Over the years, engineers have estimated the life of ball and roller bearings with the aid of industry standards and guidelines. These include the ANSI/AFBMA standards for predictin g the life of ball bearings (1950) and roller bearings (1953), which were updated in 1978 and 1990, plus the ASME rolling-element bearing life factors (1971). These standards were published by the American National Standards Institute (ANSI) and the Anti-Friction Bearing Manufacturers Association (AFBMA), which is now the American Bearing Manufacturers Association (ABMA). The American Society of Mechanical Engineers (ASME) was the first to publish life factors to compliment these standards.

Recently, the Society of Tribologist and Lubrication Engineers (STLE), with the aid of rolling-element bearing experts, took these standards and guidelines one step further by developing new bearing life factors that further reflect improvements in steel processing, manufacturing, design analysis, and lubrication over the last 50 years. These improvements have gradually increased bearing life to a level that is much higher than that predicted by both the original industry standards of the 1950s and the 1971 ASME life factors. These new bearing life factors, along with the manufacturing advances, are described in the book STLE Life Factors for Rolling Bearings, 1992.

Better steel

Research in steel metallurgy and processing has significantly improved bearing life over that obtained in the 1940s and reflected in standards of the 1950s. As summarized in Figure 1, here are the major manufacturing advances that contributed to improved life.

Heat treatment. Developments in bearing steel manufacturing began in the early 1940s with heat-treating equipment that incorporated improved temperature controls. Plus, the use of neutral atmospheres during heat treatment eliminated, for practical purposes, surface decarburization.

Bearing material research at the NASA Lewis Research Center beginning in the late 1950s culminated in the discovery of the differential hardness principle, which shows that differences between rolling element and race hardness significantly affect bearing fatigue life. For AISI 52100 steel, as an example, optimum life is achieved when the ball or roller hardness is 1 to 2 points Rockwell C higher than the races.

Melting. Major advances in melting practice evolved from 1952 to the early 1970s. Vacuum degassing and vacuum melting processes started in the late 1950s. One such process, vacuum-arc remelting (VAR), releases entrapped gases and alters the type of inclusions and trace elements in the steel so they are less likely to form failure initiation sites. At the same time, Pratt & Whitney Aircraft Div. of United Technologies started using AISI M-50 steel, which has higher operating temperature capabilities for aircraft engine bearings.

In the 1960s, argon atmosphere protection of the molten steel during teeming (pouring) substantially improved micro and macroscopic homogeneity and cleanliness, thereby reducing fatigue failures.

Testing. With the advent of vacuum processes, non-destructive evaluation (NDE) testing methods (eddy current and ultrasonic) were applied to billets, bars, and tubing to ensure the cleanliness of bearing steel.

Finishing. Before the 1950s, workers hand-polished as-ground bearing races to improve finish and appearance. But overly aggressive hand polishing created a thin layer of plastically displaced or smeared material that was softer and more prone to fatigue failure. In 1964, manufacturers replaced this manual process with mechanized honing, which finishes parts more uniformly.

Metalworking. As the need for bearing steel increased, manufacturers installed electric arc furnaces that produced larger billets. These billets had to be mechanically worked to reduce them to the size of smaller cross-section tubing or cylindrical forgings. This working process refines the grain and grain carbide and reduces the size of inclusions and segregates in the material.

A NASA Lewis Research Center study in 1958 led to the introduction of forged races with controlled fiber orientation in 1963, extending the life of angular-contact ball bearings.

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