From VEX Wiki
Rack gears are flat bars with straight gear teeth. A rack gear is normally combined with a spur gear, which is commonly referred to as the pinion. Rack and pinion setups are designed to convert rotational motion to linear motion. The circular pinion engages the teeth on the rack gear. When rotational motion is applied to the pinion, it moves the rack to the side. These gears are commonly used as part of the steering mechanism on most cars (you may have heard of rack and pinion steering). In robotic design, rack gears are great for applications where linear motion is required. The VEX System has a 19-tooth rack gear that meshes with all of the VEX spur gears. These short rack gears can be put together to form longer sections.
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