Glossary term
Keyway
A machined slot in a shaft or hub that receives a key to transmit torque between rotating components.
Definition
deviceA keyway is a slot machined in a shaft, hub, pulley, gear, or coupling to receive a key for torque transmission.
A key and keyway create a positive mechanical connection between a rotating shaft and a mounted component. The key bears against the side faces of the keyway and transmits torque by shear and bearing contact. Keyways are common in gears, sprockets, pulleys, flywheels, couplings, pump impellers, and machine shafts, but they also introduce stress concentration and reduce shaft cross-section.
A keyway is a machined slot that accepts a key. The key fits partly into the shaft and partly into the hub of the mounted component. When torque is applied, the key transmits load through shear and bearing pressure on its side faces. This prevents relative rotation between the shaft and the gear, pulley, coupling, or other rotating part.
Design checks
A keyed joint must be checked for key shear, bearing stress, shaft strength, hub strength, fit, alignment, and fatigue. A simplified torque relation for tangential force at shaft radius is:
where T is transmitted torque and d is shaft diameter. That tangential force is then used to evaluate shear stress in the key and compressive bearing stress between key and keyway faces.
The keyway weakens the shaft because it removes material and creates a geometric discontinuity. This is especially important under alternating torque, bending, shock loading, or high-speed rotation. Fillet radius, machining quality, surface finish, residual stress, and fit all affect fatigue life.
Manufacturing and fit
Keyways may be cut by broaching, milling, slotting, shaping, wire EDM, or other processes. The hub keyway and shaft keyway must align and fit the key correctly. Too loose a fit can cause fretting, impact, noise, and progressive wear. Too tight a fit can create assembly damage, distortion, or high residual stress.
Standards often define key sizes for shaft diameters, but the designer must still verify torque, service factor, reversal, shock, corrosion, lubrication, and inspection requirements. Critical rotating machinery may use splines, interference fits, shrink fits, taper locks, or other torque-transfer methods instead of a simple keyway.
Common mistakes
A common mistake is treating a keyway as a harmless detail. It is often the highest stress-concentration feature on a shaft. Another mistake is checking only static torque while ignoring fatigue, start-stop cycles, misalignment, and emergency overload. Good drawings specify keyway dimensions, tolerances, corner radii, surface finish, material, heat treatment, and inspection requirements.