Brake Tests

235 440. Incident to the breaking-in of the shoes for the test train a series of tests was made to determine the variation in hardness with respect to wear. The results of this investigation are shown in Fig. 142 which indicates that when the shoe is new a wide variation in hardness may be expected between shoes made from the same heat, but that as the shoe wears down the hardness becomes fairly uniform and remains so, decreasing slightly as the shoe wears out. Although at that time no data was available, it was thought that Brinell hardness was an indication, not only of the ultimate strength of the face metal of the shoe but also was related in some way to the coefficient of fric- tion. Later tests confirm this belief but at this time the degree of this relation is not known. The indications are, however, that the harder the cast iron material the lower will be the coefficient of friction and that the highest possible coefficient of friction from cast iron (at least under clasp brake conditions) is obtained with a material approaching the condition of the shoe when about three-fourths worn out and having a Brinell hardness number of about 190. 441. The greatest uniformity of action and the highest friction seems to be obtained when the brake shoe bearing on the wheel is best. Tins condition would naturally be expected to follow a series of relatively light applications in the course of which the effects of temper- ature in warping the shoe are kept at a minimum. 442. It may be seen here that this most desirable condition is precisely what results from the continued use of the brakes during service stops on the road and consequently the brake shoes as worn in ordinary train service are in the most favorable condition for making short emergency stops. In the tests this condition appeared at times to have been reached after several light braking power runs. Stops following such a series of runs were then shorter than similar stops following several tests at higher braking powers, other conditions being the same. This result seemed to be most consistently obtained with the single shoe train. 443. The schedule of tests with the No. 3 clasp brake was ar- ranged with this in mind but the effect was not so noticeable in this case, there being indications that a good shoe bearing with the clasp brake might follow a small number of tests made at high braking power as well as a much larger number of tests made at lower braking power. The influence of previous tests on the shoe bearing for any particular test under consideration is so obscured by other conditions, such as the general wearing in of the shoes that necessarily results at any braking power, that the observations of the effect of the shoe dressing runs are of questionable value. 444. The effect of the bearing of the shoe on the wheel is illus- trated on the graphical log on page 343 (tests 1501 to 1531). The difference in the length of stop (110 feet) is due entirely to the gradually