Draught and vertical forces obtained from dynamic soil cutting by plane tillage tools

An understanding of the relationship between tool forces and speed is important in evolving management strategies for optimum performance. The effect of speed on tillage tool forces were studied experimentally for wide (width=25.4 cm, depth=15 cm) and narrow (width=5.1 cm, depth=22.9 cm) plane tillage blades operating in a Dystric Fluvisol (silty sand texture) in a soil bin. The tools were tested at two depths (10 cm and 15 cm for wide blade, 11.4 cm and 22.9 cm for narrow blade), two rake angles (45 degrees and 90 degrees) and eight speed levels (0.25, 0.5, 0.75, 1.00, 1.25, 1.50, 1.75 and 2.00 m/s). The variables were combined in a 2x2x8 factorial experiment with three replications. The performance of three theoretical models based on the trial wedge approach in predicting the experimental results was evaluated. The first model (Model 1), based on Soehne's approach (with modification for the three-dimensional analysis) assumes that the soil fails in a series of shear planes, forming a wedge that is trapezoidal in shape. The equilibrium of the wedge boundary forces produce the force required for failure. The second model (Model 2), based on Mckyes' approach assumes that soil failure is by the formation of a centre wedge flanked by two side crescents. Equilibrium of the boundary forces on the wedge and crescents produce the forces as a function of an unknown failure angle which is obtained by minimizing the weight component of the total force. Model 3, based on Perumpral's approach assumes the same failure wedge as Model 2 but the total cutting force is minimized instead. Experimental results show that the tool force (draught and vertical force) is a function of the speed and the square of speed whereas the three models assume it to be a function of the square of speed only. The models were not very accurate in predicting the experimental results. The average percent deviation of the predicted forces from the observed values were 43%, 40% and 66% for Models 1, 2 and 3, respectively. Thus, Model 2 had more general agreement with experimental observations. The models were better in predicting the forces (draught and vertical force) far the narrow tool with average percent deviations of 33%, 28% and 46% for Models 1, 2 and 3, respectively, as compared to 53%, 51% and 85% for the wide blade. (C) 1998 Elsevier Science B.V. All rights reserved.