BIZOMAT MAKEOS · Power Transmission
Flange Coupling Design (Design Torque, Bolts, Friction Drive, Key)
Is this rigid flange coupling strong enough for the motor power, do the bolts carry the torque by friction or shear, and is the hub key long enough?
Check of a rigid flange coupling: design torque from power, speed and service factor; bolt force and bolt shear stress on the bolt circle with the shank diameter needed, and the torque the bolt preload can carry by face friction (as the flange coupling bolts tool); key force, shear and crushing stress in the hub and the least key length for each (as the key stress tool), with the key length checked against the hub length.
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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 the driver, load and duty. | |
| Bolts | ||
| Number of bolts | Number of coupling bolts. | |
| Bolt circle diameter | mm | Diameter of the bolt circle. |
| Bolt shank diameter | mm | Diameter of the bolt shank in the shear plane. |
| Allowable bolt shear stress | MPa | Allowable shear stress of the bolts. |
| Preload per bolt | kN | Tightening preload of each bolt. |
| Friction coefficient between flange faces | Friction coefficient between the flange faces. | |
| Shaft key | ||
| Shaft diameter | mm | Diameter of the shaft in the hub. |
| Key width | mm | Width of the key. |
| Key height | mm | Height of the key. |
| Effective key length | mm | Length of key engaged in the hub. |
| Hub length | mm | Length of the coupling hub. |
| Allowable key shear stress | MPa | Allowable shear stress of the key. |
| Allowable compressive stress (weakest of key, shaft, hub) | MPa | Allowable crushing stress of the key, shaft or hub, whichever is weakest. |
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
- The power the coupling transmits from driver to driven machine, normally the driver’s rated power.
- 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 30 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 with it every stress, rises in proportion.
What this means: Speed
- What it is
- The speed of the shaft the coupling is on.
- 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 1000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Design torque falls in proportion as speed rises.
- Get it right
- Use the speed of this shaft, not the motor speed if a gearbox sits between them.
What this means: Service factor
- What it is
- A multiplier on the nominal torque that allows for the driver, the driven load and the duty, such as shocks or frequent starts.
- 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 rises in direct proportion.
- Get it right
- Take it from the coupling maker’s selection table for your driver and load.
Bolts
What this means: Number of bolts
- What it is
- The number of bolts joining the two coupling flanges around the bolt circle.
- Units and format
- Enter a plain number with no unit. The form accepts 2 to 64. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Force per bolt falls in proportion to the number of bolts.
What this means: Bolt circle diameter
- What it is
- The diameter of the circle through the bolt centres on the flange.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 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 bolt circle reduces the bolt force and raises friction torque.
- Get it right
- Use the bolt circle diameter, not the flange outside diameter.
What this means: Bolt shank diameter
- What it is
- The diameter of the bolt shank where it crosses the joint face between the flanges, the section that is sheared.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 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
- Bolt shear stress falls with the square of the diameter.
- Get it right
- If the thread crosses the joint, use the thread root diameter instead of the shank.
What this means: Allowable bolt shear stress
- What it is
- The shear stress you allow in each coupling bolt, from the bolt grade and your design standard.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 10000 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.
What this means: Preload per bolt
- What it is
- The clamping force each bolt applies when tightened. Face friction from all bolts together can carry torque without shearing them.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Friction torque rises in proportion to preload.
- Get it right
- This is preload per bolt, not total clamp force; it depends on the tightening method.
What this means: Friction coefficient between flange faces
- What it is
- The friction coefficient between the two flange faces, from test or design data for the surface condition.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Friction torque rises in proportion.
- Get it right
- Paint, oil or coatings on the faces can lower it.
Shaft key
What this means: Shaft diameter
- What it is
- The diameter of the shaft where the hub and key sit.
- 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 in proportion.
What this means: Key width
- What it is
- The width of the key across the shaft, from the key standard for the 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
- Key shear stress falls in proportion to width.
What this means: Key height
- What it is
- The full height of the key. The tool takes half of it as the bearing face in the hub for the crushing check.
- 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 to key height.
- Get it right
- Type the full key height, not the depth in the hub.
What this means: Effective key length
- What it is
- The length of key actually engaged in the hub and keyway, excluding rounded ends that carry no 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 60 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 and crushing stresses fall in proportion to length.
What this means: Hub length
- What it is
- The length of the coupling hub along the shaft. The effective key length must fit within it.
- 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 70 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Allowable key shear stress
- What it is
- The shear stress you allow in the key, from the key material and your design standard.
- 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.
What this means: Allowable compressive stress (weakest of key, s…
- What it is
- The crushing (bearing) stress you allow on the key sides, taken from whichever of the key, shaft and hub materials is weakest.
- 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
- Use the weakest of the three materials.
Method and formulas
- Design torque T = 9549 P ÷ n × service factor.
- Bolt force = 2T ÷ (n_b D_bc); shear stress = force ÷ (π d² ÷ 4); friction torque = μ n_b F_i D_bc ÷ 2 (as the flange coupling bolts tool).
- Key force = 2T ÷ d_s; shear = F ÷ (w L); crushing = F ÷ (h ÷ 2 × L); least length for each (as the key stress tool).
- Example: the design torque is 429.72 N·m; the bolts see 22.797 MPa and friction can carry 960 N·m; the key needs at least 53.715 mm.
Limitations
- Steady torque with a service factor; shock and reversing loads need a fatigue check.
- Bolts in shear assume fitted (reamed) bolts; clearance bolts rely on friction.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a rigid coupling when fitting a larger motor
- Choose bolts and keys for a fabricated coupling
- Decide whether fitted bolts are needed
Who it is for
- Design engineers
- Maintenance engineers
- Millwrights
What you get
- Design torque
- Bolt force, shear stress and friction capacity
- Key stresses and least key length
Interpreting the result
- If friction alone carries the design torque, the bolts are not loaded in shear and clearance holes are fine.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2441–2450, 2460–2486, 301–302. Topic reference; no source tables or recommended settings are reproduced.
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Open the Flange Coupling Design (Design Torque, Bolts, Friction Drive, Key) calculator