Important Design Factors Influencing 

 Reliability of Relays 



By J. R. FRY 



(Manuscript received June 13, 1952) 



Relays are produced by a large number of manufacturers in this country. 

 When we survey their product, we find that there are many kinds and vari- 

 eties. They differ widely as to their size, shapes and configurations. Many 

 of these differences are dictated by the reouirements of the task they must 

 perform and by the environments under which they must work. Other differ- 

 ences are brought about from considerations of cost and by the design and 

 fabrication techniques the particular manufacturer employs. However, all 

 relays have a common objective. For whatever use they are employed, it is 

 highly desirable that they be reliable. They are expected to function each time 

 thei are called upon without failure and over the expected life of the equip- 

 ments in which they are used. This paper deals with the more important 

 design factors which all relays have in common that greatly influence their 

 reliability of performance. Contact spring pile-up stability and the impor- 

 tance of strength of screws, insulating materials with low cold flow and mois- 

 ture absorption, and manufacturing procedures and controls to achieve this 

 end are discussed. Coil construction so as to minimize the occurrence of open 

 windings due to corrosion of the wire and breakage of the lead-out wires is 

 dwelt upon. Contact reliability and how it is affected by the material used, 

 its size and shape, the method of actuation, the presence of contaminating 

 vapors, and single versus twin contacts are discussed. The degree by which 

 magnetic materials change their magnetic properties with age and treatments 

 for alleviating this effect are described. The importance of adequate structural 

 design so that the relay will be rugged and remain stable so that its perform- 

 ance is substantially unaffected by ivear, shock and vibration is stressed. 

 Methods of test to determine how well the relay meets these objectives are 

 described. 



Although a relay is conceptually a simple device, the wide range of 

 conditions under which relays are required to operate, the many different 

 characteristics they must have, and the complete dependence placed 



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