STEAM ENGINE DETAILS
Some of the most important details of a steam engine are those of its
valve gear. The simplest form of valve is that known as the plain slide
valve, and as nearly all others are a modification of this, it is
essential that the designer should first familiarize himself with this
particular type of valve in all its details of operation. After this has
been done, a study of other forms of valves will be found a
comparatively easy matter. The so called Corliss valve differs radically
from the slide valve, but the results to be obtained and the terms used
in its design are practically the same. The valve gear of a steam engine
is made up of the valve or valves which admit steam to and exhaust it
from the cylinder, and of the mechanism which governs the valve
movements, the latter usually consisting of one or more eccentrics
attached to the main shaft.
[Illustration: Fig. 8. Longitudinal Section of Slide Valve with Ports]
The Slide Valve
Fig. 8 shows a longitudinal section of a slide valve with the ports,
bridges, etc. The valve is shown in mid-position in order that certain
points relating to it may be more easily understood. The valve, _V_,
consists of a hollow casting, with ends projecting beyond the ports as
shown; the lower face is smoothly finished and fitted to the valve seat
_AB_. In operation it slides back and forth, opening and closing the
ports which connect the steam chest with the cylinder. Steam is admitted
to the cylinder when either port _CD_ or _DC_ is opened, and is released
when the ports are brought into communication with the exhaust port
_MN_. This is accomplished by the movement of the valve, which brings
one of the cylinder ports and the exhaust port both under the hollow
arch _K_. The portions _DM_ and _ND_ of the valve seat are called the
bridges.
It will be seen by reference to Fig. 8 that the portions _OI_ and _IO_
are wider than the ports which they cover. This extra width is called
the _lap_, _OC_ being the outside lap and _DI_ the inside or exhaust
lap. The object of outside lap is that the steam may be shut off after
the piston has moved forward a certain distance, and be expanded during
the remainder of the stroke. If there were no outside lap, steam would
be admitted throughout the entire stroke and there would be no
expansion. If there were no inside lap, exhaust would take place
throughout the whole stroke, and the advantages of premature release and
compression would be lost. Hence, outside lap affects the cut-off, and
inside lap affects release and compression. A valve has _lead_ when it
begins to uncover the steam port before the end of the return stroke of
the piston. This is shown in Fig. 9, where the piston _P_ is just ready
to start on its forward stroke as indicated by the arrow. The valve has
already opened a distance equal to the lead, and the steam has had an
opportunity to enter and fill the clearance space before the beginning
of the stroke. The lead varies in different engines, being greater in
high-speed than in low-speed types.
[Illustration: Fig. 9. Illustration showing Lead of Slide Valve]
[Illustration: Fig. 10. Diagrammatical View of Eccentric]
The Eccentric
The slide valve is usually driven by an eccentric attached to the main
shaft. A diagram of an eccentric is shown in Fig. 10. An eccentric is,
in reality, a short crank with a crank-pin of such size that it
surrounds the shaft. The arm of a crank is the distance between the
center of the shaft, and the center of the crank-pin. The throw of an
eccentric corresponds to this, and is the distance between the center of
the shaft and the center of the eccentric disk, as shown at _a_ in Fig.
10. The disk is keyed to the shaft, and as the shaft revolves, the
center of the disk rotates about it as shown by the dotted line, and
gives a forward and backward movement to the valve rod equal to twice
the throw _a_.
[Illustration: Fig. 11. Relations of Crank and Eccentric]
In Fig. 11 let _A_ represent the center of the main shaft, _B_ the
crank-pin to which the connecting-rod is attached (see _H_, Fig. 1), and
the dotted circle through _B_ the path of the crank-pin around the
shaft. For simplicity, let the eccentric be represented in a similar
manner by the crank _Ab_, and its path by the dotted circle through _b_.
Fig. 12 shows a similar diagram with the piston _P_ and the valve in the
positions corresponding to the positions of the crank and eccentric in
Fig. 11, and in the diagram at the right in Fig. 12. The piston is at
the extreme left, ready to start on its forward stroke toward the right.
The crank-pin _B_ is at its extreme inner position. When the valve is at
its mid-position, as in Fig. 8, the eccentric arm _Ab_ will coincide
with the line _AC_, Fig. 11. If the eccentric is turned on the shaft
sufficiently to bring the left-hand edge _O_, Fig. 8, of the valve in
line with the edge _C_ of the port, the arm of the eccentric will have
moved from its vertical position to that shown by the line _Ab'_ in Fig.
11. The angle through which the eccentric has been turned from the
vertical to bring about this result is called the _angular advance_, and
is shown by angle _CAb'_ in Fig. 11. The angular advance evidently
depends upon the amount of lap.
If the valve is to be given a lead, as indicated in Fig. 12, the
eccentric must be turned still further on the shaft to open the valve
slightly before the piston starts on its forward movement. This brings
the eccentric to the position _Ab_ shown in Fig. 11. The angle through
which the eccentric is turned to give the necessary lead opening to the
valve is called the _angle of lead_, and is shown by angle _b'Ab_. By
reference to Fig. 11, it is seen that the total angle between the crank
and the eccentric is 90 degrees, plus the angular advance, plus the
angle of lead. This is the total angle of advance.
[Illustration: Fig. 12. Piston just beginning Forward Stroke]
The relative positions of the piston and valve at different periods of
the stroke are illustrated in Figs. 12 to 16. Fig. 12 shows the piston
just beginning the forward stroke, the valve having uncovered the
admission port an amount equal to the lead. The crank is in a horizontal
position, and the eccentric has moved from the vertical an amount
sufficient to move the valve toward the right a distance equal to the
outside lap plus the lead. The arrows show that steam is entering the
left-hand port and is being exhausted through the right-hand port.
[Illustration: Fig. 13. Steam Port fully Opened]
In Fig. 13 it is seen that the valve has traveled forward sufficiently
to open the steam port to its fullest extent, and the piston has moved
to the point indicated. The exhaust port is still wide open, and the
relative positions of the crank and eccentric are shown in the diagram
at the right. In Fig. 14 the eccentric has passed the horizontal
position and the valve has started on its backward stroke, while the
piston is still moving forward. The admission port is closed, cut-off
having taken place, and the steam is expanding. The exhaust port is
still partially open.
[Illustration: Fig. 14. Valve has started on Backward Stroke]
[Illustration: Fig. 15. Both Steam Ports Closed]
In Fig. 15 both ports are closed and compression is taking place in
front of the piston while expansion continues back of it. Release occurs
in Fig. 16 just before the piston reaches the end of its stroke. The
eccentric crank is now in a vertical position, pointing downward, and
exhaust is just beginning to take place through the left-hand port.
This completes the different stages of a single stroke, the same
features being repeated upon the return of the piston to its original
position. The conditions of lap, lead, angular advance, etc., pertain to
practically all valves, whatever their design.
[Illustration: Fig. 16. Exhaust Begins]
Different Types of Valves
In the following are shown some of the valves in common use, being, with
the exception of the Corliss, modifications of the plain slide valve,
and similar in their action.
[Illustration: Fig. 17. Engine with Piston Valve]
_Double-Ported Balanced Valve._--A valve of this type has already been
shown in Fig. 2. This valve is flat in form, with two finished
surfaces, and works between the valve-seat and a plate, the latter
being prevented from pressing against the valve by special bearing
surfaces which hold it about 0.002 inch away. The construction of the
valve is such that when open the steam reaches the port through two
openings as indicated by the arrows at the left. The object of this is
to reduce the motion of the valve and quicken its action in admitting
and cutting off steam.
[Illustration: Fig. 18. Section through Cylinder of Engine of the
Four-valve Type]
[Illustration: Fig. 19. Different Types of Corliss Valves]
_Piston Valve._--The piston valve shown in Fig. 17 is identical in its
action with the plain slide valve shown in Fig. 8, except that it is
circular in section instead of being flat or rectangular. The advantage
claimed for this type of valve is the greater ease in fitting
cylindrical surfaces as compared with flat ones. The valve slides in
special bushings which may be renewed when worn. Piston valves are also
made with double ports.
[Illustration: Fig. 20. Longitudinal Section through Corliss Engine]
[Illustration: Fig. 21. The Gridiron Valve]
_Four-Valve Type._--Fig. 18 shows a horizontal section through the
cylinder and valves of an engine of the four-valve type. The admission
valves are shown at the top of the illustration and the exhaust valves
at the bottom, although, in reality, they are at the sides of the
cylinder. The advantage of an arrangement of this kind is that the
valves may be set independently of each other and the work done by the
two ends of the cylinder equalized. The various events, such as
cut-off, compression, etc., may be adjusted without regard to each
other, and in such a manner as to give the best results, a condition
which is not possible with a single valve.
_Gridiron Valve._--One of the principal objects sought in the design of
a valve is quick action at the points of admission and cut-off. This
requires the uncovering of a large port opening with a comparatively
small travel of the valve. The gridiron valve shown in Fig. 21 is
constructed especially for this purpose. This valve is of the four-valve
type, one steam valve and one exhaust valve being shown in the section.
Both the valve and its seat contain a number of narrow openings or
ports, so that a short movement of the valve will open or close a
comparatively large opening. For example, the steam valve in the
illustration has 12 openings, so that if they are 1/4 inch in width
each, a movement of 1/4 inch of the valve will open a space 12 x 1/4 = 3
inches in length.
[Illustration: Fig. 22. The Monarch Engine with Corliss Valve Gear.--A,
Rod to Eccentric; B, Governor; C, Reach Rod; D, Radial Arm; E, Steam
Valve; F, Bell-crank; G, Wrist Plate; H, Exhaust Valve; K, Dash-pot]
_Corliss Valve._--A section through an engine cylinder equipped with
Corliss valves is shown in Fig. 20. There are four cylindrical valves in
this type of engine, two steam valves at the top and two exhaust valves
at the bottom. This arrangement is used to secure proper drainage. The
action of the admission and exhaust valves is indicated by the arrows,
the upper left-hand and the lower right-hand valve being open and the
other two closed.
Side and sectional views of different forms of this type of valve are
shown in Fig. 19. They are operated by means of short crank-arms which
are attached to a wrist-plate by means of radial arms or rods, as shown
in Fig. 22. The wrist-plate, in turn, is given a partial backward and
forward rotation by means of an eccentric attached to the main shaft and
connected to the upper part of the wrist-plate by a rod as indicated.
The exhaust valves are both opened and closed by the action of the
wrist-plate and connecting rods. The steam valves are opened in this
manner, but are closed by the suction of dash pots attached to the drop
levers on the valve stems by means of vertical rods, as shown.
[Illustration: Figs. 23 to 26. Action of Corliss Valve Gear]
The action of the steam or admission valves is best explained by
reference to Figs. 23 to 26. Referring to Fig. 23, _A_ is a bell-crank
which turns loosely upon the valve stem _V_. The lower left-hand
extension of _A_ carries the grab hook _H_, while the upper extension is
connected with the wrist-plate as indicated. Ordinarily the hook _H_ is
pressed inward by the spring _S_, so that the longer arm of the hook is
always pressed against the knock-off cam _C_. The cam _C_ also turns
upon the valve stem _V_ and is connected with the governor by means of a
reach rod as indicated in Fig. 23 and shown in Fig. 22. The drop lever
_B_ is keyed to the valve stem _V_, and is connected with the dash pot
by a rod as indicated by the dotted line. This is also shown in Fig. 22.
The end of the drop lever carries a steel block (shown shaded in Fig.
23), which engages with the grab hook _H_.
When in operation, the bell-crank is rotated in the direction of the
arrow by the action of the wrist-plate and connecting-rod. As the
bell-crank rotates, the grab hook engages the steel block at the end of
the drop lever _B_ and lifts it, thus causing the valve to open, and to
remain so until the bell-crank has advanced so far that the longer arm
of the grab hook _H_ is pressed outward by the projection on the
knock-off cam, as shown in Fig. 24. The drop lever now being released,
the valve is quickly closed by the suction of the dash pot, which pulls
the lever down to its original position by means of the rod previously
mentioned.
[Illustration: Fig. 27. Governor for Corliss Engine]
The governor operates by changing the point of cut-off through the
action of the cam _C_. With the cam in the position shown in Fig. 25,
cut-off occurs earlier than in Fig. 24. Should the cam be turned in the
opposite direction (clockwise), cut-off would take place later. A
detailed view of the complete valve mechanism described is shown
assembled in Fig. 26, with each part properly named. A detail of the
governor is shown in Fig. 27. An increase in speed causes the revolving
balls _BB_ to swing outward, thus raising the weight _W_ and the sleeve
_S_. This in turn operates the lever _L_ through rod _R_ and a
bell-crank attachment, as shown in the right-hand view. An upward and
downward movement of the balls, due to a change in speed of the engine,
swings the lever _L_ backward and forward as shown by the full and
dotted lines. The ends of this lever are attached by means of reach-rods
to the knock-off cams, this being shown more clearly in Fig. 22. The
connections between the lever _L_ and cam _C_ are such that a raising of
the balls, due to increased speed, will reduce the cut-off and thus slow
down the engine. On the other hand, a falling of the balls will lengthen
the cut-off through the same mechanism.
[Illustration: Fig. 28. Dash-pot for Corliss Engine]
[Illustration: Figs. 29 and 30. Plan and Longitudinal Section of
Adjustable Piston]
Mention has already been made of the dash pot which is used to close the
valve suddenly after being released from the grab hook. The dash-pot rod
is shown in Fig. 26, and indicated by dotted lines in Figs. 23 to 25. A
detailed view of one form of dash pot is shown in Fig. 28. When the
valve is opened, the rod attached to lever _B_, Figs. 23 and 24, raises
the piston _P_, Fig. 28, and a partial vacuum is formed beneath it which
draws the piston and connecting rod down by suction as soon as the lever
_B_ is released, and thus closes the valve suddenly and without shock.
The strength of the suction and the air cushion for this piston are
regulated by the inlet and outlet valves shown on the sides of the dash
pot.
Engine Details
Figs. 29 to 37 show various engine details, and illustrate in a simple
way some of the more important principles involved in steam engine
design.
[Illustration: Fig. 31. A Typical Cross-head]
[Illustration: Figs. 32 and 33. Methods Commonly Used for Taking Up Wear
in a Connecting-rod]
A partial cross-section of an adjustable piston is shown in Fig. 29, and
a longitudinal section of the same piston in Fig. 30. The principal
feature to be emphasized is the method of automatic expansion employed
to take up any wear and keep the piston tight. In setting up the piston
a hand adjustment is made of the outer sleeve or ring _R_ by means of
the set-screws _AA_. Ring _R_ is made in several sections, so that it
may be expanded in the form of a true circle. Further tightness is
secured without undue friction by means of the packing ring _P_ which
fits in a groove in _R_ and is forced lightly against the walls of the
cylinder by a number of coil springs, one of which is shown at _S_. As
the cylinder and piston become worn, screws _A_ are adjusted from time
to time, and the fine adjustment for tightness is cared for by the
packing ring _P_ and the coil springs _S_.
The points to be brought out in connection with the cross-head are the
methods of alignment and adjustment. A typical cross-head is shown in
cross and longitudinal sections in Fig. 31. Alignment in a straight
line, longitudinally, is secured by the cylindrical form of the bearing
surfaces or shoes, shown at _S_. These are sometimes made V-shaped in
order to secure the same result. The wear on a cross-head comes on the
surfaces _S_, and is taken up by the use of screw wedges _W_, shown in
the longitudinal section. As the sliding surfaces become worn, the
wedges are forced in slightly by screwing in the set-screws and clamping
them in place by means of the check-nuts.
[Illustration: Fig. 34. Outboard Bearing for Corliss Type Engine]
[Illustration: Fig. 35. Inner Bearing and Bed of Corliss Engine]
The method commonly employed in taking up the wear in a connecting-rod
is shown in Figs. 32 and 33. The wear at the wrist-pin is taken by the
so called brasses, shown at _B_ in the illustrations. The inner brass,
in both cases, fits in a suitable groove, and is held stationary when
once in place. The outer brass is adjustable, being forced toward the
wrist-pin by a sliding wedge which is operated by one or more
set-screws. In Fig. 32 the wedge is held in a vertical position, and is
adjusted by two screws as shown. The arrangement made use of in Fig. 33
has the wedge passing through the rod in a horizontal position, and
adjusted by means of a single screw, as shown in the lower view. With
the arrangements shown, tightening up the brasses shortens the length of
the rod. In practice the wedges at each end of the rod are so placed
that tightening one shortens the rod, and tightening the other lengthens
it, the total effect being to keep the connecting-rod at its original
length.
A common form of outboard bearing for an engine of the slow-speed or
Corliss type is illustrated in Fig. 34. The various adjustments for
alignment and for taking up wear are the important points considered in
this case. The plate _B_ is fastened to the stone foundation by anchor
bolts not shown. Sidewise movement is secured by loosening the bolts
_C_, which pass through slots in the bearing, and adjusting by means of
the screws _S_. Vertical adjustment is obtained by use of the wedge _W_,
which is forced in by the screw _A_, as required. The inner bearing and
bed piece of a heavy duty Corliss engine is shown in Fig. 35. The
bearing in this case is made up of four sections, so arranged that
either horizontal or vertical adjustment may be secured by the use of
adjusting screws and check-nuts.
Engines of the slide-valve type are usually provided either with a
fly-ball throttling governor, or a shaft governor. A common form of
throttling governor is shown in Fig. 36. As the speed increases the
balls _W_ are thrown outward by the action of the centrifugal force, and
being attached to arms hinged above them, any outward movement causes
them to rise. This operates the spindle _S_, which, in turn, partially
closes the balanced valve in body _B_, thus cutting down the steam
supply delivered to the engine. The action of a throttling governor upon
the work diagram of an engine is shown in Fig. 38. Let the full line
represent the form of the diagram with the engine working at full load.
Now, if a part of the load be thrown off, the engine will speed up
slightly, causing the governor to act as described, thus bringing the
admission and expansion lines into the lower positions, as shown in
dotted lines.
[Illustration: Fig. 36. Common Form of Throttling Governor]
The shaft governor is used almost universally on high-speed engines, and
is shown in one form in Fig. 37. It consists, in this case, of two
weights _W_, hinged to the spokes of the wheel near the circumference
by means of suitable arms. Attached to the arms, as shown, are coil
springs _C_. The ends of the arms beyond the weights are connected by
means of levers _L_ to the eccentric disk. When the engine speeds up,
the weights tend to swing outward toward the rim of the wheel, the
amount of the movement being regulated by the tension of the springs
_C_. As the arms move outward, the levers at the ends turn the eccentric
disk on the shaft, the effect of which is to change the angle of advance
and shorten the cut-off. When the speed falls below the normal, the
weights move toward the center and the cut-off is lengthened. The effect
of this form of governor on the diagram is shown in Fig. 39. The full
line represents the diagram at full load, and the dotted line when the
engine is under-loaded.
[Illustration: Fig. 37. Shaft Governor for High-speed Engine]