Standard Key & Keyway Dimensions
Square and rectangular keys transmit torque between shafts and hubs (gears, pulleys, couplings). The key size is determined by the shaft diameter — there’s a standard match.
Square & Rectangular Key Sizes (ANSI B17.1)
| Shaft Diameter (in) | Key Width (in) | Key Height (in) | Key Type |
|---|---|---|---|
| 5/16 to 7/16 | 3/32 | 3/32 | Square |
| 1/2 to 9/16 | 1/8 | 1/8 | Square |
| 5/8 to 7/8 | 3/16 | 3/16 | Square |
| 15/16 to 1-1/4 | 1/4 | 1/4 | Square |
| 1-5/16 to 1-3/8 | 5/16 | 5/16 | Square |
| 1-7/16 to 1-3/4 | 3/8 | 3/8 | Square |
| 1-13/16 to 2-1/4 | 1/2 | 1/2 | Square |
| 2-5/16 to 2-3/4 | 5/8 | 5/8 | Square |
| 2-13/16 to 3-1/4 | 3/4 | 3/4 | Square |
| 3-5/16 to 3-3/4 | 7/8 | 7/8 | Square |
| 3-13/16 to 4-1/2 | 1 | 1 | Square |
Keyway Depth
| Key Width (in) | Shaft Keyway Depth (in) | Hub Keyway Depth (in) |
|---|---|---|
| 3/32 | 3/64 | 3/64 |
| 1/8 | 1/16 | 1/16 |
| 3/16 | 3/32 | 3/32 |
| 1/4 | 1/8 | 1/8 |
| 5/16 | 5/32 | 5/32 |
| 3/8 | 3/16 | 3/16 |
| 1/2 | 1/4 | 1/4 |
| 5/8 | 5/16 | 5/16 |
| 3/4 | 3/8 | 3/8 |
| 1 | 1/2 | 1/2 |
For square keys, shaft and hub keyway depths are each half the key height. The key sits half in the shaft and half in the hub.
Key Fit Classes
| Fit Class | Key Width Tolerance | Use Case |
|---|---|---|
| Class 1 (Free fit) | -0.000 / -0.002 | Easy assembly/disassembly, frequent removal |
| Class 2 (Normal fit) | +0.000 / -0.001 | Standard — most applications |
| Class 3 (Interference) | +0.001 / +0.000 | Permanent installation, no backlash |
Design Notes
- Key length: Typically 1.5× shaft diameter. Don’t exceed 3×D — a long key with poor alignment will bind.
- Woodruff keys: Half-moon shaped, self-aligning. Good for tapered shafts and small shafts. Weaker than square keys.
- Key material: Standard is 1018-1045 cold-drawn steel. Don’t hardened the key — it’s designed to shear before damaging the shaft or gear.
- Stress concentration: The keyway creates a stress riser in the shaft. For fatigue-critical shafts, use a generous fillet radius at the keyway corners and check the reduced cross-section.
- Never use set screws alone to replace a key. Keys handle the torque; set screws just keep the hub from sliding axially.
Related: Set Screw Torque Chart | Press Fit Chart | Bolt Torque Chart