Estimating Fatigue in the Main Bearings of Wind Turbines Using Experimental Data

Axial bearing failures, in modern wind turbines, have been one of the major sources of unplanned maintenance expenses for the wind energy business. Although the causes of such failures are unclear, its statistics do increase in wind farms where the wind conditions range in considerably larger spans. It is common belief that torsional vibrations, combined with axial ones resulting from these extreme wind conditions are the major causes of high cycle, fatigue affecting the bearing. In this study, the strain measurements on blades are used to estimate the internal force time histories at the location of the main bearing. A rigid body equilibrium analysis was performed using moments and forces measured at the blades to estimate the force and moments acting on the low speed shaft. Experimental wind data was used to compare the difference between strong wind speed events and low wind speed events. The approach uses the blade bending-moments to isolate the axial excitations and is validated by measured data from an instrumented turbine with four interferometric strain sensors and four temperature sensors for each blade. The instrumented turbine is part of an onshore wind farm site, in Cohocton, NY, where wind conditions span in a large range and turbines are often placed in stop conditions. Preliminary data has shown some large excitations that could eventually lead to crack initiation.