Power take-off

A power take-off or power takeoff (PTO) is any of several methods for taking power from a power source, such as a running engine, and transmitting it to an application such as an attached implement or separate machines.

Most commonly, it is a system comprising a splined output shaft on a tractor or truck, designed so that a PTO shaft, a kind of drive shaft, can be easily connected and disconnected, and a corresponding input shaft on the application end. The power take-off allows implements to draw energy from the engine.

Semi-permanently mounted power take-offs can also be found on industrial and marine engines. These applications typically use a drive shaft and bolted joint to transmit power to a secondary implement or accessory. In the case of a marine application, such shafts may be used to power fire pumps.

In aircraft applications, such an accessory drive may be used in conjunction with a constant speed drive. Jet aircraft have four types of PTO units: internal gearbox, external gearbox, radial driveshaft, and bleed air, which are used to power engine accessories. In some cases, aircraft power take-off systems also provide for putting power into the engine during engine start. See also Coffman starter

Various power transmission methods were available before power takeoffs became common, but there were applications left wanting for some of the attributes that PTOs would provide. Flat belts generally lent themselves only to applications where the engine was , such as factory steam engines, portable stationary engines, or traction engines parked in front of the work. For moving vehicles such as a traction engine or early tractor towing a farm implement, the implement could receive rotary power by taking it from one of its own wheels (whose turning was imparted by the towing) and distributing it via roller chains (to a sickle bar's crank, for example), but such a transmission ceases if the vehicle stops traveling; and the workload's resistance tends to make the wheel skid rather than turn, even if cleated. The concept of a shaft drive with easily connected and disconnected couplings, and flexibility for driving at changing angles (such as when an articulated tractor-and-trailer combination turns), was a goal to pursue.

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