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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

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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 17 of 30)
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241



VALVE GEARS



Steam enters the cylinder of the engine through ports which
must, in some manner, be opened and closed alternately, in order
to admit and exhaust the steam at the proper time. To accom-
plish this purpose a valve is moved back and forth across the port
openings. A complete understanding of the valve and valve gear
is essential to the engineer as well as to the designer, for even
though a valve be properly designed, its economy may be seri-
ously impaired by improper setting. The design and adjust-
ment of these valves plays a very important part in the efficient
action of the steam engine.

The term "valve gear" includes the valve or valves that
admit steam to and exhaust it from the cylinder of the engine,
together with the mechanism from which the valves derive motion.
There may be a single valve to regulate admission and exhaust, or
there may be a double set of valves; one set to admit the steam at
each end and another to release it. The valve may have a plain
reciprocating motion, moved
by a rod, or it may be opened
by some device that lets go at
the proper time, allowing the
valve to drop shut under the
influence of counter weights,
springs or vacuum dashpots.
To the first class belong the
plain slide valve and its modi-
fication of piston valve, gridiron valve, etc.; to the second belong
such valves as the Corliss, Brown, and others.

The simplest type of valve is the plain slide or D valve as
shown in Fig. 1.

In this figure V is the valve, R the valve rod, K the exhaust
cavity, P and, P' the steam ports, E the exhaust port, AB the valve
seat, and DM the bridges of the valve seat. The valve seat must
be planed perfectly smooth, so that pressure on the valve will




Fig. 1.



243



VALVE GEARS



make a steam tight fit, and cause as little friction as possible when
the valve slides. Furthermore, the length of the seat AB must be
a little less than the distance from the extreme right-hand posi-
tion of the right-hand edge of the valve to the extreme left-hand
position of the left-hand edge of the valve. This allows the valve at
each stroke slightly to over travel the seat, thus keeping it always
worn perfectly flat and smooth. If the valve seat were not raised
slightly above the rest of the casting, or if it were too short, the
constant motion of the valve would soon wear a hollow path in the
valve seat, and it would cease to be steam tight.

Eccentric,, The valve usually receives its motion from an
eccentric which is simply a disc, keyed to the shaft in such a




Fig. 2.

manner that the center of the disc and the center of the shaft do
not coincide. li; is evident that as the shaft revolves, the center
of this eccentric disc moves in a circle about the shaft as a center,
just as if it were at the end of a crank. The action of the eccentric
is equivalent to the action of a crank the length of which is equal
to the eccentricity of the eccentric (the distance between the center
of the eccentric and that of the shaft).

Fig. 2 represents the essentials of an ordinary eccentric. O
is the center of the shaft, O the center of the eccentric disc E, and
S is a collar encircling the eccentric and attached to the valve
rod R.



244



VALVE GEARS



As the eccentric turns in the strap, the point O moves in the
uotted circle around O', and the point A also moves in a circle.
When half a revolution is accomplished the point O will be at
O", the point A will be at A", and the eccentric strap and valve
rod will be in the position indicated by the dotted lines.

Since the revolving shaft transmits motion to the valve
through the eccentric, it will be necessary to study the relative
motions of the crank and eccentric in order to get a clear idea of
the steam distribution.

The distance of the center of the eccentric from the center
of the shaft (GO' in Fig. 2) is known as the eccentricity, or throw,
of the eccentric. The travel of the valve is twice the eccentricity.

Valve without Lap. Fig. 3 shows a section through the
steam and exhaust ports of an engine, together with a plain slide




Fig. 3.

valve placed in mid-position, and so constructed that in this posi-
tion it just covers the steam ports and no more. A valve is in
mid-position when the center line of the valve coincides with the
center line of the exhaust port.

Fig. 1 shows the same valve drawn to*a larger scale.

Suppose the valve is moved a slight distance to the right;
the port P (see Fig. 1) is then uncovered and opened to the live
steam which enters the cylinder and causes the piston to move.
Since the two faces of the valve are just sufficient to cover the
steam ports, it is evident that as the port P opens to live steam,
the port P' opens to the exhaust. The ports are closed only when
the valve is in mid-position. This allows admission and exhaust



VALVE GEARS



to continue during the whole stroke. With such a valve there is
no expansion or compression; the indicator card would be a rec-
tangle, and the M. E. P. would be equal to the initial steam pressure,
assuming no frictional losses in the steam pipe or condensation in
the cylinder.

For a theoretical discussion of valve motion, it is assumed that
the eccentric rod moves back and forth in a line parallel to the
center line of the engine. This is not the case in practice, for the
eccentric rod always makes a small angle with the center line, just
as the connecting rod does, but the eccentricity is so small in com-
parison with the length of the eccentric rod that the angularity of
the eccentric rod is very much smaller than the angularity of the
connecting rod, and its influence may be neglected without appre-
ciable error.




J



Fig. 4.



When the valve shown in Fig. 3 is in mid-position, the crank
is on dead center, the eccentric is set at right angles to it, and the
piston is just ready to begin the stroke.

Fig. 4 shows the relative positions of crank, piston, eccentric
and valve when the crank has made a quarter turn or the piston
has moved to half stroke. The eccentric is now in its extreme
position to the right, the valve has its maximum displacement and
both the steam and exhaust ports are wide open. The valve will
not close again until the piston has reached the end of its stroke.

This type of valve is used only on small and unimportant
engines, and since it allows no expansion of the steam, is very
uneconomical. Furthermore, it will be seen that this valve opens
just after the s f roke begins, which is impractical, for it means that
the piston has begun its stroke before the full steam pressure



246



VALVE GEARS



reaches it, which will cause an inclined admission line on the indi-
cator diagram.

Valve with Lap. If the face of the valve is made longer
than shown in Fig. 1, so that in mid-position it overlaps the steam
ports, we shall have a valve such, as shown in Fig. 5. The
amount that the valve overlaps the steam ports is called the lap of
the valve. In Fig. 5, DI is the inside lap, and OC the outside lap.
It will at onee be seen that both the admission and exhaust ports may
remain closed during a part of the stroke, thus making expansion




Fig. 5.



and compression possible. It is also evident that steam cannot
be admitted until the valve uncovers the port by moving a dis-
tance from mid-position equal to OC. Admission continues until
the valve returns to such a position that the outer edge of the
valve again closes the port. Release will begin when the inner
edge of the inside lap begins to uncover the port.

Fig. 6 represents a valve with lap, at the point of admission.
Since the valve must move a distance equal to the outside lap
before admission can take place, it is evident that the eccentric can
no longer be at right angles to the crank at the beginning of the
stroke, but must be ahead of the right-angle point by an amount
equal to AOC. The angle AGO is known as the angular advance.

The maximum displacement of the valve is attained when
the eccentric is horizontal as shown in Fig. 7. In this position



247



8



VALVE GEARS



both the steam and exhaust ports are wide open, and any further
motion of the piston will cause the valve to move toward its mid-
position.

Admission continues until the valve returns to the position




Fig. 6.

shown in Fig 8. Here the outside lap just closes the left-hand
steam port, cut-off takes place, and the steam already in the cylin-
der begins to expand. As the valve continues to move toward the
left, the left-hand inside lap begins to uncover the left-hand port
and releases the steam at the position shown in Fig. Id

The dotted lines of Fig. 7 show the valve in its extreme




Fig. 7.

position to the left. Any further movement of the piston will
cause it to return toward mid-position.

The dotted position of crank and eccentric in J^g- 10 show r s
the valve returned to the point of compression, which continues
until the conditions of Fig. 6 are again reached and the opening
valve allows steam again to enter the cylinder.

This process has been traced step by step for one end only;
let us now consider what is happening at the other end.



248



VALVE GEAKS



Admission is the point at which the valve opens to admit
steam to the cylinder. Cut-off is the point at which the valve
closes to cut off the admission of steam. Release is the point at
which the exhaust is opened ; and Compression is the point at
which the exhaust is closed.




Fig. 8.



"While the crank is moving from the position shown in Fig.
6 to that of Fig. 8, steam is being admitted to the head end and
being exhausted from the crank end. The inside lap being less
than the outside lap, causes the exhaust to continue longer than
the admission.

Fig. 9 shows the relative positions of crank, eccentric and
valve when the exhaust closes on the crank end and compression





Fig. 9.

begins. Between these two positions the steam is expanding in
the head end and exhausting from the crank end.

Between the positions of Fig. 9 and Fig. 10 both ports are
entirely closed, expansion is taking place in the head end and com-
pression in the crank end. Fig. 10 is head-end release. Fig. 11
shows admission at crank end of cylinder and marks the end of
crank-end compression.



10



VALVE GEAKS



By referring to Figs. 6-11, the effect of any change of lap
may at once be observed. If the outside lap is increased, the
valve must move farther from mid-position before admission will
occur, and on the return, after the maximum displacement is
reached, the outside lap, being wider, will close the port sooner,
and the cut-off shown in Fig. 8 will take place before the crank




Fig. 10.

reaches the angle there shown. A decrease of outside lap will
make cut-off later and admission earlier.

If the inside lap is increased, the valve must move farther be-
fore release occurs and the crank angle would be greater than shown
in Fig. 10. On the return of the valve to the dotted position shown
in Fig. 10, the port will close earlier and make an earlier compres-
sion; the crank angle will be less than is there shown. Decreasing
inside lap will cause earlier release and later compression.




\




/



Fig. 11.

Thus we see that it is the outside lap^that influences admis-
sion and cut-off, and the inside lap that controls release and
compression. For this reason the outside lap is often called the
steam lap^ and the inside lap the exhaust lap.



250




s*
21



!



VALVE GEARS



11




Fig. If



Lead. If a valve havimr lap is in mid-position, the port is
closed and the engine cannot start because no steam can enter the
cylinder. That the steam may be ready to enter the cylinder at
the beginning of the stroke it is necessary that the eccentric be
set more than 90 ahead of the crank and the eccentric radius will
take an angle as shown in Fig. 0, called the <uujul<tr <i(lwnn-<'. In
order that the ports and
clearance may be prop-
erly filled with steam
at the beginning of the
stroke, it is necessary
that the valve be dis-
placed from its mid-
position anvamount
slightly greater than
the outside lap. With
the piston at the end of the stroke the valve will have a position as
shown in Fig. 12. The port will be open the distance A 15. This
causes the eccentric to be moved forward a slight amount in excess
of the angular advance. This excess is called the angle of lead.

In Fig. 13, O'R' represents

^Jr_ A ( the crank at the beginning of the

stroke, LOA the angular ad-
vance, and AC) A' the angle of
lead. The eccentric, to give
lead, must be set at the angle
EGA' ahead of the crank or 90
plus angular advance plus angle
of lead. In large, quick-run-
ning engines, a liberal lead is
essential, so that the ports and
clearance may be well filled
with steam before the stroke
beoins. If there is no lead, a






\



\

-)
/'



Fig. 13.



portion of the steam will be used
in filling these places and full
pressure steam will not reach the piston until it is well advanced
on the stroke. This will give a sloping admission line as <*ho\vn



251



11'



VALVE GEARS



in Fig. 14. Too much lead, on the other hand, will cause too
early an admission as shown in Fig 15.

If the ammlar advance is increased, the eccentric will he

O

moved farther ahead of the crank, and consequently will begin its
motion sooner. It will necessarily arrive at each of the events




Fig 14. ' Fig. 15.

sooner than before. If then, the angular advance is increased, all
^f the events of t\ie stroke will occur earlier.

Inequality of Steam Distribution. In the wilve diagrams
thus far considered, the events of the stroke have been discussed
for each end separately, without reference to the relation of sim-
ilar events on the other side of the
piston. If the connecting rod
were of infinite length, so that it
would always remain parallel to
the center line of the engine, the
distribution would be the same
for both ends of the cylinder. In
practice, the connecting rod is
from 4 to 8 times the length of
the crank, which causes the con-
necting rod always to be at an
angle to the center line of the
engine, and for a given crank
angle makes the piston displace-
ment greater at the head end than at the crank end.

To Jin d the displacement <>f tie valve, let us consider Fig. 16.
The circle represents the path of the eccentric center during a
complete revolution of the engine. OC represents the crank, and
OR the corresponding position of the eccentric. The diameter
XY represents the extent of the valve travel. Since the eccentric
rod is so long in comparison to the eccentricity, we make no
appreciable error by assuming it always to be parallel to the center




Fig. 16.



252



VALVE GEARS



13



line of the engine. AYhen the eccentric is at OL, the valve is in mid-
position. At Oli the valve has moved from mid-position an amount
ON, found by dropping a perpendicular from li to the center line
XY. If the angularity of the connecting rod could be neglected,
the piston displacement could be found in the same manner.

To Jiiid tJte displacement <rf the p't&tou, a diagram as shown
in Fig. 17 must be drawn. In this figure AB represents the
cylinder, P the piston, II the crosshead, Hfl the connecting rod,
and OR the crank. Suppose now the engine should stop in this
position and then be clamped. The piston displacement would
be represented by AP. If the crank pin at II should now be
loosened so as to allow the connecting rod to fall to a horizontal posi-
tion, the point R would describe the arc of a circle UN, and XN
would represent the piston displacement and would be equal to AP




Fig. 17



Suppose now that in this disconnected way the piston, crosshead
and connecting rod were moved forward until the end of the rod
came to O. P would then be at P' and the piston would be in the
middle of its stroke. Now swing the end of the rod up to its
proper position on the crank-pin circle, the piston remaining sta-
tionary. It would describe an arc OZ. The crank pin would be
at Z, less than a quarter revolution from X, while the piston would
be in the middle of its stroke.

Suppose this engine were running with cut-off at half stroke
on the head end and that XOZ represented the corresponding crank
anale. On the return stroke the valve would cut off at the same

O

crank angle YOT = XOZ, and OT would represent the crank cut-
off on the return or crank-end stroke. The piston, as we have just
seen, will not be at half stroke except when the crank is at OZ or OS.
Consequently OT is less than half stroke and cut-off takes place
earlier at the crank end than at the head end. AVhen the crank is at
OZ the eccentric will be at OA (Fig 17</), and the valve displace-



253



14



VALVE GEARS



inent will be OB. When the crank is at OT the eccentric will be at
OA', and the valve displacement will be OB', which is equal to OB,
the displacement of the valve at cut-off in the head end. The pis-
ton displacement will be OX in the head end and "WY in the crank
end when cut-off occurs. If the connecting rod always remained
parallel to the center line, the cut -off would be the same at both ends.
Compensation of Cut=off. Jt has already been pointed out
that lengthening the outside lap makes the cut-off earlier, and short-
ening the lap makes it later. The cut-off in the case just cited may
then be equalized by altering the outside laps. If we increase the
outside lap on the head end, or decrease the crank-end lap, the
inequality will be less. By changing either or both of the laps the
proper amount, the rut-off may be exactly equalized.




Fig. 17a.



But altering the outside lap changes the lead as has already
been explained. If the lap is increased on the head end, the lead
will be less than on the crank end. If the lead becomes too small
on the head end, the angular advance may be increased but the
inequality of lead will still remain, for this increase of angular
advance will increase the lead at the crank end as well as at the
head end, and by hastening all the events of the stroke may give a
bad steam distribution if care is not taken.

Unequal lead is of less consequence on a low-speed than on a
high-speed engine. On low-speed engines the cut-off may be
equalized at the expense of lead with beneficial results, but on high-
speed engines it will not do to give too little lead at one end. A
high-speed engine requires more lead than a low-speed, for there is
relatively less time in each stroke for the clearance to fill with steam.



254



VALVE GEARS



If both inside laps are equal, compression will not occur
equally at both ends. To equalize it, the inside laps may be
changed in the same manner as the outside laps are changed to
equalize the cut-off. By altering these inside laps to equalize
compression, it may happen that the lap is reduced enough to
leave the exhaust port open when the valve is in mid-position.




This opening of the valve is called an inside clearance, or negative
lap. In Fig. 18, A is the inside clearance.

Rocker. Sometimes it happens that the valve stem and
eccentric rod cannot be so placed that they will be in the same
straight line; or it may be that the travel of the valve must be so
great as to require an excessively large eccentric. In such cases
a rocker may be used.

Fig. 19 shows a valve that is not in line with the eccentric.
This occurs in horizontal engines when the valve is set on top of




Fig. 19.

the cylinder instead of on one side. l>y means of the rocker A(i
the valve may receive its proper motion.

In case it is more convenient to place the pivot of the rocker
arm between the connections to the valve stem and those of the
eccentric rod, such an arrangement as shown in Kig. ~0 may be
used. Hen; it will be noticed that the valve stem and eccentric
i-od nre moving in opposite directions, and to give the valve the



255



16 VALVE GEARS



same motion as in Fig. 19, the eccentric must be moved 180 ahead
of the position there shown.

If AB is less than AG, the valve travel will be greater than
twice the eccentricity, in proportion as AG is greater than AB.
In all cases the valve travel is to twice the eccentricity as AG is to
AB. Thus, if the valve travel is 44 inches, AB, 15 inches, and

AG, 18 inches, then X 44 = 8-| inches, will equal twice the
eccentricity.




Fig. 20.

A valve gear may be so laid out as to make both the cut-oif
and the lead equal for both ends of the cylinder. This may be
done by a proper porportion between the rocker arms, and a careful
location of the pivot of the rocker. The eccentric must then be
set accordingly. In this manner the. Straight Line engine equal-
izes the cut-off ^and lead. A discussion of this method will be
considered later.

VALVE DIAGRAMS.

Zeuner's' Diagram. In order to study the movements of
valves, the effect of lap, lead, eccentricity, etc., diagrams of various
sorts have been devised. By the use of diagrams we may acquire
a knowledge of valve motion without the complex mathematical
expressions that such a discussion would entail. The most useful
of these various diagrams is that devised by Zeuner, and to avoid
complexity we shall confine ourselves to a discussion of this dia-
gram alone. The eccentric rod is assumed to be of infinite
length, and the positions of the crank are shown on the diagrams.
The displacement of the piston can easily be found if the ratio of
crank to connecting rod is known.

In Fig. 21 let OY be the eccentricity, then XOY will rep-
resent the valve travel, and the center of the eccentric will move



25R



VALVE GEARS



17



in the -circle XWY. Let Oil represent the position of the
crank and ()/' the corresponding position of the eccentric, which
is 90 + angle of advance 6 ahead of the crank. Draw OW
perpendicular to XY and lay off from it the angle "NVOM =
angle of advance 6 towards the crank. With OM as a diameter,
construct a circle. OM is equal to the eccentricity, and the circle
MPO is known as the valve circle. If Oil, the center line ol

w




the crank, cuts this valve circle at P, then OP is equal to the dis-
placement of the valve from mid-position.

To prove this, draw ?S perpendicular to XY. Since Of
is the position of the eccentric, OS will represent the valve dis-
placement from mid-position. Draw MP. Then by geometry
OPM is a right angle because it is inscribed in a semicircle.
OS/' is also a right angle ; the two right-angled triangles OS/'
and OMP are equal because they are similar and have two cor-
responding sides equal. O/' = OM, being radii of the same
circle. But we have seen that OS is equal to the valve displace-
ment, therefore OP is also equal to the valve displacement, for it
is equal to OS.

Now that the .truth of our proposition has been proved, let
us see how we may study the valve motion from such a diagram.
See Fig. 22. As before, let XY represent the valve travel, then
the circle XEYF will represent the path of the center of the
eccentric. Let 6 be the angular advance and lay off EO toward the
crank, making an angle 6 with the vertical. Produce EO to F,
and on OE and OF as diameters draw the valve circles as shown.
Let the outside lap be an amount equal to OV, then with O as a
center and OY as a radius draw an arc intersecting the upper
valve circle at Y and Iv. Lay off OP equal to the inside lap and



257



18 VALVE GEARS



with O as center and OP as a radius draw an arc intersecting the
valve circle at P and Q. Draw the crank line AO passing through
Y. Then, when the crank is in this position, the displacement of
the valve is equal to OV (the outside lap) and the steam is ready
to enter the cylinder. This is the position of the crank at admis-
sion, and the crank angle XOA is called the lead angle. The
valve has lead, therefore the admission takes place before the end
of the stroke. ~\Vhen the crank reaches the position OE, the
displacement of the valve is equal to OE. the eccentricity, and is




Fig. 22.

the maximum displacement. Further motion of the piston causes
the valve to move toward mid-position until, at the crank position
OC, the displacement OK. is again equal to the outside lap and
the valve has reached the point of cut-off. "When the position Oil
is reached, the crank line is tangent to both valve circles and there
is no displacement of the valve. At this point the valve is in
mid-position.

Further crank movement draws the inside lap toward the
edge of the exhaust port until, at the crank position O13, the dis-
placement is equal to OP (the inside lap) and release begins. At



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 17 of 30)