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The lower rail should have a depth (if of cast iron), of 4 to 5 inches, according to size of bridge; and the upper and inverted one, of one to two feet (the deeper the stiffer), and in both cases, they will generally be cast in segments, and those of the upper one, bolted together by flanges, so as to form a rigid hoop, over which one or more strong beams, BB, crossing at quad. rantal points ee, etc. (or at the angular points of any rectangle inscribed in the circle), should form supports for the king posts (ag, and ch, Fig. 71), the space gh, being adjusted to an equality with the side of the inscribed square or rectangle of the rail circle. And, the nearer the transverse distance between king posts comes to the length of the other sides of the said inscribed square or rectangle, the less stiffness of beams, BB, is required; that is, CC, F. 73 representing truss chords, and dd, the positions of king posts, the nearer the d points come to the e points, the less is the transverse action upon the beams BB. Hence it is desirable that the circle of rollers should pass directly under the points dd, etc.

CLXXVIII. An intermediate beam may be inserted between BB, and over the centre pivot, resting upon the circle cc, to support floor joists or rail stringers over the long stretch between BB. Or very stiff diagonal girders ee, and e'e', firmly attached by the ends to the circle cc, meeting a common nucleus at H, and so arranged as to have an adjustable bearing upon the centre pivot (5 or 6 inches in diameter, as to size of draw), enabling any desired amount of the weight of structure which such girders can support, to be thrown upon said pivot, and thereby relieving the rollers, a, of a like amount of pressure. These girders should have the greatest practicable depth, so as to suse

tain as great a proportion of the weight of superstructure as may be. But the skill and judgment of engineers in charge of specific cases respectively, will dictate as to the minutiae of these devices, and more precise detail will not be attempted in this place.

CLXXIX. This plan of turn table, as well as the one hereafter to be described, is worked by a vertical shaft attached to the superstructure, and turned by one or more sweep levers, with a pinion at the lower end, taking into toothed segments attached to the circular track b, or to the masonry of the pier p; and, in case more power be required, a gear wheel takes place of the sweeps above mentioned, and these are transferred to a second shaft and pinion working into said gear wheel.

The table above described, with slight modifications, is extensively in use, and, wben well constructed, undoubtedly works as easily and satisfactorily as can be expected. Still, it is liable to some objections, among which may be named the great weight of the ring cc, constituting or carrying the inverted rail, and the great number of rollers, a, so few of which can act with much effect at the same time. For, it is obvious that about two rollers under each king post, support essentially the whole weight. It is therefore proper that when the bridge is in place, each king post should stand centrally between two consecutive rollers; and, that the rollers be at equal distances apart. Then there will be at least 8 rollers under equal pressure at all times when loads are in transit, and when rollers receive their greatest pressure. But without discussing this plan further at present, I proceed to describe another swing bridge turn-table devised many years ago by myself, and used in a considerable number of cases with most satisfactory results.

FIG. 74.

THE WHIPPLE TURN TABLE.

CXXX. Is arranged with a two wheeled truck a, Fig. 74, directly under each king post, and the four conuected in pairs diagonally by an inverted triangular truss to each pair. These trusses consist of a bollow

cylindrical (or conic segmental) brace b, running from each truck frame obliquely downward to an abutting block c, which is common to the two trusses, with chords or ties d, from truck to truck for each pair.

The truck wheels are from 20 to 24 inches in diameter, with 5 to 6 inches width of rim, and with short axles or shafts, 3 to 4 inches in diameter, according to dimensions of bridge. The axles run in journal boxes titted to the truck-frame so as to bring the axles in the direction of radii to the circular track t, upon which the trucks are to run.

The truck frame consists of two cast iron side plates (of which g and h present an outside and an inside view), of an I formed cross section, and contour as seen at g. These plates upon the insides, have projecting portions as shown by the dark surface of diagram h, meeting from opposite plates, in the centre of the frame at a common surface of contact, and forming continuous tubes or sockets through the frame, which serve as media through which the ties d, act upon the cylindri. cal braces b, thus forming a rigid truss, which should be so proportioned as to be able to support (upon the two trusses), the whole weight of superstructure, throwing it upon the centre block c.

The chord ties d, of the two trusses, crossing one another upon the same level, are kept from mutual interference by cutting out the middle portion of one set, and replacing the removed part with two pieces to each tie bar, one passing above and the other below the single continuous rods of the other set, as shown at f.

The block c has a cylindrical cavity in the under side, 10 to 12 inches in diameter, and about 7 inches deep, into which is fitted (loosely) a solid cylinder entering about 4 inches into the cavity, and leaving a space of structure to be raised essentially free from bearing upsome 3 inches in thickness above, to be occupied by the nuts of a number of set screws s, intended to force down said internal cylinder upon the bed plate i, and thus relieve the truck wheels from nearly all the weight of superstructure.

The bed plate i, has a socket or step of an inch deep, or thereabouts, with a hardened steel plate in the bottom, to receive the lower part of the cylinder bearing upon the plate i, where the diameter of cylinder and socket should be graduated to the proportion most favorable for reducing the amount of friction. A diameter of 6 to 8 inches is thought to be suitable for draws of 60 to 100 feet opening, while the part of the pivot block within the block c should have a diameter of 10 or 12 inches, in order to afford sufficient surface for the set screws s to act upon.

The bed plate i, should have a rim about the step to retain oil, and the surfaces above and below the steel plate should have radial grooves to allow the penetration of oil; and these (grooves) should be so situated as to admit of their being probed, to prevent their getting clogger.

The pivot block should have guides to prevent its turning in the cavity of the block c; otherwise it might stick in the step, and the set screws slide upon its upper surface; which has been the case in some instances.

A groove should be formed in the under side of the block c, near the edge, to keep the water from the pivot ; and the screws s, should be kept secluded from water by a tin, or galvanized iron cap shutting over a rim or ring cast upon the block c, outside of the screw holes. Sufficient vertical movement (11 or 2 inches), should be allowed to the pivot cylinder, to enable the elasticity of the braces and ties, b and d, to be taken up, and the

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