Shaft key selection is governed by three hard facts: torque capacity scales with the product of key width × shaft diameter, the keyway is the weakest point in any retained-key joint, and a half-key mock-up is mandatory when balancing a rotor to ISO 21940-32:2012 grade G2.5 or finer [S1].
This guide covers square, rectangular (parallel), Woodruff, taper, and spline-style keys for shafts from 6 mm up to ~500 mm, drawn from the ISO 21940-32 half-key convention [S1], OEM machining practice [S2], and fatigue-method guidance from the Springer mechanical-design corpus (2025-08) [S5].
What a Shaft Key Actually Does — and Why It Fails
A shaft key is a positive-locking torque-transfer element that sits in a shaft keyway and a mating hub keyway, transmitting torque by shear across the key and by compression on the keyway flanks. ISO metric shaft-key series cover shaft diameters from 6 mm to 500 mm with matching key widths from 2 mm to 50 mm and key heights roughly one-quarter of the shaft diameter. [S1]
Failure modes, in order of frequency: hub keyway flank crushing (a compressive/bearing problem), key shear (a transverse-shear problem), key rotation in the keyway (a fit problem), and shaft keyway fatigue propagation. Springer notes the fatigue endurance limit of a keyed shaft drops to roughly 25–40% of an unkeyed, polished specimen of the same diameter because the keyway corner acts as a stress raiser with Kt values commonly reported between 2.0 and 2.7 (2025-08) [S5].
Key Type Comparison on Real Decision Criteria
Four key families cover 90%+ of industrial demand, and the choice hinges on torque, hub travel, alignment tolerance, and rework cost.
Square keys (b × b) are the cheapest, easiest to cut on a horizontal mill, and the most common in general machinery from 6 mm to 100 mm shaft diameter. Rectangular / parallel keys (b × h with h ≈ 0.6–0.75b) are used where vertical clearance is tight, typically low-profile hubs or shaft shoulders. Woodruff keys (semi-circular disc) are self-aligning, easy to replace in the field, but produce a deeper keyway stress concentration and are rarely specified above 75 mm shaft diameter. Taper keys (1:36 or 1:100 taper, gib-head on long variants) deliver the highest torque capacity per unit width because the taper preloads the flanks, but the hub position is fixed by the key-in and must be machined to ±0.05 mm to seat without binding.
For comparison, a 50 mm shaft with a 14 × 9 mm square key in C45 steel transmits roughly 350–450 N·m at a 90 MPa nominal bearing pressure, while the same shaft with a 14 × 9 × 80 mm taper key and gib-head typically reaches 600–800 N·m before the same 90 MPa flank limit is hit (values derived from standard bearing-pressure formulas in [S5], 2025-08).
Sizing Math You Can Do in a Coffee Break
The sizing check a process engineer actually runs: torque capacity T = (σ_b × w × L × d) / 2, where σ_b is allowable bearing pressure, w is key width, L is key length, and d is shaft diameter. Allowable bearing pressure for steel-on-steel, mild shock, and a stationary key typically lands at 90–120 MPa; for shock-loaded drives drop to 50–70 MPa. [S1]
Shear check: T = (τ × w × L × d) / 2, with allowable shear at roughly 0.6 × σ_b for the same material. Length-to-diameter ratio is the second quick test: L should fall between 1.0 × d and 1.5 × d for parallel keys, and the hub length is set so the key does not run out of the keyway in operation. Springer (2025-08) recommends keeping the shaft-keyway corner radius at the maximum value your key standard allows — going from r = 0.2 mm to r = 0.4 mm at the shaft keyway corner can lift fatigue life by a factor of 2–3 at the same torque [S5].
Material, Fit, and Hardness — Where the Real Cost Lives
Stock keys are cold-drawn C45 / AISI 1045 or 40Cr, with tensile strength 600–800 MPa and surface hardness 200–260 HB. For high-cycle drives (gearboxes, motor shafts above 10,000 h service life), keys are specified as case-hardened to 50–55 HRC, with a 1–1.5 mm case depth, to push the wear surface past the rotating-bending fatigue threshold.
Fit discipline: the key-side clearance class is normally P9 in the hub and N9 on the shaft per ISO 773, giving a sliding fit in the hub and a snug drive fit in the shaft. Looser (H9/h9) and tighter (P7/n6) combinations exist; the looser is for ease of assembly with light torque, the tighter is for reversing drives where key float would be a wear problem. A small but recurring failure pattern: spec'ing P9 in both shaft and hub, which makes the joint feel tight on the bench and then walks apart under cyclic load — that is a draft error, not a material problem. For reference, a related plain bearing component, the shaft collar, follows the same ISO 773 fit logic and is often used to axially locate the keyed hub.
Half-Key Convention for Rotor Balancing per ISO 21940-32:2012
ISO 21940-32:2012 specifies that when a key is fitted to a shaft but the mating component (pulley, gear, coupling hub) is balanced without its keyway, a "half-key" of height equal to half the key height must be fitted into the keyway during the balance operation to simulate the mass of the full key [S1].
This is mandatory for grade G2.5 and finer balance qualities, which covers most industrial electric motors (typically G2.5) and machine-tool spindles (G1.0 or G0.4). The half-key is placed so that its top face is flush with the shaft surface — exactly half the key stock above the shaft datum, exactly half in the keyway. Skipping this step gives a residual unbalance roughly equal to half the key mass at the keyway location, which is enough to push a motor from G2.5 to G6.3 or worse, and that shows up as a bearing-temperature rise of 5–10 °C within the first hour of run-in [S1].
Who Should NOT Use a Standard Parallel Key
If the application is reversing, high-cycle, or lives at >80% of the shaft fatigue limit, a parallel key alone is the wrong primary drive. In those cases, switch to a taper key with a gib-head and pull-up bolt, a splined shaft (involute splines per ISO 4156), or a shrink-fit hub — the keyed joint becomes a secondary anti-rotation backup rather than the primary load path. [S1]
Woodruff keys should be avoided on shafts above 75 mm, on reversing drives, and where the hub must be repositioned axially for belt tensioning. The deep semi-circular keyway creates a severe stress raiser; in one Springer test case a 60 mm shaft with a Woodruff key failed at 38% of the unkeyed fatigue limit after 10^6 cycles (2025-08) [S5]. For applications where the keyed hub must drive a flexible coupling, the shaft coupling selection then has to clear the keyway seating length and the hub OD, not just the bore.
Standards, Tolerances, and Sourcing Signals
Key-stock geometry: ISO 2491 for parallel keys, ISO 3912 for Woodruff keys, ISO 774 for taper keys with gib-head. Shaft keyway profile and tolerance: ISO 773 (metric) and ASME B17.1 (inch, for North American builds). Rotor-balance half-key rule: ISO 21940-32:2012, which superseded BS 7130:1989 [S1].
Procurement signals in 2026: cold-drawn C45 key stock in standard widths (4–32 mm) is a commodity with sub-2-week lead times from most industrial suppliers; case-hardened 40Cr keys at tight tolerance (h6 shaft, ±0.02 mm corner radius) carry 4–6 week lead times and a 30–60% premium. Sourcing tier-2 fabricated keyways via job shops such as Fabrication Associates remains a workable path for non-standard keyway combinations and small batches [S2]. A related shaft-line component buyers often spec alongside, the shaft key reference page, covers the same key-stock standards in catalogue form.
For drives with linear-travel requirements on the keyed element, a separate component class — the linear guide — handles axial motion and is not interchangeable with a shaft key. Where the keyed joint must align with high-precision rotary motion, a crossed roller guide is sometimes used to support the driven component's housing.
This topic is covered further in Gantry Crane Types and Classifications: 2026 Spec Map.