Brake Tests

294 (d) With a given nominal per cent, braking power, the actual braking power to remain constant throughout the life of the brake shoes and wheels. (e) Piston travel to be as near constant as practicable under all conditions of cylinder pressure. (f) Minimum expense of maintenance and running repairs of brake rigging between the shopping of cars. (g) The initial and maintenance cost to be as low as con- sistent with, but secondary to, the points mentioned. 15. From the data obtained on standing tests the efficiency for the various brake riggings varied from 58 to 85 per cent, when operat- ing above 40 per cent, braking power. (Fig. 76.) BRAKE SHOES. 16. The brake shoe bearing is the most difficult factor to control and at the same time it is the most potent in producing variations in brake performance. (Par. 370.) 17. The variation in stopping performance was wider with the standard single-shoe brake than with any of the three clasp brake rig- gings. (Pars. 371, 372.) 18. A comparison of air brake equipment and brake rigging may be considerably affected by the performance of the brake shoes. (Par. 373.) 19. The advantage of clasp brakes over single-shoe brakes cannot be definitely established by stops made in these road tests. (Par. 369). 20. The difference in the efficiency of the clasp and single-shoe rigging may offset the gain which might be expected from difference in coefficient of friction, and as neither of these factors could be observed uninfluenced by the other, a satisfactory comparison of the mean co- efficient of friction under the two conditions in road tests is impossible. (Par. 535.) 21. High braking powers from high initial speeds result in a great heating of the working surface of the shoe and a rapid abrasion. This effect is most marked under severe braking conditions such as obtained when heavy cars equipped with one brake shoe per wheel are stopped. (Par. 528.) 22. The brake shoes as found worn in ordinary train service and when cracked are in the most favorable condition for making short emergency stops. (Par. 442.) 23. The general performance of the shoes as observed during the road tests formed the basis of the program established for tests made at Mahwah, which work resulted in the following deductions: (a) The greater the pressure per square inch of bearing area, the lower will be the mean coefficient of friction. (Par. 502.) (b) The use of two shoes instead of one will result in a higher coefficient of friction and less wear per unit of work done. (Par. 539.)