Here's a problem I keep butting up against. Say I have a model that needs flapping wings, or arms that go up and down. What I usually do is join a linkage from the body to the arm as in the above picture. The problem has always been the limited range of movement. As the arm reaches its upper and lower position the angle of the pushrod reduces to the point where it has no pressure. The end result is that the maximum range of movement is no more than about 100 degrees. 

There are a few different things that I can try. A rack and pinion should do the job as would a belt drive

So to that end. I've mocked up a prototype using a paper belt. The central upright is glued to the belt and another point glued to the a rotor with arm attached. 

So – head goes up and down, arm movement is over 190 degrees!

I'll be completing the mock up to make sure that it all works properly with two arms then I'll make a download of the mechanism available.