Our factory has extensive experience in the processing of CNC customized parts. We consistently achieve precise tolerance matching between bearings and shafts, as well as between bearings and holes, allowing for small clearance fits that facilitate easy assembly and disassembly. Nonetheless, it is important that local parts also maintain a certain level of matching accuracy.
Fit tolerance refers to the total tolerances of the holes and shafts involved in the fit. It represents the allowable variation from clearance to interference. The size and position of the tolerance zones for both the hole and the shaft together define the fit tolerance.
The dimensions of the hole and shaft fit tolerance indicate the accuracy of the fit. Additionally, the size and position of these fit tolerance zones reflect both the precision and the nature of the fit between the hole and the shaft.
Selection of tolerance grade
The tolerance grade of the shaft or the bearing seat hole that accommodates the bearing is closely related to the bearing’s accuracy. For shafts that are paired with P0 grade precision bearings, a tolerance grade of IT6 is typically used. In contrast, the bearing seat hole usually has a tolerance grade of IT7. However, in applications where high rotation accuracy and running stability are critical—such as in electric motors—the recommended tolerance for the shaft is IT5, while the bearing seat hole should have a tolerance grade of IT6.
Selection of tolerance zone
The equivalent radial load, P, can be categorized into several types: “light,” “normal,” and “heavy.” The relationship between the radial load and the rated dynamic load, C, for the bearing is defined as follows:
- Light Load: P ≤ 0.06C
- Normal Load: 0.06C < P ≤ 0.12C
- Heavy Load: P > 0.12C
1) Shaft tolerance zone
The tolerance zone of the shaft for installing radial bearings and angular contact bearings refers to the corresponding tolerance zone table. For most occasions, when the shaft rotates, and the radial load direction remains unchanged, that is, when the inner ring of the bearing rotates relative to the load direction, a transition or interference fit should generally be selected. For a stationary shaft and the radial load direction remains unchanged, that is when the inner ring of the bearing is stationary relative to the load direction, a transition or small clearance fit can be selected (a clearance that is too large is not allowed).
2) Housing bore tolerance zone
When selecting housing bore tolerances for radial and angular contact bearings, please refer to the appropriate tolerance zone table. It’s important to avoid using a clearance fit for outer rings that will swing or rotate in the direction of the load. Additionally, the magnitude of the equivalent radial load also influences the fit selection for the outer ring.
3) Selection of bearing seat structure
Unless there is a specific need, the bearing seat for rolling bearings typically features an integral structure. Split-bearing seats are only utilized in situations where assembly presents difficulties or when ease of assembly is the primary concern. However, they are not suitable for tight or highly precise fits, such as those classified as K7 or tighter. For instance, split-bearing seats should not be used for seat holes with tolerance grades of IT6 or finer.
Tolerance standards for matching bearings and shafts
① When the tolerance zone of the bearing’s inner diameter aligns with the tolerance zone of the shaft, the tolerance codes that were originally associated with transition matching in the general base hole system will transition to interference matching. Examples of these codes include k5, k6, m5, m6, n6, etc., although the degree of overmatch is typically small. Conversely, when the tolerance codes for the bearing’s inner diameter match h5, h6, g5, g6, etc., the result is no longer a clearance fit; instead, it becomes an overmatch.
② The tolerance zone for the bearing’s outer diameter is unique because its tolerance value differs from that of a standard reference shaft. Typically, the outer ring is secured within the housing hole. However, some structural requirements of bearing CNC machined components necessitate adjustments. It is important that the fit is not too tight. It is often matched with H6, H7, J6, J7, Js6, Js7, etc.
Appendix: Under normal circumstances, the shaft is typically marked with a range of 0 to +0.005. If the shaft is not disassembled frequently, an interference fit ranging from +0.005 to +0.01 is sufficient. However, if it is disassembled often, a transition fit is recommended.
Additionally, it’s important to consider the thermal expansion of the shaft material during rotation. For this reason, it is advisable to maintain a clearance fit of between -0.005 and 0 for larger bearings, with the maximum clearance not exceeding 0.01. Another guideline is to ensure that there is an interference fit for the moving circle and adequate clearance for the stationary circle.
Bearings typically use transition fits, though interference fits can be selected in special cases, although this is rare. The fit between the bearing’s inner ring and the shaft is based on a base hole system. Ideally, there should be a complete zero fit, which we can assume in practical applications. However, to prevent the inner ring from rotating when it meets the shaft at the minimum limit size—an occurrence that could damage the shaft’s surface—our bearing’s inner ring is designed with a lower tolerance of 0 to several micrometers (μ). This ensures that the inner ring does not rotate. Consequently, bearings usually opt for transition fits, and if an interference fit is chosen, the interference should not exceed three threads.
The fit accuracy level is generally 6, and sometimes, it depends on the material and processing technology. 7 is a bit low, and five fits require grinding.
The inner ring of the bearing matches the shaft, and the shaft should be K6, and the outer ring of the bearing matches the hole, and the hole should be K6 or K7.
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Post time: Jan-07-2025