. Modern mechanism, exhibiting the latest progress in machines, motors, and the transmission of power, being a supplementary volume to Appletons' cyclopaedia of applied mechanics . ns per minute ; it is impossible to drive geared rolls, with safety, at agreater speed than 40 to 50 revolutions per minute. Since the installation at the Bertrandmill much higher speeds have been used successfully, but 100 revolutions per minute is theaverage rate now employed. The shells used on the rolls may be either chilled white iron or mild steel. The formerare the harder, but they become nicked by excessivel
. Modern mechanism, exhibiting the latest progress in machines, motors, and the transmission of power, being a supplementary volume to Appletons' cyclopaedia of applied mechanics . ns per minute ; it is impossible to drive geared rolls, with safety, at agreater speed than 40 to 50 revolutions per minute. Since the installation at the Bertrandmill much higher speeds have been used successfully, but 100 revolutions per minute is theaverage rate now employed. The shells used on the rolls may be either chilled white iron or mild steel. The formerare the harder, but they become nicked by excessively hard lumps of ore, or pieces of .steel,which may, perhaps, pass through them, and thus lose the smooth surface which is necessaryfor good work. The mild steel has the objection of retaining these pieces of steel, thus chis-eling the face of the shell as if it were in a lathe. This disfigurement and damage is usu-ally averted by the use of magnets similar to those used in flour mill grain fed properly, they, like all other shells, wear hollow in the center. It is not gen-erally known that the chilled iron may be turned, and shells of this material are usually. 582 ORE-CRUSHING MACHINES. tbrown aside when too badly worn. Dr. E. D. Peters states, however, in his 3IodernAmerican 3Iethods of Copper Smelihiq. that the hardest chilled iron may be turned with anordinary tool without difficulty if a sufficiently slow motion is used in the process. Steelshells, which may be turned quite easily, have, on the whole, ffiven greater satisfaction thanthe chilled iron, and are, at the present time, more generally in use. The size of the rollers is a matter of great importance, and the tendency of late years,on the part of many engineers, has been to increase the diameter—rollers of 86 in., 1 meter,or even 40 in. in diaraecer being now not infrequent. The larger the diameter, the largerthe size of the lumps of ore which can be crushed, and with lumps of ore of a given size,the
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