The following points highlight the seven main types of longitudinal rolling stands. The types are: 1. Two High Rolling Stand 2. Three High Rolling Stand 3. Four High Rolling Stand 4. Sendzimir Rolling Stands 5. Planetary Rolling Stands 6. Universal Rolling Stands 7. Rolling of Splines.
Type # 1. Two High Rolling Stand:
A rolling stand is the basic rolling equipment. A rolling stand having two rolls is shown in Fig. 8.3. It consists of two upright structures which house the bearings of rolls. For adjusting the roll gap the top roll along with its bearings can be moved up and down with the help of screw down mechanism which is mounted on a platform on the top of two upright structures.
The screw only pushes the roll down. The roll is kept pressed against the screw by counter weights or hydraulic cylinders. A rolling stand is designated according to the number of rolls it has. If there are two rolls in a stand as shown in Fig. 8.3, it is called two high rolling stand.
Type # 2. Three High Rolling Stand:
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Figure 8.4 shows arrangement of rolls in a three high rolling stand. The upper two rolls roll the billet in the forward direction while the lower two roll it in the reverse direction. Three or more passes may be given in a three high stand. These stands are generally placed after roughing stands in Merchant section rolling mills and in rail and structural rolling mills.
Type # 3. Four High Rolling Stand:
In rolling of very thin strips and sheets the elastic deformation of rolls and strip is a dominant factor and if we go on decreasing the thickness of strip, a stage comes when no rolling takes place. The strip only suffers elastic deformation in roll gap and on exit side recovers back to initial size.
In order to roll the strip further we must reduce the diameter of rolls. Reduction in roll diameter reduces the rolling load significantly. Reduction in coefficient of friction also helps because it also reduces the rolling load, however, the maximum possible reduction also gets reduced.
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The reduction in roll diameter makes the rolls less rigid and they tend to bend like a beam, which is not desirable for rolling of flat sheets. Therefore, back up rolls are provided to keep the working rolls straight. This makes the rolling stand a 4-high-stand (Fig. 8.5). Even with back up rolls some bending is still there and to counter this the roll surface is cambered so that after bending the roll surface in contact with sheet becomes straight. In a four high stand power is transmitted to working rolls.
Type # 4. Sendzimir Rolling Stands:
For rolling of very thin foils the diameter of working rolls is quite small and it becomes desirable to restrain the bending of rolls in the vertical as well as lateral direction. Two backup rolls are, therefore, provided for each working roll. These rolls are placed at an angle on working roll periphery. This makes the stand a 6-high rolling stand as shown in Fig. 8.6. The power may be transmitted directly to working rolls.
With further reduction of working roll diameter, the diameter of back up rolls also gets reduced and it becomes necessary to provide back up for the two back up rolls in contact with working rolls. Thus three more back up rolls are provided for each working roll and the rolling stand becomes a 12-high rolling stand. The arrangement of rolls in this stand is shown in Fig. 8.7.
For similar reasons rolling stands up to 24-high are in use for rolling very thin foils. These stands are also known as Sendzimir rolling stands. When the diameter of working rolls is very small, it may be made out of tungsten carbide.
The elastic modulus of cemented carbide is nearly two and a half times of that of steel, consequently its use reduces the elastic deformation of roll surface. Because the working roll diameter is very small the power has to be transmitted by friction between the rolls. The working and backup rolls are kept compressed together with the help of springs.
Type # 5. Planetary Rolling Stands:
The idea of using a small diameter working rolls has led to the development of planetary rolling stands which are illustrated in Figs. 8.8 and 8.9. The main advantage of the process is that very large reductions as much as 95% may be given in a single pass. The process is slow but is carried out in hot state and, therefore, planetary mill has to be installed very near the furnace such that the bar is partially in the furnace while being rolled.
The complete system consists of (i) furnace, (ii) feed-roll stand, (iii) planetary rolling stand and (iv) planishing rolling stand (Fig. 8.8). The feed rolls push the work material into the planetary rolls. The rolled stock at the exit from planetary rolls is not flat.
There are periodic variations in thickness due to pitch and circular path of planetary rolls. The planetary rolling stand is followed by a two high planishing rolling stand to even out these undulations. The problem of undulations is also solved in the improved design of planetary rolling mill shown in Fig. 8.9.
The mill consists of two stationary central rolls having double row of planetary rolls mounted in cages around them. The planetary rolls and cages rotate around the central rolls. In the rolling gap, the central rolls have flat machined surfaces on which the planetary rolls are guided during rolling. Thus the rolled stock is uniform in thickness along its length and there is no need of planishing rolling stand.
The speed of rolling of planetary rolling mills is rather low and these stand: are economical when the demand is for small quantities of various sections and alloys.
Type # 6. Universal Rolling Stands:
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In longitudinal rolling, it is often required to roll the billet in the height direction as well as in the width direction. In reversing mills the billet is turned through 90 degrees after one or more passes in order to give reduction on the other side. In continuous mills such as billet rolling mills, turning the billet is not possible because it is simultaneously rolled through several stands. The mill therefore, has both horizontal and vertical rolling stands placed one after another.
The horizontal and vertical rolls may also be arranged in the same rolling stand. Such a stand is called universal rolling stand. Universal slab rolling stands are in use in several plants. Universal rolling stands are used for rolling of slabs, rolling of wide flange beams and other sections.
Large sections such as wide flange beams are rolled in universal beam rolling stands because rolling of large flanges in horizontal mills poses a very serious problem of cutting deep grooves in rolls, which makes the rolls very weak. Figure 8.10 shows a schematic arrangement of rolls in universal beam rolling stand.
Type # 7. Rolling of Splines:
A scheme of rolling splines on round shafts is shown in Fig. 8.11. Two disc rolls, mounted on bearings, are fixed opposite each other at a desired distance. The cylindrical work piece is pushed through the rolls.
In case of hard metals or deep splines several passes may be needed, the distance between disc rolls may be adjusted after each pass. After rolling one pair of splines the shaft is indexed for the next pair of splines. In this arrangement only even number of splines may be rolled.
In another arrangement a number of disc rolls equal to the number of splines, provided the number of splines is not large, are fixed on a pitch circle and the shaft is simply pushed through the disc rolls. Thus all the splines are rolled simultaneously.