Bearing Clearance Selection
Bearing clearance, also known as “running clearance or interference”, is the space or gap between the inner and outer race of a bearing that determines the bearing’s ability to move within its housing. When specified correctly, bearing clearance can help minimize vibrations, noise, friction and heat production.
It is essential that the appropriate bearing size and clearance be chosen based on the required application. As such, it is important to understand the types of clearances available, the factors that influence bearing clearance selection, and the design components of bearings that are affected by clearance selection.
Types of Clearances
In order for bearings to operate effectively and efficiently, the races of the bearing must roll against each other without bearing destruction. When considering radial bearings, two types of clearance can be selected: caged clearance or seal gap.
Caged clearance
Most radial bearing manufacturers sell caged bearings, which is when the inner and outer race of a bearing are pre-assembled with a minimal preload. As a result, most caged bearings operate with a minimal rolling element torque, as can be seen in Figure 1.
In order to obtain ideal running clearances, the caged bearing-to-shaft and housing fits must be completed first. Generally, running clearance should fall within the ranges of .0002” to .0006” (for light load applications) and .001” to .004” (for heavier load applications).
Seal gap
This is when the races of the bearing remain loose and have minimal contact with each other until the shaft and housing are assembled, as can be seen in Figure 2. When properly sealed, seal gap bearings have less noise and friction as well as superior performance over long periods of time (in comparison to caged bearings).
Factors Influencing Bearing Clearance Selection
There are several factors that influence the design of a bearing, selection of the appropriate clearance, and the performance of the bearing. These factors include load capacity, bearing load direction, speed, temperature, and contamination.
Load capacity – The load capacity of a bearing is the maximum load that a bearing can accommodate without performing to its failure maximum. When load capacity guidelines are exceeded, a bearing’s life span is drastically reduced. For example, if the bearing load capacity guidelines are exceeded, the bearing may fail due to excessive heat production and accelerated wear.
Bearing load direction – The bearing load direction is a major factor as it alters the performance of the bearing. A bearing’s load can be in any direction (radial, thrust, or axial). For instance, when the load is radial on a bearing, the load actuates against the rolling elements, the retainer, the inner race, and the outer race.
Speed – The operating speed of a bearing is determined by the internal and external factors of its application. High speed applications tend to reduce bearing life as the bearing experiences increased friction, heat production, and wear.
For example, if a bearing is used for a high-speed application and does not have a high-speed grease, then the added friction would cause the bearing to fail. In addition, high-speed applications may require a narrower range in bearing clearance as this would prevent wide load losses.
Temperature – All bearings experience an increase in temperature when they are exposed to increased load, speed, impact, and catastrophic failure. In regards to a bearings temperature, there is a limit of 25 degrees Celsius higher than ambient temperature that a bearing should not exceed. The bearing temperature limit is determined by the bearing load, bearing size, bearing speed, and operating environment.
Contamination - Bearing contamination is when debris (such as dirt, dust, metal, lubricants, and water) are present within the bearing and alter the bearings ability to perform as expected. Contamination can alter the bearings performance and reduce its life span. In order to minimize contamination, it is essential to have the appropriate sealing products installed, such as a filled or plated seal.
Design Components of Bearings Affected by Clearance Selection
When selecting the appropriate clearance for a bearing, there are several design components that need to be taken into account.
Retainer – The retainer of a bearing guides and separates the rolling elements. If the retainer is too loose, the bearing will be prone to more vibration, noise, and friction. On the other hand, if the retainer is too tight, the bearing will experience bearing destruction due to excessive heat production and wear.
Rolling elements – The rolling elements of a bearing roll against the races and provide the bearing with its load carrying capacity. Therefore, the rolling elements require a narrow range when selecting the appropriate bearing clearance. For example, with ball and roller bearings, the rolling elements run best with a bearing clearance of .0002” to .0006”.
Inner and Outer Races – The inner and outer races of the bearing allow the rolling elements and retainer to move freely. If the bearing clearance is not within the appropriate range, then the races will experience accelerated wear and fail prematurely. On the other hand, if the bearing clearance is too large, then the bearing will experience excessive vibration, noise, and friction.
Cage – Cages are designed to reduce bearing wear by guiding, separating, and controlling the rolling elements. If the cage is too loose, then the bearing will be more prone to excessive vibration and noise. On the other hand, if the cage is too tight, then the bearing will experience accelerated wear and fail prematurely.
Conclusion
In order for bearings to operate efficiently and at their best performance, it is essential that the appropriate bearing clearance be chosen based on the required application. Factors like load capacity, bearing load direction, speed, temperature, and contamination should be taken into consideration when selecting the bearing clearance. Additionally, all the components of the bearing; retainer, rolling elements, inner and outer races, and the cage must be considered when deciding on the bearing clearance.
