**1. Introduction**

Wind turbines are typically designed for a minimum 20-year life. However, failure of the main bearing of the turbine after only a few years, perhaps 5 years, can involve the immense expense of dismantling, lowering, transporting away for repair or replacement, raising, and re-installing the new bearing. These costs are enhanced for off-shore turbines and threaten the economic case for wind energy.

It is important therefore that bearings are reliable. The engineering involved in the modern bearing designs, optimised by computer simulation, and manufacture involving precision machining ensure an extremely high standard of 'designed in' reliability. The steel is also held to within close limits of its chemical specification, generally based on the composition 1C–1.5Cr, the typical 'carbon chrome' steel which is widely used for ball and roller bearings. The use of this bearing steel for over 70 years or more has generated an optimised material backed up by an immense volume of development and production experience.

It is a source of surprise and disappointment, therefore, despite all this vast accumulation of knowledge and experience, bearing failures of large turbines still occur prematurely.

A higher strength steel, with an unusual structure of lower bainite, has more recently been available; it is hoped that this improved material will provide greater reliability and longer life. Experience with its longevity should be emerging over the next few years, so it is too early to include the new bearings in this report. This chapter concentrates, therefore, on the known behaviour of the carbon-chrome steel bearings. Even so, it seems likely that the proposals in this chapter will also benefit the low bainitic steels in due course.

This chapter draws attention to the universally neglected role of the casting process in the behaviour of the steels. Unfortunately, defects are introduced by the casting process which can be sufficiently serious to dominate the failure mechanism of the steel. This widely overlooked effect is considered in detail. The mechanisms cited in this chapter are described in more detail by the author elsewhere [1–3].
