From left: Legacy gas-turbine engine design with original torque arm connecting linear actuator to bellcrank and close-up of new, topology-optimised and connected parts.
Aerospace and defense contractor Advatech Pacific recently took on a project as part of an ongoing effort to help a customer both support and improve a family of legacy, gas-turbine engine designs – the kind of project where the only available data is stored on E-size paper blueprint sheets. This case study from PADT looks at how the company was able to combine mechanical simulation with additive manufacturing (AM) to optimise an existing part.
With previous success using high-end simulation software such as ANSYS to identify and evaluate the fine details of massive multi-body mechanical systems, whether through simple static analyses or tightly-coupled multiphysics computations, Advatech was tasked with helping to overcome part obsolescence, reduce manufacturing costs, and redesign parts to meet new exhaust-emissions requirements.
Implementing AM as an additional way to improve system design presented opportunities to cut back on tooling costs and reduce lead time for several candidate turbine-engine parts. Doing so would also alleviate the challenge of reproducing complex castings, a problem made increasingly difficult by the fact that many of the original casting providers are no longer in business.
Since Advatech was already working with the customer to transition some components to additive, the company also saw it could demonstrate the benefits of combining topology optimisation with the redesign project. For a test case, it chose an existing short torque arm (about eight inches long) that connected a linear actuator to a bellcrank assembly, which adjusted inlet guide vanes (IGV) within the gas-turbine engine. This component was a good choice for the project due to its simplicity. The geometry was a straightforward beam subjected to a transverse load and Advatech wanted to show that topology optimisation can provide non-intuitive improvements even when applied to a seemingly basic part.
Mechanical Simulation + Topology Optimisation = Novel Solutions
Original design of torque arm.
Advatech uses mechanical simulation software to quickly find optimal solutions that satisfy system constraints such as applied torque, range of motion, and maximum part dimensions. In order to achieve the most effective results, Matt Humrick, Advatech Pacific’s Engineering Manager, performed a few tests comparing ANSYS Mechanical with a competing solution.
When testing out the process between the two software packages, it became clear they took very different paths to accomplish the same analysis. For example, ANSYS is aimed more at the analyst who is already familiar with the structured ANSYS Mechanical user interface, while the competing package approached the problem from the perspective of a designer. Ultimately, the engineers found setting up the optimisation analysis was easier with ANSYS, mostly due to their familiarity with the tool’s interface, as well as an ability to include the entire IGV assembly in the optimisation run.
ANSYS Mechanical static-structural stress results of topology-optimised torque arm.
Advatech was able to include the entire assembly and even specify frictionless contact between the torque arm and shaft surfaces. By accounting for the elasticity of both the torque arm and bellcrank assembly, the software correctly predicted the uneven pressure between these contact faces with significant load transfer along the inner edge and almost no load transfer at the outer edge of the shaft. This had a significant impact on the torque arm’s final optimised shape, specifically the asymmetric web that curves directly towards the loaded edge.
In addition to the dramatically different asymmetric web geometry, the optimisation results included tapered flanges (an expected result for a beam in bending) and a simple truss structure. The latter layout eliminated even more unnecessary material within the beam web and, while crude, was still better defined than that produced with the competing software.
Back to CAD, Preparing for AM
After performing the optimisation analysis, Advatech engineers exported the organic shape into a separate CAD program. Even though the primary features were mostly present, significant cleanup was required to produce the final design, as is often the case when bridging between analysis-generated geometry and classic solid-model representations. This process actually required the most time and effort.
Final version of CAD solid model based on optimised ANSYS stress results.
The first CAD iteration of the new torque arm closely matched the organic shape produced by the software and was inserted back into the IGV assembly where a follow-up static structural analysis revealed a few areas for further improvement. After running a couple more iterations, increasing the web fillet radii slightly and adjusting the truss configuration to reduce stress, Advatech engineers chose the build direction that gave the best material properties for the applied load and made a few changes specifically to prepare the part for additive manufacturing. For printing a visual model in ABS plastic on an industrial 3D printer, this included eliminating some overhangs in the web cutouts and slicing off one side of the rounded end to create a flat build surface.
Original (below) and topology-optimised (above) designs, printed for visual comparison in ABS. Final part could be 3D printed in stainless steel.
Comparing the geometry of the original part and the new, optimised part, evaluated as if 3D printed in the same stainless-steel material, Advatech’s torque arm design realised a 45% weight reduction while matching the original part’s stiffness – the key performance parameter.
Humrick, commented: “As an engineering services provider, we are always looking for ways to add value for our customers by saving them money, implementing a capability they lack, or providing a fresh perspective. Topology optimisation and design for additive manufacturing potentially improves all of these capabilities and will continue to be something we offer our customers where appropriate."
