A prediction model based upon Mohr-Coulomb soil mechanics theory has been developed to predict the interaction between the soil and agricultural discs of different geometries and operating angles to both vertical and tilted discs. The model is based on two forms of soil failure. The magnitude of each form of failure is depen- dent upon a passive reaction on the concave side of the disc and a vertical bearing reaction on the convex side of the disc. The predicted results are in close agreement with the results of the experimental studies. Disc geometry is one of the most important factors to be considered in evaluating the soil reaction acting upon a disc tool. A theoretical analysis of the disc geometry was therefore carried out. The analysis of the disc geometry makes it possible to determine the factors required for the predicted forces acting on a disc tool. The experimental studies were conducted in a sandy loam soil at a moisture content in the friable range, under controlled soil bin conditions. The work was carried out at full-scale with 3 dimensional force measuring apparatus developed primarily for this investigation. The discs studied had a range of disc angles between 150 and 600 and a range of tilt angles between 0 and 35. The depth of cut ranged from 80 mm to 140 mm and at speeds between 0.75 m/s and 1.5 m/s. The disturbance area of the soil caused by the rotating soil cutting disc is calculated to define the disturbed area for any combination of disc angle, disc diameter, gang spacing and depth of work in order to accurately calculate specific resistance. Disturbance and soil inversion were investigated using transparent discs and soil tracers, with a variety disc and tilt angles in order to quantify inversion and mixing
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