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Why does final Sliding Mate cause assembly to be overdefined?

roland_hauffroland_hauff Member Posts: 6 ✭
in Community Support


I can drag the plunger and it animates properly but gives the error when I add the final sliding mate.





Best Answers

  • eric_pestyeric_pesty Member Posts: 2,413 PRO
    Answer ✓
    You have too many constraints stopping things from sliding "out of plane". Get rid of the limits on your cylindrical mates, and switch the first two to "revolue":

    https://cad.onshape.com/documents/d27b8fdab628a00a7061b409/w/a6edc118892b03fe0a3c9de4/e/795f4a595e47c8399152a09f


  • eric_pestyeric_pesty Member Posts: 2,413 PRO
    Answer ✓
    roland_hauff said:
    Ok, I think I get it.  The main point is ONLY 1 rotary mate should limit motion in the Z direction.  I tried making the first Revolute, and the last two Cylindrical with no limit in Z direction, and that works as well.
    Thanks #eric_pesty!
    Yes you got it, the key is to avoid having multiple mates constraining the same things. You can "sometimes" get away with it when the result is the same (i.e. two things constraining in Z at exactly the same "depth" but it often causes issues as there is always some tolerance in computer math so best to avoid it.

Answers

  • eric_pestyeric_pesty Member Posts: 2,413 PRO
    Answer ✓
    You have too many constraints stopping things from sliding "out of plane". Get rid of the limits on your cylindrical mates, and switch the first two to "revolue":

    https://cad.onshape.com/documents/d27b8fdab628a00a7061b409/w/a6edc118892b03fe0a3c9de4/e/795f4a595e47c8399152a09f


  • roland_hauffroland_hauff Member Posts: 6 ✭
    Well, that is very interesting.  I started out with the first 3 being Revolute with no limits.  Then tried adding limits as forum posts seem to imply that too MUCH freedom can cause problems.  Then switched all 3 to Cylindrical and at least got motion.

    How did you choose to make just the first 2 Revolute and leave the third cylindrical?
  • roland_hauffroland_hauff Member Posts: 6 ✭
    Ok, I think I get it.  The main point is ONLY 1 rotary mate should limit motion in the Z direction.  I tried making the first Revolute, and the last two Cylindrical with no limit in Z direction, and that works as well.
    Thanks #eric_pesty!
  • eric_pestyeric_pesty Member Posts: 2,413 PRO
    Answer ✓
    roland_hauff said:
    Ok, I think I get it.  The main point is ONLY 1 rotary mate should limit motion in the Z direction.  I tried making the first Revolute, and the last two Cylindrical with no limit in Z direction, and that works as well.
    Thanks #eric_pesty!
    Yes you got it, the key is to avoid having multiple mates constraining the same things. You can "sometimes" get away with it when the result is the same (i.e. two things constraining in Z at exactly the same "depth" but it often causes issues as there is always some tolerance in computer math so best to avoid it.
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