A Treatise On The Principles And Practice Of Dock Engineering

532 DOCK ENGINEERING. alternate faces of the piston-ram will be perceived from an inspection of fig. 535, whicli is a section showing the valve of the cylinder. A is the supply passage ; B, the constant pressure port, always open to the upper side of the piston ; C, the pressure port to the under side of the piston ; D, the exhaust therefrom ; and E, the discharge passage from the engine. F is a ring of hard metal forming the fixed working face, the upper segment of which, marked G G, is free to press up against the rubbing surface as it wears down, and is kept in contact by the pressure of the water. H is the trunnion in section, showing the pressure port on the upper side and the exhaust port on the lower; and I is the relief valve, the port to which is always open at the moment when the relief valve is required to act. A vertical section of an electrically worked capstan at the Ymuiden Locks is given in fig. 536. Quay Cranes.—Quay cranes are of all capacities, from half a ton or less to 150 tons or more. Types are innumerable, and it is quite beyond the province of this work to attempt to deal with them except on very restricted lines. For dock work, cranes may be concisely divided into two classes—viz., fixed cranes and movable cranes. The smaller class of cranes, dealing with the loading and unloading of vessels with cargo, are generally of the latter type, from the necessity of adapting them to the variable positions of the hatchways. They are subdivisible as follows : — 1. Cranes which travel upon rails all of which are at coping level. To accommodate the track, and also to ensure stability, this arrangement involves a clear space of some width—say, 10 feet—for the crane alone, and as additional tracks will generally be required for trucks, both while loading and in reserve, the width may easily be extended to 30 or even 50 feet. To reduce this large allowance, often inconvenient when space is limited, pedestal cranes have been devised, such that one or more lines of waggons can pass beneath the crane platform. A hydraulic crane of the former type is shown in figs. 537 and 538 and a pedestal crane in figs. 539 to 541. In both cases the lifting is performed by a ram and cylinder with six sheaves. The pedestal crane is the copy of one in vogue at Havre, Dunkirk, Bordeaux, and other French ports. It is adapted to two lifting capacities of 15 and 35 cwts. respectively. The different powers are obtained by concentric cylinders. A slewing motion is imparted by two hydraulic rams placed vertically behind the pivot. A single chain, common to both presses, is attached to the turning drum, so that the motion of one ram causes it to revolve in one direction while the motion of the other ram produces revolution in the other direction. 2. Cranes (fig. 393) which travel upon one rail at the coping level and upon another carried by a balcony or corbel on a transit shed at some height above the quay, generally at first floor level. This is obviously a device for