10/11/2010 9:00:00 AM | Sustainable Solutions for Wind Energy
| Mike Kotzalas
Spherical roller bearings (SRBs) have been commonly used to support main shafts in wind turbines. They were often selected because of their ability to accommodate misalignment between the shaft and the bearing housing.
However, operators are now reporting that many of these bearings are experiencing damage due to excessive wear that significantly reduces their serviceable life versus what they were designed to achieve. This results in expensive downtime, high crane mobilization costs and unplanned main-shaft repair and bearing change-out. The corresponding repair costs can be $100,000 to $300,000 per incident, depending on the condition of the gearbox, turbine location, etc.
The technical paper, "Main Shaft Bearings: Life-Limiting Wear and Solutions,"
details how the life-limiting bearing wear occurs on main-shaft SRBs and provides solutions to address this wear. It states that the predominant type of wear that limits the life of main-shaft SRBs is not classical rolling contact fatigue, but rather, micropitting wear.
In May 2010, Timken introduced a new line of wear-resistant bearings for the wind-energy market to provide a viable solution to micropitting wear.
Low-cycle micropitting is caused by high amounts of sliding between rollers and ring raceways, generating considerable shear stresses in the contact zone.
Figure 1, below, shows the evolution of low-cycle micropitting to raceway spalling of a 230/600 main-shaft spherical roller bearing. Analyses of the radial and thrust loads on a typical main-shaft spherical roller bearing indicates that the entire load is supported by the downwind row of the bearing with a full 360-degree load zone, and the upwind row is essentially unloaded. This results in higher loads on the downwind row, with an increased number of stress cycles occurring at any point on the inner raceway for every shaft revolution.
Main shafts typically rotate at slow speeds in the 10 RPM to 20 RPM range, and sometimes as low as 1 RPM during certain idling conditions. At these slow speeds, significant lubricant film may not be generated, even with high viscosity lubricants. In addition, because of the design geometry of the spherical roller bearing, the downwind rollers will slide due to Heathcote motion or deformation based micro-slip of the surfaces. The result of higher loads, more stress cycles, thinner lubricant film thickness and roller sliding on the downwind row significantly increases the risk of micropitting.
Timken has developed an engineered surface -- an amorphous carbon coating -- which, when applied to superfinished rolling elements, has been shown to work extremely well at reducing wear in rolling-element bearings. This coating is two-to three-times harder than steel and has low friction coefficients when sliding against steel. A durable coating and a superfinished surface on the main-shaft bearing rollers can eliminate or significantly delay the onset of micropitting by dramatically reducing the cyclic shear stresses. This ceramic coating continuously polishes and "wears in" the mating surface, resulting in improved oil film thickness and extended bearing life.
The mechanism responsible for the life-limiting wear mode of micropitting that affects main-shaft spherical roller bearings is roller/raceway sliding in low lambda conditions. These conditions are unavoidable for spherical roller bearings operating in the main-shaft pillow blocks of wind turbines.
In low lambda conditions, Timken testing shows that wear-resistant roller bearings have up to 350% longer life than standard roller bearings.
Utilizing the engineered surface described above on main-shaft bearing rollers can significantly reduce the shear stresses that cause micropitting by creating surfaces on the rollers and raceways that virtually eliminate opportunities for asperity interactions.
The bottom line is that we see significant benefits, in terms of improved uptime and lower total operating and maintenance costs, when using Timken's wear-resistant technologies in wind turbine main shafts which use spherical roller bearings.
Figure 1: Micropitting of 230/600 series spherical roller main-shaft bearings: (a) Onset of micropitting wear at the center of the inner raceway where its velocity is slower than the roller; and (b) Advanced micropitting wear that caused geometric stress concentration spalling initiated at the edge of the wear track. In low lambda conditions, Timken testing shows that wear-resistant roller bearings have up to 350% longer life than standard roller bearings.
Do you agree: Is life-limiting micropitting of spherical roller bearings in wind-turbine main shafts causing more downtime and need for repair than classical roller contact?