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

274 M. Ruiz-Altisent et al.
3.3.1. Tree shakers for open-trained trees
The most extended method to harvest fruits mechanically is the use of inertial
trunk or limb shakers that attach to the pertinent wood and are able to transfer
large amounts of energy in the form of vibrations. Nowadays, the shake-catch method
is the only mechanical harvest system used extensively in deciduous tree fruits.
The simplest shaking system appropriate for fruit trees is the tractor-mounted
cable shaker. In it, the motion is generated directly by the tractor p.t.o, through an
eccentric that powers the cable. Vibration is created by the returning movement
of the tree branch or trunk. Frequency is 5–10 Hz and the amplitude is large:
20–60 mm. Power: 10–30 kW (Moser, 1984).
Eccentric rotating masses are the most widely used in tree shaker machines.
Inertial shakers have to be isolated from the machine that carries them, so that no
vibration is transferred to it. The basic principle consists of transmitting to the
tree the forces generated by one or several rotating masses, or by a slider-crank
mechanism.
The slider-crank shaker transmits forces in only one direction. The magnitude
of the force depends on:
– the rotation speed and
– the mass of the housing of the shaker.
Slider-crank mechanisms are applied exclusively in limb-shakers. The frequency
is 10–20 Hz; amplitude: 20–40 mm; power: 20–40 kW. Reciprocating housing mass:
100–200 kg; clamping force approx. 5 kN. The diameter of the branches can be
maximum 30 cm, and clamping surface 2 × 30 cm2 .
In shakers provided with eccentric rotating masses, centrifugal forces are generated.
The distribution of these forces can be varied by changing:
– the size of rotating masses;
– their eccentricity and
– their rotating speed.
Normally, forces are multidirectional, although two equal-size masses, rotating at
the same speed (in opposite directions) generate a one-directional oscillating force.
Multidirectional shakers are generally used as trunk shakers (Figure 11). The frequency
is 20–40 Hz, amplitude: 5–20 mm; power: 30–70 kW. Eccentric masses:
20–60 kg; total mass of the shaker: 600–1000 kg, max. diam. of the trunks: 40–50
cm; clamp force 5–7 kN; clamp contact surface: 2 × 40 cm2 . Trunk shakers are faster
and easier to operate than limb shakers (Figure 12). The structure of the trees
needs to be adapted to limb shaking (3–4 main limbs at maximum). The use of trunk
shakers is not well suited for too large trees (>50 cm in diameter) or for trees
with hanging branches, which leads to low fruit detachment; in these cases, limb
shakers are preferred.
Vibration of fruits
When a fruit vibrates, there simultaneously appear: traction, twisting, bending and
shear forces, and also fatigue effects. As mentioned above for tomato shaking, an