72 BULLETIN 173, U. S. NATIONAL MUSEUM 



manner except that the rock levers, instead of being connected to the 

 valve stems, are provided with geared sectors that operate sliding 

 racks, and these racks are connected to the valve stems by means of 

 catches that permit the steam valves to be engaged or disengaged 

 from the rest of the valve gear. When the steam valves are closed 

 the racks move sufficiently far to engage the valve rods, and on the 

 return motion open the valve until the catch strikes a cam, which 

 disengages the valve rod and permits it to be closed quickly under 

 the force of a heavy weight provided for that purpose. The cam is 

 a helical projection on the sliding shaft of a centrifugal governor. 

 Its position determines the point in the stroke of the piston at which 

 the disengaging catch releases the steam valve and cuts off the steam. 

 When the engine runs faster than the desired speed, the governor 

 changes the position of the cam to cut off earlier in the stroke. This 

 reduces the steam supplied to the engine and it slows down. If the 

 engine runs slower than the desired speed, the cut off occurs later 

 and the speed of the engine increases. 



In addition to the automatic drop cut-off this valve gear gives 

 but little motion to the valves when they are closed and diminishes 

 the power required to operate the valves. 



CORLISS CUT-OFF GEAR, 1851 



U.S.N.M. no. 308G53 ; original patent model ; transferred from the United States 

 Patent Office ; not illustrated. 



This model was submitted with the application for the patent 

 issued to George H. Corliss, July 29, 1851, no. 8253 ; reissued July 26, 

 1859, no. 780. 



This valve gear is a more compact combination of the elements 

 of the disengaging gear of the first Corliss design (above). It ex- 

 hibits for the first time some of the characteristic arrangements that 

 have identified Corliss engines to the present time, such as the wrist 

 plate located at the side of the cylinder, separate steam and exhaust 

 valves at opposite sides and at each end of the cylinder, and valve 

 spindles or rock shafts and arms for moving the valves. The com- 

 bination of these rock shafts with flat slide valves is a transition in 

 the development of the pure rotary valve, which is so well suited 

 to the Corliss gear (see below). 



The model represents a vertical cylinder with two steam and two 

 exhaust valves, one of each on opposite sides of each end of the 

 cylinder. A wrist plate (or rock disk), located at the side and 

 center of the cylinder, is connected to an eccentric on a shaft directly 

 above the cylinder. From the wrist plate rods extend to arms 

 on the short rock shafts, which move the exhaust valves so that the 

 connection of the exhaust valves to the wrist plate and thus to the 

 eccentric is permanent and the exhaust valves will be alternatelv 



