Power Take-Off (PTO) shaft is an efficient means of transferring
mechanical power between farm tractors and implements. This
power transfer system helped to revolutionize North American
agriculture during the 1930's. It is also one of the oldest
and most persistent hazards associated with farm machinery.
This fact sheet discusses several aspects of PTO safety.
1 is a diagram of component parts of an implement PTO. Two typical
PTO system arrangements are shown. These drawings are helpful
for understanding PTO hazards, guarding, and injuries. The top
drawing is of a PTO system involving a pedestal connection,
such as is found on many types of pulled machinery (hay balers,
forage choppers, large rotary mowers, etc.). The lower drawing
is of a PTO system where the implements input driveline connects
directly to the tractor PTO stub. Examples of this type of connection
include three-point hitch mounted equipment (post hole diggers,
small rotary mowers, etc.) and augers.
following parts of the PTO have been found to be hazardous.
tractor's stub shaft, often called the PTO, transfers power
from the tractor to the PTO-driven machine or implement. Power
transfer is accomplished by connecting a drive shaft from the
machinery to the tractor's PTO stub shaft. The PTO and drive
shaft rotate at 540 rpm (9 times/sec.) or 1,000 rpm (16.6 times/sec.)
when operating at full recommended speed. At all speeds, they
rotate in proportion to the speed of the tractor engine.
incidents involving PTO stubs stem from clothing caught by
an engaged but unguarded PTO stub. The reasons a PTO stub
may be left engaged include: the operator forgetting or otherwise
not being aware the PTO clutch is engaged; seeing the PTO
stub spinning but not considering it dangerous enough to disengage;
or, the operator is involved in a work activity requiring
PTO operation. Boot laces, pant legs, overalls and coveralls,
sweat shirts, and windbreakers are clothing items that can
become caught and wrapped around a spinning PTO stub shaft.
PTO driveline hazard (sometimes identified as a machinery wrapping
point hazard) is one of the oldest and most common farm machinery
hazards, and refers specifically to the part of the implement
(machine) drive shaft that connects to the tractor. This drive
shaft is known as the implement input driveline (IID). The entire
IID shaft is a wrapping point hazard if the IID is completely
unshielded. If the IID shaft is partly guarded, the shielding
is usually over the straight part of the shaft, leaving the
universal joints, the PTO connection (the front connector),
and the Implement Input Connection (IIC, the rear connector)
as the wrapping point hazards. Protruding pins and bolts used
as connection locking devices are particularly adept at snagging
clothing. If clothing doesn't tear or rip away, as it sometimes
does for the fortunate, a persons limb or body may begin to
wrap with the clothing. Even when wrapping doesn't occur, the
affected part may become compressed so tightly by the clothing
and shaft that the person is trapped against the shaft.
machines IID shaft is coupled to the tractors PTO stub. Therefore,
it too rotates at either 540 rpm (9 times/sec.) or 1,000 rpm
(16.6 times/sec.) when at full recommended speed. At these
speeds, clothing is pulled around the IID shaft much quicker
than a person can pull back or take evasive action. Many IID
shaft entanglements happen while the shaft is turning at one-half
or one-quarter of recommended operating speed. This may be
the situation on occasions when the tractor has been stopped
but not turned off, and the PTO is left engaged. Why an operator
might do this is discussed in the paragraph below. The point
here is that even at slower speeds, once caught by a IID shaft,
a person may not have time for evasive action. A 540 rpm shaft
makes over two complete revolutions per second when operating
at one-quarter speed. Even with a relatively quick reaction
time of five-tenths of a second, the wrapping action has begun.
Once wrapping begins, the person instinctively tries to pull
away. This action simply results in a tighter, more binding
wrap. The 1,000 rpm shaft roughly cuts in half the opportunity
for evasive action.
powered machinery may be engaged while no one is on the tractor
for many reasons. Some PTO powered farm equipment is operated
in a stationary position: it needs no operator except to start
and stop the equipment. Examples are elevators, grain augers,
and silage blowers. At other times, adjustments or malfunctions
of machine components can only be made or found while the
machine is operating. Additionally, many work practices such
as clearing crop plugs leads to operator exposure to operating
PTO shafts. Other unsafe practices include mounting, dismounting,
reaching for control levers from the rear of the tractor,
and stepping across the shaft instead of walking around the
machinery. An extra rider while PTO powered machinery is operating
is another exposure situation.
wrapping hazard is not the only hazard associated with IID
shafts. Serious injury has occurred when shafts have become
separated while the tractors PTO was engaged. The machines
IID shaft is a telescoping shaft. That is, one part of the
shaft will slide into a second part. This shaft feature provides
a sliding sleeve which greatly eases the hitching of PTO powered
machines to tractors, and allows telescoping when turning
or moving over uneven ground. If a IID shaft is coupled to
the tractors PTO stub but no other hitch is made between the
tractor and the machine, then the tractor may pull the IID
shaft apart. If the PTO is engaged, the shaft on the tractor
end will swing wildly and may strike anyone in range. The
swinging force may break a locking pin allowing the shaft
to become a flying missile, or it may strike and break something
that is attached or mounted on the rear of the tractor. Separation
of the driveline shaft is not a commonly occurring event.
It is most likely to happen when three-point hitched equipment
is improperly mounted or aligned, or when the hitch between
the tractor and the attached machine breaks or accidentally
information is taken from the Purdue University source listed
at the end of this fact sheet. This reference is the most comprehensive
study of power take-off injury incidents to date. The percents
shown include fatal and nonfatal injury incidents, and are best
thought of as approximations. Generally, PTO entanglements:
the tractor or machinery operator 78 percent of the time.
was absent or damaged in 70 percent of the cases.
areas were at the PTO coupling, either at the tractor or
implement connection just over 70 percent of the time.
bare shaft, spring loaded push pin or through bolt was the
type of driveline component at the point of contact in nearly
63 percent of the cases.
equipment, such as augers, elevators, post-hole diggers,
and grain mixers were involved in 50 percent of the cases.
equipment, such as self unloading forage wagons and feed
wagons, were involved in 28 percent of the cases.
all incidents involving moving machinery, such as hay balers,
manure spreaders, rotary mowers, etc., were non-moving at
the time of the incident (the PTO was left engaged).
four percent of the incidents involved no attached equipment.
This means that the tractor PTO stub was the point of contact
four percent of the time.
a PTO system includes a master shield for the tractor PTO stub
and connection end of the implement input driveline (IID) shaft,
an integral-journal shield which guards the IID shaft, and an
implement input connection (IIC) shield on the implement. The
PTO master shield is attached to the tractor and extends over
and around the PTO stub on three sides (Figure 2). This shield
is designed to offer protection from the PTO stub and the front
joint of the drive shaft of the connected machine. Many tractors,
particularly older tractors, may no longer have PTO master shields.
Master shields are removed or are missing from tractors for
several reasons including: damaged shields that are never replaced;
shields removed for convenience of attaching machine drive shafts;
shields removed out of necessity for attaching machine drive
shafts; and shields missing when used tractors are sold or traded.
are many more injuries associated with the IID shaft than
with the PTO stub. As noted earlier, machine drive shaft guards
are often missing. This occurs for the same reasons tractor
master shields are often missing. A IID shaft guard completely
encloses the shaft, and may be constructed of plastic or metal.
These tube like guards are mounted on bearings so the guard
rotates with the shaft but will stop spinning when a person
comes into contact with the guard. Some newer machines have
driveline guards with a small chain attached to a non-rotating
part of the machine to keep the shield from spinning. The
most important thing to remember about a spinning IID shaft
guard is that if the guard becomes damaged so that it cannot
rotate independent of the IID shaft, its effectiveness as
a guard is lost. In other words, it becomes as hazardous as
an unguarded shaft (Figure 3). This is why it is important
to always spin the IID shaft guard after attaching the PTO
to the tractor (the tractor should be shut off), or before
starting the tractor if the attachment has already been made.
This is the best way to make sure that the IID shaft guard
is really offering you protection.
examples of PTO injury incidents involving Pennsylvania farmers
will help illustrate the serious nature of PTO hazards.
#1. A farmer was finished loading a load of green chop
into the silo and was approaching the tractors PTO lever
to turn off the forage blower. As he stepped onto the drawbar,
the laces on his boot became caught on the spring loaded
push pin of the forage blower PTO driveline coupling. He
was thrown backwards off the drawbar, with his boot and
denim trousers being forcibly removed. He suffered considerable
muscle damage to his right leg.
#2. A teenager was helping her family load corn into
a grain elevator when her jacket sleeve became entangled
by the elevator PTO shaft. Her body was flung around the
shaft and her arm torn from its socket before the tractor
could be turned off.
#3. A small child was killed when as an extra rider
on his fathers tractor, he slipped off and became entangled
by a spinning PTO shaft. His father grabbed for the boy
as he began to slip but was unable to hold him out of the
#4. A farmers clothing was caught by a spinning shaft,
pulled him in, flung him around the shaft a couple of times,
and then threw him clear. He suffered injuries to his head,
leg, right arm and shoulder.
not always easy or convenient, there are several ways to reduce
the risk of PTO injury incidents. These safety practices offer
protection from the most common types of PTO entanglements.
it a specific point to keep all components of PTO systems
shielded and guarded.
test driveline guards by spinning or rotating them to ensure
they have not become stuck to the shaft.
the PTO and shut off the tractor before dismounting to clean,
repair, service, or adjust machinery.
around tractors and machinery rather than stepping over
a rotating shaft.
universal joints in phase. (Check the operators manual or
with a farm implement dealer if you do not understand what
use the driveline recommended for your machine. Never switch
drivelines among different machines.
the tractor's drawbar properly for each machine used. This
will help prevent driveline stress and separation on uneven
terrain and in tight turns.
PTO shaft abuse by observing the following: avoid tight
turns that pinch rotating shafts between the tractor and
machine; keep excessive telescoping to a minimum; engage
power to the shaft gradually; and avoid over tightening
of slip clutches on PTO-driven machines.
W.P. 1987. The Condition of Agricultural Driveline System
Shielding and Its Impact in Injuries and Fatalities.
M.S. Thesis. Department of Agricultural Engineering, Purdue
University, West Lafayette, IN.
of Machine Operation: Agricultural Safety. 1983. Second
edition. Deere and Company, Moline, IL.
Operation of Agricultural Equipment: Student Manual. 1988,
Revised. Hobar Publications, St. Paul, MN.
Publication #: Fact Sheet Safety-33
This document is from the
Pennsylvania State University
Pennsylvania Cooperative Extension Service. For more information,
contact Pennsylvania State University College of Agricultural
Sciences, Agricultural Engineering Department, 246 Agricultural
Engineering Building, University Park, PA 16802. Publication
J. Murphy, professor, Agricultural Engineering Department,
Cooperative Extension Service, Pennsylvania State University,
University Park, PA 16802.
Disclaimer and Reproduction Information: Information in
NASD does not represent NIOSH policy. Information included in
NASD appears by permission of the author and/or copyright holder.