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Crankshaft Mating

tom_gullantom_gullan Member Posts: 14

I'm really new to Onshape and CAD. I have a problem and wonder if anyone can help.

I've designed an engine casing, crankshaft and conrod.

I have secured the crankshaft to the casing using the Revolute Mate and I am able to animate it using the mating menu.

I then connected the conrod to the crankshaft journal, again using the Revolute. However, when I now animate the crankshaft, the conrod rotates as part of the crankshaft, passing through the walls of the casing. The conrod does not rotate round the crankshaft journal and I am unable to get the conrod to slide parallel with the centre of the cylinder lining.

I have watched numerous Online tutorials but have not been able to find any information on this problem. Please, can anyone point me?

Kind Regards


  • matthew_stacymatthew_stacy Member Posts: 111 PRO
    @tom_gullan , it sounds like the trick is to figure out how to mate the small end of the connecting rod to keep it on the center-line of the cylinder.

    One thought is to add a piston pin that is mated revolute to the connecting rod and planar with a mate connector on the cylinder.  I'd be happy to give it a whirl if you're willing to share a link to your document.


  • tom_gullantom_gullan Member Posts: 14
    Good afternoon Matt,

    Thank you for that offer.  How do I go about doing that?

  • john_mcclaryjohn_mcclary Member, Developers Posts: 2,962 PRO
    The only mates you need for a piston/crankshaft are:
    Slider (for the piston and cylinder wall)
    Revolute (wrist pin and piston)
    Revolute (connecting rod and crankshaft)
    Revolute (crankshaft main bearing journal) In this case I just have a stray pin on the centerline of the crankshaft.


  • matthew_stacymatthew_stacy Member Posts: 111 PRO
    @tom_gullan , the ability to share documents is a unique strength of the Onshape platform.  For starters you can make your document PUBLIC and give other users the URL to it.  Then anyone can view your document and copy it.  From there you can fine tune sharing privileges for individual users (edit, link, etc.).

    To share a document click the blue Share icon in the upper right-hand-corner of your screen.  In the share dialog box select the PUBLIC tab and click the MAKE PUBLIC button if you would like to allow other users (all users) to copy the document. 

    Then click the Link Sharing tab and click the TURN ON LINK SHARING button.  Notice the COPY TO CLIPBOARD button that you can use to paste the URL for your document into a forum post, email, etc.  The share dialog box should look something like the attached screen shot.

    Like @[email protected]_mcclary showed earlier in this thread, you're going to need mate one or more additional  components to the top end of your connecting rod to constrain the motion properly.  A wrist pin and piston will do the trick nicely.  If you click on the URL that John so kindly provided you can open his document and copy the work space to make your own edits and see the inner workings behind the curtain.  Enjoy!

  • tom_gullantom_gullan Member Posts: 14
    Good evening Matthew, John,

    Thank you very much for your time and effort. 

    As I said, I'm really new to CAD and Onshape. I'm not totally sure how to to construct this but I will attempt to reverse engineer your assembly John.

    Once again, thank you.

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