A Treatise On The Principles And Practice Of Dock Engineering

HYDRAULIC MACHINERY. 517 the subject will find much entertaining and valuable information contained in a paper read by him before the Institution of Civil Engineers in 1877.* The modern hydraulic machine (for dock work) takes the form either of a direct-acting ram, working backwards and forwards in a cylinder with suitable multiplying gear for increasing the effective length of its stroke, or of a bent crank with rotary motion imparted by two or more pistons also working in cylinders. The former system is most commonly applied to gate and sluicing machines, and to cranes ; the latter, generally, to capstans, and •occasionally to gate machines. We will deal, first of all, with the ram apparatus. Primarily, this con- sisted of a ram fitting into the bore of a cylinder, the pressure being applied at one end of the ram, so that it was, accordingly, capable of acting in a forward direction only. The return stroke, being unopposed, was effected either by gravitation, if the ram were vertical, or by a small auxiliary ram, if the main ram were horizontal. One important drawback of this contriv- ance was that it admitted of no variation in the power applied. Whether the load moved were great or small, the same expenditure of energy was necessary. When the load was fairly uniform, as in the working of dock Fig. 516. —Combined Piston and Ram. gates and sluices, the objection was of little importance, and this type of machine is still largely used for that purpose. But in the case of cranes and other lifting apparatus, where loads are irregular, economy demands some modification so as to make the expenditure of water correspond approxi- mately to the actual load. This has been contrived by the use of two or three cylinders, able to act either independently or collectively. Three power values have, however, been found superfluous, or, at anyrate, unduly cumbersome in practice, and it is now customary to be satisfied with two powers at the most, and these are obtained with a single cylinder in one of two different ways :— First, by the use of a combined piston and ram (fig. 516), water being admitted to both sides of the piston for the lower power, and to the larger side only for the higher power. This arrangement is now very rarely used, one of the reasons being that a bored cylinder is required, the machined surface of which becomes corroded while out of action, with the result that the packing on the piston is cut. Secondly, by the use of two concentric rams (fig. 517), one contained within the other, in the same cylinder. For the lower power, the smaller * Armstrong on “ Water-pressure Machinery,” Min. Proc. Inst., C.E., vol. 1.