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in the ring would be exactly counterbalanced by corresponding ellipticity in the cylinder; and not only so, but the next rings that happened to be put in would fit with as great truth as the most hair-breadth system of compensation could possibly effect, since the alleged unequal expansion of the ring would then correspond exactly with the shape to which the cylinder has by that time worn.

It is abundantly obvious, however, that the effect upon the cylinder of the unequal expansion might be altogether neutralised (were it ascertained to have any proper existence), by using two rings instead of one, the position of the joints being 90° apart. It is not uncommon to have the rings placed with the joints on opposite sides of the cylinder, or 180° apart: an arrangement only useful to prevent the passage of the steam through the single break (an object which might be easier attained in another way), and incapable of exerting a correcting influence on the above-named inequality.

Under the third modification of this description of packing, we class pistons in which each ring is still in one piece, but in which some of the above modes of compensation are had recourse to. The common method is, simply to turn the ring about one third thinner toward the part where the joining is made; two rings are commonly used, the position of the joints being as above described, and instead of the rings being turned of greater than the required diameter, and then sprung in, the requisite elasticity is given by hammering the thicker semicircumference on the hollow side. Of course, the same method may be taken with rings of uniform thickness, and to derive the full effect of the hammering, it might be well to retain the original skin of the casting on the inside of the ring. The advantage of giving the requisite elasticity by hammering rather than by compressing the ring, lies in the more perfect circularity ensured by the former; but were the precaution taken to turn up the compressed rings in the lathe, this difference would no longer have place; and this is now generally done in addition to the hammering.

This arrangement has been generally found to work very well; but perhaps not better than the preceding more simple variety, when the workmanship of both is equally correct; and to deficiency in this particular, we apprehend, must be ascribed any disadvantage which the latter, as sometimes constructed, have been found to labour under. Numberless methods have been taken to prevent the escape of the steam by the open end joints which this species of ring exposes; frequently hemp packing is put behind the rings; sometimes they are merely cut without any further provision; and sometimes the evil is left in its full magnitude, without any attempt at a remedy; but by far the best plan, undoubtedly, is to rivet a piece of brass or iron, previously fitted to the proper curvature, to the inside of the ring on one side of the break, so that it shall apply correctly to the other side, and slide along steam-tight as the ring expands by wearing. Inattention to this simple precaution has been the occasion of great inconvenience, and has even led to the substitution of a much more expensive, though not in reality much more efficient system.

Deferring our remarks on some important modifications of this construction till we come to consider the pistons in detail, we proceed to consider the second principal division of our subject, comprising many subvarieties: we mean, that class of pistons where artificial springs are used, acting in most cases by the intervention of blocks or wedges. The most common construction is as follows: - Two strong cast-iron rings, of such dimensions as to have no perceptible elasticity themselves (say from 13 in. to 2 in. square in the cross-section), are cut, each into from three to six or eight segments, according to the size of cylinder, or other regulating circumstance, placed the one above the other, so as to break bond at each joint, and at each break part of the ring is cut away to admit the introduction of a wedge usually of the angle of 90°, the point of which may be in. from the surface of cylinder: a common elliptical spring is then introduced between the body of the piston and the back of the wedge. Sometimes, instead of a wedge, a simple block is used, or the spring presses immediately upon the back of the ring.

In the original form of this piston, as constructed by Barton, the points of the wedge came in contact with the cylinder, being situated in the periphery of the piston, and, in order to prevent their injurious action in grooving the cylinder, it was proposed to make them of softer metal; and, we believe, it is to Tredgold that we are indebted for the suggestion of the above-described greatly improved modification. In this description of piston, the mode of action of the wedges is what seems principally to require investigation. When the points of the wedges are not in contact with the cylinder, the mode of action is sufficiently obvious; the intervention of the wedge serving simply to multiply the energy of the springs on the principle of the wedge, as a simple mechanical power. Of course, the more acute the angle of the wedge, the greater force is imparted to the springs, on the principle of virtual velocities; and were the segments only required to expand indefinitely, instead of expanding and contracting by turns, to suit the inequalities in the cylinder, then a very acute wedge might be used: but as the wedge must be ready to spring back, to allow the packing to yield when it comes to a tight part of the cylinder, as at the top and bottom of the stroke, it is plain that the inclination must be considerably greater than the angle of repose; it is seldom, however, made less than 80° or 90°, but we believe a considerably sharper angle would be found to answer. Considering now, for a moment, the case in which the points of the wedges come in contact with the cylinder, it might seem, at first

sight, that they would exert no pressure at all upon the adjacent segments; and such would be the case, were it possible for them to wear no faster than the rings themselves; a little consideration, however, will show that this can never be the case, as the wear upon the wedge must be to that of the ring, in the proportion of the side to half the base of the wedge; that is, in the case of a right-angled wedge, as the diagonal to one of the sides of a square were the wedge and cylinder then to be made of the same material, the wear of the cylinder opposite the wedge would exceed that of the rest of its surface, in the above proportion, or probably in a somewhat higher ratio, arising from the different grain of the metal composing the wedge. The wedges however are almost always made of gun metal, which serves in a great measure to neutralise what would be otherwise the injurious tendency of this arrangement.

With these preliminary remarks, we now proceed to consider, in detail, several of the varieties of piston that have come most prominently under our notice, the better able to decide upon their respective merits from the general principles we have laid down.

Fig. 207.

Fig. 207. represents two views of the locomotive piston made by Messrs. Forrester and Co., Liverpool, for the Grand Junction Railway. The peculiarity of construction and mode of action is apparent from inspecting the drawing. By fitting a tongue piece, or tenon, into a corresponding mortice, in both wedge and ring, it is intended to prevent the passage of steam at the breaks of the segment, instead of the common method of using two rings, each of half the thickness, with the joinings of the one ring placed midway between those of the other. We believe this construction has been found to answer perfectly; but this is not paying it a distinguishing compliment, as we shall soon see that the same may be said of certain other descriptions, about which there is not above one fourth of the workmanship here displayed. In fact, display is the very idea which this piston suggests; the elegance of the design, the slenderness of the working parts, and the extreme accuracy of workmanship, necessary to its action and which the Messrs. Forrester are never known to spare rather serve to point

Scale 1 inch=1 foot.

FORRESTER AND CO.

it out as a pretty engineering toy (not without its LOCOMOTIVE PISTON BY MESSRS. share of usefulness, at the same time) than as a plan likely to be either extensively adopted or generally approved of. In the first place, there can be no doubt, that the prevention of leakage by the joints would be at least as effectually accomplished by having two rings arranged as above described; a method that will generally be found, in the hands of common workmen, to be of much easier execution; very great nicety being required in the fitting of the sliding pieces into the mortices, and the slightest imperfection at first having a tendency to wear worse from the constant passage of the steam, Again, as the sliding pieces must move forward faster than the segments (when the wear commences) in the proportion of the diagonal to the side of a square, as can easily be demonstrated, it follows, that the pieces themselves, and consequently the part of the cylinder corresponding, must wear faster too; and thus is the cylinder liable to be worn unequally. The extreme slenderness of the springs, and the slight pressure which they are generally required to exert, may indeed, for the most part, prevent this inequality from being readily perceptible; but it is evident that the action is in some measure similar to that of a flat-pointed wedge pressed forward by a spring, and itself in contact with the cylinder.

Fig. 208.

We shall further find, on comparing this piston with more simple varieties, that it is unnecessary to divide the ring into so many segments. Such a complete system of adjustment and compensation might have been very beneficially employed during the nonage of steam cylinders; but is proved to be superfluous in the present state of machinery, by the fact that a ring cut in a single place is found to give perfect satisfaction. Fig. 208. represents plan and section of a piston manufactured for common and locomotive engines, by Messrs. Dircks and Nelson, late of the Etna Foundry, Liverpool, differing from the above, in having only one wedge and break in the ring instead of four. The set screw, on the side farthest from the wedge, might be omitted; the two side screws serve in some measure to compensate for the sluggish action of the rightangled wedge, which seems rather to press the ring against the cylinder at the point immediately adjacent than to force it open, and thus make it bear equally all round. A more acute wedge would propagate the pressure of the spring more readily throughout the entire circumference. A circular spring, like that used here, while it possesses several conveniences, is yet less delicate and perfect in its action than the common elliptical kind: and perhaps, too, less easily re-set or LOCOMOTIVE PISTON BY MESSR9 re-tempered.

Scale 1 inch=1 foot.

DIRCKS AND NELSON.

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Fig. 210. represents a plan of the piston so extensively known as Edward Bury's locomotive piston. The distinctive feature in it is, that each spring acts upon one wedge in the upper ring, and another in the lower, the centre of the spring, contrary to common usage, being made the "point d'appui," from the body of the piston. This variety is free from some of the defects of Forrester's, and when managed with care is found to work with great truth and accuracy. First cost of workmanship, unnecessary complication, considerable liability to derangement, are accompanying qualities which will be found to act more or less as drawbacks in different circumstances, and which by some may rather be deemed matters of financial than of engineering science. Among the great variety used on the Gloucester and Birmingham Railway, this description has been found to give the greatest satisfaction, while on some of the other lines they are in the course of being superseded by simple forms, which are said to be every way preferable. The peculiar contour of the springs involving, as it does, the difficulty of replacing them elsewhere than where they were first manufactured, is a point almost too insignificant to be noticed, but which, we doubt not, has tended to bring them into disrepute on some of the lines of railway above alluded to.

Fig.211. represents a piston that has been much used by Messrs. Stephenson: it consists of three concentric rings, the two outer being rebated and morticed upon their edges, and together filling up the space between the flanges of the piston. The inner ring is equal in depth to the two outside rings. When the rings are turned to fit the cylinder and each other, they are hammered on the inside, to give them a tendency to spring outwards, and are then cut through, to allow them to expand. The divisions of the rings are placed so as to break joint. To provide for the wear of the outside rings, springs are placed at the back, which can be tightened up by means of screws bearing upon the centre part of the springs.

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Fig. 213.

must, however, confess, that in urging this particular we form our opinion on theoretical, we had almost said, conjectural grounds, as we have never met with a more acute angle in a piston than from 80° to 90°. The pistons on the Liverpool and Manchester Railway are generally brass. This is an advantage in horizontal or inclined cylinders, as the softness of the material, and the greater lightness of the piston, considerably reduce the wear of the cylinder. Brass pistons have been, on this account, much used for locomotive engines; one drawback, however, to their employment, is, the inferior elasticity of the metal, which renders the employment of artificial springs necessary.

Fig. 213. represents the packing-rings of Stephenson's locomotive piston, as we have seen them used on the Great Western Railway. The peculiarity of this piston consists in the two rings being grooved and tongued into each other in a manner similar to

Scale 1 inch=1 foot.

RAILWAY.

common flooring deals as we have already LOCOMOTIVE PISTON, GREAT WESTERN mentioned. In the published description of this locomotive we are told that this is intended to "keep them steady," - an end that should, we think, be sufficiently accomplished by their being enclosed within the cylinder.

Fig. 214. represents the plan of a piston used on the South-Western Railway, bearing a considerable resemblance to what we have seen on the Liverpool and Manchester line. It is superior in simplicity, and at least equal in efficiency, to several of those we have figured. A modification of this kind is used with success upon the Runcorn Gap Railway, with this difference, that, opposite the set-screw, the ring is made perfectly flat for about the fourth of the circumference. The engines made by Sharp and Roberts for the French Railways are likewise fitted according to this modification.

Fig. 215.

Fig. 214.

Scale inch=1 foot. LOCOMOTIVE PISTON, SOUTH WESTERN RAILWAY.

OO

Scale 1 inch=1 foot.

LOCOMOTIVE PISTON, GLASGOW AND AYR RAILWAY.

Fig. 216.

Fig. 215. represents the piston adopted on the Glasgow and Ayr Railway, where it has superseded the more complex varieties. It is free from all or most of the defects we have pointed out, and has given much satisfaction in practice. The position of the rings, in being placed with the breaks, 90° apart instead of 180°, is one of those trivial but judicious arrangements which often determines the character of a mechanical contrivance. Fig. 216. is a plan and section of piston similar to that used for the American or Bogie engine, manufactured for the Gloucester and Birmingham Railway, by Messrs. Nasmyth, Gaskell, and Co.two packing-rings, in. x 12in., each in three segments, pressed out by three elliptical springs with set screws, through the medium of an inner ring equal in depth to the two, and cut in three places. Mr. Nasmyth has constructed many of his locomotive pistons with spiral steel springs instead of the usual elliptical kind: they were found very liable to break, however, and are now almost entirely disused. We have seen hollow packingrings constructed by Mr. Nasmyth, with the object, we presume, of obtaining a broad steam-tight bearing without increased weight.

These then are the principal specimens of locomotive pistons we have to give: we shall next describe some of the most approved pistons for marine engines, all of which are also applicable to land engines.

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Scale 1 inch=1 foot.

LOCOMOTIVE PISTON BY MESSRS. NASMYTH AND CO.

Fig. 217. represents a variety of packing manufactured by Messrs. Maxton and Co., Leith, adapted for marine engines. The specimen we have already given by these makers has an improvement introduced or patented several years ago by Mr. M'Dowall, of Johnston, near Paisley, which consists in cutting the rings in a slanting instead of a perpendicular direction. In Mr. M'Dowall's piston the ring is cut into several segments; and, as only a single ring is used, a sliding piece is checked in on one side, to break bond, and prevent the passage of the steam through the opening. That shown in the figure would be improved by the substitution of a single ring of sufficient depth, instead of two; and, by employing a sliding piece behind the joining, thus rendering the hempen packing unnecessary. The other forms of joining shown in the above figures are possessed of no particular interest, being somewhat difficult in execution, without corresponding advantage.

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Fig. 218. represents Jessop's spiral metallic packing, as made several years ago by the late Messrs. Claud Girdwood, and Co., for two small subsidiary engines for the "Don Juan" steamer. On considering the action of the spiral coils, when the ring is compressed into a lesser diameter, it will be found that the tension of the springs is a minimum at the centre of the coil; while it increases towards each extremity in the ratio of half the number of coils to unity. This packing is but little used.

Fig.219.

Scale 1 inch=1 foot.

Fig. 219. represents an arrangement that was extensively adopted by Messrs. Claud Girdwood and Co. The packing consists of four rings, from to 14 square, according to the size of the cylinder, which may vary from 6 to 50 inches in diameter. In pistons of the latter dimensions the rings are turned fully two inches more in diameter, and afterwards cut in one place, and bent to the proper circle in a mould prepared for the purpose the ends being half checked in such a way as to be steam-tight without the necessity of using packing behind, which, however, is frequently added. In order to bring the cylindrical surface of the rings sooner to an exact bearing, a groove is turned out of the outer circumference of each: this may help, besides, to keep the piston tight, by affording a lodgment for oil and tallow, and may in certain circumstances be of considerable use. In one instance within our knowledge this kind of piston has been at work, with occasional interruption, but little or no repair, for a period of ten years. Fig. 220. represents plan and section of piston both for marine and land, manufactured by the Messrs. Maxton, of Leith. This may be regarded as a favourable, and at the same time, characteristic specimen of the most generally approved and widely adopted variety of packing in which separate springs are employed. The ease with which the springs may be bent and reset to the proper compass prevents set screws from being required: the only seriously objectionable point seems to be the expense of fitting and grinding steam-tight the ten wedges required. While, however, we may admire the capability of perfect adaptation to the form of the cylinder, which this piston presents, we must express our decided opinion, that a sufficient, if not, practically speaking, an equal degree of correspondence might be attained at a fraction of the expense. This, however, supposes the case of a new and perfect cylinder; but where that has been previously worn unequally by hempen packing, as is frequently the case, then this modification possesses decided advantages, and has, we believe, been repeatedly applied by the Messrs. Maxton in these circumstances, with great success. The use of wrought-iron nuts, fitted into the body of the piston to receive the junk ring screws, instead of tapping them into the cast iron, is a very obvious improvement, though not peculiar to Messrs. Maxton.

PISTON BY MESSRS. CLAUD GIRD

WOOD AND CO.

Fig. 221. represents a form of marine piston at one time manufactured

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Fig. 222. is the form of piston adopted by Mr. Robert Napier in many of his vessels, being similar to that used in the Halifax line of packets. The peculiar shape of the blocks against which the springs press, instead of the usual V wedge, is one point of identity between this and that given in our plates of details of a marine engine.

Fig. 223. represents the form of piston at present constructed by Mr. Robert Napier. Two rings, about 2 in. square (cylinder being 65 in. diameter), divided into two segments, the lengths of which may be in the proportion of 1 to 5, each ring being pressed out with a number of springs, generally made very stiff, and of little compass: the rings, in consequence, wear so rapidly, that we have known them to require to be eked with a considerable thickness of copper at the ends. We do not know the object of dividing the segments in the above proportion, unless it be to take out the rings without taking off the cross-head: in other respects a single break would be quite as efficient. The distinguishing feature of this piston lies in the cut on the outer circumference of the rings being made at a considerable angle with the perpendicular, while that on the inner side is vertical; an arrangement that we do not recollect to have met with before. The peculiar form of spring shown here, in an enlarged view, is, so far as we have observed, adopted almost exclusively by Mr. Napier.

Fig 222.

Scale inch 1 foot. PISTON BY K. NAPIER.

Fig. 223.

Fig. 224. represents the species of packing-ring employed by the Messrs. Seaward. Only one packing-ring is used, about 3 in. x 12in.; sprung in from a larger diameter, or hammered internally so as to have considerable elasticity. This ring is cut in only one place, and a piece, 8 in. or 9 in. in length, half the thickness, and about one-third the depth of the ring, is checked in on the upper side, in order to break bond; and at the back of the joint a block is placed, screwed to the one end of the ring and loose at the other, so as to prevent the steam from passing through the cut. In addition to the elasticity of the ring, it is pressed out by six elliptical springs. This is, on the whole, one of the best marine pistons we have met with. The ring, however, in wearing, deranges the joint between the ring and the small segment-piece. The joint opens at each end, so that the only contact comes to be at the central point, which is a defect.

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Fig. 225.

Scale inch=1 foot.

PISTON OF DON JUAN STEAMER.

Fig. 225. shows plan and section of the piston of the "Don Juan's" engines (cylinder 68 in. diameter). There are two rings cut into two segments, the lesser being one-third the circumference, with V blocks at the joinings, and stiff elliptical springs.

Fig. 226.

Scale inch-1 foot. PISTON BY MESSRS. SEAWARD.

Scale inch=1 foot.
1

PISTON OF WEST INDIA MAIL STEAMERS TRENT AND ISIS, BY MILLER, RAVENHILL, and co.

Fig. 226. shows the species of piston employed by Messrs. Miller, Ravenhill, and Co. The ring is turned the full size, measuring to the point of the tongue-piece, and squares are then cut out to allow the ends to come together, so that the tongue-piece is in one piece with the ring. The malleable iron bridle is for the purpose of expanding or contracting the ring, which is effected by driving in a cutter at the one end of the bridle or the other, whereby the ring is contracted or expanded. One end of the bridle is attached to the ring by a bolt, which slides in an oblong hole in the bridle; and if a cutter be driven between the bridle and one end of the piece ground on the back of the joint, the ring will be expanded; whereas, if driven at the opposite side, the ring will be drawn together. II

Fig. 227.

Scale inch=1 foot.

PISTON OF WEST INDIA MAIL STEAMERS THAMES AND MEDWAY, BY MAUDSLAY, SONS, AND FIELD,

Fig. 227. represents the marine-engine piston of Messrs. Maudslay and Field. Here the tongue-piece is not cast in one with the ring, but is put in with pins; and the slot is on the incline, to prevent a rut in the cylinder. The tongue-pieces, of this and the preceding variety, are of course ground in, a piece being ground on the back to prevent the steam from passing. This species of piston ring is probably among the best yet introduced in practice; yet, in the West India mail steam vessels, it was found that the pistons were not tight until springs had been introduced to press out the rings. The elasticity of these rings must therefore be reckoned insufficient; and it is a very common fault of metallic pistons that the springs are made too weak. We have often known them to be too weak, but never in any one instance knew them to be too strong.

valves, but are slide-valves sometimes. A cock is to be regarded as a circular sliding-valve. The diameter of the spindle of spindle-valves is usually made about one eighth of the diameter of the valves: in small valves the proportion is greater.

Safety-valves.We have in page 86. given a theoretical investigation of the size of orifice requisite for safety-valves; but, as is there stated, much larger orifices are employed in practice. A common proportion is a circular inch of orifice per 14 horse power, or 8 of a circular inch per horse power. In marine engines safety-valves are usually lifted by a lever, which presses up the spindle from underneath, and the weights are either wholly or partially hung from the spindle. The spindles are sometimes guided by means of an iron bar, which passes across the steam-chest; but this is not a commendable practice, as explosions have occurred from the jamming of the spindles in the guide, in consequence of an alteration of shape in the steam-chest when the pressure came on. Fig. 229. represents the safetyFig. 229.

Fig. 228.

SAFETY VALVE OF LOCOMOTIVE BOILER.

valve of a locomotive engine, which is of the steelyard kind, and the end of the lever is kept down by a spring.

Boiler explosions sometimes arise from the adhesion of the safety-valve to its seat, and numerous plans have been devised, and some of them of considerable ingenuity, for obviating this source of danger. The ordinary method of feeding land boilers by a head of water may rank among these contrivances: it is shown in fig. 230. A float, which is usually made of stone

Fig. 230.

Scale 1 inch=1 foot.

BRIDLE FOR MAUDSLAY AND CO'S PISTON VALVE.

Fig. 228. represents the piston used for Messrs. Maudslay's cylindrical slide-valves. By raising or lowering the screw, it will be evident that the rings are compressed or expanded. A flat place is of course cast in the body of the piston to leave room for the bridle.

The pistons of Messrs. Scott and Sinclair consist of a number of segments pressed out by V blocks acted on by a continuous ring spring. Messrs. Caird's are similar, but the blocks are flat, and the springs are separate, as may be seen in the plates of the details of the Clyde, Tweed, Tay, and Teviot steamers. Messrs. Blythe's consist of two eccentric rings, one above the other, without springs or packing. Messrs. Fairbairn and Co. have made several with a double tier of eccentric rings, having eccentric rings again within these, so that the interior of the inner ring is concentric with the exterior of the outer. The space behind the inner rings is also packed with hemp. This variety of piston is troublesome to keep in good repair.

In the pistons of oscillating engines, it is necessary to take precautions against any compression of the packing-ring by the weight of the piston during the inclination of the cylinder. The method pursued by Messrs. Penn is to pack the space between the metallic ring and the piston with hemp. It might be a good way to force out the ring by means of a V block placed on one side of the piston, in the line of the trunnion, a steady pin being inserted in the piston on the opposite side, with a corresponding oblong hole in the packing-ring, so as to prevent the packing-ring from turning round at the same time that it was permitted to expand. Messrs. Penn make use of a single ring and tongue piece with the block behind recessed, so as not to interfere with the hemp packing. The upper side of the ring, too, is sharpened off to an edge.

VALVES.

We now come to the subject of valves, and here we have nearly as great variety as under the head of pistons. The function of a valve is to open or close a passage, and all the varieties are divisible into the genera of lifting and sliding valves. Sliding-valves are generally employed to admit the steam alternately above and below the pistons of engines, except in the case of pumping-engines, and there lifting or spindle valves are usually employed. Safety-valves are always spindle-valves, though a slide-valve, opened by the rise of a column of water or mercury, has been proposed as a safety-valve. The valves of pumps are generally spindle, flap, or ball

STAND PIPE, float, and FEED APPARATUS FOR LAND BOILERS.

or iron, is so balanced, by means of a counterweight, that it rises or falls with the fluctuations of the water level, and in so doing opens or closes a valve in a small cistern, at the top of a stand-pipe set on top of the boiler, thereby maintaining the water at the right level. The stand-pipe is of sufficient diameter to receive a float connected with a chain proceeding to the damper; and as the water is forced up in the stand-pipe to a height corresponding to the elasticity of the steam, the float in the stand-pipe will rise and fall with the varying pressure, thereby adjusting the vehemence of

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