The failure of main bearings 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 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 that is prone to these failures is known as a three-point mount and is illustrated in Figure 1. The configuration is referred to as such 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 (SRB). SRBs are used successfully in many applications and have many advantages over some other bearing types, namely a high radial load capacity. Most importantly, SRBs demonstrate 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 three-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 the 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. 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 several damaged bearings, 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 that can be used to identify damaged main bearings. They are the following:
- Main bearing temperature trends. Damaged bearings tend to run hotter than undamaged bearings;
- Main bearing condition monitoring data. Damaged bearings can be identified using vibration data from the CMS system; and
- Main bearing grease analysis. Analysis of the size, shape and concentration of metallic particles in the bearing grease can identify damaged bearings.
There are also steps owners can take to reduce the rates of failure of main bearings. The most important, and cost-effective, measure that 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 original equipment manufacturer (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; such changes should be well validated before the change is rolled out to an entire fleet.
If a bearing has been identified as damaged, a thorough flush of the grease in the bearing and a replacement with fresh, clean grease have been shown to be effective in significantly extending the life of the bearing. The bearing’s life will be extended because 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 that 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 taper double inner (TDI), which 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 main shaft bearing failures are costly and frequent. However, as the root-cause factors responsible for the failures are identified, owners and operators can take steps to reduce the rate of failure.
Rob Budny is president of Petaluma, Calif.-based consultancy RBB Engineering. He can be reached at (805) 280-9044 or firstname.lastname@example.org.