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.
Fig. 414.
Fig. 415.
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.
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.