In districts where the winters are severe, water power may also fail and work have to be stopped; and consequently, even when water power is the prime source of energy, a steam engine must be installed as a stand-by, being, of course, only used when the main source of power gives out or proves insufficient.
The machines employed for preparing the raw materials in the manufacture of earth colours may be divided into the following groups:—
Machines operating entirely by pressure: crushers; machines acting by impact: stamps; those acting by impact and pressure: vertical mills (edge-runners), ball mills, centrifugal mills; and, finally, machines with a frictional action: grinding mills. Then there are the levigating machines, which do not reduce the material but separate the coarser particles from the finer. The construction of the foregoing machines is a matter for the machinery manufacturer rather than the maker of earth colours; but as the business of the latter is dependent on them, a short description is considered necessary. The selection depends, on the one hand, on the nature of the materials to be treated, and, on the other, on the size of the works, since a manufacturer who has to deal with large quantities of a given raw material will require different machines from those used on a small scale. The sole purpose of the following description is to indicate to the colour maker the way in which the reduction of the raw material can be accomplished.
Crushing Machinery
Crushers and Breakers.—Crushers usually consist of grooved iron rollers revolving on horizontal axes. One of the rollers is fixed, the other being adjustable by screws, in order that lumps of different sizes may be treated in one and the same machine, which may be employed either to turn out a roughly crushed product, or to reduce it to a certain degree of fineness.
If several pairs of crushing rollers be mounted in series, and each set a little closer than its predecessor, the material can be reduced progressively from large lumps to a fairly fine powder.
Each pair of rollers is geared together by pinions, and is turned in such a way as to draw the material in between. If the gear pinions have the same number of teeth, the two rollers will revolve at the same speed and will then merely crush the material into lumps of a size depending on the distance at which the rollers are set apart.
Nevertheless, by simply altering the gear ratio of the pinions, the crushing action of the rollers can be supplemented by a grinding action, a much finer powder being then obtainable than otherwise, the one roller running at a higher speed than the other.
These crushers differ in strength of construction, very strongly built machines being required for dealing with large lumps of hard material, whereas substances of low crushing strength, such as clay or other earthy materials, can be treated in much lighter machines. In any case, however, it is advisable to have the machine stronger than is absolutely necessary for the work in view; for, although the prime cost is thus increased, the outlay on repairs will be reduced, and the machines can, if necessary, be used on harder material as well. The framework supporting the rollers should always consist of a strong iron casting; and the machine should be set up as close as possible to the engine or motor, to minimise the loss of power in transmission through long shafting, etc.
