An open-ended "trunk" piston, similar in principle to that used in gas
engines, is employed; and the valve is of the piston type, which is less
complicated than the box form of valve, though less easily made steam-tight
in small sizes. The engine is single-acting, making only one power stroke
per revolution.
The cylinder is a piece of brass tubing; the piston another piece of
tubing, fitting the first telescopically. Provided that the fit is true
enough to prevent the escape of steam, while not so close as to set up
excessive friction, a packing behind the piston is not needed; but should
serious leakage be anticipated, a packing of thick felt or cloth, held up
by a washer and nuts on the gudgeon G, will make things secure. Similarly
for the built-up piston valve P may be substituted a piece of close-fitting
brass rod with diameter reduced, except at the ends, by filing or turning,
to allow the passage of steam.
CONSTRUCTION.
[Illustration: FIG. 51.--Elevation of simple reciprocating steam engine.]
The bed is made of wood, preferably oak, into the parts of which linseed
oil is well rubbed before they are screwed together, to prevent the entry
of water.
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