When selecting rolling bearings for a specific application a number of factors must be taken into account: achieving the desired performance at a reasonable cost, robustness under adverse operating conditions, life expectancy, and lubrication requirements number among them. To help evaluate these factors when putting together bearing specifications, SKF recommends following this eight-stage selection process:
Performance and operating conditions
Evaluate the operating conditions in as much detail as possible, particularly the load, speed, temperature, lubricant requirements and lubricant cleanliness. These can be determined from physical and mechanical analyses of the application, or from experience with similar applications, but make sure that any assumptions made are clearly documented. Also bear in mind that operating conditions typically vary over time due to factors such as seasonal temperature changes or increased output power.
Bearing type and arrangement
A bearing arrangement supports and locates a shaft radially and axially relative to other components; two bearing supports are typically required to position a shaft. Those arrangements that are comprised of two bearing supports can be classified as locating (providing axial location of the shaft relative to the housing) and non-locating (accommodating axial displacements resulting from thermal expansion). Other arrangements include adjusted (generally used for short shafts, where thermal expansion has little effect) and floating (allowing a degree of axial shaft movement).
The size of a bearing must be sufficient to ensure that it is strong enough to deliver the required or expected life under defined operating conditions. Size selection may be based on rating life (which is a calculable quantity) or static load, the selection criteria used being dependent on the operating conditions of the bearing.
For example, where bearings are running at normal speeds, are well lubricated and not highly or peak loaded, size selection should be based on rating life. However, for bearings running at very low speeds or which are used under stationary conditions, are badly lubricated or where occasional peak loads occur, size selection should be based on static load.
Considering how and when a bearing is lubricated is possibly the most vital step in the process of specifying a bearing for a particular application. There are choices to be made: whether grease should be used, the type and its lubrication interval; whether oil lubrication would be better as it is more effective at removing heat from the bearing. Other options might include the use of automated lubrication systems or sealed lubricated-for-life bearings.
Operating temperature and speed
Estimating the operating temperature and verifying speed limitations are also critical aspects of bearing selection. Bearing operating temperature is a function of the bearing load, size, speed and lubrication conditions. This depends as much on the application design as on the bearing generated friction. Thus, the bearing, its adjacent parts and the application as a whole should all be thermally analysed.
The speed capability of a bearing is normally determined by the bearing operating temperature. However, for certain bearing types and arrangements, the mechanical limits of the bearing components may have a significant influence. The limiting speed is the maximum valid for a standard bearing execution. It should not be exceeded unless the bearing design and the application has been specifically adapted for higher speed operation.
Bearing seats on shafts and in housings and components that locate a bearing axially, have a significant impact on bearing performance. To fully exploit the load carrying ability of a bearing, its rings or washers should be fully supported around their complete circumference and across the entire width of the raceway. Bearing seats should be manufactured to adequate geometrical and dimensional tolerances, and be uninterrupted by grooves, holes or other features.
As part of the bearing selection process, when the bearing type, size and fit have been determined, additional factors must be considered to enable the final variant of the bearing to be defined. These will include, but will not be limited by, bearing internal clearance or preload; bearing tolerances; cage design and construction; the need for seals or special coatings.
Sealing, mounting and dismounting
Seals are available in a range of types (contact, non-contact and static) and are vital to maintaining the cleanliness of the lubricant and the service life of a bearing. When mounting a bearing, special tools may be required that are compatible with its type and size, and every effort must be made to avoid damaging any part of the bearing during this process. Heat application and oil injection techniques are often used to aid mounting.
Dismounting for repair or replacement is a similarly critical process and forces must not be applied that will compromise the integrity of the rolling elements. The application of heat can aid the process, in combination with mechanical pullers, or oil injection techniques can be used.
By following this straightforward eight-stage selection process, design engineers can now achieve the desired performance at reasonable cost and robustness under adverse operating conditions. Bearing performance can now be built into any application.