DRESSING OF ORES 87 



A (fig. 663), lever hung to the cast-iron frame B at c, and pressing upon pin at D. 

 When it is required to change the rollers, tho pressure resulting from the lever A and 

 weighted box E, is relieved by means of the screw tackle F. 



The considerations which should be attended to in constructing a crushing mill, are, 

 first, to make all the parts sufficiently strong to meet tho varying resistances -which 

 continually occur in crushing. For this purpose, tho framework to receive the rolls 

 ought to bo of good cast iron, the axles of the rollers of best wrought iron, and the 

 cylinders of the hardest and most uniform metal. 2ndly. To design tho machine, so 

 that tho matter to be crushed may be readily delivered into the hopper, sized by tho 

 circular sieves, for the dressing proqess ; and such portions as are not properly crushed 

 returned to the rollers without the intervention of manual labour. In order to effect 

 this, the inverted, or raff wheel D, fig. 664, shown in section, ought to be made of 

 sufficient diameter to allow the stuff, on being discharged, to descend by its own 

 gravity, into the feed-hopper. 3rdly. To extend from the axis of the rollers, long 

 tumbling shafts, A A, fig. 664, and fix on their ends the driving wheels B B, allowing 

 a little play in the plummer blocks, so that any undue opening of the rolls may not 

 vary the pitch line of the wheels B B, to such an extent as to endanger the safety of 

 the teeth. 4thly. To construct the roll so that it may be readily changed, yet 

 maintained on its axis without slipping when in motion. One of tho most efficient 

 plans for this purpose is shown in the following woodcut, in which A is the axis or 

 arbor, and D the roll (fig. 665). 



It will be seen that the cylinder roll is fitted with four internal projections ; these 

 are of the same length as the portion of the groove marked B B', but no wider than 

 tho narrower part of the groove c c. When the cylinder is to be fixed on the axis, 

 the studs are to be introduced into the recesses c, and the cylinder advanced into its 

 working position, when it is turned until the studs fit into that portion of the recess 

 between B B', and which are then wedged to the roll by a close-fitting cutter. 



5thly. The diameter of the rolls should be decreased, and the length increased in 

 proportion to the fineness of the stuff to be crushed, since a fine material requires a 

 longer line of contact, and not so large a grip as coarser substances. 



In practice it has been found advantageous to make the roller placed on the driving 

 shaft somewhat longer than that which is opposite, and to work the rolls by spur 

 gearing rather than by friction, since the latter is proved to furnish less economical 

 results than the former. It has also been found 'injudicious to harden the rolls by 

 chilling; hence ordinary sand-cast rolls are most frequently employed. 



The speed of the rolls varies from 45 to 60 feet per minute, but this necessarily 

 differs with the character of the stuff to be crushed. Again, great variation is ex- 

 perienced in the quantities crushed within a given period, since a small amount of 

 moisture in vein-stuff of a certain class-, makes it cake, and will thus considerably 

 reduce the produce of the mill. On the other hand, if the matter operated upon, be 

 very dry, heavy, and brittle, as in the case of some varieties of lead ore, the produce 

 may be much increased, since the mill can be driven at a great speed ; a less bulk 

 will have to pass for a given weight, and there will be a smaller quantity of material 

 carried back by the raff wheel to be re-crushed. 



Variable speeds have sometimes been tried in order to produce friction together with 

 pressure at the line of contact, but it has been found that any departure from a uniform 

 speed on the two surfaces absorbs a considerable additional amount of power, without 

 materially augmenting the results. 



The various dimensions and velocities of the rolls, crushing force and power em- 

 ployed, effective value of different mills, &c. now in use, may be gathered by referring 

 to the Table at top of next page. 



^The crushing machines which are in general use at Alston Moor and the northern 

 mines of this country, and where they have been employed for upwards of fifty years, 

 differ in some respects from those described. 



This machine is composed of one pair of fluted cylinders, x x, fig. 666, and of two 

 pairs of smooth cylinders, z z, z' zf, which serve altogether for crushing the ore. 

 The two cylinders of each of the three pairs turn simultaneously in an inverse 

 direction, by means of two toothed wheels upon the shaft of every cylinder, which 

 work by pairs in one another. The motion is given by a single water wheel, of which 

 tho circle a a a represents the outer circumference. One of the fluted cylinders if 



