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ference, and that at one part the face of a swells abruptly out, when the pulley c came in contact with this portion it would be forced out, and the lever with which it is connected would have a reciprocating motion; this would act on the expansion-valve and raise it, admitting a certain portion of steam to work the piston. As soon, however, as the swelled portion of the cam, by its revolution, passed the line of the pulley c, the lever would be put into another position, and the valve would be instantly closed, thus effecting the cut-off. By having along the face or breadth of the cam a variety of swells or projections, and by mechanism by which the pulley can he brought in contact with one or other of these " steps" or grades" as they are termed, any amount of expansion can be effected. The steps are so arranged as to cut off the steam at a certain period of the stroke; and thus the engineer can command any degree of expansion required, by making the desired step come in contact with the pulley: this is done by means of a screw. The pulley can also be disconnected from the cam.

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In a form of expansion-valve adopted in locomotives, a supplementary valve to effect the cut-off is placed above the ordinary three-ported slide. This arrangement is shown

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ordinary valve-casing; the travel of the valve mm can be altered at pleasure, so as to cut off the steam at any desired point.

ported slide: ab the steam ports, c the exhaust; the lap is towards the steam side of the valves, as ee. In working engines, it is considered to promote their efficiency by giving what is called "lead" to the valve; this is done by making the port open a little before the termination of the foregoing stroke: thus, suppose the piston just about to terminate the upstroke, the upper port is opened a little, thus admitting steam before the other stroke is quite finished, and in consequence of the lap in the valve the exhaust port is opened sooner; so that by the time the piston begins its down-stroke, the steam from below the piston is escaping freely. If this arrangement was not adopted, we can easily conceive of the engine in its down-stroke having to press against the steam below the piston, which would get slowly out, inasmuch as the valve opens but slowly at the beginning of its stroke. To have the full perfection of working in the cylinder, the escape or release of the steam should be instantaneous if possible; the less steam the piston has to encounter in its motion either up or down the cylinder the better. By the arrangement of the valves now described, the " "lap" and the "lead," the speed of locomotives has been much in

creased.

To obtain the full efficiency of the expansive method of working, it is considered best to have the cut-off instantaneously effected-this the slidevalve cannot do: at the beginning of the throw of the eccentric the motion is slow, and is gradually accelerated; the valve is therefore both opened and closed slowly. In some cases, therefore, expansion-valves and gearing are adopted. One species of expansion-valve is identical in principle and construction with the "throttle-valve;" another form is shown in fig. 96,

a

fig. 96.

it is a species of double beat valve; when this is raised, the steam from the boiler passing down d goes past the upper and lower valves bc, and through e to the cylinder. The supply of steam from the boiler to the pipe d is regulated by the ordinary throttle-valve; the upper valve b is made somewhat larger than the lower c; by this arrangement the pressure is greater on the upper valve, and tends to keep it in its seat; it will be observed that little opening of this valve will admit a large supply of steam, and that it is easily worked. The expansion-valve is worked by a series of · levers and a cam. In p. 81., fig. 156, Mechanics and Mechanism, a diagram is given explaining how the revolution of a cam gives a reciprocating motion to the rod g: now suppose this rod to be connected with the expansionvalve, in such a way that it can lift it and depress it at intervals, and that these intervals are so timed as to close the expansion-valve at the exact period when the cut-off is to be effected, the system of expansive-working with an instantaneous cut-off will be carried out. It only remains for us to describe how the rod g is actuated on at the intervals required. Suppose the pulley c, attached to the end of the lever a, fig. 156, Mechanics and Mechanism, p. 81, to be in contact with the circular part a of the cam; it is obvious that no motion of a reciprocating kind would result. But supposing the circular part to be only continued for a certain distance of its circum

́ference, and that at one part the face of a swells abruptly out, when the pulley c came in contact with this portion it would be forced out, and the lever with which it is connected would have a reciprocating motion; this would act on the expansion-valve and raise it, admitting a certain portion of steam to work the piston. As soon, however, as the swelled portion of the cam, by its revolution, passed the line of the pulley c, the lever would be put into another position, and the valve would be instantly closed, thus effecting the cut-off. By having along the face or breadth of the cam a variety of swells or projections, and by mechanism by which the pulley can he brought in contact with one or other of these "steps" or "grades" as they are termed, any amount of expansion can be effected. The steps are so arranged as to cut off the steam at a certain period of the stroke; and thus the engineer can command any degree of expansion required, by making the desired step come in contact with the pulley: this is done by means of a screw. The pulley can also be disconnected from the cam.

In a form of expansion-valve adopted in locomotives, a supplementary valve to effect the cut-off is placed above the ordinary three-ported slide. This arrangement is shown

in the diagram in fig. 97: a a is the ordinary slide-valve, cb the steam-ports, d the exhaust, and a a the valve; the valve-casing is at ee, ff the valve-casing of the supplementary valve. This valve consists of a solid plate with two apertures; these, when opposite the ports in the cover e e, admit steam to the

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ordinary valve-casing; the travel of the valve mm pleasure, so as to cut off the steam at any desired point.

can be altered at

In some cases the expansion-valve is worked by the governor acting on the cam, as in Maudsley's and Whitelaw's engines. We here give diagrams explanatory of a "self-regulating motion for expansion-valves," the invention of Mr. Howson of Manchester.

In referring to the drawings, fig. 98 is a general view of the apparatus. Figs. 99 and 100 are enlarged views of tappet-rod and levers, showing the different positions they assume: a is a case or nozzle containing a common

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equilibrium or double-beat valve, through which the steam is admitted through the pipe i in connection with the boiler to the cylinder; v is the valve-spindle, u its guide; g a lever for lifting the valve, having its fulcrum on the pillar b secured to the nozzle; c is a tappet-rod, having an upward and downward motion, and actuated by any suitable means from the motion of the crank-shaft, taking care that for one stroke of the piston there are two of the tappet-rod. A recess or slot d is cut in the tappet-rod c for the reception of the lever e, which has its upper end supplied with a notch x and a projection or stopping-piece e. The lower tail of the lever e forms an inclined plane f, which, when required, is allowed to come in contact with and slide against the adjustable stud m. This stud projects from the upper tail of the double lever h, which has its fulcrum on a pillar fixed to the nozzle, the lower tail being furnished with a series of teeth forming the segment of an ordinary worm-wheel; into this gears the worm n, keyed in the shaft or spindle o. On the same shaft is the bevil-pinion p, which gears into the bevil-wheel q fixed to a second shaft, to which a partially rotary motion is communicated by the governor-rod r and lever w. The operation of the machinery is as follows: The engine being in motion, and the tappet-rod c at the extent of its upward stroke, the valve will be down, and the levers c and e in the position shown at figs. 99 and 100; but on its descent, the notch x on the lever e will come in contact with the point of the lever g, thereby raising the valve. On the further descent of the tappet-rod, the lower end or inclined plane of the lever e slides against the stud m, and has a tendency to throw the former into the position shown

at fig. 10. The lever g having its point then released from the notch x, the valve drops by its own weight, and, consequently, the supply of steam is cut off from the cylinder, the lever g assuming its former position as at figs. 8 and 9. On the return or upward stroke of the rod c, the upper part of the notch x sliding against and passing over the rounded or under part of the lever g, immediately assumes its proper position, preparatory to its making another descent. In order to prevent noise as much as possible, the projection e, which prevents the notch x from taking too great a hold on the lever g, is furnished with a small leather buffer. The variable motion obtained from the centrifugal force of the governor-balls through the action of the governor-rod r, lever w, wheel and pinion q and p, and worm n, communicates a like variable motion to the lever h,`and, consequently, to its projecting stud m. It will now be easily perceived that the inclined plane ƒ on the lever e will come in contact with and slide against the stud at different positions in the descent of the rod c, and the valve have varied lengths of time for remaining open; and consequently, varied quantities of steam will be admitted to the cylinder, according to the variations of the governor. The vibratory motion required for the stud m is so small, and the means for effecting it so powerful, that the most trifling alteration in the action of the governor will be communicated to the stud m; while the adoption of the worm and segment will form an infallible and solid bearing for the pressure of the inclined plane,

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Fig. 101 is a sketch in which the governor connection is dispensed with altogether; the stud m being adjusted by hand to a bracket fixed to the nozzle, the side of the slot in the bracket having a scale to which an index on the stud may be pointed, in order that the engineer may at once place it in a position that the steam may be cut off at the particular portion of the stroke required.

The piston-rod, in moving up and down through the cylinder-cover, is kept steam-tight by what is called a "stuffing-box." This is shown in fig. 102: a a is part of the cylinder-cover; a cylindrical hollow cup or box is cast on this of a much larger diameter than the piston-rod bb; this is curved inwards, as shown in the diagram; the aperture in the cylinder-cover is a little larger than the piston-rod, to admit of its easy working; packing, composed of plaited hemp, is wound tightly round the piston-rod bb, at the part ee; the stuffing-box "cover" nn is placed above this packing, and screwed tightly down by the screws.

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