Buchanan's Straight-line Crushing Rolls are a modification of the ordinary type of Cornish rolls. The journals of the sliding roll are cast in one piece, the standards of the journals being connected by a strong cross-bar, strengthened by heavy ribs. There are heavy slides on the bed frame, and the entire top of the latter is planed, so that the sliding roll is compelled to move in a straight line, its face being always parallel with that of the fixed roll, when giving to pass a piece of material too hard to crush, such as a drill point. The makers claim that this straight-line movement causes the rolls to wear more evenly, and avoids end play of the rolls.
Bowers' Rolls are constructed upon the principle of ordinary Coruish rolls, differing only in the shape of the roll faces. With careless and improper feeding, the faces of cylin drical rolls wear concavely. To compensate for this, the Bowers rolls are made with the face of one concave and the other convex. Rolls properly fed wear regularly enough, however, and as the advantages gained by the Bowers system are more than balanced by the disad vantages, they have not come into general use.
The Heberle Mill, one of the standard types of fine-crushing machines. consists of a cyl indrical drum, in which is a large plate slowly rotating on a horizontal axis, and two small, rapidly revolving runners, each having a plane annular grinding face and a coned center. Both of the runners are set on the same side of the turning plate, very close to and parallel with it, and in the fields of its lower quadrants. The large turning plate is pierced by a number of holes set radially and in a ring, so as to form in it a circle of apertures. Ore, supplied by launders to one side of the plate, passes, during the revolutions of the latter, through the apertures, and falls directly into the narrow slit between the inner edge of the grinding face of each runner and the wearing surface of the plate. Seized by the runners, and partly ground and partly sheared, the ore is reduced, and drops out through the bottom of the machine. The closeness of the runners to the plate. and the pressure they exert upon the ore, can be accurately adjusted by hand screws and rubber resistance buffers. The parts of the machine. are made so as to be easily replaceable. Shoes on the runners last 960 hours. According to W. B. Kunhardt, in his hook, The Art of Ore Dressing in Europe, the machine works to poor advantage and with small capacity on ore above 5 mm.
(k in.) in size. It is used for grinding to a fineness of 1 mm. in.) to 2 mm. C& in.), but the has sometimes been carried down to 0.75 mm. As the result of long-continued working at Przibram, the quantity of quartzose ore crushed clown to 2 mm. size by a Heberle mill with two runners was as follows : 2,466 lbs. of 4-mm. ore per hour ; 1,368 lbs. of 6-mm. ore ; and 1,157 lbs. of 9-mm ore. tinder the most favorable conditions the consumption of water in crushing was 5.4 gallons per minute for each runner. The percentage of slimes made is small compared with many other machines, and the Heberle mill is, consequently especially adapted for fine-crushing in dressing works. For further details see Berg uncl'Hnettennsannische Zeitung, vol. xi. p. 400, 1881.
The Sturtevant Mill (Fig. 12).—The crushing and grinding parts of this mill consist of two cylindrical heads or cups arranged upon the opposite sides of a case into which they slightly project, and, facing each other, arc made to revolve in different directions. The rock or ore to be reduced is fed into the opening at the top of the case, and passing clown to the interior, is prevented from dropping below the revolving heads by a cast-iron screen, and entering, as it must, the heads or cups in revolution, is immediately thrown out again from each cm) in opposite directions with such tremendous force that the rock or ore coming in collision is crushed and pulverized. and the grinding, which would otherwise be upon the mill, is transferred to the material, which is at once reduced to ally fineness desired, deter mined by the size of opening in the screen used. The material, as fast as ground, passes through the screen, and falls into the bin. When necessary to reduce the rock to a greater fineness than the screen outlets allow, the coarser part of what leaves the screen is reconveyed to the mill by an elevator, for regrinding, that which is already sufficiently fine being first removed by the usual apparatus adopted in milling. A suction blower causes the air to draw strongly into the mill, and prevents the escape of dust. The cast-iron screen is composed of small sections, and the worn parts are cheaply and easily replaced. It is claimed that the wear upon this screen is very slight, as it is always protected from the action of the rocks thrown from the heads or cups by a cushion of interposing material, formed by the rocks which always fill the case and cover the screen.