TELEPHONE APPARATUS SPRINGS 



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The bend test requires that when the material is bent back parallel 

 to itself to form a "U" and further compressed between flat parallel 

 surfaces (for example, between the jaws of a vise), at a rate not to 

 exceed 0.3 inches per minute, the material shall break along a straight 

 line making approximately a 90 degree angle with the axis of the 

 strip when the distance between the inside of the legs of the "U" is 

 25 to 16 times the thickness of the material. It must not break 

 before the distance is reduced to 25 times the thickness of the metal. 

 This test can be conveniently applied by drawing the looped material 

 through two of a series of graduated slots. 



Music Wire Springs 



Tinned and plated music wire is extensively used for compression 

 springs in telephone apparatus. Here the spring is in the form of an 

 open helix. Fig. 8 shows a type widely used. These springs are either 

 tinned or plated with nickel. It has been observed that the plating 

 baths adversely affect. the fatigue characteristics of music wire and 

 this is under investigation at the present time. 



Fig. 8 — Switchboard keys employing sheet metal springs and 

 helical compression springs 



The average tensile properties of the music wire employed are: 

 proportional limit — 217,000 lb. per sq. in.; ultimate tensile strength 

 — 350,000 lb. per sq. in. No chemical or tensile requirement is 



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