Which motion is converted in a rack-and-pinion mechanism?

• A. Rotary motion of the pinion into linear motion of the rack.
• B. Linear motion of the rack into rotary motion of the pinion.
• C. Pulsed motion into steady motion.
• D. Torque into speed.

Answer: (A)

In a rack-and-pinion, the turning of the pinion (a gear) drives the straight rack along a line. The teeth mesh together, so as the pinion rotates, each tooth pushes against a rack tooth and forces the rack to slide in a straight path. The amount the rack moves for each turn depends on the gear’s pitch and tooth count, but the essential idea is that rotational motion is translated into linear motion. This is the fundamental way the mechanism converts motion type, which is why it’s described as rotary motion of the pinion becoming linear motion of the rack. The other descriptions don’t describe this same rotational-to-translational transformation.