894 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



CHARGING 

 CIRCUIT 



: 80MF 



IOO>WF 



Fig. 4 — Practical welding circuit. 

 SELECTION OF A PRACTICAL CIRCUIT 



It is probably not practical to use a distributed constant capacitative 

 line having a current time relationship as plotted in Fig. 3. The line 

 can, however, be approximated to the desired degree of accuracy by 

 means of a series of r-c sections. The current discharge curve for a single 

 r-c section plotted on the semilog graph of Fig. 2 A\ill be a straight line. 

 Clearly this is not a very good approximation of the ideal curve. If the 

 constants are adjusted such that the desired initial high currents are met, 

 the current will decay too fast allowing the surface to cool prematurely for 

 long arc durations. If the constants are adjusted to match the desired 

 curve for long arc times, the initial heating will be insufficient and short 

 arc durations will produce poor welds. 



A fairly good approximation of Fig. 2 can be obtained by as little as 

 two r-c sections in parallel (Fig. 4).* Use of this circuit has resulted in 

 considerable improvement not only in the uniformity of the welds ob- 

 tained but curiously enough in the control of arc duration. The reason 

 for the latter phenomenon is that with the multiple section circuit the 

 desired bridging characteristics can be met much more closely. 



THE MECHANICAL STRUCTURE 



Relatively little is known about the effect of the mechanical design of 

 the welding apparatus on the process. Basically, the mechanical constants 

 of interest are the mass and velocity of the gun when the arc is being 

 extinguished and the forces propelling the gun. The gun contains kinetic 

 energy part of which is absorbed during the impact of the two parts to 

 be welded. The remaining part will tend to produce rebounding of the 

 gun. Clearly the weld must have cooled sufficiently when the gun draws 

 back such that it can withstand the forces tending to pull it apart. 

 The time allowed for cooling is then roughly one-half the period deter- 



* The circuit configuration shown is ecjuivalent to two L sections of a lumped 

 constant line as usually depicted. 



