Disc Clutch and Brake Torque Calculator
Torque capacity of a single or multi plate friction clutch or brake, worked out on both the uniform pressure theory for a new lining and the uniform wear theory it settles into after bedding in. The contact pressure, friction area, equivalent radius and the clamping force needed for a given torque are reported alongside the steady power.
How to use this calculator
- Enter the outer and inner diameters of the friction face, and count every surface that rubs, which is two on a single plate clutch and two n minus two on a pack of n plates keyed to the housing.
- Enter the friction coefficient for the lining, from about 0.3 dry to about 0.12 wet, and the axial clamping force from the spring or piston.
- Enter the running speed to turn the torque into a power figure, and the allowable contact pressure for the lining material.
- Read both torque values, design to the uniform wear one, and check the contact pressure is inside the allowable for the lining.
Frequently asked questions
What is the difference between uniform pressure and uniform wear theory?
A new clutch with a stiff, flat lining carries the pressure evenly across the face, which is the uniform pressure case and gives the higher torque. Once it beds in, the lining wears faster where it rubs faster, at the outer edge, and the pressure redistributes until the wear rate is the same everywhere. That is the uniform wear case, it gives the lower torque, and it is what the clutch actually settles down to. Design to it.
How many friction surfaces does a multi plate clutch have?
The number of plates rubbing against something else, counted on both faces. In a pack of n plates with the outermost plates keyed to the drum and the inner ones to the hub, there are 2n minus 2 active surfaces. Getting this wrong by one is one of the most common errors in clutch sizing, and it shows up as a torque capacity that is out by tens of percent.
What contact pressure is acceptable for a clutch lining?
Dry organic and paper linings are usually held below 1 to 1.5 MPa, sintered metal below 2 to 3 MPa, and carbon graphite can go higher. The limit is really a temperature one: the wear rate climbs steeply once the lining runs hot, so the pressure limit is a roundabout way of limiting the heat generated at the surface.
Does this tell me whether the clutch will survive a hard engagement?
No. Torque capacity is a steady state figure. The thermal problem is the energy dumped into the lining during a single engagement, which is the kinetic energy of the load and can put more heat into the plates in one second than an hour of steady running. A clutch can have ample torque capacity and still fade or burn out on a hard engagement, so the duty cycle needs checking separately.