96 POWER AND THE PLOW 



TRANSMISSION 



In transmitting power from the crankshaft to the drive- 

 wheel, we find almost our widest variation among tractors, 

 and the comparative efficiency of the various types of trans- 

 mission has never been even partially determined by accurate 

 competitive tests. This is unfortunate, for tractive efficiency 

 is one of the prime essentials of a plowing tractor. It is by 

 no means the only essential, for great tractive efficiency can 

 be obtained by any designer who is willing to sacrifice other 

 features of equal importance. All other things being equal, 

 however, it is obvious that the most desirable tractor is one 

 which will consume the least power in moving its own weight 

 and overcoming the friction of its transmission. The crank- 

 shaft makes many more revolutions than could be allowed for 

 the drive-wheel, consequently the speed of rotation must be 

 reduced and the power at the same time delivered to the 

 drive-wheel at some little distance from the engine. The 

 transmission systems may be divided into friction, chain and 

 gear drive, and the latter again into systems employing all 

 spur gear, or part bevel and part spur gear. Combinations of 

 all systems are found. The size and strength of the various 

 parts of the transmission must gradually increase as their speed 

 is reduced, owing to the greater strain upon each link or tooth. 



FRICTION CLUTCH 



It is evident that there must be a friction device which will 

 allow the load to be applied gradually. Otherwise, especially 

 in plowing, the initial effort of starting the load would require 

 much greater power and strength than ordinary requirements 

 would justify. The friction clutches which are used represent 

 practically every type found on automobiles and steam engines. 

 Probably the most common is the internal expanding clutch, 

 commonly found on steam tractors. This is equipped with 

 two or more friction shoes, which when thrown in are locked, 



