[Footnote: A paper read before the Mechanical Section of the British
Association, at Montreal, August, 1884.]
Four years ago, in August, 1880, a paper was read on this subject before
the Annual Summer Meeting of the Mechanical Engineers' Society of Great
Britain, then held in Barrow-in-Furness, describing this valve motion and
its functions, which was then comparatively new. It was, however,
illustrated by its application to a large express goods (freight) engine,
built by the London and North-Western Railway Company (England) specially
to test the advantages and the endurance of the gear. This engine had
cylinders of 18 inches in diameter and 24 inch stroke, and six wheels
coupled 5 feet 1 inch diameter, and was designed by Mr. Webb, the
Company's chief engineer, for their heavy fast goods traffic on the main
line. The engine has been running this class of traffic ever since. In
January, 1884, it was passed through the repair shops for a general
overhauling, when it was found that the valve motion was in such good
condition as to be put back on the engine without any repairs.
The main object of this present paper is to deal with the advantages of
the valve gear and its application to various classes of engines both on
land and at sea, and with the results of such applications, rather than
treating it as a novelty, to give an exhaustive description of its
construction and functions, which was done in the paper above referred to.
A very short description of its action and main features will, however,
be necessary to the completeness of the paper, and as a basis from which
the improved results to be recorded should necessarily be shown to spring.
The essential feature of this valve gear is that movement for the valve is
produced by a combination of two motions at right angles to each other;
and by the various proportions in which these are combined, and by the
positions in which the moving parts are set with regard to each other, it
gives both the reversal of motion and the various degrees of expansion
required. Eccentrics are entirely dispensed with and the time-honored link
gear abandoned, the motion is taken direct from the connecting rod, and by
utilizing independently the backward and forward action of the rod, due to
the reciprocation of the piston, and combining this with the vibrating
action of the rod, a movement results which is suitable to work the valves
of engines, allowing the use of any proportions of lap and lead desired,
and giving an almost mathematically correct "cut-off" for both sides of
the piston and for all points of expansion intermediately, as well as a
much quicker action at the points of "cut-off" and "release" than is given
by a link gear.
The machinery for accomplishing this is both less costly and less
complicated than the ordinary link motion, and is shown in elevation on
cut, which is a view of the complete motion as on the first London and
North-Western locomotive. Here E is the main valve lever, pinned at D to a
link, B, one end of which is fastened to the connecting rod at A, and the
other end maintained in about the vertical by the radius rod, C, which is
fixed at the point, C¹. The center or fulcrum, F, of the lever, E,
partaking of the vibrating movement of the connecting rod at the point, A,
is carried in a curved slide, J, the radius of which is equal to the
length of the link, G, and the center of which is fixed to be concentric
with the fulcrum, F, of the lever when the piston is at either extreme end
of its stroke. From the upper end of the lever, E, the motion is carried
direct to the valve by the rod, G. It will be evident thus that by one
revolution of the crank the lower end of the lever, E, will have imparted
to it two different movements, one along the longer axis of the ellipse,
traveled by the point, A, and one through its minor axis up and down,
these movements differing as to time, and corresponding with the part of
the movement of the valve required for lap and lead, and that part
constituting the port opening for admission of steam.
[Illustration: JOY'S REVERSING AND EXPANDING VALVE GEAR.]
The former of these is constant and unalterable, the latter is
controllable by the angle at which the curved slide, J, may be set with
the vertical.
It will further be evident that if the lever, E, were pinned direct to
the connecting rod at the point, A, which passes through a practically
true ellipse, it would vibrate its fulcrum, F, unequally on either side of
the center of the curved slide, J, by the amount of the versed sine of the
arc of the lever, E, from F D; it is to correct this error that the lever,
E, is pinned at the point, D, to a parallel motion formed by the parts, B
and C. The point, D, performing a figure which is equal to an ellipse,
with the error to be eliminated added, so neutralizing its effect on the
motion of the fulcrum, F.
The "lap" and "lead" are opened by the action of the valve lever acting as
a lever, and the port opening is given by the incline of the curved slide
in which the center of that lever slides, and the amount of this opening
depends upon the angle given to that incline. When these two actions are
in unison, the motion of the valve is very rapid, and this occurs when the
steam is being admitted. Then follows a period of opposition of these
motions, during which time the valve pauses momentarily, this
corresponding to the time when the port is fully open. Further periods of
unison follow, at which time the sharp "cut-off" is obtained.
The "compression" resulting with this gear is also reduced to a minimum,
owing to the peculiar movement given to the valves (_i. e._, the series of
accelerations and retardations referred to), as, while the "lead" is
obtained later and quicker, the port is also shut for "compression" later
and quicker, doing away with the necessity for a special expansion valve,
with its complicated and expensive machinery, and allowing the main valve
to be used for expansion, as the "compression" is not of an injurious
amount, even with a "cut-off" reduced to 15 per cent., or about 1/6 of the
stroke.
Thus, so far as the distribution of the steam and its treatment in the
cylinder is concerned, a marked advantage is shown in favor of this valve
gear. But next in its favor, as before said, is that the above advantages
are not gained at the cost of added complication of parts or increased
cost of machinery, but the reverse, as this gear can be built at a less
cost than link gear, varying according to the circumstances, but reaching
as high as a saving of 25 per cent., or, if it be compared with a link
gear supplemented by the usual special expansion valve and gear as
employed on marine engines, then the total saving is fully 50 per cent.,
and an equally good result is obtained as to the distribution and
subsequent treatment of the steam.
After accuracy of result and reduction in cost may rank saving room and
the advantages arising therefrom (though for steamships perhaps this
should have come first). Taking locomotives of the inside cylinder type,
which is the general form in use in England and the continent of Europe,
by clearing away the eccentrics and valves from the middle of the engine,
much larger cylinders may be introduced and a higher rate of expansion
employed, and this is being done. Also room is left for increasing the
length and wearing surfaces of all the main bearings with even less
crowding than is now the case with engines with the smaller cylinders.
But this advantage of saving room comes much more prominently forward in
marine engines, especially in war ships, where every inch of room saved is
valuable; and in the new type of triple-cylinder engines now coming so
much into vogue in the mercantile marine, whether those engines be only
the ordinary three-cylinder engines with double expansion, or the newer,
triple expansion engine, expanding the steam consecutively through three
cylinders--the form of marine engine which promises to come into use
wherever high-class work and economy are required. On this system, by
placing all the valve chests in front of the cylinders instead of between
them, or in a line with them, sufficient room is saved to get the new-type
three-cylinder engine into the space occupied by the old form of
two-cylinder engine.
Besides these prominent advantages there are others which, though of minor
importance, are still necessary to the practical and permanent success of
any new mechanical arrangement, such as the accessibility of all the
working parts while in motion, for examination and oiling; the ease with
which any part or the whole can be stripped and cleaned, or pinned up out
of the way in case of break down or accident, or got at and dismantled for
ordinary repair; the ease with which the whole may be handled, started,
reversed, or set at any point of expansion--all these being
recommendations to enlist the care and attention of the engineers in
charge by lightening their duties and rendering the engines easy to work.
With those advantages it is perhaps not surprising that this valve gear
has been very considerably adopted for many classes of steam engines,
especially where a high result has been required, with economy of space,
and a minimum of complication.
Having crucially tested the original engine on the London and
North-Western Railway, Mr. Webb proceeded to build others similar, and on
his bringing out his Compound Express Engine--notably the most advanced
step in locomotive design of the present day--he adopted this valve gear
throughout. There are now a number of these engines running some of the
fastest trains on the London and North-Western Railway, with the most
satisfactory results.
Following these, others of the leading railways took up the system, and
prominently among these Mr. Worsdell, of the Great Eastern Railway, built
a number of large express engines for his fast and heavy traffic, and is
now building a number of others similar as to the valve gear for his
suburban traffic, which is specially heavy. Also the Lancashire and
Yorkshire and the Midland and others of the chief railways are employing
the system specially for large express engines; the Midland engines having
cylinders of 19 inches diameter by 26 inches stroke, and four coupled
wheels of 7 feet diameter. A number of the above-named engines have run
large mileages, in many cases already exceeding 100,000 miles per engine.
For other countries also a number of locomotive engines have been built or
contracted for--both of inside and outside cylinder types--making a total
of nearly 800 locomotives built and building, many of them being of
special design and large size, up to 20 inches and 21 inches diameter of
cylinder.
In all these the absence of wire-drawing may be specially noted by the
full line at the top of the diagram, showing the admission of steam--this
fullness arising from the rapid and full opening of the port for
admission.
Passing now to the other great type of engines, those covered under the
general designation of marine engines, this gear has been applied to
nearly 40,000 H.P. indicated, built and building, and to all classes and
sizes, from the launch engine with cylinders 8 inches by 9 inches, running
at 600 to 700 revolutions per minute, up to engines for the largest class
of war ships, such as her Britannic Majesty's steel cruiser Amphion, of
5,000 H.P., with cylinders in duplicate of 46 inches and 86 inches
diameter, and 3 feet 3 inches stroke, running 100 revolutions per minute.
An examination of the indicator diagrams taken from these engines shows
that no wire-drawing takes place, and that, though the expansion is
carried to a point beyond the ordinary requirements, the compression is
but slightly increased. In all the diagrams taken from this valve motion
there is seen the clear, full upper line showing an abundant admission of
steam without any wire-drawing, and also the distinctly marked points
where "cut-off" or "suppression" and where "release" takes place, showing
the rapid action of the valves at those points.
It is well known to engineers that to obtain the maximum advantage out of
compounding, it is necessary to cut off in the low pressure cylinder at a
point corresponding to the relation between the low and the high, and that
point should be unaltered, whereas the point of cut-off in the high may at
the same time be varied to suit the work to be done.
In an ordinary link motion engine (where both links are connected to the
same weigh shaft), when linking up the high pressure cylinder to cut-off
short, the same change is necessarily made in the low. By the use of the
Joy gear, cut-off valves may be fitted to both cylinders, that for the low
pressure being fixed at the constant position required by the proportion
of the cylinders, while that on the high is adjustable; of course, in this
case, the position of the quadrants must be only changed for reversing. In
arranging the independent cut-off on the Joy gear, it is only necessary to
increase the length of the vibrating link beyond the point of attachment
for the main valve spindle connection to obtain a point from which motion
may be taken to actuate the cut-off valve; even then the cost of the Joy
gear for both cylinders is but little more than for a single set of link
gear.
This arrangement gives an absolutely perfect distribution of steam for
compounding, also equalizes the power developed by both cylinders, and is
far more simple and inexpensive than any other gear in existence.
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