fL\TALOG OF THE MECHANICAL COLLECTIONS QQ 



to be similar to one described in a patent granted to Charles H. Browm 

 and Charles Burleigh, January 15, 1856. The valve stems are oper- 

 ated by short levers, which are raised and lowered by cams on a lay 

 shaft paralleling the cylinder. The levers that operate the steam 

 valves have variable f ulcrums, which are controlled by a governor so 

 that the steam can be cut off at any point of the stroke. 



THOMPSON BALANCED AND CUT-OFF VALVE, 1875 



U.S.N.M. no, 3086S8 ; original patent model ; transferred from the United States 

 Patent Office ; not illustrated. 



This model was submitted with the application for the patents 

 issued to Joseph W. Thompson, of Salem, Ohio, April 27, 1875, 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 "auto- 

 matic" valve gears. It was introduced in the very successful and 

 well-known Buckeye engine. 



The model represents a horizontal steam engine with one fixedl 

 eccentric and one shifting eccentric driving the main slide valve and 

 the riding cut-off valve, respectively. The valve of the engine is ia 

 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 cham- 

 ber 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 pres- 

 sure 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 detennine 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 (see below) 

 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 the 

 practice. 



