Roller mill
Roller mills are mills that use cylindrical rollers, either in opposing pairs or against flat plates, to crush or grind various materials, such as grain, ore, gravel, plastic, and others. Roller grain mills are an alternative to traditional millstone arrangements in gristmills. Roller mills for rock complement other types of mills, such as ball mills and hammermills, in such industries as the mining and processing of ore and construction aggregate; cement milling; and recycling. Types. Two-roller mills Two-roller mills are the simplest variety, in which the material is crushed between two rollers before it continues on  to its final destination. The spacing between these two rollers can be adjusted by the operator. Thinner spacing usually leads to that material being crushed into smaller pieces. Four-roller mills Four-roller mills have two sets of rollers. When using a four-roller mill to mill grain, the grain first goes through rollers with a rather wide gap, which separates the seed from the husk without much damage to the husk, but leaves large grits. Flour is sieved out of the cracked grain, and then the coarse grist and husks are sent through the second set of rollers, which  further crush the grist without damaging the crusts. Similarly, there are three-roller mills, in which one of the rollers is used twice. Five- and six-roller mills Six-roller mills have three sets of rollers. When using this type of mill on grain, the first set of rollers crush the whole kernel, and its output is divided three ways: Flour immediately is sent out the mill, grits without a husk proceed to the last roller, and husk, possibly still containing parts of the seed, go to the second set of rollers. From the second roller flour is directly output , as are husks and any possible seed still in them, and the husk-free grits are channeled into the last roller. Five-roller mills are six-roller mills in which one of the rollers performs double duty. Gristmill conversion. In the 19th century roller mills were adapted to grist mills before replacing them. The mill used either steel or porcelain rollers. Between the years 1865 and 1872, the Hungarian milling industry upgraded and expanded the use of stone mills combined with roller mills in a process known as Hungarian high milling. Hungarian hard wheat so milled was claimed as integral  to the "First in the world" success of the Vienna Bakery of the 1867 Paris Exposition. Working principle. A motor or other prime mover drives the hanger of the grinding roller to rotate through a V pulley and center bearing. The roller, which is hung by a bearing and pendulum shaft, will roll along the inner circle of the roll ring while the hanger is rotating. A dust removal blower will generate negative pressure at the inlet and outlet of the grinder to prevent dust and radiating the heat in the machine. History. Sugar rolling mills first appeared in the  Mughal Empire, using the principle of rollers as well as worm gearing, by the 17th century. The Indian roller mills used the principle of rollers as well as worm gearing. Modern era roller mills were re-invented by the Hungarian engineer András Mechwart in 1874, then quickly spread to other parts of Europe and Americas.