A shaft key is a positive-locking mechanical element that transmits torque between a shaft and a hub through a fitted key and a mating keyway; selection hinges on three independent decisions — geometry (square / parallel / Woodruff / tapered / splined), material grade (typically C45 / 1045 medium-carbon or 40Cr / 4140 alloy), and the ISO 773 / DIN 6885 tolerance field that controls clearance in the keyway.
Shaft keys remain the lowest-cost, lowest-complexity torque-transmission method for general industrial gearboxes, pumps, and conveyor drives, and the catalog universe spans key cross-sections from 2×2 mm up to 50×28 mm for shaft diameters from roughly 6 mm to over 500 mm. For engineers who routinely specify shaft couplings and shaft collars, the same parallel-key logic carries over: a shaft key is the friction-free "third hand" that keeps a coupling hub from slipping on its shaft.
Key Geometry Options and Where Each One Fits
Parallel keys (square b×b or rectangular b×h, e.g. 10×8 mm on a 30 mm shaft) are the default pick for general industrial drives, manufactured to ISO 773 and seated in a keyway cut to ISO R773 / DIN 6885 with normal clearance (N9 key in Js9 keyway) or close clearance (P9 key in P9 keyway) [S1].
Woodruff keys (half-moon, sized by the W-number system, e.g. W-25 for a 25 mm shaft) are used where the keyway must be milled with a circular cutter rather than a broach or end mill; their self-aligning seating makes them common on production-line automotive shafts and small motors where a single-pass keyway operation is a throughput priority. Tapered keys (Gib-head type, with a 1:100 taper per ISO 774 / DIN 6887) handle the high-shock, reversibility, and frequent-removal cases — mill drives, crusher couplings, and large crane slewing rings — because the taper lets the fitter preload the key to eliminate backlash.
Feather keys and sliding keys (DIN 6886 flat or gib-head) are the right answer when the hub has to move axially along the shaft under load — for example a machine-tool spindle drive gear. Splined shafts (per DIN 5480 / ISO 4156 involute, or SAE J498 straight-side) supersede single keys above roughly 80 mm shaft diameter or above 50 kW transmitted power, where a single keyway would be a stress-concentration liability.
Material and Hardness Selection
Standard key stock is cold-drawn medium-carbon steel: C45 (1.0503, AISI 1045) in the 220–250 HB range for general duty, or 40Cr (1.7035, AISI 4140) quenched-and-tempered to 28–32 HRC for high-shock duty such as rock-crusher pinions and rolling-mill drives [S2].
Stainless keys (A2-70 / A4-80, 1.4301 / 1.4401) are specified for food-grade mixers, pharmaceutical agitators, and marine deck machinery where corrosion would seize a carbon-steel key in its keyway. For elevated-temperature service above 300 °C — turbine auxiliaries, exhaust-fan shafts — the move is to 1.4923 / X22CrMoV12-1 (12 % Cr martensitic) or to an austenitic A286 / 1.4980 fastener-grade material that retains hardness at temperature.
The shaft-key hardness rule of thumb is hard-key / soft-keyway: the key should be at least 10 HB harder than the shaft keyway surface, otherwise under repeated torque the softer material yields, the key rocks, and the keyway walls frett. A C45 key (≈230 HB) on a 42CrMo4 shaft (≈260 HB) violates this rule; flip the pairing or upgrade the key.
Tolerance Fields, Fits, and Why They Matter
ISO 773 and DIN 6885-1 define three standard key fits: loose (key N9 in keyway Js9), normal (key N9 in keyway P9), and tight (key P9 in keyway P9); the tighter the fit, the less backlash but the harder the assembly and the more likely the key will need a press [S1].
For general industrial gear-motor output shafts, normal fit is the catalog default. For high-cyclic-load applications (punch presses, vibrating screens), the close-clearance variant reduces fretting wear, and a key of 0.1–0.2 mm shim-stock feeler gap will not pass on a quality-control bench. On a reamed keyway for a heavy rolling-mill drive, the tight fit is mandatory because the impact loading of the first tooth meshing is otherwise taken as a hammering shock on the key.
Key-length selection is independent of cross-section: a general rule is that the key length should be 0.8 to 1.0 times the hub length, and never longer than 1.5× hub length — over-long keys introduce a step-loading stress peak at the hub end and break. Hub length itself is set so that the contact pressure on the key flank stays below roughly 60 MPa for steel-on-steel, which on a 10×8 mm key at 200 N·m torque gives a required bearing length of about 42 mm.
Sizing the Key to Shaft Diameter and Torque
The standard cross-section table pairs shaft diameter to key size: 6–8 mm shaft → 2×2 mm, 10–12 mm → 4×4 mm, 17–22 mm → 6×6 mm, 30–38 mm → 10×8 mm, 44–50 mm → 14×9 mm, 58–65 mm → 18×11 mm, 70–80 mm → 20×12 mm, and 85–95 mm → 24×14 mm, per ISO 773 [S1].
Torque capacity scales with the working height of the key (h/2 for a square key) times the key's bearing length. For a 10×8 mm square key with 50 mm working length on a 35 mm shaft, the permissible torque is roughly 160 N·m in normal service and 250 N·m in shock-free service; halve those numbers for reversing duty and halve them again for high-cycle applications. Woodruff keys derate further — typically to 60–70% of a parallel-key of equivalent depth — because the W-key's seating depth is shallower than its nominal and the load path is shorter.
For drive design, prefer shaft couplings that already incorporate a finished keyway per ISO 773 (most flexible couplings in the > 20 mm bore range do); this removes the field-machining step that is the single biggest source of fit complaints. A shaft collar with a finished keyway and a setscrew is the cheap-and-cheerful alternative for low-torque positioning rather than power transmission.
Failure Modes and When NOT to Use a Single Key
The classic shaft-key failure modes are: key shearing (cross-section too small for the torque), key crushing (keyway walls yield because the key material is too soft or the bearing length is too short), keyway wall frettage (clearance fit plus cyclic load), and keyway fracture at the shaft (stress concentration at the keyway end-radius — the #1 cause of shaft fatigue failure in gear-motor output shafts) [S2].
A single key should NOT be specified for: shaft diameter above roughly 80 mm under continuous reversing load (use two keys 180° apart or, better, an involute spline per DIN 5480); applications above roughly 5000 rpm where key unbalance will throw the rotor; any service where the key sits in a corrosive or abrasive environment and cannot be greased; or any duty requiring precise angular timing between shaft and hub (splines or interference fits are mandatory). Woodruff keys are also not recommended for thin-walled hubs, because the W-key's curved bottom pops the hub out under heavy torque.
If the duty is fundamentally axial-positioning rather than torque transmission — a gear that has to slide on the shaft — the right part is a feather key (DIN 6886) or a sliding key with a retainer, not a driven tapered key that will lock the hub in place.
Standards, Sourcing, and Cross-References That Matter
The standards to cite on a drawing are: ISO 773 (dimensions and tolerances of parallel keys), ISO 774 (tapered keys, 1:100 taper), ISO 2491 (keyways — depth and profile), DIN 6885-1 (parallel keys, the German equivalent still widely used in EU machinery), DIN 6887 (tapered and Gib-head keys), and DIN 6886 (feather keys). For US-built equipment, ASME B18.25 series keys are the direct equivalent. Woodruff keys are still named to the historical W-number list (W-3 through W-50, mapped to ANSI B17.2). [S1]
Source from stock key suppliers (Misumi, SKF, norelem, Bossard, McMaster-Carr, Travers) rather than milling from bar stock — keystock sold to ISO 773 is supplied to the ISO tolerance, ground to length, and chamfered, whereas in-house cut keys routinely violate the N9 / P9 tolerance field. For first-fit serviceability, specify the key as a separately line-itemed spare part on the BOM with the part number, length, and cross-section.
For a broader selection map of mechanical drive elements that pair with keyed shafts, the locking assembly sizing guide covers the keyless alternative when fretting or maintenance cost is the deciding factor. For gearbox and powertrain architectures that combine keyed couplings with hydraulic or electric actuators, the truck-mounted crane types and classifications spec map gives a worked example of parallel-key sizing on a planetary slew drive.
Trackable next-node signals: (1) confirm whether the connected coupling / gear / pulley is delivered with a keyway finished to ISO 773 — many "finish-bore" couplings ship with the bore drilled but the keyway left to the buyer; (2) verify the key material certificate on the MTC matches the drawing call-out (C45 vs 40Cr is a common substitution error); (3) for any drive above 200 N·m, calculate the hub bearing length explicitly rather than copying the previous project's hub length.