In operation the main piston rod commences its forward stroke, motion is communicated to the vertical arm, which moves forward on the rod until it engages with one of the cams, _E_, Fig. 259, adjusted on the valve stem. The arm coming in contact with the inclined faces of the cam imparts a rolling or oscillating movement to the stem and valve, opens one of the recesses, _C_ or _D_, cut in the side of the valve and admits steam through a port to one of the valve pistons. At the same time this oscillating movement of the valve opens the slot, _F_, Fig. 260, opposite the first, and exhausts steam through the alternate port from the other piston. Steam thus admitted to one of the pistons (while exhausts from the other piston), carries the valve over its seat, and by means of the vents, _A_ and _B_, steam is admitted through one of the main ports and exhausts through the other. When the main piston has completed one stroke, it moves backward on its return; the arm strikes against the other cam on the valve stem and communicates a reverse motion to the valve which closes and opens the alternate ports to admit and exhaust steam to and from the pistons. It also cushions the piston at the end of each stroke which prevents the piston from striking the heads.
_To set the valves._ This can best be done by starting the pump slowly and adjusting the cams, _E_, so as to open the valve at the proper time to compel the piston to make a full stroke. A little experimenting combined with good judgment is all that is necessary, unless the piston valve be badly worn, in which case _a new valve must be substituted_ and fitted to the valve chest.
The plunger in the Cameron pump is reversed by means of two plain tappet valves, shown in the accompanying engraving, Fig. 261, and the entire valve mechanism consists of four pieces, all of which work in a direct line with the main piston. This pump is simple and has no delicate parts.
As here represented—_A_ is the steam cylinder; _C_, the main piston; _L_, the steam chest; _F_, the valve piston, the right-hand end of which is shown in section; _G_, the slide valve; _H_, a lever, by means of which the valve piston, _F_, may be moved by hand when expedient; _I_, _I_ are reversing valves; _K_, _K_ are the reversing valve chamber bonnets, and _E_, _E_ are exhaust ports leading from the ends of steam chest direct to the main exhaust. The passages, _M_, _M_, lead directly into the exhaust (although the connection is not shown, being cut away in the sectional view), and closed by the reversing valves, _I_, _I_.