Thanks for all the replies so far. Rod - thanks for the pics of your telescope. Its good to see that the worm around the gear idea works in practice. jenchris - yup thats sort of what I'm trying to do. alocky - thanks. I'm still not convinced that the one would work and the other would not. Surely if they can both convert a linear force into a rotational force they would both be able to spin around the fixed gear? I'm still scratching my head...
As for the rack and pinion, I've revised the diagram. I tried to do a force diagram analysis... If the motor moves the rack to the left then there will be a resultant force to the right on the red frame. To my thinking this will make the red frame rotate (with the rack) anti-clockwise around the pinion. Is this correct?
The reason for wanting to do it this way? In the application I'm thinking of, it would use less materials plus I can mount items to the red frame rather than a smaller gear (or another part attached to the gear). It might not end up being practical but its worth looking into.
