基于离散元法的旋耕刀三向工作阻力仿真分析与试验

为分析旋耕刀所受三向工作阻力及其变化规律,该文通过实测南方果园土壤颗粒参数,逆向重构旋耕刀三维实体,基于离散元颗粒接触理论,构建了适应南方土质环境的旋耕刀-土壤相互作用仿真模型。土槽扭矩对比试验表明,仿真值与试验值的变化趋势相同,扭矩均随转速增加而变大,最大相对误差10%;扭矩先从0增加到某个最大值,接着逐步减小到一个低值,随后又快速增加到一个高值,最后回落,该变化过程同旋耕刀与土壤之间的接触状态相关。单刀受力仿真分析表明,水平阻力方向与前进方向相同,侧向阻力方向为由刀具弯折区内侧面指向刀体,垂直阻力方向为先垂直土面向上后转为向下;水平阻力和侧向阻力在最大耕深处出现最大值,而垂直阻力在入土后转动约30°时出现最大值;水平阻力和垂直阻力的仿真波形与理论计算、土槽试验结果比照表明,对应曲线的变化趋势基本一致,且仿真结果与土槽试验结果更为接近,水平阻力相对误差为11.3%,垂直阻力相对误差为16.8%;水平阻力最大值大于侧向和垂直方向阻力最大值,水平阻力是功率消耗的主要因素;随着转速的增加,3个方向阻力最大值均增大,当转速高于250 r/min时,增速加快;侧向阻力和垂直阻力随前进速度增加而平稳增大,水平阻力却出现下滑趋势;耕深对三向阻力的影响比较显著,增加耕深会急剧增大三向阻力值。相关试验数据可为旋耕机能耗分析、机体作业振动及刀片磨损等研究提供参考。

Simulation analysis and experiment for three-axis working resistances of rotary blade based on discrete element method

Abstract: During operation of a small rotary tiller, the three-axis working resistances of rotary blade will directly influence the power consumption, moving stationarity of whole machine and wear of blade. Therefore, it is important to measure values of three-axis working resistances and analyze the relative changing rules. But so far, it has been difficult to measure these resistances using a three-dimensional force sensor because the rotary blades are buried in the soil during operation. Discrete element method is a common method to study particle dynamics. Therefore, it would be a feasible solution to discuss the interaction between rotary blade and soil based on discrete element method. Firstly, physical characteristics and mechanical properties of soil from orchard in Guangdong Province were studied by experiments. The soil particles could be regarded as some spheres with a diameter of 1.25 mm. Secondly, a 3D model of rotary blade was reconstructed by reverse engineering. Comparing the side-edge curve of blade in model with Archimedes line, it showed that the 3D model had higher precision. Thirdly, through analyzing the bonding state of soil particles, the formula of computing bonding radius was presented. Lastly, the simulation model to study three-axis resistances of rotary blade was established by EDEM2.6 software, with size of 300 mm×300 mm×600 mm, and particle number of 150 000. Experimental results in the soil bin showed that the practical measured values of torque had the same changing tendency as the simulated results, and the maximum relative error was only 10%. The changing rule of rotation shaft torque was that the value increased to a maximum from 0, then decreased to a minimal value gradually, and then increased to a high value fast, finally decreased again, which agreed with the actual working conditions of blades. Simulation for resistances of single rotary blade indicated that the horizontal resistance acted in the same direction to forward speed, the lateral resistance acted in the direction from inner face of blade to back, but the vertical resistance acted in the direction of down, and then up. The maximum values of horizontal resistance and lateral resistance both occurred at the maximum plowing depth, while maximal value of vertical resistance appeared when the blade turned about 30°. Comparing values of horizontal and vertical resistances with theoretical results and experimental results, it was found that they were the same change law with rotation angle of cutter shaft; meanwhile, the simulation values were more coincident with the experimental values. Finally, single factor simulation tests showed that the maximums of three-axis resistances increased with the increase of the rotation speed, when rotational speed was more than 250 r/min, the increase speed was faster. Maximums of lateral and vertical resistance slowly increased with the increase of forward speed, but maximum of horizontal resistance was in decline. Tilling depth had significant effect on three-axis resistances, so the greater the tilling depth, the bigger the maximums of three-axis resistances, and the power consumption would also increase sharply. This study can provide the reference for saving energy, improving stationarity of rotary tiller and reducing blade wear, and also can prove the feasibility of using discrete element method to analyze three-axis resistances of rotary blade.