Brake Tests

156 a rule the greatest amount of wheel sliding occurred during the first runs of the morning, at which time the coefficient of rail friction was usually low, it was also a fact that occasionally considerable wheel sliding would be experienced when the coefficient of rail friction ob- served previous to such tests had been about at its average value. 349. A study of the action of the train where wheel sliding oc- curred and the coefficient of rail friction noted at the same time led to the conclusion that other factors, such as shock, slack action, and foreign matter on the rail surface, have a controlling influence in causing wheel sliding. It was, therefore, concluded that the readings obtained for coefficient of rail friction at the particular point on the rail used for this purpose, could not be depended upon to indicate the probability of wheel sliding. HIGH BRAKING Power on Locomotive. 350. To overcome the shock resulting from the maximum emer- gency braking power on the cars being higher and much more quickly obtained than that on the locomotive, an experimental device (Par. 147) was applied to the locomotive which gave a higher maximum emergency braking power in a shorter time than is obtained with the standard ET equipment. The brake cylinder pressure was blown down to normal toward the end of the stop. 351. Figs. 86 and 87 show comparative car and tender brake cylinder cards and slack action diagrams for 60 m.p.h. emergency stops, electro-pneumatic equipment, 150 per cent, braking power on cars, with and without the higher braking power on the locomotive. The sudden and considerable slack action on the records taken between cars 1 and 2 indicates clearly the shock received at the draft gears about two seconds after the brakes are applied when ordinary braking power is used on the locomotive. The slack action is much less severe on the diagrams taken when the locomotive is braked higher than nor- mal. A gradual change in the slack curves represents a comparatively slow relative movement between cars, which is not noticeable to passengers. 352. By referring to time pressure diagrams on these figures it is noted that at twelve seconds after the application, the pressure in the brake cylinder of the locomotive equipment is about the same in either case, and therefore the arrangement of high percentage of braking power cannot cause any more wheel sliding on the locomotive after the twelve seconds’ period than with the original installation of the ET equipment. This means that above some speed corresponding to the twelve seconds’ time (about 35 miles per hour when stopping from