7 Replies Latest reply on May 14, 2014 8:35 AM by Alison Powers

    Non massless spring in Motion Analysis

    Alison Powers

      Hi, all.

       

      I'm working on a motion analysis with a heavy die spring (20lb). I'm wondering if there is a way to model the geometry (and mass) of this spring; I think the dynamic behavior of the system is being influenced by the relatively heavy mass of the spring. I have found ways on these forums to animate the coils, but can Solidworks Motion help me to analyze the dynamic behavior of the spring itself, given the properties of it?

       

      Thanks in advance,

      Alison

        • Re: Non massless spring in Motion Analysis
          Shaun Densberger

          Can you give us more advice on your system? I'm not too fimilar with the motion analysis, so I'm not sure what features it has. However, you are correct that the mass of the spring does effect the fundamental mode of the system, but the effect depends on how much the spring weights compared to the rest of the system. If you have a mass-spring system with a block of mass m, and spring of stiffness k, and a spring weight of m', then the fundamental mode of the system is:

           

          w = [k/(m+m'/3)]^(1/2)

           

          If your system (aka the block) has a much greater mass than the spring (m >> m'), then you could probably ignore the effect of the spring's weight.

            • Re: Non massless spring in Motion Analysis
              Alison Powers

              Thanks, Shaun, for your quick reply. The equivalent mass of my system is about 20 pounds, where the mass of my spring is that or greater, depending on the spring used. My system is this way because of the need of very high forces when very small displacements are seen. I'm afraid I can't go into more detail without giving away too much of my application (this is for my work).

               

              I still wonder if Solidworks Motion Analysis can take into account the mass of my heavy spring!

                • Re: Non massless spring in Motion Analysis
                  Jared Conway

                  i believe springs in motion are massless

                   

                  http://help.solidworks.com/2012/English/SolidWorks/motionstudies/HIDD_DVE_SIM_SPRINGS.htm

                   

                  if it is connecting 2 parts, how is the mass effecting the system other than through its stiffness and damping?

                   

                  or are you concerned about the weight of spring with respect to something else in the system

                   

                  this would be a good spot to setup a test example and post it to come up with ideas for workarounds if it isn't supported in the software

                  • Re: Non massless spring in Motion Analysis
                    David Schaller

                    Have you considered adding a set of pivoting links  to your motion simulation to aproximate the mass of the spring?  It wouldn't be exact, but would certainly be closer than nothing.  Attach each end to the respective part with frictionless hinge joints between all the links and density set so all the links mass totals the same as your spring.

                     

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                    • Re: Non massless spring in Motion Analysis
                      Shaun Densberger

                      OK, then in that case you should account for the mass of the spring. As I said before, I'm not too fimilar with SW motion analysis, so you might want to wait for someone more knowledge to answer. That being said, after taking a quick look my guess is that SW doesn't provide the ability to define a spring with internal mass. However, there are two options that I can think of that you could take.

                       

                      1. Depending on the specifics of your system, you might just be able to increase the mass of your system by 1/3 of the mass of your spring. I think that this would require that you can condense the system down to a single-degree of freedom representation, but I'd need to think about that a bit more. If you can do this, then just increase the density of your part material such that you add 1/3 of the mass of the spring.
                      2. You could divide up the heavy-spring into a series of massless-springs and masses. The most basic is turn your heavy-spring into a massless-spring connect to a mass which is connected to another massless-spring. The mass would have a mass equal to your heavy-spring and each massless-spring would have a stiffness equal to twice that of your heavy-spring. Note that this method will introduce some error. However, the more sub-divisions that you make, the more accurate this representation becomes. See below.

                       

                      Heavy Spring Division.png

                  • Re: Non massless spring in Motion Analysis
                    Alison Powers

                    Thanks, everyone for your help. I will be trying some tactics you've suggested today and tomorrow and report back.