BIZOMAT MAKEOS · Power Transmission
Flange Coupling Bolt Capacity
Can the bolts of this flanged coupling carry the torque in shear?
Shear stress in the bolts of a rigid flanged shaft coupling from torque, number of bolts and bolt circle, the bolt diameter needed, and the friction torque the bolt preload can carry instead.
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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. |
| Number of bolts | bolts | Number of fitted bolts. |
| Bolt circle diameter | mm | Diameter of the bolt circle. |
| Bolt shank diameter | mm | Bolt diameter in the shear plane (fitted bolts). |
| Allowable bolt shear stress | MPa | Allowable shear stress of the bolt material. |
| Preload per bolt (friction check, 0 = skip) | kN | Bolt preload, if friction is to carry the torque. |
| Friction coefficient between flange faces | Friction coefficient between flange faces. | |
| Mean friction diameter (0 = bolt circle) | mm | Mean diameter of the contact face; 0 uses the bolt circle. |
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 highest torque the coupling must transmit, including any service factor for shock or starting loads, as decided for the drive.
- 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 2000 N·m 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 rises in direct proportion to the torque.
- Get it right
- Apply the service factor before entering the torque; the tool does not add one.
What this means: Number of bolts
- What it is
- How many bolts pass through the two flanges and share the torque. The tool assumes they are fitted (reamed) bolts that share the load equally.
- Units and format
- Enter a number in bolts: bolts. The form accepts 2 bolts to 64 bolts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 bolts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shear stress per bolt falls in inverse proportion to the number of bolts.
- Get it right
- Count only fitted bolts that bear in shear; clearance bolts carry torque by friction instead.
What this means: Bolt circle diameter
- What it is
- The diameter of the circle through the centres of the coupling bolts, measured across the coupling centre.
- 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 150 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 lowers the force on each bolt in proportion.
- Get it right
- This is the bolt circle diameter, not its radius and not the flange outside diameter.
What this means: Bolt shank diameter
- What it is
- The diameter of each bolt where it crosses the joint between the two flanges, which is 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 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 with the square of the bolt diameter.
- Get it right
- Use the plain shank diameter where it crosses the joint; if the thread crosses the joint instead, the sheared section is smaller than nominal.
What this means: Allowable bolt shear stress
- What it is
- The highest shear stress your design standard allows in the bolt material, used for the pass or fail check and to size the bolt diameter.
- 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.
- Get it right
- Use an allowable shear stress, not the bolt tensile strength or proof stress.
What this means: Preload per bolt (friction check, 0 = skip)
- What it is
- The clamping tension in each bolt after tightening, used to check whether flange-face friction alone could carry the torque. Zero skips the friction check.
- 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 30 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 capacity rises in direct proportion to the preload.
- Get it right
- This is per bolt and in kilonewtons; it comes from the tightening torque and method, not from the bolt strength.
What this means: Friction coefficient between flange faces
- What it is
- The coefficient of friction between the two flange faces where they clamp together, used only in the friction check.
- 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 capacity rises in direct proportion to the coefficient.
- Get it right
- Coatings, paint and oil on the faces change it greatly; use a value for the actual surface condition.
What this means: Mean friction diameter (0 = bolt circle)
- What it is
- The mean diameter of the ring of flange face that actually clamps and carries the friction. Zero tells the tool to use the bolt circle diameter instead.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Friction torque capacity rises in direct proportion to this diameter.
- Get it right
- A raised or recessed face can make the friction diameter quite different from the bolt circle.
Method and formulas
- Force per bolt = 2T ÷ (n Dbc); shear stress = force ÷ (π d²/4).
- Bolt diameter needed = √(8T ÷ (π n Dbc τ)).
- Friction capacity = μ n Fi × D_f ÷ 2.
- Example: 2000 N·m on 6 × 12 mm bolts at 150 mm gives τ = 39.298 MPa.
Limitations
- Equal sharing between fitted bolts; clearance bolts carry torque only by friction.
- Flange and hub strength are checked separately.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a rigid coupling for a higher-power drive
- Size coupling bolts
- Decide whether friction or bolt shear carries the torque
Who it is for
- Design engineers
- Maintenance engineers
- Automation engineers
What you get
- Shear force and stress per bolt
- Bolt diameter needed
- Friction torque capacity
Interpreting the result
- Use either friction (preload) or fitted-bolt shear as the design basis, not both added together.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2441–2450, 302. Topic reference; no source tables or recommended settings are reproduced.
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