Rack Gears can be used to turn rotary motion into linear motion.
 
  
Use any VEX spur gear as a pinion to drive these rack gears.
  
Stack multiple rack gears back to back for longer motions.
  
Combine these with a VEX linear slide and Rack Gear Bracket (sold separately) to create a simple linear actuator.
  
Now with a new design! The Rack Gear v2 is wider and thereby more compatible with VEX High Strength gears. v2 has also eliminated the brass inserts from the original design, and replaced them with molded-in nut pockets. Now you can use 8-32 screws!