To prevent the segments from falling out of their places whilst the piston is being taken out, or put into the cylinder, the periphery of it is grooved near to its upper and lower edge, in which are sunk two slight spring hoops, cleft across into forked joints, which close together simply by their elasticity. To lubricate the piston, there is a third groove, made midway between the two former for the reception of the oil; these parts are not introduced into the figures. The action is as follows: as the piston and cylinder wear away by the friction, the circular spring c presses out the wedges b, and these project the segments against the cylinder; and as the segments become reduced, the wedges fill up the increasing opening between them.
An objection has been raised against this piston, that as the wedges must move through a greater space than the segments, in order to press the latter into the circumferential line, the wedges must in consequence rub twice as much against the cylinder, and consequently score it. This objection we believe to be unfounded; and as far as our experience and observation have extended, we have found the wear very uniform. Mr. Barton, perhaps, softens the segments, or makes them of an alloy, which is more easily abraded than the segments; sometimes (we have been informed,) he obviates the supposed tendency of scoring, by cutting out a portion of the end of the wedges, so that they do not bear upon their whole depth or thickness against the cylinder, consequently they will abrade twice as fast as the segments, supposing them to be equally hard. A great variety of metallic pistons have been made of late years, but we know of none that have so fully answered the purpose as the recently patented improvement by Mr. John M'Dowall, of Johnston, near Paisley, who has a manufactory of them at Manchester, where, we understand, great numbers are advantageously working in the engines of the factories.
We have seen them in other parts of the kingdom, and can attest their superior excellence.
In the specification of his patent, Mr. M'Dowall states, that his experience in the working of cast-iron pistons led him to observe that the surfaces of the metal betweeen the segments and the plates against which they slide rapidly corrode, and become converted into a substance resembling plumbago, by which the effectiveness of the piston is of course seriously impaired. As a remedy for this defect in cast-iron pistons, he lines or covers the aforesaid surfaces with plates of brass or gun metal, which he connects by screwing or pinning to the cast-iron, and thus acquires the durability of the gun metal at a trifling additional expense above cast-iron.
Another important improvement which Mr. M'Dowall has introduced, consists in a modified construction of the segments, and in the steam stops or slides by which they are pressed outwards. In the figure on the next page is exhibited an external elevation of one of these cast-iron pistons: a is the piston rod which passes through a solid central block, the upper part of which is seen at b, and through the top plate c, and bottom plate d, the latter being made fast to the bottom of the central block through the medium of the piston rod; the top plate c, for the convenience of removal at pleasure, is fastened to the central block b, by means of screws e e e. Between the top and bottom plates, and around the periphery of the central block, are fitted an expanding ring of segments, two of which are seen at ff; these segments instead of being divided by perpendicular cuts, as usual, have these parts inclined, as seen at g, which thus overlap each other, and cause the cylinder to be equally worn, (which would not be the case were the apertures between the divisions vertical) The inclined crevices through which the steam might pass is stopped by movable sliding pieces, which are made to press continually against the segments by the agency of springs, in the same manner as the wedges are acted upon by the springs in Barton's piston, previously described.
One of these sliding pieces is seen at g, the projecting part of it being of a rhomboidal figure, that fills up corresponding notches made in the corners of the segments, and those parts which come in contact, and are represented by a single line, are faced and ground to each other, to prevent the upward or downward passage of the steam; and to stop it laterally, the slides are ground to fit the backs of the segments to which they are connected, by dove-tailed grooves, represented by the two parallel dotted lines h h. The double lines at i, both above and below the segments, indicate the brass linings before mentioned. Mr. M'Dowall's patent includes the application of the same improvements, namely, the sliding steam stops, and the brass linings to the air-pump buckets of steam engines, described under the article Valve, to which his principal improvement in this appendage relates.