Tractors and Self Propelled Machines

Operators of tractors and self-propelled machines face risk of overturns, runovers, and roadway collisions. Tractors and self-propelled machines also have operator stations that must be engineered with human factors in mind for safe and comfortable operation.


To minimize or prevent injury during an overturn, either caused by operator practice or situations beyond control of the operator, ROPS have been developed. They are not ordinarily found on self-propelled machines other than tractors, since the risk of overturn is considered by the industry to be minimal.

The ROPS are crush-proof structures designed to create a zone of protection around the operator during an overturn; ASAE standard S383.1 FEB04, "Rollover Protective Structures (ROPS) for Wheeled Agricultural Tractors," is the current standard for ROPS design. The ROPS may be in the form of a two-post structure (two nearly vertical steel posts located behind the operator connected by a crossbar on top), a four-post structure, or a cab with a crush-proof frame. Some ROPS are designed to fold to allow entrance through low doors or use in low-clearance situations. Seat belts must be worn to prevent the operator from being thrown outside the zone of protection during an overturn. Figure 6.2 shows a two-post folding ROPS in the upright position (7).

Tractor manufacturers have provided ROPS as standard equipment on all tractors in the U.S. and Canadian market since 1985. Retrofits are available for many U.S. tractors going back to the mid-to-late 1960s, but availability for tractors older than that is more limited. Federal OSHA standard 1928.51 requires ROPS only on tractors that were manufactured after October 25, 1976, and operated by employees. Farmers historically have not voluntarily clamored to purchase retrofit ROPS. A guide to retrofit ROPS is available (8,10).

Other countries have varying requirements for ROPS. Contacts should be made with the minister of agriculture or equivalent in countries of interest to determine such requirements.


Runovers can result from three primary causes:

1. Operators or extra riders falling from the operator platform during operation

2. Operators attempting to start the tractor from the ground while standing alongside it

3. Unseen bystanders being in the path of travel

Figure 6.2. Two-post folding rollover protective structure (ROPS) in upright position.

Seat belts help prevent operators from falling from the platform, but only tractors with ROPS have seat belts, and it is common knowledge that relatively few operators wear them. Cabs offer an additional safety factor but people (particularly extra riders) have been known to fall against doors or windows and fall out of the cab. Extra riders should not be allowed, but recognizing the desire of operators to bring along a second person for training purposes, manufacturers have provided training seats with seat belts in some newer tractor cabs. Backup alarms are not generally found on agricultural tractors and machines.

Roadway Collisions

Engineering to prevent roadway collisions involves providing lighting and marking to improve visibility of the machine by other motorists and identification as a slow-moving vehicle. Lighting and marking on tractors and self-propelled machines can consist of headlights, amber flashing lights (combination flashers and turn signals), and conspicuity tape (amber reflective strips) located on the front of the machine, and red taillights, amber flashing lights, and conspicuity tape (fluorescent orange strips for daytime visibility and red reflective strips) located on the rear of the machine. Trailing machines pulled behind a tractor can have much or all of the same lighting and marking, except for headlights. Newer tractors and machines have extremity lighting and marking with flashers and conspicuity tape located to mark the outer extremities of wide components.

In the United States and Canada, a slow-moving vehicle (SMV) emblem is located on the rear of vehicles normally traveling 25 mph or less on public roads. The emblem is a standard triangular sign, 350 mm high, consisting of a fluorescent orange triangle outlined with red reflective material. Other countries have different markings for road travel. As tractors are manufactured that exceed 25 mph, standards are being developed calling for additional markings to identify the additional speeds (7).

In the United States, state requirements for other lighting and markings are variable and often lag well behind the state of the art. Most states require the SMV emblem. During hours of darkness, some states require only two headlights and a single red taillight in addition to the SMV emblem. As ASAE standards have evolved, manufacturers have provided increased lighting and marking. In general, the older the machine, the less lighting and marking it will have. Retrofitting is possible but not widespread. Lighting and marking must be maintained, and it is not uncommon on older machines to see non-operational lights or faded markings.

Compatibility of lighting systems between tractors and trailing machines of different ages is a problem. In the mid-1970s, tractor manufacturers began providing turn signals on their tractors along with a standard seven-pin connector in the rear to activate lighting on trailed machines. Some trailing machines, particularly those manufactured by tractor manufacturers, had optional lighting packages, but these were not widely purchased. Conversely, while these lighting packages became standard on trailing machines in the 1990s, many of these machines are pulled by older tractors that lack the requisite seven-pin connector. Retrofitting of tractors and trailed machines is possible but not widely done.

Human Factors

Manufacturers of tractors and self-propelled machines have devoted much time to ergonomics and human factors in the design of operator stations, including cabs, seats, and controls. Ingress and egress, seating, controls, noise, and general operator comfort are some of the factors covered. Seat design has evolved into seats with complex suspensions, some of which have computerized active vibration cancellation to counteract movement by the tractor and to maintain a steady ride for the operator. Controls are placed logically and within easy reach, and colors and activation motions have been standardized. Adjustments must meet a wide range of physically small and large operators. Improvements in noise reduction inside cabs have brought sound levels well below 80 dB, as compared with 100 dB that operators may be exposed to on old tractors and below OSHA's 90-dB, 8-hour permissible exposure limit. Climate control, including dust filtration and air conditioning, is common in newer tractors with cabs. Other features like drink holders and even coolers are being incorporated. To help fight fatigue and at the same time improve accuracy, steering systems that follow existing rows or use global positioning satellites (GPSs) are available that allow the operator to let the machine follow the row without constant close attention. As with all other engineering improvements, the newer the machine, the more likely these improved features will be found.

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