332 APPLIED SCIENCE 



the shaft to the center of the crank pin is called the crank's 

 throw, and is half the piston stroke. 



371. Dead Center. When the piston rod is fully out or 

 fully in, and the connecting rod and the crank in consequence 

 lie in a straight line, the crank is said to be at a dead point or 

 dead center. When the crank is in this position the admission 

 of steam will not produce motion since the thrust would be 

 absorbed by the bearings. A locomotive engine must be 

 constructed so that it may be started in any position. In 

 order that this may be done the engine must have at least 

 two cylinders, and the cranks must be set at an angle to one 

 another, so that when the crank of one is at a dead point the 

 other has reached a position where it exerts its maximum 

 turning power. 



372. Steam Valves. The steam is admitted into the 

 cylinder of an engine by means of valves, as previously stated. 

 There are three distinct types of valve (1) slide, (2) Corliss, 

 and (3) poppet valves. 



The slide valve is a simple casting similar in its lengthwise 

 section to the letter D. By being moved back and forth over 

 the steam ports of the engine it admits and exhausts the 

 steam alternately, thus causing the piston in the cylinder to 

 work back and forth. 



Corliss valves are semirotary valves, cylindrical in shape, 

 which partly turn in cylindrical chambers. 



Poppet valves are simply disks attached to a stem, which 

 work over a circular opening. They are raised and lowered 

 over the parts. 



The mechanism controlling these valves is called the valve 

 gear. 



