Howto Sizesmooth Roll

CrushersAdam Zanker

Smooth roll crushers are widelyused as secondary crushers, operatingmost effectively when set for size re-ductions in a ratio between 3 or 4 to I.The feed supplied to these units var-ies from 13 to 102 mm in size and theproduct obtained ranges from 13 mmto about 20 mesh. Smooth rollcrushers produce very few fines andabsolutely no oversize.

The diameter ofthe rolls varies withthe clearance between them, feedsize, and the coefficient of internalfriction ofmaterial to be crushed, De-tailed calculations of this relationshipmay be found in Unit operations of

Chemical Engineering, (McCabe,W. L., and Smith, J. C., 1967, McGrawHill, NY, Chapter 26, pg. 825).

The geometrical arrangement of theroll crusher is shown on the ac-companying sketch. A relatively sim-ple relationship between the diame-ter of feed, the roll, and the gap be-tween them is as follows: - -

where R = radius ofroll(mm); d = thehalf-gap between rolls (mm); r = theradius of feed (mm); and a = the angleof friction. This e(luation may be rear-ranged as follows:

2R + 2d

Cos a = 2R + 2r.(2)

It is possible to insert other vari-ables for greater convenience,namely: 2R = D (diameter of the rollin mm); 2d = S (the gap between rollsin mm); and 2r =- k (the diameter offeed particles in mm). The rearrangede(luatiou looks as follows:

D+Scoscr=D+k, (3)

When taking into account that y is thecoefficient of internal friction of thecrushed material, and the angle a isthe angle of friction: p = tan a, a = arctan p, and cos a = cos (are tan ~). Afterinserting tl~e new value of C(JSa, andrearranging the e(luation for D, thefinal e(luation will he obtained:

~ ~ cos(arc tan p)K + Scos(arc tan ~) - 1.

(4)

Etluation (4) gives the direct de-pendency of the roll diameter fromthe gap between the rolls, feed size,and the coefficient of internal frictionof the material to be crushed.

The solution ofthe e(~uation, whilerelatively simple, is neverthelesstedious and time-consuming. There-fore, a nomograph has been con-structed which allows a graphic solu-tion.

Using the Nomograph

The simple procedure in solvingthis problem is as follows:

Step 1. Find the intersection pointof the lines of known values of K onthe grid in the central part of thenornograph. Mark this point A.

Step 2. Connect point A with aruler to the known value of S on theleft-side scale. Extend this line up tothe intersection point with the right-hand D scale. Read the final result Don this intersecting point.

Sample Calculation

It is known that a rock to be crushedhas a coefficient ofinternal friction of0.22. The average size of feed e(luals k= 460 mm. The size of the productdesired is 11.1 mm (the size 01 producte[luals the gap between rolls, hence S= 11.1 mm). What is the proper diame-ter of rolls to bti used for this opera-tion?

Solution: According to the proce-dure described, the proper diameterof crushing rolls is graphically cleter-mined at 1.43 m. ❑

Adam Zanker is a mining engineer livingin Kiriat Jam, Israel.

I

—

MINING ENGINEERING JUNE 1980 665