Modeling wind-turbine gearboxes

The Calyx gearbox model includes all components necessary to accurately predict the load distribution and stresses in the entire gearbox. The model lets users evaluate gear and bearing load distributions in the presence of systemlevel effects such as deflections and misalignments. The model also includes the intended gear tooth modifications, such as profile and lead corrections, plus bearing parameters such as crowning and clearances. The colors represent various tensile stresses.
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Gearboxes in general are a result of over 80 years of accumulated engineering knowledge. So why do gearboxes for wind turbines sometimes fail after seven to 11 years — way less than the anticipated 20-year design life of the entire turbine structure? This is a complex question with many variables to consider and no easy answer. For example, it was once thought there was an incomplete understanding of the loading conditions the gearbox sees as it suffers high torques from blades subjected to gusty winds. Also, gearboxes turn at low rpms and therefore generate a thin lubricant-film thickness, which makes them prone to micropitting and wearing of the gears and bearings. In addition, the units see a large range in operational temperatures and high vibrations at the nacelle.

To try and get a handle on the causes of early gearbox failure, the National Renewable Energy Laboratory's (NREL) Gearbox Reliability Collaborative (GRC) about four years ago began collecting data from two custom-built planetary gearboxes instrumented with over 200 strain gages, proximity probes, accelerometers, and acoustic emission sensors. One of the units is in the field; the other resides on a dynamometer in a lab. The gearboxes are big and heavy enough to require positioning via cranes.

The goals of the GRC are to reveal the causes and loading conditions that result in the premature failure of wind turbine gearboxes and recommend improvements for the industry.

A driveshaft similar to the one in the NREL gearbox is modeled in KISSsoft. Results include the pressure curve between the roller and the race way and the load distribution among bearing rollers.
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In these endeavors, the GRC is performing gearbox modeling as well as dynamometer and field testing. “The dynamometer tests let us validate computer model predictions under a controlled setting with increasing levels of complexity,” says Jeroen van Dam, a senior engineer at NREL and the GRC project lead “The field tests, of course, provide us the real-world data that the models ultimately must predict accurately. The field-test data can also be used to design a better dynamometer test for new gearbox designs. Our efforts also include condition monitoring as well as populating a gearbox-failure database. Condition monitoring should help us develop a better operation- and-maintenance strategy and reduce gearbox downtime. The failure database stores the hard data we collect on gearbox problems. This data is intended to help us understand the root cause of gearbox failure,” he says.

From a software point-of-view, the project's complexity necessitates the use of many different products. They include specialized packages intended specifically for gear design and in-house codes for studying gear micro geometries. Also seeing use are high-end commercial FEA software for predicting how the entire gearbox will react with the turbine structure.

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