496 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



Fig. 20 — Wear circles (magnified 100 times). 



of 10 pounds and a maximum tangential load of 2.04 pounds per lens 

 run for about 3 hours at 60 vibrations per second. The outer area of 

 contact is seen to be 0.03 inches in diameter. The inner area of wear is 

 a circle displaced slightly from a concentric form and has a diameter of 

 0.0175 inch. If we plot 1 — {a' /of" against the ratio of tangential to 

 normal force, where a' is the inner radius and a the outer radius, as 

 shown by Fig. 21, a pomt at 0.204 and 0.8 is obtained. A number of sets 

 of lenses were run and as shown by Fig. 21 the results can be plotted on 

 a straight line corresponding to a coefficient of friction of 0.25. This 

 value agrees well with other determinations^^ of the coefficient of friction 

 of glass on glass. Hence the assumption of slip between spheres under 

 tangential forces appears to be verified. This type of slip may be re- 

 sponsible for some types of wear, such as in ball bearings, where no gross 

 slide of one surface over another occurs. 



An attempt was also made to check the area of the loop as determined 

 theoretically by Equation (20). The applied force is measured directly 

 by the barium titanate pickup and the displacement was measured by 



