The Steam Engine Familiarly Explained and Illustrated / With an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain axioms for railway speculators

by Dionysius Lardner

This contrivance, although in the main inferior to the more simple one of the crank, is not without some advantages; among others, it gives to the sun-wheel double the velocity which would be communicated by the simple crank, for in the simple crank one revolution only on the axle is produced by one revolution of the crank, but in the sun and planet-wheel two revolutions of the sun-wheel are produced by one of the planet-wheel; thus a double velocity is obtained from the same motion of the beam. This will be evident from considering that when the planet-wheel is in its highest position, its lowest tooth is engaged with the highest tooth of the sun-wheel; as the planet-wheel passes from the highest position, its teeth drive those of the sun-wheel before them, and when it comes into the lowest position, the highest tooth of the planet-wheel is engaged with the lowest of the sun-wheel: but then half of the sun-wheel has rolled off the planet-wheel, and, therefore, the tooth which was engaged with it in its higher position, must now be distant from it by half the circumference of the wheel, and must therefore be again in the highest position, so that while the planet-wheel has been carried from the top to the bottom, the sun-wheel has made a complete revolution. A little reflection, however, on the nature of the motion, will render this plainer than any description can. This advantage of giving an increased velocity, may be obtained also by the simple crank, by placing toothed wheels on its axle. Independently of the greater expense attending the construction of the sun and planet-wheel, its liability to go out of order, and the rapid wear of the teeth, and other objections, rendered it decidedly inferior to the crank, which has now entirely superseded it.

(58.) Whether the simple crank or the sun and planet wheel be used, there still remains a difficulty of a peculiar nature attending the continuance of the rotatory motion. There are two positions in which the engine can give no motion whatever to the crank. These are when the end of the beam, the axle of the crank, and the pivot which joins the connector with the crank, are in the same straight line. This will be easily understood. Suppose the beam, connector, and crank to assume the position represented in [fig. 15]. If steam urge the piston downwards, the point H and the connector H I will be drawn directly upwards. But it must be very evident that in the present situation of the connector H I, and the lever I K, the force which draws the point I in the direction I K can have no effect whatever in turning I K round the centre K, but will merely exert a pressure on the axle or pivots of the wheel.

Again, suppose the crank and connector to be in the position H I K ([fig. 16].), the piston being consequently at the bottom of the cylinder. If steam now press the piston upwards, the pivot H and the connector H I will be pressed downwards, and this pressure will urge the crank I K in the direction I K. It is evident that such a force cannot turn the crank round the centre K, and can be attended with no other effect than a pressure on the axle or pivots of the wheel.

Hence in these two positions, the engine can have no effect whatever in turning the crank. What, then, it may be asked, extricates the machine from this mechanical dilemma in which it is placed twice in every revolution, on arriving at those positions in which the crank escapes the influence of the power? There is a tendency in bodies, when once put in motion, to continue that motion until stopped by some opposing force, and this tendency carries the crank out of those two critical situations. The velocity which is given to it, while it is under the influence of the impelling force of the beam, is retained in a sufficient degree to carry it through that situation in which it is deserted by this impelling force. Although the rotatory motion intended to be produced by the crank is, therefore, not absolutely destroyed by this circumstance, yet it is rendered extremely irregular, since, in passing through the two positions already described, where the machine loses its power over the crank, the motion will be very slow, and, in the positions of the crank most remote from these, where the power of the beam upon it is greatest, the motion will be very quick. As the crank revolves from each of those positions where the power of the machine over it is greatest, to where that power is altogether lost, it is continually diminished, so that, in fact, the crank is driven by a varying power, and therefore produces a varying motion. This will be easily understood by considering the successive positions of the crank and connector represented in [fig. 16].

This variable motion becomes particularly objectionable when the engine is employed to drive machinery. To remove this defect, we have recourse to the property of bodies just mentioned, viz. their tendency to retain a motion which is communicated to them. A large metal wheel called a fly-wheel is placed upon the axis of the crank ([fig. 15].), and is turned by it. The effect of this wheel is to equalize the motion communicated by the action of the beam on the crank, that action being just sufficient to sustain in the fly-wheel a uniform velocity, and the tendency of this wheel to retain the velocity it receives, renders its rotation sufficiently uniform for all practical purposes.

This uniformity of motion, however, will only be preserved on two conditions; first, that the supply of steam from the boiler shall be uniform; and secondly, that the machine have always the same resistance to overcome or be loaded equally. If the supply of steam from the boiler to the cylinder be increased, the motion of the piston will be rendered more rapid, and, therefore, the revolution of the fly-wheel will also be more rapid, and, on the other hand, a diminished supply of steam will retard the fly-wheel. Again, if the resistance or load upon the engine be diminished, the supply of steam remaining the same, the velocity will be increased, since a less resistance is opposed to the energy of the moving power; and, on the other hand, if the resistance or load be increased, the speed will be diminished, since a greater resistance will be opposed to the same moving power. To insure a uniform velocity, in whatever manner the load or resistance may be changed, it is necessary to proportion the supply of steam to the resistance, so that, upon the least variation in the velocity, the supply of steam will be increased or diminished, so as to keep the engine going at the same rate.

(59.) One of the most striking and elegant appendages of the steam engine is the apparatus contrived by Watt for effecting this purpose. An apparatus, called a regulator or governor, had been long known to mill-wrights for rendering uniform the action of the stones in corn-mills, and was used generally in machinery. Mr. Watt contrived a beautiful application of this apparatus for the regulation of the steam engine. In the pipe which conducts steam from the boiler to the cylinder he placed a thin circular plate, so that when placed with its face presented towards the length of the pipe, it nearly stopped it, and allowed little or no steam to pass to the cylinder, but when its edge was placed in the direction of the pipe, it offered no resistance whatever to the passage of the steam. This circular plate, called the throttle valve, was made to turn on a diameter as an axis, passing consequently through the centre of the tube, and was worked by a lever outside the tube. According to the position given to it, it would permit more or less steam to pass. If the valve be placed with its edge nearly in the direction of the tube, the supply of steam is abundant; if it be placed with its face nearly in the direction of the tube, the supply of steam is more limited, and it appears that, by the position given to this valve, the steam may be measured in any quantity to the cylinder.

At first it was proposed that the engine-man should adjust this valve with his hand; when the engine was observed to increase its speed too much, he would check the supply of steam by partially closing the valve; but if, on the other hand, the motion was too slow, he would open the valve and let in a more abundant supply of steam. Watt, however, was not content with this, and desired to make the engine itself discharge this task with more steadiness and regularity than any attendant could, and for this purpose he applied the governor already alluded to.

This apparatus is represented in [fig. 15].; L is a perpendicular shaft or axle to which a wheel, M, with a groove is attached. A strap or rope, which is rolled upon the axle of the fly-wheel, is passed round the groove in the wheel M, in the same manner as the strap acts in a turning lathe. By means of this strap the rotation of the fly-wheel will produce a rotation of the wheel M and the shaft L, and the speed of the one will always increase or diminish in the same proportion as the speed of the other. N, N are two heavy balls of metal placed at the ends of rods, which play on an axis fixed on the revolving shaft at O, and extend beyond the axis to Q Q. Connected with these by joints at Q Q are two other rods, Q R, which are attached to a broad ring of metal, moving freely up and down the revolving shaft. This ring is attached to a lever whose centre is S, and is connected by a series of levers with the throttle-valve T. When the speed of the fly-wheel is much increased, the spindle L is whirled round with considerable rapidity, and by their natural tendency[20] the balls N N fly from the centre. The levers which play on the axis O, by this motion, diverge from each other, and thereby depress the joints Q Q, and draw down the joints R, and with them the ring of metal which slides upon the spindle. By these means the end of the lever playing on S is depressed, and the end V raised, and the motion is transmitted to the throttle-valve, which is thereby partially closed, and the supply of steam to the cylinder checked. If, on the contrary, the velocity of the fly-wheel be diminished, the balls will fall towards the axis, and the opposite effects ensuing, the supply of steam will be increased, and the velocity restored.