Research Applications and Advances in Threshing and Cleaning Devices for Grain Harvesters

Against the backdrop of increasing demand for intelligent, high-efficiency, and low-loss harvesting, the operational performance of threshing and cleaning devices directly affects the loss rate, grain quality, and operational efficiency of grain combine harvesters. However, continuous variations in crop type, moisture content, feed rate, and material composition make the threshing and cleaning processes highly complex. Existing devices still encounter problems such as grain breakage, threshing losses, cleaning losses, and increased impurity content under high loads and complex operating conditions. This paper systematically reviews the current research status and development progress of threshing and cleaning devices for grain harvesters and analyzes the structural composition and working principles of threshing and cleaning systems. In terms of threshing, particular attention is given to research progress on tangential-flow, axial-flow, and combined tangential- and longitudinal-flow cylinders, as well as fixed, variable-clearance, rotary, and load-adaptive concave structures. The applications of the discrete element method (DEM) and its coupling with multibody dynamics in analyzing material motion and forces and optimizing parameters are also examined. In terms of cleaning, approaches to structural improvement are summarized with respect to vibrating screen structures, fan and air duct configurations, and related aspects. The applications of DEM, computational fluid dynamics (CFD), and coupled CFD–DEM simulations in studies of particle stratification, passage through screen openings, and gas–solid motion are analyzed. Existing cleaning airflow designs have gradually shifted from single-variable airflow rate adjustment toward zoned and multi-source air supply to accommodate material loads in different screen sections. In addition, this paper further reviews technologies for feed rate and operational load sensing, online detection of threshing and cleaning quality, and coordinated control of multiple parameters. Issues concerning material state coupling, simulation model credibility and transferability, and sensing delays are also discussed. Finally, it is proposed that threshing and cleaning devices should evolve from the optimization of individual structures and parameters toward the integration of structural design, numerical simulation, state sensing, and coordinated regulation and control. This review provides a theoretical and technical reference for the low-loss, high-efficiency, and intelligent design of threshing and cleaning devices for grain combine harvesters.