When consulting bearing specifications, one frequently encounters the designations Cr and Cor. What do these letter combinations signify? Cr denotes the bearing's rated dynamic load capacity, whilst Cor indicates its rated static load capacity. What precisely are rated static and dynamic load capacities? What relationship and distinctions exist between them? In design, three fundamental parameters are frequently employed for rolling bearings: the basic rated dynamic load Cr (radial) or Ca (axial) to meet specific fatigue life requirements; the basic rated static load Cor (radial) or Coa (axial) to satisfy certain static strength criteria; and the limiting rotational speed No to control bearing wear. What constitutes the bearing's rated static load?

The basic rated static load (radial Cor, axial Coa) of a rolling bearing refers to the nominal radial load or central axial static load equivalent to the maximum load at which a given contact stress occurs at the centre of contact between the rolling elements and raceway.

The rated static load is determined under assumed load conditions. For radial bearings, it denotes the radial load; for radial thrust bearings (angular contact ball bearings), it refers to the radial component of the load bearing on half the raceway; for thrust bearings, it signifies the central axial load.

In other words, the radial basic rated static load and axial basic rated static load denote the maximum load a bearing can withstand under static or rotating conditions.

The load-carrying capacity of deep groove ball bearings when stationary or rotating slowly (at speeds n < 10 r/min) is defined as the rated static load.

What is the bearing's rated dynamic load?

The rated dynamic load of a bearing denotes the constant radial load (or constant axial load) that a rolling bearing can theoretically withstand, with a basic rated life of 100,000 revolutions under this load. The basic rated dynamic load reflects the bearing's resistance to rolling fatigue. The basic dynamic load ratings for radial bearings and thrust bearings are respectively termed the radial basic dynamic load rating and axial basic dynamic load rating, denoted as Cr and Ca.

For deep groove ball bearings, the load capacity during rotation (rotational speed n > 10 r/min) constitutes the basic dynamic load rating.

The rated dynamic load refers to the load the bearing can withstand for a rated life of one million revolutions. The rated static load indicates the load the bearing can bear when stationary under that load. Why is the rated dynamic load greater than the rated static load?

The sum of the permanent deformation of the largest rolling element and the raceway at their contact point is one ten-thousandth of the rolling element's diameter.

Bearing service life correlates with rated load capacity. Under what circumstances should bearing selection be based on rated load capacity? Primarily in the following situations:

Note: In certain cases, bearing dimensions should be selected based on rated static load (designated as Co for descriptive convenience) rather than calculated bearing life.

1. When the bearing is stationary under continuous or intermittent impact loads.

2. When the bearing undergoes only slow reciprocating or adjusting motion under load.

3. The bearing operates at very low rotational speeds (less than 10 revolutions per minute) under load and requires only a short service life. It is particularly important to note that under these conditions, for a given equivalent load P, the life formula yields a very low required basic dynamic load rating C. Consequently, the selected bearing may be severely overloaded during actual operation.

4. In addition to normal operating loads, the bearing must withstand heavy impact loads whilst rotating.

In all the above scenarios, the load capacity of the bearing is not determined by material fatigue, but rather by the extent of permanent deformation caused to the raceway by the applied load.