Part 9 (2/2)
The rack and sector patented at this time was soon superseded by the parallel-motion; and the last claim, the ”steam-wheel” or rotary engine, although one was built of considerable size, was not introduced.
After the patent of 1782 had been secured, Watt turned his attention, when not too hard-pressed by business, to other schemes, and to experimenting with still other modifications and applications of his engine. He had, as early as 1777, proposed to make a steam-hammer for Wilkinson's forge; but he was too closely engaged with more important matters to take hold of the project with much earnestness until late in the year 1782, when, after some preliminary trials, he reported, December 13th: ”We have tried our little tilting-forge hammer at Soho with success. The following are some of the particulars: Cylinder, 15 inches in diameter; 4 feet stroke; strokes per minute, 20. The hammer-head, 120 pounds weight, rises 8 inches, and strikes 240 blows per minute. The machine goes quite regularly, and can be managed as easily as a water-mill. It requires a very small quant.i.ty of steam--not above half the contents of the cylinder per stroke. The power employed is not more than one-fourth of what would be required to raise the quant.i.ty of water which would enable a water-wheel to work the same hammer with the same velocity.”
He immediately set about making a much heavier hammer, and on April 26, 1783, he wrote that he had done ”a thing never done before”--making his hammer strike 300 blows a minute. This hammer weighed 7-1/2 hundredweight, and had a drop of 2 feet. The steam-cylinder had a diameter of 42 inches and 6 feet stroke of piston, and was calculated to have sufficient power to drive four hammers weighing 7 hundredweight each. The engine made 20 strokes per minute, the hammer giving 90 blows in the same time.
This new application of steam-power proving successful, Watt next began to develop a series of minor inventions, which were finally secured by his patent of April 27, 1784, together with the steam tilt-hammer, and a steam-carriage, or ”locomotive engine.”
The contrivance previously used for guiding the head of the piston-rod--the sectors and chains, or rack--had never given satisfaction. The rudeness of design of the contrivance was only equalled by its insecurity. Watt therefore contrived a number of methods of accomplis.h.i.+ng the purpose, the most beautiful and widely-known of which is the ”parallel-motion,” although it has now been generally superseded by one of the other devices patented at the same time--the cross-head and guides. As originally proposed, a rod was attached to the head of the piston-rod, standing vertically when the latter was at quarter-stroke. The upper end of this rod was pivoted to the end of the beam, and the lower end to the extremity of a horizontal rod having a length equal to one-half the length of the beam. The other end of the horizontal rod was coupled to the frame of the engine. As the piston rose and fell, the upper and lower ends of the vertical rod were swayed in opposite directions, and to an equal extent, by the beam and the lower horizontal rod, the middle point at which the piston-rod was attached preserving its position in the vertical line. This form was objectionable, as the whole effort of the engine was transmitted through the parallel-motion rods. Another form is shown in the sketch given of the double-acting engine in Fig. 31, which was free from this defect. The head of the piston-rod, _g_, was guided by rods connecting it with the frame at _c_, and forming a ”parallelogram,” _g d e b_, with the beam. Many varieties of ”parallel-motion” have been devised since Watt's invention was attached to his engines at Soho. They usually are more or less imperfect, guiding the piston-rod in a line only approximately straight.
The cross-head and guides are now generally used, very much as described by Watt in this patent as his ”second principle.” This device will be seen in the engravings given hereafter of more modern engines. The head of the piston-rod is fitted into a transverse bar, or cross-head, which carries properly-shaped pieces at its extremities, to which are bolted ”gibs,” so made as to fit upon guides secured to the engine-frame. These guides are adjusted to precise parallelism with the centre line of the cylinder. The cross-head, sliding in or on these guides, moves in a perfectly straight line, and, compelling the piston-rod to move with it, the latter is even more perfectly guided than by a parallel-motion. This arrangement, where properly proportioned, is not necessarily subject to great friction, and is much more easily adjusted and kept in line than the parallel-motion when wear occurs or maladjustment takes place.
By the same patent, Watt secured the now common ”puppet-valve” with beveled seat, and the application of the steam-engine to driving rolling-mills and hammers for forges, and to ”wheel-carriages for removing persons or goods, or other matters, from place to place.” For the latter purpose he proposes to use boilers ”of wood, or of thin metal, strongly secured by hoops or otherwise,” and containing ”internal fire-boxes.” He proposed to use a condenser cooled by currents of air.
It would require too much s.p.a.ce to follow Watt in all his schemes for the improvement and for the application of the steam-engine. A few of the more important and more ingenious only can be described. Many of the contracts of Boulton & Watt gave them, as compensation for their engines, a fraction--usually one-third--of the value of the fuel saved by the use of the Watt engine in place of the engine of Newcomen, the amount due being paid annually or semiannually, with an option of redemption on the part of the purchaser at ten years' purchase. This form of agreement compelled a careful determination, often, of the work done and fuel consumed by both the engine taken out and that put in its place. It was impossible to rely upon any determination by personal observation of the number of strokes made by the engine. Watt therefore made a ”counter,” like that now familiar to every one as used on gas-meters. It consists of a train of wheels moving pointers on several dials, the first dial showing tens, the second hundreds, the third thousands, etc., strokes or revolutions. Motion was communicated to the train by means of a pendulum, the whole being mounted on the beam of the engine, where every vibration produced a swing of the pendulum. Eight dials were sometimes used, the counter being set and locked, and only opened once a year, when the time arrived for determining the work done during the preceding twelve-month.
The application of his engine to purposes for which careful adjustment of speed was requisite, or where the load was subject to considerable variation, led to the use of a controlling-valve in the steam-pipe, called the ”throttle-valve,” which was adjustable by hand, and permitted the supply of steam to the engine to be adjusted at any instant and altered to any desired extent. It is now given many forms, but it still is most usually made just as originally designed by Watt.
It consists of a circular disk, which just closes up the steam-pipe when set directly across it, or of an elliptical disk, which closes the pipe when standing at an angle of somewhat less than 90 with the line of the pipe. This disk is carried on a spindle extending through the pipe at one side, and carrying on its outer end an arm by means of which it may be turned into any position. When placed with its face in line with the pipe, it offers very little resistance to the flow of steam to the engine. When set in the other position, it shuts off steam entirely and stops the engine. It is placed in such position at any time, that the speed of the engine is just that required at the time. In the engraving of the double-acting engine with fly-wheel (Fig. 31), it is shown at _T_, as controlled by the governor.
[Ill.u.s.tration: FIG. 29.--The Governor.]
The governor, or ”fly-ball governor,” as it is often distinctively called, was another of Watt's minor but very essential inventions. Two heavy iron or bra.s.s b.a.l.l.s, _B B'_, were suspended from pins, _C C'_, in a little cross-piece carried on the head of a vertical spindle, _A A'_, driven by the engine. The speed of the engine varying, that of the spindle changed correspondingly, and the faster the b.a.l.l.s were swung the farther they separated. When the engine's speed decreased, the period of revolution of the b.a.l.l.s was increased, and they fell back toward the spindle. Whenever the velocity of the engine was uniform, the b.a.l.l.s preserved their distance from the spindle and remained at the same height, their alt.i.tude being determined by the relation existing between the force of gravity and centrifugal force in the temporary position of equilibrium. The distance from the point of suspension down to the level of the b.a.l.l.s is always equal to 9.78 inches divided by the square of the number of revolutions per second--i. e., _h_ = 9.78 (1/_N_^2) = 0.248 (1/_N_^2) meters.
The arms carrying the b.a.l.l.s, or the b.a.l.l.s themselves, are pinned to rods, _M M'_, which are connected to a piece, _N N'_, sliding loosely on the spindle. A score, _T_, cut in this piece engages a lever, _V_, and, as the b.a.l.l.s rise and fall, a rod, _W_, is moved, closing and opening the throttle-valve, and thus adjusting the supply of steam in such a way as to preserve a nearly fixed speed of engine. The connection with the throttle-valve and with the cut-off valve-gear is seen not only in the engraving of the double-acting Watt engine, but also in those of the Greene and the Corliss engines. This contrivance had previously been used in regulating water-wheels and windmills.
Watt's invention consisted in its application to the regulation of the steam-engine.
Still another useful invention of Watt's was his ”mercury steam-gauge”--a barometer in which the height of the mercury was determined by the pressure of the steam instead of that of the atmosphere. This simple instrument consisted merely of a bent tube containing a portion of mercury. One leg, _B D_, of this U-tube was connected with the steam-pipe, or with the boiler by a small steam-pipe; the other end, _C_, was open to the atmosphere. The pressure of the steam on the mercury in _B D_ caused it to rise in the other ”leg” to a height exactly proportioned to the pressure, and causing very nearly two inches difference of level to the pound, or one inch to the pound actual rise in the outer leg. The rude sketch from Farey, here given (Fig. 30), indicates sufficiently well the form of this gauge. It is still considered by engineers the most reliable of all forms of steam-gauge. Unfortunately, it is not conveniently applicable at high pressure. The scale, _A_, is marked with numbers indicating the pressure, which numbers are indicated by the head of a rod floating up with the mercury.
A similar gauge was used to determine the degree of perfection of vacuum attained in the condenser, the mercury falling in the outer leg as the vacuum became more complete. A perfect vacuum would cause a depression of level in that leg to 30 inches below the level of the mercury in the leg connected with the condenser. In a more usual form, it consisted of a simple gla.s.s tube having its lower end immersed in a cistern of mercury, as in the ordinary barometer, the top of the tube being connected with a pipe leading to the condenser. With a perfect vacuum in the condenser, the mercury would rise in the tube very nearly 30 inches. Ordinarily, the vacuum is not nearly perfect, and, a back pressure remaining in the condenser of one or two pounds per square inch, the atmospheric pressure remaining unbalanced is only sufficient to raise the mercury 26 or 28 inches above the level of the liquid metal in the cistern.
[Ill.u.s.tration: FIG. 30. Mercury Steam Gauge. Gla.s.s Water Gauge.]
To determine the height of water in his boiler, Watt added to the gauge-c.o.c.ks already long in use the ”gla.s.s water-gauge,” which is still seen in nearly every well-arranged boiler. This was a gla.s.s tube, _a a'_ (Fig. 30), mounted on a standard attached to the front of the boiler, and at such a height that its middle point was very little below the proposed water-level. It was connected by a small pipe, _r_, at the top to the steam-s.p.a.ce, and another little pipe, _r'_, led into the boiler from its lower end below the water-line. As the water rose and fell within the boiler, its level changed correspondingly in the gla.s.s. This little instrument is especially liked, because the position of the water is at all times shown to the eye of the attendant. If carefully protected against sudden changes of temperature, it answers perfectly well with even very high pressures.
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.”
[Ill.u.s.tration: FIG. 31.--Boulton & Watt's Double-Acting Engine, 1784.]
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 antic.i.p.ated 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 pa.s.sing through his apprentices.h.i.+p in Scotland, sending his chief a.s.sistant, 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 princ.i.p.al losers, the former losing 6,000, and the latter 3,000.
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 ill.u.s.tration (Fig. 32) represents this valve-motion as attached to the Albion Mills engine.
[Ill.u.s.tration: FIG. 32.--Valve-Gear of the Albion Mills Engine.]
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