BIZOMAT MAKEOS · Power Transmission
Friction Clutch Engagement Design
What clamp force does this clutch need to start the load in time, and how much heat does each engagement make?
Sizing a multi-plate or single-plate friction clutch for an inertia start: torque to accelerate the load in the target time plus load torque with a service factor, clamp force for the friction faces (uniform wear), peak lining pressure against the allowable, actual slip time at the design torque, energy dissipated per engagement, temperature rise of the plates and average heat at the engagement rate.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Duty | ||
| Load inertia at the clutch | kg·m² | Inertia of everything being started, referred to the clutch shaft. |
| Driving speed | rpm | Speed of the driving side (assumed constant). |
| Load torque during engagement | N·m | Resisting load torque while starting. |
| Target engagement time | s | Time you want the start to take. |
| Service factor | Factor on the torque for wear and variation. | |
| Clutch | ||
| Friction face outside diameter | mm | Outside diameter of the friction surface. |
| Friction face inside diameter | mm | Inside diameter of the friction surface. |
| Number of friction faces | Number of friction interfaces (2 for a single plate between two faces). | |
| Friction coefficient | Friction coefficient of the lining. | |
| Allowable lining pressure | MPa | Allowable pressure for the lining material. |
| Heat | ||
| Mass of plates absorbing heat | kg | Mass of the parts that soak up the heat of one engagement. |
| Specific heat of plates | J/kg·K | Specific heat of the plates (about 460 J/kg·K for steel). |
| Engagements per hour | How often the clutch engages. | |
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.
Duty
What this means: Load inertia at the clutch
- What it is
- Total rotational inertia the clutch must bring up to speed, referred to the clutch shaft: driven shaft, gears, couplings and the machine.
- Units and format
- Enter a number in kg·m²: kilogram square metres (mass moment of inertia). The form accepts 0.000001 kg·m² to 1000000 kg·m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 kg·m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More inertia raises the starting torque, clamp force, slip energy and temperature rise.
- Get it right
- Refer inertia through gearing by multiplying by the square of the speed ratio to the clutch shaft.
What this means: Driving speed
- What it is
- Speed of the driving side of the clutch, which the tool treats as constant during engagement. The load is accelerated up to this speed.
- 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 1000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher speed raises the starting torque and the slip energy steeply.
- Get it right
- Use the speed at the clutch shaft, after any gearing from the motor.
What this means: Load torque during engagement
- What it is
- Torque the driven machine resists with while it is being started, such as friction, process load or lifting load at the clutch shaft.
- 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 20 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the torque needed and raises the slip energy per start.
- Get it right
- Refer the load torque to the clutch shaft through any gearing between them.
What this means: Target engagement time
- What it is
- The time within which the load must reach the driving speed after the clutch starts to engage.
- Units and format
- Enter a number in s: seconds. The form accepts 0.000001 s to 1000 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shorter times raise the inertia torque and clamp force.
- Get it right
- A very short time demands high torque; a long one makes more heat in the lining.
What this means: Service factor
- What it is
- Multiplier on the starting torque to cover lining wear, friction variation and shock, giving the design torque.
- 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
- Design torque, clamp force and lining pressure rise in proportion.
- Get it right
- Take it from the clutch maker or your design practice for this duty.
Clutch
What this means: Friction face outside diameter
- What it is
- Outside diameter of the friction lining ring, across its centre.
- 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 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure the lining, not the plate or the housing.
What this means: Friction face inside diameter
- What it is
- Inside diameter of the friction lining ring. It must be smaller than the outside diameter.
- 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 120 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- With uniform wear, peak pressure occurs at this inner radius.
What this means: Number of friction faces
- What it is
- Number of sliding friction interfaces carrying torque. A single plate clamped between flywheel and pressure plate has two.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 40. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More faces reduce the clamp force needed in proportion.
- Get it right
- Count interfaces, not plates: each driven plate gripped on both sides gives two faces.
What this means: Friction coefficient
- What it is
- Friction coefficient between the lining and its mating plate at operating temperature.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 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
- Higher friction reduces the clamp force needed in proportion.
- Get it right
- Use the lining maker’s design value, not the peak or static value.
What this means: Allowable lining pressure
- What it is
- Highest contact pressure the lining material may carry, from the lining maker. The peak pressure at the inner edge is checked against it.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 20 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter megapascals (1 MPa = 1 N/mm²).
Heat
What this means: Mass of plates absorbing heat
- What it is
- Mass of the plates and parts that absorb the heat of one engagement before it can be carried away.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 100000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More mass lowers the temperature rise per engagement in proportion.
- Get it right
- Count only metal close to the friction faces that heats up during slip.
What this means: Specific heat of plates
- What it is
- Specific heat capacity of the plate material: the energy needed to raise one kilogram by one kelvin.
- Units and format
- Enter a number in J/kg·K: J/kg·K. The form accepts 0.000001 J/kg·K to 5000 J/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 460 J/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Temperature rise falls in proportion as specific heat rises.
- Get it right
- Take it from the material data sheet for the plate material.
What this means: Engagements per hour
- What it is
- Number of times per hour the clutch engages under this duty. It sets the average heat the clutch must get rid of.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 100000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Average heat rises in proportion.
- Get it right
- Use the busiest hour, not the daily average.
Method and formulas
- Torque to start: T = J ω ÷ t + T_L; design torque = SF × T.
- Uniform wear: T = μ F faces (D_o + D_i) ÷ 4, so F = 4T ÷ [μ faces (D_o + D_i)]; peak pressure p = F ÷ [2π r_i (r_o − r_i)].
- Slip time at the design torque = J ω ÷ (T_design − T_L); energy per engagement = T_design ω² J ÷ 2(T_design − T_L).
- Temperature rise = energy ÷ (m c_p); average heat = energy × engagements per hour ÷ 3600.
- Example: starting 2 kg·m² to 1000 rpm in 1 s needs 7.17 kN of clamp force and makes 11.643 kJ per start.
Limitations
- Constant driving speed and constant friction; real engagement curves vary.
- Heat soak assumes the plate mass absorbs each engagement before cooling.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a clutch or brake for starting a flywheel, conveyor or spindle
- Check clutch heating at a higher cycle rate
- Choose plates and actuator force
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Design torque and clamp force
- Lining pressure and slip time
- Energy, temperature rise and average heat
Interpreting the result
- Slip energy is set by inertia and speed: a faster start does not reduce it, a lower slip speed does.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2446–2458. 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
Engaging a friction clutch
In Clutches and Brakes by the Numbers
What it covers: What happens while a clutch slips: slip time, the energy that becomes heat, the snatch on the load, plate temperature rise and engagements per hour, on the shaft where the clutch sits.
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