The research, funded by the Spanish Ministry for Agriculture, was carried out by Ignacio Arana Navarro, José Ramón Alfaro López and Carmen Jarén Ceballos, researchers of the Department of Projects and Rural Engineering of the UPNA. In the work the real safety ensured by the current approval criteria for tractor cabs was assessed, and proposals were made to amend these criteria. In most countries worldwide there are regulations requiring in different ways that tractors be equipped with a cab to protect the life of the tractor driver in the event of overturning. In order to be marketed, these cabs are tested using a specific methodology set out in most cases in three codes (known as code 4, code 6, and code 7) which are used in most OECD countries.
“For about the last forty years the codes in force have saved lives,” explains the researcher Ignacio Arana, “but in recent years their effectiveness has been called into question. What happened was that the allowable maximum masses fixed by some manufacturers for their tractors were more than double the unballasted masses. This, which was legal, made it possible for the tractor at the moment it rolled over to have, for an instant, a mass which was double the one used to carry out the testing of its structure. And this has meant that properly approved tractors equipped with commercial cabs at the moment of roll-over have caused the death of their drivers because the cab has collapsed.”
The Spanish Ministry of Agriculture funded this research project in order to resolve this problem and prevent further deaths. After evaluating the real safety provided by the tests, the authors of the piece of research submitted a proposal for amending the above-mentioned codes used until now. “It was proposed that the reference mass used should be equal to or higher than the maximum allowable mass set by the manufacturer, divided by 1.75 so that we could at least ensure that the structure providing protection in the event of roll-over would always be capable of withstanding the allowable maximum weight of the tractor by using a safety co-efficient like those normally used in building and resistance of materials,” points out Prof. Arana.
As a result, the three most important testing codes for tractor cabs were amended in January 2012. “The amendment was substantial and the proposal we defended at the international meetings of the OECD was accepted.” At the same time, three papers in connection with this work have been published in international journals.
To prevent being crushed by the cab
Most tractor deaths are caused when the vehicle turns over and crushes the driver. The first safety frame providing protection in the event of the tractor turning over was manufactured in Sweden in 1954. To find out whether these frames afforded sufficient protection, field tests were carried out, but they were very complicated, costly tests that could not be reproduced. Later on, a normalised test was developed in which the protective structure was hit by a 2,000 kg mass that was dropped onto the tractor’s structure. As in all the tests, the aim was that, following the impacts, the deformation of the structure should not be so severe as to fail to allow the safety zone that was meant to be occupied by the driver to remain free. The end of the 1960s saw the development of the first standards for static testing in which the structure was not hit by a pendulum but pushed by a piston.
By the 1970s the conclusion was reached that what needed to be achieved was to get the structures to absorb most of the total energy of the roll-over, without becoming so deformed that they could not ensure there would be a safety zone sufficiently large to accommodate the driver.
“The most important parameter,” points out Ignacio Arana, “was not the resistance of the structure in itself, but the energy needed to deform it to the point where the safety zone was affected.” That is why the structures need to be sufficiently resistant; they must not be too rigid, but must allow a certain degree of plastic deformation. “We can take formula one cars as an example: when they crash, the whole engine can end up deformed but the space occupied by the driver remains intact. The same thing happened to the Gallic walls attacked by Julius Caesar; they were built of layer upon layer of timber and stones so that the timber would provide resistance and the stones would absorb the energy from the blows of the battering ram.”