12 May, 2018 Int J Agric & Biol Eng Open Access at https://www.ijabe.org Vol. 11 No.3 Review of grain threshing theory and technology Jun Fu1,2,4, Zhi Chen2,3*, Lujia Han4, Luquan Ren1 (1. Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130022, China; 2. College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, China; 3. China National Machinery Industry Corporation, Beijing 100080, China; 4. College of Engineering, China Agricultural University, Beijing 100083, China) Abstract: Threshing is the most important function of grain harvester. Grain loss and damage in harvesting are significantly related to threshing theory and technology. There are four kinds of threshing principles including impact, rubbing, combing and grinding. Four types of contact models between grain and threshing components have been constructed correspondently. Grain damage can be regarded as a function of peripheral velocity and contact pattern of impacting. Grain loss can be regarded as a function of contact pattern of rasp bars. Grain loss coming from cleaning and separation in the subsequent process of combing threshing was significantly decreased. Tangential and axial threshing technologies have been applied in grain threshing system widely. It showed that in the combined application, tangential rolls are used to accelerate grain flow, and axial rolls are used to increase threshing quality especially lower loss and damage. Conical concave may take the place of the traditional cylindrical one. With the development of sensor technology and communication technology, intelligent harvesting robot and automatic threshing system will be integrated together to improve grain quality and operation comfort. Keywords: grain, threshing, threshing theory, threshing technology, harvester, grain loss DOI: 10.25165/j.ijabe.20181103.3432 Citation: Fu J, Chen Z, Han L J, Ren L Q. Review of grain threshing theory and technology. Int J Agric & Biol Eng, 2018; 11(3): 12–20. [24] separating . Natural loss comes from weather conditions such 1 Introduction as wind and rain[25]. Others are relevant to the mechanical Rice and wheat are major grain crops in the world. The performances of combine. As is known, the weather is planting area of rice in China is about 30 million ha and its annual uncontrollable. Therefore, reducing the mechanical loss may be a yield is more than 200 million t[2]. Almost half of the world more practicable means to improve quality of grain threshing. population feed on rice[2]. Likewise, wheat is the highest yield Likewise, damage rate is another direct index of grain threshing [26-31] food crop and ranks first among cereals[3]. Consequently, grain which negatively affects the market value and storage . harvester is an important agriculture machinery that improves Typical kinds of grain damage are mechanical damage and harvesting efficiency and reduces labor costs[4-7]. The typical kind chemical damage. Mechanical grain damage is due to the impact of grain harvester is a combine which integrates the harvesting of grain kernels and rigid surface of threshing unit of high relative [32-35] processes such as gathering, cutting, threshing, separation and speed . The method of high-speed digital imaging has been cleaning, etc., especially threshing is its most important function[8]. used to find the morphological and textural characteristics of three [36] Furthermore, grain loss and damage in harvesting are significantly views of grain when grain kernels free fall and impact . affected by threshing performances[9-12]. Hence, many researchers Chemical damage comes from a complex of fungi when grain is [37-39] put their efforts to investigate grain-threshing devices, and different harvested during the rainy season . Previous research kinds of grain threshers or threshing components have been indicates that if grain crops are harvested at physiological maturity developed since 1820s[13-18]. and then dried artificially, the grain chemical damage could [40-42] However, higher level of threshing theory and technology are decrease to the minimum level . still the unswerving pursuit, due to the fact that they always Many mechanical improvements have been made empirically [43] seriously affect grain loss and damage. Loss rate is a major to reduce the grain loss and damage. For instance, Ahmad et al. parameter to evaluate the performance of grain threshing[19-23]. modified the designs of beater and beater drum of conventional Dodds has reported that there are four resources of grain loss, wheat thresher. As a result, grain damage is reduced four times namely, nature, cutting and rolling, pick-up, threshing and and grain-cleaning efficiency is improved from 97.44% to 98.18%. In order to change the contact condition, Mesquita et al.[44] installed Received date: 2017-04-16 Accepted date: 2018-04-17 nylon cords on the rotating shafts. The result indicates that Biographies: Jun Fu, PhD, Associate Professor, research interests: bionic threshing efficiency increased from 94.8% to 99%, which was threshing technology and equipment of grain, Email: [email protected]; Lujia substantially higher than conventional threshing components. At Han, PhD, Professor, research interests: development and utilization of biomass the same time, seeds broken rate ranges from 0.6% to 0.3%, which resources, Email: [email protected]; Luquan Ren, Professor, Academician of Chinese Academy of Sciences, research interests: agricultural mechanization was also considerably lower than the conventional level (8.4%). engineering, Email: [email protected]. Another improvement of threshing component was made on the *Corresponding author: Zhi Chen, PhD, Professor, research interests: grain Thai-made combine[45]. A stripping rotor was used to take the harvesting technology and equipment. China National Machinery Industry place of cutter bar system. It consisted of eight rubber blades Corporation, Beijing 100080, China. Tel: +86-10-82688715, Email: [email protected]. fixed on an octagonal drum, and each blade had 71 teeth. As the May, 2018 Fu J, et al. Review of grain threshing theory and technology Vol. 11 No.3 13 result showed that the rice loss rate was 4% in standing crop Previous investments have developed the contact model of condition and 5.6% in lodged crop condition, and the average impact threshing[54]. It is found that peripheral velocity of teeth, percentage of crack grain was 1.6%. Additionally, to optimize the types of teeth and types of drum all have prominent effects on performance of concave, which was the general threshing threshing efficiency and kernel surface quality[55-57]. Grain crops component in grain harvester, experiments were conducted based mass obtains impact force and sustains internal resistance among on different concave clearances and drum speeds[46]. It was found the mass flow and frictional resistance at concave. When the that grain damage decreased when the concave clearance increased peripheral velocity of teeth is greater than the critical velocity of from 29 mm to 35 mm, while it increased when the drum speed impact damage, the impact force plays an important role for grain [58] ranged from 675 r/min to 875 r/min. Although above empirical damage . Otherwise, the internal resistance and frictional methods could reduce grain loss or damage to some extent, they are resistance give rise to grain damage. Modification of the surface [59] usually applied in specific conditions and partial grain crops. of threshing teeth could change the pattern of impact contact . This paper starts with a brief introduction of two main Therefore, grain damage can be regarded as a function of problems of grain threshing including grain loss and damage. peripheral velocity and contact pattern. Afterwards, four types of threshing principles were analyzed. In 2.2 Rubbing threshing particular, four threshing contact models are established. Based The contact model of rubbing threshing is shown in Figure 2. Normally, six or eight rasp bars are installed on a drum to form a on threshing theory, four threshing technologies are extended, and threshing cylinder, and a concave of wrap angle of 110° is characteristics of typical threshing equipment are summarized. underneath the cylinder (Figure 2a). A lot of ribs are distributed Finally, three development trends of grain threshing are discussed. on the threshing contact surface of each rasp bar (Figure 2b), which 2 Grain threshing principles increase the friction coefficient between rasp bar and grain ears[60-64]. Therefore, rasp bars are the key threshing components There are four kinds of grain threshing principles including [47-49] of rubbing threshing device. They are used most widely to form a impact, rubbing, pre-cut combing and grinding . threshing clearance, which spreads from the top of rasp bar to the Correspondently, four types of contact models between grain crops top of concave grate, where grain ears are threshed under the and threshing components are generated. Grain threshing influence of friction. The process of grain threshing could be performance such as loss and damage are significantly affected by identified as follows: grain kernels are detached from stalks usually contact pattern between grain crops and threshing components. by rubbing of rasp bars and concave grate, and then penetrate Contact models are constructed based on geometric structures and through the straw mat, lastly pass through the opening between mechanical motions of threshing devices. Force