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Threshing and shelling

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Definition

Threshing or shelling consists of separating the grains, or the shells in the case of groundnuts, from the portion of the plant that holds them.

This separation, done by hand or machine, is obtained by threshing, by friction or by shaking the products; the difficulty of the process depends on the varieties grown, and on the moisture content and the degree of maturity of the grain.

Threshing or shelling operations

Threshing or shelling operations follow the harvest and whatever pre-drying of the crop is undertaken.

These operations may be carried out in the field or on the farm, by hand or with the help of animals or machines.

Depending on the influence of agronomic, economic and social factors, threshing or shelling is done in different ways:

As we have observed above, the operations of harvesting and threshing or shelling can be carried out simultaneously, by combine-harvesters or picker-shellers.

Whatever the system used, it is very important that threshing or shelling be done with care. Otherwise, these operations can cause breakage of the grains or protective husks thus reducing the product's quality and fostering subsequent losses from the action of insects and moulds.

Transport of the product from the field to the threshing or shelling place must also be handled with special care, since it can bring about severe losses.

 

Hand threshing

One of the simplest systems for threshing rice is to pick up the sheaf of rice and strike the panicles against a hard surface.

Another frequently-used method of threshing rice is to trample it underfoot.

Threshing of rice, as well as of sorghum, beans and groundnuts, can be done by striking sheaves spread out on a threshing-floor with a flail or a stick.

The threshing-floors on which the sheaves are spread must have a hard, clean surface.

By using one of these methods of hand-threshing, a worker can obtain 15 to 40 kg of product per hour.

 

Hand shelling

The easiest traditional system for shelling maize is to press the thumbs on the grains in order to detach them from the ears.

Another simple and common shelling method is to rub two ears of maize against each other.

These methods require a lot of labour, however. It is calculated that a worker can hand-shell only a few kilograms an hour.

Shelling of maize, as well as of sunflowers, can be more efficiently accomplished by striking a bag full of ears or heads with a stick.

Maize and sunflowers can also be shelled by rubbing the ears or heads on a rough surface.

Small tools, often made by local artisans, are sometimes used to hand-shell maize.

With these tools, a worker can shell 8 to 15 kg of maize an hour.

 

Threshing with animals or vehicles

If draught animals are available and there are large quantities of rice, threshing can be done by driving the animals (harnessed, in that case, to threshing devices) over a layer of sheaves about 30 cm thick.

This operation, which is also called "treading out", can equally well be accomplished with vehicles.

This method of threshing rice is adopted in some Asian countries, using a tractor for power instead of draught animals.

Paddy is obtained by running the tractor twice over sheaves of rice that are spread in layers on a circular threshing-floor 15-18 m in diameter. The sheaves must be turned over between the two passages of the tractor.

If operations are alternated between two contiguous threshing-floors, yields of about 640 kg/in can be obtained.

 

Threshing with hand-driven machines

Machines driven by a manual device or a pedal are often used to improve yields and working conditions during threshing.

By means of the handle or pedal, a big drum fitted with metal rings or teeth is made to rotate.

The rice is threshed by hand-holding the sheaves and pressing the panicles against the rotating drum.

The speed of the threshing-drum must be kept at about 300 revolutions per minute (rpm).

The hand-held sheaves must all be of the same length with the panicles all laid in the same direction, and the grains must be very ripe and dry.

The machine must be continuously and regularly fed, but without introducing excessive quantities of product.

If the paddy obtained contains too many unthreshed panicles and plant residues, a second threshing must be followed by an effective cleaning of the product.

Use of these threshing machines may require two or three workers.

Depending on the type of machine, the skill of the workers and organization of the work, yields can be estimated at a maximum of 100 kg/in.

 

Maize-shelling with hand-operated machines

Manual shellers, which are relatively common and sometimes made by local artisans, permit easier and faster shelling of ears of maize.

These come in several models, some of them equipped to take a motor; they are generally driven by a handle or a pedal.

Use of manual shellers generally requires only one worker.

With yields of from 14 to 100 kg/in, they are well-adapted to the needs of small-scale production.

 

Threshing or shelling with motorized equipment

In describing operations of threshing or shelling with motorized equipment, the principal reference will be to motorized threshing-machines.

Although they are gradually being replaced by combine-harvesters, these machines still have an important place in the post-harvest production process, especially for their convertibility.

By the simple replacement of a few accessories and the appropriate changes in settings, these machines can treat different kinds of grain (e.g. rice, maize, sorghum, beans, sunflowers, wheat, soybeans, etc.).

Equipped with a rotating threshing-drum (with beaters or teeth) and a stationary counter-thresher, these machines often have devices to shake out the straw and to clean and bag the Brain.

Whether self-propelled or tractor-drawn, these threshers are often mounted on rubbertyred wheels for easy movement to the field.

The use of motorized threshers may require two or three workers.

Yields depend on the type of machine, the nature and maturity of the grain, the skill of the workers and organization of the work, and they can vary from 100 to 5 000 kg/in.

Just as a guide, the following table shows the principal technical features of the multipurpose BAMBA motorized thresher (Bourgoin-France).

FEATURES GRAINS
MAIZE RICE SORGHUM BEAN
Threshing speed (rpm) 800 800 1200 600
Yields (kg/h) 1500 to 2000 450 to 600 450 to 600 450 to 600

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