Thompson Balanced and Cut-Off Valve, Patent Model

Description:

This model was submitted to the U.S. Patent Office with the application for the patents issued to Joseph W. Thompson, of Salem, Ohio, April 27, 2875, nos. 162714 and 162715. These were assigned to the Buckeye Engine Co., of the same place.

The model represents the first form of J. W. Thompson’s balanced and cut-off valve gear, which was one of the earliest of the “automatic” valve gears. It was introduced in the very successful Buckeye engine.

The model represents a horizontal steam engine with one fixed eccentric and one shifting eccentric driving the main slide valve and the riding cut-off valve, respectively. The valve of the engine is in the shape of a hollow rectangular box the top of which works in close proximity to the valve chest cover and has a steam-tight, ring-packed opening through which steam is admitted to the inside chamber of the valve. The bottom of the hollow box forms the main valve taking steam through the chamber and into the valve chest at the ends of the valve. The opening through which steam is admitted is made enough larger than the steam pipe opening to cause the steam pressure within the chamber to exert some force to keep the main valve on its seat; otherwise the valve is perfectly balanced. A riding cut-off valve operates on the inside face of the bottom of the hollow main valve.

The main valve is operated from a rock shaft directly connected to the rod of the fixed eccentric. The riding cut-off valve is operated from a double-arm rock shaft, which is carried in the main valve rock shaft, one arm being connected to the valve rod, the other to a shifting eccentric on the engine shaft. The position of this eccentric will determine the position of the double-arm rock shaft relative to the main valve rock shaft and will in this way control the point of cut-off.

A shaft governor of the Thompson and Hunt design carries the shifting eccentric and varies its position relative to the crank with changes in speed of the shaft. The governor is mounted in a disk on the shaft and not in the flywheel as has since become practice.

Reference:

This description comes from the 1939 Catalog of the Mechanical Collections of the Division of Engineering United States Museum Bulletin 173 by Frank A. Taylor.