The fit of rolling bearings is a critical step in mechanical design to ensure stable equipment operation. Its selection directly affects the bearing’s service life, operating accuracy, and the performance of the overall mechanical system. Bearing fit primarily refers to the connection between the inner ring and the shaft neck, and the outer ring and the bearing seat bore. A proper fit ensures that the bearing operates without vibration and noise due to excessive looseness, or increased friction loss due to excessive interference, or even overheating and failure. In practical applications, the selection of fit requires comprehensive consideration of factors such as bearing type, load size and direction, speed, and operating temperature. For example, situations with heavy axial loads generally require a tighter fit, while high-speed bearings require a smaller interference fit to reduce heat generation.
From the perspective of fit properties, the fit of rolling bearings can be divided into three categories: clearance fit, transition fit, and interference fit. Clearance fit means that there is a certain gap between the shaft neck and the inner ring or between the outer ring and the bearing seat hole. This fit is suitable for scenarios where the bearing needs to move axially, such as the axial adjustment mechanism of the machine tool spindle, and can effectively compensate for dimensional changes caused by temperature changes. The transition fit is between the clearance fit and the interference fit. Slight clearance or interference may occur during assembly. It is often used in situations that require both precise positioning and easy disassembly. For example, the fit between the motor end cover and the bearing often adopts this method. The interference fit is achieved by making the size of the shaft neck or the bearing seat hole slightly larger than the inner ring or outer ring of the bearing, achieving a tight connection, capable of transmitting large torque and axial force, and is widely used in the transmission components of heavy machinery.
Rolling bearings adhere to the “base system” principle when selecting a fit datum. Typically, the inner ring and shaft journal fit uses the base hole system, while the outer ring and bearing housing fit uses the base shaft system. This is because the inner and outer rings of the bearing are already manufactured to standard tolerances. The inner ring’s inner diameter serves as the base hole, while the outer ring’s outer diameter serves as the base shaft. This base system reduces machining complexity and production costs while ensuring good interchangeability between bearings from different manufacturers. For example, when the shaft journal uses the base hole system, the shaft journal’s tolerance band is determined solely based on the tolerance of the bearing’s inner ring, eliminating the need for additional adjustments and greatly streamlining the design process.
Operating conditions significantly influence the selection of rolling bearing fits. In high-temperature environments, bearings and related components experience dimensional changes due to thermal expansion. In these cases, a fit with a slightly larger clearance is necessary to avoid the increased interference caused by expansion and prevent bearing seizure. In applications subject to impact loads or high vibration, a larger interference fit should be selected to ensure a secure connection between the bearing, shaft neck, and housing bore. This prevents relative sliding and reduces wear on the mating surfaces. Furthermore, when bearings require frequent disassembly and installation, such as those used in testing equipment, a transition fit or clearance fit should be preferred to ease disassembly and protect the mating surfaces of the bearing and related components.
The accuracy level of rolling bearing fit is also important and cannot be ignored. It is closely related to the accuracy level of the bearing. High-precision bearings require high-precision mating surfaces; otherwise, the shape errors of the mating surfaces (such as roundness and cylindricity deviation) will affect the rotational accuracy of the bearing. For example, bearings used in precision machine tool spindles typically use P4-level accuracy, and the corresponding dimensional and shape tolerances of the journal and bearing seat holes must also reach a higher level to ensure that the spindle’s rotational accuracy is within the micron range. In actual processing, precision grinding, honing and other processes should be used to ensure the roughness and shape accuracy of the mating surfaces to avoid additional loads on the bearings during operation due to poor surface quality, thereby shortening their service life.
