KEVOS® MAKEOS · Fasteners
Woodruff Key Check (Key Shear, Hub and Shaft Bearing, Weakened Shaft Torsion)
Will a Woodruff key carry the torque without shearing or crushing, and is the shaft, weakened by the deep keyseat, still strong enough in torsion?
Check of a Woodruff (half-moon) key transmitting torque: the force at the shaft surface, the shear stress across the key width over its length (as the parallel key stress tool), the bearing stress on the part of the key standing above the shaft in the hub and on the part sunk in the shaft, the shaft's torsional stress raised by a stress concentration factor for the deep keyseat, all against your allowables, and the least key length for shear and for hub bearing.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Torque transmitted | N·m | Largest torque the key transmits, including any service factor you use. |
| Shaft diameter | mm | Diameter of the shaft at the key. |
| Key | ||
| Key width | mm | Width of the Woodruff key, from the key standard or drawing. |
| Key length (chord at the shaft surface) | mm | Length of the key where it meets the shaft surface (the chord of the half-moon at the shaft diameter). |
| Key height | mm | Total height of the key. |
| Keyseat depth in the shaft | mm | Depth of the keyseat in the shaft, which sets how much of the key stands above the shaft into the hub. |
| Allowables | ||
| Allowable shear stress in the key | MPa | Allowable shear stress for the key material and duty, from your design standard. |
| Allowable bearing stress (weakest of key, hub, shaft) | MPa | Allowable bearing stress for the weakest of the key, hub and shaft materials. |
| Shaft stress concentration factor at the keyseat | Stress concentration factor for torsion at the deep keyseat, from a design reference for this keyseat shape. | |
| Allowable shear stress in the shaft | MPa | Allowable torsional shear stress for the shaft material and duty. |
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 transmitted
- What it is
- The largest torque the Woodruff key must transmit between the shaft and the hub.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 1000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Every stress rises in proportion.
- Get it right
- Include any service factor for shock or starting torque that your design practice requires.
What this means: Shaft diameter
- What it is
- The diameter of the shaft where the key sits, which sets the force at the key and the shaft’s torsional strength.
- 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 25 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, more strongly, the shaft stress.
- Get it right
- Use the shaft diameter at the keyseat, not a larger shoulder nearby.
Key
What this means: Key width
- What it is
- The width of the Woodruff key, which takes the shear across the joint between shaft and hub.
- 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 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Key shear stress falls in proportion.
- Get it right
- Take it from the key size in the key standard or on the drawing.
What this means: Key length (chord at the shaft surface)
- What it is
- The length of the key where it meets the shaft surface, the chord of the half-moon at the shaft diameter, over which shear and bearing act.
- 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 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shear and bearing stresses fall in proportion.
- Get it right
- Measure the chord at the shaft surface, which is shorter than the key’s full length if the key stands proud.
What this means: Key height
- What it is
- The total height of the half-moon key from its curved base to its flat top.
- 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 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- With the keyseat depth, sets how much of the key bears on the hub.
- Get it right
- Take it from the key standard for the key size.
What this means: Keyseat depth in the shaft
- What it is
- The depth of the curved keyseat cut in the shaft; the rest of the key height stands above the shaft and bears on the hub.
- 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 7.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A deeper seat leaves less key in the hub, raising hub bearing stress.
- Get it right
- Take it from the key standard or measure the seat; the deep seat is what holds a Woodruff key from tipping.
Allowables
What this means: Allowable shear stress in the key
- What it is
- The highest shear stress you allow in the key material for this duty.
- 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 60 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the shear pass line and the least length for shear.
- Get it right
- Take it from your design standard for the key steel and the type of load.
What this means: Allowable bearing stress (weakest of key, hub,…
- What it is
- The highest bearing (crushing) stress you allow on the weakest of the key, the hub and the shaft.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 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
- Only moves the bearing pass line and the least length for bearing.
- Get it right
- A cast iron or aluminium hub usually sets this, not the key.
What this means: Shaft stress concentration factor at the keyseat
- What it is
- The factor by which the deep keyseat raises the shaft’s torsional stress above the plain-shaft value.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The shaft stress rises in proportion.
- Get it right
- Take it from a stress concentration reference for this keyseat shape; Woodruff seats are deep, so the factor is not small.
What this means: Allowable shear stress in the shaft
- What it is
- The highest torsional shear stress you allow in the shaft material for this duty.
- 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 80 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the shaft pass line.
- Get it right
- Take it from your design standard; for fatigue loading a lower value applies.
Method and formulas
- Force at the shaft surface F = 2T ÷ d.
- Key shear τ = F ÷ (w L) (as the parallel key stress tool); least length for shear = F ÷ (w τ_a).
- Hub bearing σ_h = F ÷ ((h − t₁) L); shaft bearing σ_s = F ÷ (t₁ L); least length for hub bearing = F ÷ ((h − t₁) σ_a).
- Shaft torsion τ = K_t × 16T ÷ (π d³), against the shaft allowable.
- Example: 60 N·m on a 25 mm shaft gives 4,800 N at the key: 32 MPa shear, 76.8 MPa hub bearing and 39.114 MPa in the shaft.
Limitations
- Uniform shear and bearing over the key length; a Woodruff key bears unevenly along its curved seat.
- The stress concentration factor is your input; the shaft check covers static torsion, not fatigue.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a Woodruff key on a small shaft
- Find the least key length for the torque
- Check that the deep keyseat has not weakened the shaft too much
Who it is for
- Machine designers
- Mechanical engineers
- Maintenance engineers
What you get
- Force at the shaft surface
- Key shear stress and least length
- Bearing stress in the hub and in the shaft
- Shaft torsional stress at the keyseat
Interpreting the result
- Only the small height of a Woodruff key above the shaft bears on the hub, so hub bearing usually governs; the deep seat holds the key against tipping but weakens the shaft.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2460–2486, 301. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
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KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Woodruff Key Check (Key Shear, Hub and Shaft Bearing, Weakened Shaft Torsion) calculator