Square key, rectangular key, and Woodruff key dimensions by shaft diameter. Covers ANSI B17.1 (inch), DIN 6885 / ISO 2491 (metric), and Woodruff standard sizes with keyway depth for shaft and hub.
Updated Apr 24, 20263 min read
Keys transmit torque between a shaft and a hub (gear, pulley, sprocket). Square and rectangular parallel keys sit in matching keyways on the shaft OD and hub ID. Woodruff keys are half-moon — the keyway is milled with a circular cutter and the key self-locates, ideal for tapered shafts. This chart gives the standard key cross-sections, the keyway width, and the depths cut into shaft and hub for class-II (free) fit. For interference fits, reduce keyway depth by 0.002" / 0.05 mm.
Woodruff key numbers are a compact sizing code: the last two digits × 1/8 = cutter diameter in inches; the preceding digits × 1/32 = nominal key width.
Metric Woodruff keys use DIN 6888 — sizes given as W × D (mm), e.g. 6 × 22 means 6 mm wide × 22 mm diameter.
Design notes
Key length: rule of thumb is 1.5 × shaft diameter for full torque transmission. Less = shear / bearing failure; more = rarely a problem.
For high-speed reversing loads, use two keys 180° apart to balance the shaft.
Keyway cutting: use a slotting or broaching operation on the hub. For short runs, an end mill in a CNC or slotter is fine.
Keyway stress-concentration: sharp corners at the keyway ends are a fatigue initiation point. Fillet the ends with a radius ≥ 0.010" (0.25 mm) or cut a sled-runner keyway with lead-in / lead-out.
Taper keys (gib-head) wedge axially into a tapered keyway; used on older machinery. Forbidden on high-speed rotating equipment because of imbalance.
Set-screw alternative: for loads below ~20% of key-specified torque, a pair of cup-point set screws at 90° (or one over the keyway + one 90° off) can replace a key on cheap adapters — not for safety-critical drives.
Torque capacity (approximate)
Shear capacity (steel key)
T = (W × L × D × Ss) / 2 · Ss = 80 MPa (11.6 ksi) for soft key steel
Bearing capacity (key sides)
T = (H × L × D × Sc) / 4 · Sc = 140 MPa (20 ksi) for mild steel
Typical margin
Design to 2× max operating torque; use the smaller of shear or bearing calc
#1 common failure
Key rolling in keyway from insufficient key length; shaft key ejects axially
#2 common failure
Fatigue crack at keyway corner from sharp entry; fillet required
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