Dial Clutch of the Spring Type

' I ''HE operation of all present day machine switching telephone systems depends on the use of the telephone dial. The dial originates the current pulses required to operate the step-by-step, panel, or crossbar switching equipment and for the reliable functioning of this equipment the pulses must occur within a closely limited frequency range. The stepping pulses are produced during the unwinding of the dial from the position to which it has been wound by the subscriber and it is this unwinding which must occur at a constant speed. To accomplish the speed control a governor depending on centrifugal force is used. It is not desirable that the governor come into action on the windup of the dial as this would put an extra load on the user's finger and slow up the operation of dialing. A clutch which holds in one direction of rotation and is free in the other direction is therefore interposed between the governor and the finger wheel with its associated circuit interrupting mechanism. In the past, the most commonly used clutch consisted of a pawl and ratchet, but this has now been replaced by the spring clutch because of its quietness and lower cost. A partially assembled dial using a spring clutch is shown in Fig. 1. The ideal clutch for a dial governor would be one offering zero coupling torque during dial windup and an infinite positive coupling in the other direction. In practice the free torque in the windup direction need only be small compared to the torque of the main spring, while the holding torque in the other direction need only be great enough to withstand the main spring torque plus any helping torque that a * Essentially the same material was presented at National Meeting of Applied Mechanics Division of The American Society of Mechanical Engineers, New York, N.