Читать книгу A Catechism of the Steam Engine - C.E. John Bourne - Страница 18

111. Q.--Will you describe the principal features of the kind of steam engine employed for the propulsion of vessels?

A.--Marine engines are of two kinds,--paddle engines and screw engines. In the one case the propelling instrument is paddle wheels kept in rotation at each side of the ship: in the other case, the propelling instrument is a screw, consisting of two or more twisted vanes, revolving beneath the water at the stern. Of each class of engines there are many distinct varieties.

112. Q.--What are the principal varieties of the paddle engine?

A.--There is the side lever engine (fig. 26), and the oscillating engine (fig. 27), besides numerous other forms of engine which are less known or employed, such as the trunk (fig. 22), double cylinder (fig. 23), annular, Gorgon (fig. 24), steeple (fig. 25), and many others. The side lever engine, however, and the oscillating engine, are the only kinds of paddle engines which have been received with wide or general favor.

113. Q.--Will you explain the main distinctive features of the side lever engine?

A.--In all paddle vessels, whatever be their subordinate characteristics, a great shaft of wrought iron, s, turned round by the engine, has to be carried from side to side of the vessel, on which shaft are fixed the paddle wheels. The paddle wheels may either be formed with fixed float boards for engaging the water, like the boards of a common undershot water wheel, or they may be formed with feathering float boards as they are termed, which is float boards movable on a centre, and so governed by appropriate mechanism that they enter and leave the water in a nearly vertical position. The common fixed or radial floats, however, are the kind most widely employed, and they are attached to the arms of two or more rings of malleable iron which are fixed by appropriate centres on the paddle shaft. It is usual in steam vessels to employ two engines, the cranks of which are set at right angles with one another. When the paddle wheels are turned by the engines, the float boards engaging the water cause a forward thrust to be imparted to the shaft, which propels forward the vessel on the same principle that a boat is propelled by the action of oars.

114. Q.--These remarks apply to all paddle vessels?

A.--They do. With respect to the side lever engine, it may be described to be such a modification of the land beam engine already described, as will enable it to be got below the deck of a vessel. With this view, instead of a single beam being placed overhead, two beams are used, one of which is set on each side of the engine as low down as possible. The cross head which engages the piston rod is made somewhat longer than the diameter of the cylinder, and two great links or rods proceed one from each end of the cross head to one of the side levers or beams. A similar cross bar at the other end of the beams serves to connect them together and to the connecting rod which, proceeding from thence upwards, engages the crank, and thereby turns round the paddle wheels.

115. Q.--Will you further illustrate this general description by an example?

Q.--Fig. 26 is a side elevation of a side lever engine; x x represent the beams or keelsons to which the engines are attached, and on which the boilers rest. The engines are tied down by strong bolts passing through the bottom of the vessel, but the boiler keeps its position by its weight alone. The condenser and air pump are worked off the side levers by means of side rods and a cross head. A strong gudgeon, called the main centre, passes through the condenser at K, the projecting ends of which serve to support the side levers or beams. L is the piston rod, which, by means of the cross head and side rods, is connected to the side levers or beams, one of which is shown at H H. The line M represents the connecting rod, to which motion is imparted by the beams, through the medium of the cross tail extending between the beams, and which by means of the crank turns the paddle shaft S. The eccentric which works the slide valve is placed upon the paddle shaft. It consists of a disc of metal encircled by a hoop, to which a rod is attached, and the disc is perforated with a hole for the shaft, not in the centre, but near one edge. When, therefore, the shaft revolves, carrying the eccentric with it, the rod attached to the encircling hoop receives a reciprocating motion, just as it would do if attached to a crank in the shaft.

116. Q.--Will you describe the mode of starting the engine?

A.--I may first mention that when the engine is at rest, the connection between the eccentric and the slide valve is broken, by lifting the end of the eccentric rod out of a notch which engages a pin on the valve shaft, and the valve is at such times free to be moved by hand by a bar of iron, applied to a proper part of the valve gear for that purpose. This being so, the engineer, when he wishes to start the engine, first opens a small valve called the blow through valve, which permits steam from the boiler to enter the engine both above and below the piston, and also to fill the condenser and air pump. This steam expels the air from the interior of the engine, and also any water which may have accumulated there; and when this has been done, the blow through valve is shut, and a vacuum very soon forms within the engine, by the condensation of the steam. If now the slide valve be moved by hand, the steam from the boiler will be admitted on one side of the piston, while there is a vacuum on the other side, and the piston will, therefore, be moved in the desired direction. When the piston reaches the end of the stroke, the valve has to be moved in the reverse direction, when the piston will return, and after being moved thus by hand, once or twice, the connection of the valve with the eccentric is to be restored by allowing the notch on the end of the eccentric rod to engage the pin on the valve lever, when the valve will be thereafter moved by the engine in the proper manner. It will, of course, be necessary, when the engine begins to move, to open the injection cock a little, to enable water to enter for the condensation of the steam. In the most recent marine engines, a somewhat different mechanism from this is used for giving motion to the valves, but that mechanism will be afterwards described.

117. Q.--Are all marine engines condensing engines?

A.--Nearly all of them are so; but recently a number of gunboats have been constructed, with high pressure engines. In general, however, marine engines are low pressure or condensing engines.

118. Q.--Will you now describe the chief features of the oscillating paddle marine engine?

A.--In the oscillating paddle marine engine, the arrangement of the paddle shaft and paddle wheels is the same as in the case already described, but the whole of the side levers, side rods, cross head, cross tail, and connecting rod are discarded. The cylinder is set immediately under the crank; the top of the piston rod is connected immediately to the crank pin; and, to enable the piston rod to accommodate itself to the movement of the crank, the cylinder is so constructed as to be susceptible of vibrating or oscillating upon two external axes or trunnions. These trunnions are generally placed about half way up on the sides of the cylinder; and through one of them steam is received from the boiler, while through the other the steam escapes to the condenser. The air pump is usually worked by means of a crank in the shaft, which crank moves the air pump bucket up and down as the shaft revolves.

119. Q.--Will you give an example of a paddle oscillating engine?

A.--I will take as an example the oscillating engines constructed by Messrs. Ravenhill & Salked, for the Holyhead Packets. Fig. 27 is a longitudinal section of this vessel, showing an engine and boiler; and fig. 28 is a transverse section of one of the engines, showing also one of the wheels. There are two cylinders in this vessel, and one air pump, which lies in an inclined position, and is worked by a crank in the shaft which stretches between the cylinders, and which is called the intermediate shaft. A A, is one of the cylinders, B B the piston rod, and C C the crank. D is the crank in the intermediate shaft, which works the air pump E. There are double eccentrics fixed on the shaft, whereby the movement of the slide valves is regulated. The purpose of the double eccentrics is to enable an improved arrangement of valve gear to be employed, which is denominated the link motion, and which will be described hereafter. I I are the steam pipes leading to the steam trunnions K K, on which, and on the eduction trunnions connected with the pipe M, the cylinders oscillate.

120. Q.--By what species of mechanism are the positions of the paddle floats of feathering wheels governed?

A.--The floats are supported by spurs projecting from the rim of the wheel, and they may be moved upon the points of the spurs, to which they are attached by pins, by means of short levers proceeding from the backs of the floats, and connected to rods which proceed towards the centre of the wheel. The centre, however, to which these rods proceed is not concentric with the wheel, and the rods, therefore, are moved in and out as the wheel revolves, and impart a corresponding motion to the floats. In some feathering wheels the proper motion is given to the rods by means of an eccentric on the ship's side. The action of paddle wheels, whether radial or feathering, will be more fully described in the chapter on Steam Navigation.