A shaft key is a precisely machined metal insert that occupies a keyseat cut into the shaft and a matching keyway cut into the hub of a gear, pulley, sprocket, or coupling, blocking relative rotation and transmitting torque across the joint [S2][S3]. The keyed joint is the default low-cost power-transmission element in industrial drives, and the seven functional families — parallel, Woodruff, taper, gib-head, feather, spline, and round — cover roughly 95% of catalogued selections [S2].
Selection is governed by three engineering levers: the torque the joint must carry without slipping, whether the hub must slide axially along the shaft, and the relative hardness between key, shaft, and hub material. Parallel keys dominate general industrial use, Woodruff keys own tapered-bore and milling-friendly niches, splines carry the highest torque densities, and round keys remain the cheapest to retrofit in the field [S2][S3][S4].
How a Keyed Joint Is Built and Loaded
A keyed joint consists of a shaft keyseat (a slot milled along the shaft) and a hub keyway (a pocket broached or slot-cut into the bore of the mating element) that together trap the key and resist tangential slip [S3]. Keyways are produced on key-seating machines, by broaching, milling, planing, shaping, or slotting, with broaching the standard high-volume method for hub pockets and end-mill or side-mill cutters the default for shaft seats [S3].
The key carries load in two modes: bearing (compression on the side flanks of the keyseat and keyway) and shear (across the key's cross-section). In most industrial designs the bearing stress on the key flanks — not the shear stress — is the limiting criterion, and a soft key on a hardened shaft is intentionally specified so the key is the sacrificial element that protects the more expensive shaft and hub [S3]. Machine keys are normally the same material as the shaft and equal or greater in hardness so wear is distributed, while a "sacrificial key" is intentionally softer to fail first under overload [S3].
Parallel, Square, and Rectangular Sunk Keys
Parallel keys have uniform width and thickness across their full length, with two sub-classes: square (width = thickness) and rectangular (width > thickness), both sides flat and parallel [S2][S3]. They are the most specified general-purpose sunk keys, used where the hub is fixed axially, and are typically paired with a set screw or end plate when axial location must be positive [S2].
Square sunk keys are normally specified for shaft diameters from 0.25 to 1.0 in, with larger square stock available up to roughly 6.5 in shaft diameter; designers pick square over rectangular when they want maximum depth engagement for a given shaft size [S3]. Parallel sunk keys are taperless, which lets a gear, pulley, or coupling slide along the shaft for periodic clearance or adjustment — and that same lack of taper is why they transmit unidirectional torque reliably only at low starting loads unless an additional axial clamp is added [S2][S3]. Standard stock forms are produced to ISO 2491 (British Standard BS 4235 parallel keys) and DIN 6885 Form A (square ends) / Form B (round ends) / Form C (one round, one square) / Form D (both round) in metric sizes from 2×2 to 100×50 mm width × height.
Taper, Gib-Head, and Woodruff Keys
Taper keys are wedges with a slope of 1:100 (1%) on the height, machined along the length so that driving the key in forces the hub tightly onto the shaft, and they hold best in reverse-mount or heavy-shock applications where the hub must be both radially and axially locked by friction [S2][S4]. A gib-head key is a taper key with a protruding head at the large end, used so the key can be driven out for disassembly with a hammer or drift without damaging the shaft — a common choice on crusher shafts, mill pinions, and other maintenance-heavy drives [S2][S3].
Woodruff keys are half-discs cut from flat bar, usually with a 1:2 width-to-radius ratio, and they seat in a deep slot milled with a small-diameter Woodruff cutter, which self-aligns the key and tolerates a moderate shaft taper [S2][S3][S4]. Their disadvantages are a deeper keyseat that reduces shaft fatigue life at the bottom of the slot, and a tendency to rock in the hub under reversing load, which is why Woodruff keys are usually specified for unidirectional light- to medium-duty service rather than for reversing drives [S4]. Typical sizes run from 1/8 in (3.175 mm) up to 1 in (25.4 mm) nominal width, manufactured to ASME B18.25 (formerly USAS B17.1) Woodruff key standards.
Feather Keys, Splines, and Round Keys
Feather keys are parallel keys fastened to either the shaft or the hub so that the other element can slide axially along them — a shaft-mounted feather key rides in a closed slot in the shaft and provides a long sliding track for the hub, while a hub-mounted feather (sometimes called a double-headed feather) drives the shaft from a sliding hub [S2][S3]. They are the standard solution for change-gear boxes, clutch sliding sleeves, and any drive where the hub must reposition without breaking the torque path.
Spline keys are multiple parallel keys machined integrally around the shaft circumference, meshing with matching grooves in the hub; published ISO, DIN, and ANSI spline standards use 4, 6, 10, or 16 teeth for the most common series [S2]. Splines carry the highest torque density of any keyed joint — the load is shared across all teeth simultaneously, so the radial depth per tooth can be much shallower than a single key — and they also self-center the hub, which is critical for gear and pump drives [S2]. Round keys are circular cross-section pins that drop into a drilled half-hole in the shaft and a matching half-hole in the hub, are the cheapest key to produce because no milling is required, and are often used for low-torque farm and conveyor applications where the joint can be reamed to fit in the field [S2].
Selection Criteria and Trade-Off Table
The most useful selection lens lines the seven families up against torque capacity, axial hub movement, reversibility, shaft-stress penalty, and installation cost — these five criteria cover roughly 90% of design decisions on industrial drives [S2][S3][S4]. Parallel (square/rectangular) keys carry moderate torque, fix the hub axially, are reversible only with set-screw backup, give moderate shaft stress concentration (Kt ≈ 2.0), and install cheaply on any key-seater.
Compared with that baseline, taper and gib-head keys carry higher torque through wedging friction, lock the hub axially without a set screw, are not suited to reversing duty, give moderate shaft stress, and need a press or hammer for assembly. Woodruff keys are light- to medium-duty only, allow no axial movement, fail under reversing load due to rocking, give the highest shaft stress (Kt ≈ 2.5–3.0 from the deep seat), but install fastest with a single Woodruff cutter. Feather keys carry moderate torque, permit unlimited axial hub travel, are reversible when shaft-mounted, give moderate shaft stress, and need a closed slot broached on the shaft. Splines carry the highest torque in the group, permit axial movement with annular snap rings, are fully reversible, give the lowest stress concentration factor (Kt ≈ 1.5–1.8), and cost the most in tooling. Round keys are lowest torque, permit limited axial movement, are partially reversible, give low stress, and install fastest of all by simple drilling and reaming.
Failure Modes, Material Choices, and Standards
The four failure modes to design against are key shear (the cross-section yields), key bearing (the flank crushes), key rotation/rocking in the seat, and shaft fatigue crack initiated at the keyseat end radius [S4]. The shaft-stress concentration is controlled by specifying a generous end-mill radius or a profile keyseat (full-radius keyseat cutter, BS 4235 Form B/D) rather than a square-ended profile, and by pushing the keyseat off the shaft shoulder away from the highest bending-moment zone.
Key material is almost always a medium-carbon steel such as AISI 1045 or 4140, case-hardened to 40–50 HRC, or a stainless grade (AISI 303/304) when corrosion resistance matters; the key is usually specified equal to or slightly harder than the shaft so wear is even, while a sacrificial key is intentionally 10–20% softer [S3]. Governing standards are ISO 2491 (parallel keys, keyways, and keyseat profiles), ISO 3912 (Woodruff keys), ISO 1101 (tolerance grades for keyseat depth and width), DIN 6885 Form A/B/C/D, ASME B18.25 for Woodruff keys, and ANSI B17.1 for the older imperial parallel-key series — these should be cited on the drawing callout so the supplier delivers the correct stock and the machine shop cuts the matching seat. For a deeper look at how alternative shaft-hub locking methods compare on install time, reversibility, and concentricity, see Locking Assembly Advantages, Disadvantages, and Spec-by-Spec Selection, and for the related question of how a retaining ring holds a hub axially once the key has set the angular position, see Retaining Ring Pros, Cons, and Spec-by-Spec Selection.
Track the next two signals when specifying shaft keys: (1) whether your supplier still cuts parallel keys to ISO 2491:2024 revision tolerances or has rolled forward to the 2025 amendment on keyseat end-radius, since that change shifts the recommended Kt for shaft fatigue checks; and (2) the available stock for splined shafts in 6- and 10-tooth ANSI B92.1 series, which remains the tightest lead-time item in motor and gearbox sub-assemblies through 2026.
For the relevant spec sheets and selection criteria, see shaft key, shaft collar, and shaft coupling.