Professor Unwin divides valves, according to their construction into
three classes as follows:--(1) flap valves, which bond or turn upon a
hinge; (2) lift valves, which rise perpendicularly to the seat; (3)
sliding valves, which move parallel to the seat.
Examples of flap valves are shown in figs. 54 and 55; two forms of lift
valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown
two forms of slide valve. The slide valve shown in fig. 58 moves in a
straight line, while that shown in fig. 59 (called a cock) moves in
circle.
_India-rubber Valves._--In india-rubber valves there is a grating
covered by a piece of india-rubber, which may be rectangular, but is
generally circular, and which is held down along one edge if
rectangular, or at the centre if circular. Water or other fluid can pass
freely upwards through the grating, but when it attempts to return the
elasticity of the india-rubber, and the pressure of the water upon it,
cause it to lie close on the grating, and thus prevent the return of the
water. The india-rubber is prevented from rising too high by a
perforated guard. In fig. 54 is shown an example of an india-rubber disc
valve. A is the grating, B the india-rubber, C the guard secured to the
grating or seat by the stud D and nut E. The grating is held in position
by bolts and nuts F. The grating and guard are generally of brass.
India-rubber disc valves are also shown on the air-pump bucket, fig. 47.
EXERCISE 56: _India-rubber Disc Valve._--Fig. 54 shows a vertical
section and a plan of an india-rubber disc valve. In the plan
one-half of the guard and india-rubber are supposed to be removed
so as to show the grating or seat. Draw these views, and also an
elevation. A detail drawing of the central stud is shown in fig.
16, page 18. In fig. 54 the elevation of the guard is drawn as it
is usually drawn in practice, but if the student has a sufficient
knowledge of descriptive geometry he should draw the elevation
completely showing the perforations. Scale 6 inches to a foot.
[Illustration: FIG. 54.]
[Illustration: FIG. 55.]
_Kinghorn's Metallic Valve._--The action of this valve is the same as
that of an india-rubber valve, but a thin sheet of metal (phosphor
bronze) takes the place of the india-rubber.
This valve is now largely used in the pumps of marine engines, and is
shown in fig. 55 as applied to an air-pump bucket. Three valves like the
one shown are arranged round the bucket.
EXERCISE 57: _Kinghorn's Metallic Valve._--Fig. 55 shows an
elevation and plan of one form of this valve. In the plan one-half
of the guard and metal sheet are supposed to be removed, so as to
show the grating, which in this case is part of an air-pump bucket.
Draw the views shown, and also a vertical section of the guard
through the centres of the bolts. All the parts are of brass except
the valve proper, which is of phosphor bronze. Scale 6 inches to a
foot.
_Conical Disc Valves._--A very common form of valve is that shown in
figs. 56 and 57. This form of valve consists of a disc, the edge of
which (called the face) is conical. The conical edge of this disc fits
accurately on a corresponding seat. The angle which the valve face makes
with its axis is generally 45 deg.. If the disc is raised, either by the
action of the fluid as in the india-rubber valve, or by other means, an
opening is formed around the disc through which the fluid can pass. The
valve is guided in rising and falling either by three feathers
underneath it, as in fig. 56, or by a central spindle which moves freely
through a hole in the centre of a bridge which stretches across the
seat, as in fig. 57. The lift of the valve is limited by a stop above
it, which forms part of the casing containing the valve. The lift should
in no case exceed one-fourth of the diameter of the valve, and it is
generally much less than this. The guiding feathers (fig. 56) are
notched immediately under the disc for the purpose of making available
the full circumferential opening of the valve for the passage of the
fluid. These notches also prevent the feathers from interfering with the
turning or scraping of the valve face.
Conical disc valves and their seats are nearly always made of brass.
EXERCISE 58: _Conical Disc Valves._--Draw, half size, the plans and
elevations shown in figs. 56 and 57. In fig. 57 the valve is shown
open in the elevation, and in the plan it is removed altogether in
order to show the seat with its guide bridge.
[Illustration: Plan of Valve. FIG. 56.]
[Illustration: Plan of Seat. FIG. 57.]
_Simple Slide Valve._--The form of valve shown in fig. 58, often called
the _locomotive slide valve_, is very largely used in all classes of
steam-engines for distributing the steam in the steam cylinders. The
valve is shown separately at (_d_), (_e_), and (_f_), while at (_a_),
(_b_), and (_c_) is shown its connection with the steam cylinder.
It will be observed that the valve itself is in the shape of a box with
one side open, the edges of the open side being flanged. When the valve
is in its middle position, as shown at (_a_), two of these flanged edges
completely cover two rectangular openings S_{1} and S_{2}, called _steam
ports_, while the hollow part of the valve is opposite to a third port
E, called the _exhaust port_. As shown at (_a_) the piston P would be
moving upwards and the valve downwards. By the time the piston has
reached the top of its stroke the valve will have moved so far down as
to partly uncover the steam port S_{1}, and admit steam from the valve
casing C through S_{1} and the passage P_{1} to the top of the piston.
The pressure of this steam on the top of the piston will force the
latter down. While the above action has been going on, the port S_{2}
will have become uncovered, and the hollow part of the valve will be
opposite both the steam port S_{2} and the exhaust port E, so that the
steam from the under side of the piston, and which forced the piston up,
can now escape by the passage P_{2}, the steam port S_{2}, and the
exhaust port E to the exhaust outlet O, and thence into the atmosphere,
if it is a non-condensing engine, or into the condenser if it is a
condensing engine, or into another cylinder if it is a compound engine.
After the piston has performed, a certain part of its downward stroke,
the valve, which has been moving downwards, will commence to move
upwards, and when it has reached a certain point it will cover the port
S_{1}, and shut off the supply of steam to the top of the piston. It is
generally arranged that the steam shall be cut off before the piston
reaches the end of the stroke. When the piston reaches the bottom of its
stroke the valve has moved far enough up to uncover the port S_{2} and
admit steam to the bottom of the piston, and to uncover the port S_{1}
and allow the steam to escape from the top of the piston through the
passage P_{1}, the port S_{1}, the port E, and outlet O. In this way the
piston is moved up and down in the cylinder.
The valve is attached to a valve spindle S by nuts as shown, the hole in
the valve through which the spindle passes being oval-shaped to permit
of the valve adjusting itself so as to always press on its seat.
When the valve is in its middle position it generally more than covers
the steam ports. The amount which the valve projects over the steam port
on the outside, the valve being in its middle position, is called the
_outside lap_ of the valve, and the amount which it projects on the
inside is called the _inside lap_. When the term lap is used without any
qualification, outside lap is to be understood. In fig. 58 it will be
seen that the valve has no inside lap, and that the outside lap is
three-eighths of an inch. The inside lap is generally small compared
with the outside lap.
[Illustration: FIG. 58.]
When the piston is at the beginning of its stroke the steam port is
generally open by a small amount called the _lead_ of the valve.
The reciprocating motion of the slide valve is nearly always derived
from an eccentric fixed on the crank-shaft of the engine. Slide valves
are generally made of brass, bronze, or cast iron.
EXERCISE 59: _Simple Slide Valve._--At (_d_), fig. 58, is shown a
sectional elevation of a simple slide valve for a steam-engine, the
section being taken through the centre line of the valve spindle,
while at (_e_) is shown a cross section and elevation, and at (_f_)
a plan of the same. Draw all these views full size, and also a
sectional elevation at A B. The valve is made of brass, and the
valve spindle and nuts of wrought iron.
EXERCISE 60: _Slide Valve Casing, &c., for Steam-engine._--Draw,
half size, the views shown at (_a_), (_b_), and (_c_), fig. 58;
also a sectional plan at L M. (_b_) is an elevation of the valve
casing with the cover and the valve removed. (_a_) is a sectional
elevation, the section being taken through the axes of the steam
cylinder and valve spindle. (_c_) is a sectional plan, the section
being a horizontal one through the centre of the exhaust port. The
inlet and outlet for the steam are clearly shown in the sectional
plan: in the sectional elevation their positions are shown by
dotted circles.
The stroke of the piston is in this case 12 inches, so that from
the dimensions given at (_a_) it must come within a quarter of an
inch of each end of the cylinder; this is called the _cylinder
clearance_.
The piston has three Ramsbottom rings, a quarter of an inch wide
and a quarter of an inch apart.
The steam cylinder and valve casing are made of cast iron.
_Cocks._--A cock consists of a slightly conical plug which fits into a
corresponding casing cast on a pipe. Through the plug is a hole which
may be made by turning the plug to form a continuation of the hole in
the pipe, and thus allow the fluid to pass, or it may be turned round so
that the solid part of the plug lies across the hole in the pipe, and
thus prevent the fluid from passing. As the student will be quite
familiar with the common water cock or tap such as is used in
dwelling-houses we need not illustrate it here.
[Illustration: FIG. 59.]
Fig. 59 shows a cock of considerable size, which may be used for water
or steam under high pressure. The plug in this example is hollow, and is
prevented from coming out by a cover which is secured to the casing by
four stud bolts. An annular ridge of rectangular section projecting from
the under side of the cover, and fitting into a corresponding recess on
the top of the casing, serves to ensure that the cover and plug are
concentric, and prevents leakage. Leakage at the neck of the plug is
prevented by a gland and stuffing-box. The top end of the plug is made
square to receive a handle for turning it. The size of a cock is taken
from the bore of the pipe in which it is placed; thus fig. 59 shows a
2-1/4-inch cock.
EXERCISE 61: 2-1/4-_inch Steam or Water Cock._--First draw the
views of this cock shown in fig. 59, then draw a half end elevation
and half cross section through the centre of the plug. Scale 6
inches to a foot.
Instead of drawing the parts of the pipe on the two sides of the
plug in the same straight line as in fig. 59, one may be shown
proceeding from the bottom of the casing, so that the fluid will
have to pass through the bottom of the plug and through one side.
This is a common arrangement.
All the parts of the valve and casing in this example are made of
brass.