Themechanism of these presses is well known. A large flywheel turns a screw, which has at its lower end one side of the mould; the other part of the mould slides upon a cylindrical iron rod; in order to fill it, the workman draws it towards him, and, holding it horizontal by means of a handle, places the slab of clay into it, and then pushes it back under the screw, where it is held by catches exactly under the upper part. The flywheel is set in motion, and the stamping takes place; but two or three compressions should be made in order to force the clay into all the cavities of the mould, and to ensure that all the bubbles of air, which the shock accumulates in the lower part of the tile, may be driven out. As there are two moulds, while one is under pressure the other is being filled, and will be ready to take the place of the first as soon as the compression is over. The demoulding is done by turning the bottom of the mould round its supporting rod, and receiving the tile on a board. However well-fitting the moulds may be, the compression always leaves seams, and to remove these, the board and its tile is placed on a swivel stand (Fig. 384), and they are taken off" by means of a stretched wire, the tiles being afterwards taken to the drying-sheds.
Fig. 384. Swivel Stand for trimming Tiles (Boulet).
Fig. 385. Screw Press, worked by Steam (Johnson).
For a large output, this stand is replaced by a circular revolving table, in front of which women are seated.
The tiles come to the presses on an endless band, then pass to the trimmers, and afterwards proceed to the drying-sheds by another endless band.
Fig. 386. Screw Press, worked by Steam (Boulet).
With hand-presses about 150 tiles an hour can be made, giving three compressions to each.
In these presses (Figs. 385, 386) the upper part of the screw is furnished with a flywheel which, by means of friction discs, can be turned in either direction. A lever placed within reach of the workman's hand allows him to move the screw in the direction required. Another gearing, placed well in sight, acts upon the belting, and is used to stop the shaft on which the discs revolve.
These consist of a solid shaft furnished with two cams with three hollows and three projections, and set in motion by gearing worked by hand or steam. Each hollow in the cam corresponds to a pressure of the upper plate upon the slab of clay, and is immediately followed by a partial demould-ing caused by the projections; every turn of the shaft then produces three compressions, followed by three demouldings; and in consequence of the shape of the cams, the compressing force increases from the first to the third, being on an average 60 kilog. per square metre.
Fig. 387. Cam Press with Triple Compression (Joly).
Fig. 388. Press with Crank and Handle (Chavassieux).
Demoulding is effected by turning the movable carrier on which the mould rests round the bar supporting it, and the tile is received on a board. The carrier carries two moulds, so that one can be filled while the other is being pressed.
The Chavassieux press (Fig. 388), also worked by hand, has only one mould-carrier; the motion is transmitted to the upper mould by means of a crank and winch.
In the press represented by Fig, 389, the upper mould is moved by jointed levers. It is worked by power, and stops automatically when the tile is sufficiently compressed. It cannot be started unless the mould is exactly in its place. Its production is about 200 to 300 tiles per hour.
Fig. 389. Crank Press for Tiles (Joly).