MKASURING FORChB AM) ^VKAH IN SWITC'IilNG APPARATUS 



489 



that goes into producing wear is 



1 X 10"' X 10' 



7.4 X 10' 



^_"_ = 1.35 X 10" 



(12) 



or about 1 part in 10 . This suggests that the wire goes back and forth 

 o\er the same high points many miUions of times until the material 

 tinall}^ becomes fatigued and breaks off. This view is confirmed by the 

 oscillograph pictures of Fig. 9 which are a stationary pattern for millions 

 of oscillations. 



According to this picture, the material that will wear the best is the 

 one with the highest limiting shearing strain. If we assume that the 

 Umiting shearing strain is proportional to the limiting elongation strain 

 under repeated vibrations — of which there are tables — the wear for 

 various materials given in Table II agrees roughly with this concept. 

 Table II shows the yield stresses, the Young's moduli, the per cent 

 strains at the yield point and the relative wear at 10 cycles. It will be 

 seen that the materials with the highest yield strain will in general wear 

 longer than those with smaller yield strains. 



An exception to this rule was nylon which had a large wear even though 

 it has a large ^neld strain. However, nylon has a relatively low softening 

 temperature and a low heat conductivity. Observations showed that the 

 nylon was melted off rather than abraided off. According to this rule 

 gum rubber should wear much better than any other material since it 

 has such a high limiting shearing strain. A run was made with a two mil 

 inch motion on a gum rubber specimen and no observable wear was 

 found. The fact that a rubber tire will outwear a metal tire is also con- 

 firmation of this rule. 



All the tests showed that the wear on the stainless steel or nickel silver 



Table II 



Amount of Wear for Varioits Materials Caused by Sliding a 0.025 Inch 



Nickel Silver Wire for 2 Mil Inches, 30 Grams 



Normal Force and 10^ Cycles 



