56 POWER AND THE PLOW 



manufacturers key the eccentric immovably to the shaft. For 

 convenience, a quicker method of shifting is required anyhow. 



Reverse Gear 



The two systems of reverse in common use are the single 

 and double eccentric. The latter has two opposed eccentrics 

 connected by suitable straps and push rods to a vertical curved 

 link. Sliding in this link and connected directly to the valve 

 is the link block. By lifting the link, or lowering it, the link 

 block and valve will be actuated by first one and then the 

 other of the eccentrics. This system contains a few more 

 parts, but if provided with removable bushings it proves dur- 

 able, and its simplicity and economy of steam make it very 

 popular. In the single eccentric reverse the eccentric strap 

 carries an extension, at the end of which is carried a pivoted 

 block or roller. This is free to slide in a pivoted guide which 

 drives the push rod. The angle of this guide may be shifted 

 by the reverse lever so as to control the throw of the valve 

 and reverse the engine. 



Crankshaft 



On a single cylinder engine the crankshaft is usually pro- 

 vided at one end with a disk which carries a crankpin at a 

 point near the rim. This is also known as a side crank. A 

 double engine has also a centre crank, similar to that on gas 

 engines i.e., practically a bend, drop-forged in the shaft. 

 The crankshaft should be supported by ample bearings, set as 

 closely as possible to the crank. The proper diameter of the 

 crankpin, upon which the force of the connecting rod comes, 

 is given by good authority as at least one fourth of the cylinder 

 bore, and the length as one third that of the cylinder. 



Flywheel and Clutch 



The flywheel is commonly attached to the crankshaft so 

 that it revolves whenever the engine is in motion. It is usually 



