Fig. 248.
The valve is operated by the steel cam, C, acting on a steel pin, D, which passes through the valve into the exhaust port, N, in which the cam is located. In addition to this positive motion steam is alternately admitted to and exhausted from the ends of the valve piston through the ports, E and E1, which moves the pistons, B and B1.
Fig. 249.
Assumed that this pump is at rest with the valve, A, covering the main steam ports, F and F1, in which position the cam C, holds the main valve by means of the valve pin, D, so that ports, E and E1, admit steam to one end of the valve piston at the same time connects the other end with the exhaust port. The steam, acting on the valve pistons, moves both, opening the main ports, F and F1, admitting steam to one end of the steam cylinder and opening the other end to the exhaust. If the valve occupies any other position than the one described, the main ports, F and F1, will be opened for the admission and exhaust of steam; consequently it is evident that this pump will start from all points of the stroke.
Fig. 250.
On the admission of steam to the cylinder the main port, F, the main piston, cam and valve will move in the direction indicated by the arrows. The first movement of the cam oscillates the valve, preparatory to bringing it into a proper position for the opening of the auxiliary steam ports, E, to live steam, and E, to exhaust also to close the valve mechanically just before the main piston reaches the end of its stroke. This causes a slight cut-off and compression, and fully opens the auxiliary ports, E, to steam, and E1, to exhaust. By the admission of steam to one end, the other being open to the exhaust, the valve pistons move the valve to allow the admission and exhaust of steam from the cylinder for the return stroke.
