Brake Tests

281 D.—That for the same amount of work done flanged solid cost 16 per cent, less than plain solid shoes, and flanged slotted cost 23 per cent, less than plain slotted or 27 per cent, less than plain solid shoes. E.—That approximately 136 per cent, more stops will be required to wear out the flanged solid than will be required to wear out the plain solid, and 158 per cent, more stops will be required to wear out the flanged slotted than the plain slotted shoe and 171 per cent, more stops to wear out the flanged slotted than the plain solid shoe. Hardness. 546. An illustration of the relation between Brinell hardness and the mean coefficent of friction is shown on Fig. 167. As this investi- gation was not carried out through a wide variation of hardness numbers, no definite data as to the most desirable hardness can be given. How- ever, from observation made during these tests it is believed that under clasp brake conditions with cast-iron brake shoes, the mean coefficient of friction is a maximum and shoe wear a minimum with a Brinell hardness of about 190. This subject will be further investigated. Effect of Rate of Cooling of Brake Shoes on Mean Coefficient of Friction. 547. During the road tests it often became advantageous to cool the brake shoes with water after a test in order to save time and keep the brake shoes at a uniform temperature. It was recognized that this treatment might affect the performance of the brake shoes, and to deter- mine this point a series of laboratory tests was made under clasp brake conditions from a speed of 60 m.p.h. 548. A plain solid shoe was given three (3) tests at fifteen minutes intervals, 90 per cent, braking power without cleaning between tests, following which the test wheel was cleaned in the regular manner. These tests were made as the last part of the day’s work and on the following morning the shoe which had air cooled during the night was replaced on the wheel and tested at a nominal braking power of 150 per cent. This test was repeated three times to give an average of results which could be expected from air cooling. The effect of water cooling was deter- mined by tests made in the same manner except that after the three successive 90 per cent, braking power runs had been made as before and the wheel properly cleaned, both the shoe and the wheel were water cooled. From the results of the tests, Fig. 168, it will be noted that the mean coef- ficient of friction in the 150 per cent, braking power tests after water cooling averages slightly lower than that after air cooling. The dif- rerence is comparatively small, however, and not as great as differences