These consist of a horizontal pug-mill, in which are screws of various shapes and sizes, At the end of the pug-mill is placed the die through which the prism of clay passes. Above are usually fixed one or several rollers for different purposes; some serve to supply clay regularly to the screws and for crushing the hard portions in it. The others, when used, are true rolling machines like those we have described under the head of Preparation of Clays. They are used for clays which require to be crushed before being pugged.
Fig. 110. Vertical Pug-mill worked b. Horse-gear.
Fig. 111. Vertical Pug-mill worked by Steam.
The working of the screw machines is easily understood. The clay comes to the pug-mill in a regular and continuous stream, is seized by the blades of the screws, mixed, triturated, divided, and finally driven towards the outlet as would be the nut of a turning screw fixed at its two ends. The treatment of the clay is more or less severe according to the dimensions of the mill. In the French machines the screws are two in number and turn in opposite directions.
In order to simplify the description of the numerous screw machines, which-are, besides, all based on the same principle, we shall classify them according to the number of cylinders attached to them. We shall have then:
1. Simple screw machines.
2. Screw machines with distributing cylinders.
3. Screw machines with rollers and distributing cylinders.
These are not desirable, as they can only treat pure clays or those having already undergone preliminary working. Moreover, they have to be fed by hand, as, having no feeding cylinders, the clay is not drawn up by the screws; therefore their use is not recommended.
Fig. 112. Simple Screw Machine (Jager).
To make sure of regular distribution and continuous flow of clay to the pug-mill, it has been found advisable to join to it one or two cylinders whose rotation, opposite in direction to that of the screws, assures a constant supply.
The arrangement of these machines is similar to that of the preceding ones. The gear which transmits motion to the screws is fixed on the framework itself of the machine (Fig. 113) or separate from it (Fig. 114).
Fig. 113. Screw Machine with a Distributing Cylinder (Lieis et Cie.).
When there are two cylinders, as is usually the case, they crush small stones and lumps which may be contained in the clay, without, however, producing as thorough- effects as rolling machines specially constructed for that purpose.
Fig. 115 represents a machine with two distributing cylinders whose axes are in the same plane. It is fixed on a cast-iron framework in such a way that the die is on the level of the cutting-table. At the foot of the framework in the figure may be seen a die with water-face so that the brick may come out flat..
Fig. 114. Screw Machine with a Distributing Cylinder (Bernhardi Sohn).
Fig. 115. Machine with two Screws and two Distributing Cylinders (Joly).
The Boulet machine (Fig. 116) docs not sensibly differ from the last.
The Lacroix machine (Fig. 117) is also similar to the preceding ones. In the figure one of the workmen is throwing clay between the cylinders while the other cuts off the hollow products as they come from the die.
In other machines, notably those made by the Johnsons (Fig. 118), a special arrangement for bringing up the clay is observed. At the side is placed a windlass worked by the machine and set in motion by means of a gear lever which is seen in front of the machine. This windlass brings up the waggons full of clay close to the hopper of the machine by means of a chain; then they have only to be overturned. The distributing cylinders are arranged in different ways;sometimes their axes are in the same plane (Figs. 115, 116, 117), sometimes they are not (Figs. 119, 121).
Fig. 119. Screw Machine with Distributing Cylinders (Borner).
It may be seen from the figures that the dies have different shapes and appearance; we shall refer to them in a more detailed manner in the paragraph relating specially to them.
Fig. 120. Screw Machine with Distributing Cylinders (Jacobi).
In certain powerful machines, the output of which is considerable, the pan of the pug-mill is in two parts joined by bolts. This arrangement facilitates cleaning (Figs. 122, 123) and taking to pieces.
Fig. 122 represents a strong American machine producing 5000 to 8000 bricks an hour with a motive force of 75 horsepower.
The Chambers machine (Fig. 123) is similar to the last-named in arrangement and production.
Fig. 113. Screw Machine (Chambers Brothers).
Instead of cylinders, cones can be used, and act as rollers as well as distributors.
We have stated that the distributing cylinders of screw machines, the principal aim of which is to stuff the pan of the pug-mill with clay, could to a certain extent act as crushers. But when the crushing has to be severe it is preferable to substitute a pair of conical (Fig. 124) or cylindrical rollers (Fig. 125) which are placed above the distributing cylinder or cylinders. These rollers are worked direct from pulleys (Figs. 125, 128) or else receive their motion from the gearing of the machine by means of cog-wheels (Fig. 126).
Fig. 124. Screw Machine with Distributing Cones (Jager).
Fig. 125. Screw Machine with Rollers and Distributing Cylinders (Laeis et Cie.).
If the clay is difficult to hold, cylinders with points are used like those we have described (Fig. 27).
Fig. 126. Screw Machine with Rollers and Distributing Cylinders (Johnson).
The machine represented in Fig. 127 is so made. One or two pairs of rollers, cylindrical, conical, or fluted, are placed over it, according to the nature of the clay to be treated.