Operation
The Baker valve gear replaces the expansion link of the Walschaerts gear with an assembly of levers and links which produces the same effect of allowing continuous variation valve travel. The remainder of the gear is the same, so that the return crank and combination lever take the same form, although the proportions are usually modified. The Pilliod Co. sold the gear as a single assembly which was mounted on the frame in the location ordinarily occupied by the Walschaerts expansion link.
The Baker mechanism consisted of the following parts:
- A frame which replaced the mounting for the expansion link. It came in two basic forms, depending upon whether it was attached to the locomotive frame at both ends or only at the front. In either case it had two pivots: one forward and at the top of the frame, and one below and to the rear.
- A U-shaped reversing yoke which tilted back and forth to control reversing and cutoff. The ends of the yoke's arms pivoted on the rear frame mount, and there was also another pivot mounted near the base of the arm. The yoke was connected at the top to the reach rod leading to the reverser.
- A pair of swing links which hung from the upper pivot on the yoke.
- A main link which pivoted in the middle from the lower end of the swing links and whose lower end was connected to the eccentric rod (which in turn connects to the return crank on the driver, as in the Walschaert gear). This link had a characteristic "J"-shape.
- A bellcrank whose center pivot was the upper, forward mount on the frame, and whose ends were attached to the top of the main link and the radius rod (the connection to the combination arm and thence to the valve).
The parts were arranged so that when the yoke was centered, the connection of the main link to the bellcrank was in a line with the two pivots on the yoke. At this point, the back and forth motion of the lower end of the main link left the top relatively motionless as main link swung back and forth on the swing links.
In forward motion, the yoke was pushed forward, so that its upper pivot was in front of the bell crank-main link connection. Moving the eccentric arm back and forth lowered and raised the top of the main link. This motion was translated by the bellcrank into back and forth motion of the radius rod. The angle of the yoke controlled the relative motion, and therefore the cutoff; tilting the yoke backwards reversed the motion of the radius rod.
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