Early horse-drawn steam engines transmitted power through an endless belt from a drive pulley on the engine to a similar pulley on the thresher, grinder, pump, or other stationary machine. Many tractors today can be also equipped with belt pulleys to provide stationary power.
With the engines self-propelled, power could be transmitted by traction between drive wheels or tracks and the soil. This permitted the use of implements pulled behind the tractor or mounted on the tractor frame.
In 1918 the "power takeoff" was introduced. With this system, rotary power from the engine can be transmitted through a flexible shaft to drive such field implements as hay balers, combines, and mowing machines. Because power takeoff can also be used to drive stationary machines and is much more convenient than an endless belt, it has largely replaced the belt pulley on newer tractors.
Hydraulic power, available on nearly all new tractors, is generated by oil forced through pipes or hoses by an engine-driven pump to a point of application. Hydraulic lift is obtained by pumping oil into a piston-equipped cylinder, forcing the piston out and raising the attached pull-type or tractor-mounted implement. Reversing the procedure and removing the oil, lowers the implement. Hydraulic rotary power for machine operation is provided by forcing oil through a hydraulic motor similar to the engine-driven pump. Other applications of hydraulic power include power steering, power brakes, and raising and lowering seats.
The tractor's electrical system is used in a very minor way to power small electric motors for special applications. Primary uses of tractor electrical power—normally a 12-volt system—are for starting the engine, for lighting, for air conditioning, and for ignition on gasoline engines.
