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Static study of assemblies

Question asked by Bob Thesponge on Mar 1, 2015
Latest reply on Mar 13, 2015 by Bob Thesponge

Hello,

 

I have experience in SW simulation of parts (i.e. assemblies with no internal degrees of freedom, which I call kinematic sets. Another way to put this is to say kinematic sets are sets of parts rigidly linked together and which move together). Now I am willing to perform a simulation of an assembly with internal degrees of freedom (i.e. the system now includes different kinematic sets linked by prismatic and pivot linkages), this is all new to me. I have reviewed the different ways offered by SW simulation to solve such a system, but as this is new to me I do not feel confident on which approach I should use - i.e. which approach is the most standard.

 

Here are the methods I could think about:

  1. Simulate each kinematic set individually: compute the efforts at all internal and external links (to get to know the interactions between kinematic sets) then simulate each kinematic set individually
    • This brings me back to the situation I know how to handle in SW
    • But I need an extra stage where I compute all the link efforts. Getting these link efforts can be done in several ways:
      • A static study by hand: this is the "back to school" way where you simplify your system, write the equations and solve them
        • I find this approach time consuming and error prone
      • Perform a motion study: motion studies allow to record the efforts in links (over time) and later import these in SW simulation
        • This is more than what I want to do (as you study the system for a range of time - which I do not really need)
        • You need the Motion plugin in order to do so
      • Simulate kinematic sets one after another: simulate a single kinematic set, probe the resultant efforts in the links (this is done is SW simulation) then simulate the contiguous kinematic set (re-applying the resultant efforts probed at the previous kinematic set)
        • This may not be very accurate as the resultant efforts over a whole link may arise from a very non-uniform distribution of the constraints inside the link (there can even be opposite constraints inside the same link)
        • Hence the resultant effort may not represent the behavior inside the link
  2. Simulate links using contacts: you use the geometry of the link to simulate its kinematic effect
    • Very resource intensive and does not always find a solution
    • Need to simplify the model in case of complex geometry (rather often)
    • Seems overkill (but I am not an expert)
  3. Parametric link: use the mates put in place in the modeler to set-up constraints on the mesh nodes
    • This "gut feeling solution" does not not seem to be possible for some (dark) theoretical reasons
    • More specifically: I find it weird to be able to specify parametric mechanical links for the fixtures (prismatic, pivot...) but not to be able to do so for connection
    • I guess there is a theoretical reason behind this which finds its roots in the FEM used by the simulation (but this goes beyond my knowledge of simulation)

 

Anyone having advice on the technique I should use? What is the most standard approach used by the professionals? My gut feeling is that using contacts seems the easiest way, though is seems a bit overkill...

 

Thanks for your help,

 

Antoine.

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