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BIZOMAT 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

InputUnitTypical source
PowerkWPower transmitted.
SpeedrpmSpeed of the coupling.
Service factorService factor for the driver and driven machine (from the coupling maker).
Peak (starting) torque ÷ running torqueStarting or peak torque as a multiple of running torque (motor starting torque, shock).
Coupling nominal torqueN·mNominal (continuous) torque rating of the coupling.
Coupling maximum torqueN·mMaximum (peak) torque rating of the coupling.
Coupling maximum speedrpmMaximum 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 offsetmmMeasured or expected parallel offset.
Parallel allowancemmAllowed parallel offset.
Axial displacementmmMeasured or expected axial displacement.
Axial allowancemmAllowed axial displacement.
Bore and key
Shaft diametermmDiameter of the shaft in the hub.
Largest bore of the hubmmLargest bore the hub accepts.
Key widthmmKey width.
Key heightmmKey height.
Key length in the hubmmKey length engaged in the hub.
Allowable key shear stressMPaAllowable shear stress of the key.
Allowable key crushing stressMPaAllowable crushing stress of the key or keyway.

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
Illustration: PowerA motor driving a shaft; Power is the rate at which it does work, energy per second.PExample: 15 kWPower
What it is
The power the coupling passes from the driving machine to the driven machine in normal running.
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 15 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Running torque rises in direct proportion to power.
Get it right
Use the absorbed power or the motor rating, as your selection practice requires; the motor rating is the safer choice.
What this means: Speed
Illustration: SpeedA turning disc with a marker; Speed is how many full turns it makes in each minute.turns of the shaftper minutenExample: 1460 rpmSpeed
What it is
The speed at which the coupling turns, used to turn power into torque and checked against the coupling speed rating.
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 1460 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
At the same power, a lower speed means a higher torque.
Get it right
Use the speed at the coupling itself; behind a gearbox it differs from the motor speed.
What this means: Service factor
Illustration: Service factorTwo bars: a base value and the same value multiplied by Service factor, the factor.base valuewith factorSFExample: 1.5Service factor
What it is
The multiplier the coupling maker applies to running torque for the type of driver and driven machine, covering uneven or shock loads.
Units and format
Enter a plain number with no unit. The form accepts 1 to 4. 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
The design torque, key force and key stresses rise in direct proportion to it.
Get it right
Take it from the coupling maker’s service factor table for your driver and load type.
What this means: Peak (starting) torque ÷ running torque
Illustration: Peak (starting) torque ÷ running torqueTwo wheels of different sizes linked together; Peak (starting) torque ÷ running torque compares one with the other.iExample: 2.5Peak (starting) torque ÷ running torque
What it is
The highest torque the coupling sees, at starting or under shock, divided by the running torque. It is checked against the coupling maximum torque.
Units and format
Enter a plain number with no unit. The form accepts 1 to 6. Required: the tool does not calculate until every input is valid.
Example
The form opens with 2.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
The peak torque rises in direct proportion to it.
Get it right
Base it on the motor starting torque or the worst shock; this peak is not also multiplied by the service factor.
What this means: Coupling nominal torque
Illustration: Coupling nominal torqueA shaft with a curved arrow around it: torque Coupling nominal torque is the twisting effect, force times lever arm.Ttwisting effect= force × lever armExample: 200 N·mCoupling nominal torque
What it is
The torque the coupling can carry continuously, from the maker’s catalogue for the size and element you are considering.
Units and format
Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
Example
The form opens with 200 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Use the nominal rating, not the maximum; the maximum has its own input.
What this means: Coupling maximum torque
Illustration: Coupling maximum torqueA shaft with a curved arrow around it: torque Coupling maximum torque is the twisting effect, force times lever arm.Ttwisting effect= force × lever armExample: 400 N·mCoupling maximum torque
What it is
The highest torque the coupling can take briefly, for starting or occasional shock, from the maker’s catalogue.
Units and format
Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
Example
The form opens with 400 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Make sure it is the short-term peak rating for the same coupling size and element as the nominal rating.
What this means: Coupling maximum speed
Illustration: Coupling maximum speedA turning disc with a marker; Coupling maximum speed is how many full turns it makes in each minute.turns of the shaftper minutenExample: 5000 rpmCoupling maximum speed
What it is
The highest speed the coupling size is rated for, from the maker’s catalogue.
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 5000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Check whether the rating depends on the hub material or balancing grade.

Misalignment (actual / allowed)

What this means: Angular misalignment
Illustration: Angular misalignmentTwo lines meeting at a point with an arc between them, marking the angle Angular misalignment.θExample: 0.3°Angular misalignment
What it is
The angle between the two shaft axes at the coupling, measured or expected after alignment.
Units and format
Enter a number in °: degrees of angle. The form accepts 0° to 10°. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.3° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
More angular misalignment uses more of the combined allowance.
Get it right
Enter degrees; if your alignment tool reports a gap across the face, convert it to an angle first.
What this means: Angular allowance
Illustration: Angular allowanceTwo lines meeting at a point with an arc between them, marking the angle Angular allowance.θExample: 1°Angular allowance
What it is
The angular misalignment the coupling maker allows for this coupling size and speed.
Units and format
Enter a number in °: degrees of angle. The form accepts 0.01° to 10°. Required: the tool does not calculate until every input is valid.
Example
The form opens with 1° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Use the allowance for your running speed if the maker gives one.
What this means: Parallel offset
Illustration: Parallel offsetTwo parts separated by a small gap with a dimension across the gap, marking Parallel offset.cExample: 0.1 mmParallel offset
What it is
The sideways offset between the two shaft centrelines at the coupling, measured or expected after alignment.
Units and format
Enter a number in mm: millimetres. The form accepts 0 mm to 20 mm. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
More offset uses more of the combined allowance.
Get it right
Use the offset between centrelines, not the total indicator reading, which is twice the offset.
What this means: Parallel allowance
Illustration: Parallel allowanceTwo parts separated by a small gap with a dimension across the gap, marking Parallel allowance.cExample: 0.4 mmParallel allowance
What it is
The parallel offset the coupling maker allows for this size and speed.
Units and format
Enter a number in mm: millimetres. The form accepts 0.01 mm to 20 mm. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Use the same basis as the measured offset (centreline offset, not indicator reading).
What this means: Axial displacement
Illustration: Axial displacementTwo parts separated by a small gap with a dimension across the gap, marking Axial displacement.cExample: 0.5 mmAxial displacement
What it is
The movement of one shaft end along its axis towards or away from the other, from thermal growth, end float or setting error.
Units and format
Enter a number in mm: millimetres. The form accepts 0 mm to 50 mm. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
More axial movement uses more of the combined allowance.
Get it right
Include thermal growth of the shafts in running, not only the cold setting.
What this means: Axial allowance
Illustration: Axial allowanceTwo parts separated by a small gap with a dimension across the gap, marking Axial allowance.cExample: 2 mmAxial allowance
What it is
The axial displacement the coupling maker allows for this size.
Units and format
Enter a number in mm: millimetres. The form accepts 0.01 mm to 50 mm. Required: the tool does not calculate until every input is valid.
Example
The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Take it from the same catalogue page as the angular and parallel allowances.

Bore and key

What this means: Shaft diameter
Illustration: Shaft diameterA circle with a line straight through its centre from edge to edge, marking the diameter Shaft diameter.dmeasured acrossthe centreExample: 42 mmShaft diameter
What it is
The diameter of the shaft that fits in the coupling hub. The key force is worked out from the torque at this radius.
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 42 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
A smaller shaft raises the key force and key stresses.
Get it right
Use the larger of the two shafts if the hubs differ, or check each shaft separately.
What this means: Largest bore of the hub
Illustration: Largest bore of the hubA circle with a line straight through its centre from edge to edge, marking the diameter Largest bore of the hub.Ømeasured acrossthe centreExample: 55 mmLargest bore of the hub
What it is
The largest bore the coupling hub can be machined to, from the maker’s catalogue.
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 55 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Check whether the largest bore depends on the keyway standard; the tool rejects a shaft larger than this.
What this means: Key width
Illustration: Key widthA bar with a dimension line and arrows from one end to the other, marking the length Key width.wExample: 12 mmKey width
What it is
The width of the key across the shaft, which sets the area resisting shear.
Units and format
Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
A wider key lowers the key shear stress.
Get it right
Take it from the key standard for the shaft diameter.
What this means: Key height
Illustration: Key heightA block with a vertical dimension line from its base to its top, marking the height Key height.hExample: 8 mmKey height
What it is
The full height of the key; the tool takes half of it as the depth bearing on the hub keyway for the crushing check.
Units and format
Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 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
A taller key lowers the crushing stress.
Get it right
Enter the full key height, not the hub keyway depth.
What this means: Key length in the hub
Illustration: Key length in the hubA bar with a dimension line and arrows from one end to the other, marking the length Key length in the hub.LExample: 50 mmKey length in the hub
What it is
The length of key engaged in the coupling hub, which carries both the shear and the crushing load.
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 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
A longer engagement lowers both key stresses in direct proportion.
Get it right
Use the length actually inside the hub, less any rounded key ends.
What this means: Allowable key shear stress
Illustration: Allowable key shear stressA test bar pulled from both ends: stress Allowable key shear stress is the force carried by each square millimetre of its cross-section.σstress = force ÷ areaExample: 60 MPaAllowable key shear stress
What it is
The shear stress the key material may carry, from your design standard or key material data.
Units and format
Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 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.
Get it right
Use the allowable for the key steel and your load type, including any shock.
What this means: Allowable key crushing stress
Illustration: Allowable key crushing stressA test bar pulled from both ends: stress Allowable key crushing stress is the force carried by each square millimetre of its cross-section.σstress = force ÷ areaExample: 100 MPaAllowable key crushing stress
What it is
The crushing (bearing) stress allowed on the key sides, governed by the weaker of the key, shaft and hub materials.
Units and format
Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 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
A cast iron or aluminium hub usually limits this value, not the key.

Method and formulas

  1. Running torque T = P ÷ ω; required nominal = T × SF; peak = T × peak factor.
  2. Combined misalignment = angular ÷ allowed + parallel ÷ allowed + axial ÷ allowed (keep the sum at or below 100%).
  3. Key force F = 2 T SF ÷ d; shear τ = F ÷ (w L); crushing σ = F ÷ (h ÷ 2 × L) (as the key stress tool).
  4. 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.

Learn the Concepts Behind This Tool

These BIZOMAT Library readings explain the method this tool uses, with worked examples.

BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.

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