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Compound Motion Study


I'm interested in simulating a compound rotary unit I am designing. I want to simulate a jointed arm, which turns relative to the forces of a cable that pull it right or left, and that jointed arm is connected to a central rotating hub which allows the mechanism to rotate 360 degrees. How do I order my revolute / fastener mates in order that the one force acts both on the jointed arm as well as the main hub.

Also, is there a combination of mates that works to simulate a sub assembly travelling along a track (turning with the turns and remaining in the upright orientation)? I've tried the tangent, but the subassembly continuously rotates around that tangent line instead of maintaining its upright orientation.


  • philip_thomasphilip_thomas Member, Moderator, Onshape Employees, Developers Posts: 1,381
    Kolbi - this is not easy without an example (most people post a public link and then 5 guys fall over themselves to solve the problem as fast as possible). That said, there are some insights that i can offer you.
    1) Mates in any CAD system, define degrees of freedom that exist between any two parts.
    2) Mates do NOT define 'forces'
    3) Mates do NOT define joint precedence.
    4) Mates cannot be turned on or off based on time or contact
    5) Mates cannot have intermittent contact

    THAT SAID - I have seen some very clever work using hidden parts (tracks) to define the path of (for instance) an end effector. At that point you can usually drive a single mate (using the animate capability) to move the end effector along the path (the positions of the remaining parts are determined by the solutions derived from the 3D constraint solvers). There are no forces, but this often gives the desired results.

    Re your other question - usually a parallel mate will keep a component in the correct orientation as it is moved through a range of motion.

    Sorry if this is vague - again a document for the community to chew on is usually the fasted route to an answer.

    Good luck - Philip.
    Philip Thomas - Onshape
  • john_mcclaryjohn_mcclary Member, Developers Posts: 2,410 PRO
    Defiantly link an example model, or a screen shot.

    This sounds like a fun challenge I'd like to sink my teeth into

    The order of mates do not matter, all mates are solved at the same time. (Or so I have heard from an OnShape employee a while back)

    If what you are describing is what I think it is, you may need to model an invisible track as @philip_thomas was suggesting.

    Here is an example of a rotary piston with a valve that only opens at the 12 o'clock position. It was mated by a single tangent mate by attaching a point on the valve to the outside surface of the transparent "Track"

  • john_mcclaryjohn_mcclary Member, Developers Posts: 2,410 PRO
    Here is a quick and dirty example of a cart on a track:

    Had to use a few work arounds to get it to work properly, but every once in a while a tangent mate will flip on the wheel and put the axle out of position. but a quick suppress->rotate->unsuppress will fix it.

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