There are two basic formulae used in calculating the capacity of a clutch or brake. One is to determine the torque capacity and the other HP (horse power) absorption. The torque should be known in order to know that sufficient holding power has been provided. The HP gives us an indication as to the amount of power that will be absorbed in the unit and the amount of heat that may be generated. From this we can determine whether a reasonable amount of friction material has been provided and that this work can be performed with reasonable life.

In the following formulae:

T=maximum torque
R=Drum radius or mean radius of a ring
F=Coefficient of friction
P=Total pressure of friction material
N=Number of working faces


Torque formula:


In using this torque formula, use the coefficient of friction for the Hibbing material under consideration. Allow a safety factor of 30% for mechanical losses, spring set, and normal variations in the coefficient of friction.

There are cases, however, where, brake and clutch applications are frequently at high rate of power absorption and surface temperatures may be sufficiently high to cause greater variation than 30% in resulting torque. This can occur when surface temperatures reach more then 400 degrees F.

In multiple disc clutches using friction facings with driving teeth or lugs cut in the friction facings, we must also allow a greater factor of safety higher because of additional losses due to the drag caused by friction between the driving lugs of the facing and the housing or hub.

When the teeth or lugs are cut in a steel member supporting friction material, or when the device operates in oil, this frictional drag will be less. For multiple disc clutches, a safety factor of 50% should be used.

The work done and the heat generated by the friction material is a function of the pressure, and most friction material will withstand a tremendous pressure without fracture.


HP formula:


In using this formula use the torque derived from the torque formula. Inasmuch as most friction material is used to stop or start a machine, the slipping speed varies from maximum to zero during an application and, therefore, “S” can be taken as one-half the maximum speed.

The answer arrived at in the horsepower formula will be HP absorbed per minute, provided the device operates continuously. From it therefore, we should calculate the actual power absorbed during a minute of operation as follows:

HP X Number of seconds per operation

Friction material can absorb depending on the formulation anywhere from 3 to 25 HP per sq.. inch. Where frequent applications are encountered a greater area should be provided. Where a clutch is for starting a machine at in- frequent intervals, a smaller area will provide reasonable life.


For satisfactory operation of a friction brake or clutch, it is essential to have not only the proper friction material but also a good mating material for it to run against. Two materials, both ferrous, are in common usage,steel and cast iron. Nonferrous metal are occasionally used, but is usually not satisfactory except for a few applications.

Surface finish for the steel or cast iron is of some importance. A surface finish of from 30 to 60 micro inches is suggested.

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