METHOD AND DEVICE FOR NIP CONDITION MEASUREMENT

The invention relates to a method of measuring a condition in a nip between two rollers which are in elastic nip contact, and relates to a device for carrying out that method. In particular, the invention is applicable to a paper making machine, board making machine, paper finishing machine or printing machine.

In the above machines rollers are commonly used which form a soft nip. At least one of the two contacting rollers forming the nip there between by press contact is a so- called soft roller which has an elastically deformable jacket. Usually, such soft rollers have a hard (metal) core with an elastomeric cover thereon. The success of treatment of a web by passing it through the nip is in many cases directly related to the conditions in the nip. In order to obtain e.g. homogenous treatment of wide webs, there is big interest that nip conditions are the same or constant all over the length of the nip. Nip conditions may be controlled by several measures, however, suitable control requires a detection or measurement of the nip conditions.

As an example, in paper making industry polymer covered metal rollers which are brought together under load are utilized to squeeze out water from a paper web passing through a nip between two rollers, so as to make a common paper web from a slurry of wood fibres and other ingredients. Here, the amount of water that can be removed from the web in the nip is directly related to the load applied to the rollers. To produce a sheet paper with the same relative dryness throughout its substance, this load

must be applied relatively evenly across the entire roller contact length, i.e. across the nip length.

Also, in calendaring a paper web in a nip between two rollers it is essential to have nip conditions such that thickness of the web as well as gloss and other surface properties are uniform in the cross direction of the web.

On the other hand, the load which may be applied is limited, since polymer covered rollers are used. With these rollers, overload of the polymer covering of the rollers may cause local destruction of the polymer. Accordingly, in order to ensure the integrity of the polymer covers on the metal roll bodies, it is necessary to ensure that an even load is applied to the rollers in the nip; in particular load peaks in both the axial direction and the circumferential direction of the rollers have to be avoided.

It is noted that the rollers are loaded against each other with forces which cause an actual and significant deformation of the polymeric cover, so that the nip is not a mere line contact, but is a contact area having a certain nip width.

For the reasons given above, measurement of a contact area and/or a pressure existing between two nipped rollers is commonly utilized in the paper making industry to ensure that the rollers are evenly loaded.

A typical method used to measure this contact area is a stationary method, and involves introduction of a sandwich of plain paper and carbon paper into the nip, then bringing the rollers together under operation load in stationary condition, that is the rollers are not turning. With this

arrangement, rollers are left loaded against each other for a certain period of time and are then released so that the carbon paper and the plain paper can be removed from the nip. The carbon impression on the plain paper under load in the nip allows conclusions on the nip contact area or nip width, also allowing conclusions regarding nip pressure distribution. However, with this conventional method, only a stationary nip can be measured and the polymer materials used for the roll covering may show a certain creeping behaviour, that is a certain reversible material flow within the roll covering at the loaded portion thereof. This may cause some enlargement of the carbon trace on the plain paper which may be misinterpreted as a higher pressure than actually present. Further, depending on the quality of the plain paper and the carbon paper which has been used, the plain paper may have been stretched under pressure in the nip, which stretch of the paper may cause some distortions of the image of the contact area.

Recently, attempts have been made to replace the carbon paper/plain paper sandwich by electronic pressure sensitive devices which may be arranged in sheets or pads which are placed in the nip. However, here the static phenomenon of material creep of the polymeric cover may also occur, which adversely affects the measured results, although precise sensing devices are used. It has also been considered to introduce the electronic sensing devices into the nip of rollers which are slowly moving, in order to overcome the material creep problem in the stationary case. However, each time the sensor pad passes through the nip, inevitable variation of the thickness of the sensor pad causes the loaded rolls to separate from each other and return under load, thus causing a bounce working between the rollers. If the bounce is severe, a damage of the polymeric cover can occur.

In patent literature methods have been disclosed in which various types of sensors are embedded inside the roll cover. As an example is given EP 538221 in which PVDF sensors are embedded in roll cover and signal is transferred telemetrically. In practise, embedded sensors have so far turned out to be problematic due to insufficient adherence to the cover material and/or discontinuity in the cover thus resulting in cover delamination .

Accordingly, there is a need for a measuring method and device for carrying out that method with which method the conditions in the nip between elastically deformed rollers can be measured.

To satisfy this need, the invention proposes a method according to claim 1, and a device for carrying out that method according to claim 9.

In particular, in the method of claim 1 for measuring the condition of a nip between two rollers in elastic contact, the difference of peripheral speeds of the two rollers is measured and the nip conditions are derived from the measured peripheral speed difference. It is noted that with this arrangement, the stationary material creeping problem can be completely solved, and the rotational speed of the rollers may even reach normal operating conditions of the rollers. Thus, nip conditions between rollers operating under normal operation conditions can be measured.

It has been found that in operation, the time in which the at least one elastic roller surface is compressed is very short and the creeping phenomenon observed in static methods of nip measurement does not occur to any

significant degrees. On the other hand, it was found that the compression of the elastic material of the at least one elastic roller leads to a changing diameter thereof, and since the surface particles of the two rollers which are in mutual contact in the nip shall basically have the same speed, a difference in diameters of the rollers in the nip can be measured via the peripheral speed difference of the roller surface outside the nip.

It is further possible to measure the peripheral speed of the rollers at a plurality along a direction of the rotational axis of the rollers, so that the nip conditions at the respective location along nip extension (nip length) can be concluded, so that pressure distribution within the nip over the nip length can be obtained since the elastic behaviour of the elastic roller surface is known.

A typical roller has an elastically deformable cover which deforms elastically under nip conditions, however, such elastic deformation is usually not an ideal elastic behaviour. For the application of the method of the present invention, also, a visco-elastic behaviour of the elastically deformable cover allows the use of the invention. Also in this case the peripheral speeds are precisely measured and based on the known compression/recovery behaviour of the visco-elastic cover the nip condition can be found.

There are various possibilities for measuring peripheral speeds of the rollers/ possible solutions include a tachometer and a measurement device for the diameter of the roller.

It is noted that diameters of rollers can be very precisely measured, so that it is considered sufficient to use the

once measured diameter and the angular velocity particularly of hard roller when such roller is used for forming one side the nip; however, such an approach is useable with the soft roller as well.

Any suitable and sufficiently precise methods for measuring peripheral speeds, i.e. the surface speeds of the rollers, can be used. Such measuring methods may include optical measurement methods, radar methods or electromagnetic method using e.g. electromagnetic induction.

In a device according to claim 9, a measuring device is provided, for measuring a difference of peripheral speeds of two rollers which are in engagement in an elastic nip contact. As outlined above, such a device might include a tachometer or strobe for measuring angular velocity of the rollers and a measuring device for measuring roller diameters. One can even think of using a measurement wheel which is in direct contact with the roller surface as well as devices using the above mentioned measurement methods, such as optical, radar or other surface speed measurement methods. For example, suitable marks on the roller surfaces may be optically detected and from the tuned signals and the known distance between the detected marks, the speed may be calculated.

Another possibility could be the electro-magnetic detection of metal parts (or magnets) embedded in the roller cover.

It is noted that the speeds of the two rollers may also be detected by different measurement means for each roller.

The device may comprise a plurality of individual measuring devices which may be arranged at locations on a line parallel to an extension direction of the rotational axis

of the roller. To simplify the collection of measurement data, it may also be considered to arrange the plurality of measuring devices at locations on a line extending in a direction oblique to a rotational axis of the roller, so that each measuring mark (e.g. a line in roller axial direction) passes the respective measuring device with a slight offset in time, so that it is not required to collect or obtain all measuring data at the same time but may be collected successively.

Of course, it is also possible to provide a plurality of measurement marks along a circumference line of the roller at one or more locations of the roller, so that several measurements of the peripheral speed of the same circumference line of the roller can be made to obtain a grid-shaped surface speed pattern or distribution over the whole roller surface.

The present invention may be applied to any machine or device in which rollers are in elastic nip contact, and in which the nip condition is to be measured. The nip condition may include a nip load, a nip width and a nip pressure distribution, for example.

Here, the application of method and device according to the invention to a pair of rollers of a calender will be described as an example, not excluding the application to any pair of rollers in any device in which the rollers are press-contacted, at least one of them is elastically deformed, and in which nip conditions are of interest. The nip condition measurements according to the method and device may be used to control the nip condition by controlling suitable nip condition control parameters line roller load, roller shape, roller speed or the like.

The invention will now be described using an embodiment and with reference to the drawings in which

Fig. 1 shows a schematic arrangement of a device putting the method according to the invention into practice,

Fig. 2 shows a detail E of Fig. 1;

Fig. 3 shows a schematic arrangement for measuring nip conditions along the nip; and

Fig. 4 shows results of the speed ratio vs. roll cover deformation with various cover materials.

In Fig. 1 there is shown a hard roller 1 and a soft roller 2 which has a hard core 22 and an elastically deformable cover or covering 21, wherein the rollers 1 and 2 are in an elastic nip contact. The rollers 1 and 2 may be calender rolls in a calender. The rollers are rotatably held in a support structure which allows to suitably press-contact the rollers with each other. In a calender, the paper web passes through at least one nip or a series of nips in the calender in which nips the surface of the paper web is smoothened. Heat and/or moisture may be applied to the web in connection with calendering. In multinip calenders, a stack of rollers, each in mutual contact with two other rollers are piled, and only one of the rollers is driven. An enlarged view of the nip encircled with circle E is shown in Fig. 2.

Measuring devices 4 and 5 are provided with which the peripheral speed Vl and V2, respectively, of the two rollers are measured. The method of measuring may include measuring of the angular velocity of each roller such as a tachometer or an optical device like a strobe which operate in conjunction with a measuring device capable of precisely measuring outer diameter of each of the rollers. Also, measurement of the surface speed of the rollers may be

conducted by one or both of the speed measuring devices 4 and 5; e.g. marks on the surface can be optically detected, and, having the marks in a predetermined precise interval arranged around the circumference of the roller, the time signal of the detection can be used to calculate surface speed. Also, other optical solutions with visible or invisible light or electromagnetic detection can be used to detect the roller surface speed.

The so detected signals representative of the surface velocity of the rollers are input into a controller 6, where the measured signals are evaluated in a converting means for converting the measurement data to nip data providing nip conditions such as nip width, nip load, nip pressure distribution. Based on the information so obtained, suitable control of nip condition control parameters like roller loading, roller shape roller load distribution, or other control parameters influencing the nip can be set in order to obtain the desired nip condition profile in the nip.

An example for the evaluation of a single differential speed difference between the two rollers measured on substantially the same location with respect to the length of the rollers is given here below as an example.

From the speed difference in those measurements, at each individual location, the contact width at that location can be implied through the following relationships:

N*f(d _r )*f(S _R ) eq.[l]

For any measurement location, the contact width (N) in the nip can be approximated by a function of the radial deformation (d _r ) of the elastomeric covered rollers, which

in turn can be approximated by the difference in surface speed (S _R ) between the rollers. So, by association:

N*f(S _R ) eq.[2]

With the device described above, these measurements can be taken under dynamic conditions, in the covered roller operating environment and operating conditions, and at operating machine speeds, without actually introducing a measuring device into the nip contact area itself.

In the papermaking industry, there are many empirical and semi-empirical equations used to approximate the nip contact area between two loaded elastomeric covered rollers. An example of this type of approximation for a soft elastomeric cover loaded against a metal roller as shown in Fig. 1 is expressed in equation 3, below: ³² )

N 5.8x10 -6 LTD ₁ D ₂ P 1.35 (o.SlD,- ⁰² eq.[3]

A±A where, N = nip width, in inches

L = Load in PLI

T = working cover thickness, in inches

Di = Diameter of the elastomeric covered roll, in inches D2 = Diameter of the metal mating roll, in

Inches P = Pusey & Jones hardness, 1/8 inch ball

Indentor

It is well understood and accepted that under load, the radial deformation of a cover (s) is related to the nip contact area. Equation 4 is one of many derivable equations used in the papermaking industry to describe that relationship.

, N ² {D _X ±D ₂ ) ^d ' " AD ₁ D ₂ ^eq - ^{[ 4 ]} where, N = nip width, in inches

Di = Diameter of the elastomeric covered roll, in inches D ₂ = Diameter of the metal mating roll, in inches d _r = radial deformation of the elastomeric covered roll for the situation in equation 3.

Consequently, it can be said that the speed ratio existing between the two rollers is related to the radial deformation of a polymeric (elastomeric) cover, hence is also related to the nip contact area.

The above considerations have been applied to several different roll coverings, and it was found that there is actually a good relation between cover deformation in percent and the speed ratio of the two loaded rollers as is shown in Fig. 4. These results may be better understood by making reference to Fig. 2 which shows an enlarged schematic view of a cross-section through the nip in Fig. 1, using the symbols and indices of the above equations 1 to 4.

As can be seen, the lower roller 2 has a hard core 22 and a soft covering 21. In the area of the nip 3, there is found some bulging behaviour of the cover material as a result of the hard roller 1 pressing into the surface of the soft roller. In Fig. 2 these bulging portions 23 and 24 are almost symmetric, however, one may also expect that a non- symmetric bulging occurs on the entry side of the nip rather than on the exit side of the nip (in Fig. 2 entry

side bulging corresponds to ref. 24) especially, when the material has a visco-elastic behaviour. In this case, the formation of the bulging can be understood as a kind of partly blocking or to dam up the passage of the softer cover material 21 through the passage formed between the hard roller 1 and the hard roller 22 core of the soft roller 2.

The speed difference or the surface velocity difference of the rollers which is made use of with the present invention is assumed to occur due to the reduction of diameter d _r in the nip, where the deformation of the elastic material of the roller cover is maximum and, at the same time, there is the assumption that the surface velocity at this point of the rollers is identical, or very nearly identical. In this case, roller covering deformation causes a certain difference in diameter as compared to the non-loaded portion of the rollers, so that there is a difference of the moving speeds of the two points P (Fig. 2) on the surface of the rollers when the roller cover has returned to the original shape. As a result, by measuring the speed difference of the two points P, one may conclude on the deformation amount of the elastic cover d _r in the nip, and, as a consequence, also on the width of the nip N. This is valuable information which additionally allows to conclude on the pressure which is actually present in the nip.

Finally, Fig. 3 shows a possible arrangement of a device which may be used to measure nip conditions or nip condition profile in the nip over the roller length. Here, very schematically, two rollers 1 and 2 are shown, one of which may be a hard roller and at least one of which is a soft roller. Measuring devices 4 and 5 are only schematically shown which may consist of beams having an array of sensors arranged thereon, so as to measure

velocity of the roller surface along a circumference line of the roller in certain points along the roller length.

It is noted that drawing Fig. 3 is only schematic and the dash lines only indicate that there is a transfer of information indicative of the surface velocity of the roller exchanged between the roller surface and the sensor in its array. As is further shown with dashed lines along the circumference of each roller, it is indicated that the most accurate conclusion or nip conditions may be collected if the surface speeds of the two rollers are measured on congruent circumferential lines of the rollers. With such a distribution of sensors as is schematically indicated in Fig. 3, one may also drive a full nip profile, i.e. nip width distribution along the roller length and nip pressure distribution in that direction.

It is noted, that the arrays of sensors need not be arranged in parallel to the axis of rotation of the rollers, but it may be an advantageous solution to arrange the sensors such that they are arranged on an inclined or curved line with respect to the rotational axis of the rollers. In this arrangement the sensors supply the measured velocity signals successively during one rotation of the rollers, an arrangement which can simplify data acquisition and evaluation in the device.

For the measurement, the rollers may be rotated at any speed; preferably the intended operating speed, so that any additional speed effects stemming from centrifugal forces etc. can be automatically included in the measurements and subsequent control. In this connection, it is noted that this is a particular advantage because the roller covers are usually visco-elastic, i.e. they do not show ideal

elastic behaviour so that there is also a time component involved in deformation and return to original shape.