Gear Technology Magazine

Questioning New Techniques

January 20, 2015
[starbox] In my last blog I mentioned the great analysis tools available to today’s gear engineers. On a current project, we are eagerly awaiting the finite element analysis (FEA) of a critical part that was designed using far less-sophisticated methods, going back 40 years. It would be a great surprise if the FEA reveals any hidden flaws. Without going “full-curmudgeon” on you, I have been disappointed in FEA many times, beginning with the first FEA course ever offered at my engineering school, back in 1974. Our instructor, a newly credentialed PhD, brought FEA technology with him. It had been the topic of his dissertation and he was an evangelist for this method of stress analysis. Unfortunately, the computers of the day were far less user-friendly. They weren't as powerful, either; and between that, the dial-up/phone cradle interface and monochrome 10-inch monitors, the class struggled to get simple homework assignments done. We wondered if the box of punch cards the professor had brought from his previous school had been scrambled at baggage claim. By semester’s end, we barely managed to get a tri-axial stress on a cube to run. My plan to impress the supervisors who were paying my tuition by using FEA on some fabricated steel housings went up in error messages. It was up to Stress-coat and hydraulic jacks to back up the manual calculations derived from Omar Blodgett’s classic book, “Design of Weldments.” Fast-forward 20 years and computers were finally up to the task of analyzing a full housing — in color and on a big monitor. We couldn't afford our own installation, of course, but an important project had the budget to sub-contract the FEA to the local experts. It was almost engineering porn to watch the colors change as the housing model was checked for various loading conditions. The only drawback was that the FEA work took so long that the patterns had to be revised twice to incorporate the results. But even this “improved” FEA experience turned out to be a disappointment, as the housings were later found to be cracking in areas that FEA said were “very low stress.” The usual finger-pointing took place on whether the FEA, design, or foundry practices were to blame. But I came away skeptical of any design based upon colorful pictures. Hopefully this latest incarnation of FEA will convince me to fully embrace the 21st century.