Method for grinding a variable crown roll

TECHNICAL FIELD

The invention relates to a method according to the preamble of claim 1 and to a roll ground by means of the method.

BACKGROUND ART

Variable-crown rolls provided with a composite shell are known, among other things, from US patents 5,897,476 and 5,785,636.

One problem associated particularly with composite shell rolls but also to some extent with metal shell rolls, is inequalities in the wall thickness of the shell. The process of manufacturing metal shell rolls is controlled relatively well today, so that the inner surface of the metal shell can generally be made sufficiently straight. When the outer surface of the metal shell is after that ground to be straight, the end result is a roll in which the thickness of the metal shell at each radial cross-section is sufficiently good. However, in the metal shell there may remain stresses that, upon release, cause such deviations in the shape of the metal shell that the roll cannot be made into a commercially acceptable product by grinding the outer surface of the metal shell. On the other hand, the process of manufacturing composite shell rolls is not controlled sufficiently well so that the wall of the composite shell should become of equal thickness. During the manufacturing process, different stresses are produced in the composite shell and they cause deviations in the shape of the composite shell. Variations in the thickness of the shell are particularly problematic in variable crown rolls. For

example, in a calender, variations in the thickness of the shell of this kind of roll cause variations in the linear load in the nip. This in turn has an adverse effect on the properties of paper causing, among other things, machine-direction thickness variations and uneven gloss in the product that is being manufactured.

With today's technique, attempts are made to grind the outer surface of the shell to be straight but since the inner surface of the shell is not straight, the shell still has thickness variations although the outer surface of the shell is straight. Grinding can be done before the roll is assembled, in which case the shell is fitted with separate grinding ends, by which the shell can be attached to grinding supports. Grinding can also be done when the roll has been assembled, in which case the roll is attached to grinding supports by means of its own bearings. When the roll has been assembled, grinding can be performed such that loading elements of the roll are loaded during grinding, thus making it possible to prevent the shell from being deflected during grinding. In addition, the roll must be reground during use at given intervals. For grinding, the roll is detached from the site of use and a new roll is replaced. After this the roll is ground, and then transferred to serve as a spare roll awaiting the next roll replacement.

In the state-of-the-art arrangements, measurements made from the outer surface of the shell are used as the basis of grinding. The cross section of the outer surface of the shell is measured in the axial direction at several, preferably 3-9 locations. At each measurement location, the cross section of the outer surface of the shell is determined by means of hundreds of measurement points evenly spaced around the circumference of the shell. A 3-D model representing the outer surface of the shell is formed based on the measurement results. Based on this model, grinding is performed such that the outer surface of the shell becomes circular at each cross section. The deflection of the shell is naturally also taken into account in the grinding operation, as a result of which the centre of rotation of the shell and the actual centre of the shell differ from each other at each cross section.

DE patent 199 15 224 discloses one method for grinding the outer surface of a variable crown roll provided with a composite shell. The grinding process is performed using a cylindrical grindstone, which is pressed with a given grinding pressure against the outer surface of the composite shell at the location of grinding. A fixed support placed at the location of grinding is arranged between the inner surface of the composite shell and the support shaft of the roll. The fixed support prevents the composite shell from being depressed at the location of grinding. The grinding is accomplished such that the shell of the roll remains stationary in the axial direction of the roll and the grindstone moves in the axial direction of the roll. In that case, the fixed support extends over the entire axial length of the composite shell. The grinding can also be carried out such that the grindstone remains stationary in the axial direction of the roll and the composite shell is moved during grinding in the axial direction of the roll. This requires that the shell be mounted during grinding on a support shaft whose length is at least twice the axial length of the roll shell. In both alternatives the fixed support is located on the opposite side of the roll with respect to the loading shoes of the roll. The shell of the roll is tightened by means of the loading shoes against the slide surface of the fixed support. In this arrangement, the roll must be provided with a separate support member for the grinding operation.

US patent 6,379,227 in turn discloses a band grinding device for rolls, comprising an abrasive band rotated by means of a motor. The abrasive band forms a closed loop around a drive pulley of the motor, two guide pulleys and a contact pulley placed between the guide pulleys. The contact pulley presses the abrasive band against the shell of the roll that is being ground. The guide pulley is mounted on a separate frame such that the guide pulley can be inclined by means of an actuator with respect to the outer surface of the roll. In this way, more precise grinding can be achieved when the guide pulley and, with it, the abrasive band can be inclined according to the deflection of the outer surface of the shell.

SUMMARY OF INVENTION

The principal characteristic features of the method according to the invention are stated in the characterizing part of claim 1.

Protection is also directed to a roll ground by the method according to the invention.

In the arrangement in accordance with the invention, measurements of the shape of the shell are made from the inner surface of the shell instead of the outer surface of the shell. In other words, the grinding of the outer surface of the shell is controlled based on measurements made from the inner surface of the shell instead of controlling the grinding of the outer surface based on measurements made from the outer surface of the shell.

When measurements are made from the inner surface of the shell, the outer surface of the shell can be ground according to the inner surface. For this reason, greater deviations in the shape of the inner surface of the shell can be allowed. The grinding of the outer surface of the shell according to the inner surface of the shell enables the shell to be made of equal thickness in the circumferential direction at each cross section.

In the arrangement in accordance with the invention, the aim is that the thickness of the roll shell remains constant in the circumferential direction of the shell at each loading element. Therefore, the shell need not necessarily be of entirely equal thickness over the entire axial direction of the roll, but there can be small variations in the axial direction. The outer surface of the shell also need not be straight, but it can be in the shape of an arc, in which case the inner surface of the shell is in a similar manner in the shape of an arc in order that the thickness of the shell in the circumferential direction of the shell should remain constant at each loading element. A variable crown roll with a composite shell is allowed a

thickness variation of about 0.1 mm in the circumferential direction of the roll at each loading element. In the axial direction of the roll, the thickness variation of the shell may be about 0.2 - 0.4 mm. A variable crown roll with a metal shell is in turn allowed a thickness variation of about 0.1 mm in the circumferential direction of the metal shell at each loading element. In the axial direction of the roll, the thickness variation of the metal shell may be about 0.1 mm.

One possibility is to grind the outer surface of the shell at each cross section fully according to the inner surface of the shell, so that the shell will be, at each cross section, of equal thickness in the circumferential direction.

Another possibility is to adapt a circle to the shape of the inner surface of the shell based on measurements of the inner surface of the shell, after which the outer surface of the shell is ground at each cross section to be round according to the circle adapted to the shape of the inner surface of the shell. In that case, the circularity error of the inner surface of the shell in the circumferential direction remains at each cross section as thickness variations of the shell.

BRIEF DESCRIPTION OF FIGURES

In the following, the invention will be described with reference to the figures in the appended drawings.

Figure 1 is a schematic longitudinal sectional view of a variable crown roll.

Figure 2 is a schematic longitudinal sectional view of a shell of a variable crown roll in a grinding station.

Figure 3 is a schematic cross-sectional view of a shell of a variable crown roll in a grinding station, showing one measuring sensor arrangement.

Figure 4 is a schematic cross-sectional view of a shell of a variable crown roll in a grinding station, showing a second measuring sensor arrangement.

Figure 5 is a schematic cross-sectional view of a shell of a variable crown roll in a grinding station, showing a third measuring sensor arrangement.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows a variable crown roll 10 comprising a stationary support shaft 11 and a shell 12 rotating around the support shaft. The shell 12 is supported on the shaft

11 by means of hydraulic loading elements Zl, ZN. The number of the loading elements Z in the roll is n, in which n = 8 - 100 depending on the length of the roll and the length of a single loading element Z. The loading elements Z are located side by side in the axial direction, i.e. as close as possible to one another, and form a row. Each loading element Z is advantageously provided with its own feed of a hydraulic pressure medium, which is separately controllable, for example, by means of a valve, and which is not shown in the figure. In the outer surfaces of the ends of the shell 12 there are so-called recessed portions 13, which are made right at the initial stage of manufacture of the shell 12. During the entire manufacture of the shell 12, the recessed portions 13 are used as reference surfaces according to which the shell 12 is manufactured. At the recessed portions

13, the outer surface 12a of the shell 12 is precisely circular.

Fig. 2 shows the shell 12 of a variable crown roll in a grinding station. One end of the shell 12 is here attached to a grinding end 60 in a known manner and the other end of the shell is supported by means of a roller or sliding bearing block 70 in a known manner, so that said end of the shell 12 remains open. The shell 12 is thus rotated by means of the grinding end 60 and the roller or sliding bearing block 70.

By means of the roller or sliding bearing block 70, three support points are provided in the area of the recessed portions 13 of the outer surface 12a of the shell 12. Two support points are located on the lower semicircle of the shell 12

and one support point is located 180 degrees from a grindstone 20. The figure also shows the cylindrical grindstone 20, which is used for grinding the outer surface 12a of the shell 12. In the state-of-the-art arrangements, the grindstone 20 is controlled by means of a computer 30 based on measurements made from the outer surface 12a of the shell 12 using a measuring sensor 40. In the arrangement in accordance with the invention, the grindstone 20 is controlled in the same manner by the computer 30, but measurements are performed from the inner surface 12b of the shell using a measuring sensor 50. The grindstone 20 is movable in the axial direction Sl of the shell, so that the shell 12 can be ground over its entire length. In addition, the grindstone 20 can be moved in the radial direction S2 towards and away from the outer surface 12a of the shell 12 to control grinding pressure. The sensor 50 measuring the inner surface 12b of the shell 12 is mounted in one end of a support arm 81 and the opposite end of the support arm 81 is in turn supported on a carriage 80. The carriage 80 is movable in the axial direction Sl, so that the measuring sensor 50 measuring the inner surface 12b of the shell 12 can be moved, when needed, in the axial direction Sl.

The measurements of the inner surface 12b of the shell 12 can be made, for example, in a separate measuring station before the shell 12 is moved to the grinding station. The grinding results are stored in the computer 30, in which a grinding program is formed for the grindstone 20 from the measuring results by means of algorithms used by the manufacturer of the grinding machine. After measurements of the inner surface 12b of the shell 12, the shell 12 is moved to the grinding station, in which the grindstone 20 grinding the outer surface 12a of the shell 12 is controlled by the computer 30 according to the grinding program formed based on the measurements of the inner surface 12b of the shell 12. In a situation where the measurements of the inner surface 12b of the shell 12 have already been made in advance, the shell 12 can be attached at its both ends to grinding ends 60 in the grinding station.

The measurements of the inner surface 12b of the shell 12 can also be made in the grinding station instead of or in addition to the above-mentioned separate measuring station. If measurement is performed from the inner surface 12b of the shell 12 in the grinding station, the attachment of at least one end of the shell 12 in the grinding station must be open, as shown in Fig. 2, in order that the measuring sensor 50 may be passed into the interior of the shell 12. Measurement can be performed in the grinding station from the inner surface 12b of the shell 12 before grinding and/or during grinding.

In a situation where the measurement of the inner surface 12b of the shell 12 is performed during the grinding of the outer surface 12a of the shell 12 it is also possible to correct any changes that may occur during grinding in the shape of the shell 12. In that case, the shape of the outer surface 12a of the shell 12 is corrected according to the changes occurring in the inner surface 12b of the shell 12.

Fig. 3 is a schematic cross-sectional view of a shell 12 of a variable crown roll in a grinding station showing one measuring sensor arrangement. In Fig. 3, one measuring sensor 50 is used which is attached to a support 81 shown in Fig. 2. A grindstone 20 and the measuring sensor 50 are opposite each other.

Fig. 4 is a schematic cross-sectional view of a shell 12 of a variable crown roll in a grinding station showing a second measuring sensor arrangement. In Fig. 4, four measuring sensors 50a, 50b, 50c, 5Od are used which are attached to a measuring arc frame 82, which is in turn attached to a support arm 81 shown in Fig. 2. A grindstone 20 and the first measuring sensor 50a are opposite each other.

Fig. 5 is a schematic transverse sectional view of a variable crown roll in a grinding station showing a third measuring sensor arrangement. In this arrangement, the roll is thus ground as assembled, so that the roll is attached to grinding supports by means of its own bearings. The method according to the invention can also be used in this kind of case. A measuring sensor 50 has already

been attached to a stationary support shaft 11 of the roll when the roll is assembled. A grindstone 20 and a measuring sensor 50 are opposite each other. The measuring sensor 50 can be mounted, when needed, on support of an actuator such that the measuring sensor 50 can be moved in the radial direction out of contact with the inner surface of the shell 12 in the normal operating state of the roll. A cable 51 of the measuring sensor 50 is conducted out of the roll along a bore 11a in the shaft 11. The cable of the measuring sensor can be conducted out of the roll through the bore in the shaft.

Differing from Fig. 5, the measuring sensor 50 can be omitted from the roll although the roll is ground as assembled. In that case, the measurement of the inner surface 12b of the shell 12 is performed before assembling the roll, for example, following the principle shown in Fig. 2. The measurement results of the inner surface 12b of the shell 12 are stored in the memory of the computer 30. When the roll is mounted in the grinding station after assembly, the grinding of the outer surface 12b of the shell 12 is carried out based on the measurement information about the inner surface 12b stored in the memory of the computer 30. In that case, the measurement of the inner surface 12b of the shell 12 is not performed at all during grinding. A problem may then be posed by the fact that the shape of the inner surface 12b of the shell 12 has changed after measurement before grinding and/or during grinding.

The method in accordance with the invention can be used for rolls with a composite shell as well as for rolls with metal shell. By composite is meant in this patent application a fibre-reinforced polymer material.

The measuring method can, in principle, be any state-of-the-art measuring method, which is used in the invention for measuring the inner surface of the shell instead of or in parallel with measuring the outer surface. For example, a company called RollResearch International Ltd. markets, under the registered trademark Hybrid 3D, a four-point shell measurement method that can be applied in our

invention. A company called RollTest Oy in turn markets, under the registered trademarks RollCal Classic, RollCal 2 and RollCal 3, shell measurement methods that can also be applied in our invention.

Only some advantageous embodiments of the invention are described above, and it is clear to a person skilled in the art that numerous modifications can be made to them within the scope of the appended claims.