This article comes from engineering software developer Dassault Systèmes
Control over the position of a turbine’s nacelle and blades is essential for the most efficient conversion of the wind’s kinetic energy into electrical energy. Yaw and pitch drives adjust the physical orientation of those components in response to fluctuations in the speed and direction of the prevailing wind.
A yaw drive, at the base of the nacelle, changes the direction which the rotor faces. Where each blade meets the hub, a pitch drive changes the angle of the blades. Working in tandem, these computer-driven gear mechanisms optimize the orientation of the turbine relative to the wind so power can be generated in the most efficient manner possible.
Gearing up for large wind turbines
Precision yaw and pitch drives for reduction gears in robotic applications are a specialty of Nabtesco Corp. of Japan. The company has developed technology that allows manufacturing those components smaller and lighter than many competitive robotic drives. But when the company expanded into wind turbines and solar-panel trackers, they realized that ‘smaller, and lighter’ have limits in large industrial structures.
“Particularly in the wind industry, our biggest challenges are ensuring sufficient gear strength and long-term endurance in gusty conditions,” says Kazuhiko Yokoji, CAE manager of the company, whose team provides computer-aided engineering services for the entire Nabtesco group. “Our reduction gears are made of a lot of complicated assemblies, with many parts that come in contact with one another. For each wind turbine configuration, we have to provide customers with the best possible design that minimizes overall stress while maintaining durability.”
Every yaw and pitch drive in a wind turbine uses a pinion to transmit power from the drive to the nacelle or blade. In a utility-scale turbine, the rotation angle between drive and pinion gear teeth is particularly small, so repeated contact over time—particularly under the ‘routine’ stresses of high winds and tower vibration—has the potential to damage tooth surfaces and cause assembly breakdowns.
To minimize such hazards, company engineers work to balance the contact between ring and pinion teeth so that reduces the ‘normal’ stress of rotation and the sheer stress of the teeth against each other. Decreasing stress on teeth gives them a longer working life—and that of the entire assembly as well.
Because the thinner, outer edges of a tooth are most susceptible to damage, teeth are manufactured with a curved surface (“crowning”) so that the edge dips away to either side of the center of the tooth and contact between teeth occurs near the middle. A crowning too steep decreases the contact area and intensifies tooth stress. Too shallow crowning allows stress to extend too close to tooth edges. Damaging edge contact can also increase when the pitch drive shaft bends. “Because crowning has such a significant effect on contact area and maximum stress, pinion tooth shape is a major focus in our design process,” says Yokoji.
Finding an optimum tooth shape is a time-consuming challenge when done manually. The contact area between drive and pinion teeth can be visually identified in a test rig by coating the teeth with paint that rubs off where they engage. “But this method means manufacturing and testing a lot of pinions to identify the most durable shape,” says Yokoji. “And this method doesn’t give us the overall stress data that helps evaluate durability. We felt we could get a more complete picture using an analytical model that could simultaneously analyze tooth contact area and maximum stress.”
How simulation speeds analysis
Yokoji’s team uses Abaqus unified finite element analysis (FEA) from Dassault Systèmes to solve gear-related design challenges. To create an analytical simulation of the pinion test rig, engineers started with an Abaqus CAE model based on imported CAD geometries of rig, drive, and pinion. The bottom surface of the test apparatus was modeled as fixed. Resistance (this data, including the effect of wind velocity, was provided by a Nabtesco customer) was applied to the reduction gear shaft, and the pinion was rotated at a prescribed angle. This lets the team see when and where opposing teeth engaged as the gears rotated, along with resulting stresses.
For a better understanding of what happens when gear teeth interact, the engineers had to mesh the relatively small area of tooth-face contact with particularly fine FEA elements (C3D8R elements in Abaqus). To reduce computation time for this portion of the model, they created a submodel that contained only those areas of interest. When compared against the painted gear rig tests, the FEA results showed good agreement.
“We still wanted to get a complete picture of how the total contact area and stresses fluctuated over the course of the entire engagement period,” says Yokoji. “This would provide us with the ‘big picture’ of contact history that we needed for evaluating tooth designs for durability.” So his team developed a proprietary post-processing technique that use an Abaqus subroutine to show the history of how stress developed from the start of engagement, through changes in the rotation angle, to the end of engagement. This provided a complete toolset to fine-tune individual pinion-tooth shapes.
Let the software do the repetition
At this point the group turned to process automation and optimization software called Isight, also from SIMULIA. Isight is capable of feeding the results of one program into another and making user defined decisions along the way. It let the design team couple its own in-house program for creating contact stress distribution history into the workflow. This let the team quickly generate results from huge result files and evaluate immediate (stress) and the long-term (strength/durability) consequences of every design change. Then they plotted the results into 3D stress distribution graphs that tracked all phases of gear engagement and let them see how modifying gear crowning contours affected overall performance.
“Our CAE workflow now lets us accurately optimize the crowning of pinion-gear teeth and with less manpower than before,” says Yokoji. “This method significant reduces design times.” The company’s newly designed gears are being introduced into the field and the company is gathering data to generate accurate comparisons with previous models. “We believe that there will be a significant improvement in durability,” says Yokoji.