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A
power take-off or power takeoff (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. In aircraft
applications, such an accessory drive may be used in
conjunction with a Constant Speed Drive.
Experimental power take-offs were tried as early as
1878, and various homemade versions arose over the
subsequent decades, but International Harvester Company
(IHC) was first to install a PTO on a production
tractor, with its model 8-16, introduced in 1918. Edward
A. Johnston, an IHC engineer, had been impressed by a
homemade PTO that he saw in France about a decade
before, improvised by a French farmer and mechanic
surnamed Gougis. He and his IHC colleagues incorporated
the idea into the 8-16, and designed a family of
implements to take advantage of the feature. In 1920,
IHC offered this option on their 15-30 tractor, and it
was the first PTO-equipped tractor to be submitted for a
Nebraska tractor test. The first PTO standard was
adopted by ASAE (the American Society of Agricultural
Engineers) in April 1927. The PTO rotational speed was
specified as 536 ? 10 rpm; the direction was clockwise.
The speed was later changed to 540 rpm. The PTO was a
competitive advantage for IHC in the 1920s, and other
companies eventually caught up with PTO implementation.
In 1945, Cockshutt Farm Equipment Ltd of Brantford,
Ontario, Canada, introduced the Cockshutt Model 30
tractor with live power take-off (LPTO). LPTO allows
control of the PTO rotation independently of the tractor
motion. This was an advantage when the load driven by
the PTO required the tractor motion to slow or stop
running to allow the PTO driven equipment to catch up.
In modern tractors, LPTO is often controlled by
push-button or selector switch. This increases safety of
operators who need to get close to the PTO shaft.
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. On April 13, 2009 former Major League
Baseball star Mark Fidrych died as a result of a PTO
related accident; "He appeared to have been working on
the truck when his clothes became tangled in the truck's
power takeoff shaft," District Attorney Joseph Early Jr.
said in a statement.
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 or truck .
In some countries it is illegal to operate a PTO without
the shaft guard correctly fastened.
Agricultural PTOs are
standardized in dimensions and speed. The ISO standard for PTOs
is ISO 500, which as of the 2004 edition was split into three
parts:
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ISO 500-1
General specifications, safety requirements, dimensions for
master shield and clearance zone
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ISO 500-2
Narrow-track tractors, dimensions for master shield and
clearance zone)
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ISO 500-3
Main PTO dimensions and spline dimensions, location of PTO.
The original type
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 types, supporting higher power applications,
operate at 1000 RPM and differ in shaft size. The larger shaft
has 20 splines (1?" diameter), while the smaller has 21 (1⅜"
diameter). All three types rotate counterclockwise when viewed
from the tractor. A 10 spline type was used with some early
equipment such as the 1948 Land Rover, a six spline adapter was
usually supplied. It is customary for agricultural machines
manufacturers to provide the nominal PTO power specification, an
indication of the available instantaneous power at the shaft.
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