Keyway Design Calculator

This calculator checks whether a shaft key — the small rectangular bar that locks a gear, pulley, or coupling to a shaft — is strong enough to transmit a given torque without shearing or crushing. It first converts torque into a tangential force at the shaft surface, F = 2T/d, then evaluates that force two ways: shear stress across the key's width-times-length cross-section (τ = F/(wL)), and compressive (bearing) stress against only half the key's height, since a standard key sits half-embedded in the shaft's keyseat and half in the hub's keyway (σ = F/((h/2)L)). Allowable stresses are derived from the key material's yield strength with a safety factor of 2 baked in — shear uses the distortion-energy (von Mises) relation, 0.577 × Sy/SF, while bearing uses Sy/SF directly.

From those allowables the calculator back-solves for the minimum key length needed to survive shear and bearing separately, then reports the larger of the two as the governing minimum length, along with an overall safety factor comparing your actual key length against both failure modes. A result below about 2.0 signals the key should be lengthened, widened, or upgraded to a stronger material (or that you should switch to two keys or a spline). This follows the standard machine-design approach for parallel (square/flat) keys sized per ASME/ANSI B17.1, Keys and Keyseats, the standard that defines square and flat key cross-sections as a function of shaft diameter; it doesn't cover woodruff keys, tapered keys, or fatigue from repeated torque reversals, which need a separate check.