When a ball bearing is used, it is not used only by itself; it is always fitted to either a shaft or a housing bore. Fit is the value of tightness between the shaft and bearing bore when the bearing is installed. Fit can also be the tightness between the housing bore and the bearing outside diameter. Fits are classified into clearance fit, interference fit, and transition fit.
Fits prevent bearings from having unfavorable slip called “creep” by firmly securing the inner ring on the shaft and in the housing, respectively. fits also minimize vibration during rotation. When creep happens, abnormal heat and wear particles can be generated. Abnormal heat hastens the degradation of grease and retainers. Wear particles could migrate inside the bearing, and cause vibration and surface degradation. It is necessary to choose a proper fit for each application because improper fits can not only degrade the bearing performance, but they also could cause seizure due to heat generation and premature failure. In the case of interference fits, the interference causes a change in radial internal clearance. The change in radial clearance generated by interference can be calculated as shown below.
Interference fit of the inner ring to the shaft:
The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When press fitted with an interference ” i “, the inner ring groove diameter (d2) increases by an amount (δ). This value (δ) is also equal to the decrease in radial clearance.
Press fits of the inner ring to the shaft:
Interference fit of the outer ring to the housing
The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When it is press fitted with an interference ( I ), the outer ring groove diameter D1 decreases by an amount (Δ). This amount (Δ) is also equal to the decrease in radial clearance.
Press fits of the outer ring to the housing :
Securing with adhesive
When the bearing is fitted to the shaft and housing with adhesive without interference, it is necessary to select the proper clearance to enhance the effectiveness of the adhesive.. Proper clearance depends on the type of adhesive used, therefore, we recommend you consult the the adhesive manufacturer before using it. Please be aware that the roundness of the ring raceways could worsen because of the curing stress of the glue.
*Tolerance of Bearing Bore is based on JIS B 1514-01
**Symbol of tolerance zone class is based on JIS B 0401.
*** Tolerance of outer diameter of bearings is based on JIS B1514-1.
More Information About Interference Fits and Creeping
What Is Interference Fit?
If you’re not completely familiar with the idea of fit as it relates to a bearing, here is a brief primer to clarify matters. Essentially, the fit in a ball bearing assembly refers to how much “play” or “give” there is between the parts of the bearing. Specifically, this is the fit between the shaft and bearing bore or bearing and housing bore. This space between the two parts is known as clearance, and a fitting where there is clearance between the parts is known as a clearance fit. In the case of an interference fit, sometimes known as a press fit, the two parts are pressed together so that there is a minimum of give between the two parts of the bearing assembly; in fact, the two parts are overlapping and “interfering” with one another. This occurs typically when one part is slightly larger than the part it is supposed to fit into. Once the two parts are joined, they deform to accommodate the lack of space and are effectively fused together as one.
How Is the Fit in an Interference Fit Created?
The fit can be created in one of two ways. The first is force, where a hydraulic press or similar pressure-applying device simply forces the oversized part into the other, compelling the parts to conform under the pressure. A less violent method is through thermal expansion. The part to be inserted is cooled, causing it to contract, then it is inserted into the other part. When the metal heats up and expands, the proper fit is created. The level of interference between the two parts determines whether or not the bearing has a loose fit, a light interference fit, or an interference fit. The interference fit is the tightest fit. NMB engineers can calculate the allowance which will produce these different kinds of fits for different sizes and construction types of ball bearing assemblies. The allowance is a planned deviation from the nominal, expected size of the part and the actual size of the part.
What Is Creeping?
Creeping is the tendency of metals to move or permanently deform as a response to the constant stresses placed upon them. Ball bearing assemblies are particularly vulnerable to creep as they are often consistently subject to pressures and high temperatures for extended periods of time. As you may imagine, creep can destroy the integrity of the ball bearing assembly, which can result in a failure of the application that is making use of the ball bearings. Interference fit can defend against creeping, and this is a large part of what makes this kind of fit desirable.