A Treatise On The Principles And Practice Of Dock Engineering

5’8 DOCK ENGINEERING. ram only is put in motion ; for the higher power, the larger ram is liberated and moves simultaneously with it. In some instances, a combination of both the preceding methods has been utilised to obtain three powers from one cylinder. The outer or larger ram is fitted with a piston so as to give two powers by the first method, while the internai ram supplies the lowest power. The arrange- ment is, however, so complicated as to be of doubtful utility, and, except in extreme cases, it will be found preferable in this respect to sacrifice economy to efficiency. The uniform expenditure of water upon work of the most variable nature cannot be considered excessive when it is borne in mind that simplicity in construction and manipulation has advantages to offer nearly, if not quite equivalent, to economy in power. The second system is based on the principle of the reciprocating action of the connecting-rod and crank-shaft of the ordinary steam engine, and one type °f the apparatus consists of three small cylinders with plungers, each acting upon a three-throw crank and having mitre valves, worked by cams upon a revolving shaft. Fig. 517. —Two Concentric Rams. Another type (fig. 534), until recently much in vogue, had only two cylin- ders. These oscillated upon trunnion bearings, and were fitted with com- bined rams and pistons working on over-end cranks set at right angles to each other. The areas exposed to pressure in the cylinder were as two to one. One face of the piston had the exact moiety of the area of the other face, the difference being due to the displacement of the ram. The pressure on the smaller face was maintained constant, there being continuous com- munication with the supply pipe. The pressure on the larger face was intermittent, and alternately full and nil, according as the cylinder on that side was open to supply or exhaust. The piston, accordingly, was actuated by the difference of pressure on its two faces, the stroke in one direction being effected by unopposed pressure on the smaller face, and in the other direction by the balance of pressure on the larger face, which, by the adjustment of areas, resulted in regularity of effort. The three-crank system is adapted for large engines in situations where there is ample space at disposai. The two-crank system, on the other hand, is more compact and also less expensive in construction, in that a middle crank is obviated, but it lacks the uniformity of movement, characteristic of