Assessment of Blade Bearing Greases Performance with Water Contamination & Grease Mixtures

Roller bearings have been playing a more important role in pitch system designs in the past years due to their comparably high stiffness and load carrying capacity.

With limited operational experience, tests play an important role in any risk mitigating strategy. This study focuses on scaled wear tests. Worst-case operating conditions of a 7.5MW wind turbine with individual pitch control are scaled to smaller custom roller bearings. The contact pressure and oscillation frequencies are kept constant while the outer loads and oscillation amplitudes are scaled to the smaller bearing size. The test profile is a 13.7 h time series which reproduces worst-case operating conditions close to rated speed of the turbine with small oscillation amplitudes and no longer protection runs in between. Four different commercial greases are tested in both “dry” (no water contamination) and wet (10% vol demineralized water added) condition. This work uses customized angular contact ball bearings and 180 mm outer diameter.

The results from these bearings show that for some greases the impact of water contamination will have a significantly higher false brinelling and copper corrosion impact. While extending the 13.7 h test cycle to model the lifetime of a full blade bearing the differences become even more visible.

Secondly the test setup has been used to model the performance of grease mixtures to model the impact of a longer-term grease switch-over from one grease to another. This offers real wear testing of grease mixtures instead of only relying on compatibility testing.