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To meet the requirement that the traditional combine harvester can adjust the threshing gap in real time according to different working conditions, a hydraulic driven variable-diameter threshing drum and a synchronous hydraulic system were developed in the early stage. However, the performance of synchronous hydraulic system has not been studied in detail. Therefore, AMESim software was used to simulate and analyze the synchronous hydraulic system to verify its feasibility in this study. The results showed that the synchronization error of synchronous hydraulic system was less than 0.02 mm. When the hydraulic system was self-locking, the position fluctuation of the piston rod of hydraulic cylinder was less than 0.06 mm. When the hydraulic cylinder did not work, the logic valve effectively ensured that the hydraulic oil could directly return to the oil tank, resulting in the minimum energy loss of the system. The flow control valve could regulate the flow rate of the hydraulic system, causing the adjusting speed of the drum diameter can be adjusted. The hydraulic system pressure was determined to be 8 Mpa.

In order to solve the problem of the threshing performance of a large combine harvester being reduced due to the non-adjustable diameter of the threshing drum, a variable-diameter threshing drum with movable radial plates based on the principle of concentric regulation was studied. It was mainly composed of a mechanism for adjusting the diameter by moving the radial plates, six fixed threshing tooth rods, six retractable threshing tooth rods and the single piston rod hollow hydraulic cylinder. The threshing gap can be adjusted by a stepless change of the drum diameter. By using RecurDyn simulation and field performance tests, the adjustable ranges of diameter and gap of the movable variable-diameter threshing drum were 670~710 mm and 10~30 mm. Based on the feed amount of the combine, the rotation speed of the threshing drum and the threshing gap (the diameter of the drum) as the influencing parameters, and the grain entrainment loss rate, grain un-threshed rate and grain breakage rate as the evaluation indexes, the three-factor and three-level response surface tests were carried out, and the result data were analyzed using Design-Expert 13.0. The optimal threshing gap and rotation speed of the threshing drum were determined under different feeding quantities. A comparative test was carried out to adjust and fix the threshing gap and rotation speed of the threshing drum in real time according to the change in feeding amount. The results showed that when the working parameter combination under different feeding amounts was adjusted in real time, the entrainment loss rate was 0.65%, the un-threshed rate was 0.063% and the breakage rate was 0.47%. Compared with the threshing gap and the rotation speed of the threshing drum being fixed, the entrainment loss rate, the un-threshed rate and the breakage rate were reduced by 44.9%, 27.6% and 34.1%, respectively. A threshing drum with variable diameter was provided for a large multi-crop harvesting combine to realize the concentric stepless adjustment of the threshing gap.

The threshing gap of the thresher device for rice combine harvester has to be adjusted in real time based on different feed rates to ensure the operation efficiency in the harvesting process. However, adjusting the threshing gap by changing the position of concave grid may result in unevenness of threshing gap of the thresher device and further impact on the fluidity of material in the thresher device; in addition, it is also unavailable to adjust the threshing gap by changing the drum diameter when the rice combine harvester is in operation. In view of the above and based on axial flow threshing drum, the design of a variable-diameter threshing drum available for overall and rapid drum diameter adjustment and the research on diameter adjustment device as well as electronic control self-locking device were introduced in this study. Besides, stress analysis was implemented to the diameter adjustment device to ensure the stability of the variable-diameter threshing drum. Field experiment was implemented to identify the difference between the impacts brought to the threshing performance (grain-entrainment loss rate, damage rate, threshing efficiency, and threshing power consumption) by both methods for threshing gap adjustment. The experiment result shows that the drum adjustment method with variable-diameter drum features higher grain-entrainment loss rate, threshing efficiency, and threshing power consumption, yet stable in terms of consumption fluctuation, but a lower damage rate than their counterparts with concave adjustment method.

During the rice harvesting process of combine harvester, it is necessary to adjust the working parameters of the threshing device in time according to working conditions to ensure operational efficiency and performance. This paper designs a variable diameter threshing drum, develops a device for diameter regulation with self‐locking function, and its installation process and working principle are introduced in detail. A force analysis and modal analysis were carried out on the device for diameter adjustment of variable diameter threshing drum. It aims to ensure that the design dimensions of the constant speed spiral disc and claws in the device for diameter adjustment are in compliance with the self‐locking conditions, and effectively ensure the reliability of the device for diameter adjustment. A field experiment was carried out to check the reliability of the developed system. A regression analysis model takes the feeding rate, threshing gap and drum speed as influencing factors. The grain separation loss rate and damage rate as performance indexes were established, and calculations for multiple target optimization were performed. The experimental results showed that the threshing gap could be adjusted in real time according to the different feeding rates of the variable diameter threshing drum. This paper develops a threshing drum with variable diameter making it possible to adjust the drum diameter integrally and quickly. The developed device can provide a reference to design of threshing drum with variable diameter as well as that of self‐adaptive control system.