DF.SIGX FACTOUS IN FLrENCING UKLIAHILITY OF RELAYS 979 



Pressure clamping during manufactuiv enables tlie spring pile-up to 

 better maintain its adjustment through cycles of humidity and drying, 

 and to prevent displacement during installation and wiring. The action 

 of a pile-up under this compression is illustrated in Fig. 1. Curve AB ic])- 

 resents the application of a 1,700-lb force to the pile-up by apo\verdri\en 

 fixture prior to tightening of the pile-up screws. At the start, the relation- 

 ship is not linear due to "nesting" of the parts, but a linear slope is soon 

 reached, representing the stiffness of the pile-up without the sci-ews. .\t 

 the point B the two screws are tightened with a controlled torque; fui- 

 ther compression takes place, indicated by the jagged line BC. An esti- 

 mate of the tension put into the screws by this tightening operation can 

 l)e made by extrapolating the curve AB to the point B\ vertically above 

 the point ('. When the pressure fixture is released, the pile-up tends to 

 expand and follows the line CD. The slope of this line represents the 

 combined stiffness of pile-up parts and screws, which makes it stiffer than 

 the original compression slope. When the pile-up is released, an equilib- 

 rium point is reached where the tension in the screws equals the force 

 with which the pile-up tends to expand. 



A series of measurements on a typical relay pile-up screw and clamp 

 plate assembly is shown in Fig. 2 to illustrate the stress-strain relation- 

 ship when a force is applied axially to put the screws under tension. As 

 force is applied, Hooke's law is followed up to the point A; strain is pro- 

 portional to the stress and no permanent deformation takes place. Be- 

 yond point A the elastic limit of the metal is exceeded and permanent 

 deformation begins. WTien a point B is reached, somewhere below the 

 breaking point of the screw, and the force is released, a permanent de- 

 formation results. Note that, in Fig. 2, the high strength screw will 

 permit a higher screw tension without deformation - and its resultant 

 looseness of the pile-up - than will the lower strength screw. 



Analytical methods are available for estimating the range of screw 

 tensions that exist during the life of the relay. By taking into account 

 the known cold flow characteristics of the insulators with the relay in 

 the dry state, a minimum ^•alue can be estimated. It should be of suffi- 

 cient value to hold the springs securely in place. By considering the con- 

 ditions that obtain in the humid state, maximum value of tension can 

 \)e predicted, which should not exceed the strength characteristics of the 

 materials used. 



To determine how well the design objectives for stability are being 

 realized, accelerated la))()rat()ry tests are made upon the relay, and from 

 these results predictions can be made as to its performance during its 

 life. In the telephone system, relays are generally subjected to repeated 



