The steam being admitted to the steam-pipe passes through it to the front end of the boiler, and the pipe being enclosed within the boiler the temperature of the steam is maintained. The steam-pipe passing through the tube-plate at the front end of the boiler is carried to a small distance from the tube-plate in the same direction, where it is flanged on to a cross horizontal pipe proceeding to the right and to the left as represented in [fig. 104.] This cross pipe is itself flanged to two curved steam-pipes S ([fig. 104.]), by which the steam is conducted to the valve-boxes V V. The lower ends of these curved arms are flanged on to the valve-boxes of the two cylinders [Pg373] at the ends nearest to the boiler. The opening of one of these is exhibited in the right hand cylinder in [fig. 99.] By these pipes the steam is conducted into the valve-boxes or steam-chests, from which it is admitted by slide-valves to the cylinders to work the pistons in the same manner as has been already described in the large stationary engines.
On the upper sides of the cylinders are formed the steam-chests or valve-boxes, which are exhibited at U (figs. 97. 99. 104.). These are made of cast-iron half an inch thick, and are bolted to the upper side of each cylinder. At the front end they are also secured by bolts to the smoke-box, and at the hinder end are attached to the tube-plate. These valve-boxes communicate with the passages m and n [fig. 99.] leading to the top and bottom of the cylinder: these are called the steam-ports. They also communicate with a passage o leading to the mouth of a curved horizontal pipe p′ connecting the front ends of the two cylinders, as seen in figs. 99. 104. These curved pipes unite in a single vertical pipe p, called the blast-pipe, seen in figs. 97. 104.: this vertical pipe becomes gradually small towards the top, and terminates a little above the base of the funnel or chimney G. In the valve-box is placed the slide-valve v to which is attached the spindle l′. This spindle moves through a stuffing-box k′, and is worked by gearing, which will be described hereafter. According to the position given to the slide, a communication may be opened between the steam-chest, or the waste-port, and either end of the cylinders. Thus when the slide is in the position represented in [fig. 97.] the steam-chest communicates with the front end of the cylinder, while the waste-port communicates with the hinder end. If, on the other hand, the spindle l′ being pressed forward, move the slide to its extreme opposite position, the steam-port n would communicate with the waste-port o, while the steam-chest would communicate with the steam-port m, steam would, therefore, be admitted to the hinder end of the cylinder, while the foremost end would communicate with the waste-port. It will be perceived that this arrangement is precisely similar to that of the slide-valves already described ([133].). The slide-valve is represented on a larger scale in [fig. 106.], where A is the hinder steam-port, [Pg374] B the foremost steam-port, and C the waste-port. The surfaces D, separating the steam-ports from the waste-ports, are called the bars: they are planed perfectly smooth, so that the surfaces F and G of the slide-valve, also planed perfectly smooth, may move in steam-tight contact with them. These surfaces are kept in contact by the pressure of the steam in the steam-chest, by which the slide-valve is always pressed down. In its middle position, as represented by the dotted lines in the figure, both the steam-ports are stopped by the slide-valve, so that at that moment no steam is admitted to either end of the cylinder. On either side of this intermediate position the slide has an inch and a half play, which is sufficient to open successively the two steam-ports.
Fig. 106.
The cylinders are inserted at one end in the plate of the smoke-box, and at the other in the tube-plate of the boiler. They are closed at either end by cast iron covers, nearly an inch thick, flanged on by bolts and screws. In the cover of the cylinder attached to the tube-plate is a stuffing-box, in which the piston rod plays. The metallic pistons used in locomotive engines do not differ materially from those already described, and therefore need not be here particularly noticed. From their horizontal position they have a tendency to wear unequally in the cylinders, their weight pressing them on one side only; but from their small magnitude this effect is found to be imperceptible in practice. In the engine here described the stroke of the piston is eighteen inches, and this is the most usual length of stroke in locomotive engines. The piston, in its play, comes at either end within about half an inch of the inner surface of the covers of the cylinders, this space being allowed to prevent collision. In the foremost cover of the cylinder is inserted a cock q′ ([fig. 97.] [99.]), by which any water which may collect in the cylinder by condensation or priming may be discharged. A cock r′ ([fig. 97.]), communicating with a small tube proceeding from the branches of the waste pipe p′ ([fig. 104.]), is likewise provided to discharge from that pipe any water which may be [Pg375] collected in it. After the steam has been admitted to work the piston through the slide-valve, and has been discharged through the waste-port by shifting that valve, it passes through the pipe p′ into the blast-pipe p, from the mouth of which it issues, with great force, up the funnel G. When the motion of the engine is rapid, the steam from the two cylinders proceeds in an almost uninterrupted current from the blast-pipe, and causes a strong draft up the chimney. The heated air which passes from the mouths of the tubes into the smoke-box is drawn up by this current, and a corresponding draft is produced in the fire-box.
Fig. 107.
The piston-rods Y terminate in a fork, by which they are attached to cross heads Z, the ends of which are confined by guide-bars A′, in which they are allowed to play backwards and forwards through a space equal to the stroke of the piston. To these cross heads Z, between the prongs of the fork in which the piston terminates, are attached the foremost ends of the connecting rods B′. These rods are, therefore, driven backwards and forwards by the motion imparted to the cross head Z by the piston-rods Y. The connecting rods B′ are attached at the hinder ends to two cranks formed upon the axles C′ of the driving wheels D′. These two cranks are formed upon the axles precisely at right angles to each other. The left-hand crank is represented in its horizontal position, in [fig. 99.], and the right-hand crank is seen in its vertical position. A cranked axle is represented on a larger scale in [fig. 107.], and the two cranks are seen in a position oblique to the plane of the figure. As this axle is the instrument by which the impelling force is conveyed to the load, and as it has to support a great portion of the weight of the engine, it is constructed with great strength and precision. It is made all in one [Pg376] piece, and of the best wrought iron called Back Barrow, or scrap iron. In the engine here described its extreme length is six feet and a half, and its diameter is five inches. At the centre part A it is cylindrical, and is increased to five inches and a quarter at C, where the cranks are formed. The sides D of the cranks are four inches thick, and the crank pins B, which are truly cylindrical, are five inches diameter, and three inches in length, the brasses at the extremities of the connecting rods which play upon them having a corresponding magnitude. The distance from the centre of the crank-pins B to the centre of the axle A must be exactly equal to half the stroke of the piston, and is, therefore, in this case precisely nine inches. Upon the parts F, which are seven inches and a half long, the great driving wheels are firmly fastened, so as to be prevented from turning or shaking upon the axle. The axle projects beyond the wheels at G, where it is reduced to three inches and an eighth diameter. These projecting parts G are five inches long, having collars at the outer ends. Brasses are fixed at the outside frame of the engine which rest upon these projections G of the axle, and upon these brasses the weight of the engine is supported. The entire axle is accurately turned in a lathe, and each of the crank-pins B is likewise turned by suspending the axle on centres corresponding with the centres of the crank-pins, and made on strong cast iron arms, which are firmly fixed on the ends of the axle, and project beyond the cranks so as to balance the axle, and enable it to turn round on the centre of the crank-pin. The axle is by such means made perfectly true, and the cranks are made of exactly the proper length, and precisely at right angles to each other. The corners of the cranks are champered off, as shown in the figure, and the ends of the cylindrical parts well rounded out.