. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . i- 466 RAILROAD CONSTRUCTION. §424. ciency in such a clumsy mechanism also reduces the effectiveratio. Assuming the effective ratio as G: 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 una
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . i- 466 RAILROAD CONSTRUCTION. §424. ciency in such a clumsy mechanism also reduces the effectiveratio. Assuming the effective ratio as G: 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 w^ith 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 operation w^hen a quick stop is desiredmakes it exceedingly dangerous to attempt to run a train athigh speed unless some automatic brake directly under thecontrol of the engineer is at hand. The great increase in the. Fig. 205.—Sketch of Mechanism of Hand-brake. average velocity of trains during recent j^ears has only beenrendered possible by the invention of automatic brakes. 424. Straight air-brakes. The essential constructive fea-tures of this form of brake are (1) an air-pump on the engine,operated by steam, which compresses air into a reservoir onthe engine; (2) a brake-pipe running from the reservoirto the rear of the engine and pipes running under each car,the pipes having flexible connections at the ends of the carsand engine; (3) a cylinder and piston under each car which § 425. ROLLING-STOCK. 467 operates the brakes by a system of levers, the cyHnder beingconnected to the brake-pipe. The reservoir on the engineholds compressed air at about 45 lbs. pressure. To operate thebrakes, a valve on the engine is opened which allows the com-pressed air to flow from the reservoir
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