Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21G—CALENDERS; ACCESSORIES FOR PAPER-MAKING MACHINES
  • D21G1/00—Calenders; Smoothing apparatus
  • D21G1/002—Opening or closing mechanisms; Regulating the pressure
  • D21G1/004—Regulating the pressure
  • D21G1/0053—Regulating the pressure using magnetic forces

Definitions

• 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.
• 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.
• the non-uniform pressure causes vibrations and accordingly noisiness, as well as a consequent non-uniform wear of the rolls.
• 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.
• the material being pressed with a pressure varying along its transversal dimension is not perfectly uniform.
• 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.
• 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.
• 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.
• Figure 1 is a schematic side view of the present device and of the calender rolls.
• Figure 2 is a schematic front view of said calender on the outlet side of the processed material.
• 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.
• 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.
• 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.
• 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.
• 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
• magnet 5 is an electromagnet, but it is possible to use alternatively an electropermanent magnet or a permanent magnet
• 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
• 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
• 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
• peripheral roll speed is about 4-5 5 m/s
• 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

Landscapes

• Casting Or Compression Moulding Of Plastics Or The Like (AREA)
• Treatment Of Fiber Materials (AREA)
• Soft Magnetic Materials (AREA)
• Battery Electrode And Active Subsutance (AREA)
• Hard Magnetic Materials (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=11377195

Family Applications (1)

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Cited By (1)

Citations (4)

• 1997
  • 1997-05-22 IT IT97MI001203A patent/IT1291975B1/it active IP Right Grant
• 1998
  • 1998-05-12 WO PCT/IT1998/000118 patent/WO1998053139A1/en active Application Filing
  • 1998-05-12 AU AU72335/98A patent/AU7233598A/en not_active Abandoned

Patent Citations (4)

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