DESIGN FACTORS IXFU'KNf'ING UKLIAHIMTY OF RKLAYS '.)87 



111 the early days of relay design, contacts were attached to the si)riiigs 

 by riveting. In fact, many of the relays manufactured abroad toda}^ are 

 made in this manner. In this country, during the past 30 years or more, 

 spot welding has largely replaced the riveted construction. This was doiu* 

 for economy reasons. Spot welded contacts, unless carefully controlled 

 during manufacture, may not be so reliably attached as the riveted con- 

 tacts and the likelihood of the contacts dropping off during the life of 

 the relay may be greater. It has been found in welding the millions of 

 contacts required in the telephone system that close control is required 

 in several factors affecting the quality of welds obtained with any given 

 material. Important factors are cleanliness of the welding surfaces, pres- 

 sure between electrodes, welding current and the time during which the 

 current is applied. In order to insure that these factors are at all times 

 under control, and since the conseciuences are rather gra\'e when poor 

 quality welds are produced, it has been found desirable to institute fre- 

 (luent inspect oiis of the qualit}^ of welds at each welding machine on a 

 sampling basis. Periodically a small number of contacts are subjected 

 to a destructive test in which the force reciuired to shear off the contact 

 is measured. In this manner, any deterioration in the cjuality of welds 

 can be detected early, and corrective measures can be applied. 



One type of failure sometimes experienced with relays is contact lock- 

 ing.'* When a contact is closed by the operation of the relay it may become 

 mechanically locked to the contact member with which it is engaged and 

 fail to open when the relay is released. As a result of arcing as the contact 

 closes and opens an electrical circuit there is a transfer of metal from one 

 contact to the other. This building up and wearing away leaves both 

 contacts roughened. If the opening and closing motion were along a per- 

 pendicular to the face of the contacts, this roughening would ordinarily 

 have little effect. But with a slight sliding or rocking motion at the con- 

 tacts after they come into engagement, small projections on one contact 

 ma}' lock mechanically in a cavity on the other and thus prevent the 

 contacts from opening when they should. When contacts have locked, 

 measurements have shown that forces in excess of 100 grams may 'be 

 required to separate them. 



This kind of failure can be avoided by employing an improved method 

 of spring actuation.^ This is illustrated in Fig. 6. At the top of the figure 

 is a stud actuated contact spring assembly. The spring carrying the 

 moving contact is tensioned away from the fixed contact member and 

 exerts a force to hold the armature against the backstop when the relay 

 is unoperatcd. A stud, moved I)}' the armature, presses against the mov- 

 ing contact spring a short distance back of the contact to close the contact 



