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By Brian J. Hogan, Managing Editor -- Design News, January 22, 1997
FEA speeds actuator housing's design
Rockford, IL--The movable front parts of a wing--slats--allow a pilot to change wing configuration to produce more lift during takeoff and landing. Designed to operate the slats on a new commuter aircraft, a linear ballscrew actuator made by Sundstrand Corp. has crossed shafts. Rotating the input shaft changes the ballscrew's length. When the input shaft turns one way, the actuator's length increases and the slat deploys. Reversing the direction of input shaft rotation shortens the actuator and retracts the slat.
The commuter aircraft required three slats per wing and two actuators per slat. Each slat required an actuator with a slightly different housing. "The housings have an interesting shape," says Sundstrand Aerospace Principal Engineer John Hilgers. "They consist of two perpendicular offset housings. This configuration creates an interrupted load path that we thought might cause stress problems."
To resolve his concerns, Hilgers used ANSYS to model one of the housings using the software's solid modeler. A fairly complex part, the housing required Hilgers to combine about ten primitives to build a model. He built a parametric model, hoping to model the other two housings by simply changing parameters. Unfortunately, the complexity of the parts defeated this time-saving step.
After modeling the housing, Hilgers used ANSYS' automatic meshing capability to create an FEA model. The system produced a model consisting of 7,900 ten-noded tetrahedral elements. He applied boundary conditions and loads determined from customer specifications, and performed a static stress analysis on a DEC Alpha workstation. The static analysis required about one hour.
According to Hil-gers, results justified the design team's concern about the housing's strength. But it also presented some surprises. "The way I envisioned the load going through the part was wrong," explains Hilgers. "But there definitely was a high-stress area that we had to thicken."
Having spent about one week on the problem, Hilgers took his results to other members of the project team. After two redesigns, and after doubling the thickness of the original high-stress area, the team produced a housing strong enough to deal with the customer's loads. Furthermore, Hilgers spotted areas from which excess material could be removed.
"Analysis might add two to four weeks at the beginning of a design cycle, but it can also save two to four months at the end of the cycle by eliminating test failures," says Hilgers. A user of FEA software since 1977, the Sundstrand engineer points out that projects such as the housing design require that the user understand boundary conditions. "If you apply improper boundary conditions you get absolute garbage for output," he warns.
Additional details, housing…Contact John Hilgers, P. O. Box 7002, Sundstrand Aerospace, Rockford, IL 61125.
Additional details, ANSYS software…Contact Linda Kubik, ANSYS Inc., 201 Johnson Rd., Houston, PA 15432, (412) 873-2878.
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