The engines built by Boulton & Watt were finally fitted with the crank and fly-wheel for application to the driving of mills and machinery. The accompanying engraving ([Fig. 31]) shows the engine as thus made, combining all of the essential improvements designed by its inventor.
In the engraving, C is the steam-cylinder, P the piston, connected to the beam by the link, g, and guided by the parallel-motion, g d c. At the opposite end of the beam a connecting-rod, O, connects with the crank and fly-wheel shaft. R is the rod of the air-pump, by means of which the condenser is kept from being flooded by the water used for condensation, which water-supply is regulated by an “injection-handle,” E. A pump-rod, N, leads down from the beam to the cold-water pump, by which water is raised from the well or other source to supply the needed injection-water. The air-pump rod also serves as a “plug-rod,” to work the valves, the pins at m and R striking the lever, m, at either end of the stroke. When the piston reaches the top of the cylinder, the lever, m, is raised, opening the steam-valve, B, at the top, and the exhaust-valve, E, at the bottom, and at the same time closing the exhaust at the top and the steam at the bottom. When the entrance of steam at the top and the removal of steam-pressure below the piston has driven the piston to the bottom, the pin, R, strikes the lever, m, opening the steam and closing the exhaust valve at the bottom, and similarly reversing the position of the valves at the top. The position of the valves is changed in this manner with every reversal of the motion of the piston as the crank “turns over the centre.”
The earliest engines of the double-acting kind, and of any considerable size, which were built to turn a shaft, were those which were set up in the Albion Mills, near Blackfriars’ Bridge, London, in 1786, and destroyed when the mills burned down in 1791. There were a pair of these engines (shown in [Fig. 27]), of 50 horse-power each, and geared to drive 20 pairs of stones, making fine flour and meal. Previous to the erection of this mill the power in all such establishments had been derived from windmills and water-wheels. This mill was erected by Boulton & Watt, and capitalists working with them, not only to secure the profit anticipated from locating a flour-mill in the city of London, but also with a view to exhibiting the capacity of the new double-acting “rotating” engine. The plan was proposed in 1783, and work was commenced in 1784; but the mill was not set in operation until the spring of 1786. The capacity of the mill was, in ordinary work, 16,000 bushels of wheat ground into fine flour per week. On one occasion, the mill turned out 3,000 bushels in 24 hours. In the construction of the machinery of the mill, many improvements upon the then standard practice were introduced, including cast-iron gearing with carefully-formed teeth and iron framing. It was here that John Rennie commenced his work, after passing through his apprenticeship in Scotland, sending his chief assistant, Ewart, to superintend the erection of the milling machinery. The mill was a success as a piece of engineering, but a serious loss was incurred by the capitalists engaged in the enterprise, as it was set on fire a few years afterward and entirely destroyed. Boulton and Watt were the principal losers, the former losing £6,000, and the latter £3,000.
Fig. 32.—Valve-Gear of the Albion Mills Engine.
The valve-gear of this engine, a view of which is given in [Fig. 27], was quite similar to that used on the Watt pumping-engine. The accompanying illustration ([Fig. 32]) represents this valve-motion as attached to the Albion Mills engine.
The steam-pipe, a b d d e, leads the steam from the boiler to the chambers, b and e. The exhaust-pipe, g g, leads from h and i to the condenser. In the sketch, the upper steam and the lower exhaust valves, b and f, are opened, and the steam-valve, e, and exhaust-valve, c, are closed, the piston being near the upper end of the cylinder and descending. l represents the plug-frame, which carries tappets, 2 and 3, which engage the lever, s, at either end of its throw, and turn the shaft, u, thus opening and closing c and e simultaneously by means of the connecting-links, 13 and 14. A similar pair of tappets on the opposite side of the plug-rod move the valves, b and f, by means of the rods, 10 and 11, the arm, r, when struck by those tappets, turning the shaft, t, and thus moving the arms to which those rods are attached. Counterbalance-weights, carried on the ends of the arms, 4 and 15, retain the valves on their seats when closed by the action of the tappets. When the piston nearly reaches the lower end of the cylinder, the tappet, 1, engages the arm, r, closing the steam-valve, b, and the next instant shutting the exhaust-valve, f. At the same time, the tappet, 3, by moving the arm, s, downward, opens the steam-valve, e, and the exhaust-valve, c. Steam now no longer issues from the steam-pipe into the space, c, and thence into the engine-cylinder (not shown in the sketch); but it now enters the engine through the valve, e, forcing the piston upwards. The exhaust is simultaneously made to occur at the upper end, the rejected steam passing from the engine into the space, c, and thence through c and the pipe, g, into the condenser.
This kind of valve-gear was subsequently greatly improved by Murdoch, Watt’s ingenious and efficient foreman, but it is now entirely superseded on engines of this class by the eccentric, and the various forms of valve-gear driven by it.