1100 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



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TIME IN SECONDS 



Fig. 5 — Ratio of measured stripping force to initial stripping force as a func- 

 tion of time and temperature for tinned copper wire on nickel silver terminals. 



The dashed lines show the corresponding hoop stresses as a fraction 

 of the initial hoop stress and hence it is evident that as the hoop stresses 

 go down the stripping forces first decrease and then increase to higher 

 values than were effective originally. We shall presently show that the 

 difference between the measured stripping force and the proportionally 

 relaxed hoop stress is due to a stress caused by diffusion of the tin in the 

 plating into the wire and terminal of the solderless wrapped connection. 



One experiment which shows that the initial stripping force is caused 

 bj^ the hoop stress in the wire is the experiment shown by Fig. 6. Here a 

 terminal is made which has a slightly tapered tin plated pin in the middle 

 and is cut back for some distance beyond the pin. The terminal is wound 

 with five turns of No. 14 gauge (0.065 inch diameter) tinned copper wire. 

 The initial stripping force to pull off the winding was determined and 

 it was found to take 72 pounds force on the average to strip the wire 

 off the terminal. A similar set of measurements was made on the force 

 required to pull the pin out of the terminal and this averaged about 50 

 pounds or 70 per cent of the stripping force for the wire. Since the pin 

 and terminal had the same coefficient of friction as the wire and ter- 

 minal, and since the pin is required to support all the compressional 

 stress in the terminal due to the hoop stress in the wire, it is evident 

 that at least 70 per cent of the force required to strip the wire off the 

 terminal is plain frictional force between the wire and the terminal. 

 It is thought that the remainder of the force is due to the gouges cut in 

 the terminal bj^ the winding process. When the wire is stripped off the 

 terminal, these cuts gouge out parts of the wire and hence require a 

 higher force. This effect is equivalent to friction for a rough surface, 

 which is higher than that for a smooth surface. 



