Extended Nip Press With Special Belt Reinforcement

The foregoing abstract is not to be taken as limiting the invention of this application, and in order to understand the full nature and extent of the technical disclosure of this application, reference must be made to the accompanying drawings and the following detailed description.

This invention relates to presses for extracting water from a continuous traveling web and particularly to such a press section for extracting water from a newly formed web of paper in a papermaking machine. More particularly it relates to an extended nip press structure and an endless belt utilized in such press structure.

While the present invention relates to dewatering of a continuously running web of any material, it will be described herein with respect to the specific process of dewatering a web of paper. In the papermaking process, the web is formed by depositing the slurry of pulp fibers on a traveling wire. A large portion of the water is normally extracted from the web in the forming area by gravity or suction. The web then passes through what is known as a press section which normally would involve a series of nips of pairs of roll couples in which a substantial amount of the remaining water is squeezed out. The web will then pass on to a drying section which normally is composed of a series of heated drums to drive water off by vaporization. The web then finally passes to such finishing operations as calendering, coating, slitting, winding, et cetera.

The present invention relates specifically to a particular type of press section wherein the pressing operation in each unit is extended in time and thereby results in the extraction of significantly more water than in the heretofore nip of a roll couple. This extended nip pressing is accomplished by wrapping an endless belt about an arc of a rotating drum. The web is sandwiched between the endless belt and the drum and may have a traveling felt on one or both sides thereof for absorbing the water from the web. Additional pressure is provided to the arc of contact area by means of a pressure shoe located on the side of the belt opposite the drum.

The principles and advantages of extended nip pressing have been discussed in U.S. Pat. Nos. 3,798,121 and 3,853,698, both of which are assigned to the assignee of this invention. These principles and advantages, therefore, need not be discussed herein. The present invention, however, is related to an extended nip press of the type disclosed in U.S. Pat. No. 3,853,698 wherein a pressure shoe located on the side of the belt opposite the drum to generate high pressing forces against the web. This is to be distinguished from the type disclosed in aforesaid U.S. Pat. No. 3,798,121 in which the pressure is provided by tension in one or more belts as they pass about the drum.

In the operation of such extended nip press sections having a pressure shoe, a problem has evolved wherein a bulge or bow forms ahead of the nip. The exact phenomenon which causes this bow or bulge is not fully understood. It is clear, however, that center portion of the endless belt in the area of the shoe is compressed, heated by the oil and friction and is otherwise worked differently than the rather wide edges of the belt. The bulge will sometimes be centered on the belt and at other times will be off to one lateral side of the belt. It will sometimes appear on the downstream side of the shoe on the laterally opposite side of the belt relative to a bulge on the upstream side of the belt. Experience thus far shows that the bulge is always confined in lateral directions to the shoe area.

Needless to say, this bulge in the belt is undesirable for many reasons, among which is the fact that it can cause wrinkling or creasing of the web. While the bulge can be eliminated by increasing the tension on the belt, this is not fully satisfactory since it causes increased loading on belts, shafts, bearings and drives. This in turn results in a decrease in the service life of such components and an increase in power consumption and down time.

The complexity of the operating conditions renders a solution to the problem evasive. Presently, pressure shoes having a 10 inch arc of contact and pressures of 600 pounds per square inch are utilized in experimental machines. This means that the belt is subjected to 6,000 pounds of normal force for every inch of width of the belt in the shoe area. Further, it is contemplated that pressures may be increased to 900 pounds per square inch or above and arcs of contact might be increased to as much as 20 inches or more. A 20 inch arc of contact and shoe pressures of 900 psi would result in 18,000 pounds of normal force for each inch of width of the belt in the shoe area.

Further, since the belt is in sliding contact with the shoe and under extremely high pressure, significant heat can be generated due to the sliding friction. The hydraulic fluid in the shoe is maintained at 140 degrees Fahrenheit (46 degrees Centigrade) to maintain the proper viscosity. With the heat caused by the sliding friction and hysteresis losses in the belt added to the heat from the oil, it is believed that belt temperatures may approach 200 degrees Fahrenheit (79 degrees Centigrade).

According to the present invention, an extended nip press section is provided in which circumferentially extending cords are located in the belt only throughout the shoe area. This has permitted elimination of the bulge with substantially less tension in the belt.

Other objects, advantages and features will become more apparent with the disclosure of the principles of the invention and it will be apparent that equivalent structures and methods may be employed within the principles and scope of the invention in connection with the description of the preferred embodiment and the teaching of the principles in the specification, claims and drawings.

FIG. 1 is a schematic side view of a press section of a papermaking machine;

FIG. 2 is a partial cross-sectional view of the apparatus of FIG. 1 taken substantially along line 2--2 and illustrating the present invention.

With reference to the drawing, and in particular FIG. 1, there is illustrated a schematic side elevational view of an extended nip press section 10 of a papermaking machine. The press section 10 includes a press roll 12 rotatable about an axis 14 which extends transversely of the press section. For purposes of this invention, lateral or transverse directions shall be directions which extend parallel to the rotational axis 14 of the press roll 12. Also, longitudical or circumferential directions shall be directions which extend parallel to the direction of motion of the belt or web of paper.

A flexible endless belt 16 is trained about a plurality of pulleys 18 through 22 which are arranged in such a fashion with respect to the press roll 12 that the belt 16 wraps about a portion of the roll 12 to form an arcuate press area 24. One or more of the pulleys 18 through 22 are mounted in a known manner for movement in directions perpendicular to their respective rotational axis to permit installation of the belt 16 and adjustment of the tension in the belt 16.

An arcuate pressure shoe 26 is disposed adjacent the belt 16 on the side thereof opposite the roll 12 and press area 24. A force F is exerted on the pressure shoe by any suitable means to exert a pressure on the belt 18 in the press area. To insure even pressure P across the belt in this area, and minimize sliding friction, hydraulic pressure is supplied through a pipe 28 to a cavity 31. The pressure is regulated by means of a valve 30. The specific mechanical and hydraulic operation of the pressure shoe forms no part of the present invention and, therefore, will not be discussed herein in further detail. Further, although a pressure shoe 26 with a fluid cavity 31 is illustrated, it will be appreciated that a solid pressure shoe with an arcuate surface to mate with the roll 12 could be utilized. For a specific example of a pressure shoe, reference may be had to U.S. Pat. No. 3,853,698.

A felt 32 is trained about the press roll 12 and passes between the press roll 12 and the belt 16. A web of material 34 to be dewatered, is applied to the felt 32 and carried through the press area 24 in the direction of the arrows 36. While only one felt 32 is illustrated, it will be appreciated that a double felt system could be utilized wherein the web of paper or other similar material 34 is sandwiched therebetween.

As best seen in FIG. 2, the pressure shoe 26 is disposed in the transverse center area of the roll 12 and belt 16. The width PW of the pressure shoe is substantially less than the width BW of the belt and, therefore, exerts a pressure only over the center portion of the moving belt. This leaves the laterally outer portions 40,41 free of any normal force or pressure caused by the pressure shoe 26.

As discussed above, during the operation of such an extended nip press, a problem has arisen wherein a bulge or bow appears in the belt 16 on the ingoing side of the nip at various positions across the width PW of the pressure shoe. The bulge or bow can occur in a central location with respect to the shoe or at either lateral side of the shoe. Further, the bulge will sometimes appear at one lateral side of the shoe on the upstream side and at the opposite lateral side of the shoe on the downstream side. Attempts heretofore at eliminating this bulge have generally been directed to increasing the tension in the belt 16. While these attempts have successfully removed the bulge, they also result in undesirably increasing the forces and loads on the belt, bearings and drive.

It has been discovered quite surprisingly that by limiting all reinforcing members which are capable of resisting longitudinal tension to the area of the shoe, the tension required to eliminate the bow or bulge can be reduced quite significantly. Therefore, in accordance with the present invention, a reinforcing structure 38 capable of resisting longitudinal tension is provided in the belt and restricted to the central area BW. This reinforcing structure 38 should include a flexible reinforcing material which is capable of being flexed around the pulleys 18 to 22 and drum 12 without loss of strength. The reinforcing structure also should have enough strength and modulus to absorb the necessary tension in the belt without an unacceptable amount of elongation.

The elastomers used in making the belt should be carefully chosen to provide low hysteresis loss to minimize heat build up. It must be resistant to high temperatures and compatible with whatever hot oil is used in the pressure shoe as well as water and common chemicals used in paper machines. Further, it should have good abrasion resistance and a low coefficient of friction since it will be subjected to sliding friction as it passes over the shoe. Suggested elastomers include acrylonitrile butadienes, ethylene acrylic copolymers, polyurethanes, fluorinated hydrocarbons and epichlorohydrin rubbers.

In the specific embodiment illustrated, the reinforcing structure 38 is comprised of a single strand of rayon cord which was helically wrapped about the mandril at a rate of 15 turns per inch and under a tension of 5 pounds. The rayon had a strength of about 90 pounds per cord resulting in a tensile strength for the belt structure of approximately 1,350 pounds per lineal inch. In some applications it may be desirable to provide a layer of cords of lesser strength extending transversely of the belt for added stability.

The uncured belt structure was then wrapped with a nylon tape and cured in open steam. Subsequent to cool down the outer surface of the belt is ground down to provide the desired thickness in the belt. In prior art, extended nip press sections in which the circumferentially extending reinforcing members extended completely across the belt a tension of 75 to 100 pounds per lineal inch was required to assure that no bulges appeared in the belt. In a structure in accordance with the present invention and in specific in accordance with the embodiment illustrated herein, a tension of only 30 to 50 pounds per lineal inch was required to assure that no bulge or bow developed in the belt.

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 this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.