Machine study of a Disc Mill

DISC MILL:-

Disc mill can be for secondary or fine crushing but its use is limited to special applications only because capacity is low and it is not suitable for all types of ores economically.

Disc mills are popular in West Africa and the Sudan and operate with a greater component of shear than compression. A disc mill consists of a circular chamber made of cast iron or steel within which two Discs with a narrow gap between them are mounted face to face. The discs are grooved in order to provide a shear mechanism.

When grains are introduced into the centre of the mill, the discs shear the grains between them. One of the discs rotates and the grains revolve, working their way to the outer edge of the disc before dropping by gravity into a holding sack below.

The grains lodge in the rotating disc and are sheared by the grooves in the opposing Disc. As the grains move to the edges of the Discs, the grooves become shallower and reduce the size of the grains. The design of grooves follows a very old style developed for stone mills several thousand years ago.

Discs are usually about 200–300 mm in diameter. Discs are normally aligned in a vertical direction, but horizontal alignment is more convenient when the mill is run by a diesel engine. Disc mills can run as fast as possible but normally at about 2 500–3 500 revolutions/minute, as overheating of the discs limits the speed of the mill.

Frictional heating imposes power limits. For example, a Disc mill with 300 mm Discs cannot be driven by an engine with more than 12 kW. However, the speed of mill is not a critical factor to the mechanism of

grinding. Disc mills operate more effectively with soft and moist grains that shear easily than with hard and brittle grains. It is common in West Africa to add water at the time of grinding. The milled product has to be used very quickly in order to prevent fermentation.

The fineness of the flour ground is adjusted by increasing the pressure on the grain by narrowing the gap between the discs. This is done with a simple hand wheel connected to the outer disc by a shaft. The mill should not be run empty because grains in the mill are needed in order to lubricate the action and, thus, prevent wear. Excessive wear is caused when the discs come into contact with each other. A fine flour or meal from a disc mill is obtained by re-circulating the product in the mill for a second or third grind.

CONSTRUCTION:-

A disc consists of two saucer-shaped discs with their surface having specially shaped grooves the depth of which reduced towards the circumference. The discs are face to face mounted vertically or horizontally and revolve at different speeds and in the opposite directions. In most designs one of the discs is driven while the other flutters or gyrates during revolving. Our laboratory model has heat treated mechanite metal discs mounted vertically one revolving in a planetary manner always having a proper curvature with relation to the other which is stationary.

Like other crushing machines, a disc mill has not of the discs spring-loaded through a screw mechanics that helps in adjusting the set and also provides safety against un crushable lumps.

The material is fed through a hopper at the top and falls into the axial conic between the discs during revolving. Due to the centrifugal force the feed is pushed through the taper grooves towards the periphery and gets crushed progressively. the product is finally discharged peripherally and collected in a peripheral receptacle.

Side View:-