A conveyor drive shaft, 40 mm diameter, with a 12 mm key. The key sheared. The pulley stopped. The shaft was fine. The customer replaced the key with the same size. It sheared again in three weeks. The customer thought the key material was bad. The key was fine — it was undersized for the torque. The key is the weakest link by design, but it should be sized so it’s the second weakest link, not the first.
A key transmits torque from the shaft to the hub through shear and bearing stress. The keyed joint has three failure modes: the key shears, the shaft keyway crushes, or the hub keyway crushes. The key is usually the weakest by geometry — it’s a small rectangle in a big shaft. But if the key is too small for the torque, it shears in weeks. If it’s sized correctly, the joint lives as long as the machine.
The shear calculation
Key shear: τ = 2T / (d × b × L)
Where T is torque (N·mm), d is the shaft diameter (mm), b is the key width (mm), L is the key length (mm). The 2 comes from the key being loaded over half its height in shear… actually the standard derivation: the torque creates a tangential force F = 2T/d at the key. The shear stress is F divided by the shear area (b × L).
For the 40 mm shaft, standard key is 12×8 mm (width × height). With a 40 mm long key: shear area = 12 × 40 = 480 mm². The tangential force at 2000 N·m torque: F = 2 × 2000 × 1000 / 40 = 100,000 N. The shear stress: 100,000 / 480 = 208 MPa. A standard key material (C45, 45 steel, unmachined surface) has a shear strength of about 180-200 MPa. The key was running at its limit. Any overload — a jammed conveyor, a hard start — sheared it.
The fix: use a longer key. A 70 mm key: shear area = 12 × 70 = 840 mm². Stress = 100,000 / 840 = 119 MPa. That’s a 1.6x safety factor on the standard key material. It holds. Or use two keys at 90 degrees (standard for large drives), or a spline for very high torque. For this application, the longer key was the right answer — it used the existing shaft and hub, no machining change.
The keyway depth matters too
The keyway in the shaft and hub must be cut to depth. If the keyway is too shallow, the key protrudes and the hub can’t seat. If too deep, the key sits proud… actually the failure mode here is different: a shallow hub keyway means the key bears on a smaller area — the crushing stress rises. The rule: the keyway depth in the shaft is about 0.5× the key height, and in the hub about 0.5× as well (standard for metric keys, per DIN 6885). Cutting both to spec gives the full bearing area.
Most field failures trace to the key being too short or the keyway being cut shallow. Both are visible with calipers in 2 minutes. The sheared key was 40 mm long in a 70 mm long hub — the key didn’t fill the hub. Someone used a shorter key for convenience. The hub keyway was bearing on only 40 of its 70 mm.
Key size by shaft diameter
| Shaft dia (mm) | Key section (w × h) | Key length (torque-limited) |
|---|---|---|
| 20 | 6 × 6 | 20-30 mm |
| 30 | 8 × 7 | 30-50 mm |
| 40 | 12 × 8 | 40-70 mm |
| 50 | 14 × 9 | 50-90 mm |
| 60 | 18 × 11 | 60-110 mm |
These lengths assume C45 key steel and a standard application. For high shock loads, multiply the length by 1.5 or use two keys. For very high torque, use a spline — a keyed joint beyond 100 mm shaft diameter gets impractical.
The assembly check
After key installation: the key must fit snug in the shaft keyway (a light tap with a brass hammer, not loose). The hub must slide over the key and seat fully against the shoulder. If the key is loose in the groove, it hammers — the key, keyway, and hub all wear. A loose key fails in weeks regardless of size. The two-minute check: push the hub axially — no movement. Pull the hub radially — no lift.
Keys are the fuse in the drivetrain, but the fuse should be sized for the full torque, not for the minimum. The sheared key was 40 mm long in a 70 mm hub — half the bearing area was unused. Check key length against the torque formula, and fit the key snug.
