LEGAL CONSIDERATIONS IN INVESTIGATING TRACTOR POWER TAKE-OFF (PTO) SHAFT INJURIES

Richard L. Parish, PhD, PE

Improperly guarded tractor power take-off (PTO) shafts are the source of many serious injuries and deaths. PTO shafts are used to transmit rotary power from tractors to implements including rotary cutters, post-hole diggers, rotary tillers, flail mowers, forage harvesters, hay balers and many others. Because of the high rotational speeds involved, a common scenario involves a person getting too close to a moving PTO shaft and getting his/her clothing caught in some part of the shaft. A limb or the entire body is then wrapped around the shaft before the operator can turn off the power.

The standard operating speeds for rear PTO shafts on agricultural tractors are 540 rpm and 1,000 rpm. The speed of a front PTO on agricultural tractors, if so equipped, is 1,000 rpm. Lawn and garden tractors use a 2,000 rpm PTO, whether front, mid or rear. These rotational speeds are fast enough that once a person’s clothing is caught, the person will be wrapped up in the shaft before he/she can pull away or the operator can react. At only 540 rpm, a 3-inch diameter PTO universal joint yoke will have a peripheral speed of over 7 feet/second. Put another way, if it takes a person a half second to notice being caught and then react, a 540-rpm shaft will already have turned 4.5 times. If the driver is not watching and does not notice immediately that a person has become entangled, the shaft can continue turning for several seconds. Death or limb amputation is common with PTO accidents.

Proper Guarding of PTO Shafts

The typical PTO driveline consists of two universal joints (U-joints), one at the tractor PTO stub shaft and one at the point where the driveline connects to the implement, with a sliding connecting member between them. The two U-joints allow the tractor to turn relative to the implement and, if the U-joints are in phase and both U-joints are at the same angle, will allow constant output velocity. The sliding member is necessary to accommodate linear motion due to U-joint rotation as well as turning or vertical angle changes. The front yoke of the front U-joint will slip over the tractor PTO shaft (on agricultural tractors) and the rear yoke of the rear shaft will typically slip over a mating shaft on the implement.

Standards: Standards adopted by the American National Standards Institute (ANSI) and the American Society of Agricultural Engineers (ASAE) require three specific guards for PTO shafts.

The moving shaft itself should be guarded by an Implement Input Driveline (IID) guard that covers the two U-joints and their connecting member(s). This IID guard will have to slide linearly as the connecting member changes in length. Bells at both ends partially cover the two U-joints. On older drivelines, the bells were rigid; on current drivelines, the bells have accordion folds that allow flexing. Prior to 1970, most PTO IID guards were made of steel. Beginning in the 1970s, most manufacturers changed to plastic IID guards. All IID guards are intended to allow free rotation of the IID driveline while the guard remains stationary. Thus, if a person comes into contact with the IID guard, the guard should stop or remain stopped, allowing the driveshaft to turn freely inside. Most current IID guards are equipped with small chains attached to the front and rear bell guards. The chains should be fastened (by the operator) to stationary members on the tractor and the implement, thus restraining the IID guard from rotating at any time. Older IID guards typically rotated along with the IID driveline until contacted and stopped.
The second guarding component is the tractor master shield. This guard is a part of the tractor. It is typically a U-shaped steel guard over the tractor PTO stub shaft. In most cases, the tractor master shield can be tilted up for easier access to the tractor PTO stub shaft for attaching the IID front yoke. It should be rotated back into position for use.
The third guarding component is the Implement Input Connection (IIC) guard. This guard is mounted to the implement and surrounds (or partially surrounds) the rear yoke of the IID driveline where it fits over the mating shaft on the implement. In some cases, the IIC guard will also cover a slip clutch or other driveline component. The IIC guard should extend forward at least two inches in front of the end of the rear bell of the IID guard, thus overlapping the bell and preventing personnel contact with the rear yoke or attaching means. On agricultural field equipment, the IIC guard can surround the IIC or cover only the sides and top; on stationary equipment, the IIC guard must completely surround the IIC. Both steel and plastic IIC guards have been used. The use of an IIC guard has been required by an ASAE standard (S318) since at least 1985. The configuration of an acceptable IIC guard was further clarified by a 2002 revision of S318. Not all manufacturers have provided IIC guards. Some manufacturers have provided no guard and others have provided an extended rear bell (extending back beyond the rear U-joint) rather than an IIC guard. The long bell guards are not an effective IIC guard and are not as durable as most IIC guards.

Responsibility for proper guarding: All three of these components are critical and must be in place for safe operation. It is the responsibility of the manufacturer(s) of the tractor and the implement to provide these guards. It is the responsibility of the owner or employer to properly maintain these components and to promptly replace any components that are damaged or missing.

Common Accident Scenarios

Most accidents resulting in PTO shaft entanglement involve unsafe actions by the injured person, the operator and/or the employer. For example:

No one should be near an operating PTO shaft.
No one working around moving equipment should be wearing loose clothing.
No one should try to step over or reach over an operating PTO shaft.

Common accidents involve a person wearing loose clothing who then steps over, reaches over, or approaches a moving PTO shaft. As an example, a common PTO entanglement accident on post-hole diggers occurs when a helper stands near the operating post-hole digger and tries to guide the auger and/or apply downpressure to the auger.

Although contact with moving PTO shafts should always be avoided, such contact should not result in injury if the PTO is properly guarded as described in the standards section. Therefore, most injuries involving PTO shaft entanglement are the result of one of the following equipment issues:

Missing tractor master shield, IID or IIC guard components
Damaged tractor master shield, IID or IIC guard components
Use of an attaching bolt in the IIC that is too long and protrudes excessively

Responsibility for PTO Accidents

Missing or inadequate guard components can be the result of omission by the manufacturer (design or manufacturing defect) or failure of the owner/employer to replace missing guards (negligence). Likewise, damaged guard components should be replaced by the owner/employer and failure to do so is negligence. Also, it is possible that guards were improperly designed, thus offering insufficient durability. A very common problem occurring in PTO accident cases is the use of an overly long bolt in the IIC. Unless it was provided by the manufacturer, use of a longer bolt would be negligence on the part of the owner/employer. Many operator’s manuals specify the length of bolt to be used. It is the responsibility of employers to ensure safe operation by their employees. That responsibility includes:

Proper maintenance of the equipment including guards and correct IIC bolt
Establishing policies and training employees to not approach a moving PTO driveshaft or operate an implement without full PTO guarding
Not requiring or allowing employees to perform unsafe acts such as guiding or pushing down on a post-hole digger.

Some Relevant Standards

Current (2011) versions of these standards are listed. The version in effect at the time of manufacture should be cited when used for specific cases.

IDENTIFICATION OF STANDARDS ORGANIZATIONS:
ANSI = American National Standards Institute;
ASABE = American Society of Agricultural and Biological Engineers (formerly American Society of Agricultural Engineers;
ASAE = American Society of Agricultural Engineers (now American Society of Agricultural and Biological Engineers;
ISO = International Standards Organization

ANSI /ASABE AD500 – 1:2004 W/Cor.1 (DEC2010) Agricultural Tractor – Rear-mounted Power Take-off types 1, 2 and 3 – Part 1: General Specifications, Safety Requirements, Dimensions for Master Shield and Clearance Zone.

ANSI/ASABE AD500 – 2:2004 (DEC2010) Agricultural Tractor – Rear-mounted Power Take-off types 1, 2 and 3 – Part 2: Narrow-track Tractors, Dimensions for Master Shield and Clearance Zone.

ANSI/ASABE AD500 – 3:2004 (DEC2010) Agricultural Tractor – Rear-mounted Power Take-off types 1, 2 and 3 – Part 3: Main PTO Dimensions and spline dimensions, location of PTO.

ANSI/ASABE AD8759 – 1:1998 (DEC2010) Agricultural Wheeled Tractors – Front-mounted Equipment – Part 1: Power Take-off and Three-point linkage.

ANSI/ASABE AD8759 – 2:1998 (DEC2010) Agricultural Wheeled Tractors – Front-mounted equipment – Part 2: Stationary Equipment Connection.

ANSI/ASAE S318.17 JUN2009 Safety for Agricultural Field Equipment.

ANSI/ASAE S331.5 MAR1995 (R2010) Implement Power Take-off Driveline Specifications.

ANSI/ASAE S370.5 FEB2011 2000-RPM Power Take-off for Lawn and Garden Ride-on Tractors.

ANSI/ASAE S493.1 JUL2003 (R2008) Guarding for Agricultural Equipment.

ANSI/ASAE S522.1 JAN2005 (ISO 5674:2004) (R2009) Tractors and Machinery for Agricultural [sic] and Forestry – Guards for Power Take-off (PTO) Drive Shafts – Strength and Wear Tests and Acceptance Criteria.

ANSI/ASABE S604 JUN2009 Safety for Power Take-off (PTO), Implement Input Driveline (IID), Implement Input Connection (IIC), and Auxiliary Power Take-off (aux. PTO) for Agricultural Field Equipment.

ASAE S205.2 DEC1978 (R209) Power Take-off Definitions and Terminology for Agricultural Tractors.

ASAE S207.12 MAR1994 (R2009) Operating Requirements for Tractors and Power Take-off Driven Implements.

ASAE S431.2 FEB2011 2000-RPM Front and Mid PTO for Lawn and Garden Ride-on Tractors.

ASAE S440.3 MAR2005 Safety for Powered Lawn and Garden Equipment.