Effective Lubrication for Wind Power Bearing

A meaningful oil film thickness cannot be generated in a slowly and intermittently moving (oscillating) grease-lubricated yaw or pitch bearing. Therefore a clean grease with good boundary lubrication additives (especially for oscillating conditions) should be selected on the basis of experience for use in wind turbine yaw and pitch bearings. A seal system (integral or external) also is essential for achieving satisfactory operation.
A fretting-corrosion type of raceway and rolling element surface failure commonly is encountered in yaw and pitch bearings. The fretting corrosion appears as elliptical or rectangular footprints at ball or roller spacing in the bearing. The markings are tiny corrosion pits caused by the lubricant being forced out of the contact area (by a small load increase) and then not being able to re-enter the contact zone. The unprotected surface then is subject to corrosion pitting. Most grease rated for oscillation use can coat the rolling contact surfaces and maintain corrosion protection. In extreme cases, coating the raceways is an option. A TDC coating increases the bearing cost significantly but provides increased protection.
One manufacturer of wind turbine yaw and pitch bearings suggests using the Hertz contact stress limits given in Table 16 as means to limit fretting-corrosion types of failures.