12 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



mechanical wear. In order to decide whether this is due to impacts, sHde 

 of parts, or vibration, methods ha\'e been de\'eloped to isolate some of 

 the transient forces encountered in relay operation, and measure them 

 individually. 



Rapidly varying forces, such as impact forces, may be measured with 

 the barium titanate crystal, which acts piezoelectricallj'- to yield a voltage 

 across its surfaces when subjected to compression or shear type forces. 

 Since it can be obtained in very small sizes, it may be mounted within 

 the relay as a substitute for the part to be studied. By observing the 

 voltages that have been developed across it during relay operation, one 

 may readily measure the forces involved.'^ Such an arrangement can be 

 made to provide a faithful frequency response over a vdde range, though 

 it calls for careful amplifier circuit design. The unit shown in Fig. 5 pro- 

 vides for accurate frequency response for impact forces varying as high 

 as 50 kilocycles. Fig. 6 shows a typical force-time relation obtained with 

 this equipment. In some recent measurements on an experimental rela}^, 

 the impact forces between the driving members were measured to be 

 approximately five-fold the static force, but not sufficient to explain 

 pulverizing and other damage to certain of the functioning parts. 



Imposed Motion 



Slide between parts has been found to be a very damaging source of 

 wear in telephone relays. It has been studied in some detail by means of 



ZERO FORCE 



-STATIC FORCE 



IMPACT FORCE 



Fig. 6 — Oscillogram of impact in a relay 



