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constructed; it is balanced for all speeds and under all pressures, running or drifting, and the wear on the valve and valve seat is very slight.

The compression valve is a piston valve of the usual design except that it is very light, and provided with wide snap rings to overlap the ports and for protection against wear; it takes its motion direct from the valve gear through an independent connection with the combination lever.

The admission of steam to the cylinders and the cutoff is controlled by the main steam-valve. Steam is admitted along the entire lower edge and up one side, providing an unusually large port area for admission. Exhaust also is controlled by the main steam-valve, but to this is added 'the exhaust of the compression valve which releases simultaneously with the main valve.

Closure or compression is controlled entirely by the compression valve which delays the final closure of the exhaust until the piston has completed 90% of its stroke at the short cut-offs, thereby reducing the volume of the exhaust steam in compression from about 10 inches in the usual practice, to 2 inches in this system. The main steam-valve, although working on a plane seat and of the slide-valve type, is of inside admission and divides the central, live-steam chamber from the exhaust chambers at each end somewhat like

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the ordinary piston valve. The pistons of the compression valve pass through the longest section of the admission ports from which they exhaust past their outer ends. The compression valve and its valve stem are shown in dotted lines in the longitudinal section view in Fig. 43.

The Valve Gear.—This is an outside, radial gear that comes within the Hackworth division of valve motions, but like Marshall's modification of that principle no link is employed, the single eccentric giving a pendulum-like motion to a suspended lever from which the motion of the valve is derived; by shifting the point of this lever's suspension, the lower end will swing at different angles, and shorten or lengthen the travel of the valve, or reverse its motion. The eccentric is of the Walschaert style worked from a crank on the main pin; and a combination lever actuated by the crosshead is used to distort the motion of the valve rod for the purpose of advancing the position of the valve, to overcome the effect of the steam lap when the piston is at the beginning of its stroke, as in the Walschaert gear.

The action of this valve gear may be followed by reference to the line drawing, Fig. 45, and the diagram, Fig. 46, in connection with the following explanation:

Beginning with the eccentric crank A, motion is

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Fig. 46.—Centre Line Diagram of the Hobart-Allfree Valve Gear.

transmitted through the eccentric rod B to the transmission bars D and the radius bar C. The radius bar C is pivoted on the radius block J, which is movably mounted on the radius guide N.

When the radius block J is in centre position, the transmission bar D oscillates in unison with the radius bar C, and no motion is transmitted from the eccentric crank A. The radius block J is operated from the reverse lever by the usual reach rod, through the reverse shaft M, reverse arm L, and reversing link K, and its position upon the radius guide N determines the amount of cut-off in the forward or backward motions. When the radius block J is thus moved to a position on the radius guide N, either in front of or back of the central position, the radius bar C and transmission bar D no longer oscillate in unison, and motion is transmitted to the bell crank or rocker E to the upper end of which one end of the transmission bar D is attached.

The motion thus transmitted to the lower arm of the bell crank E is combined at this point with the motion which the combination lever F receives from the crosshead through the crosshead link G and crosshead arm H, and is transmitted to the valve by the valve rod I as usual; the motion received from the combination lever is equal to lap plus lead, as in the Walschaert motion.


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