The Analysis Tools Sanity Check

Let’s call it a ‘sanity check’.  Sometimes you need to prove to yourself, or possibly your boss, that your analysis tools are calculating results correctly.  Maybe you’re reviewing the results and something doesn’t look quite right.  There are plenty of scenarios where you may be called upon to use hand calculations to back up what your workstation and software can do for you in seconds, minutes or hours.  With our designs becoming increasingly more complex, this can definitely seem like an impossible task.  Fear not!  With Engineering knowledge, understanding how your analysis tools operate, plus a bit of diligence you can do just that.

Here is an example of using hand calculations to verify results from a SOLIDWORKS Motion analysis of a three component linkage.  I’ve simplified the analysis to consider the starting, static position because, well, you have to start somewhere!  The plot for calculated motor torque appears to be incorrect.  The Motion analysis result for motor torque at the starting position is 24 lbf-ft while the expected result should be approximately 10 lbf-ft.

When beginning your hand calculations, be sure to note any assumptions that you need make.  In this example, I am neglecting the mass of the components at the starting position, friction at the mate locations and gravity.

Now that I have the calculations in as simplified a format as possible (equation D), I can set up an Excel spreadsheet to calculate what Motion should have generated as a result.

My next steps are to investigate the original Motion study and SOLIDWORKS assembly to determine why the software didn’t calculate motor torque to be 10.8 lbf-ft.  In this example, it has to do with the mate scheme used for building the mechanism.  SOLIDWORKS Motion used the mates directly from the assembly, so how the mechanism was constructed is extremely important.  In the original design, my mate scheme had four redundancies, as indicated on the Mates folder inside the Motion study.  In SOLIDWORKS Motion, a redundant mate occurs when more than one mate constrains a specific degree of freedom for an assembly component.  The redundant mates have to be excluded from the Motion analysis before the mechanism calculation begins.  This is what caused the motor torque output to be incorrect.

Now that I have determined it was my faulty mate scheme that led to the incorrect calculation, I can modify my assembly mates to work towards a mechanism with zero redundancies.  A mechanism with zero redundant mates will allow SOLIDWORKS Motion to correctly calculate forces acting on the mechanism.

The changes I made to the assembly were adding hinge components to the assembly, fixed the hinge components in space to provide an established path to ground and changed the mate scheme for the linkages.  The linkages were originally positioned with mates attached to an assembly sketch.  By adding the pins I can replace multiple mates connecting the linkages to sketch entities with a few hinge mates.  Reducing the number of mates in the assembly helps to remove redundancy from the mechanism.

Now that my Motion study is calculating the correct motor torque at a known position, I can increase the complexity of the analysis and have a high level of confidence regarding the results.  My next steps would be to include the mass of the components by unsuppressing gravity from the Motion study.  I would then change the motor definition to move the mechanism through the appropriate range of motion.  Eventually, I would include the effects of friction on the mates.  But I could not take any of these additional steps without first verifying that I was obtaining the correct output utilizing hand calculations.  So the next time you’re not sure if your analysis tool is giving you the correct result, do the smart thing – trust but verify!

Now go make your products better with SOLIDWORKS Simulation. Use the comments section below to share any success stories. I would love to hear how you are using it in your designs!

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