(127.)

Having thus explained the principal mechanical contrivances provided by Watt for the maintenance and regulation of the rotatory motion to be produced by his double-acting steam engine, let us now consider the machine as a whole, and investigate the process of its operation. A section of this engine is represented in fig. 43.

Fig. 43.

Steam is supplied from the boiler to the cylinder by the steam pipe S. The throttle-valve T in that pipe, near the cylinder, is regulated by a system of levers connected with [Pg217] the governor. The piston P is accurately fitted in the steam cylinder C by packing, as already described in the single-acting engine. This piston, as it moves, divides the cylinder into two compartments, between which there is no communication by which steam or any other elastic fluid can pass. The upper steam box B is divided into three compartments by the two valves. Above the upper steam valve V is a compartment communicating with the steam pipe; below the upper exhausting valve E is another compartment communicating with the eduction pipe which leads to the condenser. By the valves V and E a communication may be opened or closed between the boiler on the one hand, or the condenser on the other, and the top of the cylinder. The continuation S′ of the steam pipe leads to the lower box B′, which, like the upper, is divided into three compartments by two valves V′ and E′. The upper compartment communicates with the steam pipe, and thereby with the boiler; and the lower compartment communicates with the eduction pipe, and thereby with the condenser. By means of the two valves V′ and E′, a communication may be opened or closed between the steam pipe on the one hand, or the exhausting pipe on the other, and the lower part of the cylinder. The four valves V, E, V′, and E′ are connected by a system of levers with a handle or spanner m, which, being driven downwards or upwards, is capable of opening or closing the valves in pairs, in the manner already described (116.). The condensers, the air-pump, and the hot-water pump, are in all respects similar to those already described in the single-acting engine, except that the condensing jet is governed by a lever I, by which it is allowed to play continually in the condenser, and by which the quantity of water admitted through it is regulated. The cold-water pump N is worked by the engine as already described in the single-acting engine, and supplies the cistern in which the air-pump and condenser are submerged, so as to keep down its temperature to the proper limit. On the air-pump rod R are two pins properly placed, so as to strike the spanner m, upwards and downwards, at the proper times, when the piston approaches the termination of the stroke at the top or bottom of the cylinder. The pump L [Pg218] conducts the warm water drawn by the air-pump from the condenser to a proper reservoir for feeding the boiler. The vertical motion of the piston-rod in a straight line is rendered compatible with the circular motion of the end of the beam by the parallel motion already described. The point b, on the beam, moves upwards and downwards in a circular arch, of which the axis of the beam is the centre. In like manner the point d of the rod d c moves upwards and downwards, in a similar arch of which the fixed pivot c is the centre. The joint or bar d b, which joins these two pivots, will be moved so that its middle point e will ascend and descend nearly in a straight line, as has been already explained (120.); [Pg219] opposite this point e is attached the piston-rod of the air-pump, which is accordingly guided upwards and downwards by this means. The jointed parallelogram b d g f is attached to the beam by pivots; and, as has been explained (120.), the point g will be moved upwards and downwards in a straight line, through twice the space through which the point e is moved. To the point g the rod of the steam piston is attached. Thus, the rods of the steam piston and air-pump are moved by the same system of jointed bars, and moved through spaces which are in the proportion of two to one.

Although this system of jointed rods forming the parallel motion, appears in the figure to consist only of one parallelogram b d g f, and one rod c d, called the radius rod, it is, in fact, double, a similar parallelogram and radius rod being attached to corresponding points, and in the same manner on the other side of the beam; but from the view given in the cut, the one set of rods hides the other. The two systems of rods thus attached to opposite sides of the beam at several inches asunder, are connected by cross rods, the ends of which form the pivots or joints, and extend between the parallelograms. The ends of these rods are only visible in the figure. It is to the middle of one of these rods, the end of which is represented at e, that the air-pump piston-rod is attached; and it is to the middle of another, the end of which is represented at g, that the steam piston-rod is attached. These two piston-rods, therefore, are driven, not immediately by either of the parallelograms forming the parallel motion, but by the bars extending between them.

To the working end of the beam H is attached a rod of cast-iron O, called the connecting rod, the lower end of which is attached to the crank by a pivot. The weight of the connecting rod is so made, that it shall balance the weight of the piston-rods of the air-pump and cylinder on the other side of the beam; and the weight of the piston-rod of the cold-water pump N nearly balances the weight of the piston-rod of the hot-water pump L. Thus, so far as the weights of the machinery are concerned, the engine is in equilibrium, and the piston would rest in any position indifferently in the cylinder.

The axis of the fly-wheel on which the crank is formed is [Pg220] square in the middle part, where the fly-wheel is attached to it, but has cylindrical necks at each end, which rest in sockets or bearings supported by the framing of the machine, in which sockets the axis revolves freely. On the axle of the crank is placed the fly-wheel, and connected with its axle is the governor Q, which regulates the throttle-valve T in the manner already described.

Let us now suppose the engine to be in full operation. The piston being at the top of the cylinder, the spanner m will be raised by the lower pin on the air-pump rod, and the upper steam valve V, and the lower exhausting valve E′, will be opened, while the upper exhausting valve E and the lower steam valve V′ are closed. Steam will, therefore, be admitted above the piston, and the steam which filled the cylinder below it will be drawn off to the condenser, where it will be converted into water. The piston will, therefore, be urged by the pressure of the steam above it to the bottom of the cylinder. As it approaches that limit, the spanner m will be struck downwards by the upper pin on the air-pump rod, and the valves V and E′ will be closed, and at the same time the lower steam valve V′ and the upper exhausting valve E will be opened. Steam will, therefore, be admitted below the piston, while the steam above it will be drawn off into the condenser, and converted into water. The pressure of the steam, therefore, below the piston will urge it upwards, and in the same manner the motion will be continued.

While this process is going on in the cylinder and the condenser, the water formed in the condenser will be gradually drawn off by the operation of the air-pump piston, in the same manner as explained in the single-acting engine; and at the same time the hot water thrown into the hot well by the air-pump piston will be carried off by the hot-water pump L.

Such are the chief circumstances attending the continuance of the operation of the double-acting engine. It is only necessary here to recall what has been already explained respecting the operation of the fly-wheel. The commencement of the motion of the piston from the top and bottom of the cylinder is produced, not by the pressure of the steam upon it upwards or downwards, which must, for the reasons [Pg221] already explained, be entirely inefficient; but by the momentum of the fly-wheel, which extricates the crank from those positions in which the moving power cannot affect it.

The manner in which the motion of the crank affects the connecting rod at the dead points produces an effect of great importance in the operation of the engine. When the crank-pin is approaching the lowest point of its play, and therefore the piston approaching the top of the cylinder, the motion of the crank-pin becomes nearly horizontal, and consequently its effect in drawing the connecting rod and the working end of the beam downwards and the piston upwards, is extremely small. The consequence of this is, that as the piston approaches the top of the cylinder, its motion becomes very rapidly retarded; and as the motion of the crank-pin at its lowest point is actually horizontal, the piston is brought to a state of rest by this gradually retarded motion at the top of the cylinder. In like manner, when the crank-pin moves from its dead point upwards, its motion at first is very nearly horizontal, and consequently its effect in driving the working end of the beam upwards, and the piston downwards, is at first very small, but gradually accelerated. The effect of this upon the piston is, that it arrives at and departs from the top of the stroke with a very slow motion, being absolutely brought to rest at that point.

The same effect is produced when the piston arrives at the bottom of the cylinder. This retardation and suspension of the motion of the piston at the termination of the stroke affords time for the process of condensation to be effected, so that when the moving power of the steam upon the piston can come into action, the condensation shall be sufficiently complete. As the piston approaches the top of the cylinder, and its motion becomes slow, the working gear is made to open the lower exhausting valve; the steam enclosed in the cylinder below the piston, and which has just driven the piston upwards, presses with an elastic force of 17 lbs. per square inch on every part of the interior of the cylinder, while the uncondensed vapour in the condenser presses with a force of about 2 lbs. per square inch. The steam, therefore, will have a tendency to rush from the cylinder to the [Pg222] condenser through the open exhausting valve, with an excess of pressure amounting to 15 lbs. per square inch, while the piston pauses at the top of the cylinder. This process goes on, and when the piston has descended by the motion of the fly-wheel, a sufficient distance from the top of the cylinder to call the moving force of the steam into action, the exhaustion will be complete, and the pressure of the uncondensed vapour in the cylinder will become the same as in the condenser.

The pressure of steam in the cylinder, and of uncondensed vapour in the condenser, varies, within certain limits, in different engines, and therefore the amount here assigned to them must be taken merely as an example.

The size of the valves by which the steam is allowed to pass from the cylinder to the condenser should be such as to cause the condensation to take place in a sufficiently short time, to be completed when the steam impelling the piston is called into action.

Watt, in the construction of his engines, made the exhaustion-valves with a diameter which was one fifth of the diameter of the cylinder, and therefore the actual magnitude of the aperture for the escape of the steam was one twenty-fifth of the magnitude of the cylinder; but the spindle of the valve diminished this so that the available space for the escape of steam did not exceed one twenty-seventh of the magnitude of the cylinder. This was found to produce a sufficiently rapid condensation.

It was usual to make the steam valves of the same magnitude as the exhausting valves, but the flow of steam through the former was resisted by the throttle-valve, while no obstruction was opposed to its passage through the latter.

The rapidity with which the cylinder must be exhausted by the condenser will, however, depend upon the velocity with which the piston is moved in it. The magnitude, therefore, of the exhausting valves which would be sufficient for an engine which acts with a slow motion would be too small where a rapid motion is required.

In the single-acting steam engine, where the moving force always acted downwards on the piston, the pressure upon [Pg223] all the joints of the machinery by which the force of the piston was conveyed to the working parts, always took place in the same direction, and consequently whatever might be the mechanical connection by which the several joints were formed, the pins by which they were connected, must always come to a bearing in their respective sockets, however loosely they may have been fitted. For the same reason, however, that the arch head and chain were abandoned as a means of connecting the steam piston with the beam, and the parallel motion substituted, it was also necessary in the double-acting engine, where all joints whatever were driven alternately in opposite directions, to fit the connecting pins with the greatest accuracy in their sockets, and to abandon all connection of the parts by chains. If any sensible looseness was left in the joints, a violent jerk would be produced every time the motion of the piston was reversed. Any looseness either in the pivots or joints of the parallel motion of the working beam, the connecting rod, or crank, would, at every change of stroke, be so accumulated as to produce upon the machinery the effects of percussion, and would consequently be attended with the danger of straining and breaking the moveable parts of the mechanism.

To secure, therefore, the necessary accuracy of the joints, Watt contrived that every joint in the engine should admit of the size of the socket being exactly adapted to the size of the pin, so as always to make a good fitting by closing the socket upon the pin, when any looseness would be produced by wear. With this view, all the joints were fitted with sockets made of brass or gun-metal, capable of adjustment. Each socket was composed of two pieces, accurately fitted into a cell or groove, in which one of the brasses can be moved towards the other by means of a wedge or screw. Each brass has in it a semi-cylindrical cavity, and the two cavities being opposed to each other, form a socket for the joint-pin. One of the two brasses can always be tightened round that pin, so as to enclose it tight between the two semi-cylindrical cavities, and to prevent any looseness taking place. The brasses, and other parts of such a joint, are represented [Pg224] in fig. 44. These joints still continue to be used in the engines as now constructed.

Fig. 44.

The motion of the working beam, and the pump-rods which it drives, and of the connecting rod, ought, if the whole were constructed with perfect precision, to take place in the same or parallel vertical planes; but this supposes a perfection of execution which could hardly have been expected in the early manufacture of such engines, whatever may have been attained by improvements which have been since made. In the details of construction, Watt saw that there would be a liability to lateral strain, owing to the planes of the different motions not being truly vertical and truly parallel, and that if a provision were not made for such lateral motion, the machinery would be subject to constant strain in its joints and rapid wear. He provided against this by constructing the main joints by which the great working lever was connected with the pistons and connecting rod, so as to form universal joints, giving freedom of motion laterally as well as vertically.

The great lever, or working beam, was so called from being originally made from a beam of oak. It is now, however, universally constructed of cast-iron. The connecting rod is also made of cast-iron, and attached to the beam and to the crank by axles or pivots.

The mechanism by which the four valves are opened and closed, is subject to considerable variation in different engines. They have been described above as being opened and closed simultaneously by a single lever. Sometimes, however, they are opened alternately in pairs by two distinct levers driven by two pins attached to the air-pump rod. One pin strikes the lever, which opens and closes the upper steam valve, and lower exhausting valve; the other strikes that which opens and closes the lower steam valve and upper exhausting valve.

Since the date of the earlier double-acting engines, constructed by Boulton and Watt, a great variety of mechanical expedients have been practised for working the valves, by which the steam is admitted to and withdrawn from the [Pg225] cylinder. We shall here describe a few of these methods:—