How Does a Combine Harvester Work?

The name "combine" comes from the combination of the harvesting, threshing and grain cleaning functionality in a single machine--the combine harvester. Before agricultural equipment to harvest large crops was invented, harvesting a crop of grain was labor-intensive and time-consuming. First workers had to cut the grain with a scythe or long-handled cutting tool. Next came threshing, when the cut stalks of grain were beaten to separate the grain from the chaff. Finally, the grain had to be cleaned of debris and made ready for milling. Each stage of the harvest required many hours of work, and made it difficult for small farms to make ends meet.

Relief came circa 1784 with the invention of the threshing machine by Scotsman Andrew Meikle. These horse or steam powered machines were fed cut stalks of grain by hand, and automated the process of separating grain from chaff. The machines were so much more efficient than threshing by hand that farm workers were replaced by machines in large numbers, which was partly responsible in 1830 for the Swing Riots in the United Kingdom. Mechanization of farms also contributed to the migration of the population from rural to urban areas, since farm operations required less and less labor and supported far fewer workers. Mechanization additionally put pressure on smaller farms to compete with growing numbers of commercial farming operations with large budgets and the best equipment, and today far fewer farms produce much more food and agricultural products than 200 years ago.

The modern combine harvester is in many ways quite similar to the first threshing machines. The combine is a bit like a conveyor belt that takes in stalks of grain and runs them through a series of processors. First to meet the crop is the header, which uses a pair of crop dividers to funnel shafts of grain into the combine. Different types of grain require different headers, and they can be interchanged and customized. The header pushes stalks of grain into a revolving wheel, called a pickup reel, that is strung with bars and teeth to grip the crops. The pickup reel pushes the stalks down to be cut in the correct position by the cutter bar, which runs the width of the combine just behind the wheel.

Once the crop is cut, a conveyor takes the crop into a threshing drum. A threshing drum can be configured and designed in different ways to suit particular crops, and a certain amount of scholarly work has gone into designs that yield more edible product and less chaff and debris. The basic idea is unchanged from the workers who were threshing by hand 200 years ago--beating the stalks to release the grain and sifting it from the chaff. The concept has been modified for harvesting all kinds of other crops, like corn and peas. The grain falls through a sieve and is collected in a tank, while the shafts and other chaff are transported on a conveyor called a straw walker, and dropped behind the combine as it moves. Returning chaff to the earth helps to retain the soil nutrients needed by the crop year after year. The combine can also be connected to a baling machine, which rolls the straw into bales to be used elsewhere on the farm.

Finally, when the tank holding the finished grain is full, a trailer is pulled up to the side of the combine and loaded with grain so the combine can continue the harvest. You can see, now, how the combine harvester must have changed farming when it was first introduced. In 1900, 38 percent of the workforce in the United States held agricultural jobs, but 100 years later, only 3 percent of Americans were working in the agricultural industry.