BIZOMAT MAKEOS · Power Transmission
Centrifugal Clutch Check (Engagement Speed, Torque at Running Speed, Idle Clearance)
For a centrifugal clutch with spring-held shoes, at what speed does it start to engage, how much torque does it carry at running speed, and will it stay free at idle?
Check of a shoe-type centrifugal clutch: the centrifugal force on each shoe at speed (as the centrifugal force tool), the speed at which it equals the spring force holding the shoe in (engagement speed), the net shoe force and the friction torque at the running speed from the number of shoes, friction coefficient and drum radius, against the torque needed with a service factor and the idle speed at which the clutch must stay disengaged.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of shoes | shoes | Number of friction shoes. |
| Mass of each shoe | kg | Mass of each shoe with its lining. |
| Radius to the shoe centre of gravity | mm | Radius from the clutch axis to each shoe’s centre of gravity when engaged. |
| Spring force holding each shoe in (at contact) | N | Inward force of the springs on each shoe when it just touches the drum. |
| Drum inside radius | mm | Inside radius of the drum where the linings rub. |
| Lining friction coefficient | Friction coefficient of the lining on the drum. | |
| Duty | ||
| Running speed | rpm | Speed the clutch runs at under load. |
| Idle speed (must not engage) | rpm | Engine idle speed, at which the clutch must not drag. |
| Torque needed at running speed | N·m | Torque the driven machine needs. |
| Service factor | Factor for starting and shock loads. | |
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: Number of shoes
- What it is
- The number of friction shoes that fly out against the drum.
- Units and format
- Enter a number in shoes: shoes. The form accepts 1 shoes to 12 shoes. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 shoes as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque rises in direct proportion.
- Get it right
- Count all shoes; the tool assumes they share the torque equally.
What this means: Mass of each shoe
- What it is
- The mass of each shoe including its lining, which creates the outward centrifugal force.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 100 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A heavier shoe lowers the engagement speed and raises torque.
- Get it right
- Weigh one shoe with its lining, without the springs.
What this means: Radius to the shoe centre of gravity
- What it is
- The distance from the clutch axis to each shoe’s centre of gravity with the shoe touching the drum.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the engaged position, not the rest position.
What this means: Spring force holding each shoe in (at contact)
- What it is
- The inward pull of the springs on one shoe when the shoe just touches the drum. Centrifugal force must exceed it before the shoe presses on the drum.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 1000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A stronger spring raises the engagement speed and lowers torque at speed.
- Get it right
- Measure or calculate it at the engaged position, per shoe, not for all springs together.
What this means: Drum inside radius
- What it is
- The inside radius of the drum where the linings rub, the lever arm of the friction force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque rises in direct proportion.
- Get it right
- Radius, not diameter: halve the drum bore.
What this means: Lining friction coefficient
- What it is
- The friction coefficient of the shoe lining on the drum.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 1. 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
- Torque rises in direct proportion.
- Get it right
- Take it from the lining maker; it falls when hot, wet or oily.
Duty
What this means: Running speed
- What it is
- The speed the clutch runs at when transmitting the working torque.
- 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 3000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shoe force rises with the square of speed, so torque rises quickly above engagement.
- Get it right
- Use the loaded running speed, not the engine’s maximum speed.
What this means: Idle speed (must not engage)
- What it is
- The engine idle speed, at which the shoes must stay off the drum. It is compared with the engagement speed.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1200 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the highest idle speed the engine may settle at.
What this means: Torque needed at running speed
- What it is
- The torque the driven machine needs at running speed.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 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 steady torque; the service factor covers starting and shock.
What this means: Service factor
- What it is
- A multiplier on the needed torque for starting, shock and lining wear.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. 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 torque the clutch must reach rises in direct proportion.
- Get it right
- Take it from the clutch maker or your design rules.
Method and formulas
- Centrifugal force F = m ω² r (as the centrifugal force tool).
- Engagement when m ω² r = spring force: ω = √(F_s ÷ (m r)).
- Torque = shoes × μ × (m ω² r − F_s) × drum radius.
- Example: engages at 1,232.8 rpm; carries 83.055 N·m at 3000 rpm.
Limitations
- Leading and trailing shoe effects (self-energising) are ignored; pivoted shoes can carry more torque one way.
- Linings worn so the shoes travel further change the spring force.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a centrifugal clutch for an engine-driven machine
- Choose spring force for a set engagement speed
- See whether a clutch slips under load
Who it is for
- Design engineers
- Small engine mechanics
- Machine designers
What you get
- Centrifugal force per shoe
- Engagement speed
- Torque at running speed
Interpreting the result
- Clutch torque rises with the square of speed above engagement, so the clutch slips at low speed and grips firmly at full speed.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 187–190, 2433–2446. 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.
Method explained here · BIZOMAT original book
Centrifugal clutches
In Clutches and Brakes by the Numbers
What it covers: How a centrifugal clutch engages by speed alone: shoe force, spring force, engagement speed, torque against speed, idle clearance, and the heat of slipping at part throttle.
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