. The Street railway journal . e speedincreased, were the car carried on a single set of springs ofuniform dimensions, a point might be reached where thesuccessive jolts would fall in step with the period of oscilla-tion of the springs, and thus tend to increase the amplitudeof this oscillation before the damping effect of friction hashad time to stop it. It is not necessary that such periodicjolts should follow each other with a rapidity equal to theperiodicity of the springs, but only that they should fall instep with this periodicity, one impulse to, say, every threeor four periods. This ef
. The Street railway journal . e speedincreased, were the car carried on a single set of springs ofuniform dimensions, a point might be reached where thesuccessive jolts would fall in step with the period of oscilla-tion of the springs, and thus tend to increase the amplitudeof this oscillation before the damping effect of friction hashad time to stop it. It is not necessary that such periodicjolts should follow each other with a rapidity equal to theperiodicity of the springs, but only that they should fall instep with this periodicity, one impulse to, say, every threeor four periods. This effect is at times noticeable as a kindof dancing or bobbing of the car body at certain speeds,especially with trucks that depend too largely on spiralsprings. It should be counteracted in designing by placingtwo or more sets of springs between the car body andwheels, the periodicity of the sets bein°- to each other as 5is to 7, or some other similar proportion whose least com-mon multiple is large in comparison to its FIG 6.—PECKHAM (4A XX TRUCK As pointed out above, a loaded spring started oscillating,continues to do so until damped by friction. With thespiral spring this friction must come from the rubbing to-gether of working parts of the truck, and consequently en-tails wear and lost motion. With the elliptic, the frictionfor damping is furnished by the movement of the leavesover each other, and consequently lasts as long as the springitself. The elliptic also, with its greater weight of ma-terial for a given carrying capacity, has a slower period ofoscillation. The combination of this rapid damping andslower period has led to the almost universal practice ofplacing the elliptic, the softest riding member of the springsystem, next to the car body, or immediately under thebolster. The limited space available in the side frames and October 4, 1902.] STREET RAILWAY JOURNAL. 519 1— ■■ FIG. 7.—LIFT OF WHEEL MOUNTINGOBSTRUCTION over the journal boxes has made
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectstreetr, bookyear1884
