. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . theoretically possible on tangents, it is unobtainableon curves, and especially on the approaches to curves when theouter rail is being raised. Even on tangents it is impossibleto maintain a perfect surface, no matter how perfectly thetrack may have been laid. In consequence of this, the points §412. ROLLING-STOCK, 445 of contact of the wheels of a locomotive, or even of a four- wheeled truck, will not ordinarily lie in one plane. The rougherand more defective the track, the wo
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . theoretically possible on tangents, it is unobtainableon curves, and especially on the approaches to curves when theouter rail is being raised. Even on tangents it is impossibleto maintain a perfect surface, no matter how perfectly thetrack may have been laid. In consequence of this, the points §412. ROLLING-STOCK, 445 of contact of the wheels of a locomotive, or even of a four- wheeled truck, will not ordinarily lie in one plane. The rougherand more defective the track, the worse the condition in thisrespect. Since the frame of a locomotive is practically rigid,and the frame rests on the driver-axles through the medium ofsprings at each axle-bearing, the compression of the springs(and hence the pressure of the drivers on the rail) will be varia-ble if the bearing-points of the drivers are not in one planeThis variable pressure affects the tractive power and severelystrains the frame. Applying the principle that a tripod willstand on an uneven surface, a mechanism is emploj^ed which. r>l~~^-:^ Fig. 195.—Action of Equalizing-levers. virtually supports the locomotive on three points, of which oneis usually the center-bearing of the forward truck. On eachside the pressure is so distributed among the drivers that evenif a driver rises or falls with reference to the others, the loadcarried by each driver is unaltered, and that side of the enginerises or falls by one nth of the rise or fall of the single driver,where n represents the number of wheels. The principle in-volved is shown in an exaggerated form in Fig. 195. In thediagram, ikfAT^ represents the normal position of the frame whenthe wheels are on line. The frame is supported by the hangersat a, c, /, and h, ah, de, and gh are horizontal levers vibratingabout the points H^ Kj and L, which are supported by theaxles. While it is possible with such a system of levers to make 446 RAILROAD CONSTRU
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