the development of a nine metre span gantry for the mechanized production and harvesting of...

J. ugric. Engng Rex (1989) 43, 125-135

The Development of a Nine Metre Span Gantry for the Mechanized Production and Harvesting of Cauliflowers

and Other Field Vegetables

J. B. HOLT;* N. D. TILLETT*

A wide span self propelled gantry running on crawler tracks was designed and constructed for the mechanized production and harvesting of vegetable and salad crops such as cauliflowers, cabbages and lettuces. The particular benefits are the avoidance of damage to the growing crop by the passage of wheels during selective harvesting and the ability to travel on the land at any time. The specification of the experimental vehicle is discussed and consideration is given to possible developments to meet particular requirements. Some observations are made on the performance of a commercially constructed gantry operating on a vegetable growing farm.

1. Introduction

This development relates particularly to vegetable crops such as cauliflowers and calabrese, which are readily damaged by the passage of tractors or specialized narrow track-width harvesting equipment during multi-pass harvesting. It is likely that other vegetable and salad crops such as cabbage, spring greens and lettuces would benefit from the avoidance of soil compaction or from the wide tool bar or mobile packhouse facility offered by a gantry.

2. Operational characteristics

The perceived requirement was for a vehicle which would aid the selective harvesting of cauliflowers whilst reducing crop losses due to damage and soil contamination. To meet this requirement a gantry was conceived and it was decided to construct an experimental unit for evaluation and to assist with the development of harvesting methods and equipment.‘l The following specifications were devised for the gantry.

(1) It should have adequate traction and flotation so that it could travel over a wide bed of crop regardless of the moisture content of the soil and be able to carry up to three tonnes of harvested crop.

(2) It should provide space for packing the produce from full-width harvesting and have a range of speeds from 0.13 to 8 km/h to suit operations such as transplanting and idle return or crop spraying.

(3) It should have a nominal span of 9 m. A study’ had indicated that this was an attractive compromise in a practical range which was considered to be from 6 to 12 m.

(4) It should be suitable for establishing and tending the crop, so as to ensure that the row spacing and bed widths are suitable for harvesting with the gantry, and also to avoid unnecessary soil compaction in the growing area. Beds established using other equipment

* Horticultural Engineering Division, AFRC Institute of Engineering Research, Wrest Park, Silsoe, Bedford MK4.5 4HS, UK.

Received 13 December 1988; accepted in revised form 28 March 1989

Presented at AG ENG 88, Paris, France, 2-6 April 1988

125

126 GANTRY FOR HARVESTING CAULIFLOWERS

might be of variable width which could lead to the crawler tracks of the gantry damaging some of the crop during harvesting operations.

(5) Secondary cultivations and the setting up of ridges, if required, should be possible with the gantry. There was reason to believe that if traffic could be kept off the growing area the work to be done after harvesting to prepare the ground for the following crop would be very much reduced.4 It might therefore be desirable to chop up and incorporate the crop residue with low draught gantry mounted equipment and eliminate the normal primary cultivations.

(6) The gantry should be capable of being used for the application of fertilizer, lime and chemicals for weed disease or pest control at least over its 9 m span at one pass. The wide track width was expected to provide a stable base for a 28 m wide spray boom able to span three gantry-width beds.

3. The experimental gantry

The AFRC Institute of Engineering Research designed and constructed a gantry (Fig. 1) for the evaluation and development of wide bed mechanization of vegetable production. The principal features identified on Fig. 2 are as follows:

(1) The gantry is designed to span a 9 m wide bed and consists of two 0.5 m wide crawler track units A, with an overall length of 3 m, supporting a space-frame beam, B. Crawler tracks were chosen instead of wheels because of their superior

Fig. 1. The experimental vegetable mechanization gantry of the AFRC Institute of Engineering Research

J. B. HOLT; N. D. TILLETT 127

Fig. 2. Principal parts of the experimental gantry

flotation and traction characteristics within a given plan envelope. The top of the beam is fitted with plastic duck boarding, J, to serve as a work platform.

(2) Jack-down wheels, H, are provided at each end of the beam to enable the gantry to be towed by a tractor from field to field and on public roads. The wheels at the “front” end are close together, similar to the nose wheels of an aircraft, and are steered by the drawbar.

(3) To provide flexibility in the materials handling operation, six separate fork lift units, C, using a double parallelogram linkage have been attached to the beam. This linkage was designed to swing the pallet out over a trailer as it is being lowered; the mechanism is self holding in the raised position. A float position is provided in the hydraulic system so that if the trailer settles as successive pallets are transferred to it the forks do not become trapped.

(4) To relieve the beam of avoidable bending load the 60 kW diesel engine, F, was positioned at one end of the beam over a track unit with the hydraulic oil tank, G, over the other. This necessitated large diameter suction pipes between the tank and engine driven pumps for the propulsion, services and suspension. Oil flow from the services pump can be added to that from the transmission pump to give a top speed of 8 km/h.

(5) The gantry is steered by means of a manually operated variable flow divider valve which controls the flow of oil to the two track driving motors. A solenoid operated flow reversing valve is fitted close to each track motor to control the direction of travel, but more importantly the valves enable the gantry to pirouette about its mid-point.

(6) The gantry has a leader cable guidance system. Search coils mounted on a boom extending forwards over one track pathway can cause the gantry to follow the line of a cable buried about 350 mm below that path when fed with a 2 kHz 50 mA maximum current. Trials have shown that the automatic steering is accurate to f50 mm and further refinements are possible. The purpose of such a system is to save one worker during the particularly slow speed planting and harvesting operations. Provision has been made for starting and stopping the gantry from a position readily accessible to a worker on the ground although when in automatic steering mode the gantry would normally be controlled by a worker on the platform.

(7) The beam is tiltable in respect of both track assemblies to allow for uneven ground


and so that the fork-lift units can be tilted to match the slope of the platform of a nearby trailer. The beam inclination is controlled by pairs of rams, E, which straddle each track unit. The rams are all cross connected hydraulically to a pump circuit and to a hydraulic accumulator which acts as a spring.

(8) The beam has a cross-section of 1.33 m wide x0.62 m deep and is fabricated from 5OC high yield rectangular hollow section steel. When standing on a firm surface, the underbeam clearance is 0.88 m. This is excessive for many purposes but was chosen because of the uncertainty about the conditions in which the experimental machine would be required to operate.

(9) The overall length, with the wheels down, is approximately 13 m and the width is just less than 3 m. The machine may therefore be towed on UK roads at speeds of up to 32 km/h.

4. Application of the experimental gantry

The gantry has been used for a variety of operations over three seasons on a trial area consisting of twenty-four 9 m wide beds each 110 m long, it has also been used to transplant and harvest about 2 ha of cauliflowers on a commercial farm in Lincolnshire.

4.1. Driving characteristics

It was observed that the driver would best be positioned directly over one of the track paths so that he did not have to look at this obliquely. The engine position precluded this and the driver has to stand at ‘D’ (Fig. 2), but even so, steering accuracy is generally within f70 mm.

4.2. Headland travel

The gantry was to be towed on its wheels from bed to bed but it was soon appreciated that it could easily be made to pirouette on the headland and run broadside-on to the next bed (Fig. 3). The extra amount of uncropped land resulting from this technique in comparison with the headland width required for tractors and trailers, and the manoeuvring of conventional equipment into the rows, is surprisingly small. Some of the area within the circles made by the tracks can be cropped if desired. A farmer using a gantry of similar dimensions (Fig. 4) reports that 71 cauliflowers were lost in each bed. In his second season using his gantry he cropped 9 m wide headland strips with spring cabbages or cauliflowers which were harvested before the main area.

4.3. Traction

The hydraulic motors and the 9: 1 reduction in the transmission to the crawler track sprockets had been chosen to give the gantry the ability to climb a gradient of 1 in 8, for example the step from a field on to a farm road. None of the envisaged operations called for high draught but the available draught was measured using a drawbar dynamometer. The maximum pull obtained was 17.3 kN, the limiting factor being the motor torque.

4.4. Flotation

A commercial gantry (Fig. 4) has been used on extremely wet silt soil, and despite running over the same beds a number of times the maximum depression of the track pathways has not exceeded 50 mm. A tractor-based harvesting rig operating in the same


Gantry I

-99 -

3 4

Fig. 3. The pirouetting procedure for mouing the ganrry from bed to bed

Fig. 4. A commercial vegetable harvesting gantry


field a few days earlier had sunk in the soil to the extent that it “bottomed” on the crop. On different soil the tracks of the experimental gantry have never produced ruts deeper than 50 mm.

4.5. Mounted implements

The gantry has been used for transplanting brassicas and drilling lettuces using three and five row machines attached to the implement carriage. The implement carriage has rollers to locate it on the longitudinal members on one side of the gantry beam. A hand winch is used to move the carriage along the beam but there are difficulties with the system. When operating in Lincolnshire the gantry was fitted with an additional implement carriage; one was used for planting three and the other four rows of cauliflowers. The required fourteen rows were planted on a return trip along each bed. Full or half bed width tool bars fitted with quick hitch attachments are advocated.

4.6. Harvesting operations

The gantry has been used for harvesting hand cut cauliflowers in three ways.

(1) An experimental conveyor system,5 Fig. 5, consisting of two forwardly extending compartmented finger conveyors which delivered heads synchronously into the baskets of a paternoster conveyor, was mounted on the leading face of the gantry beam. The objective of the finger conveyors is to provide the workers cutting the crop with some fore and aft float relative to the forward motion of the gantry to accommodate the variability of their task. The paternoster conveyor elevates the crop to a convenient height for the packer or packers on the gantry’s work platform and provides buffer storage to allow for box handling. A theoretical study6 indicates that fingers up to 3 m long should provide a useful benefit; studies of video recordings of workers unfamiliar with the equipment indicated that the least skilled workers were the ones who obtained the greatest benefit. This could be useful for achieving better team balance.

Fig. 5. The finger and paternoster conveyor used on the experimental gantry


(2) A cup conveyor was constructed for mounting on the gantry as a simpler alternative to the finger conveyor. The cup conveyor design incorporates a simple means of keeping the cups adequately upright7 when the conveyor is inclined at an angle of 45”.

(3) To meet the requirement to supply cauliflowers in bulk bins to processors, some bins were carried on the fork-lift units in their lowered position. Workers walking behind the gantry could easily throw the heads into them.

On one occasion, to meet a market demand, the cup conveyor was used at the front of the gantry for heads being packed into plastic crates at the same time as bulk bins were being filled at the rear. The work indicated the versatility required and how a gantry could be a flexible tool.

4.7. Crop spraying

The gantry has been used for spraying the beds in the trial area. A tank and pump unit for mounting on a tractor was adapted so that it could be fork-lifted onto the gantry beam. The pump is driven by a small hydraulic motor and is connected to a 9 m span spray boom carried by the implement carriage. Although the top speed of the gantry was less than that commonly used when spraying with a tractor, the potential for a large liquid carrying capacity and 28 m span spray boom makes the system attractive.

4.8. Cultivations with the gantry

For experimental purposes a 1.8 m width Rotavator was extensively modified so that the rotor was driven by a diesel engine of about 45 kW which is fork-lifted onto the gantry beam. Crop debris can be chopped and incorporated, and since the rotor speed and the forward speed are independently variable, different types of cultivation can be achieved. The exercise demonstrated that cultivations could be performed by a low draught gantry but it should be recognized that farmers operating on silt land, where many of the brassica crops are grown, do not favour rotary cultivation.

5. Developments arising from experience with the experimental gantry

Little attempt was made to optimize the design of the gantry beam, the wheel equipment, the transmission or the span since it was hoped that ideas for changes to the concept would emerge as a result of experience with the gantry. There are now various proposals.

5.1. Movement from bed to bed or field to field

It has been envisaged that the rear wheels might be made steerable to assist in manoeuvring round corners, for example when turning into a field gateway. However the gantry can easily be slewed by running it a short distance on its rear track and then raising it on its wheels again. This is a quick operation but would probably not be permitted on public roads. A choice between these options would depend on cost and circumstances.

Widely spaced front wheels are considered desirable to improve stability, particularly on badly rutted headlands. Although powering some or ail of the wheels would provide complete mobility along headlands and would suit some growers, the pirouetting technique which enables the gantry to run along the headland on its tracks has proved very satisfactory.

In situations where it is desirable for a gantry to be self-propelled endwise along field


Fig. 6. Possible arrangement of wheels and a rotatable track assembly self -propelled end-ways on

to enable to gantry to be

headlands, an economical solution could be to make one of the track units, A (Fig. 6), rotatable through 90” so that the gantry could run on a pair of wheels and that crawler track. It is assumed that the end of the gantry would be raised off the ground either on wheels or feet while the track unit was pivoting. Such a gantry could be towed on its four wheels on the road.

Stability would be improved if the pivoting track, T, were combined with wheels positioned as shown in Fig. 7. This illustrates a proposed gantry with a deep beam having the work platform, P, in the lower plane of the structure. The one pair of jack-down wheels would be positioned a little to one side of the centre of the gantry so that it could be towed in a similar manner to an unbalanced trailer. The same relationship between towed weight and tractor size would apply. It is a system which might be particularly suited to gantries which were lighter or of narrower span than the present one and had a materials handling system.

Wheels are not an essential feature of a tracked gantry. For travel from field to field the gantry could self-load itself on to a purpose built trailer equipped with side ramps to match the spacing of the gantry tracks. Such a trailer could be towed by a conventional tractor. Alternatively a gantry without wheels could drive over a twin-axled bogie; the gantry would be raised by lifting gear which could be on either the bogie or the gantry, Fig. 8.

Fig. 7. A deep beam gantry with two wheels, and one track rotatable


Fig. 8. A gantry with a detachable bogie for travel on roads

5.2. Harvesting and produce handling

For harvesting work, rapid mechanical unloading of the produce is important and variations on the method adopted are possible. Instead of the six separate fork-lift units, a demountable trailer can be carried by a gantry similar to the experimental one (Fig. 9). To provide the same clearance under the trailer wheels as that under the gantry beam, the deck of the trailer may have to be higher than the gantry’s work-platform. An alternative is to use a large demountable load-platform which can be transferred to a trailer. A load-platform could be lighter and involve less overhang than the equivalent trailer. An important advantage of demountable trailers or platforms over the individual fork-lifts is that with them it is easier to secure the produce containers.

If a gantry beam were approximately 3 m wide there would generally be room to carry pallet loads of produce on the beam itself and transfer them over the end of the beam on to farm transport. However the height of the present beam is somewhat greater than that of a farm trailer. A gantry with a 2.2 m deep beam is being considered where the work platform is a little above the lower plane of the beam. Fig. 7shows such a proposal with a “drawbridge” conveyor for linking powered pallet conveyors, C, on the deck of the gantry with free running conveyors on a trailer. This arrangement would seem to lend itself to the rotating track possibility, as shown, with the drawbar and drawbridge doubling as the means for lifting that end of the gantry while the track was rotated

Fig. 9. A commercial gantry carrying a trailer


Fig. 10. Method of working a gantry span bed with quarter width implements

through 90”. These designs involving pallet conveyors would only be acceptable where the overall dimensions of the pallet loads were well established.

5.3. Implement mounting

As has been stated, a half or full width tool bar would now be preferred to the implement carriage. Assuming the gantry will not always work all the rows in the bed it spans at one pass, the number of rows should preferably be divisible by four. Fig. 10 shows a convenient way of working the bed in two passes without repositioning the implements (transplanter, drill or ridging bodies etc).

5.4. Position of the power unit

Two vegetable harvesting gantries constructed commercially to specifications produced by AFRC Institute of Engineering Research have their engines positioned in the centre of the beam and accommodated mostly within the depth of the beam. This position was chosen to leave the platform above the tracks clear for a driver and to enable the hydraulic tank to be close to the pumps while maintaining even weight distribution.

5.5. Transport width

Harvesting conveyors/elevators can take various forms but in general they must move out of the way to bring the machine to a width of no more than 3 m for travel on the road. Likewise more space may be required for trimming overwrapping and packing tables than can be accommodated within the road width, and fold-away platforms may be needed. A gantry with this feature has been constructed principally for harvesting calabrese.


6. Conclusions

As with tractors, gantries could take many different forms to meet particular requirements. For the mechanization of vegetable production the authors believe important features are:

1. Crawler tracks for flotation and traction. 2. Self propulsion. 3. Load carrying capacity of about O-4 tonne per metre of span width. 4. The ability to carry and power harvesting conveyors or equipment. 5. In many but not all situations, the ability to operate equipment for crop

establishement and maintenance. 6. A span probably between 6 and 12 m. It would be unwise to extrapolate further

from the present 9 m span. 7. The ability to travel along roads without inhibiting restrictions.

The structural part of a gantry is a straight-forward fabrication representing only a small part of the total gantry cost. The variations in the dimensions and nature of this beam, which we will probably see during the next few years, are not likely to add significantly to their costs while production is in small numbers. However there are strong arguments in favour of a standard span being adopted, for example a crop spraying contractor would be able to work with his gantry on various gantry users’ farms.

This work has already led to the construction of a highly successful gantry for commercial use harvesting cauliflower? and another for calabrese harvesting. These designs closely follow specifications prepared by the authors.

References

’ Tillett, N. D.; Holt, J. B. Leaf vegetable harvesting-a review. Divisional Note DN 1269, National Institute of Agricultural Engineering, Silsoe, February 1985

’ Tillett, N. D.; Holt, J. B.; Chestney, A. A. W.; Reed, J. N. Experimental field gantry for leaf vegetable production: Specification, design and evaluation. Journal of Agricultural Engineering Research 1988, 41: 53-64

’ Tillett, N. D.; Audsley, E. The potential economic benefits of gantries to mechanise the production of leaf vegetables. Divisional Note DN 1288, National Institute of Agricultural Engineering, Silsoe, September 1985

’ Chamen, W. C. T.; Chittey, E. T.; Howse, K. R. The effect of different tyre/soil contact pressures on soil and crop responses when growing winter wheat: Year 3-1984-85. Divisional Note DN 1432, AFRC Institute of Engineering Research, Silsoe, December 1986

5 Tillett, N. D. The design and construction of a gantry mounted experimental conveyor for cauliflower harvesting. Divisional Note DN 1430, AFRC Institute of Engineering Research, November 1987

’ Tiiett, N. D. A theoretical study of alternative methods of selectively hand harvesting cauliflowers. Divisional Note, DN 1422, AFRC Institute of Engineering Research, Silsoe, August 1987

’ Holt, J. B.; Sharp, J. R. A simple inclined cup conveyor for vegetable harvesting. Divisional Note, DN 1499, AFRC Institute of Engineering Research, Silsoe, February 1989

a Walsh, H. Gantry on track to up vegetable quality. Farming Guardian, January 1988, pp. 11

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