Coupling Calculator
Free online coupling calculator for mechanical engineering. Compute bolt shear stress, bearing stress, required bolt diameter, and torque capacity for bolted flange couplings with standard coupling selection reference.
Coupling Calculator
Flange Coupling Details
Torque transmitted through the coupling
Diameter through the bolt centres
Typically 4–12 for flange couplings
Nominal bolt shank diameter
Thickness of each flange half at the bolt circle
Bolt allowable shear (before service factor)
Flange allowable bearing/crushing stress
Rated Torque Capacity
7,238N·m
Lesser of the bolt-shear limit (7,238) and the bearing limit (17,280).
Capacity exceeds applied torque22.1MPa
13.89MPa
3.619×
5.947mm
1.852mm
5.659MPa
3.619×
8.64×
22.1MPa
τ_b = 2T/(n·A_b·D_p)
13.89MPa
σ_b = T/(n·t·d_b·D_p/2)
3.619×
Capacity / applied torque
5.947mm
For target shear allowable
1.852mm
For target bearing allowable
5.659MPa
At the hub-flange junction
3.619×
τ_allow / τ_b
8.64×
σ_allow / σ_b
Result Summary
| Bolt Circle Diameter | 150.00 mm |
| Number of Bolts | 6 |
| Bolt Diameter | 16.00 mm |
| Flange Thickness | 20.00 mm |
| Bolt Shear Stress | 22.10 MPa |
| Bearing Stress | 13.89 MPa |
| Torque Capacity | 7,238 N·m |
| Verdict | Adequate (SF 3.619×) |
Interactive Diagram
Diagram updates live with your inputs — the bolt circle, bolt count, and status indicator reflect the current flange coupling.
Visualizations
Torque — Capacity vs Applied (N·m)
Utilization (Applied / Capacity)
Formula
Bolt shear stress is the transmitted torque shared equally across the bolt circle and resisted by the bolt areas at the pitch radius. Bearing stress is the compression of the bolt against the flange hole wall over the projected bearing area.
Alternative Form
Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| T | Applied torque | N·m | Torque transmitted through the coupling |
| n | Number of bolts | - | Bolts sharing the torque equally |
| A_b | Bolt area | mm² | A_b = π·d_b²/4, shear cross-section of one bolt |
| D_p | Bolt circle diameter | mm | Diameter through the bolt centres |
| t | Flange thickness | mm | Thickness of each flange half at the bolt circle |
| d_b | Bolt diameter | mm | Nominal bolt shank diameter |
| τ | Bolt shear stress | MPa | Average shear in each bolt |
| σ | Bearing stress | MPa | Crushing stress of bolt against flange |
Explanation
The bolts lie on a circle of diameter D_p, so the tangential force per bolt is F = T/(n·D_p/2) = 2T/(n·D_p). Dividing by the bolt area A_b gives the shear stress τ_b. The bearing (crushing) stress on the flange hole wall is the same tangential force spread over the projected area n·t·d_b, giving σ_b = T/(n·t·d_b·D_p/2). Each mode has its own allowable, and the rated capacity is the smaller of the two torque limits.
Derivation
Torque equilibrium about the shaft axis: T = n · F_b · (D_p/2), where F_b is the tangential force resisted by each bolt. Shear stress τ_b = F_b / A_b = (2T/(n·D_p))/A_b = 2T/(n·A_b·D_p). Bearing stress σ_b = F_b·n / (n·t·d_b) = F_b/(t·d_b) = 2T/(n·D_p·t·d_b) = T/(n·t·d_b·D_p/2). Multiplying by the respective allowable and dividing by 2 yields the torque limits T_shear and T_bearing.
Step-by-Step Calculation
Follow each step to understand how the result is calculated from your inputs.
Compute the bolt cross-sectional area from its diameter.
Compute bolt shear stress at the flange interface.
Compute bearing (crushing) stress between bolt and flange hole.
Determine torque capacity from both failure limits.
Compare rated capacity against the applied torque to get the service factor and verdict.
Verify the flange hub shear at the hub-flange junction.
Worked Examples
See how the calculator works with different scenarios.
Scenario
A small rigid flange coupling transmits a modest torque between two close-coupled pump shafts.
Inputs
Calculation
A_b = π·16²/4 = 201.06 mm²; τ_b = 2000·2000/(6·201.06·150) = 22.1 MPa; σ_b = 2000·2000/(6·20·16·150) = 27.8 MPa
Result
T_max ≈ 9066 Nm, SF ≈ 4.5× — comfortably adequate
Interpretation
The coupling has large reserve; a smaller bolt or fewer bolts would still be safe.
Understanding Your Result
Your Result: 7,238 N·m
Typical range: Healthy designs use a service factor of 1.5–3.0×. Yours: 3.619× at 27.63% utilization.
What This Means
Your flange coupling experiences a bolt shear stress of 22.10 MPa and a bearing stress of 13.89 MPa. Its rated torque capacity is 7,238 N·m, giving a service factor of 3.619× against the applied 2000.00 N·m.
Why It Matters
A flange coupling fails by bolt shear or flange crushing — both are sudden and catastrophic. Confirming the rated capacity exceeds the duty torque (with margin for shock) is the core of safe coupling and power-transmission design.
Professional Interpretation
The coupling is adequate with reserve. If the SF is very high, a smaller bolt, fewer bolts, or a tighter bolt circle would be more economical.
Governing Limit
Bolt shear is the limiting failure mode (capacity set by T_shear).
Required Bolt Size
For the target allowables a bolt of 5.95 mm (shear) or 1.85 mm (bearing) would be needed.
Status
Capacity 7,238 N·m ≥ applied 2000.00 N·m — adequate.
Recommendations
- •Always compare both the bolt-shear and bearing limits; the smaller governs the rated capacity.
- •If bolt shear governs, increase d_b or D_p first — both raise capacity strongly.
- •If bearing governs, increase flange thickness t or bolt diameter d_b.
- •Apply a duty-based service factor of 1.5–3.0× for starting and shock loads.
- •Confirm hub, key and shaft capacity separately — they may govern before the bolts.
Educational Content
Introduction
Couplings connect rotating shafts to transmit torque, accommodate misalignment and absorb shock. The bolted flange coupling is the simplest rigid type: two flanges bolted face-to-face, where the bolts resist the transmitted torque by shear at the interface and the flanges resist crushing of the bolt holes. A coupling calculator turns classical bolted-joint theory into instant checks of stress, capacity and safety margin.
Concept Explanation
Torque applied to the coupling creates a tangential force in each bolt equal to F = 2T/(n·D_p). Shear stress in the bolt is that force divided by the bolt area A_b. The same tangential force presses each bolt against its hole wall, producing a bearing (crushing) stress over the projected area n·t·d_b. Each mode has an allowable stress; the rated torque capacity is the smaller of the two limits. The service factor compares that capacity to the duty torque.
Applications
Benefits
- •Instant shear and bearing stress without manual derivation
- •Both failure modes checked; governing capacity identified automatically
- •Required bolt diameter feedback for design sizing
- •Service factor verdict for adequacy under load
- •Live diagram visualising the bolt circle and torque path
Limitations
- •Rigid flange couplings only — no flexible-element types
- •Excludes hub, key and shaft capacity checks
- •No fatigue, impact or thermal effects
- •Assumes equal bolt sharing and uniform preload
- •Not a substitute for code/standard verification
Best Practices
- •Use an even bolt count on a symmetric pitch circle
- •Torque bolts in a star pattern to the specified preload
- •Apply a duty-based service factor (1.5–3.0×) for shock loads
- •Keep flange thickness proportionate to bolt diameter
- •Align shafts precisely — rigid couplings tolerate no misalignment
Common Misconceptions
Practical Tips
- •Current service factor is 3.619×. Aim for 1.5–3.0× when the coupling must absorb starting and shock loads.
- •Bolt shear stress scales with 1/D_p — increasing the bolt circle diameter is the most effective way to raise torque capacity without changing bolt size.
- •Bearing stress is governed by flange thickness and bolt diameter; thin flanges crush before bolts shear. Keep t reasonably large relative to d_b.
- •Use an even number of bolts and a balanced bolt circle to avoid eccentric loading and vibration.
- •Verify both failure modes: the governing (lower) limit sets the rated capacity, not just the bolt shear check.
Common Mistakes
Assumptions
This calculator makes the following assumptions to provide estimates:
Limitations
Be aware of these limitations when using this calculator:
Frequently Asked Questions
Answers to common questions about this calculator.
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Glossary
Technical terms used in mortgage calculations explained in plain language.
- Bolt Circle Diameter (D_p)
- The diameter of the circle through the centres of the coupling bolts. Torque capacity is directly proportional to D_p, so a larger pitch circle yields higher capacity for the same bolt size.
- Bolt Shear Stress (τ_b)
- Average shear stress carried by each bolt at the flange interface, τ_b = 2T/(n·A_b·D_p). It must stay below the allowable shear stress of the bolt material.
- Bearing (Crushing) Stress (σ_b)
- Compressive stress between the bolt shank and the flange hole wall over the projected bearing area n·t·d_b·D_p/2. Limits flange crushing.
- Torque Capacity (T_max)
- The lesser of the bolt-shear limit and the bearing limit. The selected coupling must carry at least the maximum expected applied torque.
- Service Factor (SF)
- Ratio of rated torque capacity to the applied torque. A value above 1.0 indicates adequate margin; multiply nominal duty torque by a duty-based SF when sizing.
- Flange Thickness (t)
- Thickness of each flange half at the bolt circle. Provides the bearing area resisting bolt crushing and resists bending under bolt preload.
- Bolt Area (A_b)
- Tensile/shear cross-sectional area of one bolt, A_b = π·d_b²/4. Governs both the shear and bearing limits.
Authoritative References
Trusted sources and standards referenced by this calculator.
Applicable Standards
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Page Information
August 2026
2.1.0
Free to use for educational and professional purposes.