In a re-circulating ball screw, ball bearings rest inside a grooved channel on the drive shaft, encapsulated in a bearing housing. The groove is wound helically about the shaft (like a thread), so that when either the ball bearing housing or the shaft is turned it moves up shaft. The rotary motion provided by the motor is subsequently converted to linear motion. This system has the advantage of having very minimal backlash (a problem with sudden changes in speed). Other benefits include durability, accuracy, and low maintenance requirements. (6)
A rack and pinion system utilizes a cog moving on a toothed path. This system is commonly seen on heavy machinery or the long axis of a machine. One of the disadvantages of this system is the complicated lubrication involved. Debris (such as wood dust) has a tendency to get stuck in-between the lubricated gear and track. This debris has the potential to interfere with positional accuracy. (6)