90 POWER AND THE PLOW 



upon the opposite end of the shaft. In a few cases the two 

 flywheels are used, the pulley being attached to one of these. 

 The permissible rim speed of a cast-iron flywheel is between 

 5000 and 6000 feet per minute, hence small flywheels are the 

 practice on high speed engines. With the power strokes 

 coming closer together as in a four-cylinder, high speed engine, 

 a large flywheel is not so essential. The weight of the flywheel 

 should be as near as possible in order to give the greatest 

 possible momentum. As the flywheel is heavy, sometimes 

 weighing 1300 pounds, it is necessary that it be put on the 

 shaft in such a way as to remain there indefinitely, and yet 

 be easily removable. For this reason, and for ease in casting, 

 it is now customary to split the hub of the flywheel. It is 

 locked by means of a key seated in both the shaft and hub, 

 after which it is tightened on by two bolts through the hub 

 on either side of the shaft. 



CRANKCASE 



In stationary engines it is customary to provide a heavy 

 base, either cast or bolted to the crankcase. In tractors this 

 base is discarded, and the frame takes its place. The crank- 

 case, however, is retained to provide a foundation for the 

 crankshaft, camshaft, and cylinders. The protection of the 

 moving parts inside the crankcase requires not only the ex- 

 clusion of dust but that some form of lubrication be provided. 

 The splash lubrication commonly used necessitates a tight 

 crankcase, which in turn is frequently difficult of access. To 

 combine the two extremes of accessibility and protection is 

 a difficult problem for the designer, yet in some cases it is 

 possible to expose the entire internal mechanism of the tractor 

 by the removal of a tight-fitting cover. 



LUBRICATION 



For the lubrication of the gas engine any one or all of three 

 systems may be used namely, gravity, force-feed, or splash. 



