2026-07-09
A Woodruff key is a semicircular disc that sits in a matching semicircular slot milled into a shaft. Roughly half the key protrudes above the shaft surface to engage a straight keyway in the mating hub. Unlike a rectangular parallel key that requires a full-length milled slot, a Woodruff key drops into a slot cut by a single plunge of a circular cutter, making it fast and cheap to produce.
The defining feature is that the key can tilt inside its cylindrical seat. This self-aligning behavior is exactly what you want on a tapered shaft, where a straight key would either bind or leave a gap as the hub slides on. Woodruff keys are the standard choice for the tapered ends of small engine crankshafts, magneto rotors, outboard motor flywheels, and lawn tractor PTO shafts. If you've ever pulled a flywheel off a Briggs & Stratton and found a shiny half-moon sitting in the shaft, that's it.
Trade-offs:
Sizing rule of thumb: ANSI B17.2 designates keys by a number where the last two digits give the diameter in eighths of an inch, and the digits before that give the width in 32nds. A "#606" key is 6/32" wide by 6/8" (¾") diameter. As a quick check on torque capacity, the shear force the key can carry is:
F = τ · w · L
where w is the key width, L is the chord length engaged with the hub (roughly the key's full diameter for a Woodruff), and τ is the allowable shear stress (typically ~40% of yield for a standard 1018 steel key, ~24,000 psi allowable). Torque capacity is then T = F · (d/2), where d is the shaft diameter.
Practical example: A #606 key (w = 0.1875", L ≈ 0.75") on a ¾" shaft gives F ≈ 24,000 × 0.1875 × 0.75 ≈ 3,375 lb, and T ≈ 3,375 × 0.375 ≈ 1,266 lb-in. Plenty for a lawn mower blade; nowhere near enough for a truck driveshaft.
The Woodruff key is also intentionally the weakest link — on a small engine, shearing the flywheel key is what saves the crankshaft when the blade hits a rock.