Chapter I (10)

The mechanism by which the four valves are opened and closed, is subject to considerable variation in different engines. They have been described above as being opened and closed simultaneously by a single lever. Sometimes, however, they are opened alternately in pairs by two distinct levers driven by two pins attached to the air-pump rod. One pin strikes the lever, which opens and closes the upper steam valve, and lower exhausting valve; the other strikes that which opens and closes the lower steam valve and upper exhausting valve.

Since the date of the earlier double-acting engines, constructed by Boulton and Watt, a great variety of mechanical expedients have been practised for working the valves, by which the steam is admitted to and withdrawn from the [Pg225] cylinder. We shall here describe a few of these methods:—

(128.) The method of working the valves by pins on the air-pump rod driving levers connected with the valves has been, in almost all modern double-acting machines, superseded by an apparatus called an _eccentric_, by which the motion of the axle of the fly-wheel is made to open and close the valves at the proper times.

An eccentric is a metallic circle attached to a revolving axle, so that the centre of the circle shall not coincide with the centre round which the axle revolves. Let us suppose that G (_fig._ 45.), is a square revolving shaft. Let a circular plate of metal B D, having its centre at C, have a square hole cut in it, corresponding to the shaft G, and let the shaft G pass through this square aperture, so that the circular plate B D shall be fastened upon the shaft, and capable of revolving with it as the shaft revolves. The centre C of the circular plate B D will be carried round the centre G of the revolving shaft, and will describe round it a circle, the radius of which will be the distance of the centre C of the circular plate from the centre of the shaft. Such circular plate so placed upon a shaft, and revolving with it, is _an eccentric_.

Let E F be a metallic ring, formed of two semicircles of metal screwed together at H, so as to be capable, by the adjustment of the screws, of having the circular aperture formed by the ring enlarged and diminished within certain [Pg226] small limits. Let this circular aperture be supposed to be equal to the magnitude of the eccentric B D. To the circular ring E F let an arm L M be attached. If the ring E F be placed around the eccentric B D, and that the screws H be so adjusted as to allow the eccentric B D to revolve within the ring E F, then while the eccentric revolves, the ring not partaking of its revolution, the arm L M will be alternately driven to the right and to the left, by the motion of the centre C of the eccentric as it revolves round the centre G of the axle. When the centre C of the eccentric is in the same horizontal line with the centre G, and to the left of it, then the position of L M will be that which is represented in _fig._ 45.; but when, after half a revolution of the main axle, the centre C of the eccentric is thrown on the other side of the centre G, then the point M will be transferred to the right, to a distance equal to twice the distance C G. Thus as the eccentric B D revolves within the ring E F, that ring, together with the arm L M, will be alternately driven, right and left, through a space equal to twice the distance between the centre of the eccentric and the centre of the revolving shaft.

If we suppose a notch formed at the extremity of the arm L M, which is capable of embracing a lever N M, moveable on a pivot at N, the motion of the eccentric would give to such a lever an alternate motion from right to left, and _vice versâ_. If we suppose another lever N O connected with N M, and at right angles to it, forming what is called a bell-crank, then the alternate motion received by M, from right to left, would give a corresponding motion to the extremity O of the lever N O, upwards and downwards. If this last point O were attached to a vertical arm or shaft, it would impart to such arm or shaft an alternate motion upwards and downwards, the extent of which would be regulated by the length of the levers respectively.

By such a contrivance the revolution of the fly-wheel shaft is made to give an alternate vertical motion of any required extent to a vertical shaft placed near the cylinder, which may be so connected with the valves as to open and close them. Since the upward and downward motion of this vertical shaft is governed by the alternate motion of the centre [Pg227] C to the right and to the left of the centre G, it is evident that by the adjustment of the eccentric upon the fly-wheel shaft, the valves may be opened and closed at any required position of the fly-wheel and crank, and therefore at any required position of the piston in the cylinder.

Such is the contrivance by which the valves, whatever form may be given to them, are now almost universally worked in double-acting steam engines.

* * * * *

Having described the general structure and operation of the steam engine as improved by Watt, we shall now explain, in a more detailed manner, some parts of its machinery which have been variously constructed, and in which more or less improvements have been made.

OF THE COCKS AND VALVES.

(129.) In the steam engine, as well as in every other machine in which fluids act, it is necessary to open or close, occasionally, the tubes or passages through which these fluids move. The instruments by which this is accomplished are called cocks or valves.

Cocks or valves may be classified by the manner in which they are opened: 1st, they may be opened by a motion similar to the lid of a box upon its hinges; 2d, they may be opened by being raised directly upwards, in the same manner as the lid of a pot or kettle; 3d, they may be opened by a sliding motion, like that of the sash of a window or the lid of a box which slides in grooves; 4th, they may be opened by a motion of revolution, in the same manner as the cock of a beer-barrel is opened or closed. The term _valve_ is more properly applied to the first and second of these classes; the third class are usually called _slides_, and the fourth _cocks_.

(130.) The single clack valve is the most simple example of the first class. It is usually constructed by attaching to a plate of metal larger than the aperture which the valve is intended to stop, a piece of leather, and to the under side of this leather another piece of metal smaller than the aperture. The leather [Pg228] extending on one side beyond the larger metallic plate, and being flexible, forms the hinge on which the valve plays. Such a valve is usually closed by its own weight, and opened by the pressure of the fluid which passes through it. It is also held closed more firmly by the pressure of the fluid whose return it is intended to obstruct. An example of this valve occurs in the steam engine, in the passage between the condenser and the air-pump. The aperture which it stops is there a seat inclined at an angle whose inclination is such as to render the weight of the valve sufficient to close it. In cases where the valve is exposed to heat, as in the example just mentioned, where it is continually in contact with the hot water flowing from the condenser to the air-pump, the use of leather is inadmissible, and in that case the metallic surface of the valve is ground smooth to fit its seat.

The extent to which such a valve should be capable of opening, ought to be such that the aperture produced by it shall be equal to the aperture which it stops. This will be effected if the angle through which it rises be about 30°.

The valve by which the air and water collected in the bottom of the air-pump are admitted to pass through the air-pump piston is a double clack, consisting of two semicircular plates, having the hinges on the diameters of these semicircles, as represented in _fig._ 46.

(131.) Of the valves which are opened by a motion perpendicular to their seat, the most simple is a flat metallic plate, made larger than the orifice which it is intended to stop, and ground so as to rest in steam-tight contact with the surface surrounding the aperture. Such a valve is usually guided in its perpendicular motion by a spindle passing through its centre, and sliding in holes made in cross bars extending above and below the seat of the valve.

The conical steam-valves, which have been already described (116.), usually called spindle-valves, are the most common of this class. The best angle to be given to the conical seat is found in practice to be 45°. With a less inclination the valve has a tendency to be fastened in its seat, and a greater inclination would cause the top of the valve to occupy [Pg229] unnecessary space in the valve-box. The area, or transverse section of the valve-box, should be rather more than double the magnitude of the upper surface of the valve, in order to allow a sufficiently free passage for the steam, and the play of the valve should be such as to allow it to rise from its seat to a height not less than one fourth of the diameter of its upper surface.

The valves coming under this class are sometimes formed as spheres or hemispheres resting in a conical seat, and in such cases they are generally closed by their own weight, and opened by the pressure of the fluid which passes through them.

(132.) One of the advantages attending the use of slides, compared with the other form of valves, is the simplicity with which the same slide may be made to govern several passages, so that a single motion with a slide may perform the office of two or more motions imparted to independent valves.

In most modern engines the passage of the steam to and from the cylinder is governed by slides of various forms, some of which we shall now explain.

(133.) In _figs._ 47. and 48. is represented a slide-valve contrived by Mr. Murray of Leeds. A B is a steam-tight case attached to the side of the cylinder; E F is a rod, which receives an alternate motion, upwards and downwards, from the eccentric, or from whatever other part of the engine is intended to move the slide. This rod, passing through a stuffing-box, moves the slide G upwards and downwards. S is the mouth of the steam pipe coming from the boiler; T is the mouth of a tube or pipe leading to the condenser; H is a passage leading to the top, and I to the bottom, of the cylinder. In the position of the slide represented in _fig._ 47., the steam coming from the boiler through S passes through the space H to the top of the cylinder, while the steam from the bottom of the cylinder passes through the space I into the tube T, and goes to the condenser. When the rod [Pg230] E F is raised to the position represented in _fig._ 48., then the passage H is thrown into communication with the tube T, while the passage I is made to communicate with the tube S. Steam, therefore, passes from the boiler through I below the piston, while the steam which was above the piston, passing through H into T, goes to the condenser. Thus the single slide G performs the office of the four valves described in (116.).

(134.) The slide G has always steam of a full pressure behind it, while the steam in front of it escaping to the condenser, exerts but little pressure upon it. It is therefore always forcibly pressed against the surfaces in contact with which it moves, and is thereby maintained steam-tight. Indeed this pressure would rapidly wear the rubbing surfaces, unless they were made sufficiently extensive, and hardened so as to resist the effects of the friction. Where fresh water is used, as in land boilers, the slide may be made of hardened steel; and in the case of marine boilers, it may be constructed of gun-metal. In this and all other contrivances in which the apertures by which the steam is admitted to and withdrawn from the piston are removed to any considerable distance from the top and bottom of the cylinder, there is a waste of steam, for the steam consumed at each stroke of the piston is not only that which would fill the capacity of the cylinder, but also the steam which fills the passage between the slide G and the top or bottom of the cylinder. Any arrangement which would throw the passages H and I on the other side of the slide G, that is, between S and G, instead of being, as they are, between G and the top and bottom of the cylinder, would remove this defect. This is accomplished by a slide, which is usually called the D valve, because, being semi-cylindrical in its form, and hollow, its cross section resembles the letter D. This slide, which is that which at present is in most general use, is represented in _figs._ 49, 50.; E is the rod by which the slide is moved, passing [Pg231] through a stuffing-box F; G G is the slide represented by a vertical section, _a a_ being a passage in it extending from the top to the bottom; S is the mouth of the great steam pipe coming from the boiler; P is the pipe leading to the condenser; T H is a hollow space formed in the slide always in communication with the steam pipe S, and consequently always filled with steam from the boiler. A transverse section of the slide and cylinder is represented in _fig._ 51., where _a_ represents the top of the passage marked _a_ in _fig._ 49. In the position of the slide represented in _fig._ 49., the steam filling the space T H has access to the top of the cylinder, but is excluded from the bottom. The steam which was below the piston, passing up the passage _a_, escapes through the tube P to the condenser. When the piston has descended, the rod E moves the slide downwards, so as to give it the position represented in FIG. 50. The steam in T H has now access to the bottom of the cylinder, while the steam above the piston passing through P escapes to the condenser. In this way the operation of the piston is continued and the steam consumed at each stroke only exceeds the capacity of the cylinder by what is necessary to fill the passages between the slide and the cylinder.

In a slide constructed in this manner, the steam filling the space T H has a tendency to press the slide back, so as to break the contact of the rubbing surfaces, and thereby to cause the steam to leak from the space T H to the back of the slide. This is counteracted by the packing _x_, at the back of the slide.

In engines of very long stroke, the extent of the rubbing surfaces of slides of this kind renders it difficult to keep [Pg232] them in steam-tight contact and to insure their uniform wear. In such cases, therefore, separate slides, upon the same principle, are provided at the top and bottom of the cylinder, moved, however, by a single rod of communication.

(135.) In slides, as we have here described them, the same motion which admits steam to either end of the cylinder, withdraws it from the other end. Such an arrangement is only compatible with the operation of a cylinder which works without expansion; for in such a cylinder the full flow of steam to the piston is only interrupted for a moment during the change of position of the slide. But if the steam act expansively, it would be necessary to move the slide, so as to stop its flow to one end of the cylinder, without at the same time obstructing the escape of steam from the other end to the condenser. It would therefore be necessary that the slide should close the passage leading to the cylinder at one end, without at the same time obstructing the communication between the passage from the cylinder to the condenser at the other end. On the arrival of the piston, however, at the bottom of the cylinder, it would be necessary immediately to put the lower passage to the cylinder in communication with the steam pipe, and the upper passage in communication with the condenser. This would necessarily suppose two motions of the slide as well as some modifications in its length. Let the length of the slide be such that when the passage to the top of the cylinder is stopped, the lower part of the slide shall not reach the passage to the lower part of the cylinder; and let such a provision be made in the mechanism by which the rod E governing the slide is driven that it shall receive two motions during the descent of the piston, the first to be imparted to it at the moment the steam is to be cut off, and the second just before the termination of the stroke. Let the position of the slide, at the commencement of the stroke, be represented in _fig._ 52., and let it be required that the steam shall be cut off at one half of the stroke. When the piston has made half the stroke, the rod governing the slide is moved downwards, so as to throw the slide into the position represented in _fig._ 53. The passage between the steam pipe and the cylinder is [Pg233] now stopped at both ends; but the passage from the bottom of the cylinder to the condenser remains open. During the remainder of the stroke, therefore, the steam in the cylinder works expansively. As the piston approaches the bottom of the cylinder, another motion is imparted to the rod governing the slide, by which the latter is thrown into the position represented in _fig._ 54. Steam now flows below the piston while the steam above it passes to the condenser. In a similar manner, by two motions successively imparted to the slide during the ascent of the piston, the steam may be cut off at half stroke; and it is evident that by regulating the time at which these motions are given to the slide, the steam may be worked expansively, to any required extent.

It is easy to conceive various mechanical means by which, in the same engine, the point at which the steam is cut off may be regulated at pleasure.

In cases where the motion of the piston is very rapid, as in locomotive engines, it is desirable that the passages to and from the cylinder should be opened very suddenly. This is difficult to be accomplished with any form of slide consisting of a single aperture; but if, instead of admitting the steam to the cylinder by a single aperture, the same magnitude of opening were divided among several apertures, then a proportionally less extent of motion in the slide would clear the passage for the steam, and consequently greater suddenness of opening would be effected. [Pg234]

The great advantages in the economy of fuel resulting from the application of the expansive principle have, of late years forced themselves on the attention of engineers, and considerable improvements have been made in its application, especially in the case of marine engines used for long voyages, in which the economy of fuel has become an object of the last importance. The mechanism by which expansive slides are moved, is made capable of adjustment, so that the part of the stroke at which the steam is cut off, can be altered at pleasure. The working power of the engine, therefore, instead of being controlled by the throttle-valve, is regulated by the greater or less extent to which the expansive principle is applied. Steam of the same pressure is admitted to the cylinder in all cases; but it is cut off at a greater or less portion of the stroke, according to the power which the engine is required to exert.

The last degree of perfection has been conferred on this principle by connecting the governor with the mechanism by which the slide is moved, so that the governor instead of acting on the throttle-valve, is made to act upon the slide. By this means when, by reason of any diminution of the resistance, the motion of the engine is accelerated, the balls of the governor diverging shift the cam or lever which governs the slide, so that the steam is cut off after a shorter portion of the stroke, the expansive principle is brought into greater play, and the quantity of steam admitted to the cylinder at each stroke is diminished. If, on the other hand, the resistance to the machine be increased, so as to diminish the velocity of the engine, then the balls collapsing the levers of the governor shift the cam which moves the slides, so as to increase the portion of the stroke made by the piston before the steam is cut off, and thereby to increase the amount of mechanical power developed in the cylinder at each stroke. The extent to which the expansive principle is capable of being applied, more especially in marine engines, has been hitherto limited by the necessity of using steam of very high pressure, whenever the steam is cut off after the piston has performed only a small part of the stroke. A method, however, is now (March, 1840) under experimental trial, by [Pg235] Messrs. Maudsley and Field, by which the expansive principle may be applied to any required extent without raising the steam in the boiler above the usual pressure of from three to five pounds per square inch. This method consists in the use of a piston of great magnitude. The force urging the piston is thus obtained not by an excessive pressure on a limited surface, but by a moderate pressure diffused over a large surface. The entire moving force acting on the piston before the steam is cut off, is considerably greater than the resistance; but during the remainder of the stroke this force is gradually enfeebled until the piston is brought to the extremity of its play.

(136.) Mr. Samuel Seaward, of the firm of Messrs. Seawards, engineers, has contrived an improved system of slides, for which he has obtained a patent. A section of Seaward's slides is represented in _fig._ 55. The steam pipe proceeding from the boiler to the cylinder is represented at A A, and it communicates with passages S and S′ leading to the top and bottom of the cylinder. These passages are formed in nozzles of iron or other hard metal cast upon the side of the cylinder. These nozzles present a smooth face outwards, upon which the slides B B′, also formed with smooth faces, play. The slides B B′ are attached by knuckle-joints to rods E E′, which move through stuffing-boxes, and the [Pg236] connection of these rods with the slides is such that the slides have play so as to detach their surfaces easily from the smooth surfaces of the nozzles when not pressed against these surfaces. The steam in the steam pipe A A will press against the backs of the slides B B′, and keep their faces in steam-tight contact with the smooth surfaces of the nozzles. These slides may be opened or closed by proper mechanism at any point of the stroke. When steam is to be admitted to the top of the cylinder, the upper slide is raised and the passage S opened; and when it is to be admitted to the bottom of the cylinder, the lower slide is raised and the passage S′ opened; and its communication to the top or bottom of the cylinder is stopped by the lowering of these slides respectively. On the other side of the cylinder are provided two passages C C′ leading to a pipe G, which is continued to the condenser. On this pipe are cast nozzles of iron or other metal presenting smooth faces towards the cylinder, and having passages D D′ communicating between the top and bottom of the cylinder respectively and the pipe G G leading to the condenser. Two slides _b b'_, having smooth faces turned from the cylinder, and pressing upon the faces of the nozzles D D′, are governed by rods playing through stuffing-boxes, in the same manner as already described. The faces of these slides being turned from the cylinder, the steam in the cylinder having free communication with them, has a tendency to keep them by its pressure in steam-tight contact with the surfaces in which the apertures leading to the condenser are formed. These two slides may be opened or closed whenever it is necessary.

When the piston commences its descent, the upper steam slide is raised, so as to open the passage S, and admit steam above the piston; and the lower exhausting slide _b′_ is also raised, so as to allow the steam below the piston to escape through G to the condenser, the other two passages S′ and C being closed by their respective slides. The slide which governs S is lowered at that part of the stroke at which the steam is intended to be cut off, the other slides remaining unchanged; and when the piston has reached the bottom of the cylinder, the lower steam slide opens the passage S′, and [Pg237] the upper exhausting slide opens the passage C; and at the same time the lower exhausting slide closes the passage C′. Steam being admitted below the piston through S′, and at the same time the steam above it being drawn away to the condenser through the open passage C and the tube G, the piston ascends. When it has reached that point at which the steam is intended to be cut off, the slide which governs S′ is lowered, the other slides remaining unaltered, and the upward stroke is completed in the same manner as the downward.

These four slides may be governed by a single lever, or they may be moved by separate means. From the small spaces between the several slides and the body of the cylinder, it will be evident that the waste of steam by this contrivance will be very small.

In the slide valves commonly used, the packing of hemp at the back of the slide, by which the pressure necessary to keep the slide in steam-tight contact is obtained, requires constant attention from the engine-man while the engine is at work. Any neglect of this will produce a corresponding loss in the power of the engine; and accordingly it is found that in many cases where engines work inefficiently, the defect is owing either to ignorance or want of attention on the part of the engine-man in the packing of the slides. In Seaward's slides no hemp packing is used, nor is any attention on the part of the engine-man required after the slides are first adjusted. The slides receive the pressure necessary to keep them in steam-tight contact with the surfaces of the nozzles from the steam itself, which acts behind them.

The eduction and steam slides being independent of each other, they may be adjusted so that the engine shall work expansively in any required degree; and this may be accomplished either by working the slides by separate mechanism, or by a single eccentric.

One of the advantages claimed by the patentees for these slides is, that the engines are secured from the accidents which arise from the accumulation of water within the steam cylinder. If such a circumstance should occur, the action of the piston will press the water against the faces of the steam [Pg238] slides, and the play allowed to them by their connection with the rods which move them permits their faces to be raised from the surfaces of the nozzles, so that the water collected in the cylinder shall be driven into the steam pipe, and sent back from thence to the boiler.

(137.) Of the cocks or valves which are opened and closed by the motion of an axis passing through their centre, the throttle-valve, whether worked by hand or by the governor, is an example. But the most common form for cocks is that of a cylindrical or slightly conical plug (_fig._ 56.), inserted in an aperture of corresponding magnitude passing across the pipe or passage which the cock is intended to open or close. One or more holes are pierced transversely in the cock, and when the cock is turned so that these holes run in the direction of the tube, the passage through the tube is opened; but when the passage through the cock is placed at right angles to the tube, then the sides of the tube stop the ends of the passage in the cock, and the passage through the tube is obstructed. The simple cock is designed to open or close the passage through a single tube. When the cock is turned, as in _fig._ 57., so that the passage through the cock shall be at right angles to the length of the tube, then the passage through the tube is stopped; but when the cock is turned from that position through a quarter of a revolution, as in _fig._ 58., then the passage through the cock takes the direction of the passage through the tube, and the cock is opened, and the passage through the tube unobstructed. In such a cock the passage may be more or less _throttled_ by [Pg239] adjusting the position of the cock, so that a part of the opening in it shall be covered by the side of the tube.

(138.) It is sometimes required to put one tube or passage alternately in communication with two others. This is accomplished by a _two-way cock_. In this cock the passage is curved, opening usually at points on the surface of the cock, at right angles to each other. Such a cock has already been described, and its use illustrated in the description of the Marquis of Worcester's engine (17.); the two-way cock, as represented at K and R (_fig._ 4.), being the means by which steam and water are alternately supplied to the two forcing vessels.

(139.) When it is required to put four passages alternately in communication by pairs, a _four-way cock_ is used. Such a cock has two curved passages (_fig. 59._), each similar to the curved passage in the two-way cock. Let S C B T be the four tubes which it is required to throw alternately into communication by pairs. When the cock is in the position _fig._ 59., the tube S communicates with T, and the tube C with B. By turning the cock through a quarter of a revolution, as in _fig._ 60., the tube S is made to communicate with B, and the tube C with T; and if the cock continue to be turned at intervals through a quarter of a revolution, these changes of communication will continue to be alternately made. It is evident that this may be accomplished by turning the cock continually in the same direction.

The four-way cock is sometimes used as a substitute for the valves or slides in a double-acting steam engine to conduct the steam to and from the cylinder. If S represent a pipe conducting steam from the boiler, C that which leads to the condenser, T the tube which leads to the top of the cylinder, and B that which leads to the bottom, then when the cock is in the position (_fig._ 59.), steam would flow from the boiler to [Pg240] the top of the piston, while the steam below it would be drawn off to the condenser; and in the position (_fig._ 60.), steam would flow from the boiler to the bottom of the piston, while the steam above it would be drawn off to the condenser. Thus by turning the cock through a quarter of a revolution towards the termination of each stroke, the operation of the machine would be continued.

One of the disadvantages which is inseparable from the use of a four-way cock for this purpose is the loss of the steam at each stroke, which fills the tubes between the cock and the ends of the cylinder. This disadvantage could only be avoided by the substitution of two two-way cocks (138.) instead of a four-way cock. A two-way cock at the top of the cylinder would open an alternate communication between the cylinder and steam pipe, and the cylinder and condenser, while a similar office would be performed by another two-way cock at the other end.

The friction on cocks of this description is more than on other valves; but this is in some degree compensated by the great simplicity of the instrument. When the cock is truly ground into its seat, being slightly conical in its form, the pressure of the steam has a tendency to keep the surfaces in contact; but this pressure also increases the friction, and has a tendency to wear the seat of the cock into an elliptical shape. Consequently, such cocks require to be occasionally ground and refitted.

(140.) The four-way cock, as above described, admits the steam to one end of the piston at the same moment that it stops it at the other end. It would therefore be inapplicable where steam is worked expansively. A slight modification, however, analogous to that already described in the slides, will adapt it to expansive action. This will be accomplished by giving to one of the passages through the cock one aperture larger than the other, and working the cock so that this passage shall always be used to conduct steam to the cylinder; also by enlarging both apertures of the other passage, and using it always to conduct steam from the cylinder. The effect of such an arrangement will be readily understood.

Let the position of the cock at the commencement of the [Pg241] descending stroke be represented in _fig._ 61. Steam flows from S through T to the top of the cylinder, while it escapes from B through C from the bottom of the cylinder. When the piston has arrived at that point at which the steam is to be cut off, let the cock be shifted to the position represented in _fig._ 62. The passage of steam from the boiler is now stopped, but the escape of steam from the bottom of the cylinder through C continues, and the cock is maintained in this position until the piston approaches the bottom of the cylinder, when it is further shifted to the position represented in _fig._ 63. Steam now flows from S through B to the bottom of the cylinder, while the steam from the top of the cylinder escapes through C to the condenser. When the piston has arrived at that point where the steam is to be cut off, the cock is shifted to the position represented in _fig._ 64. The communication between the steam and the bottom of the piston is now stopped, while the communication between the top of the cylinder and the condenser is still open. During the next double stroke of the piston the position of the cock is similarly changed, but in the contrary direction, and in the same way the motion is continued. Under these circumstances the cock, instead [Pg242] of being moved constantly in the same direction, as in the case of the common four-way cock, will require to be moved alternately in opposite directions.

PISTONS.

(141.) The office of a piston being to divide a cylinder into two compartments by a movable partition which shall obstruct the passage of any fluid from one compartment to the other, it is evident that the two conditions which such an instrument ought to fulfil are, _first_, that the contact of its sides with the surface of the cylinder shall be so close and tight throughout its entire play that no steam or other fluid can pass between them; _secondly_, that it shall be so free from friction, notwithstanding this necessary tightness, that it shall not absorb any injurious quantity of the moving power.

Since, however accurately the surfaces of the piston and cylinder may be constructed, there will always be in practice more or less imperfection of form, it is evident that the contact of the surface of the piston with the cylinder throughout the stroke can only be maintained by giving to the circumference of the piston sufficient elasticity to accommodate itself to such inequalities of form. The substance, whatever it may be, used for this purpose, and by which the piston is surrounded, is called _packing_.

In steam pistons the material used for packing must be such as is capable of resisting the united effects of heat and moisture. Hence leather and other animal substances are inapplicable.

The packing used for steam pistons is therefore of two kinds, _vegetable packing_, usually hemp, or _metallic packing_.

The common hemp-packed piston has been already in part described (79.). The bottom of the piston is a circular plate just so much less in diameter than the cylinder as is sufficient to allow its free motion in ascending and descending. A little above its lowest point this plate begins gradually to diminish in thickness, until its diameter is reduced to from one to two inches less than that of the cylinder, leaving therefore around [Pg243] it a hollow space, as represented in _fig._ 65. The cover of the piston is a plate similarly formed, being in like manner gradually reduced in thickness downwards, so as to correspond with the lower plate. In the hollow space which thus surrounds the piston a packing of unspun hemp or soft rope, called _gasket_, is introduced by winding it round the piston so as to render it an even and compact mass. When the space is thus filled up, the top of the piston is attached to the bottom by screws. The curved form of the space within which the hempen packing is confined is such that when the screws are tightened, that part of the packing which is nearest to the top and bottom of the piston is forced against the cylinder, so as to produce upon the two parallel rings as much pressure as is necessary to render it steam-tight. When by use the packing is worn down so as to produce leakage, the cover of the cylinder must be removed, and the screws connecting the top and bottom of the piston tightened: this will force out the packing and render the piston steam-tight. This packing is lubricated by melted tallow let down upon the piston from the funnel inserted in the top of the cylinder, furnished with a stop-cock to prevent the escape of steam. The lower end of the piston-rod is formed slightly conical, the thickest part of the cone being downward. It is passed up through the piston, and a nut or wedge between the top and bottom is inserted so as to secure the piston in its position upon the rod.

The process of removing the top of the cylinder for the purpose of tightening the screws in the piston is one of so laborious a nature, that the men entrusted with the superintendence of these machines are tempted to allow the engine to work notwithstanding injurious leakage at the piston, rather than incur the labour of tightening the screws as often as it is necessary to do so.

To avoid this inconvenience, the following method of [Pg244] tightening the packing of the piston without removing the lid of the cylinder, was contrived by Woolf. The head of each of the screws was formed into a toothed pinion, and as these screws were placed at equal distances from the centre of the piston, these several pinions were driven by a large toothed wheel, revolving on the piston-rod as an axis. By such an arrangement it is evident that if any one of the screws be turned, a like motion will be imparted to all the others through the medium of the large central wheel. Woolf accordingly formed, on the head of one of the screws, a square end. When the piston was brought to the top of the cylinder, this square end entered an aperture made in the under side of the cover of the cylinder. This aperture was covered by a small circular piece screwed into the top of the cylinder, which was capable of being removed so as to render the square head of the screw accessible. When this was done, a proper key being applied to the square head of the screw, it was turned; and by being turned, all the other screws were in like manner moved. In this way, instead of having to remove the cover of the cylinder, which in large cylinders was attended with great labour and loss of time, the packing was tightened by merely unscrewing a piece in the top of the cylinder not much greater in magnitude than the head of one of the screws.

This method was further simplified by causing the great circular wheel already described to move upon the piston-rod, not as an axis, but as a screw, the thread being cut upon a part of the piston-rod which worked in a corresponding female screw cut upon the central plate. By such means, the screw whose head was let into the cover of the cylinder which turned, would cause this circular plate to be pressed downwards by the force of the screw constructed on the piston-rod. This circular plate thus pressed downwards, acted upon pins or plugs which pressed together the top and bottom of the cylinder in the same manner as they were pressed together by the screws connecting them as already described.

METALLIC PISTONS.

(142.) The notion of constructing a piston so as to move steam-tight in the cylinder without the use of packing of vegetable [Pg245] matter was first suggested by the Rev. Mr. Cartwright, a gentleman well known for other mechanical inventions. A patent was granted in 1797 for a new form of steam engine, in which he proposed to use the vapour of alcohol to work the piston instead of the steam of water: and since the principle of the engine excluded the use of lubrication by oil or tallow, he substituted a piston formed of metallic rings pressed against the surface of the cylinder by springs, so as to be maintained in steam-tight contact with it, independently either of packing or lubrication. Although the engine for which this form of piston was intended never came into practical use, yet it is so simple and elegant in its structure, and forms a link so interesting in the history of the steam engine, that some explanation of it ought not to be omitted in this work.

The steam-pipe from the boiler is represented cut off at B (_fig._ 66.); T is a spindle-valve, for admitting steam above the piston, and R is a spindle-valve in the piston; D is a curved pipe forming a communication between the cylinder and the condenser, which is of very peculiar construction. Cartwright proposed effecting a condensation without a jet, by exposing the steam to contact with a very large quantity of cold surface. For this purpose, he formed his condenser by placing two cylinders nearly equal in size, one within the other, allowing the water of the cold cistern in which they were placed to flow through the inner cylinder, and to surround the outer one. Thus, the thin space between the two cylinders formed the condenser.

The air-pump is placed immediately under the cylinder, and the continuation of the piston-rod works its piston, which is solid and without a valve. F is the pipe from the condenser to the air-pump, through which the condensed steam is drawn off through the valve G on the ascent of the piston, and on the descent this is forced through a tube into a hot well H, for the purpose of feeding the boiler through the feed-pipe I. In the top of the hot well H is a valve which opens inwards, and is kept closed by a ball floating on the surface of the liquid. The pressure of the condensed air above the surface of the liquid in H forces it through I into the boiler. When the air accumulates in too great a degree [Pg246] in H, the surface of the liquid is pressed so low that the ball falls and opens the valve, and allows it to escape. The air in H is that which is pumped from the condenser with the liquid, and from which it was disengaged.

Let us suppose the piston at the top of the cylinder: it strikes the tail of the valve T, and raises it, while the stem of the piston-valve R strikes the top of the cylinder, and is pressed into its seat. A free communication is at the same time open between the cylinder, below the piston and the condenser, through the tube D. The pressure of the steam [Pg247] thus admitted above the piston acting against the vacuum below it, will cause its descent. On arriving at the bottom of the cylinder, the tail of the piston-valve R will strike the bottom, and it will be lifted from its seat, so that a communication will be opened through it with the condenser. At the same moment, a projecting spring K, attached to the piston-rod, strikes the stem of the steam-valve T, and presses it into its seat. Thus while the further admission of steam is cut off, the steam above the piston flows into the condenser, and the piston being relieved from all pressure, is drawn up by the momentum of the fly-wheel, which continues the motion it received from the descending force. On the arrival of the piston again at the top of the cylinder, the valve T is opened and R closed, and the piston descends as before, and so the process is continued.

The mechanism by which motion is communicated from the piston to the fly-wheel is peculiarly elegant. On the axis of the fly-wheel is a small wheel with teeth, which work in the teeth of another larger wheel L. This wheel is turned by a crank, which is worked by a cross-piece attached to the end of the piston-rod. Another equal-toothed wheel M is turned by a crank, which is worked by the other end of the cross-arm attached to the piston-rod.

One of the peculiarities of this engine is, that the liquid which is used for the production of steam in the boiler circulates through the machine without either diminution or admixture with any other fluid, so that the boiler never wants more feeding than what can be supplied from the hot well H. This circumstance forms an important feature in the machine, as it allows of ardent spirits being used in the boiler instead of water, which, since they boil at low heats, promised a saving of fuel. The inventor proposed that the engine should be used as a still, as well as a mechanical power, in which case the whole of the fuel would be saved.

(143.) That part of Cartwright's piston which in the common piston is occupied by the packing of gasket, already explained (141.), was filled by a number of rings, one placed within and above another, and divided into three or four [Pg248] segments. Two rings of brass were made of the full size of the cylinder, and so ground as to fit the cylinder nearly steam-tight. These were cut into several segments A A A (_fig._ 67.), and were placed one above the other, so as to fill the space between the top and bottom plates of the piston. The divisions of the segments of the one ring were made to fit between the divisions of the other. Within these another series of rings, B B B, were placed, similarly constructed, so as to fit within the first series in the same manner as the first series were made to fit within the cylinder. The joints of the upper series of each set of rings are exhibited in the plan (_fig._ 67.); the places of the joints of the lower series are shown by dotted lines; the position of the rings of each series one above the other is shown in the section (_fig._ 68.). The joints of the inner series of rings are so placed as to lie between those of the outer series, to prevent the escape of steam which would take place by one continued joint from top to bottom of the packing. The segments into which the rings are divided are pressed outwards by steel springs in the form of the letter V, the springs which act upon the outer series of segments abutting upon the inner series, and those which act on the inner series abutting upon the solid centre of the piston: these springs are represented in _fig._ 67.

(144.) An improved form was given to the metallic piston by Barton. Barton's piston consists of a solid cylinder of cast iron, represented at A in section in _fig._ 69., and in plan in [Pg249] _fig._ 70. In the centre of this is a conical hole, increasing in magnitude downwards, to receive the piston-rod, in which the latter is secured by a cross-pin B. A deep groove, square in its section, is formed around the piston, so that while the top and bottom of the piston form circles equal in magnitude to the section of the cylinder, the intermediate part of the body of the piston forms a circle less than the former by the depth of the groove. Let a ring of brass, cast iron, or cast steel, be made to correspond in magnitude and form with this groove, and let it be divided as represented in _fig._ 70., into four segments C C C C, and four corresponding angular pieces D D D D. Let the groove which surrounds the piston be filled by the four segments with the four wedge-like angular pieces within them, and let the latter be urged against the former by eight spiral springs, as represented in _fig._ 69. and _fig._ 70. These springs will abut against the solid centre by the piston, and will urge the segments C against the cylinder. The spiral springs which urge the wedges are confined in their action by steel pins which pass through their centre, and by being [Pg250] confined in cylindrical cavities worked into the wedges and into corresponding parts of the solid centre of the piston, as the segments C wear, the springs urge the wedges outwards, and the points of the latter protruding, are gradually worn down so as to fill up the spaces left between the segments, and thus to complete the outer surface of the piston.

Various other forms of metallic pistons have been proposed, but as they do not differ materially in principle from those we have just described, it will not be necessary here to describe them.

[Pg251]

CHAP. IX.

CONSTITUENTS OF COAL. — PROCESS OF COMBUSTION. — HEAT EVOLVED
IN IT. — FORM AND STRUCTURE OF BOILER. — WAGGON BOILER. —
FURNACE. — METHOD OF FEEDING IT. — COMBUSTION OF GAS IN FLUES.
— CONSTRUCTION OF GRATE AND ASH-PIT. — MAGNITUDE OF HEATING
SURFACE OF BOILER. — STEAM-SPACE AND WATER-SPACE IN BOILER. —
POSITION OF FLUES. — METHOD OF FEEDING BOILER. — LEVEL GAUGES.
— SELF-REGULATING FEEDERS. — STEAM-GAUGE. — BAROMETER-GAUGE. —
INDICATOR. — COUNTER. — SAFETY-VALVE. — FUSIBLE PLUGS. —
SELF-REGULATING DAMPER. — SELF-REGULATING FURNACE. — POWER AND
DUTY OF ENGINES. — HORSE-POWER OF STEAM ENGINES. — EVAPORATION
PROPORTIONAL TO HORSE-POWER. — SOURCES OF LOSS OF POWER. —
ABSENCE OF GOOD PRACTICAL RULES FOR POWER. — COMMON RULES
FOLLOWED BY ENGINE MAKERS. — DUTY DISTINGUISHED FROM POWER. —
DUTY OF BOILERS. — PROPORTION OF STROKE TO DIAMETER OF
CYLINDER. — DUTY OF ENGINES. — CORNISH SYSTEM OF INSPECTION. —
ITS GOOD EFFECTS. — HISTORICAL DETAIL OF THE DUTY OF CORNISH
ENGINES.