THE STEAM TRACTOR 57 



wide enough to support an eight to twelve inch belt for driving 

 stationary machines. It is evident that a friction clutch must 

 be used which will allow the power to be applied gradually to 

 the traction gearing, otherwise the starting of a heavy load 

 of plows would require great care and considerable time. 

 This clutch is usually made up of two or more shoes, and the 

 necessary collar and toggle levers for holding them against 

 the inside face of the pulley or flywheel. The blocks or shoes 

 are usually of wood and frequently faced with some special 

 friction material. When the clutch is thrown in it locks 

 itself in position without strain on the clutch lever. Some 

 clutches are fitted with counter weights, which lift the shoes 

 from the face of the pulley by centrifugal force as soon as 

 the clutch is thrown out. Means are provided for 

 taking up the wear on the shoes so as to keep the clutch 

 effective at all times. 



TRANSMISSION 



The power is usually transmitted to the traction wheels 

 by a simple train of spur gears i.e., cylinders with teeth 

 cut on the circumference parallel to the axis. A driving pinion 

 is attached to the friction clutch. This engages an inter- 

 mediate gear, and this in turn a large compensating gear on 

 the countershaft. Pinions on either end of the countershaft 

 drive the large master gears, which are fastened to the traction 

 wheels by either rigid or spring connections. On a few engines 

 the power is taken from both ends of the crankshaft and trans- 

 mitted by two complete sets of gears, making what is properly 

 known as a double-geared engine. The intermediate shaft 

 on late types is attached to the same wing sheets as the engine 

 frame. 



Differential Gear 



At some point in the transmission there must be a differen- 

 tial, or compensating, gear to allow one drive-wheel to revolve 



