THE INTERNAL - COMBUSTION TRACTOR 99 



simply by reversing the engine itself. The gas engine, 

 with very few exceptions, is made to run only in one direction, 

 hence the reversing must be done by other means. The 

 reversing mechanism which may be used is limited to some 

 extent by the arrangement of the cylinder. 



On tractors which have the crankshaft crosswise to the frame, 

 reversing may be accomplished by the use of a sliding gear 

 and a separate idler shaft carrying two pinions. When the 

 sliding gear is in mesh with the differential gear the tractor 

 moves forward. When it engages the larger of the two pinions 

 on the idler shaft, the other of which constantly engages the 

 differential, the tractor is reversed. Some tractors are equipped 

 with two or more forward speeds, and one on the reverse. 

 One speed-changing system involves a combination of gears 

 and sliding clutches. This is known as the selective type of 

 transmission, since any speed or the reverse may be selected 

 without the use of a separate idler shaft. Many tractors are 

 provided, however, with two large gears on the countershaft, 

 three on an idler shaft, and two on the sliding-gear shaft. This 

 gives two forward speeds and the reverse. Separate trains 

 of gears on opposite sides of the tractor are used in certain 

 cases to obtain the forward and reverse motions, clutches being 

 provided for gripping the two sets independently. In some 

 small tractors the friction pulley is retained for the reverse 

 motion. An eccentric shaft brings the pulley into contact with 

 a similar pulley on the crankshaft, a toothed gear attached 

 to the driven pulley driving the large gear on the countershaft. 



The planetary reverse resembles somewhat a spur gear 

 differential. A "sun" gear, moving with the crankshaft, is in 

 mesh with small pinions, and these in turn with an internal 

 gear which is part of the belt pulley. The pinions are held 

 on a spider connected to a large disk by a long hub which is 

 loose on the crankshaft. The reversing process consists in 

 holding the disk stationary by a pair of clutch-blocks. This 

 holds the axles of the small pinions stationary, and since the 



