Wind Turbine Design Guideline DG03: Yaw and Pitch Rolling Bearing Life

Abstract

These guidelines are intended to be a design aid for wind turbine designers. The complete list of guidelines is provided below. • DG01 Loads Analysis • DG02 Strength Analysis • DG03 Yaw and Pitch Rolling Bearing Life Design Guideline • DG04 Gearbox Specification • DG05 Control and Protection System Specification and Design All of these documents-with the exception of the present document (DG03)-are in draft form and are available only by request sent to NREL, Sandy.Butterfield@NREL.Gov. Modern wind turbines use large turntable bearings at the root of each blade to enable pitch angle changes and thus aerodynamic performance and load control. Yaw bearings are used for angular realignment of the nacelle into the predominant wind direction. These applications require long periods in nearly stationary positions with large stochastic loads. Due to this demanding load environment and the fact that bearings exist in the critical load path, their design becomes critical to the safety and reliability of most turbine designs. This document attempts to introduce modern bearing-design practice and its relation to the unique requirements of wind turbine applications. The fundamental theory presented here is available in other textbooks referenced in this document, and included in the References section found at the end of this report.

Wind Turbine Tribology

“…Additionally, both pitch and yaw bearings experience similar types of loading-namely, an eccentrically applied thrust, which can be divided into a pure axial load and an overturning moment load [81]. As a result, the fatigue lives of both pitch and yaw bearings can be estimated using a single life rating predictive approach.…”

“…Because pitch bearings operate under slight oscillatory motion, fretting wear and/or false brinnelling can occur when the lubricant film is insufficient. Additionally, a report by the National Renewable Energy Laboratory (NREL) suggested the pitch bearing be periodically (i.e., once per day) rotated through a largeamplitude oscillation cycle in order to redistribute grease that has been previously displaced [81].…”

“…18.23). Doublerow bearings have been shown to have the advantage of lower Hertzian stresses and increased fatigue life; however, the double-row arrangement incurs higher manufacturing costs than that of single-row bearings [81].…”

“…If the speed of pitch and yaw bearings is denoted in terms of oscillations per minute, the fatigue life, expressed in terms of millions of oscillations, is calculated as [36,81]:…”

Effect of raceway geometry parameters on the carrying capability and the service life of a four-point-contact slewing bearing

“…where α o is the initial contact angle, D is the average diameter, Z is the number of rolling balls and H, I, J are coefficients modified by the users [1][2][3]. The previous models are based on an over simplified contact load distribution theory, which supposes that the actual contact angles are constant and always equal to the initial contact angles.…”