WO1998053139A1

WO1998053139A1 - Calender with a magnetic device generating the contact pressure between the rolls

Info

Publication number: WO1998053139A1
Application number: PCT/IT1998/000118
Authority: WO
Inventors: Danilo Molteni
Original assignee: Sgm S.P.A.
Priority date: 1997-05-22
Filing date: 1998-05-12
Publication date: 1998-11-26
Also published as: IT1291975B1; ITMI971203A1; ITMI971203A0; AU7233598A

Abstract

In a calender comprising at least a pair of rotatable parallel rolls (1, 2) and a mechanism capable of driving said rolls (1, 2) against each other, said mechanism comprises a magnet (5) generating a constant magnetic flux along the length of the rolls (1, 2), which are made of paramagnetic or ferromagnetic material. The magnet (5) is preferably an electromagnet with pole pieces (6, 7) being identical and of constant cross-section, which extend along the whole length of the rolls (1, 2), so as to ensure a uniform pressure along the contact line (4).

Description

"CALENDER WITH A MAGNETIC DEVICE GENERATING THE CONTACT PRESSURE BETWEEN THE ROLLS"

The present invention relates to calenders, and in particular to a magnetic device for generating the contact pressure between the calender rolls by drawing them against each other.

The calender is known to be a machine manufactured in many different types for exerting a laminating action on various types of materials (paper, rubber, fabrics, etc. ), which as finished product are in the form of continuous sheets or tapes. By means of a calender it is possible to flatten, shape, couple, taper or superficially finish in various ways one or more sheets of material passed through a pair of counterrotating parallel rolls. Such rolls are pressed against each other with a variable load depending upon the type of material and of process to be carried out, and they are generally hollow so as to allow their heating and/or cooling by means of an inner circulation of fluid.

In prior art calenders the contact pressure between the rolls is generated by driving them against each another by means of mechanical or hydraulic devices acting at the rotation and support pivots. However, this conventional arrangement does not ensure the uniformity of the pressure along the entire contact line, in particular for rolls of some length. In fact, the thrust exerted on the pivots obviously decreases upon increasing of the distance from the support, so that the rolls are generally convex at the center in order to make up for the decreased thrust in said area.

This conventional solution has various drawbacks affecting both the machine working and the product quality. First, the non-uniform pressure causes vibrations and accordingly noisiness, as well as a consequent non-uniform wear of the rolls. Furthermore, the control of the contact pressure is neither easy nor precise because there may be an excessive pressure at some points and/or an insufficient pressure at some others. As far as the product is concerned, the material being pressed with a pressure varying along its transversal dimension is not perfectly uniform. Moreover, in a determined point the pressure may also change in time due to the irregular wear, to settlings, etc. so that a batch of material may be treated in a slightly different way than the subsequent one. The object of the present invention is therefore to provide a calender provided with a device for generating the contact pressure which overcomes the aforementioned drawbacks.

Such an object is achieved by means of a magnetic device generating a constant magnetic flux along the whole roll length.

The main advantage of the device according to the present invention is to generate a perfectly uniform contact pressure between the calender rolls. This is achieved thanks to a magnet extending along the length of the rolls and drawing them against each other, these being obviously made of a paramagnetic or ferromagnetic material.

In this way the vibrations, the noisiness and the production defects are greatly reduced and the wear as well is uniform. Furthermore, a high treatment reproducibility is obtained, so that the consecutive material batches are far more homogeneous since the production conditions are constant.

A second advantage of the present device is that it contributes to the roll heating through the effect of the eddy currents (so-called Foucault's currents) which are generated on the surface of the rolls through the effect of the magnetic field rotating with respect thereto. Since such a heating is uniform along the contact line, it contributes as well to make the working conditions uniform.

Moreover, this heating method allows to reduce the overall consumption of the machine because it is more effective than the inner circulation of fluid, which it integrates but usually can not completely replace. This is true even if the braking effect on the roll rotation due to the eddy currents results in an increase of the power being necessary to keep the required rotation speed.

These and other advantages and features of the device according to the present invention will be evident to those skilled in the art by the following detailed description of an embodiment thereof with reference to the attached drawings, wherein:

Figure 1 is a schematic side view of the present device and of the calender rolls; and

Figure 2 is a schematic front view of said calender on the outlet side of the processed material.

Referring to such drawings, there is described the operation of a calender according to the present invention especially intended for the production of corrugated board. What hereinafter disclosed is obviously just by way of non- limiting example, since it could be applied, with the suitable modifications, to other types of calender, as mentioned above. Examples of other applications are embossing, coupling of different sheets (rubberized fabrics, plastificated papers. etc.), stretching, surface finishing and so on.

The calender depicted in the drawings comprises an upper roll 1 and a lower roll 2 provided with longitudinal toothing and rotating clockwise and counterclockwise, respectively, as indicated by the relevant arrows Rl, R2. The two rolls 1, 2 are engaged with each other as a pair of gears and a cardboard sheet 3 passing therethrough accordingly takes on a corrugated form. In order to obtain this, pressure P between the rolls along contact line 4 is about 20 kN per meter and their temperature approximately ranges from 130°C to 200°C.

The contact pressure P is generated through a substantially U-shaped magnet 5, arranged in a symmetric position below the lower roll 2 so as to have a pole piece 6 (North) on the outlet side of the material and a pole piece 7 (South) on the inlet side. The two pole pieces 6, 7, having an equal and constant cross-section, extend parallel to rolls 1, 2 along the whole length L thereof. Each pole piece 6, 7 exerts on both rolls 1, 2 a force of magnetic attraction in a perpendicular direction with respect to its surface, such forces being indicated by arrows Fal, Fa2, Fb 1 and Fb2, respectively.

Thanks to the structural symmetry of the device, the attraction components acting on rolls 1, 2 in the horizontal direction cancel each other, whereas those acting in the vertical direction add to each other resulting in attraction forces FI, F2. Pressure P along the contact line 4 is thus equal to P = FI + F2, such a pressure being uniform along the whole length L of rolls 1, 2.

It should be noted that pressure P would be nonetheless generated even if one of rolls 1, 2 was made of a non-magnetic material. In such case it would be equal only to FI or F2, since the non-magnetic roll would provide only the reaction force of its support and rotation pivots.

The aforementioned symmetric structure of magnet 5 is the simplest one for producing a symmetric distribution of the magnetic attraction forces, however it is not the only conceivable structure. In fact, a first alternative solution for obtaining the same distribution of forces is to provide pole pieces 6, 7 having a different section, while making up for this difference through a corresponding difference of the relating working air gaps. It is preferable that each pole piece 6, 7 has anyhow a constant section throughout its length so as to ensure the perfect uniformity of pressure along contact line 4. However, another solution may be provided even with pole pieces 6, 7 having a section varying in the longitudinal direction, through a corresponding variation of the air gap allowing to make up for such a variation of section so as to keep constant the magnetic flux and accordingly the contact pressure P along the length of rolls 1, 2

It is clear that if rolls 1, 2 have a different length and/or the width of material 3 being processed is shorter than the length of the rolls, the important thing is that pressure P is uniform throughout the working contact length L'<L, i e the roll length actually working

In the preferred embodiment illustrated in the drawings magnet 5 is an electromagnet, but it is possible to use alternatively an electropermanent magnet or a permanent magnet The use of an electromagnet makes easier the adjustment of pressure P, performed by means of a simple change of the current flowing through the winding This allows to quickly fit the machine to the production of corrugated boards having different features, whereas, when using permanent magnets oi electropermanent magnets, it is necessary to act on the working air gap thus making the adjustment system more complex and less flexible Furthermore, when rolls 1, 2 must be drawn apart, it is sufficient to turn the electromagnet off for making them free to shift, whereas when using permanent magnets or electropermanent magnets it is necessary to pull them away from the rolls or to generate a contrary flux which cancels their attraction force

Irrespective of the used type of magnet, rolls 1, 2, through the effect of their rotation, lie in a rotating magnetic field which generates on their surface eddy currents which are sufficient to heat them of some tens of degrees In fact, considering that the peripheral roll speed is about 4-5 5 m/s, there is a frequency of sinusoidal electromagnetic variation of about 25-35 Hz, with a value of overall flux of 0 08-0.12 Wb/m², obviously constant throughout the roll

This effect, as mentioned above, allows to reduce the energy consumption foi heating the rolls from inside by means of steam or other fluid, and furthermore it makes the heating more uniform The contribution provided by the eddy currents depends upon the aforementioned parameters of intensity and frequency of magnetic field variation, as well as upon the conductivity of the roll material

The herein described and illustrated embodiment of the calender according to the invention is obviously just an example liable to many variations In particular, the shape, the size and the position of magnet 5 and of rolls 1, 2 may be rather changed according to the specific needs Likewise, the calender may comprise a greater number of rolls and magnets, and the rolls may be put side by side horizontally instead of vertically or they may be arranged with a vertical or inclined axis instead of a horizontal axis

Claims

1 A calender comprising at least a pair of rotatable parallel rolls (1, 2) and a mechanism capable of driving said rolls (1, 2) against each another, characterized in that said mechanism comprises at least one magnet (5) generating a constant magnetic flux along the useful contact length (L') between the rolls (1, 2), at least one of said rolls (1, 2) being made of paramagnetic or ferromagnetic material

2 A calender according to claim 1, characterized in that the magnet (5) has a first pole piece (6) on one side of the contact line (4) between the two rolls (1, 2) and a second pole piece (7) on the opposite side, both said pole pieces (6, 7) extending at least along the useful contact length (TJ)

3 A calender according to claim 2, characterized in that the two pole pieces (6, 7) are identical

4 A calender according to claim 2 or 3, characterized in that the two pole pieces (6, 7) have a constant cross-section along their whole length

5 A calender according to claim 2 or 3, characterized in that the two pole pieces (6, 7) have a variable cross-section in the longitudinal direction

6 A calender according to claim 2 or 4 or 5, when not dependent upon claim 3, characterized in that the two pole pieces (6, 7) have a different shape and/or size

7 A calender according to one or more of claims 1-6, characterized in that the magnet (5) is an electromagnet

8 A calender according to one or more of claims 1-6, characterized in that the magnet (5) is a permanent magnet 9 A calender according to one or more of claims 1-6, characterized in that the magnet (5) is an electropermanent magnet

10 A calender according to one or more of the previous claims, characterized in that the rolls (1, 2) are provided with a longitudinal toothing and are engaged with each other as a pair of gears