SOLDERLESS WRAPPED CONNECTIONS 



PART II 



565 



first corner to bend out and lose contact with the terminal. When the 

 second corner attempts to unwind, it pulls the first corner up against 

 the sample and no umvinding beyond the second corner can occur. In 

 order to obtain the most satisfactory connections, at least five or six 

 turns should be used. 



The fact that the first two corners on each end do not make close 

 contact to the inside terminal produces a very beneficial result when the 

 connection is subject to vibration due to the handling and operation of 



Fig. 8 — Locking in effect in a 

 rectangular terminal which allows un- 

 wrapping at only two corners on each 

 ■WIRE end. 



a relay. In a soldered connection, large bending strains are caused by 

 vibrations at the point of contact between the wire and the solder and 

 in standard vibration tests when large 60-cycle vibrations are impressed 

 upon the wires, the wires fatigue and break off at the point where the 

 wire enters the solder in times in the order of fifty hours. Similar tests 

 have been carried out for solderless wrapped connections and up to 

 times of 2000 hours and longer no breaks have occurred. This is due to 

 the fact that a bending strain is not enhanced by a sharp discontinuity 

 as it is in the soldered connection and strains for a given vibration am- 

 plitude should be less than half as large as those for a soldered connec- 

 tion. Since the relation between fatigue and strain is such that a reduc- 

 tion of strain of two to one or more caused an increase in the number of 

 cycles before breakage of factors of 1000 or more, the increased life under 

 vibration for the solderless wrapped connection is not surprising. 



Next a series of measurements were made on the value of the winding 

 stress necessary to preserve a hoop stress in the wire. This was meas- 

 ured by winding wires with different weights on photoelastic samples 

 and measuring the residual hoop stress by the technique described 

 above. This was done for both copper and aluminum wires with the re- 



