The rim-type brake can be designed for self-energizing, that is, using friction to reduce the actuating force. Self-energization is important in reducing the required braking effort; however, it also has a disadvantage.

When rim-type brakes are used as vehicle brakes, a small change in the coefficient of friction will cause a large change in the pedal force required for braking. For example, it is not unusual for a 30% reduction in the coefficient of friction (due to a temperature change or moisture) to result in a 50% change in the pedal force required to obtain the same braking torque that was possible prior to the change.

The rim types may have internal expanding shoes or external contracting shoes. An internal shoe clutch consists essentially of three elements: (1) a mating frictional surface, (2) a means of transmitting the torque to and from the surfaces, and (3) an actuating mechanism. Depending upon the operating mechanism, such clutches can be further classified as expanding-ring, centrifugal, magnetic, hydraulic, or pneumatic.

The expanding-ring clutch benefits from centrifugal effects, transmits high torque even at low speeds, and requires both positive engagement and ample release force. This type of clutch is often used in textile machinery, excavators, and machine tools in which the clutch may be located within the driving pulley.

The centrifugal clutch is mostly used for automatic operation. If no spring is present, the torque transmitted is proportional to the square of the speed [Beach, 1962]. This is particularly useful for electric motor drives in which, during starting, the driven machine comes up to speed without shock.

Springs can be used to prevent engagement until a certain motor speed has been reached, but some shock may occur. Magnetic clutches are particularly useful for automatic and remote-control systems and are used in drives subject to complex load cycles.

Hydraulic and pneumatic clutches are useful in drives having complex loading cycles, in automatic machinery, and in manipulators. Here the fluid flow can be controlled remotely using solenoid valves. These clutches are available as disk, cone, and multiple-plate clutches.

In braking systems the internal-shoe or drum brake is used mostly for automotive applications. The actuating force of the device is applied at the end of the shoe away from the pivot. Since the shoe is usually long, the distribution of the normal forces cannot be assumed to be uniform. The mechanical arrangement permits no pressure to be applied at the heel; therefore, frictional material located at the heel contributes very little to the braking action.

It is standard practice to omit the friction material for a short distance away from the heel, which also eliminates interference. In some designs the hinge pin is allowed to move to provide additional heel pressure. This gives the effect of a floating shoe. A good design concentrates as much frictional material as possible in the neighborhood of the point of maximum pressure.

Typical assumptions made in an analysis of the shoe include the following: (1) the pressure at any point on the shoe is proportional to the distance from the hinge pin (zero at the heel); (2) the effect of centrifugal force is neglected (in the case of brakes, the shoes are not rotating and no centrifugal force exists; in clutch design, the effect of this force must be included in the equations of static equilibrium); (3) the shoe is rigid (in practice, some deflection will occur depending upon the load, pressure, and stiffness of the shoe; therefore, the resulting pressure distribution may be different from the assumed distribution); and (4) the entire analysis is based upon a coefficient of friction that does not vary with pressure.

Actually, the coefficient may vary with a number of conditions, including temperature, wear, and the environment.

For pivoted external shoe brakes and clutches, the operating mechanisms can be classified as solenoids, levers, linkages or toggle devices, linkages with spring loading, hydraulic devices, and pneumatic devices. It is common practice to concentrate on brake and clutch performance without the extraneous influences introduced by the need to analyze the statics of the control mechanisms.

The moments of the frictional and normal forces about the hinge pin are the same as for the internal expanding shoes. It should be noted that when external contracting designs are used as clutches, the effect of the centrifugal force is to decrease the normal force.

Therefore, as the speed increases, a larger value of the actuating force is required. A special case arises when the pivot is symmetrically located and also placed so that the moment of the friction forces about the pivot is zero.

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