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.

Bearing Design Criteria-wind turbine slewing ring

The proper design of a yaw or pitch bearing must satisfy five design criteria and the miscellaneous considerations listed below. Each of the following criterion is addressed in detail in this guide.
1. Bearing fatigue life (rolling contact fatigue)
2. Bearing static capacity
3. Adequate case depth and core hardness
4. Adequate lubrication (surface failure)
5. Friction torque
6. Miscellaneous
A. External bolting
B. Cages or separators
C. Integral seals
The relationships used in the method for determining the basic dynamic capacity in oscillation, life adjustment factors, and equivalent load are supplied in Section 4.

Effective Lubrication for Wind Power Bearing

A meaningful oil film thickness cannot be generated in a slowly and intermittently moving (oscillating) grease-lubricated yaw or pitch bearing. Therefore a clean grease with good boundary lubrication additives (especially for oscillating conditions) should be selected on the basis of experience for use in wind turbine yaw and pitch bearings. A seal system (integral or external) also is essential for achieving satisfactory operation.
A fretting-corrosion type of raceway and rolling element surface failure commonly is encountered in yaw and pitch bearings. The fretting corrosion appears as elliptical or rectangular footprints at ball or roller spacing in the bearing. The markings are tiny corrosion pits caused by the lubricant being forced out of the contact area (by a small load increase) and then not being able to re-enter the contact zone. The unprotected surface then is subject to corrosion pitting. Most grease rated for oscillation use can coat the rolling contact surfaces and maintain corrosion protection. In extreme cases, coating the raceways is an option. A TDC coating increases the bearing cost significantly but provides increased protection.
One manufacturer of wind turbine yaw and pitch bearings suggests using the Hertz contact stress limits given in Table 16 as means to limit fretting-corrosion types of failures.

Wind Power Generator Slewing Ring Cages or Separators

Most yaw and pitch bearings are supplied with plastic spacers between each rolling element. A four-point contact ball bearing has cylindrical spacers with hemispherical indented ends. The cross roller bearing has plastic saddle-type spacers, which conform to the two adjacent rollers with axes of rotation 90 degrees apart. The ball spacers for larger balls (50-mm diameter and larger) often have a steel-plate reinforcement cast into the plastic.
Use of segmented cages is very rare for yaw and pitch bearings. The opening that the cage requires between the raceways significantly reduces the available load-carrying ball path. The bearing manufacturer should be consulted about spacer design. The spacer material also must be compatible with the selected lubricant.

Wind energy Slewing Ring Integral Seals

Wind turbine yaw and pitch bearings are usually supplied with integral rubbing lip seals. The bearing manufacturer should be consulted about materials, design, and placement of the integral seals. The seal material must be compatible with the selected lubricant.

Small wind turbines

Small wind turbines may be used for a variety of applications including on- or off-grid residences, telecom towers, offshore platforms, rural schools and clinics, remote monitoring and other purposes that require energy where there is no electric grid, or where the grid is unstable. Small wind turbines may be as small as a fifty-watt generator for boat or caravan use. Hybrid solar and wind powered units are increasingly being used for traffic signage, particularly in rural locations, as they avoid the need to lay long cables from the nearest mains connection point. The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) defines small wind turbines as those smaller than or equal to 100 kilowatts.Small units often have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind.
UWE bearing provide yaw, blade/pitch bearing and yaw bearing to our customers reliably. UWE wind turbine generator bearing is for 3KW-5MW wind turbine.Each wind turbine generator comprises of one yaw bearing and three pitch bearings, the material of the rolling rings is 42CrMo with quenching and tempering. Raceway surface intermediate frequency induction hardened. Due to complexity of the stress situation, the bearing must bear impact and carry high load. 20 years is a required lifetime for the wind turbine generator, it is also required by yaw bearing and pitch bearings as the mounting cost is too high.

UWE Slewing Rings for renewable energies

UWE slewing ring has specialized exclusively on supplying system manufacturers in the renewable energy sector.
UWE slewing ring manufacturers of single-row and double-row ball slewing rings used as blade and yaw bearings, as well as for azimuth gear rims to a diameter of 4500 mm.
These are used for onshore and offshore wind turbines in the 100 kW to 7MW range. Roller slewing rings from UWE slewing ring are in use as single main bearings for gear- and shaftless wind turbines.