Use of anisotropic rubber for venta-nip rolls

A roll having an elastomeric cover for engaging another roll to form a couple, said cover having circumferential grooves and comprising an anisotropic material which has a greater modulus of elasticity as measured in the transverse direction than in the circumferential direction to prevent closing of the grooves by transverse flow of rubber in the nip area.

BACKGROUND OF THE INVENTION 
This invention relates to grooved elastomeric covered rolls and more 
particularly to a vented or grooved roll cover for use in the paper 
industry. 
In the papermaking process, and for example in the press area of a 
papermaking machine, rolls having a resilient outer cover and 
circumferentially extending grooves are used in conjunction with another 
mating roll or other backing surface or means for pressing or squeezing a 
web of material such as paper. In this operation, it is important that the 
roll cover be resilient enough to avoid marking or damage to the web of 
paper or other material passing therebetween and to avoid damage to the 
roll cover in case small foreign bodies happen to pass between the rolls. 
The roll cover, however, must also be capable of supporting large amounts 
of pressure in the nip in order to squeeze or press the web of material. 
The problem, however, becomes even more difficult since it is necessary 
that the circumferential grooves remain open in order to provide the 
proper venting action and to provide for the removal of water through the 
grooves. 
The problem which frequently occurs involves the flattening or squeezing of 
the ribs as they pass through the nip area to such an extent that they 
widen out in directions transversely of the roll and effectively close up 
or restrict the grooves. Solutions in the past tended to involve either 
making the resilient material harder or the grooves wider. Each of these 
proposals, however, has its limitations since if the grooves become wider 
or the material harder, there is more tendency to mark or damage the 
material passing through the nip. 
SUMMARY OF THE PRESENT INVENTION 
In accordance with the present invention, the resiliency of the roll cover 
and the narrowness of the grooves can be maintained by providing a 
covering material which is anisotropic in its composite form. More 
specifically, the covering material has a modulus of elasticity which is 
substantially greater in the transverse direction than it is in the 
circumferential direction. Therefore, when a pressure is applied to the 
roll in the nip area, the rubber will flow or extend in circumferential 
directions rather than in transverse directions, which would widen the 
ribs and tend to close the grooves. 
This anisotropic condition of the resilient covering material can be 
provided by calendering or stretching the elastomeric material in one 
direction in sheet form and then applying the unvulcanized or uncured 
material to the roll with the direction of stretching or calendering, 
extending transversely with respect to the roll. An alternate method of 
providing the anisotropic properties would be to provide, as a component 
of the rubber compound, reinforcing fibers, cords or other such material 
which are oriented primarily in the transverse direction with respect to 
the roll. 
It is an object, therefore, of the present invention to provide an 
elastomeric grooved covering on a roll which is resilient yet capable of 
resisting closure of narrow grooves in the pressure nip area of the roll 
cover. Other objects will be in part apparent and in part pointed out in 
more detail hereinafter. 
The invention accordingly consists in the features of construction, 
combination of elements and arrangement of parts which will be exemplified 
in the construction hereinafter set forth and the scope of the application 
which will be indicated in the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the drawing, and in particular FIG. 1, there is 
illustrated a roll couple 10 comprising an upper roll 12 and a lower roll 
14 with a web 16 passing therebetween. In a typical application, the web 
16 may comprise a web of paper to be dewatered along with a felt material 
on either or both sides of the web of paper. One or both of the rolls 12 
and 14 will include a resilient vented covering. The vented roll will 
include circumferential grooves for the removal of air and water from the 
web of material. Further, there will be a significant pressure applied 
between the two rolls 12 and 14 for the proper squeezing action on the web 
16. This pressure can be as much as a thousand pounds per lineal inch 
(1,000 PLI or 1.78.times.10.sup.4 KG/M) or more. 
Referring now to FIG. 2a, the enlarged partial sectional view illustrates 
the phenomenon occurring in prior art grooved rolls as they pass through 
the nip illustrated in FIG. 1. For purposes of clarity, corresponding 
parts illustrated in FIG. 2a will contain the suffix a. The upper roll 12a 
is illustrated as a solid surface roll, while the lower roll 14a is a roll 
containing a resilient grooved covering. A web 16a is shown passing 
between the rolls 12a and 14a. 
The roll 14a comprises a metal core or base member 18a and a resilient 
outer cover portion 20a, which is adhered to and extends circumferentially 
about the core 18a. The resilient outer cover includes a plurality of ribs 
22a alternating with a plurality of grooves 24a. The ribs 22a typically 
will be significantly wider than the grooves 24a and typically would be 
between 4 and 10 times as wide as the grooves 24a. Further, the grooves 
24a will be as deep as or deeper than the width of the ribs 22a, generally 
on the order of one to two times the width of the ribs 22a. Typically the 
ribs will be from 0.10 to 0.25 inch (2.5 to 6.4 mm) wide and from 0.10 to 
0.25 inch (2.5 to 6.4 mm) deep while the grooves generally will be from 
0.02 to 0.06 inch (0.51 to 1.52 mm) wide. 
It will be seen that under the extreme pressures or loading in the nip area 
of the roll cover the elastomeric material of the outer roll covering 20a 
will be compressed or required to flow in some direction. The tendency is 
for the elastomeric material to flow in the transverse direction in the 
nip area thus forming the bulges or bulbous portions 26a which tend to 
restrict or even close the grooves 24a. Needless to say, this restriction 
or closure of the grooves tends to reduce or altogether eliminate the 
effectiveness of the grooves in the nip area. 
With reference to FIG. 2b, there is illustrated an enlarged view similar to 
that of FIG. 2a showing the configuration of the ribs and grooves of a 
roll cover made in accordance with the present invention as they pass 
through the nip area. For purposes of clarity, parts corresponding to 
parts in FIG. 1 will contain a suffix b in FIG. 2b. Again, there is 
included a solid or non-grooved roll 12b, a grooved roll 14b and a web of 
material 16b passing therebetween. The roll 14b includes a base member 18b 
which in the particular embodiment illustrated is steel and an elastomeric 
covering material 20b which extends circumferentially about the base 
member 18b and is adhered thereto. The outer surface portion of 
elastomeric material 20b includes a plurality of alternating ribs 22b and 
grooves 24b. 
As noted above with respect to the embodiment of FIG. 2a, the ribs 22b 
typically will be significantly wider than the grooves 24b and would be 
between four and ten times as wide as the grooves 24b. Further, the 
grooves 24b will be as deep or deeper than the width of the ribs 22b, 
generally on the order of one to two times the width of the ribs 22b. The 
ribs will be from 0.10 to 0.25 inch (2.5 to 6.4 mm) wide and from 0.10 to 
0.25 inch (2.5 to 6.4 mm) deep. The grooves generally will be 0.02 to 0.06 
inch (0.51 to 1.52 mm) wide. 
In accordance with the present invention, the elastomeric material 20b is 
provided with anisotropic properties such that the modulus of elasticity 
of the material is greater in the transverse direction than it is in the 
circumferential direction. For purposes of this invention, transverse 
directions shall be directions extending parallel to the rotational axis 
of the roll. The modulus of elasticity of the ribs in transverse 
directions should be at least 20% (twenty percent) greater than the 
modulus of elasticity in circumferential directions, and preferably at 
least 50% (fifty percent) greater. These anisotropic properties can be 
provided in any one of several of ways. Specifically, thin layers of 
unvulcanized rubber or other elastic material can be formed by an 
extrusion or calendering process whereby the molecular structure is 
oriented primarily in directions extending parallel to the direction of 
the extrusion or calendering, or in other words longitudinally with 
respect to the thin layer or web of material. These thin sheets of 
extruded or calendered unvulcanized rubber are stretched in the 
longitudinal direction and then sheared to lengths which approximate the 
width or transverse length of the roll. They are then wrapped about the 
roll with their longitudinal or calendered direction extending 
transversely of the roll. This will result in anisotropic properties in 
the roll cover which include a substantially higher modulus of elasticity 
in transverse directions as compared to circumferential directions. 
Alternate methods of providing the higher modulus elasticity in the 
transverse direction include the provision of acicula, fiber or cords as a 
component of the elastomeric or rubber layers with the acicula, fibers or 
cords oriented in the transverse direction with little or no reinforcing 
material extending in circumferential directions. 
Once the elastomeric material 20b has been formed on the roll and provided 
with the anisotropic properties of higher modulus elasticity in the 
transverse direction, the roll covering can be vulcanized in any suitable 
manner known to those skilled in the art. Following vulcanization the 
outer surface is machined to a perfectly cylindrical surface and the 
grooves 20b are carved in the surface in the usual way. 
The elastomeric material along with any fiber, cord or acicular fillers is 
selected to provide a cover having a hardness with a nip equivalent to a 5 
to 20 P & J cover. This nip equivalent hardness is the hardness measured 
in directions perpendicular to the roll cover surface. 
It will be appreciated that although the roll couple illustrated includes 
one grooved roll 14b, and one solid surface roll 12b, it may be desirable 
in some instances to utilize two grooved rolls in the roll couple. It also 
may be desirable to use such a grooved roll 14b in conjunction with a 
backing shoe in an extended nip press such as illustrated in U.S. Pat. No. 
4,229,253 for example. 
While a certain representative embodiment and details have been shown for 
the purpose of illustrating the invention, it will be apparent to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the spirit or scope of the invention.