566 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



0^ 



0.3 0.4 0.5 



WINDING STRESS, W 



YIELD STRESS, Y 



Fig. 9 — Relation between remanent hoop stress, ratio of wire curvature to 

 wire diameter and the applied winding stress for copper and aluminum wire. 



suits shown by the dashed lines of Fig. 9. To obtain a hoop stress greater 

 than zero, the ratio of winding stress to yield stress must be greater 

 than 0.1. To obtain a hoop stress that is 0.2 of the yield stress, a winding 

 stress of 35 per cent of the yield stress has to be employed. This is the 

 value recommended to give the most stable terminal. The winding 

 stresses were carried up to 0.8 of the yield stress. At this stress the 

 copper wire at the corners tends to draw down to a low value and may 

 break when it unwinds and hence this is probably the upper limit for 

 winding stresses. The radius of curvature of the middle of the wire, as 

 it is bent around a corner was also measured from photographs similar 

 to that shown on Fig. 6 and the ratio of the wire diameter to the mean 

 radius of curvature is shown by the solid lines of Fig. 9. This is an 

 alternate way of specifying the necessary winding force which may be 

 useful for other shapes of terminals. 



While square and rectangular terminals are very satisfactory shapes 

 for the inside terminal, they are not the only ones that can be used. A 

 number of coined and U shaped terminals are in general use as discussed 

 by Mallina. In order to investigate the necessary requirements for such 

 shapes, a number of experiments have been made on circular and ellip- 

 tical terminals. When a wire under tension is wound around a rod of 

 circular cross section, there are two sets of opposing stresses, one of 

 which tends to make the helix smaller and the other to make it larger. 

 As shown by Fig. 10, the tension in the wire tends to make the helix 

 hug the cylinder while the bending strains introduced by the wrapping 

 of the wire around the cylinder tend to make the helix open up when the 

 constant stress is released. A number of experiments were made on 



