Principles of a Mechanical Shaker for Coffee Harvesting

D. O. Mbuge , P. K. Langat

Abstract


Coffee harvesting is largely a manual operation, mostly done using hired labour, which may be
expensive. The objective of this study was therefore to evaluate the possibility of using a powertake-
off (PTO) driven mechanical shaker for coffee harvesting. In the theoretical analysis, a
coffee tree was modeled as a cantilever beam with a concentrated mass at one end. The model
tree was subjected to a damped forced vibration, to determine the optimal amplitude-frequency
combination at which most coffee berries would be harvested. Experimental variables included
the crank throw of the mechanical shaker at which the most berries were harvested and the
duration of shaking beyond which there were few or no more berries harvested at constant PTO
speed of 540 rpm, tree height and position of shaker attachment. The mass of individual berries
and the resistance to detachment was also determined. It was found that the highest percentage of
ripe coffee berries harvested was 45 %. A crank throw of 0.04 m was found to have the highest
percentage of ripe berries harvested and beyond 10 seconds of exposure to mechanical vibration
there were no more berries harvested. Optimal height for attachment of the mechanical shaker
was found to be 0.6 m from the ground. It was also found that 1.2 N force was required to detach
unripe berries and 0.9 N for the ripe and heavier berries. The force required to shake a single
coffee tree was 12.8 N. The results indicate that the crank slider mechanism is suitable for use in
coffee harvesting and that multiple coffee trees can be harvested simultaneously by a single for
higher productivity.

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