BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Torque
- What it is
- The peak torque the key must transmit between the shaft and the hub, including your service factor.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shear and crushing stresses rise in proportion.
- Get it right
- Enter the torque at this key, in newton metres, with the service factor already applied.
What this means: Shaft diameter
- What it is
- The shaft diameter at the keyway. The torque divided by half this diameter gives the force on the key.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger shaft lowers the key force and both stresses in proportion.
- Get it right
- Use the shaft diameter at the key seat, not at a shoulder.
What this means: Key width
- What it is
- The width of the parallel key across the shaft, from the key standard for this shaft size.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shear stress falls in proportion; crushing stress is unaffected.
- Get it right
- Use the key width, not the keyway depth.
What this means: Key height
- What it is
- The height of the key in the radial direction. The tool assumes half of it bears against the hub.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Crushing stress falls in proportion; shear stress is unaffected.
- Get it right
- Enter the full key height; the tool halves it.
What this means: Effective key length
- What it is
- The straight-sided length of key in contact with both shaft and hub that actually carries load.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both stresses fall in proportion as length rises.
- Get it right
- Subtract rounded ends, and do not count length beyond the hub.
What this means: Allowable shear stress of key
- What it is
- The shear stress the key material may carry in your design, from your design data or code.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use an allowable that includes your safety margin, not the ultimate shear strength.
What this means: Allowable compressive stress (weakest of key, s…
- What it is
- The compressive (crushing) stress allowed on the key sides, taken for the weakest of the key, shaft and hub materials.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A soft hub material, such as cast iron or aluminium, often governs.
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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