A Treatise On The Principles And Practice Of Dock Engineering

434 DOCK ENGINEERING. by swing bridges to wear their pivots unevenly. Owing to the pull exercised by the turning rams behind the pivot, the bridge bears more heavily against the forward side, and in process of time creeps gradually backward from its true centre, so as eventually to cause the tail of the bridge to jamb against the masonry of the bridge pit. This movement has been known to take place to the extent of an inch or more. A remedy might perhaps be found in a movable pivot provided with a base fitting into a fixed sole-plate, where it could be adjusted at intervals by means of cotters or wedges. Figs. 416 and 417. —Bridge Pivot at Marseiiles. Balancing Bollers and Wheels.—For a swing bridge whose centre of gravity does not lie upon the axis of rotation, some additional supporting power is rendered necessary, and this is supplied by balancing wheels or rollers. In some cases (fig. 431) the weight is brought to bear upon the top