rack
A bar with teeth on its face or edge, to work with a pawl as a ratchet allowing movement in one direction only, used for example in a handbrake or crossbow.
rack: toothed bar that converts rotary motion to linear
A rack is a straight bar or rail with uniformly spaced teeth cut along one face or edge. It meshes with a pinion (a small toothed wheel) to convert rotational motion into linear movement, or vice versa. This rack-and-pinion system is one of the oldest and most reliable motion conversion mechanisms in mechanical design, appearing in everything from steering systems to linear actuators.
The teeth on a rack are cut to a specific profile, most commonly involute, matching the tooth form of the pinion that drives it. Module and pitch determine tooth spacing and size. A typical automotive steering rack uses modules between 1.5 and 2.5 mm. The rack itself is usually made from hardened steel, sometimes with a surface hardness of 58-62 HRC, to resist the contact stresses imposed by the rotating pinion. The bar may be ground after hardening to achieve the smooth operation needed in precision applications.
Beyond the pinion-driven version, racks appear in ratchet mechanisms where a pawl (a pivoting lever) engages individual teeth to allow motion in one direction only. A hand winch or the ratchet on a pipe wrench operates this way. In these applications, the rack need not be straight: circular racks are commonly called sprockets or gear rings, though the principle remains identical. The pawl overcomes the rack's resistance to prevent backslip under load.
Common problems and variants
In loaded applications, the teeth wear or chip if the pinion is misaligned or if the lubricant breaks down. Rack-and-pinion steering, widely used in road vehicles, can develop play if the meshing gap increases from wear. Internal damping or preload springs are sometimes added to maintain tight engagement. Plastic-toothed racks appear in low-load applications such as printer carriages, offering quieter operation and corrosion resistance but lower load capacity than steel.
The term "rack" has industrial ancestry: it derives from Middle Dutch rec, meaning to stretch or extend, reflecting the bar's linear character. In workshop language, "racking" a load means distributing it across a storage rack, which uses the word in a different sense entirely, yet both capture the idea of regular spacing and structure.