392 AGRICULTURAL ENGINEERING 



and the width of the tire. On soft ground, it is customary to 

 provide an extra width of tire in the form of extensions, which 

 may be removed when not needed. 



In order to grip the surface of the soil sufficiently, the 

 traction wheels must be provided with cleats, lugs, grouters, 

 or spikes, which grip the soil and enable the tractor to exert a 

 greater tractive force. The form of these lugs should be 

 adapted to the conditions under which they work. 



Rating. The size or capacity of the steam tractor is 

 designated in horsepower. Formerly it was customary to 

 indicate its tractive power in terms of horses. This rating 

 has since become known as nominal rating, and is being 

 superseded largely by the brake horsepower rating, which 

 indicates the most practical power output of the engine 

 proper. This rating is ordinarily about three times the 

 nominal rating. 



A large part of the power of the engine is used in propelling 

 the tractor and in overcoming the friction of the gearing. 

 The tractive efficiency of a tractor is the ratio between the 

 power delivered at the draw bar and the power furnished by 

 the engine. Ordinarily this is about 50 per cent, but on soft 

 ground it may run as low as 35 or 40 per cent. On hard roads 

 it may be much higher than 50 per cent. 



Control. The control of the steam tractor is placed (1) in 

 a throttle, through which the admission of steam to the 

 engine is controlled; (2) in the reverse, which controls the 

 direction of rotation of the engine; and (3) in a clutch similar 

 to that described for gas tractors which connects the engine 

 to the transmission. Some steam tractors have a brake by 

 which the tractor may be held in place. 



The Clutch. The clutch on a steam tractor universally 

 operates within the fly wheel of the engine. The friction 



