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Traction force in arable farming: agronomic and environmental aspects after field data acquisition and modelling
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Soil-tyre interaction analysis for agricultural tractors: modelling of traction performance and soil damage
Full text linkTillage operations in farming often require high traction forces applied by tractor wheels. These interact with topsoil via a stress system along the contact surface, this interaction resulting in soil and tyre deformation. Topsoil is subject to normal and tangential stresses at the contact surface. The tangential stress rises sharply with increasing traction force and may cause topsoil among tyre lugs to fail, with the consequent formation of a strengthless layer strongly exposed to erosion and an underlying layer where shear deformations contribute to the alteration of soil structure functionalities.
This work aimed to investigate mechanical conditions along the soil-tyre contact surface which lead to topsoil damage. These conditions are analysed in the light of simulations with a soil-tyre interaction model and discussed on the basis of results of specific experimental tests. A semi-empirical model of interaction between the soil and a pneumatic wheel was adapted to simulate the traction performance of mechanical front-wheel drive MFWD vehicles, taking into account the load transfer effect, the multi-pass effect, and the theoretical speed ratio between the front and the rear axles. This model was employed to simulate (i) the traction performance of tractors in terms of drawbar pull, motion resistance due to soil compaction, traction coefficient and traction efficiency as a function of slip, wheel load and tyre inflation pressure; (ii) soil stress paths along the contact surface with tyres; and (iii) the risk of soil failure corresponding to a defined slip level. Several traction tests were performed on four agricultural soils of different texture (clay, clay loam, silty loam, and loamy sand). Four tractors of wide ranging power (40.4 kW, 65 kW, 110 kW, and 123 kW) and weight (25.3 kN, 40 kN, 66.7 kN, and 68 kN) were used. Tractor configurations were varied by changing tyre inflation pressure and tractor weight, and by using dual tyres. Slip normally ranged between 0% and 35%, only in some cases higher values, up to 58%, were reached. The shearing effect on the topsoil due to slip of tractor tyres was investigated on the silty loam agricultural field by measuring longitudinal topsoil displacements along the driving corridors during traction tests. A system of strips orthogonal to the tractor track was spray-painted on the soil surface to enable easy visualisation of the topsoil displacements.
The changes in soil hydraulic properties owing to deformation caused by the passage of the 40 kN tractor, both in a self-propelled condition without wheel slip, and with high drawbar pull (21.8 kN) and high wheel slip (27%), were compared in the clay loam agricultural field. The mechanical properties of the topsoil were determined in situ on the basis of vertical plate-penetration tests and horizontal plate-shear deformation tests with a tractor-mounted bevameter. Soil stress-strain conditions at contact with a traction tyre were reproduced in the laboratory by means of a direct simple shear box. A Geonor shear box was modified in order to carry out hydraulic conductivity measurements in saturated conditions while shearing the soil sample.
Simulations with the soil-tractor interaction model matched measured traction performance with general good agreement (overall mean error of 12% and overall mean residual of 3.30 kN). As soon as the soil failure condition, as simulated by the model, was approached along the soil-tyre contact surface, longitudinal topsoil displacements measured in the silty loam agricultural field clearly increased. The slip values at which soil failure was reached were identified for three configurations of the 40 kN tractor. These slip values should be regarded as indicative limits not to be exceeded in tillage operations in order to avoid topsoil damage in the conditions under consideration.
The stress state at the soil-tyre contact surface increased significantly, mostly in terms of shear stress, when the tractor moved with slip rather than without slip. As a consequence, the severity of tractor-traffic-induced soil degradation increased appreciably. The change in soil structure and hydraulic properties measured in the clay loam agricultural field was more pronounced in the first 0.15 m where the total porosity decreases by 11% without slip and 29% with slip, with a reduction in macropores of about 60% and 100%, respectively. The saturated hydraulic conductivity of the shallow topsoil (0 - 0.04 m) turned out to be reduced by about 66% without slip and about 98% with 27% slip.
The results of the simple shear tests confirmed that shear deformations may contribute to damage topsoil structure functionalities, decreasing, in most cases, hydraulic conductivity. However, in the samples of clay, clay loam and silty loam, the major decrease in hydraulic conductivity was caused by the deformation during compression. Moreover, it emerged that the effects of shearing on the saturated hydraulic conductivity are mainly controlled by the volumetric strain coupled to the shear strain, and the variation in voids volume of the pore system affects the hydraulic conductivity more than a pure distortional deformation which may alter the water pathways in the sample.
The validated approach for modelling tractor traction performance and predicting topsoil damage from the shearing effect due to tyre slip was used as a framework for developing a new Excel module for the third edition of the TASC V3.0.xlsm software: www.agrartechnik-agroscope.ch. This module also provides the power-wheel slip relationship. Four practical tests were set up for the user to enable a fast, simple and reliable mechanical characterisation of topsoil behaviour. Different tractor configurations, soil textures and conditions can be confronted. The limit beyond which topsoil damage is expected to occur is reported in terms of net traction and wheel slip. TASC V3.0 offers a valuable support tool for identifying tractor configurations and soil conditions which optimise traction, resulting in increased fuel saving, reduced tyre wear and limited topsoil damage
TASC V3.0 - Prognose Bodengefährdung und Treibstoffverbrauch: Eine PC-Anwendung zur Beurteilung der Bodenbeanspruchung im Ober- und Unterboden in der Land- und Forstwirtschaft sowie zur Schätzung des Energie- und Treibstoffbedarfs im Ackerbau.
No full textTASC V3.0.xlsm - a PC application of Agroscope (Excel application with manual in PDF format, publication in three languages, D/F/E, download online).
No full textInfluence of tyre inflation pressure and wheel load on the traction performance of a 65 kW MFWD tractor on a cohesive soil.
Full text linkPredicting topsoil damage from slip of tractor tyres: analysis of the soil cutting effect from the tread of traction tyres
No full textTraction performance simulation for mechanical front wheel drive tractors: towards a practical computer tool
Full text linkAn analytical model to simulate the traction performance of mechanical front wheel drive MFWD tractors was developed at the Agroscope Reckenholz-Tänikon ART. The model was validated via several field tests in which the relationship between drawbar pull and slip was measured for four MFWD tractors of power ranging between 40 and 123 kW on four arable soils of different texture (clay, clay loam, silty loam, and loamy sand). The pulling tests were carried out in steady-state controlling the pulling force along numerous corridors. Different configurations of tractors were considered by changing the wheel load and the tyre pressure. Simulations of traction performance matched experimental results with good agreement (mean error of 8% with maximum and minimum values of 17% and 1% respectively). The model was used as framework for developing a new module for the excel application TASCV3.0.xlsm, a practical computer tool which compares different tractor configurations, soil textures and conditions, in order to determine variants which make for better traction performance, this resulting in saving fuel and time, i.e. reducing the costs of tillage management
Response of a clay loam agricultural soil to mechanical stress from tractor traffic with and without slip
No full textA mechanistic approach to topsoil damage due to slip of tractor tyres
Full text linkTractor traction tyres interact with soil by a system of normal and tangential stresses along the soil-tyre contact surface. In this interaction both soil and tyre deform according to their own stress-strain relationships. Soil deformation results in the formation of a rut as well as in topsoil displacement along the soil-tyre contact surface. The topsoil displacement depends on shear stress which soil undergoes at contact with tyre. The shear stress-displacement relationship characterizing the soil layer which interacts with the traction tyre has been studied for a long time as it strongly affects the relationship between traction force and wheel slip, usually referred to as traction performance of the soil-wheel system (Becker, 1956; Janosi and Hanamoto, 1961; Wills, 1963; Wong and Preston-Thomas, 1983)LM
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