A Catechism of the Steam Engine Part 6

102. _Q._--By what expedient is the piston rod enabled to pa.s.s through the cylinder cover without leaking steam out of the cylinder or air into it?

_A._--The hole in the cylinder lid, through which the piston rod pa.s.ses, is furnished with a recess called a stuffing box, into which a stuffing or packing of plaited hemp is forced, which, pressing on the one side against the interior of the stuffing box, and on the other side against the piston rod, which is smooth and polished, prevents any leakage in this situation.

The packing of this stuffing box is forced down by a ring of metal tightened by screws. This ring, which accurately fits the piston rod, has a projecting f.l.a.n.g.e, through which bolts pa.s.s for tightening the ring down upon the packing; and a similar expedient is employed in nearly every case in which packing is employed.

103. _Q._--In what way is the piston rod connected to the great beam?

_A._--The piston rod is connected to the great beam by means of two links, one at each side of the beam shown at _f g_, (fig. 21.) These links are usually made of the same length as the crank, and their purpose is to enable the end of the great beam to move in the arc of a circle while the piston rod maintains the vertical position. The point of junction, therefore, of the links and the piston rod is of the form of a knuckle or bend at some parts of the stroke.

104. _Q._--But what compels the top of the piston rod to maintain the vertical position?

_A._--Some engines have guide rods set on each side of the piston rod, and eyes on the top of the piston rod engage these guide rods, and maintain the piston rod in a vertical position in every part of the stroke. More commonly, however, the desired end is attained by means of a contrivance called the parallel motion.

105. _Q._--What is the parallel motion?

_A._--The parallel motion is an arrangement of jointed rods, so connected together that the divergence from the vertical line at any point in the arc described by the beam is corrected by an equal and opposite divergence due to the arc performed by the jointed rods during the stroke; and as these opposite deviations mutually correct one another, the result is that the piston rod moves in a vertical direction.

106. _Q._--Will you explain the action more in detail?

_A._--The pin, fig 21, which pa.s.ses through the end of the beam at _f_ has a link _f g_ hung on each side of the beam, and a short cross bar, called a cross head, extends from the bottom of one of these links to the bottom of the other, which cross head is perforated with a hole in the middle for the reception of the piston rod. There are similar links _b d_ at the point of the main beam, where the air pump rod is attached. There are two rods _d g_ connecting the links _b d_ with the links _f g_, and these rods, as they always continue parallel to the main beam throughout the stroke, are called _parallel bars_. Attached to the end of these two rods at _d_ are two other rods _c d_, of which the ends at _c_ are attached to stationary pins, while the ends at _d_ follow the motion of the lower ends of the links _b d_.

These rods are called the _radius bars_. Now it is obvious that the arc described by the point _d_, with _c_ as a centre, is opposite to the arc described by the point _g_ with _d_ as a centre. The rod _d g_ is, therefore, drawn back horizontally by the arc described at _d_ to an extent equal to the versed sine of the arc described at _g_, or, in other words, the line described by the point _g_ becomes a straight line instead of a curve.

[Ill.u.s.tration: Fig. 21.]

107. _Q._--Does the air pump rod move vertically as well as the piston rod?

_A._--It does. The air pump rod is suspended from a cross head, pa.s.sing from the centre of one of the links _b d_ to the centre of the other link, on the opposite side of the beam. Now, as the distance from the central axis of the great beam to the point _b_ is equal to the length of the rod _c d_, it will follow that the upper end of the link will follow one arc, and the lower end an equal and opposite arc. A point in the centre of the link, therefore, where these opposite motions meet, will follow no arc at all, but will move up and down vertically in a straight line.

108. _Q._--The use of the crank is to obtain a circular motion from a reciprocating motion?

_A._--That is the object of it, and it accomplishes its object in a very perfect manner, as it gradually arrests the velocity of the piston towards the end of the stroke, and thus obviates what would otherwise be an injurious shock upon the machine. When the crank approaches the lowest part of its throw, and at the same time the piston is approaching the top of the cylinder, the motion of the crank becomes nearly horizontal, or, in other words, the piston is only advanced through a very short distance, for any given distance measured on the circle described by the crank pin. Since, then, the velocity of rotation of the crank is nearly uniform, it will follow that the piston will move very slowly as it approaches the end of the stroke; and the piston is brought to a state of rest by this gradually r.e.t.a.r.ded motion, both at the top and the bottom of the stroke.

109. _Q._--What causes the crank to revolve at a uniform velocity?

_A._--The momentum of the machinery moved by the piston, but more especially of the fly wheel, which by its operation redresses the unequal pressures communicated by the crank, and compels the crank shaft to revolve at a nearly uniform velocity. Everyone knows that a heavy wheel if put into rapid rotation cannot be immediately stopped. At the beginning and end of the stroke when the crank is vertical, no force of torsion can be exerted on the crank shaft by the crank, but this force is at its maximum when the crank is horizontal. From the vertical point, where this force is nothing, to the horizontal point, where it is at its maximum, the force of torsion exerted on the crank shaft is constantly varying; and the fly wheel by its momentum redresses these irregularities, and carries the crank through that "dead point," as it is termed, where the piston cannot impart any rotative force.

110. _Q._--Are the configuration and structure of the steam engine, as it left the hand of Watt, materially different from those of modern engines?

_A._--There is not much difference. In modern rotative land engines, the valves for admitting the steam to the cylinder or condenser, instead of being clack or pot-lid valves moved by tappets on the air pump rod, are usually sluice or sliding valves, moved by an eccentric wheel on the crank shaft. Sometimes the beam is discarded altogether, and malleable iron is more largely used in the construction of engines instead of the cast iron, which formerly so largely prevailed. But upon the whole the steam engine of the present day is substantially the engine of Watt; and he who perfectly understands the operation of Watt's engine, will have no difficulty in understanding the operation of any of the numerous varieties of engines since introduced.

THE MARINE ENGINE.

111. _Q._--Will you describe the princ.i.p.al 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 cla.s.s of engines there are many distinct varieties.

112. _Q._--What are the princ.i.p.al varieties of the paddle engine?

[Ill.u.s.tration: Fig. 22.]

[Ill.u.s.tration: Fig. 23.]

_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.

[Ill.u.s.tration: Fig. 24.]

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.

[Ill.u.s.tration: Fig. 25.]

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 ill.u.s.trate this general description by an example?

[Ill.u.s.tration: Fig. 26.]

_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 pa.s.sing 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_, pa.s.ses 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 acc.u.mulated 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 c.o.c.k 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.

SCREW ENGINES.