Dial Clutch of the Spring Type * 



By C. F. WIEBUSCH 



The mathematical theory is developed for the spring clutch 

 which consists of two coaxial cylinders placed end to end and 

 coupled torsionally by a coil spring fitted over them. Relations 

 are derived whereby it is possible to design spring clutches in terms 

 of the requirements and the constants of the spring material. Ex- 

 perimental verification of the relations is given. The theory of 

 residual and active stresses as applied to the springs is discussed. 



THE 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 un- 

 winding 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 direc- 

 tion 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 has now been replaced by the spring clutch because of its quietness 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 cou- 

 pling 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. Y., June 14-15, 1939, and published in Journal of Applied Mechanics, September 

 1939, under the title of "The Spring Clutch." 



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