Brake Tests

251 magnet similar to the one used for indicating rise in temperature was mounted on the bridge of the dynamometer and its circuit was con- trolled by an observer stationed near the test wheel. The line marked “Sparks” in Fig. 149 illustrates characteristic sparking records. 486. Five observations are indicated having the following signifi- cance :— A.—Application of brake. B.—Sparks start. C.—Maximum sparking. D.—Decrease of sparking. E.—Sparks stop. 487. It was noted that the action of sparking during machine tests was not similar to the same action observed during the road tests. The sparking of the brake shoe began simultaneously with the appli- cation of the shoe in the machine tests whereas during road tests an appreciable time was noted between the instant the shoe was applied to the wheel and the time the sparks appeared. This difference was due to the strong air currents experienced in the road tests when the speed of the train is relatively high, and could not be overcome in the machine tests although it was attempted by blowing a jet of air on the wheel. BEARING AREA. 488. The brake shoe bearing area was observed by chalking the shoe before the test and after the test making a sketch of the surface of the shoe from which the chalk had been removed. The bearing area thus shown was measured by means of a planimeter and the total shoe area, which had been in bearing at some time during the stop, computed. 489. Reproductions of characteristic bearing area sketches are shown in Figs. 151 to 154. Hardness. 490. Brinell hardness readings were taken previous to each test of the first series. The observations differed very little for the various shoes throughout the entire series of tests and indicated a uniform material. No hardness readings were made during the second series of tests. WEAR. 491. The shoes were weighed before and after each series of tests and the loss in weight noted.