The distinguishing feature of this engine will be found in the fact
that, in both the cylinder and pump, the rotary type is substituted for
the reciprocating or piston principle.
[Illustration: FIG. 412.]
The larger sizes of these engines, Fig. 411, are hung on platform
truck springs in front and on half-elliptic springs in the rear, and
are braced and stayed to withstand violent shocks in the rapid driving
over pavements. Although fitted to be drawn by horses only, they can be
supplied with rope reel and drag rope.
[Illustration: FIG. 413.]
_The Silsby steam cylinder_ consists of two rotary pistons or cams,
mounted on steel shafts and working together within an elliptical
steam-tight case. Live steam from the boiler enters at the bottom
of this case, and in its passage presses apart their long teeth or
abutments, causes the two cams to rotate, and exhausts from the top
into the tank and feed-water heater; these cams are provided with teeth
or cogs, adapted to mesh with corresponding recesses in each other, so
that a steam tight joint is maintained between them and leakage thereby
prevented from passing directly upward into the exhaust.
The sides of these cams have their arcs turned to fit the heads of
the case, and are so adjusted that, while being practically steam
tight, allowance is made for expansion and contraction. In the ends of
the longest teeth of the revolving cams are placed removable packing
strips, which are forced outward into contact with the cylinder walls
by means of springs. These packing strips may be removed through
openings in the sides of the cylinder, and readjusted to take up the
wear, which is confined to the ends of these adjustable strips. This
can be done without taking the pump or cylinder apart.
[Illustration: FIG. 414.]
_The construction of the pump is similar to that of the cylinder_; in
this there are three long teeth in each cam instead of two. One shaft
of the pump is coupled to the corresponding shaft of the cylinder,
there being outside gears on both cylinder and pump to compel a uniform
motion of the cams and to equalize the pressure. This construction
secures a transmission of power at once direct and positive in Fig. 412.
The stuffing-boxes, used on both cylinder and pump, are self-adjusting,
reduce friction and insure tightness. Valves are entirely absent from
the pump and cylinder. The water ways being large, anything liable
to enter the suction will pass through the pump without injury or
interruption; the pump requires no priming, but when started will
immediately without the aid of a check valve lift water vertically any
required distance up to 29 feet.
_The construction of the boiler_ ordinarily supplied with this
engine is shown in Figs. 414-415. In the fire-box hangs a series of
circulating water tubes arranged in concentric circles and securely
screwed into the crown sheet. These drop tubes are closed at their
lower ends by means of wrought-iron plugs welded in, and within each of
them is placed a much smaller and thinner tube, which latter is open at
both ends. The cooler water in the boiler descends through the inner
tube and is thus brought directly into the hottest part of the furnace,
whence, after being for the most part converted into steam, it ascends
through the annular spaces between these inner and outer tubes.
[Illustration: FIG. 415.]
_The gases of combustion_ pass from the fire box to the stack through
smoke flues, the lower ends of which are expanded into the crown sheet,
and the upper ends into the top head of the boiler.
[Illustration: FIG. 416.]