A Treatise On The Principles And Practice Of Dock Engineering

DOCK GATE MACHINERY. 523 overhauled and examined, and should be annealed in a wood fire at least once a year. This involves the provision of spare chains, and these should be ready for instant substitution, in case of breakage or other serions accident. The advisability of the chain connections being simple and accessible is therefore apparent. For gate attachments below water level a ring at the end of the chain, of larger diameter than the staple through which the chain passes, will be found a suitable arrangement. The expansive system of the hydraulic ram witli multiplying sheaves seems, on the whole, preferable to the rotary engine, owing to the greater risk of damage to the gearing of the latter. Damage to cliains and mechanism arises principally from such causes as irregularity of movement, with abrupt jerks and stoppages, which induce momentary stresses of unexpected magnitude. Fracture or strain may easily result from an attempt to force a gate home in the face of some submerged obstruction, and as it is preferable for a gate to be brought to, rather than for a breakage to occur, it is by no means judicious to provide machinery of excessive power, unless it be carefully regulated. Gate chains are arranged on the two systems indicated in figs. 520 and 521. In the first case, chains are attached to the back and front of the gate respectively, near the bottom and, being led horizontally to sheaves set in the walls, at opposite sides of the passage, they pass vertically upwards to other sheaves near the coping level, whence they are conducted to their respective machines. In the second system, known as the “overgate system ” (fig. 521), chains (A and B) are fixed to the opposite walls of the passage and led horizontally to sheaves at the foot (C) of the gate, thence vertically upward to sheaves at the top of the gate, and, finally, in a parallel course, over a third pair of sheaves near the heel-post to the actuating gear. By this latter arrangement, each leaf of the gate is opened and closed from the same side of the passage and from one spot. Thus, the cost and inconvenience of two separate chain-ways through the walls to the machine pits are avoided. Struts or direct-acting rams were introduced by Sir J. W. Barry for working the gates at the Barry Docks in 1894. They have the possible advantage over chains of being able to hold the gate up against external pressure, and thus discharge the functions of a strut gate in minimising the effect of waves at high water. This advantage, however, is more apparent than real, as the power of gate machines, unless unduly great, is inadequate to do more than work gates under ordinary circumstances. The earliest examples of direct-acting rams worked in cylinders oscillating upon trunnions, but this type has not been repeated, at all events in this country. Recent practice has entirely favoured a fixed cylinder, with ram and connecting-rod, which latter, by means of a crosshead and vertical and horizontal pivot pins, is free to turn in any direction. The gates at Leith, illustrated in figs. 526 and 527, are worked in this manner, as also are the West India Dock gates at London, and many others.