A Treatise On The Principles And Practice Of Dock Engineering

SETTING APPARATUS. 439 weight on a ring of live rollers, a single bridge was wedged up at each end until such time as it was necessary to put it in motion, when the wedges were withdrawn. A bridge with double leaves was also wedged up at the tail ends, so that each leaf tilted forward on to bearing blocks provided at the edge of the coping. The wedges were actuated by mechanical means, such as the screw and the lever. With the advent of hydraulic power came the water pivot, which raised the bridge off its fixed bearings during the process of rotation, and after- wards allowed it to return to them. The advantages of a solid pivot have caused the transference of the hydraulic lifting rams to the extreme rear, where the wedgingup process has been followed, but with this modifica- tion, that when the rams have lifted the bridge clear off the pivot, a pair of sliding bearing blocks are inserted, and the lifting power is withdrawn until it is required once more to raise the bridge for the removal of the bearing blocks and the resumption of the pivot seating (see fig. 441). Another form of lifting apparatus is the knuckle or toggle gear, which consists essentially of two short bars linked together, and flexibly con- nected with an upper frame, constructed to move vertically, and a base which is fixed. When the two bars are in one vertical line, the upper plate is at its highest elevation, and any movement in the bars produces a depres- sion in the°level of the plate. The thrust of a hydraulic ram straightens the knuckle, so that a bearing block may be inserted as before, but in some cases the weight of the bridge continues to be borne by the gear, the links being driven slightly past the vertical position in order to preclude any tendency to a backward movement of the ram. The opposite motion is effected by another hydraulic cylinder. As a mechanical means, the toggle joint is very powerful. Eccentrics and cams on shafting and bent levers have also been employed to accomplish the necessary lift. An ingenious arrangement adopted for a double-leaved swing bridge, each leaf weighing 116 tons, over an 80 feet passage at Barrow, consists in allowing the bridge to remain continuously upon the pivot through the medium of a very shallow and flexible girder. During the passage of a load over the bridge, this girder deflects sufficiently to admit of the structure coming in contact with specially arranged fixed blocks, which themselves take up the actual weight. After the transit of the load the resilience of the girder causes it to spring back to its original position and the bridge resumes the swinging condition. By this contrivance all apparatus for lifting and setting is dispensed with. Examples of blocks provided for the centre bearing and the nose end of a bridge at Liverpool are given in figs. 423 to 428. The blocks for the tail end are similar to the centre bearing blocks, with the addition that their undersides, instead of being fixed, are arranged to slide in grooves in sole- plates, as shown in fig. 441. The upper members in figs. 426 and 427 are attached to the underside of the bridge structure.