Engineers are aware that there are more or less objections to the use
of the ordinary spring pistons, owing to the changing tension of the
springs, the necessity of frequent adjustment, and the impossibility of
the packing rings adapting themselves to the varying pressures of the
steam on the piston. A number of attempts have been made to produce a
self packing or steam expanding piston, which will act always with the
pressure of the steam and the velocity of the engine. The advantages
of such a piston will be readily appreciated by practical engineers,
especially drivers of locomotives, working, as they nearly all do, at a
very high pressure of steam. The general complaint against the several
packings in use on our railroads is, that they "pack too tight,"
and rapidly wear out the rings, while the only remedy has been, the
extremely uncertain one of contracting the openings by which steam is
admitted under the ring, or rings, to expand them. The obvious objection
to such an arrangement is, that it allows the steam to act on the rings
with its full force during slow motion, as when a train is starting,
while if effective under any circumstances, it will be so only at
comparatively high piston speed.
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