What is Rolling Mill?
Rolling Mill (or roll stand or steel rolling mill) is a machine in which all the equipment for metal rolling like rolls, bearings, housing, motors, frame, power system, etc are set up to perform a metal rolling operation.
In steel rolling mills, workpiece having greater thickness is fed through the rollers and the final workpiece with reduced thickness and better surface finish is obtained.
Parts/Components of rolling mills
Few important components of steel rolling mills are listed below.
- Gap control mechanism
- Power system
- Power transmission mechanism
How rolling mills work?
Before knowing the working of rolling mills, first you should know that there are various rolling mills according to the number of rolls, position of rolls, as well as type of rolls.
Various rolling mills used in industries are;
- Two high reversing mills
- Two high non reversing mills
- Three high rolling mills
- Four high rolling mills
- Tandem rolling mills
- Cluster rolling mills
- Planetary rolling mills
Now let’s see how these rolling mills work.
#1) Two high reversing mills
In two high reversing mills, the rotation of rollers occurs in forward direction as well as reverse direction.
Firstly, the rollers rotate in one direction and then they rotate in opposite directions so that the metal can pass several times in the back and forth direction through the rollers.
Two high reversing mills are used for rough work in flat plate, rails, etc.
#2) Two high non reversing mills
In two high non reversing mills, two rolls rotate only in a single direction.
As the name suggests, “non reversing” means that the reverse motion of the workpiece is not possible in this type of steel rolling mills.
However, the workpiece is carried back over to the inlet of the roller mill for passing it again through the rollers.
#3) Three high rolling mills
Three high rolling mills consist of three parallel rolls which are arranged one above the other (see below image).
As shown in the above diagram, the above two adjacent rollers rotate in opposite directions which forces the workpiece in the forward direction. Again this workpiece is fed between the middle and lower roller and finally the rolled workpiece is obtained.
In simple words, the workpiece first passes through the top and middle rollers and then it is returned between the middle and bottom roller.
The thickness of the metal is reduced in each pass of the three high rolling mills.
In three high rolling steel mills, the flat rollers as well as grooved rollers are used as per the applications.
#4) Four high rolling mills
Four high rolling mills consist of four rollers parallely arranged one above the other.
As shown in the diagram, two outermost rollers spin in opposite directions. Also the two inner rollers rotate in opposite directions.
The size of the inner rollers is kept smaller than the size of the outer rollers.
These outer rollers are known as backup rollers which provide rigidity to the inner rollers.
#5) Tandem rolling mills
In tandem rolling mills, the metal is passed through several stands such that there occurs a gradual reduction of thickness in every passing.
Tandem rolling mills can also be used with a continuous casting process and this gives more advantages for manufacturing longer workpieces.
#6) Cluster rolling mills
Cluster rolling mills are special types of four high rolling mills in which the working rollers are backed up by two or more larger rollers.
Cluster rolling mills are generally used for rolling of hard materials.
#7) Planetary rolling mills
Planetary rolling mills are a special type of steel rolling mills in which the smaller diameter working rollers are arranged around the single larger roller (see below image).
The advantage of planetary rolling mills is that it avoids the sliding friction during the rolling operation.
In planetary rolling mills, generally the feed rollers are sometimes used to feed the metal piece to the planetary rollers.
Each working roller of the planetary mill is used for gradual reduction in thickness of the metal.
Planetary rolling mills have the advantage that they can be used in production plants where there are frequent changes in size of metals.