58 POWER AND THE PLOW 



faster than the other at times, and both to receive power 

 equally when moving straight ahead. This is necessary be- 

 cause of the unequal slippage of the two wheels in soft ground 

 and the unequal travel in turning. The differential consists 

 (1) of a large spur gear mounted loose on the countershaft 

 or axle; (2) a series of small bevel or spur pinions mounted 

 between the spokes of the main gear; (3) two gears standing 

 in a vertical plane in mesh with the small pinions between and 

 mounted on the shaft or axle. One of the two latter gears 

 is keyed to the shaft, and the other to a sleeve revolving upon 

 the shaft. Each is connected directly or by gears with the 

 drive-wheel on that side. When both drivers move at the 

 same rate, the small pinions do not revolve. When one wheel 

 lags, the two compensating halves revolve in opposite di- 

 rections to equalize the travel, and the main gear continues to 

 transmit power to both sides. The differential, or counter, 

 shaft is usually continuous, and the compensating halves 

 and pinions beveled. When spur pinions are used, one com- 

 pensating half is internal and one external i.e., the teeth of 

 the one extend toward the main shaft and the other outward, 

 the spur pinions being arranged in pairs between. One small 

 pinion in each pair meshes with each gear, the two pinions 

 being side by side on a stud parallel with the countershaft. 



Mounting 



The majority of plowing engines now sold have the counter- 

 shaft and rear axle mounted at the rear of the boiler. The 

 bearings for these shafts are commonly supported either by 

 large cast iron brackets on the corners of the fire box or by a 

 continuous wing sheet riveted at close intervals around the 

 top and sides of the fire box. The rear mounted engines are 

 sometimes hung en springs and links, which relieve the jar 

 upon the boiler and engine without allowing the gears to become 

 unmeshed. The wheels sometimes revolve on short axles which 

 are mounted on brackets attached to the side of the fire box. 



