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The mechanization of agricultural work and work processes has relieved many workers throughout the world of onerous, back-breaking, monotonous labour. At the same time, the speed and power associated with mechanization contributes greatly toward serious traumatic injury. Throughout the world, countries that practise mechanized agriculture list tractors and field and farmstead machinery as leading agents of fatal and disabling injury in agricultural work. Power tools also contribute to the injury toll, though these injuries are usually less severe. Some machinery also presents environmental hazards such as noise and vibration.

Tractor hazards

Farm tractors have many characteristics that result in their being the most important piece of power equipment on the farm. Most tractors have rubber tyres, hydraulic systems, and power take-off (PTO), and utilize a combination of engine speeds and gear ratios. These characteristics combine to provide tractors with speed, power, flexibility and adaptability. The most serious hazards associated with tractor operation include overturns, runovers and PTO entanglement. Tractor overturns fatally injure far more victims than any other type of incident. Table 1 provides a listing of tractor hazards and how injuries occur.

Table 1. Common tractor hazards and how they occur


Type of incident

How injury occurs


Side rollovers

Operating on slopes, turning corners too fast, rear wheel drops into a hole or off-road surface.


Rear rollovers

Hitching to a point other than the drawbar, rear wheels are stuck in mudhole or are frozen to the ground.


Passenger (extra rider) falls off

Most tractors are designed only for one operator; therefore, there is no safe location for an extra person on a tractor.


Operator falls off

Knocked off by low-hanging tree limb, bounced out of seat by traversing rough ground.


Operator is run over while standing on the ground

Jump starting tractor with tractor inadvertently in gear. Tractor rolls while mounting/dismounting. Tractor rolls during hitching/unhitching of equipment.


Bystander or on-ground helper is run over

Bystander incidents often involve small children the operator does not see. On-ground helper incidents are similar to operator-on-the-ground incidents.

Power take-off (PTO)

Entanglement with PTO stub shaft

Master shield is missing and PTO is left engaged while tractor is running. Operator may be mounting/dismounting from rear of tractor.

Slips and falls

Mounting/dismounting from tractor

Wet and/or muddy feet, first/last step is high off the ground, difficult to reach handholds, hurrying, facing wrong way when dismounting.

Noise-induced hearing loss

Operating tractor

The tractor muffler may be missing, damaged, or is a non-recommended replacement; tractor engine is not maintained properly; metal weather cab redirects sound back to the operator. Damaging noise level may come from a combination of tractor and attached machine. (Older tractors generally produce louder sounds than newer tractors.)



The central concept in tractor stability/instability is centre of gravity (CG). A tractor’s CG is the point on the tractor where all parts balance one another. For example, when a two-wheel-drive tractor is sitting with all wheels on level ground, the CG is typically about 25.4 cm above and 0.6 m in front of the rear axle and in the centre of the tractor body. For four-wheel-drive and centre-articulated tractors, the CG is located slightly more forward. For a tractor to stay upright, its CG must stay within the tractor’s stability baseline. Stability baselines are essentially imaginary lines drawn between points where tractor tires contact the ground (see figure 1). A tractor’s CG as such does not move, but its relationship with stability baselines may change. This most often occurs as the tractor moves out of a perfectly level position, such as onto a slope. A changing relationship between CG and stability baseline means the tractor is moving toward an unstable position. If the CG-stability baseline relationship changes significantly (e.g., the tractor CG moves beyond the stability baseline), the tractor rolls over. If equipment such as a front-end loader, a round bale lifting fork or a chemical side-saddle tank is mounted on the tractor, the additional weight shifts the CG toward that piece of equipment. As mounted equipment is raised, the CG is raised.

Figure 1.  The stability baseline of a tricycle tractor and a wide front-end tractor, respectively


Other factors important to tractor stability/instability include centrifugal force (CF), rear-axle torque (RAT) and drawbar leverage (DBL). Each of these factors works through the CG. Centrifugal force is the outward force nature exerts on objects that move in a circular fashion. Centrifugal force increases both as the turning angle of the tractor becomes sharper (decreases) and as the speed of the tractor increases during a turn. The CF increase is directly proportional to the turning angle of the tractor. For every degree the tractor is turned tighter, there is an equal amount of increased CF. The relationship between CF and tractor speed, however, is not directly proportional. Finding the increase in CF from turning a tractor at a higher speed                                                                                                                                       (assuming the turning radius stays the same) calls for squaring                                                                                                                     the difference between the two tractor speeds.

RAT involves energy transfer between the tractor engine and the rear axle of a two-wheel-drive tractor. Engaging the clutch results in a twisting force, called torque, to the rear axle. This torque is then transferred to the tractor tyres. Under normal circumstances, the rear axle (and tyres) should rotate, and the tractor will move ahead. In lay terms, the rear axle is said to be rotating about the tractor chassis. If the rear axle should be unable to rotate, the tractor chassis rotates about the axle. This reverse rotation results in the front end of the tractor lifting off the ground until the tractor’s CG passes the rear stability baseline. At this point the tractor will continue rearward from its own weight until it crashes into the ground or another obstacle.

DBL is another principle of stability/instability related to rear overturns. When a two-wheel-drive tractor is pulling a load, its rear tyres push against the ground. Simultaneously, the load attached to the tractor is pulling back and down against the forward movement of the tractor. The load is pulling down because it is resting on the earth’s surface. This backward and downward pull results in the rear tyres becoming a pivot point, with the load acting as a force trying to tip the tractor rearward. An “angle of pull” is created between the ground’s surface and the point of attachment on the tractor. The heavier the load, and the higher the angle of pull, the more leverage the load has to tip the tractor rearward.


There are three basic types of tractor runover incidents. One is when a passenger (extra rider) on the tractor falls off the tractor. A second is when the tractor operator falls off the tractor. The third type occurs when a person already on the ground is run over by the tractor. The person already on the ground may be a bystander (e.g., a non-working adult or a small child), a co-worker or the tractor operator. The tractor runover event often involves trailing machinery hitched to the tractor; it may be the trailing machinery that inflicts the injury. Extra rider injury incidents occur because there is no safe location for an extra person on a tractor, yet the practice of taking extra riders is common, as a means of saving time, for convenience, work assistance or baby-sitting. Whether an extra rider can be justified for any reason is strictly in the eye of the beholder. Safety experts and tractor manufacturers strongly recommend against an operator carrying an extra rider for any reason. This advice, however, conflicts with several factors that farmers must face daily. For instance, it is human nature to want to complete work tasks as easily and quickly as possible; different transportation may call for added expenditure of a meagre money supply; other baby-sitting options simply may not exist; and new tractor drivers must be taught how to operate tractors.

Persons already on the ground, usually tractor operators or children, are occasionally run over by tractors and their attached equipment. Tractor operators sometimes try to start their tractor from the ground, instead of from the operator’s seat. Most of these incidents occur with older tractors that will start with the tractor in gear, or on newer tractors where the starting interlocks built into the tractor have been by-passed. Small children, usually under the age of five, are sometimes run over by tractors and machinery that is moved around the farmstead. Often, the tractor operator is unaware that the child is even near the equipment. A loud noise, such as the start-up of a tractor, is often attractive to young children and may draw them near. And the practice of allowing extra riders may bring them running to the tractor.

Tractor safety rules include:

  • The most important safety device for a tractor is a rollover protective structure (ROPS). This device, along with a properly buckled seat-belt, prevents an operator from being crushed by the tractor during a rollover.
  • A ROPS enclosed cab provides even more protection, as cabs also provide protection from adverse weather elements and from falling off the tractor.
  • A master shield over the PTO stub shaft protects against PTO entanglement.
  • The one seat–one rider rule and other safe operating practices must be followed.
  • Operator manuals must be read to learn how to safely operate the machine.
  • Workers must be physically, psychologically and physiologically capable of operating a given machine.


Machinery Hazards

There are a multitude of machines used in mechanized agriculture. These machines are powered in many different ways including PTO shafts, hydraulic oil pressure, electrical power, engine power and ground traction. Many machines have several types of hazards. Table 2 gives machine hazards, descriptions of the hazards and examples of where the hazards occur on various machines.

Table 2. Common machinery hazards and where they occur




Pinch points

Two machine parts moving together with at least one of them moving in a circle

Where drive belts contact pulley wheels, drive chains contact gear sprockets, feed rolls mesh together

Wrap points

An exposed/unguarded rotating machine component

Power take-off (PTO) drive shafts, beater bars on self-unloading ensilage wagons, blades of some manure spreaders

Shear/cutting points

The edges of two moving parts move across one another, or a single edge moves against a stationary edge or soft material

Mowers and forage harvesters, small-grain combine heads, bedding choppers, grain augers

Crush points

Two moving objects moving toward each other, or one moving object moves toward a stationary object

The front and rear tires/sections of articulating tractors, hitching machinery, a hand caught under a piece of hydraulically-controlled equipment

Free-wheeling parts

Machine parts that continue to move after power to the part has stopped, usually from the continuing rotation of knife or fan blades

Forage harvesters, feed grinders, rotary mowers, ensilage blowers

Thrown objects

The chopping, grinding, cutting, and flinging motions of machines. Small objects such as rocks, metal, glass, sticks, and vegetation may be picked up and thrown with great force

Rotary mowers, feed grinders, combines with straw choppers, and manure spreaders

Stored energy

Energy that is confined and released unintentionally or unexpectedly

Machine springs, hydraulic systems, compressed air, electrical systems

Burn points

Skin burns from contacting hot parts of machines

Hot mufflers, engine blocks, pipes, fluids (fuel, oils, chemicals)

Pull-in points

Occurs at the point where the machine takes the crop material in for further processing

Corn pickers and combines, forage choppers, and hay balers

Noise-induced hearing loss

Operating machinery

Tractors, field machinery, grain augers, dryers, silo blowers, bedding choppers, feed grinders. Damaging noise level may come from a combination of one or more machines. Older machines generally produce louder sounds than newer machines.


Machinery power and speed

Though workers may understand that machinery is powerful and operates at very high speeds, most workers have not stopped to consider just how powerful machines are in comparison to their own power, nor do they fully comprehend how fast machines are. Machinery power varies considerably, but even small machines generate many times more horsepower than any person. A quick, pull-away action of a human arm normally generates less than 1 horsepower (hp), sometimes much less. A small 16-hp machine, such as a walk-behind mower, may have 20 to 40 times more power pulling a person into the machine than that person can generate pulling away. A medium-sized machine operated at 40 to 60 hp will have hundreds of times more power than a person.

This power and speed combination presents many potentially hazardous situations to workers. For example, the tractor’s PTO stub shaft transfers power between the tractor and PTO-powered machinery. Power transfer is accomplished by connecting a drive shaft from the machinery to the tractor’s PTO stub. The PTO stub and drive shaft rotate at 540 rpm (9 times/second) or 1,000 rpm (16.7 times/second) when operating at full recommended speed. Most incidents involving PTOs stem from clothing suddenly caught by an engaged but unguarded PTO stub or driveline. Even with a relatively quick reaction of 1 second (i.e., the worker tries to pull away from the shaft) and a shaft with a diameter of 76 mm operating only at half speed (e.g., at 270 rpm (one-half of 540), the victim’s clothing has already wrapped 1.1 m around the shaft. A faster-operating PTO and/or a slower reaction provides even less of an opportunity for the worker to avoid entanglement with the shaft.

When a machine is running at full recommended PTO speed, crop material moves into the machine intake or processing area at approximately 3.7 m/s. If a worker is holding onto crop material as it begins entry into the machine, he or she is usually unable to let go quickly enough to release the material before being pulled into the machine. In 0.3 second, the worker will be pulled 1.1 m into the machine. This situation most often happens when crop material plugs the intake point of the machine and the worker attempts to unplug it with the PTO engaged.

Machinery safety

Machinery safety is largely a matter of keeping the guards and shields that came with the original in place and properly maintained. Warning decals should be used as a reminder to keep guards and shields in place. If guards or shields must be removed for maintenance, service or adjustment, they must be replaced immediately upon completion of the repair. Safe operating practices must be followed. For example, the tractor must be shut off and the PTO or block hydraulic systems disengaged before unplugging or servicing equipment. Operator manuals must be read and their safety instructions followed. Workers must be properly trained.



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