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.

Bearings selection and calculations for wind power plants

The demands placed on bearings for wind power plants are extremely high. Therefore, the procedures for bearing selection and lifetime calculations are fundamentally different than selecting bearings in other application areas.
When selecting bearings for industrial use, design engineers can choose bearings for a defined bearing location relatively easily, working together with their suppliers if necessary, and basing their decisions on parameters such as rotary speed and torque, radial and axial load, and the required rigidity and preload or clearance. 
In wind power technology, the procedures leading up to the selection of a bearing are fundamentally different, essentially because of a single parameter: the bearing lifetime. The manufacturers of wind power plants and the gears for those plants require a bearing lifetime of 20 years (i.e. 175,000 hours) for their bearings. This is already an extreme value, but it does not cover all requirements. In addition, it must be taken into account that external loads acting on the bearing are highly dynamic (that is, irregular) because of the wind. The unfavorable environmental conditions for offshore facilities, used ever more frequently, are another factor: sea air is highly corrosive. For generator plants, electrical corrosion must also be taken into account. Finally, the service options available to wind farm operators are extremely limited, and any unscheduled downtime for a bearing will result in significant costs.

UWE heat treatment process introduction

1. Heat treatment is the key process for UWE manufacturing,100% inspection for all process.Detection adopts advanced magnetic particle inspection equipment to ensure the quality of key working procedure processing effectively controlled.
2. Using medium frequency induction heat quenching technology.Equipment was purchased from Shanghai Heatking Machine-electrical equipment Co., Ltd., induction arc automatic quenching, working with electric heating well type tempering furnace, using special water-soluble quenching agent (sodium nitrite + organic polymer).
(1) induction heating is to use electromagnetic induction to produce eddy current and the workpiece in the artifacts and localized heating (electrical energy into heat energy).Induction heating surface quenching has the following advantages compared with common quenching: heat source on the workpiece surface, fast heating, high thermal efficiency,small deformation because of non whole heating;short artifacts heating time, little surface oxidation decarburization;High surface hardness, small notch sensitivity, impact toughness, fatigue strength and wear resistance are greatly improved;Conducive to play the potential of materials, save material consumption,improve the service life of the parts;compact equipment,easy to use,good working conditions.
(2) water soluble quenchant is light yellow transparent liquid, with anti-rust, anti-corrosion, wetting, sterilization and other additives, has inverse soluble in water.It overcomes the quick water cooling speed, easy cracking of components, slow oil cooling speed, poor hardening effect and inflammable shortcomings and so on.Water soluble quenchant is more stable than water quenching, more secure than oil (not quench oil inflammable), generally use concentration from 17% to 23%, and non-toxic, no lampblack, uniform quenching hardness,deep hardening layer, and is not easy to aging, deterioration, long service life.

Slewing Ring Bearing Overhauling

Revolving inspection is necessary to check correct installation after the slewing ring is intalled. For small size machinery, we can  revolve the slewing bearing by hand, to ensure whether can run smoothly. Inspection items include revolving not smoothly due to foreign objects, bruises and indentation; moment force unstable due to unproper installation and unproper processing of support; over moment force due to backlash too small, installation error and sealing friction. If there is no abnormal status, dynamic run of the slewing bearing can be proceeded.

UWE slewing ring for excavators

UWE excavator slewing bearings/slewing rings have a high load carrying capacity, a versatile range of applications and are highly cost-effective. Due to their design, a single bearing can reliably support radial, axial and tilting moment loads. It is therefore possible in many cases to replace bearing arrangements comprising a combination of radial and axial bearings by a single bearing.

This reduces, in some cases considerably, the costs and work required in the design of the adjacent construction and the fitting of bearings. It is made up of mounting holes, inner gear or outer gear, grease hole and sealing device. It has many positive characteristics; compact structure, light weight, good rigidity, steady speed and high precision.

UWE can offer a wide range of  excavators, like  Komatsu, Hitachi, Kobelco, Caterpillar, Hyundai and so on. Small size four-point contact ball slewing bearings often used for this application.Single row four point contact bll bearing is composed of two seatings. This type slewing bearing ring/slewing bearing/slewing ring features compact in design,light in weight, the balls contact with the circular raceway at four moment may be simultaneously. 

UWE bearing-"one hundred days no accident"

In order to continue to pursue and achieve high quality slewing ring objectives, raise the quality and safety awareness of employees, enhance cohesion and competitiveness in the market, the company  has held a working sessions about “ one hundred days no accident” in our conference hall. UWE president  Wang Jianjun, and relevant department heads, and other shop workers attended the meeting.
Conference topic was "improving the quality awareness, improve quality standards to ensure safe production, improve safety awareness, and promote sound and rapid development". The meeting pointed that the aim is to improve the quality and ensure safety in production and increasing the training of staff, and strengthen the assessment about production discipline supervision, implementation of safe production and strengthen production safety management.
Advantage of the activities to enhance employee safety awareness, constantly improve production quality in the case of safe production, promote the prosperity and development for the company.


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