**1. Introduction**

Cotton stalk is a byproduct of cotton that can be utilized as a high-quality renewable resource. China is a large cotton-producing country. In 2020, the national cotton planting area was 3.1699 million square hectares [1,2]. More than 40% of the world's cotton is produced in China, and this area produced a total output of 3.05 × 107 t of cotton stalks [3,4]. However, only approximately one-tenth of this total output was subsequently utilized. The lack of effective cotton stalk harvest machinery is one of the main reasons for this low utilization rate [5,6].

The core component of cotton stalk harvester machinery is the stalk-pulling structure, the performance of which directly influences stalk-pulling efficiency. Thus, to increase the cotton stalk removal ratio, many researchers have investigated the characteristics of cotton stalks, the mechanism of the stalk-pulling structures, and the dynamic characteristics of stalk pulling. Regarding the mechanical properties and pull-out characteristics of cotton stalks, Chen et al. [7,8] examined how the cotton stalk pulling force changed in the field

between autumn and the following spring. They obtained the following results: (1) the tensile failure load was 10.9 to 29.8 times larger than the bending failure load and (2) the pulling resistance of the cotton stalk is significantly influenced by the diameter of the cotton stalk root, the soil hardness, and the harvest time. Using a pull-out force measuring device, Li et al. found that the cotton stalk pulling angle significantly affects the pulling force [9,10]. Additionally, the findings reported by Demian T. F. et al. indicate that, within a certain cotton stalk height range, a higher pulling height mandates a larger pulling force [11]. Currently, there are two general types of straw-pulling structures: the row-controlled pulling structure, which includes the tooth-disc type, double-roller type, and chain-clamp type; and the non-row-controlled structure, which includes the knife-roller type and Vshaped toothed roller type. To date, non-row-controlled structures have been preferred over row-controlled structures because they have advantages such as lower operational requirements for the driver and higher working efficiency [12–17].

Among the non-row-controlled stalk-pulling structures, the knife-roller type has a high removal ratio; however, because it penetrates the soil during operation, it is associated with high energy consumption and problems related to residual film and mud, which hinders the follow-up work. The results of theoretical research have shown that, although it is still in an exploratory phase, the V-shaped toothed roller type not only has the advantage of a high removal ratio commonly associated with the knife-roller type but also the advantage of low energy consumption [17]. Tang et al. designed an all-in-one V-shaped toothed roller machine by integrating the functions of cotton stalk pull out, soil clearance, transportation, chopping, and collection; this design yielded a high extraction rate [12]. However, an in-depth theoretical analysis of the V-shaped toothed roller was not performed. The Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, and Binzhou Agricultural Mechanization Research Institute jointly developed a V-shaped toothed roller-type cotton stalk harvester that could perform the functions of stalk pull-out, soil clearance, chopping, and bundling [18–20]. This collaboration resulted in the first application of an integrated elastic collision and simple beam theory to stalk pulling [21]. Thereafter, He et al. and Dai et al. used this theory to study and test other types of stalkpulling machines [22,23]. However, to date, how the forces applied to V-shaped toothed rollers vary as the stalks are extracted remains to be unknown, and the reasons for cotton stalk fracture and non-pulled stalks lack clear theoretical explanations.

A small-scale test bench is designed in this work, which is driven by hydraulic pressure to pull the stalk roller, and the motor drives the reel and is equipped with torque, speed sensors, and other components that can control the motion parameters very accurately so as to facilitate the research of the experiment. In order to elucidate the operational stress variation profile for V-shaped toothed roller stalk-pulling machines and explore the mechanisms of cotton stalk pull-out, fracture, and missed extraction, elastic collision and simple beam theories were applied to develop a comprehensive stalk-pulling theory. The theoretical results revealed acceleration to be a significant factor affecting the stalkpulling process. Thus, acceleration was taken into account to increase the accuracy of the mechanical model and yield a more accurate description of the formation mechanism of the three states of stalk pulling. This optimized V-shaped toothed roller model was then applied in a field experiment. Specifically, an orthogonal experiment was carried out to optimize the rotational speed, cogging angle, and ground clearance of V-shaped toothed rollers with respect to the removal ratio. It is believed that the findings of this study can be used as a reference for the development of a highly efficient cotton stalk harvester.
