Brake Tests

162 a series of so called check runs was scheduled, one test to be made at the beginning and another at the end of each day’s work, all con- ditions being kept the same throughout the entire series of tests as far as possible. These stops were all made from a speed of 60 m.p.h. with the complete train, standard braking power on the locomotive, electro-pneumatic equipment, 150 per cent, braking power, and plain shoes on the cars. 367. As anticipated, it was found that even when the tests were run under supposedly identical conditions a variation in performance was obtained. On pages 302 to 313 will be found a record of all the check runs made during the tests. It was observed that after the brake shoes were well worn in and no change was made in apparatus or manip- ulation for a considerable period of time the check runs of such a group of tests would show but little variation (see tests Nos. 594, 595, 604, 610 and 621 which averaged 1,181 feet, with a maximum only eight feet longer and a minimum only eleven feet shorter than the average). However, when any change was made, such as in locomotives used, in per cent, braking power of locomotive and tender, or in brake shoes, such as the replacement of a number of worn shoes by new ones, or the gradual wearing in of the brake shoes on the whole train, during the early part of a new series of tests, the length of stop obtained would vary, showing that the effect of new factors so introduced might be considerable. The effect of these influences could not heretofore be well defined, due to a lack of a sufficient number of tests to indicate the effect of various changes in conditions. 368. A series of check runs made with the No. 2 clasp brake affords a striking example of how brake shoe condition alone may affect the length of stop when other conditions were apparently constant. The average of the first five runs with this train was 1,310 feet. From the experience gained in previous tests it was evident that the shoe con- dition during the period when these tests were made, was not as good as could be expected after a further wearing in of the shoes. This shoe condition would not have attracted particular attention in any pre- vious series of tests and, furthermore, the five tests under the same set of conditions would ordinarily be considered ample to establish the average performance of the equipment used, without further question. 369. However, knowing the importance of what would ordinarily be considered but a slight change in brake shoe condition, many more runs were made with the result that the shoe bearing was greatly improved and the average of the last five comparative check runs was 1,121 feet. Here then, is a difference of nearly 200 feet between the average of five tests after the shoe condition was known to be good and the average of tests made some time before the shoes had been well worn in. This shows the necessity for constantly checking by