Theoretical and experimental analysis of a continuous-blade cutting system for leafy vegetables

To address the high costs associated with maintaining current reciprocating cutter bars used for harvesting spinach, a project was initiated to develop a lower-cost bandsaw cutting system. A model of the stresses generated in the saw blade under several loading scenarios was developed and used to evaluate the potential for fatigue failure with different support pulley and blade designs. For hardened steel blades, fatigue failure does not seem likely for pulley diameters equal to or greater than 300 mm. Torsional stiffness of a wide-span blade was modeled and used to determine the critical in plane feed force exerted by the crop above which the blade will buckle. Laboratory tests with an instrumented bandsaw blade testing apparatus confirmed the predictions from the stability model that the critical forces necessary to induce buckling are low. The effects of blade thickness and tension on stability were examined and indicated that the resistance to buckling could be improved with increasing thickness and tension at the expense of fatigue life. For spans greater than 1.5 m, the blade tension required to prevent buckling was not practical, which suggested that the blade must be supported across the cutting width in order to achieve the required forces for cutting. A prototype bandsaw-type harvester was developed that included a continuous polymer blade guide to prevent blade twisting and buckling. Field capacity averaged 1.6 ha/h, corresponding to a throughput capacity in excess of 26 Mg/ha in spinach grown for processing. Infield recovery measurements and an N-way analysis of variance revealed no significant difference in harvest loss between semi-Savoy and smooth leaf spinach varieties. On average, harvest loss was about 5% of the commercial yield; approximately 1% of this loss was attributed to the harvester cutting mechanism as leaves still attached to the spinach crown. The remaining losses were significantly higher, consisting of loose leaves cut by the bandsaw but not gathered by the intake conveyor.