994 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



Moving parts, such as the armature and its suspension, together 

 with the associated actuating members and springs, should move freely 

 under all conditions without binding or friction. Since friction is in- 

 herently a variable quantity and difficult to control, it should be kept as 

 small as possible, otherwise it will be a cause for instability. If the 

 friction component is an appreciable part of the total load, the relay 

 will be unsatisfactory, particularly for marginal operation. The friction 

 part of the load on the moving system of a relay can be determined 

 readily by an improved measuring technique which automatically plots 

 the force required to move the armature, and its displacement, as it 

 moves from its unoperated to its operated position. This is illustrated 

 in Fig. 9. The top curve is the force required to mo\'e the armature and 

 operate its associated contact springs as it moves from its back-stop 

 to its fully operated position. The lower curve is the force acting on the 

 armature that allows it to restore to its unoperated position. The vertical 

 displacement between the two curves represents double the friction. 

 Since friction always opposes motion, it adds to the force required to 

 operate the relay and detracts from the force releasing the relay. If the 

 ideal of no friction were obtained the two curves would coincide. When 



300 



250 



^ 225 



< 



O 200 



175 

 150 

 125 



100 

 75 

 50 



0.01 0.02 0.03 0.04 



ARMATURE MOTION IN INCHES 



Fig. 9 — Mechanical load of a relaj' and its friction component. 



