ABSTRACT
Existing millet threshers have not been performing up to expectation. Based on this, the physical properties of pearl millet grains were determined and used to design an axial-flow millet thresher. The length, width, thickness, geometric diameter, sphericity, surface area of the grains were found to decreased with decrease in moisture content of the grains from 11.6 to 7.8 %. The bulk and true densities of the grains were gotten as 0.73 and 1.14 kg m-3 respectively and the coefficient of friction of the grains on sheet metal and aluminium were obtained as 0.35 and 0.36 respectively. The thresher was made from locally available materials and evaluated at 3 levels of crop moisture content of 7.8, 9.7 and 10.3 % (dry basis), 4 levels of manual feed rates of 60, 90, 120 and 150 kg hr-1 and 4 levels of cylinder speeds of 5.5, 7.3, 9.2 and 11.0 ms-1. The collected data were analyzed using analysis of variance and Duncan multiple range test. The thresher had 96.5 % and 95.2 % threshing and cleaning efficiencies respectively. The mechanical grain damage and scatter loss from the thresher were 2.65 % and 27.6 % respectively. The throughput capacity of 60 kg hr-1 was obtained at 9.7 % crop moisture content and 150 kg hr-1 feed rate. The analysis of variance showed that for threshing efficiency, cleaning efficiency, grain mechanical damage, scatter loss and throughput, there was no significant difference at 1 % level of confidence in the interactions of the three independent variables. But the analysis showed that the main effects of crop moisture, cylinder speed and feed rate had high significant differences on each of the performance indices evaluated. The Duncan multiple range test showed that the best crop moisture content for threshing was 7.8 %. It yielded 38kg per hour throughput, 94.8 % threshing efficiency, 94.4 % cleaning efficiency and grains damage of 2.5 %. The highest feed rate (150 kg hr-1) and the two uppermost speeds gave best results.