Principal units of the combine

(continued)

III. Threshing Unit. Threshing removes the grain or seed from the head or pod. The major parts of this unit are the cylinder, the concaves and the grates.

A. CYLINDER: three different types are used (Fig. 2).

(a) Spike-tooth. This is the oldest type: it was used in stationary threshers that preceded the combine. It has proved quite satisfactory for threshing most grain crops. The spike teeth, carried in the transverse bars of the cylinder, thresh out the grain as they revolve through similar teeth in the concaves.

(b)

Fig. 2. Three types of cylinders: (a) spike-tooth cylinder, (b) rasp-bar cylinder, (c) angle-bar cylinder (USDA).

(b) Rasp-bar. This type has transverse bars with grooved metal faces. These grooves are cut diagonally, in opposite directions, across adjacent bars. Threshing is done by the rasping action between the cylinder bars and the solid concave bars below the cylinder.

(c) Angle-bar. The spiral bars of this cylinder are rubber-faced, the rubber being vulcanized to the metal. They flail out the grain between the revolving cylinder bars and the stationary shelling plate and stationary block rubber concaves.

B. CONCAVES AND GRATES. These stationary bars and open grates extend across the full width of the cylinder (Fig. 4). They are located below the cylinder and surround about one-quarter of its circumference. Hence they are called "concaves."