A "rack screw" (often referred to in the context of "rack and pinion") is a mechanical component that works as part of a system used to convert rotational motion into linear motion.

In a rack and pinion setup:

• Rack: This is a flat, linear gear that moves horizontally.
• Pinion: This is a small, round gear that meshes with the rack to convert rotational movement (from the pinion) into linear movement (of the rack).

In some contexts, a "rack screw" might refer to a screw used in the assembly of a rack and pinion system or other gear-based systems. It might also refer to screws used in rack mounting (e.g., in server racks) or as a fastening method in mechanical applications involving racks.

If you meant something more specific about a "rack screw," feel free to clarify!