The root cause of wind turbine main bearing failures in a certain drivetrain arrangement used by many wind turbine manufacturers has become one of the costliest sources of unplanned maintenance for many wind turbine operators. Failure rates vary widely, and depend on factors such as wind turbine model, rotor diameter, main bearing manufacturer, and site wind conditions. As the root cause mechanism of the failures has become more well understood, so have means of detecting failures well in advance, along with means of reducing the number of failures and delaying their onset. The drivetrain arrangement which is prone to these failures is known as a “3 Point Mount”, and is illustrated in Figure 1.
Figure 1. 3 Point Mount Drivetrain (Source: NREL)
The configuration is called a 3 point mount because the gearbox is supported in three locations, one on the main bearing and two on the gearbox torque reaction arms. The main bearing in this configuration is known as a Spherical Roller Bearing, often abbreviated as SRB. SRBs are used successfully in many applications, and have many advantages over some other bearing types, namely a high radial load capacity, and most importantly, the ability to tolerate relatively large misalignments. However, in addition to the advantages offered by SRBs, they have some disadvantages as well. One disadvantage is that SRBs require a relatively high ratio of radial load to axial load. Another is that the design has high levels of sliding due to a phenomenon known as Heathcote slip. This means that large segments of the bearing rollers are both rolling and sliding along the bearing rings. The sliding portion of the contact results in high rates of wear, and is a risk factor in a gear and bearing failure mode known as micropitting, which is very common in wind turbine gears and bearings. These two disadvantages combine to result in the high rates of failure of SRB main bearings in 3 point mount configurations. The thrust loads on a wind turbine rotor vary with wind speed, and can be very large. By design, load in the bearing is supposed to be carried by both the upwind and downwind rows of rollers in the SRB. If the thrust load is too high, the downwind rollers carry all of the load, and the load is concentrated on the end of the roller. This condition results in micropitting damage to the bearing, which generates metallic particles. Figure 2 shows a typical example of a damaged SRB main bearing.
Figure 2. Damaged SRB Main Bearing (Source: Timken)
The metallic debris particles generated by the micropitting contaminate the grease in the bearing, and result in abrasive wear, which generates even more particles, further increasing the rate of wear, and ultimately resulting in failure of the bearing. As these failures have become more common, operators have developed several ways of detecting damaged bearings. Identifying damaged bearings in advance of complete failure can result in large cost savings. If a large site has more than one damaged bearing, they can all be replaced with a single crane visit, therefore avoiding the cost of crane mobilization for each and every damaged bearing. There are three basic methods which can be used to identify damaged main bearings:
1. Main bearing temperature trends
2. Main bearing condition monitoring data
3. Main bearing grease analysis
In addition to detection, there are steps owners can take to reduce the rates of failure of main bearings. The most important, and cost effective, step than an owner can take is to ensure that the bearing is greased frequently, and with an adequate amount of grease. Many owners have found that the amount of grease specified by the turbine OEM to be added to the bearing at each maintenance period is not sufficient, and have increased the amount of grease that they are adding to the bearing during maintenance. Some owners have also found that a grease other than what was originally specified by the OEM provides better protection to the bearing. Note that changing the amount and type of grease can have negative consequences as well, and any change must be made with the participation of the bearing and grease supplier, and such changes should be validated on a small number of turbines before the change is rolled out to an entire fleet. If a bearing has been shown to be damaged, either through monitoring of the bearing temperature data, through a grease analysis, or a bore scope inspection, a thorough flush of the grease in the bearing and a replacement with new grease has been shown to be effective in significantly extending the life of the bearing. The reason for this extension of life of the bearing is that the metallic debris generated by the damaged bearing, which causes abrasive wear, is removed from the bearing by the purge, thus reducing the rate at which the damage progresses. If a bearing has failed, and needs to be replaced, several bearing companies have developed replacements which are designed to be resistant to micropitting damage. In most cases, the replacement bearing is an SRB that has been specially engineered to improve its performance in wind turbine main bearing applications, usually through a combination of bearing geometry changes and the application of wear resistant coatings. One company however, has developed a type of taper roller bearing known as a TDI (taper double inner) that is a drop in replacement of the original SRB. The TDI bearing has a number of design advantages over the original SRB, including less sliding and skidding, and increased stiffness, which helps improve gearbox life.
SRB mainshaft bearing failures are costly, and not infrequent. However, as is typical with many wind turbine reliability issues, as the root cause factors responsible for the failures are identified, steps that owners can take to reduce the rate of failures are developed. This article provides an overview of the root cause factors responsible for SRB bearing failures, how damaged bearings can be identified well in advance of failure, and steps that owners can take to reduce the number of failures of this type.