Gearbox Bearing Service Life: A Matter of Mastering Many Design Parameters

The service life of a gearbox is determined by many factors. Bearings, for example, play a major role since they contribute an important function while also interacting with the shafts, casing and oil. Without a doubt, the sizing of the bearings is of great importance in gearbox reliability. For over 50 years, bearing dynamic carrying capacity has been used to determine a suitable size needed to deliver a sufficient fatigue life. But despite the existence today of advanced calculation methods, they do not fully predict service life. Producers of high-quality bearings have introduced better ways to express (quantify) improved performance, but only in terms of increased dynamic carrying capacity. This article will cover the following: • Sizing of bearings based on dynamic carrying capacity and how this relates to service life • How the design of the interface between bearing and shafts should be adapted to modern shaft materials • How the design of the interface between bearing and gearbox casing influences service life of the gearbox • Influence of modern electric motor speed controls on bearing-type selection Sizing of rolling element bearings based on dynamic carrying capacity. For modern high-quality bearings, the classic basic rating life can deviate significantly from the actual service life in a given application. Generally speaking, service life in a particular application depends not only on load in relation to bearing size, but also on a variety of influencing factors, including lubrication, the degree of contamination, misalignment, proper installation and environmental conditions. The first method accepted by ISO for determining a suitable bearing size is the classic Lundberg and Palmgren equation L10 =(C/P) P , making it possible to determine a suitable dynamic C-value (which in turn defines the bearing size) needed to satisfy a need for a fatigue life L10.

The influence of material properties and lubricant film thickness was introduced in the 1960s, represented by the a23 factor (Fig. 1). (1) L10 =a23 (C/P)P [a23 is a function of k and the material] As an attempt to take some of those factors into account when determining a suitable bearing size, the DIN ISO281:1990/AMD2:2000 contains a modification factor aSLF to the basic rating life L10 =(C/P) P . The method makes provision for bearing manufacturers to recommend a calculation methodology for this life modification factor to be applied to a bearing based on operating conditions. Some life modification factors apply the concept of a fatigue load limit Pu analogous to that used when dimensioning other machine components. Furthermore, the life modification factor makes use of the lubrication conditions and a factor ηc for contamination level to reflect the application’s operating conditions.