The Steam Engine Explained and Illustrated (Seventh Edition) / With an Account of Its Invention and Progressive Improvement, and Its Application to Navigation and Railways; Including Also a Memoir of Watt

by Dionysius Lardner

(119.)

To explain the principle of the mechanism called the parallel motion, let us suppose that O P ([fig. 36.]) is a rod or lever moveable on a centre O, and that the end P of this rod shall move through a circular arch P P′ P″ P‴ a vertical plane, and let its play be limited by two stops S, which shall prevent its ascent above the point P, and its descent below [Pg196] the point P‴. Let the position of the rod and the limitation of its play be such that the straight line A B drawn through P and P‴, the extreme positions of the lever O P, shall be a vertical line.

Fig. 36.

Let o be a point on the other side of the vertical line A B, and let the distance of O to the right of A B be the same as the distance of o to the left of A B. Let o p be a rod equal in length to O P, moving like O P on the centre o, so that its [Pg197] extremity p shall play upwards and downwards through the arch p p′ p″ p‴, its play being limited in like manner by stops s.

Now, let us suppose that the ends P p of these two rods are joined by a link P p, the connection being made by a pivot, so that the angles formed by the link and the rods shall be capable of changing their magnitude. This link will make the motion of one rod depend on that of the other, since it will preserve their extremities P p always at the same distance from each other. If, therefore, we suppose the rod O P to be moved to the position O P‴, its extremity P tracing the arch P P′ P″ P‴, the link connecting the rods will at the same time drive the extremity p of the rod o p through the arch p p′ p″ p‴ so that when the extremity of the one rod arrives at P‴, the extremity of the other rod will arrive at p‴. By this arrangement, in the simultaneous motion of the rods, whether upwards or downwards, through the circular arches to which their play is limited, the extremities of the link joining them will deviate from the vertical line A B in opposite directions. At the limits of their play, the extremities of the link will always be in the line A B; but in all intermediate positions, the lower extremity of the link will be to the right of A B, and its upper extremity to the left of A B. So far as the derangement of the lower extremity of the link is concerned, the matter composing the link would be transferred to the right of A B, and so far as the upper extremity of the link is concerned, the matter composing it would be transferred to the left of A B.

By the combined effects of these contrary derangements of the extremities of the link from the vertical line, it might be expected that a point would exist, in the middle of the link, where the two contrary derangements would neutralise each other, and which point would therefore be expected to be disturbed neither to the right nor to the left, but to be moved upwards and downwards in the vertical line A B. Such is the principle of the parallel motion; and in fact the middle point of the link will move for all practical purposes accurately in the vertical line A B, provided that the angular play of the levers O P and o p does not exceed a certain [Pg198] limit, within which, in practice, their motion may always be restrained.

To trace the motion of the middle point of the link more minutely, let P P′ P″ P‴ be four positions of the lever O P, and let p p′ p″ p‴ be the four corresponding positions of the lever o p. In the positions O P o p, the link will take the position P p, in which the entire link will be vertical, and its middle point x will therefore be in the vertical line A B.

When the one rod takes the position O P′, the other rod will have the position o p′; and the link will have the position P′ p′. The middle point of the link will be at x′, which will be found to be on the vertical line A B. Thus one half of the link P′ x′ will be to the left of the vertical line A B; while the other half, p′ x′, will be to the right of the vertical line; the derangement from the vertical line affecting each half of the link in contrary directions.

Again, taking the one rod in the position O P″, the corresponding position of the other rod will be o p″, and the position of the link will be P″ p″. If the middle point of the link in this position be taken, it will be found to be at x″, on the vertical line A B; and, as before, one half of the link P″ x″ will be thrown to the left of the vertical line, while the other half p″ x″, will be thrown to the right of the vertical line.

Finally, let the one rod be in its lowest position, O P‴, while the other rod shall take the corresponding position, o p‴. The direction of the link P‴ p‴ will now coincide with the vertical line; and its middle point x‴ will therefore be upon that line. The previous derangement of the extremities of the rod, to the right and to the left, are now redressed, and all the parts of the rod have assumed the vertical position.

It is plain, therefore, that by such means the alternate motion of a point such as P or p, upwards and downwards in a circular arch, may be made to produce the alternate motions of another point x, upwards and downwards in a straight line.

(120.)

Fig. 37.

Let the lever represented by O P in [fig. 36.] be conceived to be prolonged to twice its length, as represented in [fig. 37.], so that O P′ shall be twice O P. Let the points P p be connected by a link as before. Let a link P′ x′, equal in length to the link P p be attached to the point P′, and let the extremity x′ of this link be connected with the point p by another link, equal in length to P P′, by pivots at x′ and p, so that the figure P P′ x′ p shall be a jointed parallelogram, the angles of which will be capable of altering their magnitude with every change of position of the rods o p and O P. Thus, when the rod O P descends, the angles of the parallelogram at P and x′ will be diminished in magnitude, while the angles at P′ and p will be increased in magnitude. Now, let a line be conceived to be drawn from O to x′. It is evident that that line will pass through the middle point of the link p P, for the triangle O P x is in all respects similar to the greater triangle O P′ x′ only on half the scale, so that every side of the one is [Pg200] half the corresponding side of the other. Therefore P x is half the length of P′ x′; but P′ x′ was made equal to P p, and therefore p x is half of P p, that is to say, x is the middle point of P p.

It has been already shown, that in the alternate motion of the rods o p, O P in ascending and descending, the point x is moved upwards and downwards in a true vertical line. Now since the triangle O P x is in all respects similar to O P′ x′, and subject to a similar motion during the ascent and descent of the rods, it is apparent that the point x′ must be subject to a motion in all respects similar to that which affects the points x, except that the point x′ will move through double the space. In fact, the principle of the mechanism is precisely similar to that of the common pantograph, where two rods are so connected as that the motion of the one governs the motion of the other, so that whatever line or figure may be described by one, a similar line or figure must be described by the other. Since, then, the point x is moved upwards and downwards in a vertical straight line, the point x′ will also be moved in a vertical straight line of double the length.

If such an arrangement of mechanism as has been here described can be connected with the beam of the steam engine, so that while the point x′ is attached to the top of the steam piston, and the space through which it ascends and descends shall be equal to the length of the stroke of that piston, the point x shall be attached to the rod of the air-pump piston, the stroke of the latter being half that of the steam piston, then the points x′ and x will guide the motion of the two pistons so as to preserve them in true vertical straight lines.

The manner in which these ideas are reduced to practice admits of easy explanation: let the point O be the centre of the great working beam, and let O P′ be the arm of the beam on the side of the steam cylinder. Let P be a pivot upon the beam, at the middle point between its centre O and its extremity P′; and let the links P p, P′ x′, and P p be jointed together, as already described. Let the point or pivot o be attached to some part of the fixed framing of the engine or engine house, and let the rod o p, equal to half the arm of the beam, be attached by a pivot to the corner of the parallelogram at [Pg201] p. Let the end of the steam piston-rod be attached to the corner of the parallelogram x′, and let the end of the air-pump be attached to the middle point x of the link P p; by which arrangement it is evident that the rectilinear motion of the two piston-rods will be rendered compatible with the alternate circular motions of the points P′ and P on the beam.

Among the many mechanical inventions produced by the fertile genius of Watt, there is none which has excited such universal, such unqualified, and such merited admiration as that of the parallel motion. It is indeed impossible, even for an eye unaccustomed to view mechanical combinations, to behold the beam of a steam engine moving the pistons, through the instrumentality of the parallel motion, without an instinctive feeling of pleasure at the unexpected fulfilment of an end by means having so little apparent connection with it. When this feeling was expressed to Watt himself, by those who first beheld the performance of this exquisite mechanism, he exclaimed with his usual vivacity, that he himself, when he first beheld his own contrivance in action, was affected by the same sense of pleasure and surprise at its regularity and precision. He said, that he received from it the same species of enjoyment that usually accompanies the first view of the successful invention of another person.

"Among the parts composing the steam engine, you have doubtless," says M. Arago, "observed a certain articulated parallelogram. At each ascent and descent of the piston, its angles open and close with the sweetness—I had almost said with the grace—which charms you in the gestures of a consummate actor. Follow with your eye alternately the progress of its successive changes, and you will find them subject to the most curious geometrical conditions. You will see, that of the four angles of the jointed parallelogram, three describe circular arches, but the fourth which holds the piston-rod is moved nearly in a straight line. The immense utility of this result strikes mechanicians with even less force than the simplicity of the means by which Watt has attained it."

The parallel motion, of which there are several other varieties, depending, however, generally upon the same [Pg202] principle, formed part of a patent which Mr. Watt obtained in the year 1784, another part of which patent was for a locomotive engine, by which a carriage was to be propelled on a road. In a letter to Mr. Smeaton dated 22d October, in the same year, Watt says,—

"I have lately contrived several methods of getting entirely rid of all the chains and circular arches about the great levers of steam engines, and nevertheless making the piston-rods ascend and descend perpendicularly, without any sliding motions or right-lined guides, merely by combinations of motions about centres; and with this further advantage, that they answer equally well to push upwards as to pull downwards, so that this method is applicable to our double engines which act both in the ascent and descent of their pistons.

"A rotative engine of this species with the new motion which is now at work in our manufactory (but must be sent away very soon) answers admirably. It has cost much brain work to contrive proper working gear for these double engines, but I have at last done it tolerably well, by means of the circular valves, placed in an inverted position, so as to be opened by the force of the steam; and they are kept shut by the working gear. We have erected an engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield, London."

Fig. 38.