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A Text Book on Civil Engineering
Copyright 1897, 1898, 1899 by The Colliery Engineer Company

 

 

TURNTABLES.

1821. A turntable, as shown in Fig. 638, is a platform usually from 50 to 70 feet long, and from 8 to 10 feet wide, upon which a locomotive and tender may be run and then turned horizontally through any portion of a circle, and thus be transferred from one track to another forming any angle with it. The table is supported by a pivot under its center, and by wheels or rollers under its ends. Beneath the platform is excavated a circular pit 4 or 5 feet deep, having its circumference lined with brick or stone masonry 2 feet in depth, and capped with either cut stone or wood. The diameter of the pit in the clear is about 2 inches greater than the length of the turntable. The masonry lining is usually built with a step (see elevation B), which supports the rail upon which the end rollers travel. At the center of the pit is a substantial foundation of masonry, upon which the pivot rests. This foundation should be 4 or 5 feet in depth and composed of large, regularly shaped stones laid in cement mortar and well bonded together. This foundation is capped by a single stone 6 ft. square and 12 in. in thickness. The pivot, shown in detail at C, is fastened to the foundation by heavy anchor bolts reaching the full depth of the masonry. Sometimes the pit is floored over with plank, but this so greatly increases the weight of the table, besides involving the expense of renewal, that it should be dispensed with unless circumstances make a floor necessary. Usually, only a walk of planks, supported by the projecting ties, is allowed.

The turntable should be somewhat longer than the total length of both locomotive and tender, so as to permit the engineman to move his engine a few feet in either direction from the pivot in order to secure an equilibrium. With a little practice, such an equilibrium is easily obtained. By this means the friction while turning is confined chiefly to the center of motion.

Probably the best turntables in use in America are manufactured by Wm. Sellers & Co., of Philadelphia, Pa. The turntable shown in Fig. 638 is a copy of their recent standard. They are expensive in first cost, but most economical both in operation and in the matter of repairs. Being composed chiefly of metal they are very enduring, and as the parts are readily duplicated, repairs are simple and expeditious. One man can readily turn one of these tables, loaded, without the assistance of machinery. They consist of two heavy cast-iron girders, perforated by circular holes to reduce weight and cost. Each girder consists of two parts, a and b, fastened to a heavy central boxing, shown in cross-section at C.

The girders are fastened to it by means of heavy iron bars c, d, 3¾ in. square, of rolled iron, fitted into sunk recesses on top of the boxing, and tightened in place by means of wedges, and also by means of two 2¼-in. key bolts at the base of the girders, passing through the holes e, f, and confined by the keys g, g. The central portion of the boring is a hollow cone h, open at top and bottom, and surrounding the hollow conical pivot post k. The pivot shell is about 1¾ in. thick. On top of the post rests a heavy, loose, cast-iron cap l, which permits of a slight rocking motion of the entire platform as the engine enters and leaves the turntable. This cap supports the steel box (see detail D) containing the friction rollers m. There are fifteen of them, each about 2¾ in., both in length and greatest diameter. They have no axles, but lie loosely in the lower part of the box, filling its circumference, save a half-inch of space left for the free movement of the rollers. In the direction of their axis they have but one-eighth in. play in the box. The lid n of the box rests directly upon the rollers themselves and does not come down to the lower part o of the box by ½ inch. Both the rollers and the box enclosing them are finished with mathematical accuracy, so as to ensure a perfect bearing between them. The rollers are kept constantly oiled, as ease in turning depends entirely upon their being well lubricated. On top of the rollers is the cap p, which is secured by heavy bolts. This cap does not rest directly upon the boxing, but is separated from it by wooden wedges q, q, by means of which the table may be raised or lowered and its height exactly adjusted to the connecting track.

When the engine is properly balanced, the cap bolts sustain all the load placed upon the turntable, excepting the small amount carried by the tracks at the end of the platform.

When properly balanced on a Sellers' turntable, the end wheels should only just touch the rails. The diameter of the roller box being 15 in., it is not difficult to balance the locomotive and tender. All turntables should be provided with the means of being raised or lowered, and so adjusted as to give the proper bearing upon the circular track.


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