1. Generally speaking, bearings are fitted with a transition fit; however, in special circumstances, an interference fit may also be selected, although such cases are rare. As the fit between the bearing and the shaft refers to the fit between the bearing inner ring and the shaft, and the base bore system is used, the bearing must be perfectly aligned (i.e. the tolerance zone reference lines must be aligned); this can also be understood in this way in practical applications. However, to prevent the inner ring from rotating and wearing the shaft surface when fitted to the minimum limit dimension, a clearance of 0 to several micrometres (μm) is provided to ensure the inner ring does not rotate; therefore, a transition fit is generally sufficient for bearings. Even when a transition fit is selected, the clearance must not exceed 3 si (1 si = 0.01 millimetres).
The fit accuracy grade is usually Grade 6, although this may sometimes need to be determined based on the materials and machining processes. Theoretically, Grade 7 accuracy is slightly lower, whilst a Grade 5 fit requires grinding to achieve.
For the fit between the bearing inner ring and the shaft, the shaft is generally selected with a K6 or K7 tolerance band.
2. Tolerance Standards for the Fit Between Bearings and Shafts
1. When the tolerance band of the bearing’s inner diameter forms a fit with that of the shaft, the tolerance designations for intermediate fits in the standard bore system (such as k5, k6, m5, m6, n6, etc.) become interference fits, albeit with a small amount of interference; when the bearing’s inner diameter fits with shaft tolerance bands such as h5, h6, g5 or g6, the fit is no longer a clearance fit but an interference fit.
2. As the tolerance values for the bearing outer diameter differ from those of the standard reference shaft, these also fall under the category of special tolerance zones. In most cases, the bearing outer ring is fixed within the housing bore, whilst certain bearing components require adjustment in accordance with structural requirements; therefore, the fit should not be too tight. Commonly used tolerance zones include H6, H7, J6, J7, Js6 and Js7.
3. Practical Examples
Typically, shaft tolerances are specified as 0 to +0.005 millimetres (mm). If disassembly is infrequent, an interference fit of +0.005 to +0.01 mm may be used; if frequent disassembly is required, a transition fit should be adopted. At the same time, the thermal expansion of the shaft material during rotation must be taken into account; consequently, the larger the bearing size, the more suitable a clearance fit of –0.005 to 0 mm becomes, and the maximum clearance must not exceed 0.01 mm. Furthermore, a distinction must be made between the rotating ring interference (the interference fit between the rotating component and the shaft) and the stationary ring clearance (the clearance fit between the fixed component and the housing).
4. Fit Tolerance
Fit tolerance refers to the sum of the tolerances of the bore and the shaft that constitute the fit; it represents the permissible variation in clearance or interference. The size and position of the tolerance zones for the bore and shaft together constitute the fit tolerance: the magnitude of the bore-shaft fit tolerance reflects the fitting accuracy; whilst the size and position of the bore-shaft fit tolerance zones reflect both the fitting accuracy and the nature of the fit (clearance, transition or interference). Specifically: the magnitude of the fit tolerance = the size of the tolerance zone; the size and position of the fit tolerance zone = the nature of the fit.
5. Selection of Tolerance Grades
The tolerance grade of the shaft or bearing housing bore mating with the bearing must be matched to the bearing’s precision. For shafts mating with P0-grade precision bearings, the tolerance grade is typically IT6, whilst the bearing housing bore is typically IT7; for applications with high requirements for rotational precision and operational stability (such as electric motors), the shaft tolerance grade should be IT5 and the bearing housing bore IT6.
6. Selection of Tolerance Zones
Based on the magnitude of the equivalent radial load P, loads can be classified into three categories: ‘light’, ‘normal’ and ‘heavy’. Their relationship with the bearing’s rated dynamic load C is as follows: light load P ≤ 0.06C, normal load 0.06C < P ≤ 0.12C, and heavy load 0.12C < P.
1. Shaft Tolerance Zones
For shafts fitted with radial bearings and angular contact bearings, the tolerance zones must be selected by referring to the relevant tolerance zone tables. In most cases, when the shaft is rotating and the direction of the radial load is fixed (i.e. the bearing inner ring rotates relative to the direction of the load), a transition fit or interference fit should generally be selected; when the shaft is stationary and the direction of the radial load is fixed (i.e. the bearing inner ring is stationary relative to the direction of the load), a transition fit or a fit with a small clearance may be selected (excessive clearance is not permitted).
2. Housing Bore Tolerance Zones
The tolerance zones for housing bores of radial and angular contact bearings must be determined by referring to the relevant tolerance tables. When selecting, note that: for outer rings that oscillate or rotate in the direction of the load, a clearance fit should be avoided; the magnitude of the equivalent radial load also influences the choice of fit for the outer ring.
3. Selection of Bearing Housing Design
Unless there are special requirements, bearing housings for rolling bearings generally adopt a one-piece design. Split bearing housings may only be used where assembly is difficult, or where ‘ease of assembly’ is a primary consideration; they are not suitable for tight or high-precision fits (such as K7 and fits tighter than K7). For example, split bearing housings must not be used for housing bores with a tolerance class of IT6 or higher.
