Techlink :: Tech Practice :: Electronics :: The Kimbolton Carrot-Crowning Machine

Techlink would like to thank Monty Brown and Terry Hawkins for their contributions to this case study.

Growing vegetables is a favourite hobby of New Zealanders, and if, at harvest time, the new potatoes are not all the same size, or the broccoli are not the same shade of green, no one cares too much.

Growing vegetables for sale is a different story, and growing vegetables for the big food processing companies is a demanding exercise. A key attribute of a commercially grown crop is consistency. To achieve this, strict procedures and controls are required. Apples and kiwifruit are good examples of crops with very strict growing procedures, because both are export crops with discerning customers.

In the case of carrots, the preparation of the ground, the variety of carrot, the quality of the seed, and the harvesting methods, are all specified by the processing company contracting for the crop. The procedures have been developed and refined by both growers and processors, to ensure a quality crop and minimum expense to both parties.

Occasionally, significant changes occur, due either to changes in other parts of the system (new materials handling systems in the factory, for instance) or, more rarely, because someone involved in the business sees a way of doing things cheaper, better or quicker. This case study describes the development of one such improvement.

Until about five years ago, most carrots were harvested with some of the green, growing, carrot-tops still on the carrot. The whole carrot was taken to the processing plant, and the tops were removed as one of the first processing steps. Done manually, it adds great cost to the finished product, and it is not an easy task to mechanise. Carrots come in different shapes and sizes, are tapered, and easily damaged so standard automation techniques aren't suitable.

COP Brief development

If carrots could be harvested without the tops, there would be big savings in several stages of the process. The transport requirements would be reduced. Handling at the factory would be simplified. There would be much less waste to dispose of from the factory site. The obvious place to put the cut-off tops is on the paddock they were grown in. There was one big problem stopping the implementation of this idea, the lack of a cost-effective way of “crowning" (i.e. cutting the greenery off) the carrots prior to digging them out of the ground. To have the job done manually would be even more costly than doing it in the factory because of the need to transport the workforce, and the reduced productivity due to the adverse working conditions.

COP Outcome development and evaluation

A search was made for a mechanical way of doing the job, and turned up an American machine, locally modified. This was tried and found to be expensive, unreliable, and hard to repair, due to the remote location of both skills and parts. Using this machine would simply transfer the factory de-crowning costs to the farmers, who would require better prices to compensate.These problems brought a temporary halt to hopes of on-farm carrot crowning.

About this time a farmer in Kimbolton, Monty Brown, was growing carrots for the specialised juice market. The juice manufacturer was ill-equipped to handle carrot tops, and was prepared to pay extra for ready-crowned carrots. The high cost of overseas equipment caused Mr Brown to join forces with an engineer in Kimbolton to develop a suitable machine, incorporating the good features of the existing machine and eliminating its faults (Conceptual statement).

Terry Hawkins, the engineer, has had an agricultural machinery build-and-repair business in Kimbolton for many years, and has proven design skills so was a natural partner for the venture. The successful outcome of this venture, the Kimbolton Carrot-Crowner, soon came to the notice of the main food processing companies, who identified its potential for the whole industry, not just the juice market.

COP Brief development

• The machine must cut the carrots cleanly.

• No cracking, splitting, crushing, bruising, etc. allowed. (Ideally, the carrot should look like its been crowned by a chef). The amount removed must be adjustable, to suit the processors requirements. Within quite close limits, all the carrots must be equally trimmed. This is a very difficult requirement. The cut off tops must be moved away from the bed (into the trenches between beds) so that they are not picked up by the harvester following behind. Any greenery included with the crop will cause problems at the factory.

• The machine must operate with carrots grown in beds (4 to 6 parallel rows along the bed) or ridges (like potatoes) 2 rows per ridge.

• It must be light enough to be pulled by a medium-sized tractor, and consume no more power than such a tractor can supply via the power-take-off shaft.

• One man operation is essential.

• The cutting mechanism must work above and below ground level. Often the carrot tops start below ground level.

• The machine must be robust - It is required to work in very bad conditions : cold, wet, muddy, and will sometimes run into hidden obstructions, like drains, stones, and old posts.

• It must be easy to repair – ideally, the operator, rather than a mechanic, should be able to repair all but the most serious damage. Also, specialised items, such as bearings, should be obtainable locally (tractor agents, stock and station agents) and breakage should be fixable by the local garage or farm workshop.

• The machine must be easy to maintain – they may be stored for months between harvests, and need to be readied for use quickly.

• It must be easy to transport – often the machine is hired to many farmers over the harvesting season, who may be some distance apart. Most farmers and contractors have above averagely strong trailers, so this was a good target to aim for.

• It must be affordable – carrots are not a high value crop, such as kiwifruit or asparagus. Consequently there is a limit to how much technology you can afford, to achieve a desired result. The design philosophy used by the developers goes a long way to satisfying this requirement.

COP Outcome development and evaluation

The basic problem facing the designer of a carrot-crowning machine is the variability in the position of the carrots, relative to any point on the machine. There is no fixed datum from which the cutting height can be measured. Despite the best efforts of the grower when preparing the ground, the bed will not be exactly flat, or level. Nor will it be at a fixed height above, for example, the tractor wheels point of contact with the ground.

Even if this level of precision could be achieved, the carrots themselves can grow at different heights in the bed, so the point-of-intersection between the body of the carrot and its green top is only known within a few centimetres.

In cases such as this the designer has to resort to approximations. It was observed that the variation in the level of the soil surface itself, V1,was much greater than the variation of the point-of-intersection relative to the soil surface,V2 (Diagram 1).

V1 = variation in height of surface from datum

V2 = variation in height of bottom of crown from the soil surface.

This lead to the decision to locate the cutter blades relative to the soil surface. This leads naturally to the need for a way of following the soil surface. This was achieved by having large-diameter rollers at the front and back of the machine, with the blades mid-way between.

The large diameter of the rollers reduces the tendency of the machine to sink into the soil, or to damage the carrots. Their distance apart averages-out any change of level in the soil surface, ignoring short-wave-length disturbances, but following long-wave-length ones.

The way in which the machine is attached to the tractor allows the machine to follow changes in the contour of the bed, both lengthways and side-to-side, without constraint. This contributes greatly to its ability to crown the carrots accurately.

The second big challenge is the actual cutting mechanism. Anyone who has tried to cut a carrot with a blunt knife will be very surprised to learn that a very sharp edge is not required, providing the edge is moving fast enough. This is why rotary lawn mowers work so well. This is a very fortunate phenomenon, because no matter how sharp a blade you start with, a few minutes in contact with soil will soon blunt it.

Unfortunately, the cutting action of a lawn mower blade (which is like that of an axe) is no good for carrots, because it tends to split the carrot near the cut. A further problem is the tendency of such systems to scatter the carrot tops far and wide. This can only be overcome by fitting additional mechanisms to collect the tops, at extra cost and complication. What is needed is a much gentler action, rather like a saw. There are many ways of achieving this, most of which are too complicated to work for long near (let alone under) the ground.

The solution is to use small circular blades revolving around their own centres and also around a much larger circle. Such motions are usually described as orbital. This was one of the best features of the original machine. Note that the blades do not have teeth, even though they use a slicing action.

As the orbiting discs advance, they slice into the carrot, rather than just striking square-on. The discs themselves are adaptations of disc-plough blades, easily obtainable, made from 3mm thick spring steel, which remains sharp enough in-use to give clean cuts. This arrangement works well in conjunction with flaps fastened to the big wheel, which push the cut-off tops to the side of the bed.

The ingenious mechanism, which causes the blades to rotate as they orbit around the 'big wheel', is shown in the photographs, and involves a friction drive via pneumatic tyres. These have the advantage of being wholly resistant to clogging by debris, and thus stalling. This is a huge benefit in the field.

As a result of the development of a successful machine, Heinz Wattie's, the major processor, has now made it a requirement for all its contracted growers to crown their carrots prior to harvesting. This has lead to the sale of six machines so far, with more to come.

COP Outcome development and evaluation

Problems and future developments

Despite the careful design and construction, extended testing and field-trials have revealed a few problems, which are being worked on.

The rollers are obviously crucial in fixing the cutting height. In certain conditions mud can build-up on the rear roller, causing it to increase in diameter and thus lift the cutting height. A simple scraper blade removes the mud, but causes increased torque and can cause the roller to stall and skid over the carrots, rather than roll over them. A two piece roller, with a gap in the middle, is being tested, to eliminate the problem.

The most serious problem, solved in later machines, was clogging of the lower bearing on each cutter shaft. These have been moved further up the shaft, away from the dirt and now give no trouble.

Regarding future developments, the wide variety of conditions encountered on New Zealand farms, and the individual preferences of farmers, means that a constant stream of suggestions for improvements and new features reaches the manufacturer.

Many of these can be incorporated into the design (at a cost) for individual customers, because the designer is also the builder. It is hoped to develop a “standard" machine, which can be built by sub-contractors at lower cost, which would satisfy the basic requirements of the industry, leaving the designer to concentrate on development, rather than production.

Possible future developments of the “standard" model include:

• Inclusion of height-of-cut markers on each jack – this would allow quicker and more consistent adjustment of the cutting height.

• Replacement of the shaft-and-gearbox drive by a hydraulic motor – this would allow the rotation speed of the blades to be adjusted independently of the forward speed of the tractor.This would be useful to cope with extra strong growth in the carrot tops.It would also make it possible to push the machine in front of the tractor,allowing a carrot harvester to be pulled along behind,thus completing the job in one pass.

• In-cab cutting height adjustment – this would allow the cutting height to be adjusted without stopping the machine,or leaving the cab.It is easiest to do this hydraulically,so this development would probably be implemented at the same time as the previous one.

• The implementation of any or all of these improvements depends on the continued commercial success of the machine, but with Australia just over the pond, this is not unlikely.