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American Technical Society.

Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. (page 20 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 20 of 30)
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or change the cut-off. The pivot P, is located precisely as in the
Hackworth gear, and the lever FP can be turned from "full gear
forward", as shown in the figure, to " full gear backing", as shown
by the line BP, intermediate positions give different cut-offs as
with the Hackworth gear. Since FA is made equal to FP, the
point A will always swing through P, no matter where F may
be, and will coincide with P, when the engine is on dead center.
The leads therefore will remain constant, as in the preceding case.

The Marshall gear is sometimes made with at the right of
A on a prolongation of the line EA. In this case if the same
kind of valve is to be used, the eccentric E must move with the
crank instead of 180 from it. The Marshall gear is frequently
used on marine engines, the one eccentric being simpler than the
t\vo required by the Stephenson.

Joy Gear. Perhaps the most widely known, and certainly
one of the best radial gears is the Joy, outlined in Fig. 4H. It
is frequently used on marine engines and on some English loco-
motives. No eccentrics are used, the valve motion being taken
from C, a point on the connecting rod. II is a fixed pivot sup-
ported on the cylinder casting. The lever ED has a block pivoted
at A, which slides back and forth in a curved slotted guide. The
guide and the lever PF are fastened to the reverse shaft P, and
by means of a reverse rod leading off from F, can be turned from
full gear forward, as shown, to full gear backing when the pin F
moves over to 13. Motion is transmitted to the valve stem by
means of the radius rod EG. The proportions are such that when
the crank is on either dead point, the pivot of block A coincides



VALVE GEARS



B3



with I*, so that the curved guide may then be set in any position
without moving the valve; therefore the leads are constant. This
gear gives a rapid motion to the valve when opening and closing
and a more nearly constant compression than the Stephenson gear,
and the cut-off can be made very nearly equal for all grades of
the gear. Its many joints cause wear and its position near the
crosshead, makes a careful inspection of the crosshead and piston
exceedingly difficult while the engine is running.




Fig. 46.

Walschaert Gear. This radial valve gear, although seldom
seen in the L'nited States, is the valve mechanism most commonly
used on locomotives built on the continent of Europe. Like all
other radial gears, it gives constant lead, and a distribution of
steam very nearly alike for each end of the cylinder. In this
respect it is superior to the Stephenson link, and gives without
doubt better economy, but its mechanical construction is compli-
cated, and not well adapted to the American type of locomotive.
Fig. 47 illustrates this type of gear. S is the center of the driver
axle. The crank pin K has forged on its center end an arm KE,
on which the pin E is fixed. This arm lies parallel to the plane
of the driving wheels, and being fixed to the crank pin, turns with
the wheel, allowing the connecting rod to pass between it and the
driving wheels. In this manner the point E moves around S in a
circle, and moves the rod EH back and forth just as if it were an
eccentric. It is so made that ES is perpendicular to the crank
KS, and therefore the action of the pin E is equivalent to an eccen-
tric with no angular advance.



293



VALVE GEARS



This arm reaching back from the outer end of the crank pin
is one of the most objectionable features on the construction, and
is sometimes replaced by the regular type of eccentric put on the
shaft between the driving wheels.

The eccentric rod EH causes the box link IIP to oscillate on
fixed trunions P. This link has a groove curved to a radius equal
to GD, the length of the radius rod. A block pivoted at G, on
one end of the radius rod, is free to move up or down in this




Fig. 47.

groove. The valve derives its motion from C, a pivot on the float-
ing lever CA. Point A receives motion from the crosshead; point
D from the eccentric and the curved link ; and a combination of
these two imparts motion to C (which can slide only along the
dotted line). A bell-crank lever pivoted above the link shifts
the mechanism from "full gear forward" when F is moved to B,
thus raising G above the link pivot or saddle pin.

ADJUSTABLE ECCENTRICS.

The position of an eccentric for a plain slide valve is 00 plus
the angular advance ahead of the crank, in the direction in which
the engine is to turn. Thus A, Fig. 48, is correctly placed, rela-
tive to the crank C, if the engine turns right-handed. For run-
ning in the opposite direction, the position of the eccentric is at
D. Some engines are provided with a reversing mechanism which
causes the eccentric to shift from A to D, either along the are



294



VALVE GEARS



55



ABD, or along the straight line AEI). Such engines provide,
not only for reversing, but for changing the cut-off as well If
the eccentric moves on the arc to OB, the angular advance is




Fig. -18.

increased and all the events of the stroke are hastened as well as
the cut-off, but the travel of the valve is not changed.

Zeuner's diagram, Fig. 49, is lettered to correspond with Fig.
48, and shows the effect of changing the angular advance from
FOA to FOB. If OK rep-
resents the lap, the crank
angle at cut-off will decrease
from HOK to IIOL, and the
lead will increase very much,
viz., from GF to IIF. If the
eccentric is shifted on the
straight line to E (Fig. 48),
a different valve motion will
result. The angular advance
is increased as before, so that
all the events are hastened,
but the eccentricity is now

only OE instead of OB and Fic , 49

the valve travel is conse-
quently reduced. Zeuner's diagram for this case, Fig. 50, shows a
decrease in crank angle at cut-off from IOM to ION, and no




56



VALVE GEARS



change in the lead IF. Let
us consider the eccentric posi-
tion OA, Fig. 48. In this
position OI represents the
displacement of the valve*
from mid-position when the
engine is on center. If the
eccentric moves to OB, the
displacement will be OM,
which is greater, showing an
increase in lead equal to IM .
but if the position is OE in.
stead of OB, the displace-

[,ijcr 5Q

ment from mid-position will
be OI as before. It is evident that the eccentric can move on the
straight line from A to J), without changing the lead, while the
decreased valve travel will result hi an earlier cut-olT. If thn





Fig. 5L

shifting eccentric is to be used for an automatic cut-off, as in
the various types of iiy-wheel governor, the curved path is not



VALVE GEARS



r,7



desirable on account of excessive lead at short cut-off, but if it is
to be used only as a means of reversing, it is preferable to the
straight line.

All fly-wheel governors operate by shifting the eccentric,
either to change the angular advance, the travel of the valve, or
both. Fig. 51 illustrates the principle of a governor arranged to
give decreasing lead, but as these mechanisms are described in
the Steam Engine Part I, under the head of governors, a further
discussion will not be given here.

The device shown in Fig. 52 is often used for reversing
engines of small launches. The eccentric E is loose on the shaft
between a fixed collar G, and a hand wheel JI. A stud project-
ing from the eccer.tric passes through a curved slot in the disc of
the wheel, and can be clamped by a hand nut F. When running
forward with the crank at C,
the center of the eccentric is at
A, and the nut clamped at F.
To reverse, steam is shut off,
and when the engine stops, the
nut F is loosened, and then
moved to B and clamped; or
after F is loosened, the wheel,
shaft, crank and propeller are
turned over by hand until B



strikes the stud at F, where it is
clamped. The engine will then
run astern.

To study the application of
the Zeuner diagram to this
form of mechanism turn again
to Fig. 51. If OA is the de-
sired eccentricity for a normal
position of the governor, the
perpendicular distance of A

from OF is made equal to the lap OI, plus the desired lead. Pivot
D is then located equally distant from A and I. Zeuner's dia-
gram for this gear, drawn to an enlarged scale, is shown in Fig. 53.
The angular advance F'OB is laid back toward OC. OB is the




297



58



VALVE GEARS



maximum eccentricity; OI, the lap or the desired least eccen-
tricity. An arc, with proper radius, described through B and I
shows the path of the eccentric. If the eccentric moves in to A,
the crank angle at cut-off is decreased from COD to COE, and
the lead decreased from FI to GI. A slight decrease in lead is




not objectionable, since the speed is not allowed to increase more
than two or three per cent; and further, as the lead increases, com-
pression decreases, so that one influence helps to counteract the other.
The decrease in maximum port opening from BII to AK is un-
avoidable, but it is permissible, since it occurs only when the load
decreases, and when less steam should be admitted to the cylinder.

DOUBLE VALVE GEARS.

It has been shown in the preceding discussion, that a plain
slide valve under the control of a gear that gives a variable
cut-off, such as a shifting eccentric or a link motion, will not
give a satisfactory distribution of steam at short cut-off owing
to execessive compression, variable lead, or early release. These
difficulties are overcome in a measure by the use of the radial
gear ; and also by the use of two valves instead of one. The main



VALVE GEARS




valve controls admission, re-
lease, and compression ; the
other, called the cut-off valve,
regulates the cut-off only,
which may be changed with-
out in any way affecting the
other events of the stroke.
This cut-off valve may be
placed in a separate valve
chest, or it may be placed on
the back of the main valve.

Meyer Valve. The most
common form of double valve
gear is the Meyer Valve, Fig.
54. The cut-off valve is made
in two parts and works on the
back of the main valve. The
two parts are connected to a
valve spindle with a riglit-
and left-hand thread, so that
their positions may be altered
by rotating the valve spindle.

A swivel joint is usually
fitted in the valve-spindle be-
tween the steam chest and the
head of the valve rod, and the
valve spindle prolonged into
a tail rod which projects
through a stuffing box on the
head of the steam chest. See
Fig. 55. The end of this tail
rod is square in section and
reciprocates through a small
hand wheel by means of which
it can be rotated while the en-
gine is running, whatever the
position of the valve may be.



VALVE GEATCS



Each valve is under the control of a separate eccentric. The
eccentric moving the main valve is usually fixed, while the cut-off
valve eccentric may be under the control of a governor. Since
a slight compression is desired, the main valve is set to give late
cut-off. This will give late release and late compression, and allow
a wide range of cut-off for the cut-off valve. With this gear, lead,
release, and compression are entirely independent of the ratio of
expansion, and the cut-off is much sharper, because the cut-off
valve, when closing the ports, is always moving in a direction




Fig. 55.

opposite to that of the main valve. The valve may be designed
by means of Zeuner's diagrams.

Design of a Meyer Valve. Let us design a Meyer Valve
having an eccentricity of 2 inches. Let the eccentricity of the
cut-off valve be 2|- inches and the relative travel of the cut-off
valve in relation to the main valve be 3 inches. This will make
the relative motion of the cut-off valve equivalent to the travel of
a plain slide valve with an eccentricity of 1^ inches. Let the out-
side lap on the main valve be | inch, the lead - 3 V inch, the com-
pression 95 per cent of the stroke, and let the ratio of the length
of the crank to connecting rod be six.



300



VALVE GEARS



fil



In Fig. 56 draw XOY equal to 4 inches, the main valve
travel. Lay off YD = 95 per cent of 4 3.8 inches, and with
a radius of 12 inches, and the center on YX produced draw the arc
DII.K. II.K.O is the crank position at compression. C.K.O, the
crank position at cut-off, is found in a similar manner. Layoff OI
c'qual to the lap plus the lead, and draw the valve circle for the
main valve through I and O with a diameter equal to its eccen-
tricity of 2 inches. To do this take a radius equal to 1 inch, an.d
draw arcs from I and (.) as centers that shall intersect at B. B is



C.K.




H

Fig. 56.

tne center of the valve circle and OBE is the eccentricity, 2 inches.
With E as a center, and with a radius equal to half the relative
travel of the cut-off valve (in this case 14- inches), draw an arc.
With O as a center and with a radius equal to 2^ inches, the
eccentricity of the cut-off valve, draw another arc intersecting the
first one at F. On OF as a diameter construct a valve circle.
This valve will represent the absolute motion of the cut-off valve,
independent of the motion of the main valve. This circle then
will show the displacements of the cut-off valve from the center of
the steam chest. With E as a center and with a radius equal to
FO draw an arc, and with as a center and with a radius equal



801



VALVE GEARS



to EF draw another arc intersecting the first at G. On OG as a
diameter construct a valve circle. This circle will then represent
a travel of the cut-off valve moving on the main valve. That is,
it will represent the displacements of the cut-off valve from the
center of the main valve. This circle is not, properly speaking,



MIN.CUT

OFF 15 '



K.




H.



Fig. 57.

a valve circle and OG is not an eccentricity, but simply represents
the relative motion of the two valves. This can be proved by
analytical geometry, but an inspection of the figure shows that
this must be true.

Draw the crank line OC at any position, cutting the valve
circles at a and and c. O represents the absolute displace-
ment of the cut-off valve, that is, from the center of the steam
chest and O^ represents the displacement of the main valve, The



VALVE GEARS 63



relative displacement of the cut-off valve, that is, from the center
of the main valve, will be the difference between Oc and O^, since
both valves are moving in the same direction. By careful meas-
urement it will be found that ()/; = Oc - O<7, and any arc as Ol
on the auxiliary circle OIG will correctly represent the displace-
ment of the cut-off valve from the center of the main valve at the
corresponding crank angle.

Fig. 57 shows the crank angle at head-end compression U.K.,
and at crank-end compression C.K., the main valve circle, and the
auxiliary circle which are transferred from Fig. 56. The con-
struction lines and all lines not essential to the figure are omitted
to avoid confusion.

Lay off on Fig. 57, OI equal to the outside lap | inch and
draw the head-end lap circle II.E.O. It will intersect the valve
circle for the main valve at L and M. Through L draw the crank
position at admission (head-end) II.A. and the crank position at
cut-off through M. This gives the greatest possible cut-off. The
cut-off valve may be set to give a much earlier cut-off than this,
but of course a later setting would be of no avail for the port
would be closed by the main valve at this angle. The crank line
OMII cuts the auxiliary circle at .N, so that ON (lj|- inches) is
the clearance of the cut-off valve. That is, the edge of the cut-off
valve must be set 1* :> inches from the edge of- the main valve port
in order to cut-off at this crank angle. The full lines of Fig. 54
show the cut-off valve placed in this position.

The intersection of II.K.O with the lower valve circle, gives
the inside lap at the head end of the cylinder. This line comes so
nearly tangent to the valve circle that the intersection can be
determined only by dropping a perpendicular to II.K.O. from F 1 .
This cuts the circle at P and OP -^ inch equals the head-end
inside lap, and II.E.I. represents the corresponding lap circle.

The crank-end angle at compression is O.K. which cuts the
upper valve circle at N', giving an inside lap for the crank end of
OX' = -JJ inch. To make this intersection more apparent the
perpendicular can be drawn from as previously explained.

Suppose that it was required that the minimum cut-off should
be 15 per cent. Find the crank position at 15 per cent of the stroke
111 the same manner as the crank position was found at compression.



64 VALVE GEAKS



Produce 4his line through () until it cuts the auxiliary circle at S.
Then ()S = -^- inch the required lap for the cut-off valve in
order to cut-off at 15 per cent of the stroke. The dotted lines in
Fio\ 54 show the cut-off valve drawn in this position.

For a valve of this sort, the cylinder port should be 14 inches
wide and the valve port 1 inch wide. Fig. 54 shows this valve laid
out to scale, but as this process is in all respects similar to that
described for laying out a plain slide valve, it will not be described
in detail.

DROP CUT=OFF GEARS.

The ordinary slide valve controls eight different events of the
stroke, that is. admission, cut-off, release, and compression for both
ends of the cylinder. A change in the setting of a plain slide
valve that affects any one event on the crank end, let us say, will
also change to a greater or less degree every other event of the
stroke, on the head end as well as on the crank end; so that in
setting a slide valve the desired position for one event must
usually be sacrificed in order to make the others less objectionable.

In order to provide a better distribution of steam than is pos-
sible with a single valve, some engines have four valves, two at
each end of the cylinder. In horizontal engines, two are placed
above the center line of cylinder and two below. The upper are
for admission and cut-off, the lower for release and compres-
sion. Since each valve controls but two events, a very satisfactory
adjustment can be made and the extra complication and cost for
large engines are more than overbalanced by the advantages
gained, vi/.: A very much better distribution of steam, short
steam passages and small clearances, separate ports for the admis-
sion of hot steam and the exhaust of the same steam after expan-
sion when its temperature has fallen, and finally the possibility of
opening and closing the ports very rapidly, thus preventing wire-
drawing. The small clearances, short ports and separate admis-
sion and exhaust materially reduce the cylinder condensation, and
thus effect a large saving in the steam consumption.

AVhen four valves are used, for high speeds, the motions of
all must be positive, that is, they must be connected directly to
some mechanism that either pushes or pulls them through their
entire motion, but for speeds up to 100 revolutions or so a disen-



304



VALVE GEARS



gaging mechanism may be used, and the valves may shut of them-
selves, either by virtue of their weight or by means of springs or
dashpots. The valve is usually opened by means of links or rods,
moved by an eccentric, and at the proper point of cut-off the
rod is disengaged from the valve which drops shut, hence the term
'drop cut-off" gears.

Reynolds=CorIiss Gear. The most widely known drop cut-
off gear is. the Reynolds -Corliss, shown in Figs. 58 and 5U; it is
often referred to as the Iunjii.old.<} hook -rcl cast inj gear. An eccen-
tric on the main shaft inves an oscillating motion to a circular disc




Fig. 58.

called the wrist plate, pivoted at the center of the cylinder. It
transmits motion to each of the four valves through adjustable
links known as strain rods or exhaust rods, according to whether
they move the admission or exhaust valves.

The valves which are shown in section in Fig. CO oscillate on
cylindrical seats, and the position of the rods is so determined that
they give a rapid motion to the valve when opening or closing, and
hold it nearly stationary when either opened or closed.

The Reynolds hook is shown in detail in Fig. 59. The steam
arm is keyed to the valve spindle which passes loosely through a
bracket on which the bell-crank lever turns, and the spindle is
packed to make a steam-tight joint where it enters the cylinder.
Motion of the steam rod toward the right will turn the fiell-cratik
lever and raise the Jiook stud. The hook (from which the gear
derives its name) pivoted on this stud, has at one end a hard-



305



66



VALVE GEARS




j



VALVE GEARS 67



ened steel die with sharp, square edges, and at the other end,
a small steel block with a rounded face. As the hook rises, the
fwok die engages the stud die which is fastened to the steam arm,
and one end of the steam arm is thus raised. This turns the
valve in its seat and admits steam. As the hook continues to
rise, its stud moves in an arc above the valve spindle, and the
round-faced block at its left-hand end strikes the knock-off cam
which causes the hook to turn about its stud and disengage the
Iwok die from the stud die. In raising the steam arm, the dash-
pot rod also is raised and a partial vacuum is created in the dash-
pot. As soon, therefore, as the dies become disengaged, the
dashpot rod quickly drops under the force of this vacuum, thus
turning the steam arm and closing the valve. The striking of the
left-hand end of the hook against the knock-off cam determines
the point of cut-off, by releasing the valve at that instant.

This cam, is a part of the knock-off lever to which the governor
cam rod is fastened. Any action of the governor which would
cause the cam rod to move toward the right would cause this
knock-off lever to turn on its axis, the steam arm, and conse-
quently lower the position of the knock-off cam. This would
cause an earlier contact between the cam and end of hook, and
consequently an earlier cut-off. By lengthening or shortening
the governor cam rod, the point of cut-off can be adjusted to suit
the engine load without changing the speed.

There is a limit to this adjustment, for it can be shown that
a Corliss gear operated by a single eccentric cannot be arranged to
cut-off later than half stroke. Suppose the eccentric is set just 90
ahead of the crank. Then it will reach its extreme position just
as the piston gets to half stroke. If by that time the Jiook which
was rising and opening the admission valve, has not yet struck
the knock-off cam, it cannot strike it at all, for any further motion
will cause the hook to descend to its original position, that is its
position at the beginning of the stroke; the hook will not disen-
gage from the steam arm stud at all and the bell crank will
return, closing the valve in the same manner in which it opened
it. Cut-off will then take place near the end of the stroke, but it
will not be the sharp cut-off produced by the sudden drop when
the dies are disengaged.



807



VALVE GEARS



If the eccentric were set less than 90 ahead of the crank, the
cut-off could be arranged to occur later than half stroke, but this
is decidedly impracticable, for with such a position of the eccen-
tric the action of the valves at release and compression is spoiled.
When it is necessary to cut-off later than half-stroke, as some-
times happens on low.-pressure cylinders of compound engines, it
may be arranged for by means of two eccentrics, one set more
than i)(r ahead of the crank to operate the exhaust valves, and one
less than ( JO ahead to operate the admission valves.

The safety cam shown in Fig.
51) is an important part of a Cor-
liss gear. If for any reason the
engine governor should fail to
act, due, for instance, to the
breaking of its driving belt, the
governor would drop to its low-
est position, .supply more steam
to the engine and allow it to
run away. The ,sv//v/y <-<iin pre-
vents this by moving so far to
the right that it strikes the hook
when it descends to pick up the
steam arm. The hook is conse-
quently turned toward the rioht
all( l tnt>n lifted without engaging
the stud die; the valve conse-




. go.



quently remains closed and the engine stops.

Brown Releasing Gear. In addition to the livyiiohlx Jiook.
several other devices are in use for opening and releasing Corliss
admission valves. Among them the I3rown releasing gear shown
in Fior. ()1 may be noted. The steam rod and dashpot rod are
arranged much the same as in the Reynolds gear. The governor
earn rod operates a plate cam having a curved slot so shaped that it
takes the place of both the knock-off and the safety cam of Fig. 51).



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 20 of 30)