No abstract

In Kenya, threshing of common beans is mainly by traditional method using sticks and animal tramping, which are slow, inefficient and tedious. Consequently, there is a need to develop portable threshers locally available in the market for small and medium-scale farmers. The objective of this study was to simulate design variables effect on the performance of a common beans portable thresher. Sizing of design variables and parameters was key in development of bean thresher. This could be achieved by costly experiments or use of prediction mathematical model equation. The later method was used by developing mathematical models from combination of Buckingham pi theorem and reference to other similar work in literature. The predicting equation for power requirement, grain losses, grain damages, efficiency and throughput capacity were developed and validated using experimental thresher from the same study. The results showed that there was a positive correlation with R2 of 0.9. Based on actual data and 10% absolute residual error interval, the prediction performance of the developed models were above 77%. The results noted that increase in cylinder peripheral speed of the pegs resulted into increase in power requirement, bean grains damages, threshing efficiency and throughput capacity. Also increase in effective cylinder diameter caused increase in threshing efficiency and grain damages.

Threshing and cleaning of common beans are some of the main post-harvest practices. Others are drying, grading and packaging. Threshing of beans can be done using heavy combine harvesters, use of stationary thresher or by use of traditional methods. The intercropping of beans combined with small scale farms in Kenya makes the use of combine harvesters unsuitable for beans threshing. Traditional threshing methods are also tedious, time consuming and labour intensive. Therefore, the focus of this study was to develop a portable common beans thresher customized to the needs of small-scale farmers in Kenya and other developing countries.The design process was through simulation, optimization, fabrication and evaluation. The thresher recorded a threshing efficiency of 99 % for all the drum speeds above 600 rpm at crop moisture content of 18% w.b. In addition, throughput capacity of 72 kg/hr and 125 kg/hr at a feed rate of 1 kg/min and 2.5 kg/min respectively was achieved at a drum speed of 800 rpm. Mechanical grain damage was less than 3.3% for drum speeds between 600 and 1000 rpm. At drum speed of 800 rpm and below, mechanical grain damage was 0.7%. The developed common beans thresher power requirement is 15.6 kWh per ton. Finally, the thresher had a cleaning efficiency of 78% at 800 rpm drum speed. Based on performance, the developed common beans thresher is recommended for use by small scale famers in beans production.