DESIGN FACTORS INFLIMCXriNG UKLIA HILITY OF RELAYS 



085 



vised for use during manufacture which will detect au incipient failure 

 of this kind. By pulsing the relay in its normal fashion the self-generated 

 coil voltage on breaking the circuit can be observed on a cathode ray 

 tube; any deviation in this \'oltage caused by momentary breakdown or 

 shorted turns can be detected readily. 



Another source of coil failure is lead bi-eakage, caused i)rincipally l)y 

 fatigue of the small copper wires. Copper has a low fatigue strength and 

 if it is subjected to repeated bending strains, eventually it will break. 

 -Vs the relay operates and releases, impact of the armatiu'e against the 

 core and backstop causes shock and vibration of the coil; coil construc- 

 tion therefore needs to be such that strains are not imposed upon the 

 fine wire by this motion. 



On spool wound coils, being individually wound, the fine winding 

 wires can be reinforced by stranded lead-out wires for connecting to the 

 i-elay terminals. Besides, the coil is generally womid tightly on the relay 

 core. These factors, to a large extent, preclude lead breakage. With the 

 cellulose acetate filled coil it is desirable to bring the winding wire di- 

 rectly to terminals on the terminal spoolhead to which the end of the 

 coil is later bonded. Since the filled coil must slide loosely over the core 

 for assembly reasons, it can have a small lateral motion at the non-ter- 

 minal end. To eliminate this movement a "motion limiting" washer has 

 been provided to fit snugly over the core and which is bonded to this 

 end of the coil.^ The washer and the way it is used is illustrated in Fig. 

 5. In assembly, the thin cellulose acetate faced phenol fibre washer and 

 the non-terminal spoolhead are forced over the knurl on the core and the 

 washer is bonded to the end of the coil. The tight fit of the washer on 

 the knurl is the feature that pre\'ents lateral movement of the coil. There 

 is always some slight shrinkage of the cellulose acetate filled coil in the 



KNURL 



Fig. 5 — Relay coil employing a molion limiting washer to prevent lead breakage. 



