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What is Winnowing?

Winnowing is the process of separating grain from a mixture of grain, chaff and other impurities in an air stream from a natural or artificial source

Where is the winnowing operation generally carried out?

It is done on the threshing floor where the entire harvested crop is threshed

How many people are required for winnowing operation?

Usually more than oone person is required

1. One persoon for pouring the threshed material with a basket from slightly above their own height

2. One person is required to collect the grains and keep the chaff away from mixing with the grains

What is to be done before starting the winnowing operation?

Before the winnowing is started, the threshing floor is thoroughly cleaned and sometimes plastered with cow dung, so that the soil may not mixed with the grain

How many times, winnowing operation is to be carried out for one crop?

Winnowing of crops like paddy, in which chaff is very small fraction is accomplished in one operation only

But the second winnowing becomes necessary for cops like wheat, barely, gram etc, in which bhusa (chaff) constitutes about 1/2 to 2/3 of the total weight of the mixture

What is the Principle of winnowing?

Separation is achieved by allowing the air stream to pass through the mixture falling vertically down

Density difference of the components of mixture, namely, heavier grain, lighter chaff and bhusa, is responsible for separation when suspended grain is in air steam

The grain being the heavier material gets deposited almost at the place of dropping whereas the lighter material (chaff) is blown away to a greater distance

How to do winnowing?

We can do winnowing with the help of the following instruments / devices which are  commonly adopted for winnowing process

1. Winnowing basket

2. Winnowing fan

3. Winnower

1. Winnowing basket

It is used by traditional and small farmers and when the produce is in low or manageable quantity

Winnowing baskets

The use of winnowing basket is the oldest and common method of winnowing in India

It makes use of the natural wind, which is very frequent and effective at the time of harvesting

For this purpose, a convenient place is selected, high enough from the general field level

The thresher grain is collected near the winnowing platform

One man stands on the platform with the basket full of grain to be winnowed

Keeping his hand up and continuously stirring the basket, very slowly, he drops the grain

As the grain falls, the broken straw and chaff, being lighter than the seed, are taken at a far distance by the blowing wind

The grain being heavier than the straw, falls very near to the platform

Thus two heaps are formed, one of grain and the other of straw, which are collected separately

2. Winnowing Fans

They are used when there are no proper wind velocity or direction of wind is wrong

They are also used when there is considerable quantity of produce

Winnowing fan hand operated

 

When the natural wind velocity is not adequate, artificial means are used to create a sufficiently strong air blast

Winnowing fans either manually operated or mechanically operated, are mostly used

Manually, operated fans of various types are available for use in India

Most often, the hand operated or pedal operated winnowing fans are used by farmers

In either case, the fan blades create a steady blast of air at the front side of the fan

One or more workers standing on the front side can drop the threshed material for effective separation

The distance where the threshed material is poured on the front side of the fan has the most important effect in winnowing

Some of the experiments performed with a pedal operated winnowing fan at about 1.8 m distance from the fan, along its central line

It was found that, the most effective cleaning can be obtained in that zone. Various type of hand operated and pedal operated winnowing fans are being manufactured by many firms in India

The number of blades on their impellers is either three or four

The diameter of the impellers varies from about 0.90 to 1.25 m

Mostly, the blades are made of mild steel sheet and their frames are made of either wood or welded steel or a combination of the two

In order to increase the rotational speed, various driving mechanisms are employed, namely sprocket and chain, V-belt pulleys and single or double reduction gears

All these fans are without casing around the impeller and they are expected to be operated in the speed range of 200-350 rpm

3. Winnower

When huge quantity of produce is there, winnower is used

Paddy winnower

The winnower consists of wooden or angle iron frame, housing, blower, feed hopper, feed and purity adjustment mechanisms, grain outlet, immature grain outlet, chaff and dust outlets and blower hand wheel with gears and pulley arrangement

Walls of the feed hopper taper towards inside from all sides and ensures gravity flow. Flow of the mixture is regulated by the feed rate adjustment mechanism, which makes the mixture to fall as a thin stream perpendicular to the air current, is essentially a gate obstructing the flow below the feed hopper

Blower consists of four curved blades mounted horizontally on a shaft with two ball bearings

The shaft carries a small gear (pinion) at its one end that meshes with a bigger gear

The bigger gear is rotated by handle provided at its periphery

On the other end of the shaft, a V-belt pulley is attached for possible mechanized operation and also it acts as a flywheel

Ring gear has 66 teeth and the pinion has 20 teeth giving a speed ratio of 3.3: 1 or 4:1 i.e., the diameter of big gear is 3.3 or 4 times bigger than the pinion

For one revolution of bigger gear will give four revolutions to the blower

Thus, if the handle is turned at a speed of 50 revolutions per minute (rpm), the blower will rotate at a speed of 165 or 200 rpm

The sieve is placed in front of the fan at the lower end of the winnower

A set of sieves are supplied with machine to suit the different sizes of grains

These sieves have holes of 12, 8, 6, 4, 3 and 2 mm diameter

The sieve is reciprocated by an eccentric roller which gets its motion from the axle of the blower

The reciprocating action of the sieve permits the grains and dirt particles to fall down, while the straw is blown out

Working of winnower

As the mixture falls into the air current, the lighter materials and chaff are blown through the opening for chaff and husk outlet by the air blast, while the good grains being heavier fall directly below and guided out by the grain outlet

Immature grains that are relatively lighter will be carried by the blast a little away from the good grain outlet and collected by the immature grain outlet

Purity adjustment mechanism is a wooden plank kept in between the grain and immature grain outlets in the air flowing path and is raised or lowered by a rack and pinion arrangement

It diverts major portion of the immature grains to the good grain outlet when lowered and immature grains are also collected; when raised the free flowing air will carry some quantity of good grains also to the immature grain outlet but improves the purity of grains in the good grain outlet.

Proper rpm of the blower and feed rate and purity adjustments are essential for efficient winnowing of the grains from the other lighter fractions

Further reading and references

Handbook of Agriculture

The Question of the Transmission of the Rotary Winnowing Fan from China to Europe: Some New Findings, Hans Ulrich Vogel, 8th International Conference on the History of Science in China

Münzenberg, Hessen. Chapel and Palas (G.Binding, Burg Münzenberg, 1962)

 

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