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STEELWORK BEING ERECTED on the centre of the swing span. The deep girders rest on a drum, or circular girder. On the bottom edge of the drum girder is fixed a cast-steel track which rests on sixty cast-steel rollers.
The superstructure comprises two enormous Warren girders and the floor carrying the roadway consists of cross girders 5 feet deep. The weight of the span is carried at the centre on two longitudinal girders 8 ft. 4 in. deep and on two transverse girders 11 feet deep. These girders rest on a drum, or circular
girder, which is stiffened by eight radial girders that support the floor of the machinery room. This is situated between the girders and has for a roof the floor of the deck. Beneath the lower edge of the drum girder is secured a cast-steel tapered track which rests on sixty caststeel rollers. These rollers are bored through the centre to take a phosphorbronze sleeve, inside which is the outer end of a steel radial rod joined to a central pivot. There are thirty of these rods that prevent the rollers from being forced, in the manner of an orange pip between the fingers, in an outward direction by the combined action of the tapers. The rods serve also to support a huge circular cage that spaces out the rollers. The whole roller system with its cage resembles, on an immense scale, a roller thrust bearing as used in motorcar practice.
Turning Machinery
The rollers run on a lower track, also of cast steel, mounted on steel beams partly embedded in the concrete of the pier. This lower track is level to within a limit of two-thousandths of an inch and the rollers are machined to within a liinit of three-thousandths in taper and in diameter. Accuracy of this degree in such large components explains the remarkably small amount of power that is required to swing the huge span.
A cast-steel rack, to engage with a gear system leading into the machinery room, is bolted to the outer edge of the lower roller track and to this in turn is secured a circular guard plate to prevent the ingress of rain or snow to the rollers. A similar guard, projecting downwards, is attached to the drum girder above the rollers.
The setting out of the roller track was done in February 1936. The load on each roller is 62 tons and lubrication is by grease under pressure. The centre pivot is a steel casting, with a diameter of 7 ft. 6 in. at the base and a height of 7 feet. Round this pivot revolves an upper collar, lined with phosphor bronze, which carries the inner ends of the eight radial arms of the drum girder. A similar lower collar carries the tie rods from the rollers.
At either end of the swing span, beneath the centre of the roadway, is a steel locking bolt mounted on a rectangular shaft and operated hydraulically at a pressure of 1,000 lb. per square inch. The tapered bolt-heads drive into cast-steel bevelled sockets in the outer faces of Piers 10 and 12.
The great length and weight of the swing span cause a slight sag at the ends. To counteract this, when the bridge is set for road traffic, four hydraulic rams are placed under the girderwork, one at each corner. These rams are fitted with tapered wedges that are forced over rollers mounted on the end piers. A lift of about 2 in. is obtained at each roller under a pressure of 52 tons. These wedge rams work at a pressure of 3,000 lb. per square inch, but this figure can be exceeded by 50 per cent if necessary.
The turning machinery comprises two separate units, each consisting of a 50 horse-power direct-current motor geared to the rack on the pier. The pinions of the sets of gearing are used at opposite sides of the rack and the bridge can be swung completely round in either direction.
Direct current for the motors is obtained from a generating set which also supplies current for the electrical control devices. Alternating current for driving the generating set and for other purposes is supplied by cable from the Fife Electric Company. This cable goes down through Pier 10, along a trench in
the river bed and up inside the central pier to a junction box in the roof. From there the cable is taken up through the swing pivot to a fixed column, equipped with a number of slip rings, in the centre of the machinery room. Contact brushes that can revolve...
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