6 Replies Latest reply on Jul 2, 2014 7:21 AM by J. Mather

    On the Job Stress Analysis

    Jesse Evangelista

      I was looking at Norton's Machine Design textbook, and the Stress Analysis of a bike lever and cable was real complicated. The other examples were complicated too. Have you ever done such complicated Stress Hand Calculation Analysis at work? I would just be tempted to model it in Solidworks or Proe, and then use the CAD FEA to do the Stress and deflection analysis for me. But it seems I'm very rusty right now in doing hand calculations. I don't recall doing such complex stress calculations during my college courses.

       

      If your job requires complex stress calculations, do you sometimes get help from a Sr Eng to do these calculations? Or do they have a specialized Stress Analysis Engineer for doing stress analysis?  Or do you just rely on Solidworks FEA to do your Stress Analysis?

        • Re: On the Job Stress Analysis
          Robert D.

          Hand calc's are best in my opinion. SW is a round about the right answer or close enough to some companies. You should always back it up with your own calculations. I would ask senior engineer for help if your in doubt of anything.

          • Re: On the Job Stress Analysis
            Mike Pogue

            I do a lot of hand calcs for back of the envelope type stuff. I also do hand calcs for situations that FEA handles poorly.

             

            In general, hand calcs involve canned solutions to highly simplified problems (see Roarke's Formulas for Stress and Strain). There are not closed form solutions to general geometries, so what you wind up doing is making heroic assumptions to get a linear, solvable equation. Heroic assumptions are not usually good assumptions.

             

            So, it is easy to get the hand-calc  von Mises stress for a prismatic, cantilever beam with almost arbitrary loading conditions, but your reality will involve a series of approximations and simplifications that eventually tend to invalidate the solution. In my experience, engineers very rarely carry errors through their calcs, and I think most would be surprised at what they find if they did. The advantage, though, is that the equations are easy to understand, and intuitive--they tend to give you an feel for the situation that is frequently good enough for design work.

             

            The advantage of FEA is that the equations are entirely general. You can solve an arbitrary problem using exactly one equation for stress. You can also model arbitrarily complicated situations--if you know what ypou are doing. The disadvantage is that most users don't understand the equations, and, as a result, have little feel for the problem, or whether the solution they've achieved is correct, or even meaningfull.

            • Re: On the Job Stress Analysis
              Jesse Evangelista

              I modeled the Norton Vise Grip problem in Solidworks, but I don't know how to setup the analysis.  I'm playing around with it to try an FEA assembly model analysis of forces at the pins.  Seems like I need a Solidworks Training Guide on this, too.

               

              I guess if I had to do SW FEA at work I need them to train me or pay for my training.

              • Re: On the Job Stress Analysis
                Jared Conway

                It depends on what you are doing. Shaun d puts it well, if life is at risk, you should do everything you can do to validate your design before implementing.

                 

                Hand calcs, fea and physical testing.

                 

                Hand calcs and fea are similar in that they are approximations. In either  case if you want to use them as absolute you need to correlate them to physical. But up to that point, they can be used to compare design under know approximations and assumptions.