Fig. 311—Valve Motion.
The cover plate is provided with recesses on its inner surface which exactly correspond with the ports in the valve face, and the corresponding ports and recesses are kept in communication with each other by means of relief passages in the valve. From this it will be seen that the valve is subjected to equal and balanced pressure on each of its sides, and hence, is in equilibrium.
The valve is operated through the valve motion, shown in Figure 311, the eccentric rod of which hooks on a slightly tapered block that turns on the pin of the rock arm, like an ordinary journal box.
The expansion, or cut-off, is automatically regulated by the operation of the governor in swinging the slotted eccentric in a manner substantially equivalent to moving it across the shaft, but is however favorably modified by the arrangement of the rock arm, which, in combination with the other motions, neutralizes the unfavorable operation of the usual shifting eccentric, and which, in connection with the large double port opening, provides for a good use of steam from 0 to 3/4 stroke.
The governor shown in Figure 312 is of the disc and single ball type, the centrifugal force of the ball being counteracted by a powerful spring. Friction is reduced to a minimum in the governor connection, by introducing steel rollers and hardened steel plates in such a manner as to provide rolling instead of sliding motion.
In order that a governor shall correctly perform its functions, it is unquestionably necessary that it have power largely in excess of the work required of it, and also that the friction shall represent a very low percentage of that power. In respect to this, especial means have been employed to reduce the friction; the valve being balanced, requires but little power to move it, while the governor ball being made heavy for the purpose of counterbalancing the weight of the eccentric and strap, its centrifugal force when the engine is at full speed is enormous, the spring to counteract it having to sustain from two to three thousand pounds. Under these circumstances, as might be expected, the regulation is remarkably good. This is a very important consideration in an engine working under the conditions of a roll-train engine.
