Brake Tests

121 Features Open for Improvement. 260. Under a high percentage braking power or repeated appli- cations with the present emergency braking power of 113 per cent. - at high speeds, the tendency for shoes to slide off the wheel is pre- vented but to a small degree by the tie rods joining the brake heads. This action is not confined to the higher percentages of braking power alone, it being a matter of time until the same condition of shoe bearing is reached with the lower percentages of braking power used in service brake application. As a result of this action, 62 shoes were changed in one day, during which eight (8) tests were made from 60 m.p.h. at 150 per cent, braking power. In several cases shoes not bearing properly were worn through to the brake head in one emergency brake appli- cation from a speed of 80 m.p.h. at 150 per cent, braking power. In other words, the action which has been observed in road operation with this rigging was reproduced during these tests by subjecting the shoes to continuous severe treatment. The same effect, however, would have been produced, but after a much longer period of time in the course of ordinary road operation. 261. To prevent the tendency to displace the journals due to the heavy unbalanced shoe pressures resulting from the use of a single shoe brake on a car of this weight, it is necessary to resort either to deep brasses or to hang the shoes so low that the resulting force passes within the bearing area of the brass. Both of these methods are ob- jectionable on account of supplying lubricant to deep brasses and the tendency of low hung shoes to cause considerable false piston travel and consequently a low efficiency of brake rigging. 262. The amount of energy which must be absorbed per shoe, by a single shoe brake when stopping modern heavy cars at high speed, taxes the material in the shoe beyond its capacity. Therefore, there is a tendency for the single shoe to break down and wear away more rapidly than was formerly the case with lighter cars, resulting in a correspondingly longer stop. These conditions are more fully discussed in Chapter VII. 263. Under the heavy loads imposed on the various members of the brake rigging system, the effect of deflection, pin wear and lost motion between parts requires especial attention when applying a rigging to a car weighing upwards of 120,000 pounds, as these factors directly affect running piston travel. 264. The present practice in design, governing the unit fibre stress, although it provides ample strength for the brake rigging parts, causes deflection in some of the members, resulting in an increase in piston travel. The present investigations point clearly to the advantage of using a fibre stress which reduces deflection and elongation to a minimum with as little increase in weight as possible.