Fig. 18. Mechanized grain harvesting: The invention of the combine, a unitized mechanism for harvesting grain and freeing it of chaff and detritus, has had a tremendous impact on agriculture. A single machine substitutes for hand cutting, shocking, flailing (threshing), winnowing, and associated procedures of primitive agriculture. (top) In the self propelled wheat combine an adjustable reel holds grain bearing stalks against the basal cutter bar; the auger guides cut stems to a central beater and eventually to the rasp bar cylinder that rotates within the open grate concave (just above the front wheels). 90% of the grain separation occurs at the concave; the grain is abraded free from the seed head by the cylinder, and falls through open slots in the concave to conveyor belts below. Additional mechanisms in the upper rear part of the combine further agitate stems and chaff to retrieve remaining grain, the spent straw being issued from the rear of the machine. Screening mechanisms in the lower rear part of the machine clean the grain further by siftings and air blasts before it is elevated to the grain tank (behind the operator). The clean grain can be unloaded through the unloading auger (behind the operator) directly into trucks drawn alongside the combine. (bottom) The corn combine, similar in principle, has a gathering mechanism designed to accommodate rows of corn. Ears are freed from stems by a snap bar mechanism (instead of cutting the whole stalk). The rasp bar cylinder and concave are designed to shell the corn from the cob. Grain is retrieved and cleaned as in the wheat combine. [Courtesy John Deere Co.]