1. Atlantic Offshore Wind Consortium: Drivetrain Reliability
Ben Gould, David L. Burris, Alex Szela, F. Feng
Department of Mechanical Engineering, University of Delaware
Motivation
Figure 2. Top: close up illustrating the three-piece thrust bearing assembled in the tester under an applied load. bottom: Potential pressure distributions. Right: wear rate calculation using interferometric surface characterization.
Wind power is free, clean, and sustainable. However, cost of wind-generated electricity is relatively due to the considerable expenses associated with the turbine converter; gearbox failure is a major contributor. Gearboxes fail after 5 years on average for unknown reasons. The failures are often unpredictable and catastrophic causing significant damage to other components. We are participating in the NREL led Gearbox Reliability Collaborative (GRC) to collectively understand and solve this industry-wide challenge.
Methods
Characterizing Loads
Understanding gearbox failure requires knowledge of the loads on the gearbox components. A major thrust of the GRC is the prediction of these loads. The potential for ‘non-torque’ loads on the gearbox input shaft is a distinguishing characteristic of the wind turbine application and a likely culprit.
We have been working to model and directly measure these non-torque loads. We estimate, for example, that a 10m/s wind applies a thrust force of 270 kN and a mean pitch moment of 3MNm with a 10% (peak to valley) fluctuation 3-times per rev. We are in the process of mounting accelerometers and strain gages on the UD G90 to directly asses the load environment and tune our modeling effort.
Results
One can see from the figures below that wear is significantly increased when contaminants are added to oil, all differences are proven significant. It is also important to note that these test were only run for 100,000 cycles, as one can see below there is significant pitting on part of the oil contaminated with water. Therefore we would expect the differences between the oil and water wear rates to be greatly exacerbated should testing continue
Methods
Small Scale Test Rig
Figure 3. Different wear tracks corresponding to oil (top left), oil and water (top right), and oil and particles (bottom).
Figure 4. Used thrust washer with clear wear tracks on top and bottom pieces.
The surfaces in a radial bearing are inaccessible. Suspected failed bearings must be sectioned for surface analysis making evolution studies impossible. We have developed a laboratory test rig to study surface failure using a thrust washer arrangement with continuous access to surfaces. Temperature, speed, force, lubricant, materials, contaminants, surface roughness, and roll/slip ratio can be controlled. Uncertainty about the lubrication environment of interest is currently our primary challenge.
Paths Forward
Figure 4. Effects of water and particle contamination of lubricant on wear rate. .1% contamination levels were used, and one micrometer alpha alumina particles
This is a complex problem in an enormous design space. Load characterization studies and system modeling will be necessary to appropriately scale the mechanical environment for controlled laboratory studies. We will continue to study the effect of load, speed, contamination, roughness, etc. on the wear morphology. Our goal is to gain basic insights into the failure of wind turbine drivetrains and provide improved design strategies.
Acknowledgements
The authors grtefully acknowledge funding for the DOE and First State Marine Wind. We would also like to thank Mike Hospod, Willett Kempton, DeAnna Sewell, Gamesa Wind, and Blue Hen Wind.
Figure 1. Surface failure test rig. The thrust washer arrangement provides access for surface characterization throughout testing.