A shear pin is the sacrificial fuse of a mechanical drive: it is sized to shear at a known torque so the rest of the machine survives an overload. This calculator converts a target overload torque into the pin diameter needed, given the pin’s moment arm, the material’s ultimate shear strength, and a safety factor.
How it works
Torque on the shaft produces a shear force on the pin at its moment arm; the pin must just reach its shear strength at the target torque:
shear force = target torque / moment arm
required area = (shear force * safety factor) / ultimate shear strength
(divide area by 2 for double shear)
pin diameter = sqrt(4 * area_per_plane / pi)The result is the minimum diameter; the nearest standard stock size at or above it sets the actual breakaway torque, which the tool reports along with a recommended working torque below the shear point.
Example and tips
To protect an auger that should never see more than 800 lb-ft, with the pin on a 1.5-inch radius, in single shear, using mild steel at 35,000 psi and a 1.3 safety factor: the shear force is about 6,400 lb, the required area near 0.24 square inches, giving a pin about 0.55 inches in diameter. Round to a standard 9/16-inch pin. Use a clean shear groove or undersized necked-down pin so the failure plane is predictable, and keep the working torque comfortably below the breakaway value.