Slide-valve.
A valve which opens or closes by sliding over the port or ports.
The valve-rod
a is operated from the crankshaft of the eccentric by an intermediate lever
b pivoted at
c; as the pitman
d advances by the down stroke of the piston, the valve is drawn back, opening the lower port, which then becomes the eductionport, and
vice versa.
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Slide-valve. |
The
long slide, of large condensing-engines, is a slide-valve of such length as to govern the ports at the ends of the cylinder, and thereby bring them alternately in connection with the middle or bridge part at which the steam is inducted, and with the open part of the valve-chamber, which has a connection, through the hollow back of the valve, with the eduction passage to the condenser.
The long slide was invented by
Murdoch, of the firm of
Boulton and
Watt.
The slide-valve was substituted for the steam-cock by
Murray, of
Leeds, England, in 1810.
The slide-valve is placed in the steam-chest to work over the steam-ports by which live steam from the boiler is admitted to the cylinder, and exhaust steam from the latter is allowed to pass to the condenser or to atmosphere, as the case may be. Its form is arched, and it has a flat face all round, which works steam-tight on the valve-seat.
The arched cavity in the valve is constantly in connection with the steam-induction pipe
s, and the live steam from this cavity is alternately admitted at the ports
a and
b as the valve shifts.
Fig. 5191.
In the three figures the valve is shown
in situ at the respective ends of its stroke and detached.
At
A, the steam admitted at
s passes by the port
b below, and elevates the piston, the latter driving before it the exhaust steam, which is educted at the port
a, and passing through or around the valve, departs by way of the escape-pipe
e, which is secured by flanges and bolts
o p to the end of the steam-chest.
The valve-rod
t is secured by a collar and nut
d to the valve, and passed through a stuffing-box in the cap
m of the steamchest.
While the faces
h of the valve slip upon the seat, the back of the valve has depressions
k k, which contain a packing, so as to bear against the top plate of the chest.
B shows the valve at the other end of its stroke, the port
b being the eduction, and the port
a the induction.
C shows the valve detached and destitute of packing in the recesses
k.
The
balanced slide-valve, so called, is one in which the steam pressure which keeps the valve on its seat is partially relieved by an upward pressure of steam on the under side, or by a plate, which prevents access of steam to the larger portion of the upper area of the valve.
The
lead is the width of opening of the steam-port when the
[
2210]
piston is at the end of its stroke.
It may be effected by turning the eccentric on the axle a little in front of the crank.
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Slide-valve. |
The
lap is
inside or
outside. The former is the difference between the width of the shell part of the valve and the distance between the inside edges of the adjacent ports.
The
outside lap is the excessive breadth of the valve over the width over all the ports.
The
travel is the length of stroke of the valve.
The
setting is the arrangement of the eccentric to give the requisite
lead.
