The plunger in the Cameron pump is reversed by means of two plain
tappet valves, shown in the accompanying engraving, Fig. 261, and the
entire valve mechanism consists of four pieces, all of which work in
a direct line with the main piston. This pump is simple and has no
delicate parts.
[Illustration: FIG. 261.]
As here represented—_A_ is the steam cylinder; _C_, the main piston;
_L_, the steam chest; _F_, the valve piston, the right-hand end of
which is shown in section; _G_, the slide valve; _H_, a lever, by means
of which the valve piston, _F_, may be moved by hand when expedient;
_I_, _I_ are reversing valves; _K_, _K_ are the reversing valve chamber
bonnets, and _E_, _E_ are exhaust ports leading from the ends of steam
chest direct to the main exhaust. The passages, _M_, _M_, lead directly
into the exhaust (although the connection is not shown, being cut away
in the sectional view), and closed by the reversing valves, _I_, _I_.
The piston, _C_, is driven by steam admitted under the ^B^ slide valve,
_G_, which, as it travels backward and forward, alternately supplies
steam to opposite ends of the cylinder, _A_. The slide valve, _G_, is
operated by the valve piston, _F_; _F_ hollow at the ends, which are
filled with steam, which, issuing through a hole in each end, fills the
spaces between it and the heads of the steam chest in which it works.
The pressure being equal at each end, the valve piston, _F_, under
ordinary conditions, is balanced and motionless; but when the main
piston, _C_, has traveled far enough to the left to strike and open the
reverse valve, _I_, the steam exhausts from behind the valve piston
through the port, _E_. The pressure being now unequal behind the two
ends, the valve piston immediately shifts accordingly and carries with
it the slide valve, _G_, reversing the piston. No matter how fast the
piston may be traveling, it must instantly reverse when it touches the
valve, _I_.
In its movement the valve piston, _F_, acts as a slide valve to close
the port, _E_, and is cushioned on the confined steam between the
ports and steam chest cover. The reverse valves, _I_, _I_, as soon as
the piston, _C_, leaves them, close by the constant pressure of steam
behind supplied direct from the steam chest through the ports, _N_,
_N_, shown by dotted lines.
_To set the valve_, it is only necessary to keep valves tight by
occasional grinding. The piston as it nears the end of each stroke
strikes the stem and lifts the valve off its seat; this allows the
exhaust steam behind the piston valve to escape. The live steam pushes
the piston towards the exhausted end carrying the main slide valve
along with it.