Fig. 353.
Extending through the ends of the steam chests are short piston rods, g, which are connected to a centrally pivoted vibrating lever, h, mounted on a pivot. When the main steam piston has moved from the top to the bottom of the steam cylinder, the corresponding valve rod has moved in the opposite direction and the auxiliary slide valve has moved upward, opening the port, i, to steam and the port, k, to the exhaust port. At the moment the main steam piston has completed its downward stroke the auxiliary piston is forced upward and carries with it the main slide valve, l. This opens the main steam port and exhaust port, which reverses the movement of the main piston. When the main piston reaches the upward limit of its stroke the auxiliary valve has moved downward, opening the port, k, to steam and the port, i, to the exhaust, causing the auxiliary piston to move downward, thus reversing the movement of the main valve and piston.
Fig. 354.
By this arrangement the valve operating piston, m, is held at all times immovably at one end of the stroke, except when the main piston is nearing the end of its stroke and is ready to reverse. Supposing the left-hand main piston has not quite reached the upper limit of the stroke, the steam would still be on the lower side of its auxiliary or main valve operating piston and the exhaust open to the other side. We now have steam on the bottom side of both auxiliary pistons, and as they are of equal diameters and are connected by the lever, h, they are balanced and cannot move the main steam valves. The right-hand main steam piston must wait until the left-hand piston has completed its stroke before it can reverse, and consequently the movement of the main pistons will always be in opposite directions, and neither can reverse until both have completed their stroke.
There are three ways that this apparatus may be operated: First, the pumps may be operated in conjunction with each other, as is hereinbefore described. Second, the lever, h, may be detached from the auxiliary or main valve operating pistons, and the two pumps may then run independently of each other or in the ordinary and well-known manner, each performing its own independent work. Third, by further detaching the link, c, on one of the valve gears the auxiliary slide valve, n, will remain at rest and the corresponding pump will not move while the other pump continues to operate. These are important features, because, as in case of accident, it may be necessary to use one pump while the other is disabled, and in some cases it may be desirable to operate the pumps independently. The engineer will appreciate this feature, as the stoppage of an air pump is a serious matter.
Fig. 355.