. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . air of brake-shoes on the wheels of any axle shall notI a certain per cent, of the weight carried by that axle\e car is empty, 90% being the figure usually adoptedsenger-cars and 70% for freight-cars. Consider the. freight-car of 100000 lbs. cap|pity, weighing 33100 lbs.,tlbs. on an axle,- and equipped with a hand-brake whichthe levers and brake-beams, which are sketched in 1 . The dead weight on an axle is 8275 lbs.; 70% of 404 RAILROAD CONSTRUCTION. § 335. this is 5792 lbs.,
. Railroad construction : theory and practice : a textbook for the use of students in colleges and technical schools . air of brake-shoes on the wheels of any axle shall notI a certain per cent, of the weight carried by that axle\e car is empty, 90% being the figure usually adoptedsenger-cars and 70% for freight-cars. Consider the. freight-car of 100000 lbs. cap|pity, weighing 33100 lbs.,tlbs. on an axle,- and equipped with a hand-brake whichthe levers and brake-beams, which are sketched in 1 . The dead weight on an axle is 8275 lbs.; 70% of 404 RAILROAD CONSTRUCTION. § 335. 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 16 to IJ. 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 efh-. 5792 Fig. 205.—Sketch of Mechanism of Hand-brake. 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 whenthe car is empty, much less when it is loaded. Note that thepressures at A and B are unequal. This is somewhat objec-tionable, but it is unavoidable with this simple form of brake-beam. More complicated forms to avoid this are sometimesused. Hand-brakes are, of course, cheapest in first cost, andeven with the best of automatic brakes, additional mechanismto operate the brakes by hand in an emergency is always pro-vided, but their slow oj^ration when a quick stop is desiredmakes it exceedingly dangerous to attempt to r
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