Quote Originally Posted by m_c View Post
Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)
Having recently designed a machine with rack and pinion, where the requirement was speed as opposed to smooth running/accuracy, I would suggest you go with the smallest economical one you can. Nonlinearity will be a real concern with anything larger than 2.

To avoid contamination, mount the rack, with the teeth pointing down

For a visual comparison: