Eric Hallman
Cornell Agricultural Health and Safety Program
The power take-off (PTO) drive, found
on most farm tractors, is a major power
source for agricultural equipment. The
PTO system efficiently transfers
mechanical power from the tractor to the
implement, but it may present serious
hazards to persons who are unfamiliar
with its operation.
When hooked up to a piece of
equipment, the complete PTO system or
implement input driveline may connect
either directly to the tractor spline or stub, or indirectly through a
pedestal connection as illustrated in
figure 1. Coupling devices are used to
attach the shaft to the tractor and to the
implement.
A spline collar slides over the tractor's
spline, which extends from the tractor
differential. The collar is held in place by
a spring-loaded pin that latches into a
recess on the spline. The telescoping
feature of the shaft allows the collar to
slide easily onto the spline. Together with
the universal joints, the telescoping shaft
allows the PTO system to flex and adjust
when the tractor turns or travels over
uneven terrain.
When operated at full recommended
speed, a PTO shaft will rotate clockwise
at 540 revolutions per minute (rpm),
which equals 9 revolutions per second, or
at 1,000 rpm (16.7 revolutions per
second). As shown in the Table 1, a 540
rpm shaft travels 7.1 feet per second,
while a 1,000 rpm shaft turns 13.1 feet
per second. A person caught in a
revolving PTO shaft has little chance to
escape.
Shields are essential to the safe
operation of the PTO system. To prevent
operators from coming in contact with
spinning parts, every component of the
PTO system should have guards,
including a shaft shield, a master shield,
and an implement shield.
Most modern shaft shields are tubular,
although older equipment may have
inverted U-shaped shielding devices.
Tubular shields are recommended
because U-shaped shields
guard against contact only from the sides
and top of the shaft. Metal or plastic
tubular shields completely enclose the
shaft. Bearings prevent the shield from
spinning when contact is made. Some
models use a chain connected to a
stationary part of the machine to prevent
the shield from spinning constantly. Even
if tubular guards are in place, a hazard
may exist if the shield is damaged and
cannot spin independently from the shaft.
A damaged guard can be as hazardous
as an unguarded shaft.
The tractor's master shield and the
implement shield are particularly
important because they prevent
equipment operators from getting
entangled in the system's dangerous
rotating connections. The master shield
extends over the top and around the
sides of the spline and protects the
operator from the rotating spline and front
universal joint. People become entangled
in this area when boot laces or other
articles of clothing catch on the locking
pin, bolt, or grease fitting. The implement
shield is similar in appearance to the
master shield and offers protection from
the rear universal joint and implement
connection.
One or more of these shields are
frequently missing. Operators sometimes
remove shields because they make
hitching up equipment inconvenient or
impossible. They may remove damaged
shields and not replace them. Shields
may be missing when used tractors or
equipment are purchased.
Most PTO accidents and injuries occur
when a person's clothing or hair becomes
entangled with a part of the spinning PTO
system. Protruding components such as
the locking pin, bolt, cotter pin, grease
fitting, nails, universal joint, and tractor
spline readily hook and grab loose or
dangling clothes or hair. Boot laces, pant
legs, coat or shirt cuffs and tails,
drawstrings on windbreakers or hooded
sweatshirts, and scarves frequently get entangled. Cases have
also been reported of long hair getting
caught in PTOs.
Table 1: Lapsed Time vs. Distance Traveled
And how it relates to a tractor PTO turning at 540 and 1000 rpm-shaft/shield diameter of 3 inches
|
540 RPM |
1000 RPM |
Lapsed time in seconds |
Revolutions Turned |
Distance Traveled (ft.) |
Revolutions Turned |
Distance Traveled (ft.) |
0.15 |
1.4 |
1.1 |
2.5 |
2.0 |
0.25 |
2.3 |
1.8 |
4.2 |
3.3 |
0.33 |
3.0 |
2.4 |
5.5 |
4.4 |
0.40 |
3.6 |
2.8 |
6.7 |
5.2 |
0.50 |
4.5 |
3.5 |
8.3 |
6.5 |
0.60 |
5.4 |
4.2 |
10.0 |
7.9 |
0.70 |
6.3 |
4.9 |
11.7 |
9.2 |
0.80 |
7.2 |
5.7 |
13.3 |
10.5 |
1.00 |
9.0 |
7.1 |
16.7 |
13.1 |
1.50 |
13.5 |
10.6 |
25.0 |
19.6 |
3.00 |
27.0 |
21.2 |
50.0 |
39.3 |
5.00 |
45.0 |
35.3 |
83.3 |
65.5 |
10.00 |
90.0 |
70.7 |
166.7 |
130.9 |
60.00 |
540.0 |
424.1 |
1000.0 |
785.4 |
Once clothing or hair is caught, the PTO's speed of rotation, combined with the power exerted by the tractor, makes escape unlikely and injury almost certain. Following are some typical outcomes:
- Clothing is torn completely off the
person, resulting in minor injuries
such as friction burns, scrapes,
sprains, and bruises.
- Clothing is torn away, and in the
process the PTO catches and tears
loose or hanging skin. Injury to the
scrotal area is common with this type
of accident.
- Clothing is torn away, and part of the
victim's body becomes wrapped up in
the shaft or wedged against the
machine resulting in severe injury,
including lacerations, broken bones,
strangulation, loss of limbs, fingers,
or toes.
- Clothing is not completely torn away,
and the victim's body rotates with the
shaft, causing multiple serious
injuries or death.
- Hair becomes entangled, resulting in
partial or complete scalping.
PTO accidents can happen when the
tractor is moving or stationary. Under
normal conditions, the PTO should be
disengaged, cutting the power from the
tractor, before the operator dismounts.
There are several reasons, however, for
keeping the PTO engaged when the
tractor Is stationary:
- Some machines, such as augers and
silage blowers, are designed for is
stationary use of PTO power.
- Maintenance procedures such as
lubricating chains, sharpening knives,
or checking the condition of moving
parts may require that power be run
to the machine.
- On some older tractors it may be
impossible to reach for control levers
without coming in close proximity to
the spinning PTO.
The operator, the machine, and the
environment can contribute singularly or
together to a PTO accident.
The Operator
The physical and mental condition of the
operator may lead to poor decision
making, unsafe actions, or impaired
capabilities. Age can affect both the
physical and the mental condition of an
operator. Young operators have limited
experience and knowledge, and their lack
of aware- ness of hazards increases the
possibility of an accident. Older workers
may be experienced but often have
slower reaction times and lack the agility
to react physically to some situations,
particularly when the work area is
slippery.
Farmers are often in a hurry to get a
job done. But trying to rush or getting
frustrated with delays and breakdowns
can cause mental strain. Preoccupation
with completing work or other distractions
can divert attention from job hazards.
A person's attitude about safety is also
very important in preventing accidents.
Does the operator perceive PTO
operation as hazardous? Does he or she
take the time to install or replace missing
guards? Does the operator knowingly or
unknowingly place himself or herself in a
hazardous situation, particularly when
mounting, dismounting, or adjusting
controls from the rear of the tractor?
Knowledge of hazards alone is not
enough to prevent accidents. A positive
attitude about safety is needed to make
use of the knowledge.
The Machinery
PTO accidents are not always the result
of human error. Often the machinery may
contribute to a PTO accident. The tractor
or implement may have been purchased
without shielding. Older tractors,
designed to be mounted and dismounted
from the rear, make it impossible for
operators to avoid coming close to the
PTO. Equipment that requires removing
the master shield to tie onto the PTO may
also present hazards to the operator.
The Environment
The environment may contribute to a
PTO accident. Slippery conditions
caused by rain, mud, snow, frost, or ice
can cause an operator to lose his or her
balance while mounting or dismounting a
tractor or implement. Temperature
extremes, noise, and vibration from the
machine can adversely affect operators
by reducing their physical capabilities or
adding to mental strain. When working in
a tight space, an operator may have to
come close to the PTO or perform an
activity differently than usual. Finally,
because the home and workplace are
often the same on a farmstead, children
are regularly exposed to dangerous
equipment, increasing the possibility of
accidents for non-operators.
- Whenever possible, shut the PTO off
before dismounting the tractor.
- Keep all tractor and implement
shields and decals in place. Replace
missing shields as soon as
possible.
- Do not wear loose or bulky clothing
around the PTO or other moving
parts.
- Make sure all operators of PTO driven
equipment are trained and
knowledgeable about safe operating
procedures and hazards that may not
be readily apparent.
- Be extra cautious when using
stationary PTO-powered equipment.
- Periodically check on PTO operators
to ensure a quick response in the
event of an accident. Injuries that
would not ordinarily be fatal can be
life threatening if the victim is not
found until hours later. PTO accident
victims require immediate medical
care.
- Always walk around equipment to
avoid coming near an active PTO.
Stepping over, leaning across, or
crawling under a live PTO can lead to
entanglement.
- Keep all bystanders away from PTO driven
equipment, and never allow
children to ride on tractors or
equipment.
- Position the tractor drawbar properly
for each machine used to prevent
driveline stress and separation.
- Regularly test the shaft guard by
rotating it to ensure that it spins
freely.
The PTO system is one of the most
useful and most dangerous pieces of
farm machinery. Entanglement with a
PTO can result in dismemberment,
scalping, or death. Shields for PTO
components are essential for the systems
safe operation.
Careful forethought should be given to
taking precautions related to the
environment, the machinery, and the
operator. By adopting a positive
approach to safety and following these
guidelines, you can help ensure safe
machine operation.
Lightning Protection for Farms
Electrical Safety on the Farm
Slow Moving Vehicle Emblems
Safer Farm Environments for Children
Safe Animal Handling
This publication is issued to further Cooperative
Extension work mandated by acts of Congress of
May 8 and June 30, 1914. It was produced with the
cooperation of the U.S. Department of Agriculture;
Cornell Cooperative Extension; the New York State
College of Agriculture and Life Sciences, New York
State College of Human Ecology, and New York
State College of Veterinary Medicine, at Cornell
University.
Designer: Dennis E Kulis
Editor: David A. Poland
Illustrations by Jim Houghton
For additional information: call 1-877-257-9777
Cornell Agricultural Health & Safety Program
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. More
NASD Review: 07/2005
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