Combine harvester

The combine harvester, or simply combine, is a machine that harvests, threshes, and cleans grain plants. The desired result is the seed (such as canola or flax) or grain (such as oats, wheat, or rye); a byproduct is loose straw, the remaining husk of the plant with all nutrients removed. The combine was patented in 1834 by Hiram Moore, the same year as Cyrus McCormick was granted a patent on the mechanical reaper.

The Threshing Process

Despite great advances mechanically and in computer control, the basic operation of the combine harvester has remained unchanged almost since it was invented.

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First of all the header, described above, cuts the crop and feeds it into the concave. This consists of a series of rasp bars fixed across the path of the crop and in the shape of a quarter cylinder, guiding the crop upwards through a 90 degree turn. Moving rasp bars pull the crop through the concave and separate the grain from the straw. The grain heads fall through the fixed rasp bars onto the sieves. The straw exits the top of the concave onto the straw walkers.

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There are usually two sieves, one above the other. Each is a flat metal plate with holes set according to the size of the grain. The holes in the top sieve are set larger than the holes in the bottom sieve. Anything that falls through the bottom sieve is assumed to be grain, and is transported to the grain tank.

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The sieves are mounted at a slight incline and shake, further separating the grain. A powerful fan blows from the front of the sives to remove chaff (wheat) or haynes (barley). If the fan is set too high it will blow grain off the back of the sives and out of the combine. If it is set too low then chaff/haynes will be present in the grain.

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Heavy material, e.g., unthreashed heads, fall off the front of the sieves and are returned to the concave for re-threshing.

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The straw walkers are located above the sieves, and too have holes in. Any grain remaining attached to the straw is shaken off and falls onto the top sieve.

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When the straw reaches the end of the walkers it falls to the floor. A straw chopper may be fitted if the straw is unwanted.

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Setting the concave clearance, fan speed, and sieve size is critical to ensure than the crop is threshed properly, the grain is clean of debris, and that all of the grain entering the machine reaches the grain tank. ( Observe, for example, that when travelling uphill the fan speed must be reduced to account for the shallower gradient of the sieves.)

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