922 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



shear test data. In a limited number of samples which were tension 

 tested failure occurred more frequently in the wire than in the weld. 



JAW LIFE 



Jaw life is dependent upon many factors, but one of importance is the 

 prevention of accidental conditions which establish an arc directly to the 

 jaws. The jaws are adjusted so they will not touch the wire end and 

 thereby discharge the capacitors if no contact is in the jaws. However, 

 if only a very short length of contact metal is cut off and put in the 

 jaws it may start the arc but there may not be sufficient material or the 

 material may not be held securely enough to keep the arc from burning 

 the jaw surfaces. Another possible trouble condition occurs when the 

 end of the wire is misplaced so it touches a jaw^ A safety circuit is pro- 

 vided with microswitches which trigger thyratrons to discharge the 

 capacitors when either of these conditions occur. Signal lamps are pro- 

 vided to indicate at which microswitch the trouble is occurring. A reset 

 microswitch is used in the test for shorted contacts so the indication can 

 be held to a later time in the cycle. At the end of the stroke this switch 

 is reset. After the part is nested approximately a quarter of a second is 

 available to test for safe welding conditions. A cam-actuated microswitch 

 controls the time of test and, if conditions are at fault, the weld energy is 

 discharged through a thyratron before the parts are brought together for 

 welding. 



SAFETY 



Safety for personnel from high voltage is provided by door switches, 

 solenoid released shorting bars and bleeder resistors on the capacitors. 

 Safety from mechanical jams is provided by a slip clutch on the main 

 drive gear and by a pull out clutch and automatic stop switch located 

 in the comb transfer drive mechanism. 



CONCLUSIONS 



After making millions of welds by automatic percussive welding 

 methods, it is found to be a method well suited to this job. Accuracy of 

 location and good weld strength are obtained. This welding method is 

 especially useful where speed and precision are desired, and where joints 

 must be made between dissimilar metals. Metals of high heat con- 

 ductivity and high electrical conductivity join readily by this method. 



