The pan is of cast-iron and has in the middle a hollow covered by a sloping sieve, the fineness of which varies with that of the product required. The vertical shaft which works the grindstone carries with it a chain of buckets supported by a shaft perpendicular to the first one, and rotated by means of a bevelled gear as shown in Fig. 33. The buckets pick up the pulverised matter into a wooden gutter which guides it on to the sieve; the portions which are too large to pass it fall back again into the pan, and the powder is received into a lower receptacle, whence it is removed by various means according to the locality.
If it has to be carried to a higher level, a kind of shaker is used, moved by a belt, and constructed to throw the pulverised substance into a trough from which it is carried by a bucket-chain to the required height.
The apparatus here represented may have two grindstones instead of one. The weight of these stones varies according to their size from 350 to 3500 kilos (6 cwt. to over 3 tons), and as they are hollow, this weight is increased by bricking them inside. Thus a simple grindstone like that in Fig. 33 weighs 350 kilos; filled with bricks like that in Fig. 34, it weighs as much as 850 kilos. The crushing mill of Fig 34 differs very little from the preceding one.
Fig. 33. Crushing Mill with Central Sieve and Automatic Feeder (Jannot).
Fig. 34. Crushing Mill (Luce).
These may either have fixed or movable pans. The one in Fig. 35 has a fixed pan pierced with a series of holes; the grindstones set in motion by the bevelled year crush by their weight and speed the substance to be pulverised. The powder is collected in a closed space, whence it can be raised by a bucket-chain to be distributed, if required, to other machines. In other mills the pan is movable and the grindstone is fixed on an axis which is held between slides and so can be raised or lowered. The grinders thus follow the level of the substance to be crushed, and turn round on their axis in consequence of the friction caused by the pulverisation.
Fig. 35. Crushing Mill with Fixed Pan (Groke).
Fig. 36. Criishing Mill with Movable Pan (Johnson).
The arrangement of the pans naturally depends upon the makers. Figs. 37 and 38 show us the different arrangements. In the machines represented the motion of the pan is transmitted from above by means of a vertical shaft which passes through, but with slight friction, the axis of the two grindstones. On the other hand, in the mill represented in Fig. 39 the pan carries a cog-wheel, and the motion is transmitted below.
The perforated bottom of the pans is made up of several segments which can easily be replaced when worn out, the parts being interchangeable.
A special crushing mill, represented in Fig. 40, is used to reduce agglomerated sand to powder. The peculiar shape of the grindstones or rather of the fluted cylinders allows of easy crushing of the lumps.
Fig. 37. Crushing Mill with Movable Pan (Whitehead].