|
The feed size being a fixed for any given application, way to decrease nip angle crease the roll diameter. Shocks incidental to shattering the particles of any given size of feed increase with speed, and with the crushing strength of the material. Large diameter rolls, because of their greater' mass, can absorb these shocks better than smaller, lighter rolls; therefore they are more suited to high speed operation. It is equally clear that high-spring-pressure, heavy-duty rolls are better fitted, because of their superior shock-absorbing capabilities, to stand up to high-speed crushing than rolls of more modest proportions.
Lastly, we have the character of the material to consider; that is, its resistance to crushing and its coefficient of friction. Except for occasional special cases the latter is not apt to diverge greatly from the normal; therefore it do is not as a rule inject any special complication into the problem. On the other hand, hardness and toughness do vary widely and must be taken into consideration in selecting the proper size and class of rolls.
It would be exceedingly difficult, if not impossible, to incorporate all of these variables into one comprehensive chart 01' formula. It can, however, be done
for one type or duty-class – of roll if we assume a reasonably uniform coefficient of friction and base our values, for safety's sake, on hard rock. Such a chart is shown in Fig. 4. This chart was prepared for 1'0115 of the heavy-duty class, with spring pressures in the approximate range of 5 10 8 tons per in. of face. |
