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With dozens of types of bearings and hundreds of thousands to choose from, selecting the correct bearing for your application can be a daunting task.
However, if the right factors are carefully considered, choosing the right bearing becomes much easier. That being said, performance issues and unexpected events can arise, causing a reassessment of bearing selection. In these instances, solutions might range from maintenance to the use of a new bearing, or the complete redesign of the application system.
Generally speaking, the following factors should be considered in the following order: size restrictions, precision requirements, bearing load, operating speeds, durability, and maintenance issues.
The space allowed for bearings is usually rather limited. In many instances, shaft diameter or bearing bore diameter is established based on design requirements. Therefore, a bearing’s type and dimensions must fit established specs.
For instance, the bearing used for a dentist’s drill must obviously be a lot smaller than the bearing used in an oil rig.
The second big consideration when selecting a bearing is precision. Some applications, like bearings for a CNC machine, will require a high degree of precision. In other applications, like in the wheels of a shopping cart, precision won’t be a major concern.
Flex, housing variations, and fabrication mistakes can result in some degree of misalignment. In instances where the level of misalignment is fairly large, bearing failure can occur due to an inability to tolerate the misalignment. Ball bearings or bearing units with self-aligning qualities are the correct choices when misalignment is a significant issue.
The amount of load that bearings must sustain will have a big impact on what type of bearing should be used for a particular application. Moreover, the various loads managed by a bearing can significantly affect on-going performance and for that reason must be carefully assessed during the selection process.
Some bearings must handle radial loads, which act perpendicularly to the axis of rotation. Some bearings must handle an axial load, which acts parallel to the axis of rotation. If the load on a bearing is offset from these axes, it creates a moment load.
The qualities, degree, and direction of acting loads can be highly varied. Generally speaking, the fundamental load ratings indicated in bearing dimension tables show load capacity. Yet, in deciding the correct bearing type, thought should also be given to load type.
For instance, if ball and roller bearings of relatively similar dimensions are being assessed, roller bearings should be considered for their bigger load capacity and greater capability to tolerate both vibration and shock loads.
Every particular bearing has specified speed limits, all established from empirical testing conducted across a wide array of conventional applications. The allowable speed can differ based upon bearing size, type, degree of the load, lubrication, and temperature.
A key factor in a bearing’s high-speed performance is the kind and amount of lubrication that has been applied. In general, deep groove ball bearings and roller bearings are best for high-speed uses.
There are a number of factors to consider with respect to durability, including rigidity and temperature tolerance. Under loading, deformation along a bearing’s contact surfaces can occur and deformation can be a serious issue in some applications. From the sub-zero temperatures that an Alaskan oil rig must handle to 100-degree deserts of Saudi Arabia, the range of operating temperatures that a bearing will tolerate should also be a major selection concern.
Some bearing applications call for regular disassembly and reassembly to allow for inspections and maintenance. In these applications, bearings with separable rings are the most suitable.
The importance of thorough lubrication cannot be understated when it comes to prolonging and supporting the performance of any bearing type. Too much lubrication, too little lubrication, contamination, and combined lubricants can all have an effect on bearing performance. It’s also crucial to think about “grease service life,” the quantity of time a bearing can properly perform between lubrications.
Sources and Further Reading