SOLDERLESS WRAPPED CONNECTIONS 



PART II 



567 



wrapping copper wire 20 mil inches in diameter on a steel cylinder hav- 

 ing a diameter of 0.124 inches. In all cases even up to stresses of 80 per 

 cent of the yield stress, the helix failed to grip the cylinder. Some fur- 

 ther measurements were made with smaller sized inner cylinders down 

 to 20 mil inches in diameter. At this small radius the wire barely gripped 

 the cylinder and it took about 70 grams stripping force to pull the 

 wrapped mre off the cylinder. As seen from Fig. 11, the normal force 

 per unit length against the cylinder is balanced by the hoop force in 

 the wire according to the equation 



BFur = 2Fn sin 



or 



rFs = Fh 



(7) 



The stripping force SF, when the wire does not dig into the terminal, 

 should be equal to 



SF = 2wmFj,f = 27rn/Fj 



(8) 



where n is the number of turns, / the coefficient of friction which is about 

 0.15 to 0.2 between metals. For a stripping force of 70 grams for 6 

 turns, the remaining hoop stress is equal to 



Fn = 9.3 grams. 



(9) 



Since the wire was wound with a 700-gram force, it is evident that only 

 about 1 per cent is maintained in the wire, which is entirely inadequate. 

 In order to obtain a good wrapped connection with high hoop stress 

 in the wires, some means has to be employed to eliminate the unwrap- 

 ping effect of the strain due to bending. This can be accomplished by 

 changing the shape of the terminal from a circular cylinder to a dis- 

 symmetrical shape. For then, as shown by Fig. 8, the tendency to un- 



-WIRE 



Fig. 10 — Strains in a wire 

 wrapped around a circular ter- 

 minal. 



Fig. 11 — Relation between 

 hoop stress and stripping force 

 for terminal not indented. 



