. Train resistance : a consideration of its elements and a discussion of experiments made with the university dynamometer car . tt If . . ^ JL fcj tf ff tt tt ...... tl ff tf ff ft 4 1-4 tt ff tt 5 ft 42 ft 1-2 ff it ff 3-4 tt tt ff 4 tt tt tt 4 1-4 ft tt ff r M From the table it is seen that draw-bar resistance may vary fiftypercent between 30-inch and 42-inch wheels. However this matter is not of nuch moment. Thirty-threeinch wheels have been adopted as a standard for freight cars in Amer-ica and practic^y so for passenger cars,a few (33-inch and 42-inchwheels be
. Train resistance : a consideration of its elements and a discussion of experiments made with the university dynamometer car . tt If . . ^ JL fcj tf ff tt tt ...... tl ff tf ff ft 4 1-4 tt ff tt 5 ft 42 ft 1-2 ff it ff 3-4 tt tt ff 4 tt tt tt 4 1-4 ft tt ff r M From the table it is seen that draw-bar resistance may vary fiftypercent between 30-inch and 42-inch wheels. However this matter is not of nuch moment. Thirty-threeinch wheels have been adopted as a standard for freight cars in Amer-ica and practic^y so for passenger cars,a few (33-inch and 42-inchwheels being in use for passenger cars. The 33-inch wheel has beenadopted because it has been found to give the best results in mile-age,and it is to be expected that there will not be much differencein train resistance on account of difference of sizes of wheels andof axles. The pressure on journals is a matter of loaded and emptycars. The coefficient of friction decreases as the load increasesup to a certain point. This point is determined by the long as there is a sufficient film of lubricating material betv/een. 15 the journal and the bearing,the coefficient varies inversely as thepressure; but as soon as the pressure is sufficient to force thelubricant out,the coefficient quickly rises,and is then approximate-ly constant for al1 pressures higher than this. This is of coursefor perfect lubrication. In Plate 1 are plotted some resistancesper square inch of bearing for mineral oil at different pressures,as found by Tower in the experiments mentioned on page 15. He useda bath of oil,which is as near perfect lubrication as it is possibleto have. V/e see from plate ] that between pressures of 100 and 500pounds per square inch the resistance is practicallyjconstant. There-fore between these pressures we have practically perfect lubrication Pressures found on freight car journals may be estimatedfrom the following table of weights kindly furnished by a prominentwestern railroa
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