One of the simplest ways of converting rotary to linear motion is a crank.
An early Jason Krugman Studio prototype:
Crank slider - http://vimeo.com/13097366
Another way is with a power screw. Power screws convert speed into torque. They are often very strong but move very slowly.
Scissor Lift Prototype using a linkage and a captive nut / screw mechanism.
http://youtu.be/vXZSMdeQVG0
