Baker valve gear

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Baker valve gear was the main competitor to Walschaert valve gear for steam locomotives in the United States. It used more pivot bearings, but avoided the sliding connection of Walschaert gear, thus somewhat lessening wear and the need for service.

[edit] History

In the very early 1900s there were many efforts to create a new valve gear to replace the by-then-standard Walschaert valve gear. In the United States the Young, Southern, and Caprotti patterns were used on a few classes, but only the Baker pattern won more than limited acceptance.

The Baker Locomotive Valve Gear was produced by the Pilliod Co. of Swanton, Ohio, with the first patents being issued in 1903. Subsequent versions were produced up to the end of steam service. It was particularly popular on the Norfolk and Western Railway, and almost all later N&W engines used it (notably NW 611). Other extensive users included the Chesapeake and Ohio Railroad and the Nickel Plate Road. The New York Central Railroad and the Baltimore and Ohio Railroad also had large classes which used Baker gear. Western United States and British railroads tended to prefer to use the Walschaert pattern, as did the Pennsylvania Railroad.

[edit] Operation

The Baker valve gear replaces the expansion link of the Walschaert valve gear with an assembly of levers and links which produces the same motion. The remainder of the gear is the same as that of the Walschaert, so that the return crank and combination lever take the same form. The Pilliod Co. sold the gear as a single assembly which was mounted on the frame in the location ordinarily occupied by the 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 th the Johnson bar or power reverse.

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 aline 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.