Optimizing double-screw burying device for rape straw returning

Rape straw is characterized by high stubble, thick stalks and difficulty to decompose. In this study, the opposing double-screw burying device was designed to combine the crushing-rotary tillage-mulching machine for the rape straw returning. The coefficient of variation of straw mass distribution and the percentage of straw mass in different soil layers were taken as the evaluation standard of straw returning. A four-factors and four-level orthogonal test was then conducted to determine the influencing parameters, such as the helix angle of the opposing double-screw burying device, rotary speed, forward speed, and the length of the straw. The results show that the coefficient of variation of straw mass distribution in different soil layers slowly decreased and then sharply increased with the increase of the helix angle. The better uniformity was obtained for the rape straw returning when the helix angle was below 60°. Furthermore, the coefficient of variation of straw mass distribution in different soil layers decreased and then increased with the increase of the rotary speed. Specifically, the displacement of the straws increased, when the rotary speed was below 210 r/min. The reason was that the kinetic energy of the straws increased with the increase of the rotary speed. Once the rotary speed exceeded 210 r/min, the displacement of the straws increased with the increase of the straw kinetic energy, and then the straw was screened and aggregated from the soil. Different specific gravity thus caused the uneven distribution of the straw in the soil. There was a similar trend of decreasing and then increasing in the coefficient of variation of straw mass distribution in different soil layers, as the forward speed increased. The straw was separated from the soil under excessive disturbance at the low forward speed, where the machine stayed put. By contrast, the variation coefficient also increased at the high forward speed with the weak function of the opposing double-screw mulching device. There was a positive relationship between the variation coefficient and the straw crushing length, indicating the shorter the length was, the smaller the variation coefficient was. The significant degree in the coefficient of variation of straw mass distribution was ranked in the descending order of the straw length, helix angle, rotary speed, and forward speed. The highest proportion of straw mass was in the upper layer (0-50 mm). There was a gradual decrease with the deepening of the depth. The uniformity of rape straw returning was also achieved to increase the straw in the deep soil (＞100-150 mm). The percentage of straw mass varied in the different soil layers with the four factors between the upper layer (0-50 mm), indicating the opposite in the deep soil (＞100-150 mm). But there was a small change in the percentage of straw mass in the intermediate soil layer (＞50-100 mm). The uniformity of the mixed straw mulching was improved to reduce the length of straw crushing reducing, while improving the degree of mixing between the straw and the soil without excessive disturbance. In terms of the helix angle of 60°, the rotary speed of 210 r/min, the forward speed of 0.6 m/s, and the length of straw of 6 cm, the percentages of straw mass in the depth of 0-50, ＞50-100, and ＞100-150 mm were 43.97%, 33.28%, and 22.75%, respectively, where the coefficient of variation of straw mass distribution of straw mass was 26.43%. The percentage of straw mass in the upper layer was reduced by 2.36 percentage points, whereas, the percentage of straw mass in the deep depth increased by 7.18 percentage points, and the coefficient of variation of straw mass distribution was reduced by 12.6 percentage points, indicating the best uniformity of rape straw returning and the best quality of the seed bed, compared with the orthogonal experiment groups. The finding can provide a strong reference to improve the quality of rape straw returning.