| Collision and friction are one of the main causes of potato damage in the process of mechanized harvesting,cleaning and grading.Potato tuber tissue collision damage and peel friction damage affect the storage,appearance quality and sales of potato,and bring huge economic losses to the potato industry.The study on the collision characteristics and damage mechanism of potato tuber and peel is of great significance to promote the rapid development of potato industry.In this dissertation,theoretical analysis,experimental research,finite element analysis and discrete element simulation were used and the mechanical properties of potato tuber and peel,the mathematical model of potato impact contact force and its influencing factors,the potato impact contact force and collision acceleration,the kinematic and dynamic characteristics of collisions between potato and separating screen,the mathematical model and influencing factors of impact friction force between potato and rigid plate and cylindrical rod,the impact friction acceleration between potato and cylindrical rod,and the changes of impact contact force and friction force between potato and separation screen in the process of potato soil separation were analyzed.The main research contents were as follows.1.The compression,puncture,tensile and shear mechanical properties of potato cylindrical sample,whole stem and peels were studied by using DDL200 electronic universal testing machine and TMS-PRO food physical property analyzer.And the effects of different test factors on the elastic modulus,yield stress,maximum tensile force,tensile strength,maximum shear force and shear strength of potato were obtained.The results showed that the compression direction had a great influence on the elastic modulus and yield stress of potato,while the sample diameter and compression rate had little influence on the results.The average elastic modulus and yield stress of potato along the long axis were 3.3497MPa and 0.8776MPa,respectively,which were larger than the corresponding values along the short axis.The maximum puncture force and puncture strength of potato peels were 5.14N and 1.64MPa,respectively,which were less than the corresponding value of the whole stem.When the tensile and shear test rate of potato peels increased from l0mm/min to 30mm/min,the maximum tensile force,elastic modulus,tensile strength,maximum shear force and shear strength increased gradually,and their ranges were 4.2N-8.1N,5.27MPa-8.83MPa,0.7MPa-1.35MPa,17.2N-28.7N and 2.87MPa-4.78MPa,respectively.The collision recovery coefficient of potato was tested by the single pendulum collision recovery coefficient test device.The results showed that the collision recovery coefficient between potato and 65Mn steel plate increased with the increase of potato drop height and decreased with the increase of potato quality.When potatoes collided with 65Mn steel plate,65Mn leather,65Mn rubber and 65Mn soft plastic,the recovery coefficient decreased gradually.2.The relative collision speed and height between potato and separating screen during the normal operation of rod chain-swing screen potato soil separating device were analyzed based on the 4SW-170 potato excavator.When the potato was thrown from the lifting chain to the separation screen,the maximum relative collision speed of potato with the separation screen were 2.675m/s and 2.625m/s in sand and clay respectively,which were equivalent to the collision speed between the potato and the stationary screen after free dropping from 365mm and 352mm height respectively.When the potato was moving on the separation screen,the maximum relative collision speed was 0.777m/s,which was equivalent to the free dropping collision speed from a height of 31mm between the potato and the stationary screen.The collision process between potato and rod and other potatoes on the screen surface were theoretically analyzed.The mathematical model of the maximum impact contact force between potato and rod was(?).The mathematical model of the impact contact force between potato and potato was(?).3.The impulse theorem was used to analyze the potato collision process,and a simplified calculation model of potato impact contact force was established.The maximum impact contact force was obtained as Fm=km(?).The hammer tips were improved and designed as a force sensor,and the potato impact contact force test system was established.The potato mass,drop height,the combined variables of potato mass and drop height m(?).and the impact material on the maximum impact contact force were tested.The results showed that both the maximum impact contact force and damage depth increased with the increase of potato mass or drop height.The results of regression analysis showed that the maximum impact contact force was linear positive relationship with m(?),which was consistent with the theoretical analysis.The maximum impact contact force of potato colliding with stainless steel,stainless steel-leather,stainless steel-rubber and stainless steel-soft plastic decreased in turn.Taking the maximum impact contact force and damage depth as the evaluation indexes,and potato mass,drop height,impact material and impact times as the experimental factors,the orthogonal test was conducted on two potato varieties,Xiabodi and Gaoyuanhong.The results showed that the influence order on the maximum impact contact force of Xiabodi was potato mass,impact times,drop height and impact material,and that was potato mass,impact material,impact times and drop height for Gaoyuanhong.The influence order on the damage depth of Xiabodi and Gaoyuanhong was the same,and that was drop height,impact material,impact times and potato mass.Under the same conditions,the maximum impact contact force of Xiabodi was smaller than that of Gaoyuanhong,and the damage depth of Xiabodi was larger than that of Gaoyuanhong.4.The effects of drop height,potato mass and tuber temperature on the potato collision acceleration and damage area were studied based on the single pendulum impact test device and acceleration test system.The results showed that both the collision acceleration and damage area increased with the increase of drop height,and with the increase of potato mass,the collision acceleration decreased and the damage area increased.When the temperature of potato increased from 5? to 23?,the fluctuation range of collision acceleration was 3.77%of the average value,but the damage area decreased by one fifth.The results indicated that the collision acceleration was not significantly affected by the tuber temperature,but the damage area was more significantly affected.The relationship between the maximum collision acceleration and the damage area of potato was analyzed.The results showed that the damage area and the maximum acceleration had a linear positive correlation within the test range of drop height.In the test range of the potato mass,the damage area was negatively correlated with the maximum acceleration.5.The potato solid model was established based on the 3D scanning and surface modeling technology.The acceleration of potato impacting with the single rod,steel plate and double rods were studied using the ANSYS Workbench LS-DYNA,and the average errors compared to the testing results were 5.3%,3.95%and 5.04%,respectively.The effects of rod diameter,rod tilt angle,rod spacing and the number of contact surfaces between potato and rod on potato collision characteristics were studied.The results showed that the maximum collision compression displacement of potato decreased linearly with the increase of rod diameter,and increased linearly with the increase of rod tilt angle and rod spacing.The maximum collision acceleration and maximum impact contact force were opposite to the maximum collision compression displacement.Compared with no soil,the collision characteristic indexes were significantly reduced when there was soil covered on the rod.The results showed that the collision characteristics of potato with single rod and double rods were similar,and the indexes of double rods collision were significantly lower than that of single rod.When the potato mass was 250g,the drop height was 200mm for single rod collision or 250mm for double rods collision,the potato would be damaged.6.Based on the principle of single pendulum impact scratch test and theoretical analysis,the impact friction models of potato with rigid plate and cylindrical rod were established,and the mathematical models of potato peel impact friction force were obtained as Ffm=?Nm and(?)tan2?1.According to the results of theoretical analysis,the relevant factors affecting the impact friction force of potato peel were obtained,including potato mass,drop height,distance from pendulum center to the potato collision center,maximum elastic displacement of potato,impact contact time,curvature radius of potato collision surface,material and roughness of contact surface,etc.7.The structure of potato peel impact friction test device was designed,and the potato peel impact friction acceleration test system was established.Taking potato mass,drop height,rod direction,rod state and rod material as the experimental factors,and the maximum acceleration of potato as the evaluation index,the impact friction process between potato peel and rod was studied.The results showed that the maximum impact friction acceleration decreased linearly with the increase of potato mass,but increased linearly with the increase of drop height.Under the same conditions,the maximum impact friction acceleration was smaller than that of impacting along the vertical direction of rod compared with the horizontal direction,and smaller than that of impacting with the fixed rod compared with the rolling rod.The maximum impact friction acceleration of potato was the smallest when it collided with the 65Mn rod,and that of the largest with the 65Mn-rubber rod.The model of energy loss ratio in the process of impact friction between potato and rod was established.The results showed that the energy loss ratio decreased with the increase of potato mass and increased with the increase of drop height when the rod was fixed.The energy loss ratio increased with the increase of potato mass when the rod was rolling.The potato peel damage acceleration threshold was tested.The results showed that the damage acceleration threshold gradually decreased with the increase of potato mass,and the drop height had no significant effect on the damage acceleration threshold.Under the same conditions,the potato peel damage acceleration threshold was larger when the rod was rolling than that when the rod was fixed.The damage acceleration threshold between potato and 65Mn rod was the smallest,and that was the largest between potato and 65Mn-rubber rod.8.The discrete element models of potato,soil and swing separation screen were established.The motion process of potato on the separation screen was analyzed by discrete element method.The variation laws of impact contact force of potato dropping from lifting chain to the separation screen and the friction force of potato conveying on the screen surface under three working conditions were analyzed.The results showed that both the maximum impact contact force and friction force between potato and separation screen increased with the increase of potato mass.Covering with 2mm thick rubber pipe on the screen surface of 65Mn rod or increasing the minimum rod spacing from 34mm to 39mm can both reduce the maximum impact contact force and increase the maximum friction force.Under the two conditions,the maximum reduction of the maximum impact contact force were 30.19%and 12.61%respectively,and the maximum increase of the maximum friction force were 8 times and 62.75%respectively.Compared with no soil condition,the friction force of potato moving along the screen surface would increase when there was soil on the screen surface.And the maximum increased ranges of the maximum friction force under the three working conditions were 74.12%,35.9%and 47.23%,respectively. |
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