CourseWiki/Engineering Fundamentals/Power Transmission

Keys and Keyways in Shaft-Hub Connections

Trace torque through a keyed shaft-hub joint, identify common key forms, and distinguish key fit from axial retention.

A key joins the shaft slot to the hub slot to transmit torque

A key is a separate piece fitted between a shaft and the bore of a hub, the part of a pulley, sprocket or gear that fits around the shaft. Contact between the key and slot faces transfers turning force. Here, keyseat means the recess in the shaft and keyway means the matching slot in the hub. Other references sometimes use keyway for both; always identify which part is being cut.

Separate end views of a shaft keyseat and hub keyway, followed by an assembled section with a key spanning both recesses. — Schematic parallel-key joint. Clearances are exaggerated; obtain dimensions and fits from the drawing.

Trace the turning load through the connection: shaft slot face → key → hub slot face, or the reverse when the hub drives the shaft. At the rim, the pulley, sprocket or gear transfers the load through its belt, chain or mating gear teeth. Not every hub is keyed; check how the actual assembly connects the hub to the shaft.

Identify the key by its cross-section and seating shape; Practice

Identify the key by its cross-section and seating shape

Key	Recognizing feature
Square parallel key	Equal width and height in cross-section; uniform section along its working length.
Rectangular parallel key	Unequal width and height; uniform section along its working length.
Woodruff key	A segment of a disk, with a curved bottom seated in a matching circular recess in the shaft.
Gib-head key	A tapered key with a projecting head that provides a place to engage it for removal.

Public reference for key forms

Engineering Institute of Technology, Fundamentals of Mechanical Engineering introduces keys, keyways and the common key types in its keys section. No standard size table or shaft-to-key size rule is reproduced here.

Practice identifying keys

Check key fit and axial retention as separate requirements

For a parallel key, the side fit controls how the turning load is carried. Excessive side clearance permits movement; an overly tight fit can prevent assembly. Verify the specified top clearance too. A tapered key has a different seating action, so do not apply a parallel-key fit rule to it.

Use the drawing's key designation, dimensions and tolerances. Do not choose a key size from shaft diameter alone or assume that half the key height belongs in each part.

The key does not by itself define how the hub is held against movement along the shaft. Check the assembly drawing for axial retention as well as torque transmission. A hub that turns with the shaft is not necessarily secured against sliding along it.

Bearings support the shaft relative to the housing; the keyed hub transfers torque between the shaft and the attached rotating part. For the bearing arrangement and the shaft's axial location, see Shaft Support and Axial Adjustment.

To make and inspect the recesses, continue with Machining Keyseats and Keyways. That lesson connects shaft milling to broaching a through hub keyway.

Appears in

Belt, Chain and Gear Drives DET 330 DET 403 DET Machining Keyseats and Broaching