STATISTICAL NATURE OF BEARING LIFE
The life of rolling bearings is rated on the basis that 90 percent of a large number of bearings in particular surroundings will survive a specific life. This life is usually designated as B10, which is interpreted as a failure rate of 10 percent expected within B. However, other catalogs give the rated load for 50 percent survival B50 (median life). A commonly accepted relationship is
Median life (5) (90% life) B50 5B10
The design life of bearings varies considerably, depending on the machine and service. Table 21.1 illustrates the variations in design life depending on the machine and service.
MATERIALS AND FINISH
The most common material used in bearings is SAE 52100. It is an alloy steel of nominally 1 percent C and 1.5 percent chromium, hardened to Rockwell C58-65. Alloys such as nickel and molybdenum are used with chromium.
The bearing life depends heavily on the hardness. The average life of a bearing when the hardness is Rc 50 is about one-half of that when Rc 60.
The operating temperature of SAE 52100 and similar steels is usually held to 300°F. However, the maximum temperature for the installation is 200°F.