=The Piston= is another very important part in the construction of
an engine, and it conveys the power of the steam to the crank. It is
composed of a piston head, on which are placed the piston rings held in
position by the follower plate, and is securely attached to the piston
rod. Great care should be taken in the construction of the piston rings
to have them fit the cylinder perfectly tight, at the same time to have
the least possible friction. Piston rings should always be made of a
softer metal than the cylinder so that the greater part of the wear
will be upon the rings instead of the cylinder, as the rings can easily
be replaced.
There are a great many kinds of packing for piston rings, but the most
commonly used at the present time are the steam packing rings. The
character, accuracy in construction and condition of the piston make
a great difference in the quantity of fuel consumed and the amount of
power developed by the engine.
=The Piston Rod= connects the piston to the cross-head and is generally
made of steel. Where the piston rod enters the cylinder, a steam tight
joint is obtained by the use of a soft, pliable packing placed in the
stuffing box, and held in position by the stuffing box gland.
This box is kept packed just tight enough to prevent leaking, by
drawing up the stuffing box gland when required. This can be repeated
until the packing is all used up, when box must be repacked.