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.