Optimization of threshing performance of a spike tooth sorghum threshing unit

Abstract

Sorghum threshing in Kenya is characterized by high grain breakages and accounts for grain loss of 4% of total production. Existing threshing methods are time consuming and yield a low throughput. Grain losses mainly occur due to grain damage and incomplete removal of grains from panicles. In order to reduce the losses, optimal levels of machine and operational parameters influencing threshing need to be established. A prototype spike tooth sorghum thresher was therefore developed using engineering principles with the objective of optimizing threshing performance of its threshing unit. The performance tests of sorghum threshing were conducted at three levels for drum diameters (200, 300 and 400 mm), spike spacings (50, 75 and 100 mm) and drum peripheral speeds (8, 10 and 12 m s-1) using a factorial experimental design. The data were subjected to graphical and statistical analysis of variance (ANOVA); and optimization was done using the Taguchi signal/noise ratio method. It was observed that threshing sorghum at spike spacing of 50 mm, drum diameter of 400 mm and drum peripheral speed 12 m s-1 produced the highest threshing efficiency of 96%. Minimum mechanical grain damage of 3% was obtained using drum diameter 400 mm, spike spacing 100 mm and drum peripheral speed 8 m s-1. Threshing sorghum at drum diameter of 400 mm, spike spacing 50 mm and drum peripheral speed of 10 m s-1 produced maximum throughput per unit energy consumption of 153 kg h-1 (kWh)-1. Although throughput of the thresher increased as the drum peripheral speed was increased from 8 to 12 m s-1, throughput per unit energy consumption reached a maximum at 10 m s-1. From the study it was concluded that optimal threshing performance could be attained with drum diameter 400 mm, spike spacing 50 mm and peripheral speed 10 m s-1. The study recommends that the performance of sorghum threshers be based on throughput per unit energy consumption rather than throughput as throughput does not take into account the energy consumed during threshing. This recommendation could be extended to other grain threshers since their principles of design and operation are the same. Future studies could be done to determine the influence of drum length, concave clearance, feed rate, sorghum variety and moisture content on the performance of sorghum threshers.