SOLDERLESS WRAPPED CONNECTIONS 



PART II 



563 



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9:9^\ X 6700 = 15,000 pounds/sq in. or a; = 0.0235 in. (4) 



Hence, plastic flow should occur for about 23 mil inches into the plastic. 

 Unwrapping the wires from the terminal, it is found that depressions 

 of this order are cut in the terminals. Since one of the requirements of 

 a better connection is that an air tight bond shall be formed between 

 the wire and terminal, it is obvious that the terminal should have a 

 low enough yield stress so that a sizable groove can be cut in it by the 

 hoop stress of the wire. This rules out such terminals as hardened steel 

 in the most satisfactory connections. It has been found that copper, 

 brass, aluminum, soft iron and nickel silver are soft enough to meet 

 this requirement. Any material with a plastic flow limit in compression, 

 much lower than photoelastic bakelite, would probably have such a 

 deep groove that it would be difficult to maintain the desired hoop 

 stress. 



Using the photoelastic technique as a tool, considerable data has been 

 obtained on desirable shapes for the terminal and limiting winding 

 stresses that can be used. One of the most used terminals is the rectan- 

 gular terminal and Fig. 6 shows a photoelastic picture of a terminal 0.8 

 inches by 0.4 inches wound with nineteen turns of 0.050-inch copper 

 wire with a winding load of 28 pounds. This figure is particularly easy 

 to analyze for stress across a line half way down the long edge since the 

 stress along this in the direction of the line varies only a little. Hence, 



Fig. 6 — Isochromatic lines for a rectangular model 0.4 inches by 0.8 wrapped 

 with nineteen turns of 0,050 inch copper wire with a constant load of 28 pounds. 



