Technical Services Notes Sheets

The design of industrial friction clutches, discs and similar devices.

One common use of friction materials is to assist in the generation of a torque; this torque may be required either simply to clamp a rotatable member, to transmit power continuously with take-up as required, to have a maximum oz value before slip occurs (a torque limiting clutch), or to restrain the constant rotation of a shaft (a tensioning brake). The following notes and comments give some advice on the design of the friction elements of such devices and on the category of friction material required.

As with most kinds of friction brake, the devices considered here can be divided into two main classes, depending on whether the friction material is in contact with a curved surface or a flat one; for convenience, these will be referred to as drum (either internal or external) or disc, even though in some cases the design permits only part of a revolution.

Drum Applications

The case considered here is that of a piece of friction material in contact with a curved surface of radius; this could be used as a clamping device, a tensioning brake with the drum rotating continuously or incorporated into a torque limiting device. The use of shoes having freedom to align themselves to the drum is a complex matter and is not dealt with here.

The torque generated by such a device will be the product of 'F' (the force applied to the friction material), 'MU' (the coefficient of friction of the material) and 'R' (the radius at which contact is made); depending on the system of units used, the torque may be expressed as either Newton metres (Nm) or pounds feet (Ibft).

For a clamping device which is required not to slip, the value of the coefficient of friction to be used is the static one, which may be taken as the initial point on the friction / temperature curve. Provision should be made for adjusting the applied force 'F' and the contact area must be such that the unit pressure on the friction material is kept within the recommended limits.

A tensioning brake with constant slip will generate a great deal of heat, so the value of 'P' 'which should be used in any calculations must be somewhat (say 10%) less than that shown by the friction / temperature curve, at the recommended maximum continuous temperature for the friction material (taken from the data sheet). In almost every case, cooling will have to be provided to dissipate the heat generated and limit the working surface temperature; drums working externally are sometimes cooled internally by water, circulated by a means allowing for drum rotation, but in most cases a powerful forced flow of cooling air will be needed.

If the working temperature rises above the recommended value, both the friction level and the wear rate will be affected; some reduction of the surface temperature can be achieved by an increase in contact area.

For a clamping device, a high friction level is desirable to limit the applied force 'F', but temperature and wear resistance are not important; flexibility may be needed when forming the friction material (which need only be thin) to the radius of the drum and will ensure good contact. On a tensioning brake a moulded material having good wear resistance will be needed, and this will entail acceptance of only a moderate friction level with less flexibility, the volume of the block must be sufficient to give acceptable life, determined by trial and error with attention to the cooling.

Disc Applications

The case considered is of a disc of friction material. In using a disc there is often a conflict between the need to maximise 'R', and yet have a sufficient area to keep the unit pressure and surface temperature within recommended limits, the reasoning being as follows.

The Radius 'R' is not now predetermined, as with a drum application, but will lie between the maximum (R1) and minimum (R2) radii, being called the effective radius; the effective radius is that at which, for calculation purposes, the friction material may be assumed to be concentrated. The usual practice is to assume that the effective radius equals the mean radius, found by calculating R = R1 - R2, but under particular conditions it may be more or less; allowance for this and any other uncertainties must be made by the application of a suitable 'factor of safety', the magnitude of which also allows for the safety aspects of the installation.

The torque generated by the device will be the product of 'F' (the applied force), 'MU' (the coefficient of friction), 'R' (the effective radius) and 'N' (the number of working faces); as before, a suitable system of units should be used.

For maximum torque output from a given force 'F', 'R' needs to be large, but there will often be a limitation on disc outside radius, imposed by considerations of space, stresses on rotating friction discs or cost; 'R' therefore can approach - but not exceed - 'R1' in value but in increasing 'R2' so as to maximise 'R' the area is progressively reduced, thus raising unit pressure and (for a dynamic application) working temperature and wear rate. If consistency of torque is particularly important, it is desirable that (R1 - R2) is relatively small in relation to R2, so that if the effective radius 'R' varies during operation, the possible variation is minimised; when a segment of a disc or some other shape is used, it is usually assumed that 'R' is taken to the centre of area.

When high torque is needed from a device of limited size, it may be possible to increase the number of working surfaces; two friction discs may be attached to a metal carrier plate mounted on the rotatable member and clamped between stationary mating surfaces, and a number of such assemblies can be used in parallel. It should be noted that a disc located between one fixed and one rotatable metal part, but not attached to either, will only slip at one face at any moment, giving only one working surface.

As before, a high coefficient of friction is desirable for a clamping device, the cold value from the friction / temperature curve being used; temperature and wear resistance are not important, but flexibility will help the friction material conform to the mating surface. The same category of friction material will serve for light duty dynamic applications, involving minimal generation of heat, but the value should be taken as 10% less than the data sheet cold value.

For a clutch to be used intermittently, with moderate generation of heat, a friction material having good temperature and wear resistance will be needed; this will then be a rigid material having a medium friction level. Forced cooling will not normally be necessary, so surface temperatures may briefly rise towards the maximum permitted value for the friction material; for calculation purposes, 'MU' should be 10% less than the lowest point on the friction / temperature curve. The same category of friction material will be suitable for torque limiting clutches and tensioning brakes; the vlaue of 'mu' for calculation will then be 10% less than that on the friction / temperature curve at the recommended maximum continuous temperature.

A torque limiting clutch is not intended to slip for long, so forced cooling may not be necessary, but such cooling is always necessary for tensioning brakes.

General considerations regarding Friction Material

For a dynamic application, the unit pressure 'W' should remain within the limits 10-100 lb/in (70--7OOkN/m ), in order that the surface condition may be maintained, but without fear of overheating. In applications involving only limited movement, pressures may be higher, and much higher still for clamping devices. See also technical note sheet 'Bedding - Normal Condition - Glazing'.

In all cases the friction material must be adequately attached to the metal part which transmits the force, either by bonding or riveting, advice being available regarding these processes on request.

Contact us now for guidance and advice, or to discuss a specific project, by telephone on 01254 397561, by fax on 01254 389722 or by email at david@frictioncomponents.co.uk

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Friction Components and Systems Ltd - Solving Industries Braking Point
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