When selecting a rolling bearing, two complementary factors are usually in the foreground:

• the desired service life of the rolling bearing in the application as a result of the relationship between the basic dynamic load rating and the forces occurring, their direction(s) of action and the speed,
• the size of the rolling bearing in relation to the available installation space in the application.

There are other function-critical bearing features that are not always considered with the necessary attention and can therefore lead to problems. These include the internal clearance, the cage, the lubrication and the sealing variants offered, particularly in the case of radial deep groove ball bearings. The following is an overview of these bearing characteristics and associated issues.

The nominal internal clearance is the dimension by which the two rings of a bearing can be moved from one end position to the other when not installed. For most radial rolling bearings, the radial displacement or radial internal clearance is specified, while for paired or double-row angular contact ball bearings and tapered roller bearings, the axial internal clearance is specified. The bearing clearance is summarized in bearing clearance classes that are standardized for standard radial bearings.

Important parameter: the internal clearance

The bearing clearance influences the function and performance of the rolling bearing and, with the exception of special applications, it can be assumed that a rolling bearing must have an operating bearing clearance, the so-called operating clearance. This operating clearance results primarily from the selected bearing clearance class in conjunction with the shaft and housing fits required for the application, the materials used and their coefficients of thermal expansion as well as the temperature difference between the shaft and housing during operation. As a rule, these factors lead to a "constriction" of the bearing, which reduces the nominal internal clearance and can lead to unintentional preload. As this results in considerable friction losses, additional heat generation and often premature bearing failure, the operating clearance should be considered more closely when selecting bearings.

Generally, manufacturers supply rolling bearings with a standard cage that is suitable for "normal" applications. A thorough check of the cage design is advisable under the following operating conditions:

• operating temperature. For example, the application limit of PA6.6 plastic cages is reached at a maximum operating temperature of 120 °C. In addition, metal cages or cages made of special plastics are required.
• High accelerations and low loads. In these cases, depending on the bearing design and size, lightweight plastic or sheet steel cages or ring-guided cages should be used to prevent deceleration or blocking of the rolling elements due to weight-related slip damage.
• This also applies to applications that are exposed to shocks, vibrations and high centrifugal forces.
• Lubrication situation. In general, rolling element-guided cages must be used for grease lubrication, as the formation of the necessary lubricating film in the gap between the cage and ring is not guaranteed with ring-guided cages. Oil-lubricated bearings are not subject to these restrictions. However, when using fully synthetic oils in particular, care must be taken with regard to compatibility, especially with engineering plastics and rubber.

The influence of lubrication

The selection of a correct lubricant and the classification of the performance and service life of standard lubricants used in pre-greased and sealed radial deep groove ball bearings is often underestimated. On the one hand, the best possible lubricating effect must be ensured. On the other hand, every lubricant is subject to ageing depending on the operating conditions, whereby its performance and service life are typically exhausted before the rolling bearing. The operating temperature is of decisive importance here.