Mechanics of soybean threshing

Two experimental devices were designed and manufactured to study the use of mechanical impact and rubbing (friction and compression) to thresh soybeans pods. Threshing efficiency and seed damage were evaluated. The energy absorbed by pods as they shatter and the energy required to operate both devices were estimated and compared with the energy used by conventional combines to thresh soybeans. The impact device was basically made of two parallel walls which hit the plants in a transverse reciprocating motion. The rubbing device connected pairs of vertical rollers driving two, facing each other, endless belts at different speeds and with adjustable gaps between them. The plants were fed either between the walls or belts to simulate the movement of both devices over a row of plants. The impact device threshed 92.5% of the soybeans. Threshing efficiency increased significantly with decreasing levels of pod moisture content. The seed damage was within acceptable levels despite the trend to increase with increasing impact speeds and decreasing pod moisture content. The estimated energy absorbed per pod at 10% moisture content was 0.0127 J. The energy used by conventional combines was 20% higher than that required by the impact device, and around 30 times higher than the total energy absorbed by pods during shattering. The rubbing device threshed over 92.5% of the soybeans from 10% moisture pods at any belt speed. There was no significant seed mechanical damage. The energy absorbed per pod was 0.0123 J. The energy used by conventional combines was over 100 times higher than that absorbed by pods as they shattered, 12 times higher than that required by plants (pods and stems), and more than 4 times that required by the rubbing device. Both devices performed satisfactory threshing with little seed mechanical damage and a relatively low level of energy consumption. These results support the conclusion that there is potential to improve soybean harvesting equipment