iMEASrUING FOKCKS AM) WIOAK 1 .V S\\ IT( III N'(! ATPAHATUS 



-1S7 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 



BILLIONS OF CYCLES 



Fig. 13 — Typical wear curves for a luinihcr of materials. 



times the length of slide. Any of these three materials give sufficiently 

 small wear to produce a long relay life, but the best performer under all 

 conditions of force and slide appears to be the D cotton flock filled 

 phenolic with graphite added. 



In order to determine the causes of wear over a greater range of 

 parameters a number of other materials were run by means of the barium 

 titanate transducer. The wear for 2 mils motion, 30 grams (0.0665 

 pounds) force, and 10 cycles are shown by Table II. 



C. Wearing Energy and Causes of Wear 



A rough estimate of the energy reciuired to break off pieces of the 

 material shows that most of the energy goes into producing heat and 

 \'ery little into wear, i.e., into breaking pieces from the material. To 

 show this let us consider a small cube fixed at one end and with a tan- 

 gential force at the other. The force will cause the top surface to move 

 with respect to the bottom surface as shown by Fig. 14, and a shearing 

 strain S is set up in the material whose value is equal to 



F = fxS dx dij 



(5) 



where dx and dy are the cross section dimensions and fi the shear stiff- 

 ness. In this displacement work is done by the sidewise displacement 

 II equal to 



W 



kuF 



(6) 



