2023 April the Third Week KYOCM Technical Knowledge: Experience in selecting rolling bearing fits

Abstract: Introduce some factors and selection experience that affect the fit of rolling bearings, providing a basis for mechanical design or maintenance.


Keywords: Rolling bearing matching selection experience


0 Preface

Rolling bearings are standardized components that have the advantages of low friction, easy starting, and easy replacement. It is crucial for us to choose a reasonable and correct bearing fit during daily maintenance or mechanical design.


1. Selection method for bearing fit

The correct selection of bearing matching is closely related to ensuring the normal operation of the machine, improving the service life of the bearings, and fully utilizing the bearing capacity. The selection of rolling bearing fit is mainly based on the nature and size of the bearing ring bearing load, and combined with factors such as bearing type, size, working conditions, material and structure of the shaft and shell, and working temperature.


(1) Does the collar rotate

When the inner or outer ring of a bearing is a rotating ring, a slightly tight fit should be used. The magnitude of the interference should ensure that the mating surface does not "crawl" under working load, because once crawling occurs, the mating surface will wear and slide, and the higher the speed of the ring, the more severe the wear.


When the bearing is in operation, if its inner or outer ring is a non rotating ring, for the convenience of disassembly and adjustment, it is advisable to choose a loose fit. Due to different working temperature rises, the shaft neck or housing hole will experience different elongation in the longitudinal direction. Therefore, when selecting a fit, the principle is to achieve free movement of the bearing along the axial direction and eliminate internal stress in the support. However, excessive clearance can reduce the rigidity of the entire component, cause vibration, and exacerbate wear.


(2) Load type

The bearing ring bears radial load and can be divided into the following three types according to the relative motion relationship between the load and the ring.


① Local load

Local load refers to the composite radial load Fr acting on the bearing, which is relatively stationary with the ring, i.e. Fr is borne by the local raceway of the ring.


② Cyclic load

Cyclic load refers to the composite radial load Fr acting on the bearing, which rotates relative to the ring, i.e. Fr sequentially acts on the entire circumference of the ring raceway.


③ Swing load

Swing load refers to the combined radial load acting on the bearing and the relative swing of the ring in a certain area. The bearing bears a radial load Fr and a rotating load Fc that remain unchanged in the direction. If Fr>Fc, their combined radial load F oscillates in opposite directions in a fixed ring raceway. Ferrules that bear local loads should choose loose transition fits or fits with smaller gaps to reduce friction between the ferrules and raceways.


Moment drives the rotation of the ring, ensuring even force on the ring and extending the service life of the bearing. The ferrule bearing cyclic load shall be interference fit or tight transition fit. The size of the interference shall be based on the principle of not causing creeping phenomenon on the mating surface between the ferrule and the shaft or housing hole. When bearing swing load, its coordination requirements are the same or slightly loose as the cyclic load.


(3) Load size

The minimum interference between the bearing ring and the journal and shell depends on the size of the load. When Pr/Cr ≤ 0.07, it is a light load; When 0.07<Pr/Cr ≤ 0.15, it is normal load; When Pr/Cr>0.15, it is considered heavy load.


Ferrules that bear impact loads or heavy loads are prone to deformation, causing uneven stress on the mating surface and causing loose fits. Therefore, tighter fits should be selected, which means the minimum interference should be greater. For rings that bear light loads, a loose fit should be selected.


(4) Other factors

① The impact of working temperature

During the operation of bearings, the temperature of the ring is higher than that of the mating parts due to friction heating and other heat sources. The thermal expansion of the inner ring will cause it to loosen its fit with the journal, while the thermal expansion of the outer ring will cause it to tighten its fit with the casing hole. Therefore, when the working temperature of the bearing is high, the selected fit should be appropriately corrected.


② The impact of rotation accuracy and speed

For bearings that bear large loads and require high rotational accuracy, in order to eliminate the effects of elastic deformation and vibration, it is necessary to avoid using a clearance fit. For lightweight bearings of some precision machine tools, in order to avoid the impact of shape errors on bearing accuracy, clearance fits are often used. It is generally believed that the higher the rotation speed of the bearing, the tighter the fit should be.


③ Conditions for installing and disassembling bearings

Considering the convenience of bearing installation and disassembly, it is advisable to use a loose fit, which is particularly important for large and oversized bearings used in heavy machinery. If convenient assembly and disassembly are required but tight fitting is required, separable bearings can be used, or bearings with tapered holes in the inner ring, tight sleeves, and withdrawal grooves can be used. In addition, the bearing fit should be suitable for tightening in the following situations: larger bearings are more suitable than smaller bearings; Hollow journal compared to solid journal; Thin walled shells are more than thick walled shells; A light alloy shell is better than a steel or cast iron shell; The integral shell is better than the partial shell.


3 Conclusion

In short, there are many factors that affect the selection of rolling bearing fits. When choosing a fit, various factors must be comprehensively considered and combined with the analogy method of actual work to achieve the best fit state.


More about KYOCM Spherical Plain Bearing

A spherical plain bearing is a bearing that permits angular rotation about a central point in two orthogonal directions (usually within a specified angular limit based on the bearing geometry). Typically these bearings support a rotating shaft in the bore of the inner ring that must move not only rotationally, but also at an angle.


KYOCM The Spherical Bearing is a heavy-duty self-aligning sliding bearing. Highly resistant to impact loads, this product is most fitted to the oscillating section under slow, heavy loads.