That part in a steam engine on which the elastic force of the steam exerting itself puts it into motion; and which, through the medium of the piston rod connected thereto, actuates the entire machine. The term piston is likewise sometimes employed to designate what is more generally termed the "bucket" of a pump. There is no part of the steam engine in which correct principles of construction and accurate workmanship are so essential. If the sides of the piston which rub against the cylinder or steam-way do not touch in every part, the steam escapes, power is lost, and fuel is wasted. If the piston rubs hard in one place, and softly in another, the cylinder becomes unequally worn, and its utility impaired or destroyed. To obviate these difficulties, therefore, the rubbing surfaces of a piston should not only be made as uniformly as possible, but also elastic, in order that it may expand and fill up all inequalities of surface with a gentle pressure. The usual mode of effecting this object is represented in the subjoined section of a common piston for a low pressure engine; a is the lower face of the piston made of metal, to which is fixed the piston rod b, that passes through the top plate c, which is made fast to the lower by screws dd; at e e is the packing (as it is termed,) made of hemp, saturated with tallow, which is wound and bound round the annular cavity made between the plates a and c; this elastic packing, as it wears away by friction against the cylinder, is occasionally screwed up, by turning the screws dd, which forces it out against the sides of the cylinder; and when entirely reduced, it is renewed by repacking with fresh materials.
From want of due care and skill in this kind of packing, a great loss of power in an engine is often sustained, either by the steam passing the piston, or by its being squeezed so tight as to cause great friction, and soon wear itself out. If the steam of an engine be weak, and the packing of the piston press tightly against the cylinder, the whole, or nearly the whole of the power may be expended in giving it motion, especially in cylinders of small diameter. On the contrary, if the packing presses very weakly against the cylinder, and the steam be very strong, the steam will push the packing away, and pass to the other side of the piston; and it should be borne in mind, that when this happens, it is not only the loss of the steam, but the reaction which it exerts on the other side, neutralizing an amount of force equal to its own volume. For these reasons, it becomes obvious that pistons should possess another property, that of being tight in proportion to the force of the steam which presses upon them.
Several plans have been proposed to construct pistons on that principle, amongst which are the following, proposed by a correspondent in a periodical journal in 1823, which perhaps deserve notice, as furnishing useful hints to the practical man.
In the annexed Fig. 1, A A is a metal plate sliding upon the piston rod; D D is the solid part of the piston, connected with the plate by a band nn; the space C C is to be filled with oil or other oleaginous fluid. By this arrangement it will be evident that the greater the force of the steam, or other pressure, upon the surfaces of the plates, the more closely will the packing be pressed against the sides of the cylinder.
The annexed Fig. 2 is a variation from the last; the top and bottom plates are fixed, and the steam acts upon short bolts or plungers b b, which, by being pressed inwards, force out the packing at the sides. Fig. 3 represents a mode of applying the principle to metallic pistons. A A is a metallic plate sliding on the piston rod, and made in the form marked by the dark line. At B B are metallic rings of a triangular form, divided and breaking joint: D D is the solid part. The pressure of the steam will cause the plates to descend, which, pressing upon the inclined planes of the rings, of course causes them to expand, and adhere to the sides of the cylinder; a small space is left between the solid part and the plates, to allow for the descent of the latter, but in no way so great as represented; the figures being only designed by the inventor to show the principle, and not the details of construction. Pistons packed with hemp and tallow continued in use, with very little variation, from the time of Captain Savery to that of Dr. Cartwright, a period of ninety-nine years.
That scientific clergyman had, however, the honour of first introducing, in an ingenious engine of his own construction, an expanding or elastic piston made entirely of metal,- an invention of indispensable utility in all engines working at high pressure.
As this piston has been considerably modified by various engineers, which we shall have to notice hereafter, we shall here state briefly that it consisted of two rings of brass, of the full size of the cylinder; these rings were each cut into three or more segments, and laid one over the other, so as to break the continuity of the vertical joints between the segments; concentric with those segments were a similar arrangement of segments inside the others, which were intended to stop the steam from passing horizontally; in the cavity between all the segments were placed feather springs, designed to press the segments outward to fill up the cylinder as either that or the piston wore. These pistons, however, proved very defective; for as the exterior segments wore, and conformed to a larger circle, the inner segments, which had no wear, were no longer concentric with those outside of them, and crevices were thus opened, through which the steam escaped past the piston.
Amongst those persons who directed their attention to the improvement of this important part of the engine, Mr. John Barton was the most successful. In the annexed engravings, Fig. 1 represents a plan of the piston, with the top plate removed, and Fig. 2 a vertical section of the same, taken on the line b e d b on the plan. a a a a are four metal segments; bbbb four right-angled wedges interposed between the segments, their points forming a portion of the periphery; cccc is a thin steel spring, formed into a single broad hoop, and pressed into the undulated form represented, by which it is found to act with uniform energy upon the wedges, until they and the segments become so much worn in the course of time, that the steel spring recovers itself into its original circular figure; d is the frame-work cast in one piece with the lower plate of the piston; e is the piston rod; the dark spaces shown on the plan within the circular frame d are cavities to lessen the weight of metal; the other dark spaces are cavities to allow of the free action of the circular spring.