KEVOS® MAKEOS · Power Transmission
Parallel Key Shear and Crushing
Is this parallel key long enough for the torque in shear and crushing?
Shear and compressive (crushing) stress in a parallel key transmitting torque, the minimum key length for each, compared with allowable stresses.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Torque | N·m | Peak torque including service factor. |
| Shaft diameter | mm | Shaft diameter at the key. |
| Key width | mm | Key width from the key standard for the shaft size. |
| Key height | mm | Key height from the key standard. |
| Effective key length | mm | Effective (straight-sided) length in contact; subtract rounded ends. |
| Allowable shear stress of key | MPa | Allowable shear stress for the key material from your design data. |
| Allowable compressive stress (weakest of key, shaft, hub) | MPa | Allowable compressive stress for the weakest of key, shaft and hub materials. |
Method and formulas
- Force at the shaft surface F = 2T ÷ d.
- Shear τ = F ÷ (w L); crushing σ = F ÷ (h/2 × L) (half the key height bears in the hub).
- Minimum length = F ÷ (w τ_allow) and F ÷ (h/2 × σ_allow).
- Example: 400 N·m on a 40 mm shaft with a 12 × 8 key 50 mm long: τ = 33.333 MPa, σ = 100 MPa.
Limitations
- Uniform load along the key; real keys are loaded more at the torque input end.
- Fatigue at the keyway in the shaft is checked with the shaft stress concentration, not here.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose key length for a gear, pulley or coupling
- Check an existing key after a power increase
- Decide between a key and an interference fit
Who it is for
- Design engineers
- Maintenance engineers
- Automation engineers
What you get
- Shear stress and minimum length for shear
- Crushing stress and minimum length for crushing
- Governing mode
Interpreting the result
- Keys longer than about 1.5 × shaft diameter carry load unevenly; use two keys or a spline instead.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2460–2486, 301. Topic reference; no source tables or recommended settings are reproduced.
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