Effects of impurity on the efficiency of a legume threshing machine

A motorized legume thresher was redesigned to utilize coefficient of friction to separate grains from impurities, the thresher performance was evaluated and best combination of parameters for highest separation efficiency determined. The study was carried out using a randomized design of three batch weights (BW): 100, 150 and 200 g, four surfaces (S) mild steel, plywood, rubber carpet and rug at an angle of inclination of 25° and a height fall of 90 mm, two impurity levels (I) using cowpea (Vigna unguiculata) IT84S-2242 and soybean (Glycine max L) 1448-2E variety. Sorghum seeds at 10% of each batch weight were mixed in with each test samples as additional impurity. Data obtained was analyzed using SAS and Duncan tests. Results showed, cleaning efficiency varied between 90 and 63.28% for soybean samples, threshing efficiency varied between 87 and 97% with an average minimal damage of 0.78% from using carpet surface. For cowpea threshing and cleaning efficiencies were 97.44 and 97.16%, respectively with average loss 2.60%. Analysis of variance (ANOVA) showed that impurity level and feed rate affected cleaning efficiency at both 1 and 5% significance, type of surface affected threshing percentage and broken seeds at 5% significance. The best combination of batch weight, surface and impurity level to obtain cleaning efficiency, threshing efficiency, low grain damage and grain losses was 100 g batch weight using carpet surface. In conclusion, coefficient of friction could be utilized to increase separation efficiency of thresher, this would aid the development of appropriate technologies for legumes processing. Key words: Threshing, threshing efficiency (TE), cleaning efficiency (CE), coefficient of friction, percentage impurity.