New Techniques for Measuring 



Forces and Wear 



in Telephone Switching Apparatus 



By WARRt:N P. MASON AND SAMUEL D. WUrrE 



(Manuscript received February 15, 1952) 



One of the main 'problems in obtaining long life in telephone switching 

 equipment is the wear caused by large momentary forces. In order to in- 

 vestigate this problem several new techniques have been devised for measur- 

 ing normal and tangential forces and for producing and controlling normal 

 and tangential motions for wear studies. The forces are measured by in- 

 serting small barium titanate ceramics between the points of application 

 of the forces and observing the voltages generated on a cathode ray oscillo- 

 graph. Barium titanate ceramic is about fifty times as sensitive as quartz 

 and has a high enough dielectric constant so that with conventional ampli- 

 fiers time intervals as long as a tenth second can be measured. Both normal 

 and tangential forces can be measured by using properly poled ceramics. 

 By using weights on top of the crystals, normal and tangential accelerations 

 can be measured. With these ceramics, forces have been measured for relays 

 and for frictional sliding of a wire over a plastic. By employing a barium 

 titanate transducer capable of a large amplitude at 18,000 cycles it has 

 been shown that no wear occurs for normal forces, and that all the wear 

 observed in a relay is due to tangential sliding. Quantitative measurements 

 of wear have been made for a variety of materials, and it has been shown 

 that materials with a large elastic strain limit will wear better than materials 

 with a small elastic strain limit even though the latter have a higher yield 

 stress; materials such as plastics and rubber will outwear materials such as 

 metals or glasses. 



As the length of slide is reduced there is a threshold of motion for which 

 there is no gross slide and very little wear. This region is determined by the 

 condition that the tangential force is smaller than the normal force times 

 the coefficient of friction. Theoretical and experimental results are obtained 

 for this region and an equation is derived which deterynines the possible 

 displacement without gross slide. The stress strain curve occurs in the 



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