mill for the cement industry are controlled by the Atlas Portland Cement Company. The mill has in consequence been used only in the plants of that company, where it seems to have given good satisfaction.

In the Huntingdon mill three heavy rollers are suspended from a circular horizontal head, the attachment being such as to allow free radial swing to the rollers. The rapid revolution of the head causes the rollers to diverge, swinging outward, and being pressed by centrif

ugal force against a horizontal annular die ring of steel. The material fed in is pulverized between this fixed ring and steel-head rings attached to the bottom end of each roller. The mill, therefore, differs from the Griffin mill, chiefly in the fact that the single roller of the latter mill is revolved positively by power applied directly to its upper end, while in the Huntingdon mill the individual rollers are not positively rotated.

A Huntingdon mill requires about 40 H.P. If fed with material varying from to 1 inch in size, its output on clinker will be about 8 barrels per hour, ground so as to pass 92 per cent through a 100-mesh sieve. On raw material its output will vary from 15 to 25 barrels per

hour, ground to pass 93 per cent through a 100-mesh sieve. For grinding clinker to a fineness of 90 or 92 per cent on 100-mesh, the Huntingdon mill has given good satisfaction, but for the greater fineness now required by many specifications, it is probable that the finishing work can be done more economically with the tube mill. The rate of grinding clinker, quoted above, is equivalent to an expenditure of 5 H.P. hours per barrel of cement, which is about the same as the work done by the Griffin mill under similar conditions.

Griffin mill.—If we disregard the enormous Atlas plants, the Griffin mill is by far the most extensively used of the class of centrifugal grinders. It is shown in section in Fig. 94.

On reference to this figure it will be seen that the power is received by a pulley (17) running horizontally. From this pulley is suspended the shaft (1) by means of a universal joint (9), and to the lower extremity of this shaft is rigidly secured the crushing roll (31), which is thus free to swing in any direction within the case. This case consists of the base, or pan (24), containing the ring, or die (70), against which the roll (31) works, and upon the inner vertical surface of which the pulverizing is done.

In dry pulverizing, this pan, or base (24), has a number of openings through it downward, outside of the ring, or die, which lead into a pit, or receptacle, from which it is delivered by a conveyor.

Upon this base is secured the screen frame (44), which is surrounded with a sheet-iron cover (45) (in the wet mill this cover is not used), and to the top of which is fastened a conical shield (25), open at the apex, through which the shaft works.

The cut shows the pulverizing roll attached to the lower end of the shaft (1), and just above the roll is the fan (7), which is used in the dry mill, but not in the wet. On the under side of the roll are shown shoes, or plows (5), which are used in both, and varied in shape according to the nature of the work to be done.

The pulley (17) revolves upon the tapered and adjustable bearing (20), which is supported by the frame composed of the standards (23). Two of these standards (23a) are extended above the pulley to carry the arms (22), in which is secured the hollow journal pin (12).

Within the pulley is the universal joint from which the shaft (1) is suspended. This joint is composed of the ball, or sphere (9), with trunnions attached thereto. These trunnions work in half boxes (11) which slide up and down recesses in the pulley-head casting (16).

The joint in the pulley is enclosed by means of the cover (13), thus keeping the working parts away from all dust and grit.

The lubricating oil is supplied for all parts needing it through the hollow pin (12).

The roll is revolved within the die in the same direction that the shaft is driven, but when coming in contact with the die it travels around the die in the opposite direction from that in which the roll is revolv

ing with the shaft, thus giving the mill two direct actions on the material to be ground. There is a pressure by centrifugal force of 6000 lbs. brought to bear on the material being pulverized between the roll and die, the united actions being very effective in their combination.

When a quantity of the material to be reduced has been fed into the mill sufficient to fill the pan as high as the shoes, or plows, on the lower side of the roll, they work in it, stir it up, and throw it against the ring, so that it is acted upon by the roll; and when fairly in operation the whole body of loose material whirls around rapidly within the pan, and, being brought between the roll and die, is crushed, and all that is sufficiently fine passes at once through the screen above the die, the coarser portion falling down to be acted upon again.

The universal joint, by which the shaft is connected with the pulley, allows perfect freedom of movement to the roll, so that it can safely pass over pieces of iron, steel, etc., such as are usually found in all rock to be pulverized, without damage to the mill.

The fan attached to the shaft above the roll draws air in at the top of the cone, forcing it through the screens and out into the discharge, thus effectually keeping all dust within the mill.

In working dry the screen which surrounds the pulverizing chamber is of much coarser mesh than the delivered product; for instance, a 16-mesh screen delivers a product over 90 per cent of which will pass a 60-mesh screen. Two sizes of the Griffin mill are made:

When running on clinker which has been previously crushed to about -inch size the Griffin mill will handle from 5 to 10 barrels per hour, using 25 to 30 H.P. For clinker-grinding the mill is usually equipped with 30- or 32-mesh screens, giving a product of about 95 per cent through a 100-mesh, and 70 to 80 per cent through a 200-mesh.

In grinding raw materials, 24- or 28-mesh screens are used, which,

however, give a product practically equal in fineness to the 30- or 32mesh screens used in clinker-grinding. With these screens the mill will turn out 2 to 3 tons of raw material (equivalent to 8 to 10 barrels) per hour, taking slightly less power than when running on clinker.

The repair costs of a Griffin mill were stated, at a plant which has always used these mills extensively, to vary with the material crushed in about the following ratio:

Repair costs on clinker : repairs on raw mix: repairs on coal.

Kent mill. The Kent mill is a comparatively untried machine, but deserves mention here because of the favorable results reported for it by Professor Newberry, and G. H. Fraser (see p. 468).

The Kent mill is shown in Fig. 95, the casing being broken out, and one fixed check-ring B being partly broken away to show the feed chutes A, the free revolving ring C, the three crushing-rolls G, and the bottom discharge outlet F.

Referring to the interior view, A is the feed-chute, which enters the casing at opposite sides above one of the three rolls G and feeds into the angle between this roll and the ring C. The three rolls G (of which one is driven) are within and support the ring C, being drawn yieldingly against its concave inner face at three points by stiff springs acting against the bearing yokes carrying the shafts of the rolls. These yokes slide in lugs on the casing and pull outward according to the adjustment of the springs by their screws. The convex faces of the rolls fitting the concave inner face of the ring hold it in position sideways, so that the ring always tracks on the rolls, but it is also checked against too much side play by fixed check-rings B fastened on the inside of the casing at a slight distance from the edges of the free ring, so as to leave a free space D between in which the free ring can play and through which the fine material may escape to the discharge chamber E, which surrounds the ring C and at its lower part meets the discharge outlet F. The rings B are cut away for a space above the outlet F. The fixed rings can be easily replaced if ever worn out.

In operation, the free ring is cushioned by the rolls and held centrally, both axially and laterally, thereby, but can yield to pass a hard substance or to cushion unequal thrusts, and can play sideways between the fixed rings B to equalize variations of charge between any roll and the ring. The driven roll drives the ring by contact with its inner face, the other rolls being passive and free to revolve by contact with the ring. The rolls and ring run on each other at like surface speeds. The charge streams in between the ring and one roll, passes the latter,