Roll set for thin metal strip

The bridle or bridles of a stretching or levelling line for a thin metal strip, e.g. of a thickness of up to 1 mm, can have at least one roll of a periphery variable between an outwardly convex and an inwardly concave bulge to compensate for dishing of the strip or edge waviness.

FIELD OF THE INVENTION 
The present invention relates to a roll set or bridle for thin metal strip, 
i.e. metal stripping having a thickness up to 1 mm. More particularly this 
invention relates to an apparatus used in the handling of thin metal strip 
which includes a pair of rollers, frequently referred to as a bridle, 
about which the strip passes in a S-pattern and to apparatuses embodying a 
roll bridle and possibly two or more roll bridles in conjunction with the 
levelling or flattening of the strip. 
BACKGROUND OF THE INVENTION 
For the levelling or flattening of the metal strip, various straightening 
processes are Used, e.g. stretch or tension straightening, tension 
levelling, stretcher levelling and stretch-bend levelling. 
Generally, these straightening or levelling processes are carried out 
continuously and, although the levelling process has been greatly improved 
in recent years, even today, it is necessary to tolerate slight central 
dishing of the strip or edge corrugation following the straightening, 
flattening or levelling process. The residual lack of planarity results 
from the nonuniform transverse stresses distributed over the width of the 
strip and which are superimposed on the tensile stresses in the 
longitudinal direction of the strip. The smaller the strip thickness and 
the modulus of elasticity of the strip, the greater is the risk of 
residual lack of planarity. The same applies to cold rolling of metal 
strip, especially for the after-rolling or dressing or final rolling 
stages. Because of the bending of the rolls, as a result of the rolling 
force, and because the incoming thickness profile of the metal strip is as 
a rule not truly rectangular, nonuniform deformations can arise in the 
rolling gap or nip of the rolling mill over the width of the strip, 
leading to corrugation or waviness in the strip following cold rolling. 
As a result, it has been the practice to try to correct the roll gap 
geometry to eliminate the corrugation or waviness. These efforts, however, 
have proven to be unsuccessful or insufficient. It appears that the 
deformation that occurs in the rolling gap does not depend exclusively on 
the rolling force distribution but is also a function of the tensile 
stress distribution over the width of the strip. That distribution could 
not be readily modified heretofore. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide a roll set, 
especially a bridle over which the strip passes in an S-pattern, so that 
in conjunction with the levelling or flattening of a metal strip, 
especially a thin metal strip, both the edge waviness and central dishing 
of the strip can be significantly reduced and even eliminated in many 
cases. 
Another object of the invention is to provide an improved levelling or 
flattening system for thin metal strip whereby drawbacks of earlier 
systems are obviated. 
It is also an object of the invention to flatten or level metal strip and 
especially metal strip having a thickness up to 1 mm utilizing at least 
one bridle over which the strip passes in an S-pattern. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the invention by providing an apparatus which 
has at least one bridle over which the strip passes and around which the 
strip is looped in an S-pattern so that at least one of the rolls of the 
bridle can have its periphery selectively deformed between an outwardly 
convex bulge and an inwardly concave bulge, while the respective metal 
strip passes around the adjustable-periphery roll with a predetermined 
looping angle or arc. 
More particularly, a roller bridle for metal strip having a thickness up to 
1 mm can comprise a pair of rolls over which said metal strip passes in an 
S pattern, at least one of said rolls having a rotatable support, a 
flexible periphery with which said metal strip is in contact over a given 
wrapping angle, and means for imparting to said periphery a selected 
deformation ranging between an outwardly convex and an inwardly concave 
bulge. 
According to the invention, when a thin metal strip passes continuously 
through the bridle of the invention and a convex bulge is imparted to the 
variable-periphery roll of the bridle, a tensile stress concentration is 
effected in the center of the strip. By contrast, with a concave inner 
bulge, a tensile strength concentration is effected along the edges of the 
strip. At the locations of the tensile stress concentration, there is a 
locally higher plastic deformation. As a result, edge waviness or 
corrugation and central dishing which can arise in the strip because of 
different degrees of stretch across the strip width and the different 
lengths of individual strip zones, can be reduced or completely 
eliminated. 
With a convex outward bulging or curvature of the roll, the center of the 
strip is stretched to a greater extent and in this manner edge waviness 
can be eliminated. With a concave inwardly bulging of the roll, the edges 
of the strip are more highly stretched to compensate transverse stretch 
stresses which increase from the strip edges toward the center of the 
strip. The central region of the strip and edge regions are thus stressed 
to similar degrees and a central dishing can be eliminated. 
The tensile strength which is applied during the flattening and levelling 
prevents excessive stretch in either of the central or edge regions and 
thus both the dishing and the edge waviness can be eliminated. 
According to a feature of the invention which is of independent inventive 
significance, two bridles or pairs of rolls are provided and the web is 
looped in an S-pattern around the rolls of both bridles. Along the stretch 
of strip between the bridles, the strip is subjected to tension and can be 
levelled or flattened, e.g. by a cold rolling mill or an array of stretch 
bend levelling rollers. 
Each bridle may have a drive for the downstream roller and a brake for the 
upstream roller or the strip may be simply pulled through the upstream 
roller of the bridle braked to a greater degree than any braking applied 
to the downstream roller. 
These systems have been found to be especially effective in the flattening 
of metal strip which is intended for use in lithography. Where surface 
distortion is to be avoided, as in this case, the strip can simply be 
flattened by displacing it through the two bridles without the cold 
rolling mill or the array of stretch-bend rollers. A surface free from 
distortion is particularly important for the lithographic strip. 
When the cold roll stand and/or the array of stretch bend rollers are 
provided along a stretch of the strip between the upstream and downstream 
bridles, at least the rolls of the bridle proximal to the aforementioned 
stretch, i.e. directly upstream and downstream of the latter, are provided 
with the adjustable periphery capable of adjustment between the outwardly 
convex bulge and inwardly concave bulge. In this case, the effects 
obtained are similar to those with cold rolling and stretch bend 
flattening but with elimination of the central dishing and the edge 
waviness. 
The concentration of the tensile stresses either in the central region of 
the strip or in the edge regions of the strip, as a consequence of a 
higher plastic deformation with a higher degree of stretch are able, not 
only during dimension levelling but also during stretch bend flattening, 
to operate with a greater thickness reduction during cold rolling to 
eliminate edge waviness and central dishing. 
According to another feature of the invention, all of the rollers of the 
bridle or bridles can be equipped so as to have their peripheries deformed 
between convex and concave peripheries. The rolls of the bridle can be 
arranged one above another in a vertical position or with one adjacent the 
other in a generally horizontal orientation. 
To insure a high degree of plastic deformation in the central or edge 
regions of the strip, the roll diameter of each of the adjustable 
periphery bridle rolls can have a diameter of at least 250 mm while the 
looping angle of the strip about each bridle roll is at least 90.degree. 
and preferably is greater than 150.degree.. 
According to a feature of the invention, the means for adjusting the 
periphery of the bridle roll can include a pressurizable chamber within 
the roll between the roll core and a peripheral shell and which can be 
supplied with a hydraulic or pneumatic fluid pressure medium. Upon 
pressure relief, the shell can assume an outwardly concave configuration. 
The configuration can be effected by a negative grinding of the peripheral 
shell which can be unitary with the remainder of the roll. 
The core can have two stub shafts at opposite axial ends by means of which 
the roll is mounted in the usual journals and an axial bore or passage in 
one of these shafts can be connected to the chamber to supply a 
pressurizable fluid thereto. A venting bore can be provided at least 
temporarily for the chamber to allow, for example, the chamber to be 
filled with the pressurizable fluid. 
By appropriate selection of the inner contour of the roll shell, upon 
pressurization of the chamber it is possible to obtain a parabolical shape 
of the periphery. The negative grinding can provide a paraboloidal inward 
bulge such that the bulging to either side of the cylindrical shape can be 
readily effected in a positive or negative sense. The venting bore is only 
required for the initial filling of the pressurizable chamber.

SPECIFIC DESCRIPTION 
From the drawing, it can be seen that the basic element of the view is a 
bridle generally represented at 20 which comprises two rolls 1, 2, 
referred to as looping rolls or deflection rolls, about which the thin 
metal strip 3 is looped in an S-shaped pattern. As can be seen from this 
Figure, a stretching zone Z may be provided between the upstream roll 1 
and the downstream roll 3 and the stretching force may be provided by 
applying a brake 21 to the upstream roll 1 and either driving the 
downstream roll 2 with a drive 22 or applying traction as represented at 
23 via other rolls to the strip. An alternative to braking the upstream 
roll 1 is to apply a retardation force to the strip 3 as represented at 
24, upstream of that roll 1. The traction may be applied by another bridle 
roll pair and the retardation may be applied by another bridle roll pair. 
The angle of looping is represented at alpha in FIG. 1 and is 
approximately 225.degree.. Looping angles are in excess of 90.degree. and 
preferably in excess of 150.degree.. 
The meter strip can have a width in excess of 800 mm and a thickness of up 
to 1 mm and especially can be metal strip utilized in the lithographic 
industry. 
At least one of the looping rolls 1 or 2 and preferably both of them has a 
variable periphery which is selectively alterable from a convex bulge 
(FIG. 5a) to a concave bulge (FIG. 5b), the periphery being represented at 
6. 
As can be seen from FIGS. 2 and 3, 2 bridle roll pairs 20 and 20' can be 
provided on upstream and downstream sides of a horizontal stretch 25 of 
the strip 3 at which a cold rolling stand 9 or an array of stretching 
bending rolls 10 can be provided. The stretch bending rolls 10 include 
rolls 10a, 10c and 10e located on one side of the strip 3 with the rolls 
10b and 10d staggered so as to lie on the opposite side in the spaces 
between the rolls 10a and 10c and the rolls 10c and 10e, respectively. 
In the embodiments of FIGS. 2 and 3, the bridles 20 and 20' are shown to be 
vertically disposed and substantially horizontally disposed, respectively 
and the proximal rolls 1 and 2 to the stretch 25 at least are of the 
variable periphery described so that the periphery can be changed from an 
outwardly convex to an outwardly concave periphery and vice versa. 
The diameters of at least the rolls 1 and 2 and preferably all of the rolls 
1, 2, 7, 8 has a diameter of 250 mm in the cylindrical configuration of 
the variable periphery rolls and here as well the looping angle should 
exceed 90.degree. and is preferably more than 150.degree. to generate the 
sufficiently high plastic deformation in either the edge regions of the 
strip or in the central region of the strip that is desired. 
FIG. 4 shows, in highly diagrammatic form, a deflection or looping roll 1 
of one of the bridles and which can have a convex to concave periphery 6. 
The periphery is formed on a shell 26 which is unitary with the roll core 
11 forming a support from which stub shafts 15 and 27 project axially. 
Between this shell and the core 11 is a pressurizable chamber 12 connected 
by a passage 13 to an axial passage 28 in the stub shaft 15. The shafts 15 
and 27 are received in journals represented diametrically at 14 which can 
be provided with a feeder for the pressurizable fluid. The pressurizable 
medium is drawn by a high pressure pump from a reservoir 30. 
A vent passage 16 can be used for filling the chamber 12 and can then be 
closed by an appropriate plug or the like for pressurization. As is 
apparent from FIGS. 4 and 5b, in the depressurized state of the chamber 12 
the periphery 5 has a concave contour as represented at 5 and the 
periphery has a convex contour 4 when liquid in the chamber 12 is 
pressurized. The range of variation of the periphery of the roll has been 
illustrated in FIG. 5c. It will be apparent that all of the rolls 1, 2, 7 
and 8 can be of similarly variable periphery.