You have a motor shaft spinning at 1,750 RPM and a pulley that needs to spin with it. A set screw will slip under load. Welding makes it permanent. The answer for a hundred years has been a keyway: a rectangular slot milled into both the shaft and the hub, with a hardened steel key wedged between them to transmit torque through shear.

Square and rectangular keys are the workhorses. A standard rule: key width equals roughly 1/4 of the shaft diameter. A 1-inch shaft gets a 1/4" × 1/4" key. The key sits half in the shaft, half in the hub. Torque is transmitted by the key shearing across its mid-plane and by the key sides bearing against the keyway walls.

Woodruff keys are half-moon shaped and sit in a semicircular pocket. They self-align and are easier to machine on small shafts (think tapered motor shafts on lawn equipment), but they cut deeper into the shaft and weaken it more than a parallel key.

Splines are what you use when one key isn't enough. Instead of a single slot, the shaft has many integral teeth (straight-sided or involute) that mesh with matching grooves in the hub. Splines distribute torque around the entire shaft circumference, handle reversing loads without backlash issues, and allow axial sliding under load — which is why every manual transmission, driveshaft slip yoke, and PTO shaft uses them.

Quick shear calculation: For a key of width w, length L, on a shaft of diameter D, the torque capacity in shear is:

T = (w × L × τ_allow × D) / 2

For a 1/4" × 1/4" × 1" key on a 1" shaft with allowable shear stress of 10,000 psi (typical for low-carbon steel keystock with safety factor):

T = (0.25 × 1.0 × 10,000 × 1.0) / 2 = 1,250 in-lb

That's about 104 ft-lb — plenty for a 3 HP motor at 1,750 RPM (which produces ~9 ft-lb).

Practical gotchas:

• Keystock is intentionally softer than the shaft — the key is the sacrificial element. If something jams, you want to shear a $2 key, not crack a $200 shaft.
• Always check bearing stress on the keyway walls separately — keys often fail by crushing before they shear.
• Use a setscrew over the key (radially) to prevent axial migration. A key that walks out under vibration will destroy a hub.
• For high-cycle reversing loads, prefer splines or interference fits — keyways concentrate stress and are notorious fatigue starters.