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Mates: Geneva Mechanism

Lewis_MainaLewis_Maina Member Posts: 9
Hi, am trying to simulate Spherical Geneva Mechanism as per the attached Onshape link:  https://cad.onshape.com/documents/f93d944f6e541c2b5367d3ea/w/a181825716223b594e276455/e/c718ecc2d5f5ee8409e0bca0?renderMode=0&uiState=62c342f05073a76df8bbfe79

Please help with the steps I should use.
Thanks

Answers

  • MichaelPascoeMichaelPascoe Member Posts: 1,698 PRO
    This one will be tricky. @john_mcclary may know how to do something like this.

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  • eric_pestyeric_pesty Member Posts: 1,462 PRO
    That's where having the ability to use equations in gear relations would come in handy! Although I do wonder about the performance implications!
    You should be able to get "one cycle" using a tangent mate but I don't see how you'd make it to keep going.
    Otherwise you'd have to create a single "helper" surface describing the follower path and mate it to that but I'm not sure how you'd go about that!
  • MichaelPascoeMichaelPascoe Member Posts: 1,698 PRO
    Knew you could do it @john_mcclary.

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  • eric_pestyeric_pesty Member Posts: 1,462 PRO
    That's cool!
    I was thinking along the line of a more "direct" mate. This involves more "cheating" (I don't mean that in a bad way at all!) but works great!

    I can see how the straight section of sketch2 is 1" long and the "double rad" bump is also 1" long as these each correspond to 1/2 turn of the driver. I guess the .25 bump height is arbitrary and just has to match with the second rack relation. I guess 1in per revolution implies 1/4 in step for a 1/4 turn.

    I guess I understand how it works but I just wasn't seeing how you would go about coming up with it but I think I'm starting to see it...
    Link the horizontal sliding of the helper part to the input rotation, and then "create" a different degree of freedom that is going to move in steps and drive the output.
    I was going to ask how the step work out to be circular segments but I see they are only approximations. 
    I guess you could make this more accurate by taking a few measurements along one cycle and using a spline instead of circular arks in the helper profile...
  • john_mcclaryjohn_mcclary Member, Developers Posts: 3,890 PRO
    yea, it's difficult to put into words, so I just gave the result.
    The spline and some math would probably more accurate, but this appeared to be accurate enough for the scale of the model.

    If there is some additional gearing attached to the output gear, it may begin to show inaccuracies, at which point the sketch that defines the helper surface won't break if you want to modify the crap out of it. (at least that seems to be true with past models I've done this with)
    So, you can try some spline math and get it perfect.

    Honestly, I did this at lunch one handed eating a pizza over my right shoulder, while drawing on the monitor off my left shoulder, in-between solidworks save/regens trying to get a customer approval drawing out... So, I was all criss-crossed just going for a quick concept, before the Boss peaked around the corner  :smile:

    There is no other way to do this that I am aware of. Since there is no collision detection in Onshape, we need to explicitly define every motion, then link them together with a 'relation' so we only have one mate being animated.
  • bryan_lagrangebryan_lagrange Member, User Group Leader Posts: 792 ✭✭✭✭✭
    What can't @john_mcclary animate. Wow!
    Bryan Lagrange
    Twitter: @BryanLAGdesign

  • john_mcclaryjohn_mcclary Member, Developers Posts: 3,890 PRO
    What can't @john_mcclary animate. Wow!
    literally this:
    https://www.youtube.com/watch?v=pR7khnO-J7U
    I tried in Onshape, but it just wouldn't compute. So I had to call on Alnis to put it blender :)

  • eric_pestyeric_pesty Member Posts: 1,462 PRO
    This was my thought but not sure how to get it to get around the bottom corner... it works on one "cycle" though...

    https://cad.onshape.com/documents/5e3f3f6f4023599ee470e163/w/f02f97b18b159f71a3e48597/e/3169373f38b50f56a14820a6

  • john_mcclaryjohn_mcclary Member, Developers Posts: 3,890 PRO
    that's the nice part of the method I use, it scales easy.

    The helper path is only 1 rotation of the large gear.
    So, pattern it out for how many rotations and you get a full animation.

    here are 12 rotations

  • matthew_stacymatthew_stacy Member Posts: 475 PRO
    That is a phenomenal animation that John McClary created. 

    However, I for one, remain hopeful that Onshape will develop to the point where we can incorporate sliding and/or rolling contact mates sufficiently robust to actually create functional-digital-prototypes to evaluate the functionality of our designs.




  • john_mcclaryjohn_mcclary Member, Developers Posts: 3,890 PRO
    That would be nice, but I won't hold my breath. A better animation studio would be more likely to come first.
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