diameter for a low-pressure cylinder, are usually made cellular-that is, with two thicknesses of metal stiffened or connected by ribs and webs, and either by the thickness of metal or by the depth of body made strong enough structurally to safely withstand, not only the steam pressure exerted on it and transmitted to the rod, but also the shocks to which it is liable when priming

occurs.

The piston body must be so designed, too, that it may be safely cast, for in the early days of large pistons it was not at all an uncommon thing for a piston to break in cooling, or mysteriously afterwards. For this reason any rules must of necessity be empirical which set out the thickness of metal of the different parts of the body; but care must always be exercised that no one part is too small for the strains to which it is subject. For example, there must be sufficient metal in the immediate neighborhood of the piston-rod boss to resist the tendency to force out this part by shearing the metal. Again, the piston may be taken as consisting of a number of sectors, and by considering one of such small sectors loaded with the pressure on its area at the centre of gravity of its figure, the bending moment at any section may be found, and the thickness of metal tried whether it be sufficient for the purpose.

For the section of an ordinary piston having a

single rod, the following table gives the multipliers for obtaining the thickness of metal and sizes of the different parts.

Details of Construction of the Ordinary Piston. -Let D be the diameter of the piston in inches, the effective pressure per square inch on it, x a constant multiplier, found as follows:

flange inside packing-ring. = 0.23 X x.

packing-ring

flange at edge.

=0.25 X x.

=0.15 X x.

=0.23 X x.

=0.35 X x.

inside packing-ring=0.21 X x.

metal around piston edge. . =0.25 X x.

When made of exceptionally good metal, at least twice melted, the thicknesses may be as

The

much as 20 per cent. less than given by the rules; but, on the other hand, if made of other than really good metal they should be thicker. piston should be made of good metal always, and for fast-running engines it is better made of steel. The packing ring is sometimes made much thicker in the part opposite the cut than given above, in order to have sufficient elasticity of itself to press steam-tight against the cylinder; but it is better to let the springs perform their function wholly, and leave the ring to act only as the packing.

PISTON RINGS AND SPRINGS.

FIG. 26.

Ramsbottom's Rings (Fig. 26).—Mr. Ramsbottom was the first to pack pistons by one or more narrow metal rings, turned somewhat larger in external diameter than that of the cylinder bore, and which, after being cut across so as to be capable of being compressed to suit the bore of the cylinder, are fitted into recesses turned in the piston edge. The rings fit accurately into these recesses, and as they are so placed that no two of the joints are in a line, the piston is practically steam-tight, and works very

well in locomotives and other quick-working engines of small size; but for large engines, and engines undergoing the same vicissitudes as those on shipboard, there is an objection to this form of piston. It will be seen that the rings cannot be removed without drawing the piston, and that there is no means of preventing steam from passing where the spring is cut across, besides which the rubbing surface is very small, and the spring is always exerting its maximum effort. The first of these objections is overcome by fitting a junk-ring, having cast with it a

FIG. 27.

spigot or ring, which goes down into the recess around the piston for the packing ring, and made steam-tight; into grooves turned in the outer surface of this spigot the Ramsbottom rings are fitted.

For small engines these rings are made of steel; for such engines as may be standing unused for many days, some engineers prefer to fit hard brass rings. When for larger engines where the section may be three-quarters of an inch square and upwards, the rings are better of tough and hard cast iron.

Common Piston Rings (Fig. 27) consist only of a single hoop made of very tough, closegrained, cast iron, made on the same principle as the Ramsbottom rings, but fitted between the piston flange and the junk-ring, so as to be free to move laterally steam-tight. This packing ring is usually turned to a diameter about 1 per cent. in excess of that of the cylinder, and cut across diagonally.

The ring is then fitted to the piston flange steam-tight by scraping both surfaces; the ring is raised by interposing very thin pieces of paper between it and the flange, and the junk-ring is then fitted steam-tight to the piston and packing ring by scraping, etc. Some makers of pistons profess to turn the piston and rings so accurately as to require no scraping, but it is doubtful if there is economy in the practice if carried to the perfection professed; other engineers prefer to grind the rings tight after coming from the lathe. In whatever way the object is attained is of small moment compared with the necessity of having the ring perfectly steam-tight between the flange and follower.

Piston Springs.-When the piston is of comparatively small diameter, the elasticity of the packing ring itself is sufficient to keep it steamtight against the cylinder sides for a very considerable time after it is fitted; and even larger rings may be made of sufficient strength to do