Licensed Professional Engineers

FORENSIC CLUES # 32- "Agricultural Machinery - PTO Hazards" by John L. Ryan


A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.

Issue 32: Vol. 1 July/August 2009

"Agricultural Machinery—PTO Hazards"

By John L. Ryan and L.D. Ryan


Agricultural Machinery and PTO Accidents

Every year there are thousands of debilitating injuries and deaths on farms. Many of these are related to power take-off’s (PTO’s). PTO shafts are used to power various farming implements, from mowers, hay balers, augers, and many other types of farming machinery. The PTO shaft was invented in the 1930’s. The PTO shaft is essentially a shaft powered by the tractor engine that rotates at high speeds, providing power for a wide array of implements. While this provides a convenient source of power to drive farm implements, serious accidents can occur due to entanglement in PTO’s. PTO’s must be properly equipped with guards including master shields which covers the stub shaft and universal joint. These shields prevent inadvertent contact with the rotating machinery. Contact with unshielded rotating PTO parts can instantly pull a person into the PTO, causing catastrophic damage.


Typical PTO Powered Implement

What Happens

PTO implements are powered by the PTO shaft stub located on the tractor. This shaft stub connects to machinery and provides the power for the machinery. Clothing can become caught in an unguarded PTO stub that is still engaged. This is uncommon, occurring only in about 4% of reported incidents.

PTO drivelines are the shafts that run from the tractor’s PTO shaft stub to the machine that is to be powered. The driveline rotates with the PTO shaft stub, at rates of 540 to about 1000 rpm. There are numerous points where an operator can become entangled in the rotating driveline. There is a universal joint on the driveline that is an entanglement hazard. The coupling where the driveline meets the machinery drive is another location where clothing can become entangled.

When a person’s clothing becomes caught in a rotating entanglement hazard such as that caused by PTO’s, the person will not be able to react in time to prevent becoming entangled. This is due to the rotational speeds of the shaft and our limited reaction times. Once the clothing gets caught in the rotating parts, the machine will continue to pull additional clothing into the driveline.


A rotating entanglement hazard of PTO’s

At times the clothing will tear away, but usually people get pulled into the rotating shaft, causing severe injury or death. People can get thrown violently around once stuck in the PTO, instead of being pulled further into the rotating element.

Accidents also occur when driveline shafts become separated, leaving the powered end that is still connected to the tractor PTO swinging wildly.

Disengaging the PTO

Ideally, tractor operators would always disengage the PTO shaft before leaving the operator’s position. A disengaged PTO will not be rotating, and will present no threat to personal safety. A vehicle that comes to a stop and is still running will still have the PTO rotating. Equipment that is operated with the tractor in motion is supposed to be disengaged before getting off of the tractor. This doesn’t always happen. Nearly all PTO accidents that occur with moving equipment occur when the tractor was not moving. Half of all PTO accidents occur in stationary equipment. Equipment that remains stationary while attached to the PTO is even more hazardous, since the operator is likely to be working near the hazardous entanglement points.

What’s the Solution?

Physical guarding is the standard solution to the rotating entanglement hazard of PTO shafts. Most PTO powered implements have a shaft guard that is a tube that either rotates with the shaft and slips upon contact, or is kept from rotating. Master shields guard the rotating PTO stub. These are simple box type barriers that surround the rotating shaft on the sides and top. These can also help guard the front part of the driveline. Master shields get damaged and removed, or they may be removed for convenience of attaching PTO implements. Used implements are sometimes sold without these guards.

Drive shaft couplings and universal joints can also be guarded with physical barriers, preventing clothing or other objects from becoming entangled in the rotating machinery.

The problem of missing guards can be reduced by using tamper-proof interlocking that would prevent engagement of the PTO if the guards were removed.

Advances in PTO Safety

Mead Pneumatics is producing a power take off valve specifically designed to prevent accidental engagement of PTO’s. This valve requires an additional signal in order for the PTO to be activated. This valve will prevent accidents that occur from unintentional engagement of the PTO system. This valve will do nothing to prevent the majority of accidents that occur when the PTO shaft is purposely engaged.

Technology that is currently available could be applied to prevent many PTO accidents. Most accidents that occur with a PTO implement that performs its function when the tractor is moving, like hay balers, mowers, manure spreaders, occur when the machine is not moving. Many of these accidents involve the operator of the tractor. An operator presence detection system, similar to that found on many riding lawnmowers, could be interlocked with the PTO engagement. When the operator left his seat,

The PTO shaft would be disengaged. The difficulty with this idea is seamlessly integrating the application for all uses of the PTO system, both stationary and mobile implements.

What do the Standards Say?

OSHA standards require PTO guarding. Field working standard 29 CFR 1928.57 (b)(1)(iii) states that “Power take-off driven equipment shall be guarded to protect against employee contact with positively driven rotating members of the power drive system…” 29 CFR 1928.57 (c)(1)(i), covering farmsteading, states “All power take-off shafts, including rear, mid-, or side-mounted shafts, shall be guarded either by a master shield as provided in paragraph (b)(1)(ii) of this section or other protective guarding.” The next section repeats the same warning as in 29 CFR 1928.57 (b)(1)(iii). The problem with OSHA standards is that they only apply to employers, they do not impose their safety standards on the manufacturers. As a result, manufacturers are not required to provide guarding on PTO devices. Many farmers and tractor operators are working their own land, thus making any OSHA regulation not-applicable.


Most PTO injuries and deaths are preventable by proper guarding techniques. Focusing on producing guards that will last the lifetime of the product, as well as not interfere with normal product use will help reduce accidents. Interlocking these guards will ensure that the product is not used without them.

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