Farm tractor drivers' safety
For more than 50 years, Roll-Over Protective Structures (ROPS) protect operators in case of tractor rollover. Their integration into closed, air-conditioned, soundproofed cabs also insulated from vibration and contamination by pesticide significantly increases driver’s comfort
The agricultural sector has always disputed the construction sector for the sad record of fatal accidents on the job. The leading cause of death is tractor rollover. Unofficial but widely and repeatedly substantiated estimates report 110-140 fatal events annually in Italy.
The leading causes of rollovers include tractor architecture (high center of gravity, very high torque development), operating characteristics (coupling with heavy overhanging loads, front and/or rear), and ground characteristics (steep slopes, uneven surface, potholes, bumps, unevenness, etc.). All these variables, which are to be added, of course, to incorrect and/or dangerous maneuvers, result in a loss of vehicle stability, leading to rollover.
Due to the typical "form factor" of the tractor+equipment train, there is a clear predominance of side rollover (75%) in the statistical frequency of accidents, followed by soaring (20%) and the unlikely front overturn (5%). As a matter of fact, the majority of accidents are caused by a combination of these dynamics.
Protective structures (ROPS)
The problem has been well-known for more than 60 years now. Several solutions have been developed over time to limit the (unfortunately often lethal) consequences on tractor drivers. After some attempts at active prevention, there has been a shift to more effective passive protection, i.e., installing devices that do not prevent the accident from happening but prevent or reduce damage to the operator.
To this end, installing protective structures, i.e., sturdy "cages" called ROPS (Roll-Over Protective Structures), has become widespread worldwide. These structures, combined with the presence (and diligent use) of appropriate lap-type safety belts, ensure the maintenance of a certain volume of safety around the driver's seat in which the operator can remain in the event of a rollover, thus averting severe injury. Such structures have different conformations, ranging from a simple frame of shaped and welded tubular sections to a completely enclosed, air-conditioned, soundproof cabin that is also protected against vibration and penetration of harmful chemicals.
More specifically, the simplest structures are two-post fixed or collapsible frames installed in front or rear of the driver's seat and 4-post frames, with or without a suspended platform. Cabs are more common on medium- to high-power models since the related additional economic burden compared to a simple frame is more absorbable in the overall cost of the machine.
Mandatory type approval
Since they have to ensure a certain level of mechanical strength and, therefore, safety in the event of rollovers, ROPS structures must be tested and consequently type-approved in accordance with what is defined by dedicated standards. The testing procedures are both dynamic and static. Both are based on the simulation principle, reproducing the stresses to which the ROPS is subjected in the event of a rollover.
Dynamic tests consist of a series of shocks and compressions; precisely, to simulate rollovers, shocks are inflicted on the ROPS mounted on the complete tractor. The tractor is anchored to the ground while a cubic-shaped mass driven by pendulum motion strikes the protective structure at the points that would come into contact with the ground in the event of a rollover. The fall height of the pendulum (and thus the energy developed) is a function of the mass of the tractor, thus the severity of the impact on the ground in the event of a rollover. This procedure faithfully simulates what happens during an accident, but in its implementation by different laboratories, it has been subject to significant approximations over time, resulting in outcomes that are not always superposable.
More recently, the static procedure, in which the shocks inflicted by the pendulum are replaced by progressive thrusts, performed, for example, by hydraulic cylinders, has become more popular as an alternative. Although this solution is less similar to what happens in reality, it is undoubtedly more reproducible, ensuring the fairness of the outcome of equivalent tests performed at different times by different laboratories.
Various supranational bodies and institutions (EU, ISO, OECD, etc.) have issued a number of dedicated standards for ROPS (and FOPS) testing over the decades. The most widely used are the codes produced by a specific Permanent Working Group of the OECD (Organization for Economic Cooperation and Development) (https://www.oecd.org/agriculture/tractors/codes). Specifically, the regulations dedicated to ROPS to be installed on distinct categories of agricultural and forestry tractors are Code 4 for open-field models, Codes 6 and 7 for narrow-track tractors, Code 8 for tracked models, and Code 9 for telehandlers. Code 10 is related to the testing of FOPS.
Maintenance and future developments
Type-approval standards for ROPS cover new units placed on the market and the fitting of protective structures on second-hand tractors.
On the other hand, it is essential to highlight that the compulsory fitting on open field tractors in Italy dates back to 1.1.1974, so no doubt a considerable number of machines with protective structures that are now 30 and even 40 or more years old are operational.
Cabs and frames are made from welded and bolted steel plates and tubular sections, which are then treated with rust-resistant paints.
However, because of the agricultural environment, which is extremely aggressive both because of weather factors and the particularities of some agricultural inputs (including organic and inorganic fertilizers and plant protection products), corrosion is a constant and sometimes serious pitfall for this material.
Old protective structures thus suffer significant areas where the integrity can no longer be the original, a factor thus reducing their overall strength. It is, therefore, necessary to promptly replace the damaged element with an identical one to restore the original strength. An additional factor that could compromise the integrity of ROPS involves accidental impacts or tears of structural elements, which are far from unlikely for long-term use over the years under harsh conditions such as those in the field.
Active rollover protection experiences
The problem of farm tractor rollover has been known since the 1950s, when the first attempts to curb the occurrence of the phenomenon were implemented, with the development of devices to warn the driver of the emergence of imminent rollover conditions.
Some of these solutions included installing an inclinometer, through which light and/or sound signals could be sent (similar to current car parking sensors) to alert the operator when potentially dangerous slopes were reached. Other cases involved the interruption of the delivery of diesel fuel with the resulting stopping of the vehicle or even the release of a lateral strut to ensure a foothold or anchor point to stop the vehicle from rollover.
A further technical way of maintaining vehicle stability, still applied today (e.g., to grape harvesters), was the introduction of self-leveling axles: However, due to the technical evolution of the time, such a mechanical innovation was considered to have little or no applicability, not least because of its excessive cost. These solutions were all soon abandoned for several reasons, including inadequate reliability, assembly complications and heavy economic burden.
For some particular agricultural (but also forestry) operations, ROPS approval tests cannot ensure an adequate level of operator safety, for instance, when handling suspended loads using the tractor's front loader or in logging operations. In such cases, the risk of falling objects from above is, in fact, far from negligible.
Therefore, in addition to protection in the event of a rollover, the structures installed in the driver's seat must also provide appropriate safety at the top, typically the roof. Therefore, it is a matter of approving chassis and cabs also under the Falling Object Protective Structure (FOPS) tests. Still, in the rollover simulation, these tests involve dropping a steel device of known mass from a predefined height to test the strength of the top of the structure. Depending on the extent of the risk for which FOPS must be approved, dedicated standards establish objects of about 45 or 225 kg mass, respectively, with fall heights varying between about 3 and 5 m.
Protection structures for second-hand tractors
Installing an approved protective structure on agricultural tractors is a legal requirement that has been in effect on all categories since the mid-1980s.
In 1996, former ISPESL (later merged into INAIL, the National Institute for the Prevention of Accidents at Work) issued appropriate guidelines for constructing and fitting simplified ROPS (two- or four-post frames) on so-called " second-hand tractors" (https://www.inail.it/cs/internet/comunicazione/pubblicazioni/catalogo-generale/linstallazione-dei-dispositivi-di-protezione.html).
It consists of highly detailed directions for manufacturing and applying these protective structures, the strength of which has been carefully commensurate with the category and mass class of the vehicles for which they are intended.
This seeks to bring obsolete machinery, which, however, is still fully operational, up to the safety level of newly manufactured ones.
Among the various categories of agricultural tractors, narrow-track models are undoubtedly the most problematic when it comes to fitting ROPS, both because of the lack of physical space on such compact machines and because these models are often intended to work on sloping surfaces.
In order to comply with the legal requirement, it is simpler (and cheaper) to install a simple rollbar and subsequently equip the vehicle with an additional weather protection-only cab. However, in the event of a rollover, this artifact is not designed to withstand the stresses that occur in such a circumstance. Breaking it could represent a serious (even fatal) danger to the driver's safety.
That is why, in order to address this potential risk as well, a few years ago, with the coordination of INAIL and with the support of CUNA (Technical Commission for Federating Data in Motor Vehicles), the problem was addressed in order to propose solutions able to give also to these cabs an adequate level of safety, comparable with that of the ROPS already in place.
Category 4 cabs
Some tractor cabs (but also of self-propelled implements, such as some models of sprayers and grape harvester-mounted equipment) are specially equipped to protect operators when carrying out phytosanitary treatments effectively, thus constituting a valid PPE instead of uncomfortable masks or ventilated helmets.
The adequate protection of the cabs thus prepared (so-called in "category 4") is certified with the fulfillment of requirements provided by dedicated regulations which prescribe the ensuring of the maintenance of sufficient internal overpressure (minimum 20 mm of water) and the provision of one or more activated carbon filters, for adequate filtration of chemical type, by adsorption, of harmful substances. An issue that still needs to be fully resolved concerns the service life of such filters, which depends on several factors, including the toxicity of the active ingredients used in the treatments and their concentration in the prepared mixture.
Generally speaking, manufacturers of activated carbon filters claim a "service life" of a few hundred hours of use. However, a further (considerable) uncertainty about their efficiency concerns the diligence with which users replace them. Some improvement has been acquired recently, thanks to "smart" filter management, which keeps track of maintenance intervals and their level of exhaustion.