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A shaft attached to the PTO.

A power take-off (PTO) is a splined driveshaft, usually on a tractor or truck that can be used to provide power to an attachment or separate machine. It is designed to be easily connected and disconnected. The power take-off allows implements to draw energy from the tractor's engine.

Semi-permanently mounted power take-offs can also be found on industrial and marine engines. These applications typically use a Cardan 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.

The PTO and its associated shafts and universal joints are a common cause of incidents and injury in farming and industry. According to the National Safety Council, 6 percent of tractor related fatalities in 1997 in the USA involved the PTO. When a piece of clothing, which can be as small as a single thread, touches a spinning part it can be pulled around the part. The clothing and the person wearing it are pulled into the shaft often resulting in loss of limb or death. Some implements do use plastic guards to try to keep a person from becoming entangled in a PTO shaft, but even with guards people need to exercise caution around PTO shafts when they are plugged into a tractor. In some countries it is illegal to operate a PTO without the shaft guard correctly fastened.^[1][2]

A tractor PTO

Agricultural PTOs are standardized in dimensions and speed. The original specification calls for operation at 540 revolutions per minute (RPM). A shaft that rotates at 540 rpm has 6 splines on it, and a diameter of 1⅜". Two newer specifications, supporting higher power applications, operate at 1000 RPM and differ in shaft size. The larger shaft has 20 splines (1¾" diameter), while the smaller has 21 (1⅜" diameter). All three types rotate counterclockwise when viewed from the tractor. It is customary for agricultural machines manufacturers to provide the nominal PTO power specification, an indication of the available instantaneous power at the shaft.^{[citation needed]}

The manner in which the power is transmitted to the PTO varies, which affects the operation.

In a tractor with a transmission PTO, the PTO is driven directly off the transmission, and the PTO is disengaged by the tractor's clutch. This configuration is the simplest, but it has the disadvantage that if the tractor needs to be slowed, the PTO will stop. This configuration often requires an overrunning clutch which disengages the PTO if the PTO is rotating faster than the transmission. This prevents PTO driven attachments with high inertia from driving the tractor once the clutch has been disengaged on the tractor.

In a tractor with a live PTO, there is a two-stage clutch. Pressing the clutch half-way will disengage the transmission while pressing it fully will disengage the transmission and the PTO. This configuration allows the operator to slow down or change gears while the PTO is still operating.

In a tractor with an independent PTO, there is a separate clutch for the PTO. As with a live PTO, this allows for full control over the tractor while separately controlling the PTO. Independent PTO's may have either mechanical or hydraulic controls. ^[3]