Design and experiment of chopping device with dynamic fixed knife coaxial for rice straw

Due to long stubble remained in farmland after harvesting as well as hardened and impervious soil in southern farmland of China, the tillage and land preparation parts of multifunctional rape-planting machine tend to wind grass and obstruct with mud, which impairs the working performance of ridging equipment and reduces the percentage of rapeseed seedling germination. In order to solve these problems, a rapeseed planting method is proposed, which combines the straw mulching cultivation with the direct seeding. The method is used to collect and cut the rice stubbles, and then the chopped rice stubbles are evenly covered on the inter-row spaces without involving the tillage apparatus. A multifunctional rape-planting device is mounted to a rice combine-harvester by replacing screening device, using the original harvester header to collect the rice stubbles and replacing the threshing cylinder to chop and scatter the rice stubbles. In this paper, an air-assisted chopping mechanism was designed, in which the moving knives and the transplanting blades were coaxially installed. The performance of the air-assisted chopping mechanism was evaluated by simulating the chopping operation of "stalk-standing" and "crop residue" in farmland through a bench test driven by an electric motor. A quadratic orthogonal rotating experiment was designed by Design-Expert software. Based on the variance analysis and regression fitting, a multiple regression equation was obtained, showing the effects of the rotating speed of feeding press rolls, the rotating speed of rotation shaft of moving knives and the gaps between moving knife and stationary knife on the qualified rate of chopped stalk length and the power attenuation. It was shown that the rotating speed of rotation shaft of moving knives had the most influential effect, followed by the gaps between moving knife and stationary knife, and the rotating speed of feeding press rolls had the least effect on the qualified rate of chopped stalk length and the power attenuation. The response surface method was applied to optimize the operation conditions. The optimum factors for "stalk-standing" and "crop residue" chopping operation were as follows: The rotating speeds of feeding press rolls were 399.2 and 401.64 r/min, the rotating speeds of rotation shaft of moving knives were 968.12 and 977.23 r/min, and the gaps between moving knife and stationary knife were 0.52 and 0.49 mm, respectively. Under the optimal condition, the corresponding qualified rates of chopped stalk length for "stalk-standing" and "crop residue" were 95.89% and 97.09%, as well as the power attenuations were 3.04 and 2.6 kW, respectively. The differences between the experimental and the software analytical value of the qualified rate of chopped stalk length for "stalk-standing" and "crop residue" were both 0.11% and those of power attenuation were 0.05 and 0.08 kW, respectively, which indicated that the software analytical value was in great agreement with the experiment value. Based on the theoretical analysis, the higher qualified rate of chopped stalk length could be achieved under the faster rotating speed of rotation shaft of moving knives and the smaller gaps between moving knife and stationary knife. Taking into consideration the existing form of rice straw in farmland, the rotating speed of feeding press roll was designed to be 401.64 r/min, the rotating speed of rotation shaft of moving knife 977.23 r/min, and the gap between moving knife and stationary knife 0.49 mm. It can provide technical support for the design of rapeseed planter with the combination of straw-mulching cultivation and direct seeding.