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In 1800, something like 90 percent of the entire US population was employed working the land; fast-forward 200 years and you'll find only 2 percent of people are now working this way. What caused that amazing change in society? One important factor was the development of huge, automated machines such as combine harvesters that made each agricultural worker vastly more productive. Let's take a closer look at how they work!
Photo: A typical combine harvester, or "combine," made by John Deere; other makes include Case IH, Gleaner, New Holland and Claas. You can see how wide the header (front cutting mechanism) is compared to the main body of the machine. The biggest combines have headers about 12m (40 ft) wide!
Photo: Left: Top: The enormously wide header at the front of a combine makes it impossible to drive down a narrow country lane, so how do you move it from field to field? Bottom: Fortunately, the header can be removed and towed on a special trailer, sideways behind a tractor. It's a simple job that takes about two minutes.
What does harvesting involve?
Photo: Wheat is one of the world's most important cereal crops. All we eat are the little grains at the top of each stalk (shown in the small piles alongside). Photo by Scott Bauer courtesy of US Department of Agriculture/Agricultural Research Service (USDA/ARS).
The crops we grow in our fields, such as wheat, barley, and rye, are only partly edible. We can use the seeds at the top of each plant (known as the grain) to make products like bread and cereal, but the dry coatings of the seeds (the chaff) are inedible and have to be discarded, along with their stalks.
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.
Photo: Combines have interchangeable headers so they can harvest all kinds of crops. This John Deere is fitted with a corn header, a special type of header with large, sturdy prongs for harvesting corn cobs. Left: Looking from the front; Right: looking down from the cab toward the incoming corn crops. Photos by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Inside a combine harvester
There's an awful lot going on inside a combine harvester—gears, blades, augers (screws that move cut crops), conveyors, belts, levers, and wheels—so we've vastly simplified everything to make it easier to follow. Roughly speaking, here's how a combine harvester works:
- Cereal crops are gathered in by the header at the front, which has a pair of sharp pincers called crop dividers at either end. Generally speaking, the wider the header, the faster and more efficiently a harvester can cut a field. Different headers are used for cutting different crops; the header is often hydraulically powered and can be raised, lowered, and angled in different ways from the cab. The header can be removed and towed behind the harvester lengthwise so it can fit down narrow lanes.
- A slowly rotating wheel called the reel (or pickup reel) pushes the crops down toward the cutter. The reel has horizontal bars called bats and vertical teeth or tines to grip the plant stalks.
- The cutter bar runs the entire length of the header underneath the reel. Its teeth (sometimes called mowing fingers) open and close repeatedly to cut off the crops at their base, a bit like a giant electric hedge cutter sweeping along at ground level.
Photo: Left: The wide, sweeping header on a John Deere combine harvester. Right: A closeup view. Photo by Carol M. Highsmith courtesy of Gates Frontiers Fund Colorado Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
- Behind the cutter bar, the cut crops are fed toward the center by spinning augurs (screws) and travel up a conveyor to the processing mechanism inside the main part of the combine.
- A threshing drum beats the cut crops to break and shake the grains away from their stalks.
- The grains fall through sieves into a collecting tank below.
- The unwanted material (chaff and stalks) passes along conveyors called straw walkers toward the back of the machine. More grain falls through into the tank.
- When the grain tank is full, a tractor with a trailer on the back pulls alongside the combine. The grain is carried up from the tank by an elevator and shoots out of a side pipe (sometimes called the unloader) into the trailer.
- The unwanted stalks and chaff tumble from the back of the machine. Some combines have a rotating spreader mechanism that throws the straw over a wide area. Sometimes the straw is baled up by a baling machine and used for animal bedding.
Photo: Unloading the grain tank of a combine harvester into a trailer pulled by a tractor driven alongside. The combine's header (cutting blade) has been raised right up so the driver can circle around the parked trailer and fill it up evenly. Look closely at the header and you'll see both the reel (black) and the cutter bar (green) underneath it.
Have combine harvesters always looked like this?
Not exactly! Here's a drawing of a Gleaner harvester from the 1930s, which I've colored and simplified into four basic parts:
- At the front of the machine, on the right, we have the reel (red) that draws the crops in.
- Next we have the cutter unit (orange), including the scythe (blue) that chops the crops.
- Once the crops are cut, they're smashed apart in the thresher (yellow).
- All that remains is to split the wheat from the unwanted stalks and chaff in the separator unit (green).