MEASURIN'O FOIU'KS AXD \\I:a1{ I\ SWri'CHIXCi API" AHATTS 



483 



5000 0) 



2000° 



1000 2 



0.4 as 0.6 



BILLIONS OF CYCLES 



Y'\^. K) — Typical wear curve for A phenol fibre plotted as a function of the 

 nunil)er of cycles. 



traA'el. Since the picture is a trace of ati oscillograph pattern which is 

 being repeated 18,000 times a second and since a t\vo second exposure 

 is reciuired to produce the picture it is obvious that the wire goes back 

 and forth over the same points for a large number of times. Most of the 

 energy is lost in producing elastic vibrations in the points of contact. 

 These oscillations are produced by the bending of the areas of contact 

 by the bonding force between them and by the motion. When the bond 

 is broken the plastic forming the point is free to vibrate and the elastic 

 energy goes into mechanical vibrations and eventually into heat. Since 

 a pattern such as that for the 0.5-mil inch or the 0.75-mil inch displace- 

 ment lasts inichanged for a number of minutes, it is obvious that very 

 little of the energy goes into breaking the plastic points of contact and 

 producing wear. This is confirmed by a rough calculation given later 

 which shows that only about 1 part in lO'"* of the energy goes into pro- 

 ducing wear. For displacements above a mil-inch motion it appears that 

 groups of point contacts are broken at one time, and the pattern changes 

 rather rapidly indicating that there is more wear at these amplitudes. 

 Over a two-second interval the pattern is changing fast enough so that 

 sharp pictures are not obtained. 



Quantitative values of wear for \'arious materials were obtained by 

 nnining the barium titanate unit for various periods of time, different 

 lengths of strokes and different normal forces. Fig. 10 shows a typical 



