The Smooth Roll Crusher plays a key role in bulk raw materials preparation, processing clays, coal fines, coke breeze and gypsum waste. Smooth roll crushers are used to particularly suitable for medium and fine grinding processes,Â
Principal of Smooth Roll Crushers: Size reduction is achieved by compression (i.e. it employs compressive force for size reduction).
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Construction
A smooth Roll crusher consists of two heavy metal rolls of the same diameter placed side by side each other in the horizontal position. The rolls are rotated towards each other at the speed. One of the shafts moves in fixed bearings while others move in movable bearings. The clearance between the rolls can be adjusted according to the size of the feed and the required size of the product.Â
One of the rolls is driven directly, the other by friction with the solids. The rolls have relatively narrow faces and are large in diameter, therefore they can nip moderately large lumps. The material fed to the machine is protected by spring loading (i.e. by mounting the bearings of one of the roll shafts against a coiled spring) against vibration damage due to tramp and very hard material.Â
The speed of rolls varies from 50 to 300 rev/min. Crushing rolls are secondary crushers accepting feed 12 to 75 mm in size and yielding products -12 mm to about 20 mesh.
Roll crusher diagramÂ
These machines are characterized by the production of few fines and by a smaller reduction ratio of the order of 4 to 1.
Working Principle
The material to be crushed is fed from the top. As the rolls rotate, the material gets caught between them and gets reduced in size by compression and discharged from the bottom.
Selection of Crushing Rolls
In selecting the rolls for a certain duty, it is necessary to know the size of the feed and the size of the product.
Consider a system as shown in Fig wherein the spherical particle B of a material is just being caught between the rolls. A and A' are centres of two rolls.
Line AB passes through the centre of the left-hand roll, through the centre of the particle, and through point C where the particle is in contact with the roll. Let the angle between line AB and the horizontal i. e. line joining the two centres of rolls be α. The line OD is tangent to the roll at point C. As the line OD is perpendicular to line AB, it makes the same angle α with the vertical. Neglecting gravity, the two forces are acting at a point C, the tangential frictional force T having vertical component T cos α, and the radial force N, having vertical component N sin α. Force T is related to force N through the coefficient of friction μ, so T = μN.
The vertical component of forces T and N are opposed. Force N sin α tends to expel the particle from the rolls and force T cos α tends to draw the particle between the rolls. If the particle is to be drawn between the rolls and crushed,
Nip Angle FormulaÂ
T cos α ≥ N sin αÂ
T and N are related through
T = μNÂ
μN cos α ≥ N sin αÂ
μ ≥ tan αÂ
In other words, the tangent of angle α must be less than the coefficient of friction. The value of μ varies from material to material, but for all practical purposes, the value of the angle α is usually taken at about 16°. The angle DOE, Which is twice the angle α is called the angle of nip.
Advantages of Roll Crushers
1. High tonnages up to 12000 MTPH.
2. Compact in size and therefore reduced installation cost.
3. Can not mobile: lower capital cost than any other crusher.Â
4. Large reduction ratio in relation to tonnages.Â
The disadvantage of Roll Crushers
1. Low reduction ratio.Â
2. Required a high-performance electrical system as there are peak power loads up to eight times the original installed power.
3. Can not be used economically for low tonnages.Â
4. Will not handle trash metal.Â
Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane
