Types of Rolling mills
Rolling mills may be classified according to the number and arrangement of the rolls.
(a): Two high rolling mills
(b): Three high rolling mills
(c): Four high rolling mills
(d): Tandem rolling mills
(e): Cluster rolling mills
1: Two high rolling mills
Two high rolling mills may further classified as
· Reversing mill
· Non reversing mill
A two high rolling mill has two rolls only.
Two high reversing mill:
In two high reversing rolling mills the rolls rotate ist in one direction and then in the other, so that rolled metal may pass back and forth through the rolls several times. This type is used in pluming and slabing mills and for roughing work in plate , rail , structural and other mills.
These are more expensive compared to the non reversing rolling mills. Because of the reversible drive needed.
Two high non reversing mill:
In two high non reversing mills as two rolls which revolve continuously in same direction therefore smaller and less costly motive power can be used. However every time material is to be carried back over the top of the mill for again passing in through the rolls. Such an arrangement is used in mills through which the bar passes once and in open train plate mill.
2: Three high rolling mill:
It consists of a roll stand with three parallel rolls one above the other. Adjacent rolls rotates in opposite direction. So that the material may be passed between the top and the middle roll in one direction and the bottom and middle rolls in opposite one.
In three high rolling mills the work piece is rolled on both the forward and return passes. First of all the work piece passes through the bottom and middle rolls and the returning between the middle and the top rolls.
So that thickness is reduced at each pass. Mechanically operated lifted tables are used which move vertically or either side of the stand. So that the work piece fed automatically into the roll gap.
Since the rolls run in one direction only a much less powerful motor and transmission system is required. The rolls of a three high rolling mills may be either plain or grooved to produce plate or sections respectively.
3: Four high rolling mill:
It has a roll stand with four parallel rolls one above the other. The top and the bottom rolls rotate in opposite direction as do the two middle rolls. The two middle are smaller in size than the top and bottom rolls which are called backup rolls for providing the necessary rigidity to the smaller rolls.
A four high rolling mill is used for the hot rolling of armor and other plates as well as cold rolling of plates, sheets and strips.
4: Tandem rolling mills:
It is a set of two or three stands of roll set in parallel alignment. So that a continuous pass may be made through each one successively with change the direction of material.
5: Cluster rolling mills:
It is a special type of four high rolling mill in which each of the two working rolls is backup by two or more of the larger backup rolls for rolling hard in materials. It may be necessary to employ work rolls of a very small diameter but of considerable length. In such cases adequate of the working rolls can be obtained by using a cluster mill.
