Rack and pinion is one of the oldest and most reliable ways to convert rotary motion into linear motion: a pinion (a small spur gear) rolls along a straight rack, and every rotation of the pinion advances the rack by a fixed distance set by the tooth geometry. It shows up in machine tool feeds, positioning tables, presses, and long-travel automation where a leadscrew would be impractical.
Pressure angle is the first spec to match
The single most important specification when ordering gear rack is the pressure angle. Stock rack is commonly manufactured in both 14½-degree and 20-degree pressure angles, and the two systems are not interchangeable. The pinion must be cut on the same pressure angle as the rack, or the teeth will not engage correctly and the drive will run noisy, wear rapidly, and may even bind. Before you order either component, confirm which pressure-angle system your existing drive uses.
Material, length, and face width
Stock rack is typically made from low-carbon cold-drawn steel and is available in 14½-degree and 20-degree pressure angle in 2, 4, and 6-foot lengths. Allowance is made for cutting and machining, so a standard length can be trimmed to fit the travel you need. Rack in lengths up to 12 feet is available on request for long-travel applications.
Face width matters because it determines how much tooth area engages with the pinion — too narrow a face for the load will quickly wear or break teeth. Match the face width to the pinion face and to the load and duty of the drive. Part-number conventions identify the pitch and length directly; for example, the R-series designates standard steel rack and the TR-series the 20-degree equivalent.
Choose the pinion from the spur gear section
Pinions to run with the rack can be selected from the standard spur gear section of the catalog. Because the rack is straight, the pinion effectively acts as a spur gear rolling on a flat surface — so the same pitch, pressure angle, and tooth-form considerations apply. If you are replacing only the pinion, bring the rack pitch and pressure angle so we can match it. If you are replacing only the rack, we can identify the correct pressure angle and pitch from the pinion.
Special rack can be supplied in other materials, sizes, and pitches for unusual applications, but for most plants a stock rack in the correct pressure angle, cut to length, is the fastest and most economical path back to running.
Catalog item plug: RBC Industrial stocks Martin machined gear rack in 14½-degree and 20-degree pressure angles, in 2, 4, and 6-foot lengths, plus matching spur-gear pinions.