There are a great many kinds of valves used on steam engines, namely,
the Corliss, Slide, Rocker, Balance, Rotary, etc., but the one most
commonly used on farm engines is the plain slide valve, which has been
generally adopted by all the larger engine builders in this country.
It is simple in design, and when properly set does its work very
efficiently. They are less complicated than others and are easily set;
they are made in many different designs, but the principle of each is
the same.
The slide valve is constructed to slide upon the smooth surface of
the valve seat, in which are contained the two steam ports for the
admission of steam to each end of the cylinder, and also the exhaust
port through which the exhaust or waste steam passes out of the
cylinder.
The slide valve is operated by the eccentrics, which are attached to
the main crank shaft of the engine and revolve with it, the object of
the eccentrics being to move the slide valve back and forth upon its
seat to admit the steam alternately through the steam ports to the
cylinder.
The valve gear is a most important detail and one upon which the
economy of fuel in a great measure depends, and any derangement in this
part of an engine causes an immediate increase in the fuel consumed and
decrease in the power of the engine.
In a properly constructed valve the slide upon the seat should be
reduced to the smallest possible amount, and should be so designed as
to give an equal cut-off and release at both ends of the cylinder,
whether working full gear or notched up.
The engine should also have the same power whether working forward or
backward, and the cut-off should be as sharp as possible.
The more perfect the valve gear the more the engine can be notched up,
and thus allow the steam to expand in the cylinder to its utmost. The
engine which can be notched up the most is the most economical in fuel
and water.