The rollers are in the form of conic frusta, or seg. ments of cones having their vertices meeting at the axis of motion of the bridge; and are retained in position by arms radiating from a central hub, and serving as axles for the rollers; or secured by a circular frame, FIG. 73. ff, formed of two concentric iron rings (shown complete in the upper, but only in section in the lower diagram of Fig. 73), one inside, and the other outside of the circle of rollers. The rollers may either turn upon pins through their centres, and through said rings, or the pin or shaft may be fast in the roller, and turn with it upon journals running in gudgeon boxes attached to or formed in the circular frame f. The pins or axles may be quite small (say 1" to 1 in diameter), as they support but a nominal weight, and are only required to maintain the proper positions and directions of the axes of the rollers. The roller frame, as well as the upper circular rail running upon the rollers, must be connected with a central hub for each (as they do not turn together), turning upon a journal or pivot attached to the masonry of the supporting pier. The rails, or surfaces between which the rollers work, are beveled to fit the conical faces of the rollers, and, in order to work in the most perfect manner, they should be of cast iron, and turned off by a tool carried by the arm of a heavy revolving vertical shaft. to of the The diameter of the circle should not probably be less than to the span of the water channel, nor less than to the width of superstructure, and the dia meter of the rollers, not greater than radius of the circle upon which they travel. Greater diameter would give so much obliquity of face as to produce too strong a centrifugal tendency. The face of the rail should have a width of 2 to 3 inches generally, and for some 30° opposite each king post (transversely of the bridge) when the draw is in position, a width about twice as great, and as great as the face of the rollers. This is to give sufficient bearing surface while loads are passing, when nearly the whole weight will be concentrated upon two or three rollers near each of those positions. 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 quadrantal 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 sus tain as great a proportion of the weight of superstruc ture 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, when 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, |