IOO SCIENCE OF SUCCESSFUL THRESHING. 



spur gears on the traction wheels. The four bevel pinions 

 are carried by the center casting, and mesh with two bevel 

 gears, one of which is cast in one piece with the right-hand 

 counter-shaft pinion, (which is loose upon the shaft), and 

 the other of which is keyed to the counter-shaft and drives 

 the left-hand counter-shaft pinion, (which is also keyed to 

 the shaft). It will be seen that when the engine travels 

 straight ahead, both counter-shaft pinions turn with the shaft 

 and the whole differential revolves as one piece. In turning 

 corners, however, the bevel pinions revolve, permitting one 

 of the counter-shaft pinions to revolve faster than the other, 

 thus allowing the traction wheels to accommodate themselves 

 to the curve of the road. The differential spur wheel is a 

 separate piece from the center casting, the power being trans- 

 mitted from the rim to the center casting through coil 

 springs, which relieve the gearing of the shocks of starting 

 and stopping the engine. 



Locking the Differential. When both traction wheels 

 have resistance, they pull equally, but if the engine be "jacked 

 up" until one of them is off the ground and free to turn, 

 then when the engine is started, the differential gear will 

 allow the free traction wheel to revolve at twice its usual 

 speed, while the traction wheel on the ground will scarcely 

 pull at all. Revolving at twice its usual speed means that 

 the free traction wheel makes one revolution to nine of the 

 fly-wheel, instead of, to the usual eighteen. Often, when 

 one wheel is in a slippery place, it will spin around, while 



