In a power thresher, the separation of grain from its stalk is carried out by passing the crop between a revolving drum and a stationary concave. The impact of the drum striking the grain against the concave beats the grain from the ears. In the shape of a half-cylinder, the concave forms a cradle below the threshing drum. It is either solid or grated and is always made of very strong wrought iron or heavy steel.
The revolving drum or cylinder carries a number of steel spikes or fine rasp bars as threshing teeth. Spiked teeth are often found in combines and threshers designed for rice processing. Because most small threshers are designed in rice-producing countries, they generally come equipped with spike-tooth drums which are not as efficient for threshing wheat. Rasp bar drums with fine-tooth spacings on the threshing bar are generally preferred for separating finer seeds from chaff. The threshing body of most modern combines comes equipped with a rasp bar, but both types of cylinders are usually available interchangeably.
Balance is an important factor. A thresher should he set on level ground with the drum carefully balanced to avoid vibration. If it's not level, the movement of grain across the width of the drum is uneven and threshing efficiency is impaired. The gap between the threshing drum and the concave is adjustable, depending upon the grain to he threshed. If the gap is too wide, the grain passes out of the machine with the straw, while too close a setting will break or bruise the grains.
Some small machines operate on a principle similar to the treadle rice threshers, with the crop being held against the revolving drum. But generally the whole crop is passed through the thresher. Spike-tooth drums are found particularly in Asia, where they are favored because they produce thoroughly crushed straw, or bhusa, which is fed to animals.
Threshers without cleaning equipment are often favored by small farmers because they combine high rates of threshing with low power requirements. They can often be driven from the power take-off (PTO) shaft of a two-wheeled tractor or by a gasoline or electric engine. Winnowing is done at the farmstead during slack periods. At this point, threshing is complete.
Plans for this build-it-yourself thresher may be obtained by
Organic Gardening Readers'- Service 33 E. Minor St. Eromaus, PA 18049 Enclose a stamped, self-addressed envelope.
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