by Chris Woodford. Last updated: July 27, 2016.
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?
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 rest
of the plant (the chaff) is inedible and has to be discarded.
Photo: Right: 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).
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
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 spurts from a big
exit pipe at the back and falls back down onto the field.
Inside a combine harvester
There's an awful lot going on inside a combine harvester—gears,
blades, augurs (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: Close-up of the cutter on a John Deere combine harvester. Left: Looking from the front; Right: looking down from the cab toward the incoming crops. Photos by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
- 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 chaff (unwanted material) 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 straw chaff tumbles 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 chaff in the separator unit (green).
Artwork: A Gleaner harvester designed by Perren J. Hanson and patented on June 21, 1932. You can read more detail about how it works in US Patent #1,863,691: Combine harvester (via Google Patents).
Artwork courtesy of US Patent and Trademark Office.
Find out more
On this website
- Combine Harvesters: Theory, Modeling, and Design
by Petre Miu. CRC, 2014. If you think combine harvesters are simple, take a look at this fascinating introduction to the surprisingly complex science, technology, and mathematics lurking inside.
- An Illustrated History of Combine Harvesters by Jim Wilkie. Ian Allan, 2001. A 126-page guide ranging from early machines to the latest models.
- Combines & Harvesters: Photographic History by Jeff Creighton. Motorbooks, 1996. From horse-, steam-, and gas-powered combines to modern machines.
- Massey Tractors by C.H.Wendel and Andrew Morland. Motorbooks, 1992. Covers the history of Massey-Harris and Massey-Ferguson tractors, combines, and agricultural implements.
- Combines & Harvesters by Hans Halberstadt. Motorbooks, 1994. Another photographic guide to combines old and new.
- A Claas combine harvester in action: A short (2 minute video) of a Claas harvester with some interesting details if you watch closely. Notice how the drive lifts the header to turn the machine at the end of the row. See how the chaff is sprayed out evenly from side to side behind the machine?
- Case IH 9120 And Magnum 310 With Hawe Grain Cart: This slightly longer (6 minute video) shows a Case harvester in action. Notice the huge header again. At about 2:50, a tractor drives alongside and the harvester unloads, while continuing to harvest more crops.
- Guided tour of new S-Series combine cab: A John Deere demonstrator gives us a tour of a typical harvester's cab controls. There's even a touchscreen computer!
If you're looking for a really detailed description of how a combine works, patents are a good place to begin. Here are a couple you might find useful:
- US Patent #4,450,671: Combine harvester with modified feeder house by Mahlon L. Love, Deere & Company, patented May 29, 1984. A modern-style combine.
- US Patent #1,863,691: Combine harvester by Perren J. Hanson, Gleaner Combine Harvester Corporation, patented June 21, 1932. A Gleaner combine typical of those used in the period between the two world wars.
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.