. The Bell System technical journal . oltage of 3,000 \olts (4,750 \olts cm) was used. On the other hand ifthe driver works against no stress, the expansion in the plated length of4 inches is x 10~ inches. The actual force applied depends on how much the relative slip be-tween the glass lenses amounts to. To measure this force, a poled leadtitanate barium titanate disk is placed between the driver and themetallic bracket which clamps the middle lens as shown by Fig. 18. Allthe force exerted on the lens has to be exerted through the disk andhence the \-oltage generated by the disk is a meas
. The Bell System technical journal . oltage of 3,000 \olts (4,750 \olts cm) was used. On the other hand ifthe driver works against no stress, the expansion in the plated length of4 inches is x 10~ inches. The actual force applied depends on how much the relative slip be-tween the glass lenses amounts to. To measure this force, a poled leadtitanate barium titanate disk is placed between the driver and themetallic bracket which clamps the middle lens as shown by Fig. 18. Allthe force exerted on the lens has to be exerted through the disk andhence the \-oltage generated by the disk is a measure of the force exertedon the middle lens. This voltage is calibrated by attaching a spring loadof known constants and measuring the displacement of the load by meansof a microscope. Using a 60-cycle driving voltage, a number of sets of disks were runwith varying tangential and normal loads and the wear patterns ob-served. Fig. 20 is a photograph (magnified 100 times) for a normal load 496 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952. Fig. 20—Wear circles (magnified 100 times). of 10 pounds and a maximum tangential load of pounds per lensrun for about 3 hours at 60 vibrations per second. The outer area ofcontact is seen to be inches in diameter. The inner area of wear isa circle displaced slightly from a concentric form and has a diameter inch. If we plot 1 — {a/of against the ratio of tangential tonormal force, where a is the inner radius and a the outer radius, asshown by Fig. 21, a pomt at and is obtained. A number of setsof lenses were run and as shown by Fig. 21 the results can be plotted ona straight line corresponding to a coefficient of friction of Thisvalue agrees well with other determinations^^ of the coefficient of frictionof glass on glass. Hence the assumption of slip between spheres undertangential 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
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