We have seen that cylinders are used for crushing the hard bodies contained in clays. For this purpose the cylinders, which are of equal diameter, are fairly distant from one another, and turn with the same velocity. But if different speeds are given to them, or if, while making the same number of turns, their diameters are unequal, the velocities at the circumference will be different and a tearing of the clay will ensue which will effect a mixture. Care is taken to leave between the rollers some clay to the thickness of a few millimetres by placing scrapers to remove the excess. This layer of clay sticks to that contained in the hopper and draws it between the cylinders, where it is united by pressure at the same time as it is being drawn out by the differential speed; this causes a special pugging different in its effects from that produced by the ordinary pug-mills.

Several arrangements are utilised to arrive at the desired result.

1. Use of cylinders with equal diameters and unequal speed, or equal speed and unequal diameters.

2. Use of cones.

3. Use of fluted cylinders.

4. Use of perforated cylinders.

Cylindrical Roller Pug-Mills

These machines, which are not very complicated, consist of two solid cast-iron supports fixed upon wooden joists or a block of masonry. Between these supports are placed two cast - iron rollers, one of which receives the motion direct and transmits it to the second by means of a differential gearing (Fig. 69). Over the two rollers is placed a hopper into which the clay is put.

Fig. 69. Pug-mill with Rolling Cylinders (Jacobi).

At a certain distance (which can be regulated at will) from each roller is a steel blade acting as a scraper. Above the rollers is the hopper, as can be seen in Fig. 70.

Cone-roller Pug-mills.- - These differ from the foregoing only in the conical form of the crushers and their arrangement. The smaller diameter of the one being placed in front of the larger diameter of the other, an unequal speed at the circumferences is caused for the same number of turns; hence blending occurs (Fig.71).

The use of cones would be more advantageous, from the point of view of output, than that of cylinders of the same diameter. To regulate the speed a flywheel is attached to the gear-shaft (Fig. 71).

Fig. 70. Pug-mill With Rolling Cylinders (Groke).

Rolling Machines With Fluted Cylinders Or Cones

The blending of the clays is increased by the use of fluted rollers; the projections of one roller enter the depressions of the other. The shape of these flutings is, however, variable (see Figs. 72 and 73). The speed of the rollers being the same, there is a difference of speed between the surface of a hollow and the surface of a projection which produces a tearing apart of the clay simultaneously with the flattening.

By using fluted cones this effect is increased. Thus it is that with the Boulet machines {Fig. 73) a single passage through it turns a mixture of differently coloured clays into a paste of uniform colour.

The Outings get filled with clay during the operation, but scraping combs, placed above, constantly clean them.

Rollers With Perforated Cylinders

These machines are specially constructed for blending hard clays. They were invented by M. Dumont, and are now built by M. Lacroix. They consist of strong cylinders pierced with holes which are very close together and have a diameter of from 12 to 13millimetres (Fig. 74). The two cylinders touch one another, and the one receiving motion transmits it to the other by contact.

Rollers With Perforated Cylinders 11

After the clay has been passed between ordinary cylinders, it is thrown by means of the hopper between the perforated cylinders, and as they press one against the other it is forced to pass through the holes when drawn down by the rotation.

Thus it penetrates into the interior of the cylinders in the form of endless curls, splitting up into small pieces which roll round one another before reaching the ground. This process develops the plastic qualities of the clay.