SEPARATE CUT OFF VALVES.
Fig. 3306.
Meyer’s cut off valve is constructed as shown in [Fig. 3306], m being the main valve, and v v the two cut off valves, whose sole duty is to cut off the steam at an earlier point than the main valve would do. If the engine is to have a fixed point of cut off, or, in other words, if the cut off is always to occur at some one particular point in the stroke, the valves may be set to do so, and equalize the points of cut off.
Variable points of cut off with the Meyer’s valve may be obtained by shifting the position of the eccentric that operates the cut off valve, but it is usually done by means of moving the valve by a right and left hand screw, such as shown in [Fig. 3306]. The cut off eccentric is set ahead of the main eccentric, so that the cut off valve will close the ports before the main valve would do so; thus, in the figure the cut off valve is shown to have effected the cut off for port a by the time the main valve has fully opened port a, and is reversing its motion. If the engine requires to reverse its motion, the cut off eccentric is set exactly opposite to the crank, but otherwise, it may be set 8 or 10 degrees either ahead of or behind the crank, but if set too little ahead of the crank, the port may reopen after the cut off has been effected.
Fig. 3307.
Gonzenback’s cut off valve is constructed as in [Fig. 3307], the steam chest having two compartments. a, a are the cylinder steam ports, c the main valve, and e the cut off valve, whose ports (as g) are made wider than the ports f.