ABSTRACT
To eliminate pod-picking, it has become necessary to replace the conventional cowpea (Vigna unguiculata (L.) Unlp) pod thresher with whole shoot thresher. This research work deals with selected physical and mechanical properties of cowpea stalk, pod and grain, as related to the mechanical threshing of the entire cowpea shoot. In the study it was found that the cowpea varieties differed significantly in their grain size which ranged from G.35 to 9.18mm in length; and the suitability of the varieties to whole shoot threshing decreased with the grain size. The mechanical properties of the cowpea stalk were found to increase with decrease in moisture content and increase in the rate of loading. The mean stalk elastic modulus and crushing/bending strength at 25mm/min rate of loading were 30.4 MPa and 5.10MPa respectively, in radial compression; and 4l70MPa and 38.6MPa respectively, in simple bending at about 8% moisture content, wet basis (MC,wb), the moisture level at which the stalk became more brittle. The minimum force of 7.25 to 29N required to crack the dry pod was very low when compared to grain and stalk. While the elastic modulus,of grain in uniaxial compression (mean 454MPa at 7.5% MC, wb) was independent of variety and grain size. The IAR cowpea pod thresher was tested for whole shoot threshing and was redesigned. It was confirmed that the stalk moisture content of about 8% wb was very suitable for Whole shoot threshing, although threshing at slightly higher moistures would be possible. The crushing and breaking energy requirements of single stalk of cowpea at 8% MC, wb, were 0-00393 and 0.109 J respectively, while the threshing energy requirement of the whole shoot thresher was estimated as 139 J at 45Om/min cylinder speed. Finally, an air flow rate of 16m2/min was found to be adequate for pneumatic cleaning of threshed whole shoot cowpea.