A Treatise On The Principles And Practice Of Dock Engineering

POWER OF GATE MACHINES. 525 increased. The chain system is preferable to the ram in this respect, for it is quite feasible to attaeh the chains at the centre of gravity of the displaced fluid, which is the ideal position. One advantage which the rotary engine possesses over the ram is that in certain cases of breakdown—viz., those not involving the gearing, barrel or chain—the former can still be worked by hand, whereas it is never possible to work a ram in this way. The expedient then generally resorted to is to attaeh a rope to the head of the mitre-post of the gate and lead it to the nearest capstan. This is by no means a desirable or convenient arrange- ment, but it should nevertheless be looked upon as a likely contingency and provided for accordingly. A point which must not be overlooked is that chains reduce the effective draught of water over dock sills, and that, in order to allow the former to lie perfeetly flat, so that vessels passing over them may not foul, it is necessary to provide a large amount of slack chain. Chases have been cut in the sill to receive the chain, but it is by no means certain that the latter will lie in them. The accompanying illustrations (figs. 522 to 527), showing the applica- tion of hydraulic power, by means of both chains and rams, to recently constructed gates at Leith, are reproduced from drawings kindly furnished by Messrs. Sir W. G. Armstrong, Whitworth & Oo., with the courteous sanction of Mr. Peter Whyte, the harbour engineer of that port. Power of Gate Machines.—While the determination of the amount of power necessary to work cranes, capstans, and other dock appliances is a matter of comparatively simple calculation, the paucity of existing data in reference to the forces at work upon dock gates renders the problem in this last instance apparently incapable of an exact or, at any rate, a general solution. There can be little doubt that, in the majority of cases, a large margin of power has to be provided to cover unknown contingencies. The resistances to be overcome are three in number. At the moment of starting there is the inertia of the gate, and during movement there are the friction of pintles, collars, wheels, rollers, &c., as the case may be, and the resistance of the water to disturbance by the motion of the gate. The force required to overcome the first of these may be estimated as follows:—Calculate the moment of inertia of the gate about its axis of rotation; for the purpose it may be treated, without serious error, as a weighted rectangle revolving about one edge. Then I = 1 M Z2, where M is the mass of the leaf and I its length. From this we find the radius of gyration, which is and the mass of the leaf may accordingly be considered concentrated at a point distant -577 7 from the axis of rotation.