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I tried in math.stackexchange.com and they send me here. First: Don't crucify me for not "doing math", or asking a dumb question. School was a long time ago, and lose what you don't use. My current math knowledge ends with "rule of proportion" (Correct term? I'm not a native English speaker).

Anyway: I recently came across a video speaking about 4-bar linkages, which linked this site: https://dynref.engr.illinois.edu/aml.html, which had this lovely image I had to try to recreate: Peaucellier-Lipkin linkage: Peaucellier-Lipkin linkage
However, I failed to make it work in onshape. Apparently, one or more bar ratios are wrong. How do I calculate those? All I can find about that topic is so far over my head in terms of math, it might be as well Swahili written in Chinese characters.

This is where I am right now:This is where I am right now: 8-Bar-Link_OnshapeCAD

AB = BC = Arbitrary chosen 50mm
CD = CE = EF = DF = Arbitrary chosen 60mm
AD = AE = … Whatever mm. I just connected the points in my sketch.

Everything moves as it should until I connect bar AD to D, then it is over constrained for … Having the CD too short? … Too long? Wrong starting angle between CD and CE? (Hence AD/AE wrong length)
The example image 1 does look like it has 90° angles all around in that square.


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  • $\begingroup$ youtube.com/watch?v=lDIYCJlfZWg $\endgroup$
    – jsotola
    Apr 5 at 16:38
  • $\begingroup$ AB is the body. $\endgroup$
    – joojaa
    Apr 6 at 14:02
  • $\begingroup$ What is weird in the model, when I tried to move the disconnected arm at point D, the rest of the mechanism also moves. This I think should not happen. So there might be an extra constraint or it might be a bug in OnShape. $\endgroup$ Apr 6 at 16:36
  • $\begingroup$ In the original, AB presumably cannot move, but in your version it looks like it can. $\endgroup$
    – RC_23
    Apr 7 at 4:44

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