Rotating bodies like grindstones, fly-wheels, etc., are built to run at a certain maximum speed. If this speed is exceeded the body may fly to pieces, as there is a tendency for particles of a rotating body to fly off in straight lines. The force that causes this movement away from the center of gravity is, as we have noted, called centrifugal force. This force is overcome by the cohesive force of the material that composes the fly-wheel, or the adhesive material that holds the particles of the grindstone together. This cohesive or resisting force is called centripetal force and is directed toward the center.
The principle of centrifugal force is utilized to great advantage in the construction of hydroextractors, i.e., machines designed to throw off the water contained in dyed or scoured fabrics, in sugar in a liquid state, and in bolts and nuts or other small metal parts. All centrifugal machines operate on essentially the same principle. Figures 37 and 38 show a machine designed to extract the liquid from solid or semisolid matter by centrifugal force; this type of machine is known as a chip wringer. Figure 37 shows the basket about to be lowered into its casing; the machine is then ready for use. Figure 38 shows the machine open with the basket inverted, the material having been dumped out.
Fig. 37. - Centrifugal Machine. Showing basket about to be lowered into casing.
The operation of the chip wringer is comparatively simple. Suppose the liquid is to be extracted from a pile of bolts and nuts. The bolts and nuts are placed in the basket, suspended above its casing by means of a device. The basket is then lowered into its casing. When in this position, ready to be set in motion, there is a very narrow slit between the rim of the basket and the casing. This slit is so narrow that, although liquid can readily flow through it, the passage of any solid particles is prevented.
The basket is then set in motion and made to revolve at a high speed. The centrifugal force thus generated forces the bolts and nuts to the sides of the basket and throws off the liquid. (The amount of centrifugal force generated increases with the revolutions per minute [R. P. M.] of the basket.) All the water thrown off is ejected from the basket through the narrow slit just described. When all the water has been removed in this way, the machine is stopped. The basket is then raised, carried along to a convenient place, and dumped.
Fig. 38. - Inverted Basket of Centrifugal Machine.