1052 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



operation of associated high speed equipment and also indirect advan- 

 tages in prolonging the life of the contacts. The impro\'ement is due 

 largely to the type of card operation of the completely independent, 

 low-mass twin wires and also to the low mass of the moving system which 

 excites less vibration of the rela}^ structiu^e as a whole. The placement of 

 the card close to the contacts allows the full contact force to be devel- 

 oped within a very short time, and the Ioav mass of the twin wires stores 

 little kinetic energy to cause reopening due to wire vibration. 



A particularly troublesome type of chatter occasionally experienced 

 is caused by rebound of the armature after striking the backstop. This 

 chatter is objectionable because of its long duration which is of the order 

 of a millisecond and ma}^ occur several milliseconds after the initial 

 opening or closure of the contacts. This increases the possibility of false 

 operation of associated circuits. Accordingly, a fundamental study was 

 made of the means for reducing armature rebound, as previously men- 

 tioned. As a result, changes were made in the suspension of the armature 

 and in the position of the backstop which substantially eliminated chatter 

 due to armature rebound. 



Electrical erosion of the contacts is reduced on the AF relay because 

 of less chatter and because of the lower energy levels controlled by the 

 contacts, where these are used to operate other AF relays. Tliis improved 

 performance not only reduces maintenance but permits the use of less 

 expensive, smaller size contacts. 



Contact locking is substantially eliminated on the AF relay because 

 of the card operation, where the static and dynamic forces associated 

 with the card and armature are aA^ailable to break loose any incipient 

 lock. Open contacts are reduced by (1) protecting the contacts from 

 dust with a small cover, (2) rounding the twin contact surfaces to reduce 

 the effective areas o)i which particles must lodge to cause opens and to 

 increase the pressure on the areas, (3) the use of palladium contacts, 

 (4) the dynamic characteristics of the wire springs, and (5) the use of 

 twin contacts on completely independent twin wires. The complete 

 separation of the twin wires is an important feature in reducing open 

 contacts. As shown in Fig. 22, the flat punched springs of the U relay 

 carry twin contacts but these are mounted on tips which are separated 

 by a relati\'ely short punched cutout. This limited separation did not 

 achieve the full advantage of twin contacts as a sufficiently large particle 

 of dust under one contact could cause both contacts to be held open. A 

 subsequent design, known as the UB relay ,^ used a longer cutout, re- 

 sulting in greater independence of the twin contacts with a significant 

 reduction in contact opens. The AF wire spring relaj^ achieves complete 



