Brake Tests

153 338. With the standard brake train and flange shoes a high percentage of wheel sliding is shown, but on only one test (No. 1014) was there excessive sliding. This may be partially attributed to the fact that it was run at 6 a. m. and subjected to rail conditions different from those prevailing during other tests made with this type of brake rigging. 339. A total of 282 emergency tests at 150 per cent, braking power were made with the various types of brakes; of this number 22 per cent, had wheel sliding, 10 per cent, occurring during the tests of the No. 1 clasp brake. 340. At 180 per cent, braking power with plain shoes, wheel sliding occurred on but 7 out of 36 tests. With flange shoes sliding occurred in 11 out of 23 tests. In only one test of the 59, with 180 per cent, braking power, was there wheel sliding amounting to over 3,000 feet. 341. From the above it follows, that the determining factor in wheel sliding is not high braking power alone but rather the uncon- trollable conditions of rail and weather in connection with it, against which no permanent provision can be made without a sacrifice in the length of emergency stops during those favorable periods of the day or seasons of the year when conditions warrant the use of high braking power. 342. The effect of wheel sliding is to lengthen the stop, but the extent of the lengthening depends upon the amount of the wheel sliding and the braking power being used. (Par. 375.) 343. Whether the sliding of wheels will or will not cause flat spots of a size sufficient to produce rough riding of the car depends entirely on circumstances; for example, a condition of rail surface which will cause a considerable amount of wheel sliding, with relatively low percentages of braking power, is a condition which at the same time will permit long slides to occur without producing noticeable flat spots. On the other hand, when the rail is in good condition, or in the extreme case of a sanded rail, a very short slide may produce flat spots of a size requiring prompt attention. The amount of flattening is further contributed to by the weight upon the wheels and the material in the wheels and rail. The effect of rail conditions on the amount of flattening produced was brought out during the tests; there being many cases observed where the wheels picked up and slid for a considerable distance on a bad rail producing but small flat spots, whereas with better rail conditions instances of wheel sliding were observed to produce flat spots of considerable size when the wheels slid a much shorter distance. No flat spots of sufficient size were obtained to necessitate changing wheels during the tests, although, on account of the number of small spots accumulated upon the wheel tread, it was found advisable to change some wheels before the cars were put back into regular service.