978 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



the better grades of phenol fibre have been found adequate. They should 

 have generous clamping surfaces, so that when the spring pile-up is 

 clamped under force, high pressures on the insulators are avoided - thus 

 minimizing cold flow and keeping well below the crushing strength of 

 the material. 



Pile-up screws, clamping plates, and screw threads should be porpor- 

 tioned such that permanent deformation under any condition does not 

 take place, i.e., the maximum stress does not exceed the elastic limit 

 of the metal. It has been found advantageous to use high tensile strength 

 steel for these parts - a tensile strength of 100,000 lbs per square inch 

 or higher. 



In the manufacture of relays, the desired pile-up tightness requires 

 certain procedures and controls. Insulators are baked in an oven at a 

 temperature of about 150°F for a minimum of 24 hours and assembled 

 in the relay while in the dry condition. Dvu'ing assembly, prior to tight- 

 ening the screws, the pile-up is pressure clamped under a hydraulic or 

 air powered fixture to a controlled force of 1,300 lbs to 3,200 lbs, depend- 

 ing upon the size of the relay; -while under this pressure, the screws are 

 tightened using a controlled torciue. To assure that the processes and 

 materials are under control, the relay is then tested on a "no-go" basis 

 in a fixture that applies a definite force on the contact springs in a direc- 

 tion to rotate them in the pile-up. 



0.010 0.015 0.020 0.025 0.030 0.035 

 COMPRESSION IN INCHES 



Fig. 1 — Compression cycle of a relay contact spring pile-up assembly. 



0.005 



