. Railroad construction. Theory and practice . brake-beam, which is operated by means of ahand-windlass and chain operating a set of levers. It is desir-able that brakes shall not be set so tightly that the wheelssh^ll be locked, and then slide over the track, producingiiat places on them, which are very destructive to therolling-stock and track afterward, on account of the impactoccasioned at each revolution. With air-brakes the maximumpressure of the brake-shoes can be quite carefully regulated,and they are so designed that the maximum pressure exertedby any pair of brake-shoes on the w^heel
. Railroad construction. Theory and practice . brake-beam, which is operated by means of ahand-windlass and chain operating a set of levers. It is desir-able that brakes shall not be set so tightly that the wheelssh^ll be locked, and then slide over the track, producingiiat places on them, which are very destructive to therolling-stock and track afterward, on account of the impactoccasioned at each revolution. With air-brakes the maximumpressure of the brake-shoes can be quite carefully regulated,and they are so designed that the maximum pressure exertedby any pair of brake-shoes on the w^heels of any axle shall notexceed a certain per cent, of the weight carried by that axlew^hen the car is empty, 90% being the figure usually adoptedfor passenger-cars and 70% for freight-cars. Consider thecase of a freight-car of 100000 lbs. capacity, weighing 33100 lbs.,or 8275 lbs. on an axle, and equipped with a hand-brake whichoperates the levers and brake-beams, which are sketched inFig. 205. The dead w^eight on an axle is 8275 lbs.; 70% of. -&5792 Fig. 205.—Sketch op Mechanism of Hand-brake. this is 5792 lbs., which is the maximum allowable pressureper brake-beam, or 2896 lbs. per brake-shoe. With the dimen-sions shown, such a pressure will be produced by a pull of about1158 lbs, on the brake-chain. The power gained by the brake-wheel is not equal to the ratio of the brake-wheel diameterto the diameter of the shaft, about which the brake-chainwinds, which is about 10 to 1|. The ratio of the circumfer-ence of the brake-wheel to the length of chain wound up byone complete turn would be a closer figure. The loss of efli- 374 RAILROAD CONSTRUCTION. § 336. ciency in such a clumsy mechanism also reduces the effectiveratio. Assuming the effective ratio as 6:1 it would require apull of 193 lbs. at the circumference of the brake-wheel toexert 1158 lbs. pull on the brake-chain, or 5792 lbs. pressureon the wheels at B, and even this will not lock the wheels whenvthe car is empt}^. mu
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