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Combine harvesters are used to harvest various crops. The combine harvester cuts the crop, threshes it, and separates the grain from the straw and chaff. The mechanism of a combine harvester can be divided into three sections. They are cutting, threshing and finally separating the grain from the straw and chaff. To cut the crop a reciprocating tyre cutter bar is used. There is a divider at each side of the cutter bar. It separates the crop to be cut from that which will be left for the next round. The crop is cut while held against the cutter bar by the reel. After the crop is cut, the reel directs it to the cutter bar platform. The reel is one of the main parts of a combine harvester. It has tines which can be angled to provide better cutting of the crops. A large auger moves the crop to the centre of the platform. By means of tines the auger directs the crop to the main elevator which lifts the crop to the threshing mechanism. The threshing mechanism consists of a front beater, a heavy rotating drum, a concave, and a rear beater. The main elevator is used to lift the crop to the front beater. It delivers the crop to the drum and concave. The front beater increases the speed of the crop as it moves to the drum. Some combines do not have a front beater. In these combines the work of the front beater can be done by the main elevator. Threshing takes place between the drum and concave. There are spaces between the concave bars, so the threshed grain can fall through on the grain pan. To reduce the speed of the crop as it leaves the cylinders the task of the rear beater. The rear beater is the part of the threshing mechanism which both reduces the speed of the crop and directs it to the separating mechanism. To separate the grain from the straw is the main function of this mechanism. The separating mechanism consists of two parts: the straw walkers and the grain sieves