Design and experiments of the axial-flow spiral drum threshing device for the edible sunflower

Sunflower is one of the four major oil crops in the world. There are oil and edible sunflower at present. Among them, there is a large planting area of edible sunflowers in China. The seeds of edible sunflowers are thin and delicate easy to thresh, and the skin is mostly black with white lines and short hairs. Particularly, the skin of seeds is easily scratched, as the full ripe stage stalks and back of sunflower disc turn yellow and leaves dry up. The traditional threshing elements are also easily scratched during threshing. The resulting flowery-skinned seeds can greatly reduce the economic value, where the sunflower discs are seriously broken and hard to clean. It is very necessary to balance the structure of the threshing element and the key operating parameters of the system for the better threshing performance of sunflowers. In this study, an axial, spiral, and cylindrical threshing device was designed for the low breaking and un-threshing rate of edible sunflowers during harvesting and threshing. A theoretical and experimental investigation was carried out, according to the current status of sunflower threshing. The sunflower threshing device was mainly composed of a threshing drum, top cover, power transmission system, frame, grain collection, concave screen, threshing gap adjustment mechanism, and material conveying component. The threshing element was a spiral tube with an external diameter of 32 mm, in order to reduce the scratching of grain skin during threshing. The concave screen used the grid with the wrap angle of 180° for the larger separation area. The kinematics and dynamics of material transport were then analyzed in the threshing space. The spiral lift angle was 63° for the spiral tube of the threshing element, where the pitch was 2 800 mm. Taking the sunflower 3638 as the object, the field test was performed on the self-developed sunflower threshing bench. A single-factor experiment was carried out to determine the influence of working parameters on the threshing performance. The results show that an optimal combination was achieved, with the range of feeding rate of 1.2-1.6 kg/s, drum speed of 250-350 r/min, and threshing clearance of 30-40 mm. There were generally low un-threshing rate and damage rates in this case. A multi-objective optimization test was conducted to evaluate the threshing performance under the optimal combination of the parameters in the threshing device. A secondary rotation orthogonal test was carried out, with the feeding rate, drum speed, and threshing clearance as the influencing factors, while the un-threshing rate and breaking rate as the response indices. Design-Expert software was then selected to establish the mathematical model between the response index and the influencing factors. The optimal combination of parameters was optimized using the response surface method. The optimized parameter set was determined as the feeding rate of 1.4 kg/s, and the drum speed of 300 r/min with a threshing clearance of 35 mm. At the same time, the un-threshing rate and the breaking rate were 0.55%, and 1.76%, respectively. A verification test was carried out on the test bench. After that, the un-threshing rate was 0.59%, and the breaking rate was 1.77%. Among them, the relative error was less than 5% in the predicted value of the model, indicating the suitability of the prediction model. This finding can provide a theoretical reference for the mechanized harvesting equipment of edible sunflowers.