THE SPRING CLUTCH 



725 



Fig. 1 — Dial governor find front and back view of partially assembled dial. 



subscriber may impatiently exert. The first limit is easy to set up 

 on the basis of the mechanical constants of the dial; the fixing of the 

 latter limit required measurements on the strength of a considerable 

 number of persons. It was found that the maximum force that an 

 ordinary man can exert with any finger at the finger hole of a dial 

 is about six pounds. When the proper factors of safety have been 

 added to these limits it is possible to specify exactly the requirements 

 on the dial clutch. The remainder of this paper is devoted to the 

 development of the relations and a discussion of the problems involved 

 in designing a spring clutch to meet a given set of requirements. 



I™3. 



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Fig. 2 — Telephone-dial clutch. 



The type of spring clutch to be discussed here consists of two cylin- 

 ders placed end to end, rotating on a common axis, and torsionally 

 coupled by the friction between the cylinders and a coil spring fitted 

 over the cylinders. A photograph of such a clutch, assembled and 

 apart, from a telephone dial is shown in Fig. 2. In spite of widespread 

 use there seems to be little theoretical discussion of this device in 

 the literature. 



