Yaw and Pitch Rolling Bearing Design Types

Modern wind turbines use large slewing rings 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.
Large wind turbines (those rated at more than 250 kW) use ball or roller bearings with special configurations for blade retention pitch bearing and yaw bearing locations. The bearings consist of two ring-rolled forgings forming the outer and inner raceways and a complement of either balls or rollers. The inner and outer continuous-ring forgings have mounting holes that allow the bearing to be bolted directly to the supporting structures. The balls or rollers are inserted into the bearing through a radial cylindrical hole in one of the rings. The hole then is closed using a removable loading plug con-toured to the ball path or roller path surface. 
It is common practice to cut a spur gear integral with one of the bearing rings, especially for the yaw bearing application. Individual plastic spacers or thin section cage arc segments are used to separate the balls. The spacers are cylindrical with a hemispherical end. The individual rollers in the cross-roller bearing are separated by plastic, saddle-shaped spacers. The rollers in the cross-roller bearing alternate in their orientation to carry load.
The inner and outer rings are hardened from 250 to 300 Brinell hardness (HB). This is referred to as the “core hardness” of the ring. This core hardness should provide adequate core yield and fatigue strength, yet remain at a hardness low enough to facilitate machining of the rings, gear teeth, and mounting-bolt holes. The actual ball or roller path (rolling contact surface) is induction heated, quenched, and tempered to provide a hard surface or “case.” The surface hardness of the raceway is a minimum of 58 HRC (Rockwell C scale hardness). The depth of the hardened case is defined as the depth to a hardness of 50 HRC.
The two-row, eight-point contact ball bearing type is more costly to manufacture than the single-row, four-point contact ball bearing. In addition to having a second row of balls and separators, the two-row bearing must be repeatedly assembled and disassembled during manufacture to accurately measure and match the internal diametral clearance or preload of the two ball rows.
The main advantages of the eight-point contact ball bearing, as compared to the four-point contact ball bearing, are:
• Lower ball loads;
• Lower Hertz stresses;
• Less required case depth; and
• Increased fatigue life.