54 POWER AND THE PLOW 



poppet valves, such as are used in gas engines, are used on 

 tractors abroad in connection with a system of superheating, 

 and will undoubtedly be introduced into this country. The 

 American plowing engines are all double acting i.e., steam 

 is admitted alternately at either end of the cylinder. The 

 valve must admit the steam, shut off the supply at the proper 

 time, and hold the pressure while the piston moves to the 

 other end of the cylinder, whereupon it must release the ex- 

 haust steam. The valve is often double ported so as to take 

 steam from both ends of the steam chest at the same time. The 

 valve is sometimes provided with a friction ring which fits 

 tightly against the cover of the steam chest so as to prevent 

 steam from getting on top of the valve and increasing the 

 friction. On some tractors what is known as a balanced valve 

 is used in order to minimize the valve friction. The valve 

 usually opens the inlet port before the end of the stroke so 

 that full pressure may be exerted the instant the piston starts 

 on its return trip. On practically all engines the exhaust port 

 is closed before the steam has entirely escaped, to form a 

 cushion and bring the piston to rest gently at the end of its 

 stroke. 



Cylinder and Piston 



The cylinder is usually of cast iron, with one head cast on 

 and one bolted. The piston is usually a hollow cast iron 

 disk, fitted with two or three expanding rings for preventing 

 the passage of steam between it and the cylinder wall. Since 

 the engine is double acting, and both ends of the cylinder are 

 closed, the piston rod moves back and forth through a stuffing 

 box in the cast-on cylinder head, remaining parallel to the 

 engine frame. The connecting rod necessarily assumes various 

 angles during the revolution of the crankshaft. The wrist- 

 pin joining the piston rod and connecting rod is carried by 

 the cross-head, the shoes of which move back and forth between 

 curved guides, which are usually cast with the cylinder as 



