Power-take-off shafts (PTOs) were developed to provide farming implement manufacturers and tractor manufacturers with a common method of powering implements. As the name implies, these shafts are designed to take power away from the engine, through the transmission.
Most often found in farming equipment, PTO shafts provide power for attached farming implements, which are basically anything a farming tractor pushes or pulls. This includes rotary tillers, finish mowers, sickle mowers, hay tedders, post hole diggers, fertilizer spreaders, and the ever popular manure spreaders. PTOs can also be used to power electrical generators, which are attached in the same way as regular farming implements. A big tractor parked in the lot of one of the grocery stores I deliver ice to used a PTO shaft to power a salt spreader, attached to the rear of the tractor; PTO shafts in tractors can be found in front or rear mounted applications, however. Another notable point is that live or continuous PTOs do exist, which provide power to the attached implement, even without the tractor being in motion. This is generally accomplished by using a separate transmission for the PTO and the tractor.
Rotational speeds of farming PTOs are 540 rpm or 1,000 rpm.
PTOs are also used in aircraft, most notably in Lockheed Martin's "new" F-22 Raptor Tactical Jet Fighter Aircraft, which utilizes stealth technology. Here they're used to take power from the engines to produce electricity, which in turn provides power to many in-flight electrical systems. The unit also serves as a measurement for aircraft horsepower. Currently, it is used in only the left engine of the aircraft, as a failure of the PTO shaft would result in lost power to many required flight systems, which in turn produces a fiery explosion and a blackened patch of earth.
Rotational speeds for this unit are around the 18,000 rpm mark, considerably higher than those found on John Deere's tractor.
Medium to heavy-duty single and multi-axle trucks also use PTO shafts. Applications here include powered winches (no, not wenches), salt spreaders, and small dumpers. Most likely the man who came to tow your car away used a PTO shaft to assist in towing your vehicle. Other specialized applications include providing power to the hydraulic systems of “snubbing trucks,” which pressurize and cap oil wells, in concrete pumping trucks, and by the telephone company to string wire. Construction companies may use PTO shaft powered air compressors, to provide a reliable, transportable supply of compressed air to the job site, and your local fire company probably uses PTO shaft power to pump the water they use as their weapon.
Rotational speeds for these types of PTO shaft applications vary, due to the variety of uses available, and to the fact that many of them are controlled by pneumatic or manual gear shifters, which allow the operator to control the speed and torque provided to the PTO implement.
PTO shafts are moving parts, and need to be treated cautiously. Since they do provide a lot of power, they can and will rip or crush anything that tries to mess with them. Guards and shields have been manufactured to reduce the likelihood of serious or often fatal injuries, but care still needs to be taken whenever one is using PTO powered equipment.
I highly recommend downloading and watching the video located here:http://cufp.clemson.edu/farmsafetyandhealth/movies/PTOd2.mpg, as it shows a PTO shaft in action. Plus, as a nifty bonus, the newspaper filled dummy really gets knocked up and spilled around.
29 July 2007 Edit:
Regrettably, the video previously mentioned is no longer available. It was definitely worth seeing, and recognizing this, it makes me slightly less hesitant to accept some of the new changes to the website. If video was available, well, I would have hosted that file on the E2 server.
The video found here (http://www.youtube.com/watch?v=ZSKz4Lj8HmQ) shows a smaller tract towing a brush-hog type implement, which utilizes a PTO shaft for power. The shaft is clearly visible as the spinning yellow object during parts of the video, which includes some nice jazzy music. Not as much fun as the destruction of a paper-filled dummy, but it will have to do.