Brake Tests

278 with standard brake conditions than might be expected if only the amount of energy to be absorbed per shoe were considered. 539. In view of the above analysis of the factors affecting shoe performance under the two conditions, it follows that the use of two shoes instead of one will result in a higher coefficient of friction and less wear per unit of work done. This conclusion is supported by the re- sults obtained during the road tests, but the difference shown by the machine tests is larger than obtained in the road tests, the latter being a function of the combined performance of the brake shoes and brake rigging, while the former is a function of brake shoe performance only. Wear of Brake Shoes. 540. The loss in weight of brake shoes as observed in machine tests with clasp brake conditions is shown in the following table. The relative wear of the various shoes is expressed in the amount of metal worn away per unit of work done, which unit is taken as 100,000,000 foot pounds. The wear factor was determined by dividing the amount of metal lost by the total work done in all the tests of each shoe. Comparative Wear of Various Types OF BRAKE SHOES UNDER Clasp Brake Conditions. 100 MILLION Ft. Lb. Work Done Types of Brake Shoes Plain Flanged A B C* Ci D E F G Solid Slotted * Area full length % Area full length % Area full width Solid Slotted Area area plain solid Wear factor. 3.105 2.937 2.896 5.124 4.630 2.502 2.170 2.935 * Column C shows figures including shoe No. 97 which was excessively hard. Column C1 is made up with this shoe excluded. 541. The important facts in the above table, when considered in connection with Figs. 165 and 166 are:— A.—That with shoes of the same type and of approximately the same hardness, the wear per unit of work done is less with the slotted shoe then with the solid shoe. The stops with slotted shoes were always shorter and the mean coefficient of friction higher than with solid shoes. B. —That with shoes which differ in type but are not modified with respect to their normal bearing area, the shoe which gives the best per- formance has a lower rate of wear per unit of work done. This is shown by a comparison of the wear factors for plain solid shoe “A” and plain slotted shoe “B,” as compared respectively with flange solid "E" and flange slotted shoe “F.” C.—That when the available bearing surface of any type of shoe is reduced the resultant wear is higher than on shoes of the same type not