New Techniques for Measuring Forces and Wear in Telephone Switching Apparatus

In obtaining long life in telephone equipment such as relays, switches, selectors and other mechanical devices subject to large momentary forces, one of the main problems is the wear encountered in various parts. This is particularly true in such small motion devices as relays where even a few mil inches of wear increases the distance that the armature has to travel and may eventually cause the relay to fail to make contact. To obtain a design objective of one billion operations requires a very careful minimizing of deleterious forces and a careful selection of the best wearing materials. As a step toward investigating this problem several new techniques have been devised for measuring normal and tangential forces and for producing and controlling normal and tangential motions for wear studies. These methods have been applied to relays and have given considerable information on the types of motion to be avoided and on the best types of materials to select for various parts of the relay to obtain long life. Specifically they have shown that normal forces cause very little wear and that tangential sliding of one part over another is the principal cause of wear. Fortunately, by designing the motion of the armature and contacts correctly, tangential sliding can be largely eliminated with a corresponding reduction in wear. To aid in the quantitative evaluation of wear produced by tangential sliding two devices have been used. One is an electromechanical vibrator 1 driven at 500 cycles per second which is capable of several mil inches of motion and the other is a barium titanate longitudinal vibrator coupled to a metal "horn" 2 which is capable of a two mil inch motion at 18,000 cycles.