LOCOMOTIVE ENGINE.
431
These arms and the centre boss stand up 2-J inches from the plate A, and another
plate, Z>, of the same thickness as the first, is put upon them, making the whole thick
ness of the piston 3f inches. The plate D is held steady in its place by a projection
on the centre boss passing through it, and is fixed by screws C C, tapped into the
bosses at the ends of the arms C, the heads of the screws GG being countersunk
into the plate D. The plates A and D are turned so as to be just capable of
moving in the cylinder without touching it, and three brass rings FF are placed
between them. The inner ring is three-eighths of an inch thick, and is the same
width as the space between the plates; the two outer rings are half an inch thick,
and of half the width; and one of them has a projecting ring or rebate upon its
edge, fitting into a corresponding groove in the other to keep them steady. The
rings are turned exactly to fit the cylinder and each other, and cut through in one
part, having been first hammered a little all round on the inside, which gives them
a tendency to expand, and causes them to fly open on being cut; when, therefore,
they are put in their places in the cylinder, they press against the cylinder by their
elasticity, and keep in close contact with it, so as to make a steam-tight joint during
the motion of the piston. The divisions in the rings are placed in opposite positions,
or break joint, in order to prevent the escape of steam through them; for if they
were to coincide, a passage would be left for the steam through the piston.
The elasticity of these rings is found to be quite sufficient to keep the piston steam-
tight when moved in the cylinder, and it continues so for a long time ; when, however,
the rings become so much worn by the friction as to have expanded nearly to the ut
most, some other means is necessary to press them against the cylinder. For this pur
pose the three steel springs GGG are placed in the piston; they are of the same
width as the inside ring, against which they bear, and one-eighth of an inch thick in
the middle, and a pin is put through each of them, having a collar bearing against
the spring, and screwed at the other end into the centre boss of the piston; by un
screwing the pin a little the spring can be made to press harder against the ring
when required, and the pin is then fixed by screwing up the set-nut upon it against
the boss. When the piston is first made and the rings are new, these springs are
not required, and they are set so as only to touch the rings; but as the rings wear
and become too loose in the cylinder, the springs are screwed up more and more,
and made to press harder against the rings; and when they are very much worn
they are kept tight to the cylinder by the springs only, as they have expanded to
their utmost. Access is readily obtained, when necessary, to the inside of the
piston, by taking off the front cylinder cover and unscrewing the front plate D of
the piston.
The pistons are often made upon different modifications of Barton’s prin-