Design and Experimental Evaluation of a Counter-Rotating Double-Layer Roller Screen for Tuber–Impurity Separation in Tiger Nut Harvesters

To address inefficient screening of high-soil-content tuber–impurity mixtures during tiger nut harvesting, a coupled-disturbance screening principle based on counter-rotation of the inner and outer roller screens was proposed. A corresponding double-layer roller-screen separation device incorporating a separating screw conveyor was developed. A kinematic model was established to characterize the equivalent relative linear velocity between the screen surfaces, showing that counter-rotation produced a greater value than co-rotation at identical rotational speeds. Comparative discrete element method simulations showed that, at identical rotational speeds, counter-rotation alleviated localized accumulation, increased the separation rate by 3.34 percentage points, and reduced impurity content by 4.07 percentage points relative to co-rotation. A Box–Behnken design with three factors and three levels optimized the speeds of the outer screen, inner screen, and screw conveyor to 20, 53, and 122 r/min, respectively. The predicted separation rate and impurity content were 97.87% and 24.43%. Field trials conducted at the optimized parameter combination yielded a mean separation rate of 91.71% and an impurity content of 22.51%, with absolute relative deviations of 6.29% and 7.86% from the predicted values, respectively. The proposed method and device provide a practical basis for improving tuber–impurity separation in tiger nut harvesters.