An agricultural machine which cuts, threshes, and cleans all types of cereals, oilseeds, and legumes. Most combines are now self-propelled, and are equipped for handling grain and seed in bulk. Cereal crops, including pulses, oilseed, and other seed crops, are now universally harvested by the combine harvester. The machine combines four functions: it gathers the crop, threshes the seed from the ear, separates the seed from the straw, and cleans the seed of the chaff, weed seeds, and other unwanted material. There are two major types of combine: the most common type is based on a drum and concave threshing mechanism; the axial flow type has higher work rates, but is more suited to dry conditions such as those experienced in parts of the USA. Forage harvesters are used to cut and gather grass and other crops used for animal feed, afterwards chopping and blowing them into high-sided trailers.
The combine harvester, or simply combine, is a machine that harvests, threshes, and cleans grain plants.
History
The combine was patented in 1834 by Hiram Moore, the same year as Cyrus McCormick was granted a patent on the mechanical reaper.
Early combines, some of them quite large, were drawn by horse or mule teams and used a bull wheel to provide power. Tractor-drawn, PTO-powered combines were used for a time. These combines used a shaker to separate the grain from the chaff and straw-walkers (grates with small teeth on an eccentric shaft) to eject the straw while retaining the grain. Tractor drawn combines evolved to have separate gas or diesel engines to power the grain separation. Today's combines are self-propelled and use diesel engines for power. A significant advance in the design of combines was the rotary design. Rotary combines were introduced in the late 1970s.
Combine Process
Header
Combines are equipped with removable heads (called headers) that are designed for particular crops. The standard head, sometimes called a grain platform (or platform header), is equipped with a sickle bar mower, and features a revolving reel with metal or plastic teeth to cause the cut crop to fall into the head. The grain platform is used for many crops, including grain, legumes, and many seed crops.
Wheat heads are similar except that the reel is not equipped with teeth. Draper heads keep the crop orientation uniform, feeding grain headfirst into the throat, which allows slightly more efficient threshing.
Dummy heads or pick-up headers feature spring-tined pickups, usually attached to a heavy rubber belt.
While a grain platform can be used for corn, a specialized corn head is ordinarily used instead.
Occasionally rowcrop heads are seen that function like a grain platform, but have points between rows like a corn head.
Self propelled Gleaner combines could be fitted with special tracks instead of tires to assist in harvesting rice. Some combines, particularly pull type, have tires with a diamond tread which prevents sinking in mud.
Conventional combine
The cut crop is sent up the feeder house by a series of chains, then dropped into the threshing cylinder.
Sidehill levelling
An interesting technology is in use in the Palouse region of the Pacific Northwest of the United States in which the combine is retrofitted with a hydraulic sidehill levelling system. This allows the combine to harvest the incredibly steep but fertile soil in the region. Gleaner, Case/International Harvester, John Deere and others all have made combines with this sidehill levelling system and local machine shops have fabricated them as an aftermarket add on.
The first levelling technology was Developed by Holt Co., a California firm, in 1891. still produces levelling systems exclusively for John Deere combines.
Sidehill levelling has several advantages. Without levelling grain and chaff slide to one side of separator and come through the machine in a large ball rather than being separated, dumping large amounts of grain on the ground. Case International produced the 453 combine which leveled both side-to-side and front-to-back thus enabling efficient threshing whether on a sidehill or climbing a hill head on.
Secondarily, levelling changes a combine's center of gravity relative to the hill and allows the combine to harvest along the contour of a hill without tipping over. The danger is very real on the steeper slopes of the region and it is not uncommon for combines to roll on extremely steep hills.
Currently sidehill levelling is on the decline with the advent of huge modern machines which are more stable due to their width. These modern combines use the rotary grain separator which makes leveling less critical. Most combines on the Palouse are equipped with dual drive wheels on each side to stabilize the machine.
Maintaining threshing speed
Another technology that is sometimes used on combines is a continuously variable transmission.
Self-propelled combines started with standard manual transmissions that provided one speed based on input rpm. Deficiencies were noted and in the early 1950's combines were equipped with what John Deere called the "Variable Speed Drive". An extra control was provided to the operator to allow him to speed up and slow down the machine within the limits provided by the variable speed drive system.
Later, as hydraulic technology improved, hydrostatic transmissions were introduced by Versatile Mfg for use on swathers but later this technology was applied to combines as well. This drive retained the 4 speed manual transmission as before, but this time used a system of hydraulic pumps and motors to drive the input shaft of the transmission.
Most if not all modern combines are equipped with hydrostatic drives. In fact, it was the downsizing of the combine drive system that placed these drive systems into mowers and other machines.
The Threshing Process
Despite great advances mechanically and in computer control, the basic operation of the combine harvester has remained unchanged almost since it was invented.
First of all the header, described above, cuts the crop and feeds it into the concave. Moving rasp bars or rub bars pull the crop through the concave and separate the grain and chaff from the straw.
Since the International 1440 and 1460 came out in the 1970s, combines have rotors in place of concaves. 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 that all of the grain entering the machine reaches the grain tank. ( 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.
When the straw reaches the end of the walkers it falls out the rear of the combine. Most modern combines are equipped with a straw spreader. Conventional Design
For a considerable time, combine harvesters used the conventional design, which used a rotating cylinder at the front-end which knocked the seeds out of the heads, and then used the rest of the machine to separate the straw from the chaff, and the chaff from the grain.
In the decades before the widespread adoption of the rotary combine in the late seventies, several inventors had pioneered designs which relied more on centrifugal force for grain separation and less on gravity alone. Advantages were faster grain harvesting and gentler treatment of fragile seeds, which were often cracked by the faster rotational speeds of conventional combine threshing cylinders.
It was the disadvantages of the rotary combine (which were increased power requirements and pulverization of the straw by-product) which prompted a resurgence of conventional combines in the late nineties. Conventional combines, which use straw walkers, preserve the quality of straw and allow it to be baled and removed from the field.
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