Four key families cover the vast majority of industrial keyed joints: parallel (square and rectangular), Woodruff (semicircular), taper, and Gib-head, with feather keys acting as a sliding-fit variant of the parallel family [S1][S2][S5].
Selection is dictated by shaft diameter, torque direction, axial load, and whether the hub must slide or be removed repeatedly, not by tradition or machinist preference alone [S4][S5].
Parallel keys: the default for general-purpose shafts
Parallel keys carry a constant cross-section along their full length and are cut from square or rectangular key stock, with square sections typically specified for shafts from 0.25 to 1.0 in. diameter and rectangular sections used on shafts between roughly 1 and 20 in. diameter [S2].
The shaft keyway can be milled with an end mill, and a keyslot is end-milled or circular-saw cut, making this family the cheapest and fastest to produce in a maintenance shop [S2][S5]. Parallel keys are recommended where torque is unidirectional, starting loads are modest, and the hub may need to slide axially or be withdrawn periodically, otherwise the hub is supplementarily secured with set screws [S1][S2]. A common failure mode is the key walking out of the assembly under vibration or reversing rotation, after which it can fall into the machine and the previously secured component spins free [S2]. For more on shaft fastening options, parallel keys sit at the low-cost, medium-duty end of the available methods.
Woodruff keys: semicircular pockets for small and tapered shafts
Woodruff keys are semicircular discs, formed by a circular cutter, and they seat in a deep pocket milled into the shaft, with their flat top protruding into a standard keyway in the hub [S3][S4].
They are commonly stocked for shafts from 0.25 to 2.5 in. diameter, and their self-aligning curved base automatically centres on tapered shaft ends, which is why they are the standard on small-engine crankshafts, motor shafts, and machine tool spindles [S2][S5]. Because the pocket removes more material from the shaft than a parallel keyway, Woodruff keys weaken the shaft and are restricted to light-duty service; mounting two in line to carry higher loads is rarely cost-effective [S2][S5]. A typical Woodruff key installation cannot walk out of the shaft because the disc is captive in its pocket, which is the main reason it survives the harsh vibration environment of chainsaw and lawnmower drives [S3][S5].
Taper and gib-head keys: friction-locked for reversing and heavy loads
Taper keys are wedge-shaped along their length, typically on a 1:100 slope, and are hammered into place so that the taper generates a clamping force between hub and shaft, locking everything by friction rather than by side-fit contact alone [S4][S5].
A Gib-head key adds a protruding head on one end, which acts as a stop during installation and provides a purchase point for a puller or screwdriver during removal, eliminating the need to strike the shaft face with a hammer and the risk of damage that creates [S1][S3][S4]. This head also serves as a visual indicator: a tight fit with the gib head sitting proud of the hub is a sign of an adequate connection, while a loose or recessed head points to an under-driven key [S1][S4]. Taper and gib-head keys are preferred for heavy, reversing, or shock-loaded drives such as mill gearing, crusher shafts, and large gear hubs, where a parallel key would quickly work loose [S2][S5]. The trade-off is that the hub position is fixed by the key, the keyway is harder to machine accurately, and the joint cannot tolerate much axial movement of the hub on the shaft. A practical note for layout: couplings and pillow blocks often do not leave room to remove a gib-head key from behind, which forces a parallel key in those locations [S2].
Feather keys: sliding-fit variant for adjustable hubs
Feather keys are a parallel-key sub-type, rectangular with no taper, fixed either to the shaft or to the hub so that the other component can slide axially without losing torque transmission [S1][S4].
When fixed to the shaft, a closed mating groove is cut in the shaft surface to capture the key, and the hub runs back and forth over it; when fixed to the hub, a double-ended key is pressed into the component and the whole assembly slides on a plain shaft [S1][S4][S5]. The fit is always clearance between the key and the moving keyway, with the contact that transmits torque restricted to the parallel opposite faces [S4]. Common applications include sliding gear selectors in manual gearboxes, clutch hubs, and variable-position sprockets where the drive position has to change under load [S4][S5]. Feather keys are usually retained to the shaft with one or two socket-head cap screws through the key body to stop the key migrating with the hub [S5].
Selection criteria: shaft size, torque direction, and hub removal
The four families line up against four practical decision criteria, drawn directly from the research, as follows: shaft diameter range, suitability for reversing or shock loads, whether the hub must slide axially, and whether the hub must be removable from the end of the shaft [S2][S4][S5].
Parallel keys win on availability and cost across the widest shaft range (0.25 to 20 in.), but lose on reversing and shock duty because they can walk out under vibration [S2]. Taper and Gib-head keys win on reversing and shock duty because the wedge locks by friction, but lose on hub sliding and on end-of-shaft removal where there is no room behind the hub for the head [S2][S4]. Woodruff keys win on small and tapered shafts (typically 0.25 to 2.5 in.) where self-aligning fit and captive installation matter more than torque capacity, and lose on large-diameter, high-torque drives because the deep pocket weakens the shaft [S2][S5]. Feather keys win uniquely where the hub must slide axially under torque, and lose everywhere else because the clearance fit lowers the torque capacity relative to a side-fitted parallel key [S4][S5]. For a related materials decision, the Shaft Key Material Selection for High Torque: Grade Match Rules guide covers how key stock grade interacts with these geometric choices.
Standards, fits, and common failure modes
Key and keyway sizes are controlled by ISO, BS, DIN, and ANSI standards, which is what lets a hub from one manufacturer accept a key supplied by another as long as both conform to the same standard [S4][S5]. Standard parallel key sizes in steel key stock are produced in lengths over 1 ft, then cut to length, so spares are cheap and stocked globally [S3][S5].
Two fit classes dominate: a close fit in both shaft and hub keyways, used where the key must locate precisely (typical gearboxes), and a sliding fit in the hub with a close fit on the shaft, which is what defines a feather-key arrangement [S5]. Shaft keys with square corners will not fully seat in filleted keyways, a common cause of stress concentration and premature fatigue, so keyway corner geometry has to be specified to match the key [S3]. The recurring failure modes across all four types are: key shearing from overload, key rolling or walking out under reversing load, hub slip from a worn keyway, and shaft cracking initiated at the keyway stress concentration, all of which are addressed by matching the key type, fit, and material to the duty rather than defaulting to the cheapest stocked parallel key [S1][S2][S3][S5]. For broader context on how keys relate to the rest of the shaft-hub assembly, see the overview of shaft key geometry and the shaft coupling page, which covers when a keyed joint gives way to a flexible coupling in the drivetrain.
Trackable signals for 2026 specification updates to watch: any revision to ISO 773 or ISO 2491 parallel-key dimension tables, and any new high-cycle fatigue data for Woodruff-keyed small-engine shafts, which together govern the four families covered here. For related geometry decisions, shaft collars and taper bush selections often interact with key choice when a hub is both axially located and keyed.