Rolling bearings in rotating machinery support shaft loads, reduce friction with rolling elements, and provide shaft location and system rigidity. Whether a bearing will be suitable for an application will hinge, in part, on the particular application requirements for the bearing and the bearing’s design features and associated capabilities. When specifying bearings, designers must consider and evaluate a myriad of application-related factors to realize the most proper match. Here is an overview of some of the more important factors to consider when selecting a rolling bearing for an application.
Available space (design envelope)
In many cases, one of the principal dimensions of a bearing – the bore diameter – is predetermined by a machine’s power design and the resulting required shaft diameter. In general, small-diameter shafts typically incorporate all types of ball bearings, while roller bearing types usually will be specified for applications featuring larger-diameter shafts. When radial space is limited, bearings with a small cross-section – especially those with a low cross-sectional height – can be a good fit. When axial space is limited, designers will want to specify bearings that can handle radial, axial, or combined loads.
Bearings must always be subjected to at least a given minimum load to provide for proper rolling element rotation and enhanced lubricant film formation in rolling contact areas. The magnitude of the load is one of the factors that usually will determine the type and size of the bearing for an application. Generally, roller bearings can support heavier loads than similarly sized ball bearings, and bearings incorporating a full complement of rolling elements can accommodate heavier loads than corresponding caged bearings. Ball bearings are mostly used where loads will be relatively light or moderate. For heavy loads and where shaft diameters are large, roller bearings typically will be recommended.
The direction of the load also will come into play in selecting bearing type and size. Some bearings can only support pure radial loads, while all other radial bearings can accommodate some axial loads in addition to radial loads. Other types have been engineered to handle purely axial light or moderate loads.
The speed limits for a bearing are governed by the specific design and material of the bearing components and the permissible operating temperature for the type of lubricant and lubrication system being used. Bearing type and size, internal design, precision, loads, lubrication regimens, and cooling conditions – as well as cage design, accuracy, and internal clearance – will combine to establish the speed capability of a bearing. Basic thermal reference speeds will provide designated values (according to ISO standards) to determine the permissible operating speed of a bearing at a defined operating temperature when subjected to specific loads and lubrication conditions. Bearings can potentially operate at speeds above the reference speed when bearing friction is reduced (using lubrication systems dispensing small, accurately measured quantities of lubricant) or when heat is removed (using circulating oil lubrication, cooling ribs on the housing, or with directed cooling air streams). In some cases, changes in component designs and materials can yield even higher permissible operating speeds.
The stiffness of a bearing is similar to the stiffness of a spring and is characterized by the magnitude of the elastic deformation (resilience) in the bearing under load. In general, this deformation is small and can be neglected. In some cases, however (such as spindle bearing arrangements for machine tools or pinion bearing arrangements in automotive axle drives), stiffness becomes a critical factor. In general, the contact conditions between rolling elements and raceways in roller bearings provide a higher degree of stiffness compared with similarly sized ball bearings. Bearing stiffness can also be affected by preload.
A bearing preload will enhance system stiffness and provide other advantages by reducing running noise, promoting the accuracy of shaft guidance, compensating for wear and settling (relaxation) processes in operation, and contributing to longer service life. A preload basically imposes a “negative operational clearance” in a bearing arrangement to achieve these benefits. Depending on bearing type, the preload may be either radial or axial. When bearings operate without any load or under light loads and at high speeds, preload should be imposed to provide a minimum load on the bearing to prevent bearing damage that may result from sliding movements of rolling elements. Preload can be accomplished in several ways, including the use of springs, washers, friction torque, or adjustment procedures, among others. Application parameters must be clearly understood when specifying preloaded bearing arrangements as the increased friction resulting from a preload arrangement can lead to higher bearing operating temperatures.
Commonly specified arrangements include locating and non-locating, adjusted, or floating versions.
The locating bearing at one end of the shaft provides radial support and simultaneously locates the shaft axially in both directions, carrying any axial loading on the shaft. The locating bearing must be fixed in position both on the shaft and in the housing. Suitable bearing types include radial bearings that can accommodate combined load or combinations of radial bearings that can support pure radial load. The non-locating bearing at the other end of the shaft provides radial support and must also enable axial displacement (not carry axial load) to prevent the bearings from mutually stressing each other (such as when the shaft length changes due to thermal expansion). Designers can choose from a wide range of locating/non-locating bearing combinations to achieve specific performance objectives consistent with application demands.
With adjusted bearing arrangements the shaft is located axially in one direction by the one bearing and in the opposite direction by the other bearing. This type of arrangement is referred to as “cross-located” and will generally be used for short shafts. Suitable bearings include all types of radial bearings that can accommodate axial loads in at least one direction. Floating bearing arrangements are similarly cross located and usually specified where demands regarding axial location are moderate or where other components on the shaft serve to locate it axially. In these arrangements, one ring of each bearing should be able to move on or in its seat, preferably the outer ring in the housing.
Filed Under: Motion Control Tips