A Treatise On The Principles And Practice Of Dock Engineering

SWING BRIDGE AT MARSEILLES. 451 the bridge may rest evenly on all three when slightly raised. This arrange- ment necessitates two circular roller paths of radii, 64 feet and 67 feet 7 inches, respectively. The bridge is turned by a chain passing round a cast- iron slewing drum, 46 feet in diameter, the motive power being supplied by two hydraulic cylinders, with rams, each 11'8 inches diameter, and 9 feet 2^ inches stroke, one of which serves to open and the other to close the bridge. The operation of turning consists in first releasing the wedges at the tail end, by which means the rollers at that part are lowered on to their tracks. The pivot press then lifts the bridge until the nose end is raised from its supports, and everything is ready for rotation. A hydraulic cylinder, 13'8 inches in diameter, actuates the wedging apparatus under a pressure of 700 Ibs. per square inch, which is the same as that obtaining in the slewing cylinders. The kentledge is arranged to throw a weight of 15 tons on the guiding rollers while the bridge is being swung. The pivot is enclosed in a press, 6'3 inches thick, which is secured by keys to a cast-iron base, from which it can be withdrawn for repairs. The prismatic top of the pivot inserts itself into a bearing plate fixed to the underside of the pivot girder. The surface of contact is made slightly convex, so that the bridge may always have a good bearing on its axis, despite any slight displacements during the process of lifting. A leather lining makes a watertight joint between piston and cylinder, but in order to prevent a tendency to tear from the turning stress imparted by the adherence of the rotating pivot, the interstice between the edges has been fitted with a band of india-rubber, which, by the interior adhesion it gives to the opposite edges of the leather, causes its exterior surface to slide on the metal. A horizontal sector is fastened to the head, of the piston, which rests against two rollers with light movable axles, supported by a cast-iron bracket to counteract the lateral strain caused by the chains in turning the bridge. Commenting in the Annales des Ponts et Chaussees, May, 1875, on the arrangements described above, M. Barret, then engineer to the Marseilles Dock Company, adds : — “ If a similar bridge had to be constructed for an important line of railway, and over a channel through which there was a considerable traffic, it would be desirable to substitute a double line for the single tramway, and to make a footway on eacli side of the cart-road within the girders, which, though increasing the widtli to 59 feet, would make the bridge more symmetrica! and easier to balance. The raising and lowering of the ends might be regulated by making the rollers at the tail end fall and rise in the roller boxes, keeping them always in contact with the roller paths by means of a counterpoise. The diameter of the piston (pivot) of the press might be increased to 4'9 feet, so that the bridge could be raised with the ordinary water pressure. The guide rollers might be increased in number, and placed higher up, so as to act all round the bearing plate. Also, if the webs of the girders were made of plate iron, the strains would be more evenly distributed, and the construction simplitied with a slight