. The Street railway journal . radually increases as the speed decreases until just be-fore the car stops the deceleration suddenly reaches a highmaximum value () and then drops back to zero whenthe car has stopped. It is this last sudden increase whichan experienced motorman will avoid by releasing the brak-ing pressure somewhat just before the car settles to rest. 20 30 40 50 CO Speed (Miles per Hour) . „ , , ^ ^ ^ Street FIG. 5.—COEFFICIENT OF FRICTION ASAFFECTED BY SPEED AND TIME approximately what braking force each kind of equipmentis capable of exerting. Thus, for example
. The Street railway journal . radually increases as the speed decreases until just be-fore the car stops the deceleration suddenly reaches a highmaximum value () and then drops back to zero whenthe car has stopped. It is this last sudden increase whichan experienced motorman will avoid by releasing the brak-ing pressure somewhat just before the car settles to rest. 20 30 40 50 CO Speed (Miles per Hour) . „ , , ^ ^ ^ Street FIG. 5.—COEFFICIENT OF FRICTION ASAFFECTED BY SPEED AND TIME approximately what braking force each kind of equipmentis capable of exerting. Thus, for example, in case ofemergency the ordinary hand brake might exert a retard-ing force of 90 to 100 lbs. per ton weight of the car andwe would refer to the i m. p. h. p. s. curve. With an airbrake the retardation might be somewhere between m. p. h. p. s. The acceleration (Fig. 4) is the rate the car changes itsspeed. This change may be positive when it is caused bythe motors, or negative when it is due to train resistance. -BRAKING TEST OF A SINGLE-TRUCK CITY CAR WITHMAGNETIC BRAKES The reason for this peculiarly shaped deceleration is moreevident from Fig. 5, which shows how the coefficient offriction is affected by the speed and the duration of brak-ing. Each point is the average of from twenty to ninetyseparate observations. These are the famous tests madeby Galton and Westinghouse in 1878. Notice how rapidlythe coefficient diminishes with time and how much less itis at the higher speeds. These tests were made with cast-iron brake-shoes on steel tires. Fig. 6 is also from the Galton-Westinghouse tests, and June i, 1907.] STREET RAILWAY JOURNAL. 963 shows clearly why the braking force should never approachtoo near the maximum limit. In these diagrams the line Prepresents the pressure applied to the brake blocks. Theline F shows the retarding force of the brake blocks uponthe wheels before the wheels slipped, and / shows the forcewhile the wheels were sliding on the rails
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectstreetr, bookyear1884
