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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 19 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 19 of 30)
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Moving the eccentric on the shaft makes the action of the valve
earlier or later as the angular advance is increased or decreased.

To Put the Engine on the Center. It is usual to put tin-
engine on center before setting the valve. First put the engine
in a position where the piston has nearly completed the outward




stroke, and make a mark M on the guide opposite the corner of
the crosshead or at some convenient place. Also make a mark,
with a center punch, on the frame of the engine near the crank
disc or on the floor. With this punch mark Pas a center, describe
an arc C on the wheel rim, v/ith a tram. A tram is a steel rod
with its ends bent at right angles and sharpened.

Turn the engine past the center until the mark on the guide
again corresponds with the corner of the crosshead, and make
another mark D on the wheel with the tram, keeping the same
center. "With the center of the pulley or crank disc as a center,
describe an arc CD on the rim, which intersects the two arcs drawn
with the tram. .Insect the arc CD on the rim, included between
the two short arcs, and turn the engine until the new point E is at



276



VALVE GEARS 37



a distance from the point on the frame equal to the length of the
tram, in which position the engine will be on the center.

The engine should always be moved in the direction in which
it is to run so that the lost motion of the wrist pin and crank pin
will be taken up the right way. In ease the engine has been
moved too far at any time, it should be turned back beyond the
desired point and brought up to that point while the engine is
moving the right way.

To Set the Valve with Equal Lead. Set the engine on the
dead point and give the eccentric the proper angular advance.
Adjust the length of the valve spindle to give the proper lead for
that end. Kow place the engine on the other dead point and
measure the lead at that end. If the leads are unequal, correct
half the error by changing the length of the valve spindle and
the other half by altering the angular advance. In case the valve
gear has a rocker, the length of the spindle should be such that
the rocker will move as designed. The angular advance should
not be changed, but the equal lead should be obtained by means
of the valve spindle or the eccentric rod.

Second Method. In case it is difficult to turn an engine the

^

following method may be used. First _loosen the eccentric on
the shaft and turn it around until it gives maximum port opening
first at one end and then at the other. If the maximum port
openings are not equal, make them so by changing the length of
the valve spindle by half the difference. When the above adjust-
ment has been made, set the engine on dead center and give the
valve the proper lead by turning the eccentric on the shaft. The
angular advance is thus adjusted.

To Set the Valve for Equal Cut=off. Place the engine on the
dead point, give the eccentric the proper angular advance and
the valve the proper lead. Move the engine forward until cut-off
occurs, then measure the displacement of the crosshead from the
befinninc" of the stroke. Continue moving the emnne forward,

f~\ n n ^

until cut-off takes place on the return stroke and measure the dis-
placement of the crosshead from the beginning of this stroke to
this point. ,

In case the cut-off is earlier at the crank than at the head-end,
the valve spindle is too short. Adjust the length of the spindle



277



38 VALVE GEARS



so that the inequality will be corrected. Now set the engine on
the dead point again and give the valve the proper lead by means
of the eccentric. By repeating the process, making slight changes,
the desired result will be obtained.

MODIFICATIONS OF THE SLIDE VALVE.

The ordinary slide valve is suitable for small engines; but for
large sizes some method must be employed to balance the steam
pressure on the back of the valve. With large valves, such for
instance as those of locomotives or large marine engines, a great
force is exerted by the steam, and the valve is forced against its
seat so hard that a large amount of power is necessary to move it.
This excessive pressure causes the valve to wear badly and is a
dead loss to the engine. The larger the valve, the greater this
loss will be.

Piston Valve. To prevent excessive pressure on the back of
the valve, the piston valve is commonly used, especially in marine
engines. This valve consists of two pistons, which cover and
uncover the ports in precisely the same manner as the laps of the
plain slide valve. These pistons are secured to the valve stem in
an approved manner and are fitted with packing rings.

The valve seat consists of two short cylinders or tubes
accurately bored to fit the pistons of the valve. The port open-
ings are not continuous as in the plain slide valve, but consist of
many small openings, the bars of metal between these open i no's
preventing the packing rings from springing out into the ports.

Steam may be admitted to the middle of the steam chest and
exhausted from the ends or vice versa. With the former method,
the live steam is well separated from the exhaust, and the valve-
rod stuffing box is exposed to exhaust steam only. This is a good
arrangement for the high -pressure cylinder; if used for a cylinder
in which there is a vacuum, air may leak into the exhaust space
through the valve-rod slutting box. AVith this arrangement the
steam laps must be inside and the exhaust laps on the outside ends.

The piston valve may be laid out and designed by means of
the Zenner diagram just as if it were a plain slide valve, and the
action is the same except that it is balanced so far as the steam



278



VALVE GEAKS



39



pressure is concerned; the power to drive it being only that neces-
sary to overcome the friction due to the spring rings.

Fig. 33 shows a section of the piston valve and the high-
pressure cylinder for one of the engines of the U. S. S. " Massa-
chusetts." This valve consists of two pistons connected by a
sleeve through which the valve rod passes. This valve rod is pro-
longed to a small balancing piston, placed directly over the main




valve. The upper end of the balancing cylinder does not admit
steam, so that the steam pressure below the balancing piston will
practically carry the weight of the piston valve, thus relieving the
valve gear and making the balance more nearly complete.

DoubIe=Ported Valve. Sometimes it is impossible to get
sufficient port opening for engines of large diameter and short
stroke, especially those having a plain slide valve with short travel.



279



VALVE GEARS



This difficulty may be overcome by means of the double-ported
valve shown in Fig. 34. It is equivalent to two plain slide valves,
each having its laps. The inner valve is similar to a plain slide
valve except that there is communication between the exhaust
space and the exhaust space of the outer valve. Each passage to
the cylinder has two ports; a bridge separates the exhaust of the
outer valve from the steam space of the inner valve, and the outer
valve is made long enough to admit steam to the inner valve.




Fig. 84.

This valve may be considered as equivalent to two equal slide
valves of the same travel, each having one-half the total port
opening. To admit the same amount of steam as a plain slide
valve, the double-ported valve requires but half the valve travel;
this is advantageous in high-speed engines.




Fig. 35.

To balance the excessive steam pressure, the back of the valve
is sometimes provided with a projecting ring which is fitted to a
similar ring within the top of the valve chest. These rings are
planed true, and fit so that steam is prevented from acting on the
back of the valve. The space inside the rings is sometimes placed
in communication with the condenser.



280



VALVE GEARS



(1



The Trick Valve. The defect of the plain slide valve, due to
the sloAvness in opening and closing, is largely remedied in the
trick valve, which is so made that a double volume of steam enters
during admission. Thus a quick and full opening of the port is
obtained with a small valve travel.

In Fig. 35 the valve is shown in mid-position. It is similar
to a plain slide valve except that there is a passage PP through it.
It has an outside lap () and an inside lap I. The seat is raised
and lias steam ports SS, bridges BB, and exhaust port E. If the
valve moves to the right a distance equal to the outside lap plus
the lead, it will be in the position shown in Fig. 30. Steam will
be admitted at the extreme left edge of the valve just the same as
though it were a plain slide valve; also, since steam surrounds the
valve it will be admitted through the passage as shown in Fig. 30.




Fig. 36.



Fig. 37.



If the lead is the same as for a plain slide valve, T ^ inch for
instance, this valve would give double the port opening, that is
inch, when the valve was open a distance equal to the lead.

Fig. 37 shows the valve when it is in its extreme position to
the right and the port is full open to steam.

Piston valves are also made with a passage similar to that of
the trick valve for double admission. The valve used with the
Armington and Sims engine is perhaps the best example.

Balanced Valves. Since there is a wide difference between
the pressure of admission and exhaust, there must always be a
great pressure acting upon the valve, causing it to run hard and
wear excessively. The greater the steam pressure, the lower the
pressure at exhaust and the larger the valve, the greater this pres-
sure will be.



281



42 VALVE GEARS



Piston valves are commonly used on the high and intermedi-
ate cylinders of triple-expansion engines, and if well made and
fitted with spring rings, should not leak. Small piston valves are
often made without packing rings; but even if they fit accurately
when new, they soon become worn and cause trouble.

The double-ported valve, the trick valve, and others often have
some device for relieving the pressure, such as a bronze ring or
cylinder, fastened to the back of the valve. This ring is pressed
by springs against a finished surface of the valve chest cover, and
the space thus enclosed by the ring may be connected to the
exhaust. There are numerous devices for balancing valves, but
they are usually more or less expensive and are liable to cause
trouble from leakage.

O

STEPHENSON LINK MOTION.

One of the earliest, and at present one of the most common
mechanisms for reversing engines, or changing the ratio of expan-
sion, is the Stephenson link motion, shown in Fig. 38. This illus-
tration is taken from the drawings of a recent battleship engine,
and may be considered the typical arrangement of the Stephenson
gear as applied to marine practice.

The two eccentrics E and E', whose centers are at C and C,
respectively, are shown in their relative positions when the crank
OA is at dead center. The eccentric rods R and R' are connected
by forked ends to the link pins H and G. The link consists of
two curved bars bolted together in such a manner that they may
slide by the link block N. On the link are three sets of trunions;
the two outer ones, or link pins, are fitted into the forked end of
the eccentric rods, and the middle one, known as the saddle pin,
is fitted into the end of the drag links FM.

The valve stem has, at its lower end, a pivoted block N, called
the link block, provided with slotted sides through which the links
can slide from right to left. The reverse shaft, or rock shaft, K,
here shown in full gear " forward," may be turned until F moves
over to B; in this position the link will be pushed across the link
block, and the valve will get its motion from the rod R' instead
of from R as before. The link in this position would be full gear
astern."




si

8

Q 3

E!



VALVE GEARS




Fig. 38,



II



VALVE GEARS



In all large engines, such as marine, the reverse shaft is
turned by power, but in smaller engines, such as locomotives, the
engineer can turn the shaft by means of a lever.

AVhen set full gear forward, as in Fig. 38, the valve admits
steam to the crank end of the cylinder, and the crank revolves as




Pig. 39.

shown by the arrow. As the crank turns, both eccentrics impart
motion to the link, but the "go ahead" link pin II approximately
coincides with the link block, so that nearly all its up-and-down
motion is transmitted to the valve stem, while the "go astern"




Fig. 40.



eccentric exerts but little eft'ect upon the link block. Moving the
drag links, over to the extreme right reverses all these conditions
by bringing the other link pin under the link block. In this posi-
tion, steam will be admitted to the other end of the cylinder, and
the engine will run in the opposite direction. This will be clearly
seen by referring to Fig, 38.



VALVE GEARS 45



When at full gear, either forward or backing, the valve moves
as if there were really but one eccentric, while at intermediate
points its motion is the result of the combined influence of both
eccentrics, one tending in a measure to counteract the other. The
effect of this is to shorten the valve travel the same as if the valve
were driven by a new eccentric having less throw than either of
the other two.




Fig. 41,



Decreasing the valve travel causes cut-off to occur earlier
compression is earlier, release later, and the lead is reduced some-
what. If every point of the link moved in the arc of a circle
when the drag link is shifted, the lead would not alter; but, since
the eccentric rods about which each end swings are centered at
different points, C and C', this is impossible.

Figs. 89 and 40 show the two principal ways of arranging
the eccentric rods of a Stephenson gear. The first is said to have
"open rods", the second "crossed rods"; referring to whether
the rods are crossed or open when both the eccentrics face the link.
It can easily be seen that when the eccentrics shown in Fig. 39
have turned through 180 they will be in the position shown in
Fig. 41, but this is the same arrangement as before and is " open "
rods. The full lines show the positions in full gear forward, while



285



46



VALVE GEAKS



the dotted lines indicate the positions in mid gear. With open
rods it will be seen that when at full gear the link block is at G,
and that if, without turning the crank, the link is shifted to mid
gear, then the link block moves to J, Fig. 39, and the valve must
consequently be moved toward the right an amount equal to GJ,
thereby increasing the lead on the crank end of the cylinder.
With crossed rods, moving the link from full to mid gear moves
(he link block from G to J, Fig. 40, thus reducing the lead. It
follows then that open rods give increasing lead from full toward
mid gear, and that crossed rods give decreasing lead. With crossed
rods there will be no lead when in mid gear. It will be apparent
that the shorter the rods the greater this increase or decrease
will be.



REVEPSE ROD




Fig. 42.



Nearly all marine engines, and some English locomotives,
have their link blocks carried directly on the valve rod. Ameri-
can locomotives commonly use a rocker, one end of which carries
the link block while the other moves the valve rod. This arrange-
ment indicated in Fig. 42 makes it possible to place the valve and
steam chest above the cylinder. The position of the crank for the
same valve position is just opposite that shown in Fig. 39 because
the rocker reverses the valve motion ; this gives an arrangement of
crank and eccentrics that is identical with that indicated in Fig.
41 and the rods, although apparently crossed, are in reality of the
open rod arrangement, giving increasing lead tow r ard mid gear.
A rod from the bell-crank lever on the reverse shaft E, leads baek
to the engineer's cab and connects with the reverse lver. This
ever moves over a notched arc, and may be held by a latch in any



VALVE GEARS 47



one of the notches, thus setting the link in any position from mid
gear to full gear, either forward or back.

The Stephenson link is designed to give equal lead at both
ends of the cylinder; but to accomplish this, the radius of the link
arc (that is an imaginary line in the center of the slot) must be
equal to the distance from the center of this slot to the center of
the eccentric. In Fig. 38 the radius of the link arc is equal to
CII and C'G.

Exact quality of lead is not essential, and the radius of the
link arc is sometimes made greater or less than stated above in
order to aid in equalizing the cut-off; but the change should never
be great enough to affect the leads.

Stephenson originally intended to use the link simply as a
reversing gear, but soon found, however, that at intermediate
points between the two positions of full gear, it would serve very
well as a means of varying the expansion and cut-off. Very soon
the link came to be used not only on locomotives and marine
engines, but on stationary engines as well, in connection with the
reverse shaft which was under the control of the governor. The
mechanism proved to be too heavy to be easily moved by a gov-
ernor and it has gradually fallen into disuse on stationary engines
excepting as a means of reversing.

In marine practice, the variable expansion feature is of little
value, for marine engines run under a steady load and the link is
set either at full gear or at some fixed cut-off. For locomotives,
however, the variable expansion is nearly as important as reversing.
Locomotives are generally started at full gear, admitting steam for
nearly the entire stroke, and then exhausting it at relatively high
pressure. This wasteful use of steam is necessary to furnish the
power needed in starting a train,, After the train is under way,
less power is required per stroke, and the link is gradually moved
toward mid gear, or "notched up" by the engineer, thus hasten-
ing the cut-off; the expansion is increased and the power is reduced
in proportion to the load.

As the cut-off is changed, it is desirable to maintain an
approximately equal cut-off at each end of the cylinder; this can
he secured in the Stephenson gear by properly locating the saddle
pin and the reverse shaft. When used without a rocker, as in



2R7



48 VALVE GEARS



Fin;. 8s, the- saddle pin should be on the arc of the link or slightly
ahead of it. When used with a rocker, the saddle pin should be
behind the link arcs, and to give symmetrical action for fonvard
and backward running, it should be opposite the middle of the arc,
that is, equally distant from each link pin.

The Stephenson link cannot be designed directly from the
Zeuner diagram, but a systematic investigation can be made by
using a wooden model of the proposed link. This can be mounted
on a drawing board, and the effect of changing the position of
pins and the proportions of rods and levers can be determined
without difficulty. By a system of trials a combination can be
found best suited to obtain the desired results. Moreover, a
model makes it possible to measure directly the slip of the link
block along the link. This slip should be kept as small as possi-
ble to prevent rapid wear. It can be controlled to some extent by
properly locating the link pins, by avoiding too short a link, and
by choosing a favorable position for the reverse shaft.

The Gooch Link. Another form of link motion, known as
the Gooch Link, is illustrated in Fig. 43. It has been extensively
used on European locomotives, although it is gradually being
replaced by a type of valve gear known as the Walschaert. which
will be described later.

The Gooch link has its concave side turned toward the valve
instead of toward the eccentric. The radius of curvature of the
link is equal to AB, the length of the radius rod. The link is
stationary and the link block slides in the link. The engine is
reversed by means of the bell-crank lever on the reverse shaft E
which shifts the link block instead of the link, as is the case with
the Stephenson. The link is suspended from its saddle pin JV1,
which is connected by a rod to the fixed center F, so that the link
can move forward and back as the eccentricity is changed, or it
can pivot about its saddle pin as the eccentrics revolve.

Since the radius of the link arc is equal to AB, it is apparent
that the block can be moved from one end of the link to the other,
that is, from full gear "forward" to full gear "back" without
moving the point A, which is on the end of the valve rod. Tlie
lead then is constant for all positions of the block, and the distri-
bution of steam for locomotives is slightly preferable to that



VALVE GEARS 49



obtained by the Stephenson; but the gear is more complicated and
requires nearly double the distance between shaft and valve stem.

The variable lead is perhaps a slight advantage to the loco-
motive, which is a slow-speed engine in starting, thus requiring
but little lead. As the speed increases, and the link is " notched
up ", the lead is increased as the cut-off is shortened, and at high
speed we have a large lead. With the Gooch link, the lead can be
set for the average running speed, and although a little too great
for good work at slow speed, it is a matter of small consequence,
because the engine runs at slow speed but a very small fraction of
the time it is in service, and the loss due to large lead at slow
speed is of no consequence whatever in a day's run.

Several other link motions have been used; but at the present
time probably more Stephenson link motions are used than all




Fig. 43.

the other forms of reversing gear combined, and when a "link
motion " is mentioned, the Stephenson is usually meant unless
otherwise specified.

RADIAL VALVE GEARS.

In general, it would be desirable to have precisely similar
steam distribution at each end of the cylinder, and it would often
be of great advantage with an expansion gear like the Stephenson,
if the cut-off could be shortened without changing any other event
of the stroke. A Stephenson gear can be made to maintain equality
of lead for both ends of the cylinder as the cut-off is shortened,
but we have seen that in so doing, the lead of both ends is either



50



VALVE GEARS



increased or diminished according as the link is arranged with
" open rods " or " crossed rods ". Moreover, the compression is
hastened by bringing the link to mid-gear, all of which in many
instances is undesirable.

This disadvantage of the Stephenson link motion lead to the
design of the so-called "Radial Valve Gears", many of which are
so complicated as to be impracticable, but all of which obtain a
fairly uniform distribution of steam.




Fig. 44.

Hackworth Gear. The essential features of the llackworti:
Gear are indicated in outline in Fig. 44. In this figure, S is the
center of the shaft, and the eccentric E is set 180 from the crank
SII. At the right-hand end of the eccentric rod EA, is pivoted a
block which slides in a straight, slotted guide. The guide remains
stationary while the engine is running, but can be turned on its



290



VALVE GEARS



51



axis P, to reverse the engine or change the cut-off. P is a pivot,
located on the horizontal through S in such a position that
DP = EA. If these two distances are equal, A will coincide
vvith P \vhen the crank is at either dead point and the slotted
guide may be turned from " full gear forward ", as shown in the
figure, through the horizontal position to " full gear backing ", as




Fig. 45.

shown by the line BL, without moving the valve. Therefore the
leads are constant for all positions of the guide. The valve rod
running upward from C, connects with the valve stem which it
moves in a straight line. The valve stem is made just long
enough to equalize both leads, and if the point C has been properly
chosen, the two cut-offs will be very nearly equal for all grades of
the gear.



291



VALVE GEARS



A somewhat better valve action is obtained by slightly curv-
ing the slotted guide, with its convex side downward. This gear
is sometimes used . on marine engines and on small stationary
engines.

Marshall Gear. The most objectionable feature of the Hack-
worth gear, is the slotted guide, for the sliding of the block causes
considerable friction and wear. The Marshall gear, shown in
outline in Fig. 45, is designed to obviate this feature. The point
A moves in the desired path by swinging on the rod FA about F
as a center. "While the engine is running, the lever FP remains
stationary, but can be turned on its axis P to reverse the engine,



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 19 of 30)