EP4653615A1

EP4653615A1 - A drive unit for a web forming machine and a web forming machine

Info

Publication number: EP4653615A1
Application number: EP24177291.2A
Authority: EP
Inventors: Antti Poikolainen
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2024-05-22
Filing date: 2024-05-22
Publication date: 2025-11-26
Also published as: CN121006721A

Abstract

Invention relates to a drive unit (10) for a web forming machine comprising an end-less fabric loop (12) wherein a forming area (14) is configured when in use, in which web is in contact with the fabric, and a number of rolls (18.1,18.2,...,18,N) which are arranged to guide the fabric loop (12), a drive unit (10) for running the fabric loop (12). The drive unit (10) comprises a first roll (20) and a second roll (22) arranged at opposite sides of the fabric, wherein at least one of the rolls (20,22) is a driven roll, and a driving nip is formed about the rolls (20,22) and the fabric outside the forming area (14) of the fabric loop (12). Invention relates also to a web forming machine provided with a drive unit according to the invention.

Description

Technical field

The present invention relates to a drive unit for a web forming machine according to the preamble of claim 1.
The present invention relates to a web forming machine according to the preamble of the second independent claim.

Background art

A web forming machine, depending on its configuration, comprises one or more endless fabric loops which are used for forming the web on the fabric, or between two fabrics, by predetermined dewatering process. The fabric loop is arranged to run via several lead and/or guide rolls at desired speed. Power needed for running the fabric is transmitted to the fabric by one or more driven rolls which are connected at their end to a controllable electric motor. Transmission of roll torque to the fabric is made possible by friction between the fabric and the roll. In order to transmit required power to the fabric without slippage, the fabric tension needs to at adequate level, being for example at a magnitude of 8-10 kN/m. The high tension of the fabric requires strong and stiff rolls as well as framework supporting the rolls.
An object of the invention is to provide a drive unit for a web forming machine which is considerably improved compared to the prior art solutions.
An object of the invention is to provide a web forming machine which is considerably improved compared to the prior art solutions.

Disclosure of the Invention

Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.
A drive unit for a web forming machine according to the invention comprises an endless fabric loop wherein a forming area is configured when in use, in which web is in contact with the fabric, and a number of rolls which are arranged to guide the fabric loop, a drive unit for running the fabric loop wherein the drive unit comprises a first roll and a second roll arranged at opposite sides of the fabric, wherein at least one of the rolls is a driven roll, and a driving nip is formed about the rolls and the fabric outside the forming area of the fabric loop.
By means of the present invention tension of the fabric need not be so high as before because force transmitting friction can be effectively improved by combination of contact surface to the fabric and nip load used in the driving nip.
According to an aspect of the invention the first roll is a lead roll for the fabric loop with a predetermined fabric wrap angle, when in use, and the second roll is a pressing roll having a nip contract with the fabric.
This way the second roll can be designed putting more emphasis on providing traction than guiding the fabric.
According to an aspect of the invention the first roll and the second roll are arranged in a nip contract with the fabric with zero wrap angle. In other words, both of the rolls meet the fabric in substantially zero wrap angle. This aspect is particularly useful in rebuild application where the drive unit can be easily installed between two successive, existing rolls.
According to an aspect of the invention only one of the first roll and the second roll is provided with a drive, being a driven roll. This way the drive unit structure is very simple.
According to an aspect of the invention the area in the second roll which is in contact with the fabric is provided with soft cover, having its hardness lower than hardness of the first roll. This way the force transmission from the second roll to the fabric is at adequate level improving friction and this also reduces required nip force.
According to an aspect of the invention the second roll comprises one or more fluid cavities in the cover, hardness of which is controllable with pressure of the fluid in the cavity.
By making use of controllable cavity or cavities the traction for force transmission is controllable in straight forward manner.
According to an aspect of the invention the second roll comprises one or more separate loading zones in width direction of the roll. This way there is a possibility to control force transmission in width direction.
According to an aspect of the invention that second roll comprises an assembly of several roll surface segments. This way the second roll may be constructed of smaller modules and use modularized manufacture.
A web forming machine according to the invention comprises an endless fabric loop, and a number of rolls which are arranged to guide the fabric loop, a drive unit for running the fabric loop, wherein a forming area is configured when in use, in which web is in contact with the fabric, when in use. The fabric loop is operated such that in that in an area outside the forming area the fabric loop is provided with a drive unit which comprises a first roll and a second roll arranged at opposite sides of the fabric, wherein at least one of the rolls is a driven roll, and a driving nip is formed about the rolls and the fabric outside the forming area of the fabric loop.
By means of the present invention tension of the fabric need not be so high as before because force transmitting friction can be effectively improved by combination of contact surface to the fabric and nip load used in the driving nip. According to the invention, tension of the fabric is 2-7 kN/m, advantageously 3-6 kN/m.
In the web forming machine according to an aspect of the invention the first roll is a lead roll for the fabric loop with a predetermined fabric wrap angle, when in use, and the second roll is a pressing roll having a nip contract with the fabric. This way the second roll can be designed putting more emphasis on providing traction than guiding the fabric.
In the web forming machine according to an aspect of the invention only the first roll or the second roll is provided with a drive, being a driven roll.
In the web forming machine according to an aspect of the invention the area in the second roll which is in contact with the fabric is provided with soft cover, having its hardness lower than hardness of the first roll. This way the force transmission from the second roll to the fabric is at adequate level improving friction and this also reduces required nip force.
In the web forming machine according to an aspect of the invention second roll comprises one or more fluid cavities in the cover, hardness of which is controllable with pressure of the fluid in the cavity. This way the traction for force transmission is controllable in straight forward and reliable manner.
In the web forming machine according to an aspect of the invention second roll comprises at least one or more separate loading zones in width direction of the roll. This way there is a possibility to control force transmission in width direction.
In the web forming machine according to an aspect of the invention that second roll comprises an assembly of several roll surface segments. This way there is a possibility use modularized construction.
The web forming machine may be implemented in various configurations. In the web forming machine according to an aspect of the invention the fabric loop is guided around at least three rolls, one of which rolls is a roll of the drive unit. This provides very compact and space saving web forming section.
In the web forming machine according to an aspect of the invention the fabric loop is guided around at least three rolls, one of which rolls is the first roll of the drive unit. This way the second roll can be designed putting more emphasis on providing traction than guiding the fabric.
In the web forming machine according to an aspect of the invention the fabric loop is guided around four rolls, one of which rolls is the first roll of the drive unit. This provides the smallest feasible web forming machine configuration according to the invention.
In the web forming machine according to an aspect of the invention in the area outside the forming area the fabric loop is supported underneath the fabric with at least one stationary support beam arranged in cross direction in respect of the normal running direction of the fabric. This way the fabric can be supported more efficiently without use of the additional rolls.
In the web forming machine according to an aspect of the invention the area outside the forming area the fabric loop is supported underneath the fabric with at least two successive stationary support beams. To avoid long unsupported draws the fabric may be supported by several support beams.
In the web forming machine according to an aspect of the invention the drive unit is arranged at vicinity of the forming area after the web has been transferred away from the support of the fabric. This is an advantageous location of the drive unit.
In the web forming machine according to an aspect of the invention that web forming machine comprises more than one forming section configured to manufacture web with more than one layer, wherein at least one, preferably all of the forming sections is provided with a drive unit according to the invention.
The present invention has several advantages. When tension of the fabric is lower, several additional benefits will emerge. For example, with lower tension needed in the fabric the rolls may be designed less robust, which in turn means lighter rolls. Also the frames of the web forming machine can be build lighter. Additionally, number of rolls in the web forming machine can be decreased, for example because guiding of the fabric can be realized with a single roll, to which also fabric tensioning functionality or means is possible to integrate. Secondly when number of roll of the forming section is reduced the forming section required less horizontal space and therefore the building, machine hall, can be smaller than before.
The drive unit according to the invention is applicable to run a web forming machine which is configured to manufacture for example paper, board, tissue, pulp or non-woven web which is produced by laying material, such as fibers, on a permeable fabric.
The web forming machine according to the invention is applicable manufacture for example paper, board, tissue, pulp or non-woven web which is produced by laying material, such as fibers, on a permeable fabric.
It should be noted that in the context of the present invention the web forming machine may also mean a section of a web forming machine, which is arranged making use of an endless fabric loop. Advantageously the fabric is a forming wire through which water can leave the web on the forming section.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings

In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates a drive unit for a web forming machine according to an embodiment of the invention,
Figure 2 illustrates a exemplary embodiments of cover of the second roll of the drive unit according to embodiment of the invention,
Figures 3 to 8 illustrates embodiments of a web forming machine according to the invention, and
Figure 9 illustrates a drive unit for a web forming machine according to another embodiment of the invention.

Detailed Description of Drawings

Figure 1 depicts schematically a drive unit 10 for a web forming machine according to an embodiment of the invention. It should be noted that in the context of the present invention the web forming machine may also mean a section of a web forming machine, which is arranged making use of an endless fabric loop 12. The fabric loop 12 in the figure 1 is shown for better understanding the drive unit and its operation and it is merely an exemplary presentation. A forming area 14 is configured in a web forming machine, when in use, to a region of the fabric loop 12 where the web 16 under formation is in contact with the fabric12. There is a number of rolls, generally referred to by numeral 18 which are arranged to guide the fabric loop 12 constituting a desired web forming machine configuration or a lay-out. The rolls 18 may have different structure and functionality in the web forming area, but in this connection the rolls 18 are freely rotatable by the fabric which is in contact with the roll 18. The drive unit 10 which is configured to run the fabric loop 12, comprises a first roll 20 and a second roll 22 arranged at opposite sides of the fabric 12. Even if it is not necessary that both of the rolls would be provided with a drive, that may be the case if so desired. As is depicted in the figure 1, it is however preferable that only one of the rolls is driven or provided with a drive. Thus, preferably only the first roll 20, which has a predetermined fabric wrap angle, is provided with a drive 24. This way the driven roll has langer contact area with the fabric improving transmission of force to the fabric. Alternatively, it is possible that only the second roll 22 is provided with a drive 24. There is a driving nip 26 formed about the rolls 20,22 and the fabric 12 which transmits the torque provided by the drive 24 to pulling force subjected to the fabric. The drive unit 10 is positioned in a run of the fabric outside the forming area 14 of the fabric loop. In other words, drive unit 10 is positioned in a run of the fabric which is free from contact of the web 16.
As it can be seen in the figure 1, the first roll 20 is a lead roll for the fabric loop 12 and it acts as a counter roll for the second roll 22, which is acts as a pressing roll. The second roll 22 has only a nip contact with the fabric, substantially without wrapping by the fabric. The fabric loop 12 passes over the roll 20 with a predetermined fabric wrap angle, when assembled for use. The nip contact of the second roll may be a long-nip contact because the area in width of the second roll which is in contact with the fabric is provided with a soft cover 22.1 arranged on the body 22.2 of the roll 22. The cover 22.1 is elastically compressible at the circumstances in use. Radial thickness of the cover may vary depending on practical application but preferably the thickness is 10 - 100mm, preferably 15 - 50mm. Hardness of the roll cover is preferably 25-100 P&J. The cover structure may vary depending on the practical application. First roll 20 cover material is typically rubber or polyurethane with hardness less than 20 P&J, typically 5-20 P&J. First roll 20 cover can be also steel or ceramic coating and those are much harder than rubber or polyurethane covers. Second roll 22 cover 22.1 is typically softer than first roll 20 cover. Depending on the application, second roll 22 can be a single roll, extending from drive side to tending side of the web forming machine. It is also conceivable that instead of one roll, there can be several successive, coaxially arranged in cross machine direction between drive side and tending side edge of the fabric, arranged at a distance from each other. Contact surface width of such narrow roll in cross machine direction (or length in roll's axial direction) can be 100-500 mm.
Figure 2 shows some exemplary embodiments of the soft cover 22.1 of the second roll 22, by the views A to F which depict the cross section II-II in the figure 1. The view A on left top, shows an embodiment of the cover 22.1 which comprises a substantially solid soft layer on the body 22.2 of the roll. The roll cover covers the roll body preferably over the width of the roll. However, there may also be arranged a number of separate segments (views D and F) which may even have a clear gap between the segments. The gap may be even longer that width of the segment. This way the surface in the second roll which is in contact with the fabric is comprised of segments and the contact area in the width of the roll is less that width of the roll and width of the fabric 12. As a minimum number of cover segments the roll is provided with a first cover segment at, or near, a first end of the roll and a second cover segment at, or near, a second end of the roll. This also means that the first cover segment is at first edge of the fabric and the second cover segment is at the second edge of the fabric. In such a case the first and the second segments have substantially equal width which is preferably at least 5% of width of the first roll 20. Suitable cover materials for the second roll 22 in this embodiment are for example rubber, polymers, such as polyurethane, . Views B to F disclose various embodiments of the cover where the cover is provided with one or more cavities 28 inside, still having substantially smooth outer surface. The cavities may be simple cavities acting as resilient elements, like shown in the view B. The cavities maybe filled with gas, liquid or another elastic substance or structure, such as a foam. Hardness of the cover 22.1 may be arranged to be controllable by pressure applied inside the cavity or cavities 28. The cover may be gas inflatable or hydraulic liquid may be used instead. In the embodiment shown in the views C to F the roll 22 is provided with one or more fluid conduits 30 by means of which gas or hydraulic liquid may be fed to the cavities 28 and pressurize the cavities to a desired pressure. There may be a common fluid conduit for all of the cavities, separate conduits for each of, or groups of the cavities coupled with a shared fluid conduit, depending on the practical application. This way the hardness of the cover 22 can be adjusted even during the drive unit is in operation.
In the views B and C the cavities 28 in the cover 22.2 are formed of several separate cavities which each circumscribe the roll body 22.2, and which are arranged in a common, continuous cover 22.1. In the view C, each one of the cavities is provided with a dedicated fluid conduit such that pressure in the cavities can be independently controlled. Alternatively, the cavities may have a common fluid conduit for all of the cavities. Thus, the cover of the second roll 22 of the drive unit comprises at least two separate loading zones in width direction of the roll formed by the successive cavities.
The cover 22.1 disclosed in the view D second roll comprises an assembly of several roll surface segments which are open at their radially inner side. The segments comprise sidewalls and an outer wall between the sidewalls which outer wall forms contact with the fabric and transmits traction force to the fabric. Inner edges of the sidewalls are arranged against rims in the surface of the roll body, such that the inner edges of the sidewalls hold the segment at its place, when inflated. Also in this embodiment the second roll 22 comprises at least two separate loading zones in width direction of the roll formed by the successive cavities.
The cover 22.1 disclosed in the view E discloses an embodiment where the cavity 28 extends over whole width of the roll. The roll is also provided with a fluid conduit for pressurizing the cavity and control the pressure during operation of the drive unit.
The view F discloses a cover 22.1 which is formed of several separate surface segments arranged one after the other covering the width of the second roll 22. Each segment is provided with a cavity circumscribing the roll. Also in this embodiment the second roll 22 comprises at least two separate loading zones in width direction of the roll formed by the successive cavities.
Figures 3 to 8 disclose embodiments of a web forming machine 100 which comprises an endless fabric loop 12 and a number of rolls 18.1,18.2,18.3,18.4 which are arranged to guide the fabric loop 12. The rolls which are inside the loop 12 are numbered in increasing order beginning from the roll adjacent to a location where fiber suspension is laid on the fabric, making use of a head box 34 of the web forming machine 100. The forming area 14 of the fabric is configured when in use, in area where the web is in contact with the fabric, between the first roll 18.1 and the second roll 18.2, at upper section of the fabric loop where two rolls form a horizontal draw of the fabric. The forming area is provided with dewatering elements 32 known as such. The head box 34 is in proximity of the first dewatering element 32 in the running direction of the fabric loop 12. The fabric is moved, or rotated as a loop over the rolls 18.1,18.2,18.3,18.4, by means of a drive unit 10 according to the invention. The drive unit 10 is arranged to the fabric loop 12 outside the forming area 14 of the fabric loop. An embodiment of the drive unit is disclosed in the figures 1 and 2 in more detailed manner.
The present invention makes it possible arrange a web forming machine in which the number of rolls guiding the fabric is decreased. In the figures 3 to 8 a guiding and tensioning functionality is shown by an arrow pointing two directions, and it can be seen that with the present invention controlling tension and guiding the run of the fabric can be performed with only one roll. In the figures 3 to 8 a roll provided with a drive, such as an electric motor, is illustrated with hatched fill pattern.
In the embedment shown in the figure 3 the the fabric loop 12 is guided around three rolls 18.1,18.2,18.3, one of which rolls 18.3 is also the first roll 20 of the drive unit 10. The first roll 20 of the drive unit is inside the fabric loop 12. This configuration results in very straightforward web forming machine 100 with several benefits. For example, number of rolls is minimum and vertical height of the machine is very small. This also makes it possible to set the pick-up point 36 of the web from the fabric loop 12 to position upstream to, or before the second roll 18.2 at opposite end of the forming area 14 to the headbox 34, where the fabric is running horizontally.
The fabric's return run from the end 40 of the web forming machine where the web has been transferred to a next machine section, to the beginning 42 is a straight return run 44 of the fabric between second roll 18.2 and the third roll 18.3. The straight run 44 may be substantially long because the web forming machine 100 comprises at least one stationary support beam 46 arranged in cross direction in respect of the normal running direction of the fabric 12 under the fabric loop 12. The support beam 46 is positioned to the area in straight return run 44, such that the fabric loop is supported underneath the fabric 12 by the at least one stationary support beam 46. The stationary beam is preferably provided with a support gliding foil, or foils attached removably to the beam against which the fabric is arranged to run.
In the embodiment shown in the figure 3 the drive unit 10 is positioned immediately upstream the roll 18.1 in the running direction of the fabric when in operation. The first roll 18.1 which is in vicinity of the head box 34.
In the embedment shown in the figure 4 the web forming machine 100 is otherwise similar to that shown in the figure 3, but in this embodiment the fabric loop 12 is guided around four rolls 18.1,18.2,18.3,18.4, one of which rolls is the first roll 20 of the drive unit 10. In this embodiment, the drive unit 10 is positioned immediately downstream to, or after, the last the roll 18.2 in the configuration downstream the forming area 14, i.e. the second roll 18.2. Like in the figure 2, in this embodiment the straight return run 44 of the fabric loop 12 is also supported underneath the fabric 12 by one stationary support beam 46. There may be more than one stationary support beam if so required or desired.
Figure 5 discloses a still further embodiment of the web forming machine 100 according to the invention. In this embodiment the fabric loop 12 is guided around four rolls 18.1,18.2,18.3,18.4, i.e. there are four guide rolls inside the loop. Here the drive unit 10 is arranged in the return run of the fabric, such that the first roll 20 of the drive unit is outside the fabric loop 12, and it changes direction of travel of the fabric 12 between the third roll 18.3 and the fourth roll 18.4, the fabric forming a predetermined wrap angle over the first roll 20.
Figure 6 discloses a variant of the embodiment in the figure 4, where the pick-up point 36 of the web from the fabric loop 12 is at a position between the second roll 18.2 and the third roll 18.3 where the fabric 12 is running substantially downwards. upstream to, or before the second roll 18.2 at opposite end of the forming area 14 to the headbox 34. Also, in this embodiment the return run 44 between the rolls 18.3 and 18.4 is provided with the stationary support beam 46.
The web forming machine 100 in the figures 3 to 6 are based on only one fabric loop 12 being configured to manufacture a single layer web. Such a machine may also be called fourdrinier forming section. In the figures 7 and 9 the web forming machine 100 comprises several forming sections 100.1 -100.3 arranged in co-operation with each other, where both web forming machines 100 are configured to produce 3-layer web, as an example. Such a multiply web forming machine may be preferably configured to produce multiply board. In the figure 7 the machine section 100.1 for manufacture bottom ply, on which the top and middle ply are laid, is a web forming machine of substantially same configuration to that shown in the figure 4, while the machine sections 100.2 and 100.3 for manufacturing the middle ply and the top ply are web forming machines of substantially same configuration to that shown in the figure 5. In the figure 7 it has been disclosed that each one of the fabric loops 12 is provided with the drive unit 10 according to the invention as is described in figures 1 and 2. However, it is also conceivable in the scope of the invention that only one or two of the web forming sections 100.1 -100.3 is provided with the drive unit 10 according to the invention as is described in figures 1 and 2.
In the figure 7 it has been disclosed that each one of the fabric loops 12 is provided with the drive unit 10 according to the invention as is described in figures 1 and 2. However, it is also conceivable in the scope of the invention that only one or two of the web forming sections 100.1 -100.3 is provided with the drive unit 10 according to the invention as is described in figures 1 and 2, and the other one is driven with conventional means.
In the figure 8 the machine section 100.1 for manufacture bottom ply, on which the top and middle plies are laid, is a web forming machine of substantially same configuration to that shown in the figure 4. The web forming machine section 100.1 is longer than that in the figure 4 and therefore there are several support beams 46. The machine sections 100.2 for manufacture the middle layer is of substantially same configuration to that shown in the figure 6, where the section is further provided with a top forming unit 102. In top forming unit 102 fabric tension can be according to prior art solution (8-10 kN/m), if there is need to use dewatering based on wire tension. The machine section 100.3 for producing the top ply is configured such that the fabrics of the top ply machine section 100.3 and the bottom ply machine section 100.1 run parallel, attached to each other over a predetermined length. The drive unit 10 in the top ply machine section 100.3 is arranged such that the first roll 20 is inside the fabric loop 12 and the second roll 22 is outside the fabric loop, but the second roll is horizontally higher than the first roll and the fabric form a wrapping area on both of the rolls.
Figure 9 discloses schematically a drive unit 10 for a web forming machine, according to another embodiment of the invention. This embodiment differs from that shown in the figure 1 such that the second roll 22 is arranged inside the fabric loop and the first roll 20 outside the fabric loop 12. Both the first roll 20 and the second roll 22 are arranged only in a nip contract with the fabric, with substantially zero wrap angle. In other words, the fabric comes to the nip and leaves the without changing direction of run of the fabric. This aspect is particularly useful in rebuild application where the drive unit can be easily installed between two successive, existing rolls. Here the second roll 22 provided with soft cover, having its hardness lower than hardness of the first roll 20, is provide with a drive 24. Preferably only one of the first and the second roll is provided with the drive 24, however the driven roll can be selected based on the requirements of the practical application. Of course, both of the first and the second rolls can be provided with a drive, but usually than is not required.
While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is obvious to the skilled person that, along with the technical progress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.

Claims

A drive unit (10) for a web forming machine comprising an endless fabric loop (12) wherein a forming area (14) is configured when in use, in which web is in contact with the fabric, and a number of rolls (18.1,18.2,...,18,N) which are arranged to guide the fabric loop (12), a drive unit (10) for running the fabric loop (12), characterized in that the drive unit (10) comprises a first roll (20) and a second roll (22) arranged at opposite sides of the fabric, wherein at least one of the rolls (20,22) is a driven roll, and a driving nip is formed about the rolls (20,22) and the fabric outside the forming area (14) of the fabric loop (12).
A drive unit (10) according to claim 1, characterized in that the first roll (20) is a lead roll for the fabric loop (12) with a predetermined fabric wrap angle, when in use, and the second roll (22) is a pressing roll having a nip contract with the fabric.
A drive unit (10) according to claim 1, characterized in that the first roll (20) and the second roll (22) are arranged in a nip contract with the fabric with zero wrap angle.
A drive unit (10) according to claim 1, 2 or 3, characterized in that only one of the first roll (20) and the second roll (22) is provided with a drive (24), being a driven roll.
A drive unit (10) according to anyone of the preceding claims, characterized in that the area in the second roll (22) which is in contact with the fabric is provided with soft cover, having its hardness lower than hardness of the first roll (20).
A drive unit (10) according to anyone of the preceding claims 1 - 4, characterized in that the second roll (22) comprises one or more fluid cavities (28) in the cover, hardness of which is controllable with pressure of the fluid in the cavity.
A drive unit (10) according to claim 5 or 6, characterized in that the second roll (22) comprises at least one or more separate loading zones in width direction of the roll (22).
A drive unit (10) according to claim 5 or 6, characterized in that second roll (22) comprises an assembly of several roll surface segments.
A web forming machine (100) comprising, an endless fabric loop (12), and a number of rolls which are arranged to guide the fabric loop (12), a drive unit (10) for running the fabric loop (12), wherein a forming area (14) is configured when in use, in which web is in contact with the fabric, when in use, characterized in that in an area outside the forming area (14) the fabric loop (12) is provided with a drive unit (10) according to anyone of the claims 1-7.
A web forming machine (100) according to claim 9, characterized in that the fabric loop (12) is guided around at least three rolls (18.1,18.2,...18.N), one of which rolls is the first roll (20) of the drive unit (10).
A web forming machine (100) according to claim 8, characterized in that the fabric loop (12) is guided around four rolls (18.1,18.2,...18.N), one of which rolls is the first roll (20) of the drive unit (10).
A web forming machine (100) according to anyone of the preceding claims 8-11, characterized in that in the area outside the forming area (14) the fabric loop (12) is supported underneath the fabric with at least one stationary support beam (46) arranged in cross direction in respect of the normal running direction of the fabric.
A web forming machine (100) according to claim 12, characterized in that the area outside the forming area (14) the fabric loop (12) is supported underneath the fabric with at least two successive stationary support beams (46).
A web forming machine (100) according to anyone of the preceding claims 8-13, characterized in that the drive unit (10) is arranged at vicinity of the forming area (14) after the web has been transferred away from the support of the fabric.
A web forming machine (100) according to anyone of the preceding claims 8-13, characterized in that web forming machine (100) is configured to manufacture web with more one layer and it comprises more than one forming sections (100.1,100.2,100.3), wherein at least one, preferably all of the forming sections is provided with a drive unit (10) according to anyone of the claims 1-7.