IL CONFORT DEL POSTO DI LAVORO SULLE MACCHINE AGRICOLE: ESAME DELLE CRITICITA' E FORMULAZIONE DI SOLUZIONI.

Introduction. The study of agricultural operator’s health must consider also the mechanical vibration risk. The continuous and various use of machinery through the year makes its exposure evaluation very difficult. In particular, for the tractor drivers the risk is quite high: the vibration transmission to the body occurs primarily through the seat, depending the levels on several factors relevant to the machine structural characteristics and operative conditions. On vehicles, Whole Body Vibrations (WBV) and Hand-Arm Vibration (HAV) are usually measured. Vibrations are generally characterized through the following parameters: - perceived axis: vibrations are very often multidirectional, but 3 axes are conventionally considered: vertical (z), longitudinal (x), transversal (y); if combined, they produce rotational vibrations; - intensity, expressed in terms of acceleration (m/s²); - frequency: the sensitivity of the human body along the 3 axes is a function of the frequency. At very low values (<2 Hz), the whole body reacts as a single and homogeneous mass; between 2 and 20 Hz the spinal column is more stressed and digestive and circulatory disorders may occur; at high frequency, from 20 to 1000 Hz the hand-arm district is particularly charged; - exposure time: the limits specified by the Standards are generally referred to the 8 hr workday. The exposure risk to mechanical vibration is essentially related to the individual sensitivity. As a consequence, the study of the exposure effects is very complex. The relevant literature highlights a correlation between the exposure intensity and some specific diseases in the short, in the medium and in the long period: - considering the hand-arm district, exposure causes vascular, neurological and muscular-skeletal problems; - considering the whole body, the risk is related to diseases involving the spinal column; in particular, the range 2-10 Hz appears to be the most dangerous in the agricultural sector. Materials and methods. The Italian D.Lgs. 81/08 provides specific requirements for workers about the identification, evaluation and prevention of mechanical vibration exposure risks. Several vibration sources should be recognized in the agricultural activity; moreover, the variability of operative conditions makes the situation much more critical. In order to quantify the exposure level and evaluate the risk, the access to dedicated databases is allowed, but in this case the obtainable results are frequently very poor, because the values found show frequently a low accuracy in representing the real situation. This enduring lack of data and of a standardized approach was highlighted also by the public institutions dealing with workers’health (INAIL, ASL, etc.), having them strongly requested the development of tools to solve the problem. The goal of this research is the creation of a predictive model, in order to quantify the operator vibration exposure level on agricultural machinery, without measuring, nor instrumentally and systematically, the acceleration levels. Taking into account the enormous variability of machines and operations carried out all over the year, the building of a unique general model, applicable to every agricultural chain is not logical. Therefore, the most suitable solution seems to be to develop many dedicated models. The research was developed in different steps, all characterized by an experimental approach, both in the field and in laboratory. This led to an intense work of acquisition and analysis of the data, in order to create validated models, able to predict precisely the vibrational exposure levels considering the various operative conditions. To acquire really comparable data, in the first part of the research a standardized collection method was defined, and was later applied on viticulture chain. 6 operative tasks are then identified and characterized through their different parameters, related to structural characteristics of the tractor, the coupled implements and the operative conditions. Vibration levels were measured on tractor seats (of both wheel and tracklaying tractors) with a 3-axes accelerometer operated by a 4-channel human vibration meter. Data about operative conditions were collected using a set of ad hoc tables. Due to the high variability of the agricultural sector was not possible to analyze all tasks, and therefore is necessary to find alternative solutions. In this scenario, two vibration test rigs were used. With the first the tractor riding during ploughing was simulated; this operation was taken into account because is probably the most diffused all over the world, representing also one of the most dangerous in terms of comfort. In fact the vibrational levels detected during ploughing are often very high. Vibration levels were instrumentally recorded on the field following different operating condition and were later reproduced on the rig, in order to classify and quantify the specific parameters influence. To collect a good amount of data to perform the statistical analysis, several test repetitions were carried out, New and more efficient devices to reduce the vibration level are nowadays available. During the period of study spent abroad, the rig installed at the Technische Universität of Berlin was used to evaluate the comfort on a 4-isodiametric wheel tractor equipped with a hydropneumatic suspension system. The comfort level was measured simulating the transfer and ploughing tasks. During ploughing were considered two different tyres inflation pressures. The hydropneumatic suspension has been tested in 3 different working modes: - unlocked: the hydraulic cylinders are moving freely; - locked: the cylinders remain in a fixed position; - semi-active: a proportional valve adjusts continuously the damping rate as a function of the vibration input. Also in this case, to guarantee the best data analysis a suitable number of repetitions have been performed. Finally, in order to quantify the influence of considered parameters, the AHP (Analytic Hierarchy Process) multi-attribute model was applied on 4 real ploughing cases. Results and discussion. The vibrational levels measured in the viticulture chain do not show a dramatic situation, even if the vertical axis results the most stressed, with a high load on the spinal column. The introduction of rest periods during the workday may effectively reduce fatigue, which often causes a decrease of the operator’s attention level, so facilitating the accidents occurrence. Travelling speed, soil surface profile, tractor-implement mass, mass distribution and soil compaction are the parameters mostly influencing the vibration during ploughing. Thanks to calculated interactions, a comprehensive algorithm was developed to evaluate the exposure level. The tests carried out on the isodiametric tractor equipped with hydropneumatic suspension show a low efficiency of the valve when operating the hydraulic cylinders (probably due to a poor adjustment): in fact, the data resulted very similar to those obtained with the suspensions in unlocked mode. Also the frequency analysis evidenced the low efficiency of the suspension device in semi-active mode. As expected, the locked mode does not sufficiently damp the vibrations, and the peaks have a frequency value very close to that of the spinal column resonance. During ploughing simulation, a significant reduction of the tyres inflation pressures (e.g. from 1.6 to 0.8 bar at the rear) improves significantly the comfort. Finally, the multi-attribute model application allowed to quantify precisely the parameters influence: tyres or tracks, soil surface profile and travelling speed seem influence remarkably the operator’s comfort. Conclusions. The different experimental approaches (both in field and in lab) led to the planned goals: - the standardized data collection method allowed the definition of the parameters influencing the vibration level in different agricultural tasks; - the work on the test rigs led to: a) the quantification of the identified parameters on the operator comfort during ploughing; b) point out the efficiency of a hydropneumatic suspension working in different modes; c) the definition of the influence on vibration levels of the tyres inflation pressure variation. - the multi-attribute model contributed in offering further evaluation of the cases studied. Some of the parameters must be analyzed more in detail (e.g. soil compaction and tractor-implement mass distribution), as well as a new and more accurate adjustment of the suspension system seems necessary to optimize its performance.