Rolling bearings are the most widely used bearings in motor products. Rolling bearings are standardized components with advantages such as low friction, easy starting, and simple replacement. In daily maintenance or mechanical design, the rational and correct selection of bearing fits is crucial. Many factors influence the selection of rolling bearing fits; therefore, a comprehensive consideration of various factors, combined with analogies from actual working conditions, is necessary to achieve the optimal fit.
Properly selecting bearing fits is crucial for ensuring normal motor operation, extending bearing life, and fully utilizing bearing load capacity. The selection of rolling bearing fits primarily depends on the nature and magnitude of the load borne by the bearing rings, combined with a comprehensive consideration of factors such as bearing type, size, operating conditions, shaft and housing materials and structure, and operating temperature.
A key factor affecting bearing operation is the bearing's operating clearance. The main factors influencing bearing clearance include the fit between the inner ring and the shaft, the fit between the outer ring and the bearing housing, the bearing's own clearance under unassembled conditions, the motor's operating conditions, and the surrounding environment. The same motor can have different fates under different operating conditions. To ensure the bearing's performance during motor operation, a comprehensive consideration should be given to bearing selection, determining the fit, understanding operating conditions, lubrication, and maintenance.
Expanding knowledge about electric motors
1. Whether the bearing rings are rotating
When the inner or outer ring of a bearing is in operation, it is a rotating ring and should be fitted with a slightly tighter fit. The amount of interference should be such that the mating surfaces do not "creep" (commonly referred to as "ring running") under working load. Once creep occurs, the mating surfaces will wear and slip, and the higher the ring speed, the more severe the wear.
When a bearing is in operation, if its inner or outer ring is a non-rotating ring, a looser fit should be selected for ease of disassembly, assembly, and adjustment.
Different operating temperatures will cause varying longitudinal elongation of the journal or housing bore. Therefore, when selecting a fit, the principle should be to allow the bearing to move freely axially and eliminate internal stress in the support. However, excessive clearance will reduce the rigidity of the entire component, causing vibration and accelerating wear.
2. Bearing load type and matching relationship
Bearing rings bear radial loads and can be classified into three types according to the relative motion relationship between the load and the rings.
●Local load—Local load refers to the combined radial load acting on the bearing that is stationary relative to the raceway, meaning the radial load is borne by a localized portion of the raceway.
For bearings subjected to localized loads, a looser transition fit or a smaller clearance fit should be selected so that the frictional torque between the raceways of the bearings can drive the bearings to rotate, resulting in uniform stress on the bearings and extending the service life of the bearings.
●Circulating load—Circulating load refers to the combined radial load acting on the bearing that rotates relative to the raceway, meaning that the radial load acts sequentially on the entire circumference of the raceway.
For bearing cyclic loads, the races should be selected with an interference fit or a tighter transition fit. The amount of interference should be such that the races do not cause creeping between the races and the mating surfaces of the shaft or housing bore.
● Oscillating load—Oscillating load refers to the combined radial load acting on the bearing and the raceway oscillating relative to each other within a certain area. The bearing bears a radial load in a constant direction and a rotational load, and their combined radial load oscillates relative to each other within a section of the raceway of the fixed raceway.
When subjected to oscillating loads, the matching requirements are the same as or slightly looser than those for cyclic loads.
3. Bearing load and fit relationship
The minimum interference fit between the bearing race, journal, and housing depends on the load. Races subjected to impact loads or heavy loads are prone to deformation, causing uneven stress on the mating surfaces and resulting in loosening of the fit. Therefore, a tighter fit should be selected, meaning the minimum interference fit should be larger. Races subjected to light loads should be selected with a looser fit.
4. The influence of other factors on coordination
●Operating Temperature—During bearing operation, due to frictional heat and the influence of other heat sources, the temperature of the bearing race is higher than that of the parts it mates with. Thermal expansion of the inner race can cause it to loosen its fit with the journal, while thermal expansion of the outer race can cause it to tighten its fit with the housing bore. Therefore, when the bearing's operating temperature is high, the selected fit should be adjusted appropriately.
● Rotational Accuracy and Rotational Speed – For bearings subjected to heavy loads and requiring high rotational accuracy, clearance fits should be avoided to eliminate the effects of elastic deformation and vibration. However, for light-load bearings in some precision machine tools, clearance fits are often used to avoid the influence of shaft shape errors on bearing accuracy. It is generally believed that the higher the bearing's rotational speed, the tighter the fit should be.
●Conditions for installing and removing bearings—For ease of installation and removal, a looser fit is preferable, especially for large and extra-large bearings used in heavy machinery. If easy installation and removal are required but a tight fit is necessary, a separable bearing or a bearing with a tapered bore in the inner ring, a retaining sleeve, and a withdrawal groove can be used.
In addition, the bearing fit should be tightened in the following cases:
Larger bearings are better than smaller bearings;
Hollow journals are better than solid journals;
Thin-walled shells are better than thick-walled shells;
Light alloy casings are better than steel or cast iron casings;
An integral shell is better than a partial shell.
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