7. MILLILITERS COLLECTED = LITERS PER HECTARE BEING APPLIED
1.6.15. Principles and Devices for the Detachment
The application of well-directed energy is necessary to effect detachment; the pro- cedure in which this energy is applied is the first and basic consideration in the aim of mechanical detachment and removal and depends on the commodity in question. Sev- ering the attachment forces requires that the ultimate fatigue, tensile, or shear strength threshold must be exceeded [2] (Table 1.70).
Low-height Herbaceous Structures (Vegetables, Strawberries, etc.) We differentiate between root and surface crops.
Table 1.69. Comparative damage thresholds for selected fruits Drop Height onto
Hard Surface Causing
Visible Bruisea(mm) Average Mass (g) Observations
Apple 10 220 High turgidity, increased
damage susceptibility
Pear 20 175
Peach 25 150 More susceptible
in compression
Apricot 150 60
Tomato 200 75 Skin most important
Orange, lemon 400 200 Rough surface, increased
damage susceptibility due to friction
Olive (table) 200 30
Small fruits
Rubus 50 10
Ribes 120 10
Strawberries 50 30
Sources: Various authors and ref. [6].
a 10 mm in diameter; wide variations for different fruit conditions; time of harvest was selected when data were known.
Table 1.70. Detachment forces for selected fruits Detachment
Force (N) Observations Apple, pear 40a
Almond 1.5a
Cherries 12–16 8–10 Na
Grape 3–5 Single berry
25–50 clusters
Peach, apricot 1–5 9–15 Na
Tomato 20–30
Orange 20–30
Lemon 30–40
Strawberries 4–9
Small fruits 0.5–2.5
Olive 2–8
Sources: Various authors and refs. [6–8, 12].
a Limb-peduncle detachment, shearing action.
Root Crops
Two different principles are used for the harvesting of these crops: digging (potatoes, carrots, and so forth) and pulling (carrots, leeks, etc.) [1, 5].
a) Digging consists of the uprooting and lifting of the crop together with considerable amounts of soil, from which it must be separated, by the thrust of a digging or plow- ing share. Working parameters of this system are length and inclination angle of the share and working speed and cutting depth, determined by the location of the recov- erable product in soil, together with the above-mentioned parameters; these constitute the regulations of the system. Following the coulters, a vibrating rod-chain conveyor is needed to clean and to feed the product to the complementary operations of handling, cleaning, and loading units. The aboveground plant parts, very fragile in comparison with the subsurface parts, may be recovered or dispersed, before or at the same time as the harvesting of the roots. This harvesting system is appropriate for any roots or tubers (e.g., carrots or potatoes), below-ground fruits (e.g., peanuts) or bulbs (e.g., onions and garlic).
b) Pulling the aerial portion of the plants often is used for the harvest of some root crops (e.g., carrots) and also some surface crops (e.g., leeks and salad greens); the struc- ture and the strength of the plants must permit in this case the engagement of the above ground leaves and the uprooting of the entire plant, aided by a subsurface coulter; the big advantage is that very little soil is extracted with the product (Fig. 1.361).
The equation for the pulling force is as follows:
Fp=2àN. (1.117)
For pulling speed:
v¯p=v¯a+v¯r (1.118)
Figure 1.361. Pulling principle and units. Velocities (a) and forces (b) in the pulling belts [1].
where
à=friction coefficient
N=normal force exerted by the belts on the plant v¯a=working speed
v¯r=speed of belts
The aerial parts have to be well aligned in the row and uniformly spaced, and they are engaged and grasped by a pair of elevating belts, which, due to the combination of the effects of advance speed and velocity and inclination of the belts, exert a vertical pulling force to the plant. Functional parameters of this system are then belt speed, working speed, belt angle of inclination, and the pulling force exerted by the belts due to friction and compression between the belts and the plants. At the upper end of the belts, a pair of topping implements (counterrotating bars or rotating disks) are placed to remove the tops.
Surface Crops
Cutting, combing, stripping, vibration, and threshing are different functions used for the detachment of the very wide range of aerial parts found in the diverse commodities to be harvested.
a) Cutting action is applied mainly to the so-called leafy products: cabbage, lettuce, spinach, endive–escarole, Brussels sprouts, celery, broccoli, cauliflower, artichokes, chard, chicory, mustard, parsley, watercress, and any other so-called greens or simi- lar plant parts. Each of these products needs multiple harvests for maximum yield for fresh market, except perhaps spinach, which is mostly grown and harvested for process- ing. Their common properties are that they have to be cleanly removed and handled
Figure 1.362. Cabbage harvester. (1) guiding discs, (2) Guiding surfaces, (3) driving belt, (4) coulter bar,
(5), (6), (7) band conveyors, (8), (9) separation devices [1].
softly for minimum leaf loss. Cutting is the most effective method of removal, and it is also used in combination with other harvesting systems (such as the cutting of whole plants of tomato; cutting undesired aerial parts of root crops; cutting of Brussels sprouts stalks for subsequent stripping of the sprouts, etc.). The conventional cutting devices are applied for this function: rotating discs with dented edge, flat or with some concav- ity, and cutting bars, simple or double. In some cases the objective is to remove entire plants, even below the surface (tomatoes, pickling cucumbers) to be subjected to further detachment of the fruits; in others, to recover the tender edible green parts (spinach, cabbage) (Fig. 1.362). Frequently, a powered reel or rotating side cylinders are added to help feeding and further conveying of the product up the band or bar-chain conveyor.
Functional parameters of cutting devices are
• Cutting speed
• Advance speed
• Sharpness of the cutting edge
• Speed and position of the feeding reel or cylinders, with respect to the cutting elements
b) Combing is based on particular properties of the plants:
• The plant is firmly attached to the soil by its root system and grows erect.
• The portions to be detached and removed have different size, shape, or rigidity from the leaves and stalks (e.g., green beans or pea pods).
• The portions to be detached possess a zone of abscission, susceptible to being severed by traction or by flexural forces.
• The parts to be recovered (e.g., strawberry fruits, green-bean pods) are capable of resisting the action of the combing fingers.
Functional parameters of combing are
• Speed of the extreme of the combing fingers, be around 4 m/s for a forward speed of approx. 0.5 m/s.
• Speed and position of the conveying device situated behind the combing fingers, to assure the removal of the product from further impacts.
• Speed of the feeding reel, conveying band, or brushes, which are installed in front of the combing fingers (Fig. 1.363).
Figure 1.363. Cylindrical reel-type snap-bean harvester.
(a) Pods are removed by contact of fingers with the pedicle (b) Detachment originated by the cylindrical reel and the
concave sheet in the plant [1].
Figure 1.364. Strawberry harvester by the combing principle. (1) Finger band to
comb the strawberry plant. (2) Fan.
(3) Discharge valve. (4) Conveyor and filling device [1].
As mentioned above, harvesters for strawberries (fruits that must be classified as aerial herbaceous commodities) may be provided with combing fingers attached to slow- moving elevating bands, and the operation is aided by air currents that elevate fruits and leaves to the fingers for easier combing (Fig. 1.364). Instead of combing, some strawberry harvesters use a cutterbar–reel principle, which cuts the entire plant and sep- arates the fruits in the machine by again air-lifting and cutting, or by stripping. The combing effect attained by two counterrotating brushes is used in harvesters for some varieties of paprika pepper for the detachment of the fruits (the concept derives from the combers/strippers for harvesting cotton capsules).
c) Stripping by the action of counterrotating rollers pulling the plant down, stripping is a procedure by which the commodity (fruit) is separated from the plant, based on the differential properties between fruit and plant: size, shape, and attachment strength. This principle is used to detach cucumbers (pickling) from their already cut supporting plants, in pepper harvesters, and in combination with other functions in selected harvesters:
Figure 1.365. Stripping machine with counterrotating rollers. (a) Front view,
(b) side view [1].
destemming of onions, strawberries, olives, and cherries; and detachment and deleafing of (sweet) corn cobs. The principle consists of the pulling action of two counterrotating rollers, provided with some roughness condition: It can be the surface itself (rough rubber rollers), or helix-shaped structures attached to the rollers. The rollers trap the long and thin plant parts (stalks, with leaves) and sever them from the commodity by a pulling action (Fig. 1.365, see also Fig. 1.364). The product is conveyed in different ways, such as inclination of the rollers or conveying by the helicoidal attachments on them, or by falling onto separate conveyors (as in some onion harvesters). Functional parameters of this system are turning speed of the rollers, separation between the rollers, and engagement speed (forward speed) to the plant. Helix-shaped open cylinders, counterrotating, have been used in paprika-pepper harvesting machines, having the effect of combing rather than stripping, as the helixes comb the fruits rather than stripping the plant. Stripping also is used for the detachment of Brussels sprouts from the previously cut stalks. In this case, the effect is achieved with rubber cords.
d) Shaking is done to plants previously cut, in tomato harvesters. Industrial mechanical tomato harvesting is the most advanced mechanical vegetable harvesting, in that world- wide there are harvesters used of numerous makes and designs, which harvest a very high percentage of the industry tomato surface. Tomato plants are cut at the soil surface and plants with fruits are conveyed to the top of a vibrating platform, on which, with the tomatoes in a hanging situation, a shaking energy is exerted on them (Fig. 1.366).
Because the values of the detachment forces, Fd, of tomato fruits are in the range of 20 to 30 N, and the masses, m, of the fruits are 0.05 to 0.20 kg, the minimum applied accelerations for fruit detachment are on the order of 10 to 60 times the acceleration of gravity (see also Table 1.70).
nãg (m/s2)=Fd(N)/m (kg) (1.119) This relationship means that only those products with a relatively high mass and relatively low detachment force are easy to detach by vibration, as is the case with the present tomato varieties. In general, fruits are easy to detach by shaking if n is between 1 and 10 [6]. Inertial application of vibratory energy for detachment results from accelerating the plant commodity with a suitable machine to attain a pattern of vibration and a frequency that are suitable for that commodity. In this sense it has to be designated as “the only noncontact” principle for detachment of fruits [1]. In the case of tree fruits (see further ahead) the necessary acceleration has to be applied at the lowest-force abscission point
Figure 1.366. Tomato harvester. (a) Cutting unit, (b) elevating chain, (c) shaker, (d) selection band,
(e) loading conveyor, (f ) shaker unit [1].
of the fruit, and that is difficult for small fruits. These considerations can be applied to other vegetable crops for which shaking could be a good solution for detachment.
The rest of the functions and units of tomato harvesters combine complementary operations and manual or automatic optical VIS (visible, color) or NIR (nearinfrared) sensors for color grading and for soil-clod separation, respectively.
e) Threshing. This separation procedure uses a rotating drum to remove peas (also some beans) from their pods in pea harvesters. Similarly to one of the effects used for grain threshing, friction is used to separate the pea grain from their hulls. Friction is applied by slow-rotating rubber rollers or bands against a drum, which sometimes also is itself rotating and behaves as the concave as it sieves the peas through it (Fig. 1.367).
From there, peas are cleaned by air, conveyed, and loaded in refrigerated bins.
Bushy Structures (Small Fruits, Wine Grapes)
As mentioned above, small fruits are at the same time difficult to detach by shaking (noncontact) due to their small mass, and damage-susceptible if detached by combing or stripping (contact), as described for some vegetable harvesters. Therefore, the detach- ment of small fruits has been accomplished by a combination of contact and noncontact actions applied by vibrating tools.
Small-Fruits Harvesters
A number of species are referred to as “small fruits:” raspberries, blackberries (Rubus species), and blackcurrant and redcurrant (Ribes species), along with gooseberries, blue- berries, boysenberries, and so forth. They have in common their way of growing in bushy structures, their small size, and their use mainly for processing. In the past 10 years there has been considerable activity in the application of mechanical harvesters to collect these fruits, which are the only fruits produced in some cold (northern) areas. Contact
Figure 1.367. Pea-threshing principle (dehulling) and pea harvester. C, concave drum;ω1,ω2, rotating speeds
of bars and concave drum; AB, shearing effect on pod;
1, cleaning brush; 2, concave sieving drum; 3, pulling bolt; 4, threshing cylinder; 5, inclined band; 6, air
cleaner; 7, slope-adjustment mechanism [1, 6].
principles (combing and stripping) have been tested for the detachment of small fruits in bushy as well as tree structures, with little success. Present small fruit harvesters use a combination of shaking with soft combing, based on vertical, tilted, or horizontal drums provided with fingers or spikes and oscillatory motion, which applies a shaking effect on the fruiting structures. They work on plants trellised in different systems: “T,”
“V,” or “Y” (Fig. 1.368) [7, 8]. The trellis system to be used and the shaking functional parameters depend on the fruit species and the conditions of fruiting. Drums are 50 to 100 cm in diameter, and the shaking is created by inertia. The frequencies depend on the fruit species: 5 to 10 Hz for raspberries (Rubus spp.) and 10 to 25 Hz for currant (Ribes spp.) with amplitudes 40 to 75 mm. Travel speed are 1 to 1.7 km/h (lower speeds for lower shaking frequencies). The quality that one may expect for the mechanically harvested small fruits is highly dependent on fruit species and variety, yield, maturity, trellis system, and the functional control capabilities of the harvester units.
Grape Harvesters
Mechanical grape harvesters were designed in the United States during the late 1950s. The fruit-detachment system consists of a series of vibrating horizontal rods that are free to move on their rear end and mechanically driven with an oscillatory mo- tion on their front end (Fig. 1.369). The number of rods depends on the height of the plants, with 8 to 16 rods for vines that are 1.2 to 1.8 m high. The effect of the rods on the vines is a combination of vibration and impact on the branches and on the vinegrapes themselves. The frequency of the vibration is 9 to 10 Hz, with an amplitude of 88 to 140 mm [9].
In the United States, the latest grape harvesters have a detachment unit consisting of two sliding bars, provided with horizontal stroke. The bars engage the vine trunks at a height of 50 to 70 cm; the vines are forced to vibrate at a higher frequency (10 to 20 Hz), causing the detachment of clusters and also, more often, of individual grapes [10].
In the late 1980s, in France, a new design was introduced that is capable of a gentler handling of the vine shoots. In a similar arrangement to the former system, the rods are substituted by arched rods, which in their rear end are all fixed to the same member, which
Figure 1.368. Blackberry harvester. Side (a) and front (b) view of “T” trellis, (c) “V” trellis, (d) “Y” trellis [8].
Figure 1.369. Over-the-row grape harvester.
(1) Overlapping spring-loaded plates; (2) shaking rods; (3) leaf blowers; (4) grape conveyors; (5) diving
port [17].
Figure 1.370. Oscillatory shaker with arched rods [2].
is pivotally mounted on an axis (Fig. 1.370). The motion of these rods corresponds to the connecting bar of a bell-crank four-bar linkage. In this type of shaker, the frequency is higher, 15 to 23 Hz, and the amplitude smaller, 25 to 70 mm [11]. This means that whereas the former system applies peak accelerations of 130 to 180 times the force of gravity in the three perpendicular directions, on the vine trunk, the new types apply lower accelerations, 50 to 100 times gravity. The result is a reduction in deleafing, broken woods, and torn wood, of 50% to 90%, as well as a better quality of grapes. Additionally, these harvesters can be driven at a higher speed along the row, therefore increasing their productivity. All these grape harvesters are straddle type (over-the-row) (see Fig. 1.369) machines.
Functional parameters of the detachment system are therefore frequency and ampli- tude of vibration, and travel speed. In these harvesters, the shaking (detachment) unit is mounted on a hanging frame, which engages the trellised vines loosely. In the lower part of this frame, a series of overlapping plates forms a catching platform. As the harvester moves forward, the retractable plates embrace the trunks, closing the catching surface tightly. The system has been developed to a very efficient level, where no more than a 5%
of the crop is lost for most grape varieties. The machine incorporates one or two lateral conveying bands and a loading elevator to load the grapes into the hopper, after one or two steps of air cleaning, which eliminates leaves, dust, and shoot pieces. The harvesters work over-the-row, and some are provided with sensing elements in front, to align them to the vine rows. Present harvesters are able to work in vineyards with narrow row spacing (down to 2 m) and are able to engage vines as low as 15 to 20 cm from the ground [9].
Tree Structures (Fruits, Nuts)
We differentiate between open-trained trees and high-density orchards.
Tree Shakers for Open-Trained Trees
The most widely used 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 power-take-off (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 to 10 Hz; the amplitude is large, 20 to 60 mm;
and power is 10 to 30 kW [6].
Eccentric rotating masses are the most widely used in tree-shaking machines. In- ertial shakers have to be isolated from the machine that carries them, so that no vi- bration 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 mecha- nism.
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 to 20 Hz; ampli- tude, 20 to 40 mm; and power, 20 to 40 kW. Reciprocating housing mass is 100 to 200 kg, and clamping force is approximately 5 kN. The diameter of the branches can be a max- imum 30 cm, and the clamping surface is 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 sizes of rotating masses, their eccentricity, and their rotating speeds. Normally, forces are multidirectional, al- though two equal-sized masses, rotating at the same speed (in opposite directions), gen- erate a one-directional oscillating force. Multidirectional shakers generally are used as trunk shakers (Fig. 1.371). The frequency is 20 to 40 Hz; amplitude, 5 to 20 mm; power, 30 to 70 kW. Eccentric masses are 20 to 60 kg; total mass of the shaker, 600 to 1000 kg maximum diameter of the trunks, 40 to 50 cm; clamp force, 5 to 7 kN; and clamp contact surface, 2×40 cm. Trunk shakers are faster and easier to operate than limb shakers (Fig.
1.372). The structure of the trees must be adapted to shaking (3–4 main limbs at maxi- mum). The use of trunk shakers is not well suited for large trees (>50 cm in diameter) or for trees with hanging branches, which lead to low fruit detachment; in these cases, limb shakers are preferred.
a) Vibration of Fruits. When a fruit vibrates, there simultaneously appear traction, twisting, bending, and shear forces, and also fatigue effects. As mentioned for tomato shaking, an acceleration has to be applied to the fruit (expressed in number of ‘g’s’) to produce its detachment from the limb or the peduncle. Table 1.71 summarizes vibration parameters typically used for shaker design.
Damage to the fruits is a limiting factor for the application of the shaking method, and also in the case of processing fruit. The use of abscission chemicals has not had widespread application due to the high cost and to problems related to timeliness of application, as well as to concerns about chemical residues on the fruits.