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Although its brief period of popularity passed some thirty-odd years since, and only a few sets were installed before interest reverted to other types, the high-speed double roll crusher deve10ped by Thomas A. Edison shortly before the end of the last century warrants a p1ace in any discussion of crushing equipment. The largest machine of this type-the 6- x 7-ft "giant" rolls-are huge crushers, judged even by present-day standards; they have an unobstructed receiving opening 7 x 7 ft and their capacity on individual skip-1oads of stone is enormous, a1though, as will be explained, they cannot maintain this peak capacity over a period of time.
Mechanically, the Edison roll crusher is a very simple machine. The two rolls are carried in bearings, supported on two very heavy and rigid bed castings which are secured on the concrete foundation by a number of large anchor bolts. The bearings, in addition to being bo1ted to these bed castings, are prevented from spreading by pairs of 1arge tie-rods which pass through them above and be10w the roll shafts. Unlike the smooth face crushing rolls we have described, these tension rods are not cushioned by springs. The machine is surmounted by a heavy cast rectangular hopper, all sides of which are vertical. Each roll is independently driven by a flat belt pulley.
The roll-centers are octagonal in cross section, each face being provided with a spline groove and a series of tapped holes for securing the chilled-iron wearing plates. These wearing plates have the sledging knobs, or teeth, cast on their outer surfaces.
Thus we have a roll surface that resemb1es that of the Fairmount crusher, except that the faces of all teeth are sloped instead of radial on the advance side. The usual practice is to fit one roll entirely with so-called regular teeth, and the other roll with six rows of regulars and two rows of higher (slugger) teeth.
The peripheral speed, or tip-velocity, of these rolls is much higher than that of any of the machines we have previously described. The range of the smooth-face rolls, for example, is from about 400 ft. per min. for the small 12-in. rolls, to 2000-2200 ft per min. for the heavy-duty 72-in. machine. (The single-roll crusher has a tip speed of 400-450 ft. per min) The 6- x 7-ft Edison crusher has a normal (no-Ioad) surface speed of just under 3500 ft per min. It can be readily appreciated that this high velocity induces an extremely violent crushing action, in conjunction with the 3- to 4-in. knobs which protrude from the roll surfaces.
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