BIZOMAT MAKEOS · Power Transmission
Keyed Shaft and Hub Design
How long must the key be to carry this drive, does the hub fit it, and is the keyed shaft strong enough?
Design of a parallel-keyed shaft-to-hub connection from the power and speed: design torque with service factor, force at the shaft surface, key length for shear and for crushing on the half-depth bearing face, the effect of a second key, the hub length needed, and the shaft torsional stress against 75% of the allowable as the handbook recommends for keyed shafts.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Power | kW | Power transmitted. |
| Speed | rpm | Shaft speed. |
| Service factor | Service factor for shock and starting loads. | |
| 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. |
| Hub length available | mm | Length of the hub bore. |
| Keys | choice | One key, or two keys at 180° (assumed to share the load at 75% effectiveness). |
| Allowable key shear stress | MPa | Allowable shear stress for the key material. |
| Allowable crushing stress (weakest of key, shaft, hub) | MPa | Allowable bearing (crushing) stress of the weakest of key, shaft and hub. |
| Allowable shaft shear stress (unkeyed) | MPa | Allowable shear stress for the shaft without a keyway. |
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: Power
- What it is
- Power passing through the keyed joint from shaft to hub, such as the rated output of the motor or the power absorbed by the driven machine.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Design torque and the key length required rise in proportion.
- Get it right
- Use shaft power, not electrical input; shock and starting loads are covered by the service factor.
What this means: Speed
- What it is
- Rotational speed of the keyed shaft. With the power it sets the torque the key must carry.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque, and the key length required, fall as speed rises for the same power.
- Get it right
- Use the speed of this shaft, after any gearing, not the motor speed.
What this means: Service factor
- What it is
- Multiplier on the steady torque to allow for shock, starting peaks and load variation, giving the design torque.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Design torque and key length required rise in proportion.
- Get it right
- Take it from your design practice or the driven machine’s service factor tables.
What this means: Shaft diameter
- What it is
- Diameter of the shaft where the key sits. The torque is turned into a tangential force at the shaft surface.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 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 in proportion and the shaft shear stress with the cube.
- Get it right
- Use the diameter at the keyway, which may be smaller than the shaft’s largest diameter.
What this means: Key width
- What it is
- Width of the key across the shaft, the dimension that resists shear.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
- A wider key shortens the length needed for shear.
- Get it right
- Take it from the key standard for your shaft diameter rather than choosing freely.
What this means: Key height
- What it is
- Height of the key, measured radially. Half of it is taken as bearing on the hub side for the crushing check.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
- A taller key shortens the length needed for crushing.
- Get it right
- Enter the full key height; the tool uses half of it for crushing.
What this means: Hub length available
- What it is
- Length of the hub bore that the key can engage. The key length required is checked against it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Deduct chamfers and any relief grooves from the hub length.
What this means: Keys
- What it is
- Whether one key or two keys spaced half a turn apart carry the torque. Two keys are assumed to share it unevenly.
- Units and format
- Choose one of 2 options from the list: One key; Two keys at 180°.
- Example
- The form opens with “One key” selected as a worked example. Choose the option that matches your case.
- Get it right
- The tool credits two keys with one and a half times the capacity of one, not double.
What this means: Allowable key shear stress
- What it is
- Allowable shear stress for the key material, used to find the key length needed so the key does not shear across its width.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 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.
- Effect on the result
- A higher allowable shortens the length needed for shear.
- Get it right
- Take it from the key material’s strength and your design code or practice.
What this means: Allowable crushing stress (weakest of key, shaf…
- What it is
- Allowable bearing (crushing) stress on the key flanks, taken as the lowest 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 2000 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.
- Effect on the result
- A higher allowable shortens the length needed for crushing.
- Get it right
- Check all three materials; a soft cast-iron or aluminium hub often governs.
What this means: Allowable shaft shear stress (unkeyed)
- What it is
- Allowable torsional shear stress for the shaft material without a keyway. The tool takes 75 % of it as the limit for the keyed shaft.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the plain-shaft allowable; the keyway reduction is applied by the tool.
Method and formulas
- T = P ÷ ω × SF; force F = 2T ÷ d.
- Shear length L_s = F ÷ (w τ); crushing length L_c = F ÷ (h ÷ 2 × σ_c).
- Two keys: required length ÷ 1.5 (each key assumed 75% effective).
- Keyed shaft: τ = 16T ÷ πd³ against 75% of the unkeyed allowable (handbook rule for a standard keyway).
- Example: 15 kW at 1450 rpm on a 40 mm shaft needs a 18.522 mm long 12 × 8 key.
Limitations
- Uniform load along the key; long keys (beyond about 1.5 d) carry load unevenly.
- Fatigue and fretting of keyed joints need separate attention for reversing loads.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a key for a pulley, gear or coupling
- Decide between a longer hub and a second key
- Check a keyed shaft after a power increase
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Design torque and force at the shaft surface
- Key length for shear and crushing
- Hub length and keyed shaft stress
Interpreting the result
- If the key would exceed about 1.5 shaft diameters, use two keys, a spline or a shrink fit 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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