98. Pivot, Track, Rack, and Rollers for Centre Bearing Swing Spans

In centre bearing swings, the centre pivot shall carry the entire dead load of the span both when swinging and when closed. This pivot shall be composed of a cast steel base supporting a disc-bearing, roller-bearing or ball-bearing that carries the top casting on which the structural work is directly supported. The disc-bearing has given the best results and shall generally be used. The phosphor-bronze disc shall be made convex on both faces and shall lie between two hardened steel discs which have curved surfaces bearing on the centre disc, but of slightly larger radius. The other surfaces of these discs shall be plane and shall bear on the upper and the lower castings and be doweled to them so as to insure that the sliding shall take place on the bronze disc. An oil-tight, cast-steel box shall be placed around these discs and attached to the base casting. This box shall be of substantial construction with a total clearance of one-thirty-second (32) of an inch between the discs and the box. It shall be made in sections and bolted together so as to permit removal for the inspection and the renewal of the centre disc whenever necessary. Semi-circular vertical oil grooves of 11⁄2 inch radius shall be placed around the inside of the boxing and connected with an oil space around the top. Oil holes feeding into this oil space shall be provided in the top casting. The latter shall completely protect this oil space from dust, etc. Oil grooves of three-eighths (3) inch radius shall be cut in diametral lines across both faces of the centre disc. A hole one (1) inch in diameter shall be drilled in all three discs at the centre. This hole shall feed into oil grooves cut on diametral lines across the top face of the base casting. Holes shall be drilled into this casting at the ends of these grooves and tapped for drain pipes. The sliding contact faces at the discs shall be polished, whereas all other surfaces shall be merely finished. The base casting shall be well anchored to the pier by not less than eight bolts, each one and one-half (11⁄2) inches in diameter and three (3) feet long.

The circular track for steadying the span when in motion and the operating rack are to be cast separately in segments, and bolted together effectively so that broken rack segments may be easily replaced. The previous specifications are to govern the design of these tracks and racks, excepting that the track may be as narrow as seven (7) inches. Generally there are to be six (6) or eight (8) trailing wheels not less than eighteen (18) inches in diameter and six (6) inches face, the axles being not less than three (3) inches in diameter. The wheels are to be set in a radial position so as to run truly on the track; and they are to be securely fastened in correct position. Provision shall be made for adjusting the rollers so that they will just clear the track when the span is swinging. The rollers and support shall be designed to take the reaction from a fifteen (15) pound wind tending to overturn the span about the pivot.

99. Centre Wedges for Centre-Bearing Swing Spans

When the span is closed, the live load shall be carried on the centre wedges. They shall not be designed to lift the trusses but merely to provide a good, firm bearing. For this reason a flat bevel of about one (1) to ten (10) is desirable. Two wedges, one at each side of the pivot near the trusses, will generally suffice. Proper provision shall be made for adjustment. The wedge shall bear on an upper casting provided with guides engaging lips on the wedge. These guides and lips shall be so arranged that the wedge will be supported by the upper casting during the swinging of the span. The wedge shall bear on a base casting substantially bolted to the pier.

100. End Lifts for Swing Spans

All swing spans shall have an arrangement to lift the ends thereof so as to make the span continuous over the centre supports for all conditions of loading. Wedges, toggle-joints, eccentrics, and rollers with links may be used for this arrangement. Whatever detail is employed, it shall be able to lift the ends to the desired elevation and form a solid, substantial support as for fixed spans. In figuring the amount of movement to be provided for, the possibility of the top chord's having a temperature 30 degrees F. greater than that of the bottom chord must be duly considered.

Wedges give very satisfactory supports. When used they shall move in the line of the trusses and bear directly under the same in the line of the end floor-beams. The upper surface of the wedge shall be beveled about one (1) to five (5), and shall engage guides in the upper bearing casting, which is directly attached to the truss so that the wedge will be supported by the span when swinging. The lower surface of the wedge shall be horizontal, and shall bear on the base casting that is bolted directly to the pier. The base casting shall have guides to engage the wedge, but these guides must not interfere with the span while swinging. All surfaces in contact are to be finished and polished.

Where roller bearings are employed, rollers are to be provided beneath the end-pins of the trusses and attached to the span by means of links which are operated by struts attached to the pins passing through the rollers. The axes of the rollers shall be parallel to the trusses and shall be moved during operation in a transverse direction. The rollers must be bored so as to provide a fairly close fit over the pins at the bottom of the links. Both the pins and the insides of the rollers must be finished very smooth; and provision must be made for oiling the bearings between them. No roller shall be less than six (6) inches in diameter, and the pins inside of them shall not be less than three and one-half (32) inches. In all draw bridges where, on account of infrequent operation combined with great changes of temperature, there is a tendency to drag

the rollers on their bearings, the links and all details must be made strong enough to overcome the friction and thus allow the ends to adjust themselves, or else special longitudinal roller bearings must be provided. The bearings for the rollers shall be cupped one-eighth (%) inch or more in depth so as to provide ample bearing area; and shoulders must be provided on the shoes to furnish a side bearing for the rollers when lowered into place. Each shoulder must be machined so as to fit the roller exactly when in its final position with the end of span fully raised. The height of these shoulders shall be about one-third (3) of the diameter of the roller, but never enough to interfere with the span while swinging. The shoes at each end of the span shall be connected by adjustable rods not less than one and one-half (12) inches in diameter, and strong enough to take up the entire thrust from the operating struts.

A toggle arrangement may be used, the upper end thereof being fastened to the truss by a pin, and the lower end connected by another pin to a shoe moving in vertical guides. The reaction at each corner is carried upward through the toggle links to the upper pin and into the truss. The operating arm is to be attached at the centre of the toggle and operated in the plane of the truss. In connection with this apparatus a nest of segmental rollers with transverse axes may be used at the ends for cases in which the spans open very infrequently and when temperature changes are great, so as to minimize the effect of the expansion and contraction. of the steelwork. These rollers shall be provided with a device for throwing them into correct vertical positions when the ends are free. Ample provision shall be made for adjusting the bases.

For bob-tailed draw spans it is usually best to place the end lifts at the end of the long arm only.

Proper provision for adjustment must be made in all cases.

101. Latch for Swing Spans

To bring the draw span to rest and proper position when closing, an automatic latch of the Pencoyd type shall be used. One shall be placed at the centre of each end floor-beam.

102. Suspending Cables for Vertical Lift Spans

In vertical lift bridges the movable span shall be balanced by a counterweight at each end, connected to the span by wire ropes passing over sheaves at the tops of the towers.

These counterweight ropes shall be of plow steel, and shall consist of six (6) strands of nineteen (19) wires each, laid around a hemp centre. They shall be of approved make and shall conform to the values given in Table 16a as to their elastic limits and ultimate strengths. The ropes shall be designed as noted under "Unit Stresses." They shall have as small side leads as possible, in no case exceeding one (1) in forty (40).

The stretch in the ropes due to direct load shall be figured by the

The manufactured length shall be the calculated length minus the stretch in the ropes. The manufactured lengths of the ropes shall not vary from the dimensions indicated on the drawings by greater amounts than those given in Table 78g.

Rope sockets shall be either open or closed as required. They shall be of the standard dimensions shown in Tables 16c and 16d.

104. Equalizers

The counterweight ropes shall, as a rule, be connected directly to the steelwork on the lift-span side, but on the counterweight side they shall be attached to equalizers, which, in turn, are attached to the counterweight. Each equalizer bar shall be made with the centre pin below the end pins. This distance will depend on the layout and design of the bars, because no vertical links shall be used between the bars except to change the direction, as noted below. The layout of the equalizers will depend on the type of counterweight used, whether sectional or solid. In the sectional type four ropes shall generally be attached to each section, and the equalizer bars shall be placed parallel to the axis of the bridge. In the solid counterweight, the upper equalizer bars, to each of which two ropes are attached, shall, as a rule, be placed transversely to the axis of the bridge. They shall be attached to the lower equalizer bars,

which run parallel to the axis of the bridge, by criss-cross links. On the span side, open sockets shall generally be used; and on the counterweight side, closed sockets. All pins connecting the various equalizer bars shall have a head one-quarter (1⁄44) inch thick on one end and a split cotter on the other. The pin connecting the bottom bars to the counterweight hanger shall have the ends threaded for Lomas nuts. Provision shall be made for removing any pin connecting the sockets to the bars or lift span in case it becomes necessary to replace a rope, and the equalizers shall be so designed that a rope can be replaced without supporting the counterweight. All pins in the upper equalizer bars shall be of the same size, viz., that used for the sockets. The clearance between any two parts shall be greater than one-half (2) inch, which amount shall be considered no clearance.

105. Tower Sheaves

Tower sheaves having a pitch diameter of thirteen and a half (13.5) feet and under shall be made of cast steel, and those of greater diameter shall be built up of structural steel with a cast-steel rim and hub. The pitch diameter of the sheave shall not be less than sixty (60) times the diameter of the rope. The ropes shall be spaced on the sheave a distance apart equal to the diameter of the rope plus one-eighth (%) of an inch. The grooves in the sheaves shall be made to fit the ropes. The metal between the grooves shall be rounded off with the top of each ridge from one-eighth (%) to one-quarter (4) inch below the pitch line. The distance from out to out of rim shall be equal to the distance between the centres of the end ropes plus two (2) diameters of the rope. The outside of the rim shall project one-half (2) the diameter of one rope above the pitch line. The inner face of this lip shall make an angle of fifteen (15) degrees with the vertical.

Tower sheaves shall have not less than eight (8) spokes, which may be tee, cross, elliptical, or H in section. Each rib shall have sufficient area to carry the load on the sheave, distributed over a length equal to the distance from centre to centre of spokes, and to resist the bending due to the friction on the journals. In all cases where the spoke consists of two ribs, one at each side of the sheave, the rim must be properly supported between these sections for its full width. It must also be properly supported longitudinally between the spokes. In cast sheaves the hub diameter shall be one and eight-tenths (1.8) times the diameter of the shaft, but shall not exceed the said diameter by more than ten (10) inches. The hub shall have a greater length than the distance from out to out of rim. It shall be made to bear on the shaft only as required on each side under the spokes.

Structural sheaves shall be built up of plates conforming to the outline of the said sheaves, one or more being used at each side, supporting a segmental cast-steel rim and connected by cross diaphragms. The