT6 or higher.
Standard Tolerance Fit for Bearings and Shafts
1. When the tolerance zones of the bearing inner diameter and shaft form a fit, tolerance grades originally belonging to transitional fits in the general basic hole system become interference fits, such as k5, k6, m5, m6, n6, etc. However, the interference is minimal. When the tolerance zones of the bearing inner diameter form a fit with h5, h6, g5, g6, etc., it becomes an interference fit rather than a clearance fit.
2. The tolerance zones of the bearing outer diameter constitute a special tolerance zone due to differing 2) tolerance values from standard reference shafts. In most cases, the outer ring is fixed in the housing hole, requiring adjustments for certain bearing component structures without excessively tight fits. Fits with H6, H7, J6, J7, Js6, Js7, etc., are common.In summary, fits between bearings and shafts are typically transitional, occasionally interference fits, but rarely so. Bearing fits involve the inner ring of the bearing and the shaft, using a basic hole system. While bearings ideally fit perfectly, in practical usage, a slight undersized tolerance is introduced to prevent the inner ring from rotating and damaging the shaft surface. Therefore, transitional fits are generally sufficient, with the interference fit not exceeding three wire units.
A tolerance grade of 6 is commonly chosen for precision fits, considering material and machining processes. Grade 7 is generally considered lower and may require grinding for fitting. Typically, for the fit between the bearing inner ring and the shaft, a k6 shaft and a K6 or K7 hole are selected.
