Research On Combined Maize Threshing And Separating Device With Low Damage

The maize grain harvesting can help reduce the drying in the sun and threshing of maize ears,decrease the labor intensity,improve production efficiency,save cost and increase benefit.The maize grains are put in storage through the drying process,which can save the grain air-drying process,reduce grain pollution.As the advanced way of maize harvesting in the world,the grain harvesting will become the development direction of maize harvesting in China.At present,there are many problems such as high damage rate,low threshing rate and serious loss in the process of maize grain harvesting.On the one hand,the growth period of maize is long and the kernel moisture content is high in China,especially in the Huang Huai Hai planting areas in two seasons of one year where the kernel moisture content is at 30%～40%when the maize is in harvesting.On the other hand,there are some problems in our maize grain combine harvester,and the technologies of maize grain harvesting need further research and development.In view of the above problems,it is necessary to study the low-damage technologies of maize grain harvesting under high kernel moisture content condition,and to solve the problems in the process of maize grain harvesting in China.In this paper,the threshing and separating device of the longitudinal axial flow maize grain harvester was taken as the research core to reduce the damage rate and improve the threshing rate,and the threshing and separating device with low-damage round-head spike tooth and the piecewise combined round-tube-type threshing concave was designed.By means of the theoretical analysis,modeling and simulation,the structural parameters of the key components of the device were determined and optimized.Then,based on the bench tests,the optimum structural parameters and working parameters of the device were obtained.Finally,the threshing performance of the device was verified by field experiments.The device has achieved high efficiency and high-performance threshing and separating operation,which also has important guiding significance to the development of maize grain harvesting technology in China.The main research results of this paper are as follows:(1)A longitudinal axial flow maize threshing and separating test bench has been designed and developed independently,which serves as the carrier for bench tests of the low damage combined threshing and separating device.The test bench consists of two parts:the mechanical structure and control system.The mechanical structure includes 4 parts—conveying device,feeding device,longitudinal axial flow threshing and separating device and receiving device,which are used for delivery,feeding,threshing and separating of maize ears and receiving the maize kernel and mixed residue.The control system includes control cabinet,frequency converter,frequency conversion motor and hydraulic control part.It is used to control and adjust the feed rate of maize ears,spiral propeller speed,chain rake conveyor speed,threshing cylinder speed,the position and inclination of feeding device and threshing and separating device.The test bench as the carrier for bench tests provides an important support for optimization and selection of the different structural parameters and working parameters of the low-damage combined threshing and separating device.(2)Based on the collision contact theory,the collision contact mathematical models between the key parts of the threshing and separating device and the maize were established,and the longitudinal axial flow maize threshing and separating device with the low-damage round-head spike tooth and piecewise combined round-tube-type threshing concave was designed.The contact damage model between the threshing element and maize was established to obtain the maximum contact force and relative critical speed when mechanical stress damage of maize kernel produced.By establishing the mathematical models and theoretical analysis of the compression and contact area during the collision process of the threshing element and maize ear,the results showed that the round-head spike tooth innovatively designed was the best threshing element.Through the establishment of the mechanical model on the collision between the grid type concave and maize ear,and the theoretical analysis,the conclusion was that the height and the shear force of the transverse plate should be both reduced when the concave is designed,and a piecewise combined round-tube-type threshing concave was designed.(3)Based on the EDEM simulation analysis,the key structural parameters of the low-damage combined longitudinal axial flow maize threshing and separating device were optimized.By establishing the simulation analysis models of the threshing and separating device and maize ear,taking account of contact force,contact area,collision frequency and maize ear movement time,the different forms of the ball head radius of the low-damage round-head spike tooth,the different forms of the piecewise combined round-tube-type threshing concaves and the different helix lift angles of threshing element arranged on the cylinder were simulated and analyzed.The results demonstrated that the optimum radius of the ball head was 12.5mm and helix lift angle of the threshing element was 45°and the piecewise combination round-tube-right-type threshing concave can help to reduce the damage rate.(4)Bench tests were carried out on the designed low damage combined longitudinal axial flow maize threshing and separating device.By testing the key components and working parameters of the threshing and separating device,the optimum structural parameters and working parameters were determined:the threshing concave selected the round tube right combined with straight round tube(front sparse and dense type),the threshing element was determined as the round-head spike tooth(equal height),the threshing cylinder speed was 300 r/min,and the threshing clearance is 55 mm respectively.The distributions of the maize grain and mixed residue along the axial direction and radial direction of the threshing cylinder were obtained by the distribution tests of the degenerated mixture,which has verified the reasonableness of the designed low damage combined longitudinal axial flow maize threshing and separating device,and also provided the reference for the design of the subsequent cleaning device.(5)Field experiments were carried out on the designed low-damage combined longitudinal axial flow maize threshing and separating device.Under the optimal combination of parameters,field validation experiments were carried out based on two maize varieties:Liyu 86 and Jufeng 339,and the comparison experiment was implemented between the designed device and the other threshing and separating device consisting of conventional trapezoidal bar tooth and the grid type threshing concave.The results showed that:The designed low-damage combined maize threshing and separating device has a significant reduction in the damage rate and the un-threshing rate compared with conventional threshing and separating device.Under the Liyu 86 maize varieties,the damage rate was reduced from 13.73%to 8.64%,which was reduced by 37%compared with the conventional threshing device.And the un-threshing rate was reduced from 0.6%to 0.2%,which was reduced by 67%compared with the conventional threshing device.Under the Jufeng 339 maize varieties,the damage rate was reduced from 14.58%to 9.37%,which was reduced by 36%compared with the conventional threshing device.And the un-threshing rate was reduced from 1.14%to 0.65%,which was reduced by 43%compared with the conventional threshing device.In addition,the design of the piecewise combined round-tube-type threshing concave has solved the problem that the conventional grid type concave was blocked by maize bracts,which reduced the un-threshing rate and the entrainment loss during the process of maize grain harvesting.The superiority of the threshing performance on the designed threshing and separating device as well as the stability and reliability of the work was verified.