keyway

A keyway is a rectangular slot machined into a shaft, hub, or bore to receive a key, a small metal piece that locks two components together against rotational slip. The keyway itself is purely passive, it is simply the negative space, the recess cut to precise dimensions, that constrains the key and prevents it from moving sideways during torque transmission. Without the keyway, the key would have nothing to grip; without the key sitting in the keyway, the coupling would spin freely.

Keyways are dimensioned to match standard key sizes. A common example is the Woodruff keyway, a semicircular slot used on smaller shafts; another is the parallel keyway, which has flat sides and is far more common in industrial machinery. The slot is typically milled or broached into the shaft during manufacture. Dimensions include the width, depth, and sometimes the radius of the slot bottom, all specified in technical standards that vary by country and application. A mismatch between keyway and key dimensions will cause the joint to bind, rattle, or fail under load.

The keyway is always cut into the weaker of the two parts being joined, typically the shaft rather than the hub, since a shaft is more easily accessible to cutting tools and easier to replace if damaged. Some designs cut keyways into both shaft and bore, creating a double-seated joint with better concentricity but higher manufacturing cost. High-speed or precision applications sometimes avoid keyways entirely in favor of splined shafts or shrink fits, which distribute load over a larger surface and allow no axial play.

Problems arise when the keyway is cut off-center, cut too deep (weakening the shaft), or when corrosion or wear enlarges it. A loose keyway allows fretting, the small relative motion that generates wear debris and heat. Under severe load or shock, a key can ride up and out of a shallow keyway. Regular inspection of keyway condition, particularly in agitated or impact-prone machinery, is necessary to prevent unexpected decoupling.

The term "keyway" refers specifically to the slot in the shaft or hub; the corresponding slot in a coupling or bushing is sometimes called a keyseats, though keyway is used loosely for both. The depth and exact geometry of a keyway are set by industrial standards, most commonly ISO 1575 for parallel keyways and various national standards for Woodruff, tapered, and other specialty forms. Correct keyway design is foundational to any power transmission joint that must remain locked under torque.