Combine harvesting combines several operations into one: cutting the crop, feeding it into threshing mechanism, threshing, cleaning, and discharging rain into a bulk wagon or directly into a bags. Straw is usually discharged behind the combine in a windrow.
Before modern-day machines were developed, agricultural workers had to harvest crops by carrying out a series of laborious operations one after another. First they had to cut down the plants with a long-handled cutting tool such as a scythe. Next, they had to separate the edible grain from the inedible chaff by beating the cut stalks—an operation known as threshing. Finally, they had to clean any remaining debris away from the seeds to make them suitable for use in a mill. All this took a lot of time and a lot of people.
Thankfully, modern combine harvesters do the whole job automatically: you simply drive them through a field of crops and they cut, thresh, and clean the grains all by themselves using rotating blades, wheels, sieves, and elevators. The grain collects in a tank inside the combine harvester (which is periodically emptied into carts pulled by tractors that drive alongside), while the chaff and the stalks spurt from a big exit pipe at the back and fall back down onto the field.
The combine header cuts and gathers the crops from the field. Most headers have rotating teeth or blades that cut the crop and then a mechanism to move the cut crop to the feederhouse.
Modern headers are typically controlled with hydraulics, which enables them to raise, lower and angle. The ability to move the header is very important because not all fields are flat and even.
Headers are typically crop-specific, and most are detachable. The most common headers include corn headers, draper headers, and knife bar headers.
Most combine headers are measured by their width - wider headers are able to harvest more field at a time. Most combine headers are too wide to travel on open roads - as a result, the combine header must be detached and trailed behind the machine when traveling on a road shared with other vehicles.
The widest combine header in the world is 60 feet wide and is manufactured by MidWest Durus Premium.
Most of the lower belly of the combine is dedicated to separating the grain from everything else. The basic mechanical function of the separation is similar to panning for gold - mixed contents are moved over a porous surface while it shakes. The large materials (stalks, leaves, roots, etc) then separate from smaller materials. Smaller materials such as grains, fine particles and dust fall down into a collection area.
The combine blows fast-moving air over the collection area to remove very lightweight items such as dust and other fine particles. The remaining material is mostly grains.
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