190 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



B. Design Criteria 



1. Type of Armature Structure. 



The selection of the number of degrees-of-freedom for an armature 

 structure depends on the expected coefficient of rebound as well as 

 practical considerations. 



It can be shown without great difficulty that for very low coefficients 

 of rebound the one-degree-of -freedom system is preferable. This is quite 

 obvious when one considers the limiting value of /c = 0. In this case the 

 one-degree-of-freedom system will have no rebound w^hatsoever, while 

 the two-degree-of-freedom system has a heel bounce followed by re- 

 bound at the front. The value of k below which the one degree system is 

 preferable varies with the mass distribution relative to the stop points, 

 being 0.18 for a rectangular plate armature with stops located at its 

 edges. 



Experience indicates that k in most practical relays and similar 

 mechanical structures varies from 0.3 to 0.6. Hence the two-degree-of- 

 freedom system is superior in all practical cases to the solidly hinged 

 armature. 



As far as three and higher degree-of-freedom systems are concerned, 

 it may be said that generally the greater the number of modes resulting 

 in impacts, the quicker the rebound energy can be diverted and dis- 

 sipated and the lower theoretical rebound values can be obtained. This 

 consideration would favor systems containing many degrees of freedom. 

 However, while multi-degree-of-freedom systems can reach very low 

 rebound values, their motion (phasing) must be very closely controlled 

 or they may prove to be inferior to simpler systems particularly under 

 vibratory (pulsing) operation. It is this difficulty which makes it appear 

 that the two-degree-of-freedom system offers the best promise with the 

 three-degree system also quite promising. By the same reasoning, 

 additional spurious rocking modes should be minimized. 



2. Armature Mass. 



The armature mass should be as low as possible. This will minimize 

 stop and structure vibration. In addition, in relay applications light 

 armatures tend to increase magnetic "drag" losses of the armature 

 during the release motion. 



3. Stops and Mounting Structure. 



As discussed before, it is desirable to reduce the amount and duration 

 of stop and mounting structure vibration. 



4. Coefficient of Restitution. 



The coefficient of restitution should be kept low. Stops having low 

 stiffness should, therefore, be avoided. 



