. Ingegneria della trazione e della trasmissione elettrica. R showedche in condizioni di pista variabili è stato richiesto uno sforzo di trazione dida 25 a 54 libbre per tonnellata per avviare lecar su una carreggiata rettilinea, la media essendo 40. Togeep questa macchina appena che si muove percettibilmente ha richiesto 22 tonnellate di Poundsper, che è circa il doppio dello sforzo di trazione needat 5 miglia all'ora. La resistenza del treno nel funzionamento del tunnel e della metropolitana supera il valore per i treni che corrono all'aperto a causa dell'aumento del vento. Vari test indicano che se le cevalue derivate dall'equazione precedente sono incrementate di circa il 20 per cento, Th
1807 x 1383 px | 30,6 x 23,4 cm | 12 x 9,2 inches | 150dpi
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. Electric traction and transmission engineering . r showedthat under varying track conditions a tractive effort offrom 25 to 54 pounds per ton were required to start thecar on a straight level roadway, the average being 40. Tokeep this car just perceptibly moving required 22 poundsper ton, which is about double the tractive effort neededat 5 miles per hour. The train resistance in tunnel and subway operationexceeds the value for trains run in the open because ofincreased windage. Various tests indicate that if thevalues derived from the preceding equation be increasedby about 20 per cent., they will be found suitable for sub-way operation. 9. Grades. — If grades be encountered additional trac-tive effort must be exerted. If a car be on a grade ofinclination a to the hori-zontal plane, Fig. 8, the com-ponent of its weight alongthe direction of motion isW sin a, the other compon-ent being balanced by thereaction of the rails. Tomaintain uniform motionup the grade a force equaland opposite to W sin a. must be exerted. For small values. 20 TRACTION AND TRANSMISSION. of a, such as are met with in railway work, sin a = tan aapproximately, whence grades may be expressed as the ratioof the vertical rise to the horizontal length of grade. It iscustomary, therefore, to consider that a grade of g per centmeans a rise of g feet in a hundred feet. The tractive effortnecessary to propel each ton of car weight up a one per cent grade is therefore X 2000, or 20 pounds, and to draw a car of W tons up a grade of g per cent with uniformspeed requires G = 20 qW pounds tractive effort. For a down grade G is considered negative.For routes having numerous grades, g should be takenas the equivalent percentage grade. On the assumptionthat only half of the kinetic energy acquired by a car ortrain in descending a grade is utilized in ascending the nextup-grade because of stops on grades, non-alternate distri-bution of up- and down-grades, and the necessity of brakingto avoid excessive spee
