Small bearing changes can save wind power operators big money

By Caleb Chovan, Senior Wind Bearing Application Engineer, The Timken Company

Ten years or less; that is how long you can expect a standard mainshaft spherical roller bearing (SRB) to last in a wind turbine, based on field performance.

Given that most wind turbines are built for a life span of around 20 years, this quickly becomes an expensive issue. If a hard-to-access mainshaft bearing begins showing signs of performance problems in the first few years, hundreds of thousands of dollars in replacement costs and lost productivity could be right around the corner for many U.S. wind energy producers.

The issue is fundamentally one of design—modern wind turbine design is still influenced by legacy designs of smaller turbines that experienced less load on the bearings than today's turbines. While designers could settle for sub-optimal bearing options with relatively few consequences in the past, it has become clear that better options are needed as wind turbines have grown bigger, more powerful, and more remotely located.

Mainshaft bearing failures are both commonplace and problematic. However, a solution to the problem already exists that can allow wind turbines to function their entire planned life span without a bearing failure.

The answer lies not necessarily in a total redesign of mainshaft systems, but rather in reinforcement of the bearing itself. Timken has found a way to offer the strength needed through a specialized coating on mainshaft SRBs, and recent field testing of seven-year-old bearings with the diamond-like coating (DLC) revealed just how well the solution is performing in real-world applications.

The Challenge … and Solution

SRB performance can be compromised by a geometric constraint known as Heathcoate slip, which sometimes contributes to micropitting on metallic bearing surfaces. As illustrated in Figure 1, because surface velocities between the inner ring and the rollers match at locations 1 and 3, the surface velocities must differ at location 2, which means that there is sliding between the roller and the center of the raceway. Speci?cally, the inner raceway will have a slower velocity, making the risk of micropitting extremely high for the downwind row.

Often, engineers working in turbine design or maintenance facility settings attempt to address this issue by replacing problematic mainshaft bearings with larger, higher-fatigue, load-rated bearings. However, Timken scientists have observed similar damage patterns in different turbine models over the years due to mainshaft bearings experiencing high-loaded and repeated metal-to-metal contact that creates "adhesive wear." This revealed that the problem is not necessarily the bearing size, but the bearing surfaces.

Figure 1 - Physical description of the source of Heathcoate slip in a spherical roller bearing.

In 2010, Timken engineers sought to address this problem by applying a proprietary coating called ES302 to SRBs designated for use in wind turbine mainshafts. This diamond-like coating is effectively a crossover between a polymer and a ceramic, featuring a nanostructure optimized for performance in lubrication-starved, low-speed, high-load environments. It creates a material dissimilarity between bearing rollers and raceways, reducing adhesive wear—the leading cause of mainshaft bearing failures in three-point mount turbines.

Based on testing, Timken engineers expected ES302-coated bearings would last up to six times as long as uncoated versions, performing well for the full 20-year life span of a turbine. However, given the difficulty in accessing mainshaft bearings in the field, it was not until recently that more extensive field test results were available.

The Proof Point

Engineers were presented with a recent opportunity to analyze an ES302-coated mainshaft SRB that had been in service inside a three-point mount turbine in New Mexico for more than seven years. Once the bearing was removed from the turbine (due to nonbearing-related issues), grease sample analysis, inspection of the bearing outer and inner rings, bearing roller inspection, brass cage inspection, quantitative analysis of surface finishes, and hardness tests were conducted to find out whether an ES302-coated bearing was truly as robust as expected.

The results were clear—the ES302-coated SRB showed little to no adhesive wear, with the expected surface finish and consistency within the original design profile. Researchers concluded the bearing was in "very good" condition for this stage of operation, and it showed no signs of entering into the next stages of damage. The engineering team felt confident that the tested bearing would have continued to provide reliable, trouble-free operation into the 15- to 20-year time frame had the turbine remained in operation.

As long as SRBs are used in wind turbines, it will remain important to ensure they have the strength to stand up to the evolving stresses of the application. As U.S. turbine fleets continue to age, Timken will have access to more field data from the 1,000-plus ES302-coated bearings in use today. In the meantime, know that the "secret" to avoiding the heavy costs that come with bearing failure may be right on the surface.

Learn more about how coatings and alternative mainshaft bearing designs are solving performance issues at www.timken.com/wind-energy.