Adding Output of Two Rack and Pinion Relations

steve_cohen

I have a mechanism that I would like to model, and am looking for advice on how to do it in Onshape.

I have a belt between two pulleys (see figure). 

Pulley A has a revolute joint to a fixed point P.  Pulley B is attached to one end of an arm, and the other end has a revolute joint to the same fixed point P.  I have a pen attached to the belt.

When I rotate Pulley A (blue arrow), I want the pen to move.  I can do this with a Rack and Pinion relation.  When I rotate the arm (gray arrow), with Pulley A fixed, I want the pen to move as it would in a real system.  This is where it gets tricky.   When the arm moves, the belt winds up on one side of Pulley A and unwinds on the other.  That has a similar effect of rotating Pulley A, moving the pen closer or farther from Pulley A.  I can also do this with a Rack and Pinion relation.

But I want to rotate the arm and Pulley A at the same time, so I need to add those two inputs (pulley rotation and arm rotation). 

Any suggestions?

Also, how do I put an image into the body of this post instead of attaching?

S1mon

Use the insert image tool. If you have an image on your clipboard you should be able to paste into this box. Otherwise you can drag and drop or choose files from your local drive. It supports JPG, PNG and GIF but I don't think it supports PDF directly, which is what you attached.