As their effect does not differ from that of those worked by steam, we will postpone a description of the inner mechanism till we refer to the latter. The only difference consists in the way in which the shaft is rotated. On the beam is fixed a pole 3 to 4 metres long (Fig. 55), having two vertical shafts at the end.
It is better to substitute for these shafts a semicircular piece of iron movable about an axis passing through the pole; in this way the motion is more free, and the horse feels shocks less (Fig. 56).
The pans of these pug-mills are of sheet-iron (Fig. 55) or of cast-iron (Fig. 56).
Double-pan pug-mills (Fig. 57) are also made, each pan being furnished with a knife-shaft. Their advantage is that they pug rapidly and well, and thus give a large output.
Like the foregoing, these are essentially formed of a sheet or cast iron pan, in the centre of which moves a shaft fitted with screw-shaped blades to cut the clay (Fig. 58). Upon these blades are fixed steel knives so arranged as to cut the clay again in a direction perpendicular to the first. At the bottom there is a strong blade of 10 or 15 centimetres in height, and shaped like a reclining CO, the use of which is to facilitate the expulsion of the clay through the orifice in the side of the pan.
The shaft is rotated in such a direction that the blades make the clay descend by pushing it in front of thsm.
The blades are arranged so as to form a kind of endless screw, hence the clay driven downwards by one blade is taken on by the second before the effect of the first ceases, and its motion is thus continuous. When it reaches the bottom of the machine the CO-shaped blade expels it by the orifice made for that purpose.
This orifice is provided with a door, the opening of which is regulated at will. Another much larger opening is used for cleaning the interior of the machine; it also is closed with a door.
Fig. 57. Pug - mill with two Pans worked by a Horse (Joly).
Sometimes the orifice of issue is placed in the cleaning-door itself, sometimes it is on the opposite side (Fig. 60) or underneath (Fig. 61).
The arrangement of the pug-mills in Fig. 62 is intended to facilitate the introduction of the clay by diminishing the height.
To make the machines lighter the pans are made of sheet-iron instead of cast-iron. But it must not be forgotten that, if we are working with hard paste, the wear is much more rapid in the case of sheet-iron in consequence of its thinness.
Fig. 62. Vertical Cast-iron Pug-mill worked from above (Sachsenberg).
The prism of earth expelled from the pug-mill, if it is not immediately cut (Fig. 57), or absorbed by another machine, is driven forward by the clay behind it; but as its weight causes friction on the horizontal surface, there comes a moment when this friction and the pushing of the clay cause equilibrium, the prism advances no further, and the supply from the pug-mill is stopped. This inconvenience must then be prevented by cutting the prism as soon as it attains a certain length. The mode in which the shaft of the blender is moved varies with the manufacturer; an examination of the figures will show these differences.
In certain installations it is sometimes advantageous to use pug-mills worked from beneath. The upper orifice is thus quite free, which facilitates feeding, especially when this orifice can be so placed that the moistened clay can be poured direct into it,
The arrangement of these machines varies with the make(see Figs. 63, 64, 65). The pug-mill in Fig. 63, however, having its upper orifice completely free, seems to exemplify most thoroughly the advantage of this type of machine, that is to say, ease of feeding.r
Fig. 63. Vertical Cast-iron Pug-mill worked from below (Jager).
The Boulet pug-mill (Fig. 65) is specially reserved for the mixing of hard clay, which, owing to the work entailed, requires strong and well-constructed apparatus.
These are made in different ways, and are used advantageously for clays difficult to mix. The height of vertical pug-mills cannot exceed a certain limit, while the great length possible with horizontal machines facilitates the incorporation of one clay with another. The introduction of the clay takes place above and the expulsion below (Fig. 66), or on one side (Fig. 67). The gearing is either fixed to the pug-mill or separated from it.
Fig. 64. Vertical Cast-iron Pug-mill worked from below (Joly).
Fig. 65. Vertical Cast-iron Pug-mill worked from below (Boulet).
Fig. 66. Horizontal Pug-mill (Jager).
Fig. 67. Horizontal Pug-mill with attached Gear (Jager).
The issue of the clay is regulated as in the vertical machines by a movable door by which the opening may be varied at will; it is held in position by a rod fixed to it (Fig. 68).
Fig. 68. Horizontal Pug-mill with attached Gear (Sachsenberg).