quality:correction
The operating principle

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

Power requirements over the years have increased due to larger capacities and some processes such as rotary threshing and straw chopping take considerable power. This is sometimes supplied by a large tractor in a pull-type combine, or a large gasoline or diesel engine in a self-propelled type. A frequent problem is the presence of airborne chaff and straw, which can accumulate causing a fire hazard and to radiators which can become plugged. Most machines have addressed these problems with enclosed engine compartments and rotary centrifugal inlet screens which prevent chaff buildup.

First, the header, described above, cuts the crop and feeds it into the threshing cylinder. This consists of a series of horizontal rasp bars fixed across the path of the crop and in the shape of a quarter cylinder. Moving rasp bars or rub bars pull the crop through concaved grates that separate the grain and chaff from the straw. The grain heads fall through the fixed concaves. What happens next is dependent on the type of combine in question. In most modern combines, the grain is transported to the shoe by a set of 2, 3, or 4 (possibly more on the largest machines) augers, set parallel or semi-parallel to the rotor on axial mounted rotors and perpendicular on axial-flow combines.

In older Gleaner machines, these augers were not present. Those combines are unique in that the cylinder and concave is set inside feederhouse instead of in the machine directly behind the feederhouse. Consequently, the material was moved by a "raddle chain" from underneath the concave to the walkers. The clean grain fell between the raddle and the walkers onto the shoe, while the straw, being longer and lighter, floated across onto the walkers to be expelled. On most other older machines, the cylinder was placed higher and farther back in the machine, and the grain moved to the shoe by falling down a "clean grain pan", and the straw "floated" across the concaves to the back of the walkers.

Since the Sperry-New Holland TR70 twin-rotor combine came out in 1975, most manufacturers have combines with rotors in place of conventional cylinders. However, makers have now returned to the market with conventional models alongside their rotary line-up. A rotor is a long, longitudinally mounted rotating cylinder with plates similar to rub bars (except for in the above-mentioned Gleaner rotaries).

There are usually two sieves, one above the other. The sieves are basically metal frames that have many rows of "fingers" set reasonably close together. The angle of the fingers is adjustable, to change the clearance and thereby control the size of material passing through. The top is set with more clearance than the bottom to allow a gradual cleaning action. Setting the concave clearance, fan speed, and sieve size is critical to ensure that the crop is threshed properly, the grain is clean of debris, and all of the grain entering the machine reaches the grain tank or 'hopper'. (Observe, for example, that when travelling uphill the fan speed must be reduced to account for the shallower gradient of the sieves.)

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

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

When the straw reaches the end of the walkers it falls out the rear of the combine. It can then be baled for cattle bedding or spread by two rotating straw spreaders with rubber arms. Most modern combines are equipped with a straw spreader.

Rather than immediately falling out the rear of the combine at the end of the walkers, there are models of combine harvesters from Eastern Europe and Russia (e.g. Agromash Yenisei 1200 1 HM, etc.) that have "straw catchers" at the end of the walkers, which temporarily hold the straw and then, once full, deposit it in a stack for easy gathering.