High on the list of O&M crews are ways to extend bearing life and reducing wear. To handle the challenge, Timken engineers developed a bearing that reduce life-limiting wear problems that trouble main shaft and gearbox bearings. Common wear problems include micropitting on main shafts, spherical-roller bearings and micropitting, smearing and brittle flaking for wind-turbine-gearbox bearings.
Wear that limits the life of main shaft spherical-roller bearings is not classical rolling contact fatigue, but mostly micropitting wear. It is caused by interaction of the raceway and roller asperities, leading to high stresses in the contact. The normal stress alone is not typically sufficient to cause initiate a crack at or near the surface early in a bearing’s life cycle. However, the addition of frictional shear stress increases the bulk contact stress values and brings the maximum values closer to the surface, allowing these localized stresses under the asperity contacts to become significant. This type of interaction typically occurs when the lubricant film is insufficiently thick to separate the contacts and when there is relative sliding between the two contacting surfaces. It is termed low-cycle micropitting.
A team at the bearing company performed an analysis of typical radial and thrust forces on a 230/600 main shaft spherical roller bearing along with the radial clearances inherent in the bearing design. Results indicate that the entire load is supported by the downwind row of the bearing leaving the upwind row essentially unloaded. This uneven load distribution results in higher loads on the downwind row, as well as a full 360° loaded arc of rollers. The fully loaded arc of rollers increases the number of stress cycles occurring on a point on the inner raceway for every shaft revolution.
Main shaft bearings typically rotate at 10 to 20 rpm, not enough to generate a substantial lubricant film thickness even with higher viscosity lubricants. Consequently, higher loads, more stress cycles, and thinner lubricant films are present on the downwind roll increasing the risk of micropitting, especially when roller/raceway sliding is present.
Wear-resistant bearings provide up to 3.5 times greater L10 life for main shaft and gearbox bearings. The company says benefits of the new bearing include resistance to:
- Smearing, scuffing, and false brinelling damage
- Life reduction from debris damage
- Low cycle micropitting
Timken says the bearings reduce micropitting, smearing, and brittle flaking often seen in competitors’ original equipment bearings for main shaft and gearbox applications. The company also provides support services for O&M crews.