KEVOS® MAKEOS · Power Transmission
Flexible Coupling Selection (Torque, Speed, Misalignment, Bore and Key)
Is this flexible coupling right for the drive: torque with the service factor, peak torque, speed, combined misalignment, bore size and hub key?
Selection check of a flexible shaft coupling: running torque from power and speed, torque with the service factor against the coupling’s nominal rating, starting or peak torque against its maximum rating, speed, the combined misalignment as the sum of angular, parallel and axial shares of their allowances, bore against the largest bore, and the hub key stresses (as the key stress tool).
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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 | Speed of the coupling. |
| Service factor | Service factor for the driver and driven machine (from the coupling maker). | |
| Peak (starting) torque ÷ running torque | Starting or peak torque as a multiple of running torque (motor starting torque, shock). | |
| Coupling nominal torque | N·m | Nominal (continuous) torque rating of the coupling. |
| Coupling maximum torque | N·m | Maximum (peak) torque rating of the coupling. |
| Coupling maximum speed | rpm | Maximum speed rating of the coupling. |
| Misalignment (actual / allowed) | ||
| Angular misalignment | ° | Measured or expected angular misalignment. |
| Angular allowance | ° | Allowed angular misalignment for the coupling size. |
| Parallel offset | mm | Measured or expected parallel offset. |
| Parallel allowance | mm | Allowed parallel offset. |
| Axial displacement | mm | Measured or expected axial displacement. |
| Axial allowance | mm | Allowed axial displacement. |
| Bore and key | ||
| Shaft diameter | mm | Diameter of the shaft in the hub. |
| Largest bore of the hub | mm | Largest bore the hub accepts. |
| Key width | mm | Key width. |
| Key height | mm | Key height. |
| Key length in the hub | mm | Key length engaged in the hub. |
| Allowable key shear stress | MPa | Allowable shear stress of the key. |
| Allowable key crushing stress | MPa | Allowable crushing stress of the key or keyway. |
Method and formulas
- Running torque T = P ÷ ω; required nominal = T × SF; peak = T × peak factor.
- Combined misalignment = angular ÷ allowed + parallel ÷ allowed + axial ÷ allowed (keep the sum at or below 100%).
- Key force F = 2 T SF ÷ d; shear τ = F ÷ (w L); crushing σ = F ÷ (h ÷ 2 × L) (as the key stress tool).
- Example: 15 kW at 1460 rpm needs 147.16 N·m nominal against 200 N·m; misalignment uses 80% of the allowance.
Limitations
- The combined misalignment rule is a common simplification; follow the maker’s own interaction chart.
- Torsional vibration and resonance need a separate check for engines and variable-speed drives.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a coupling size for a motor and pump
- Check whether measured misalignment is acceptable
- Check that the bore and key carry the torque
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Running, service and peak torques against the ratings
- Speed and combined misalignment
- Bore and key checks
Interpreting the result
- Misalignment, not torque, is the usual reason flexible couplings wear out early.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 302–304, 2460–2486. Topic reference; no source tables or recommended settings are reproduced.
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Open the Flexible Coupling Selection (Torque, Speed, Misalignment, Bore and Key) calculator
