Production Practices and Quality Assessment of Food Crops. Vol. 1

To calculate the number of directions of vibration (shaking pattern) let us consider
(Figure 15) that the speed ω 1 is slightly higher than ω 2, and rotating in opposite
directions: ω 1 t = 2π + α; ω 2 t = 2π – α, and the number of directions of shaking
is (Ortiz-Cañavate and Hernanz, 1989):
n = 2π/α = (ω 1 + ω 2)/(ω 1 – ω 2)
Fruit and Vegetables Harvesting Systems 279
Damage to the trees
The zone of attachment of the shaker to the tree (trunk or limb) has been found
to be the most dangerous for tree damage. During shaking applied by the clamp
to the wood, there are not only radial stresses, but also longitudinal and tangential
(Moser, 1984). The admissible stresses that can be absorbed without damage under
the shaker clamp are very much related to bark humidity. For fruit trees, the admissible
values for tangential, radial and longitudinal stresses are, respectively: σ t ≤
600 kPa; σ r ≤ 1700 kPa and σ l ≤ 1800 kPa.
Figure 16 shows the comparative variation of bark resistance (N/cm 2 ) for different
times of tree growth (times of the year) and irrigation, all which influence bark
humidity.
Excessive radial stresses are likely to cause damage in selected fruit trees. The
causes of excessive stress can be due to a bad design of the clamp, as well as to
careless operation. For example, damage can be caused by inadequate adjustment
of clamping pressure or inadequate contact area between the bark and the clamp.
To avoid tangential stresses (the worst for the tree) some manufacturers place a
flap on the clamp, keeping the clamp-flap contact surface well lubricated.
3.3.2. Harvesting fruits in high-density orchards with over-the row machines
A new possibility for fruit production is the high-density dwarf-tree plantation (Trees
planted at 2 × 1 or 2.5 × 1 m 2 ; this means 4000–5000 trees/ha, with a higher and
Figure 16. Bark-resistance (tangential-resistance) variation along time of the year and irrigation
(plum tree).