EngineeringMechanical EngineeringPart 06 of 15
Shaft Coupling Selection
A coupling is specified against four independent misalignment conditions, not one. Selecting on torque alone produces couplings that pass the power check and fail in service within weeks.
- Four misalignment modes
- Rigid vs flexible
- Reference-speed rating
- Selection procedure
Executive summary
Couplings divide cleanly into rigid and flexible. A rigid coupling accepts no misalignment or relative movement at all and is rated simply to transmit the same torque and power as a mild steel shaft of the same diameter. A flexible coupling is designed to accommodate four distinct misalignment conditions and should be used wherever a prime mover is directly coupled to a gearbox or machine shaft.
Flexible coupling ratings are commonly published at a reference speed — typically 100 rev/min — so the selection power must be converted to an equivalent selection power at that reference before the tables can be used.
The four misalignment conditions
Angular misalignment
The shaft axes are inclined to one another. Its magnitude is measured at the coupling faces.
Axial or parallel misalignment
The driving and driven shaft axes are parallel but laterally displaced.
End float
The ability to accommodate relative axial displacement of the connected shafts, achieved by sliding members or by flexure of resilient components.
Torsional flexibility
A design feature that allows shock and impulsive loading to be absorbed rather than transmitted directly into the driven machine.
A flexible coupling absorbs initial assembly inaccuracies, foundation settlement, movement of shafts and machinery under load, and thermal growth. What it does not do is compensate for careless alignment. Shafts should still be aligned as accurately as good engineering practice permits; coupling flexibility is a tolerance allowance, not an alternative to alignment.
A coupling that satisfies the power requirement can still be wrong for the duty if its allowable angular or parallel misalignment is below the design misalignment. The misalignment check is a separate gate, and it fails different couplings from the ones the power check fails.
Coupling families
| Family | Principle | Character |
|---|---|---|
| Rigid | Solid connection, often with taper lock bushes. | No misalignment capability. Rated as a shaft of equal diameter. |
| Spider / jaw | Elastomeric spider between two jawed hubs. | Compact, torsionally resilient, widely used on small and medium drives. |
| Pin and bush | Pins in one hub engaging resilient bushes in the other. | Good shock absorption, simple element replacement. |
| Tyre | Flexible rubber tyre element clamped between hubs. | High misalignment and shock capability; softest torsionally. |
| Disc | Flexing steel disc packs. | Torsionally stiff, no wearing elements, low maintenance. |
| Chain | Duplex chain wrapped over two sprocket hubs. | High torque density, requires lubrication and a cover. |
Manufacturers also offer high-misalignment gear types, brake drum and disc brake gear types, shear pin and buffer shear pin types, telescopic types and hydraulic couplings for specialised duty.
Rigid coupling selection
The procedure is short because there is little to decide.
- Match the coupling to the shaft sizeRigid couplings are rated to transmit the same torque and power as a mild steel shaft of the same diameter, so shaft size effectively determines the coupling.
- Check the maximum allowable speedConfirm the operating speed is below the coupling's speed rating.
- Confirm the bush boreWhere taper lock bushes are used, confirm the shaft size is a standard bore for the bush.
- Record both designationsCoupling and bush, with the bore size, form the complete specification.
Rigid couplings suit shaft extensions inside a single rigid frame where both bearings are bored in one operation and alignment is guaranteed by manufacture. They do not belong between a motor and a separately mounted machine.
Flexible coupling selection
- Set out the duty dataMaximum design power and operating speed, maximum speed if different, nature of prime mover and load, average operating hours and starts per day, expected design misalignment, and both shaft sizes.
- Classify the loadSteady, medium impulsive or highly impulsive, based on the driven machine.
- Obtain the service factorFrom the load classification and duty hours.
- Obtain the start factorFrom the number of starts per day and the starting method.
- Calculate selection powerDesign power multiplied by both factors.
- Convert to equivalent selection powerCorrect to the reference speed at which the coupling range is rated.
- Select from the coupling tablesSmallest coupling of the chosen type with capacity above the equivalent selection power. If the type is not fixed, list every family that suits.
- Check design misalignmentAgainst the allowable misalignment for that coupling in each of the four modes.
- Check maximum boreCoupling or taper bush bore must exceed the actual shaft size.
- Check maximum speedCoupling speed rating above the maximum operating speed.
- Record the full specificationCoupling and taper bush designations with bore sizes, plus the alignment tolerance to be achieved at installation.
- P
- design power, kW
- fp
- service factor from load classification and duty hours
- fs
- start factor from starts per day
- Ps
- selection power
- Nref
- reference speed at which the coupling range is rated, commonly 100 rev/min
- N
- actual operating speed, rev/min
- Pe
- equivalent selection power, used against the coupling tables
Worked example
Two 30 mm shafts are to be coupled, rotating at 1450 rev/min, driving a uniformly loaded conveyor from a direct-on-line electric motor with six starts per day and a sixteen-hour operating day. Transmitted power is 4 kW and expected assembly misalignment is small but non-zero.
The reference-speed correction is the step most often missed. At 1450 rev/min the equivalent selection power is a small fraction of the selection power, because the published table capacity is a torque capacity expressed as power at 100 rev/min. Applying the selection power directly to the table would oversize the coupling by more than an order of magnitude.
For the equivalent rigid case — two 30 mm shafts at 1450 rev/min inside a single rigid frame — the selection reduces to matching a rigid coupling and taper bush to the 30 mm bore and confirming that the coupling's maximum speed comfortably exceeds 1450 rev/min.
Selection checklist
- Rigid or flexible decided on the basis of whether misalignment or relative movement can occur.
- Load classified as steady, medium impulsive or highly impulsive.
- Service factor and start factor both applied — they are separate multipliers.
- Selection power converted to the reference speed at which the coupling range is rated.
- Design misalignment checked against the allowable value in all four modes.
- Coupling or bush maximum bore confirmed against both shaft diameters.
- Maximum coupling speed above the maximum operating speed, not just the running speed.
- Alignment tolerance to be achieved at installation stated on the drawing.
- Element replacement access considered where a resilient element will wear.
- Both coupling and taper bush designations recorded with bore sizes.