A combined load occurs when radial and axial loads act simultaneously. The most common bearing solutions for these situations are tapered roller bearings and single- and double-row angular contact ball bearings (although deep groove ball bearings may also be appropriate, depending on the load magnitude and ratio of axial to radial loading). When the axial component of combined loads proves especially large, it may be supported independently from the radial load by a separate bearing. In addition to thrust bearings, some radial bearings (such as deep groove ball bearings or four-point contact ball bearings) can handle this task.
When a load acts eccentrically on a bearing, a tilting moment will occur. For these "moment" loads, designers may want to consider double-row bearings. Suitable standard options include deep groove angular contact ball bearings, paired single-row angular contact ball bearings, or tapered roller bearings (arranged face-to-face or back-to-back).
Misalignment. Angular misalignment between the shaft and housing of rotating equipment can occur when the shaft bends (or flexes) under the operating load, when the bearing seats in the housing are not machined to the same height, or when shafts are supported by bearings in separate housings too far apart. Rigid bearings (such as deep groove ball bearings, tapered roller bearings, and cylindrical roller bearings) can go only so far to counteract misalignment (and even then only very minor misalignment).
To compensate for misalignment, various types of self-aligning rolling bearings -- including spherical roller bearings, toroidal roller bearings, and self-aligning ball bearings -- provide solutions, whether misalignment is the result of operating loads or errors in machining or mounting. Designers should consult bearing misalignment value ranges available from the bearing manufacturer during the selection process.
Precision. High-precision bearings most often are specified when arrangements must exhibit high running accuracy (such as in machine tool spindles or robotics) and for very high-speed applications. (Dimensional and running accuracy tolerances of rolling bearings have been standardized internationally). Tolerance classes to which bearings are produced offer designers a reliable blueprint for specifying the proper high-precision bearings for an application's requirements.
Speed. The speed limits for a bearing will be governed by the specific design and material of the bearing components and the permissible operating temperature for the 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 to cool the oil either with 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.
Stiffness. 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 very 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 ball bearings. Bearing stiffness is also affected by preload.