996 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



sioned, before assembly, toward the stationary spring by an amount 

 necessary to give the desired contact force. Two supplementary springs 

 are provided to support the card and tensioned to restore the armature 

 and contact springs to their unoperated position. Upon operation, the 

 motion of the card permits the contacts to close, and when engagement 

 of the contacts occurs, the contact force reaches its predetermined value 

 very rapidly. Further motion of the card, provided for by the width 

 of the slot in the card, allows for wear of the contact and card without 

 appreciably affecting the contact force or the load on the armature. 



The effect of this wear on the contact force is shown in Fig. 11 for 

 both types of actuation. For the stud actuated relay, as the contact and 

 stud wear continues, the contact force decreases very rapidly. After 

 0.010 inch wear only about 6 grams remains out of an original 26 grams. 

 This is accounted for by the fact that the combined stiffness of the 

 moving spring in engagement with the stationary spring is 2 grams per 

 0.001 inch deflection. This recjuires 0.013 inch contact follow to establish 

 a contact force of 26 grams when the relay is adjusted initially. For the 

 card "lift-off" actuated relay where the moving spring had been pre- 

 tensioned to give a contact force of 25 grams initially, after 0.010 inch 

 wear of the contacts, the contact force will have decreased about 1 gram. 

 This is because the stiffness of the moving spring is about 0.1 gram per 

 0.001 inch deflection. Card wear does not affect the contact force so long 

 as it is provided for by the width of the slot in the card. 



0.010 ^^ 



DO 

 li-Z 



i-z 



0.008 ^- 

 hZ 

 ZO 



0.006 ^5 



o 



0.004 QjE;^ 



UJx 



o> 



0.002 '-' 



0.002 0.004 0.006 0.008 0.010 0.012 



STUD AND CONTACT WEAR IN INCHES 



Fig. 11 — Comparison of effects of wear on contact pressure of a relay. 



