DESIGN FACTORS INFLUENCING RELIABILITY OF RELAYS 977 



upon thorn in many circuit applications, make them a subject of continu- 

 ous study. 



In the telephone industry, for example, the completion of a single call 

 may bring into play a thousand or moi-e relays. While their principal 

 function is to close electrical contacts, there are many facets to the prob- 

 lem of doing this satisfactorilj\ Relays are produced by many manufac- 

 turers in this country. When we survey their pi'oduct we find that there 

 are many kinds and varieties. Shapes, sizes and configurations of relays 

 may differ in accordance with the requirements of the tasks they must 

 perform, and the environments under which they may work; othei" differ- 

 ences may be brought about by cost considerations and by design and 

 fabrication techniques of the manufacturer. 



All relays, however they may be used, have one common objective - 

 they must be reliable. They are expected to function each time they are 

 called upon, should do this without failure, and should continue to do so 

 o\'er the expected life of the equipment in which they are used. It is the 

 l)urpose of this paper to discuss the more important factors that are 

 common to all relays and which have considerable influence on their 

 loliability of performance. The design considerations discussed in this 

 paper are presented in the following order. 



(1) Contact Pile-up Stability, 



(2) Coil Construction, 



(3) Contact Reliability, 



(4) jNIagnetic Stability, and 



(5) Structural Stability. 



CONTACT SPRING PILEUP STABILITY 



Stability of the contact spring pile-up contributes in a large degree to 

 the reliability in performance of the relay. Since contact springs may be 

 assembled into pile-ups of from two springs to a dozen or more, they 

 must be secured so that they will not shift position during the life of 

 the relay, even when it is subjected to relatively large changes in tem- 

 perature and humidity, and to vibration and shock tluring shipment, 

 installation, wiring, and under operating conditions. It is also important 

 that the dimensional relations between the contact ends of the springs 

 and the actuating members of the relay do not change appreciably; 

 otherwise, changes in contact separation, contact follow, contact pres- 

 sure, and operating and releasing current values may cause faulty opera- 

 tion of the relay. 



Insulators for securing the springs should be made of matx^'ials having 

 low cold flow and moisture absorption characteristics ; in telephone relays, 



