The invention relates to a grooved and perforated turn roll of a fiber web machine including a shell and grooves formed on the shell surface with suction holes extending through the shell on the bottom of the grooves in the inward area of the edges .

A turn roll is also called a turn suction roll, a vacuum roll or a VacRoll, a trademark of the applicant. A turn roll is used in the single-fabric run system in the dryer section of a fiber web machine, where a turn roll is located between two dryer cylinders. A dryer fabric led through dryer cylinders and turn rolls presses the fiber web produced against the dryer cylinders. At the turn roll, the fiber web is then outermost without support, on top of the dryer fabric. However, a suction effect created with a turn roll makes it possible to keep the fiber web and the threading tail cut therefrom, when necessary, attached to the dryer fabric even at high speeds .

FI patent No. 83680 proposes a cylinder used for transferring a fiber web; in practice, a turn roll described above. Here, too, grooves provided with suction holes are disposed side by side on the surface of the turn roll shell. Using a certain type of dimensioning of grooves and suction holes, it has been attempted to ensure a uniform holding force acting on the fiber web. A groove defined by two straight walls and a flat bottom is deep. Due to manufacturing technical reasons, a small chamber is present between the wall and the bottom of the groove.

However, known turn rolls have problems. Specifically, the use of recycled paper as raw material has increased dryer section soiling. Especially in the initial part of the dryer section, fine-grained loose material together with sticky materials has led to clogging of grooves and suction holes. Small suction holes in particular become easily soiled and even clogged. Deep and square grooves also collect impurities and these, too, can become clogged. Due to a great number of suction holes, a large air volume is transferred, which increases the consumption of energy. In addition, manufacturing a turn roll is expensive and slow. Regardless of the special dimensioning of the edge area, the suction effect could be better in the tail threading area.

The object of the invention is to provide a novel grooved and perforated turn roll of a fiber web machine, which remains clean in a better way than heretofore and can produce a more efficient suction effect with a smaller amount of air. The characteristic features of the turn roll according to this invention become apparent from the appended claims. In a turn roll according to the invention, the dimensioning of grooves and suction holes is arranged in a new and surprising way. Firstly, clogging of suction holes and grooves is avoided. Secondly, the suction effect can be made efficient and uniformly spreading while aspirating a smaller amount of air than before. The turn roll is also easier to clean than heretofore. In addition, the turn roll shell can be made thinner, if necessary, thus reducing the weight of the turn roll .

The invention is described below in detail by referring to the appended drawings, which illustrate an embodiment of the invention, in which: Figure la is a basic view of one dryer group of the dryer section of a fiber web machine, equipped with turn rolls ,

Figure lb is a partial enlargement at one turn roll,

Figure 2a depicts a part of the shell of a turn roll according to the invention, shown in the machine direction,

Figure 2b depicts a part of the shell surface of a turn roll according to the invention,

Figure 3a is a cross-sectional view of the shell of a turn roll according to the invention,

Figure 3b is a top view of a part of the shell of a turn roll according to the invention,

Figure 4a is a cross-sectional view of the shell of a turn roll according to the invention over an area of several suction holes,

Figure 4b is a partial enlargement of Figure 4a.

Figure la depicts separately one dryer group 10, of which there are several in succession in the dryer section of a fiber web machine. The dryer group consists of dryer cylinders 11 and turn rolls 12, via which a dryer fabric 13 is led as an endless loop. Today, a single-fabric run system shown here is increasingly used, having only one dryer fabric per a dryer group. In practice, the fiber web travels between the dryer fabric and a heated dryer cylinder. Correspondingly, at the turn roll, the fiber web travels on top of the dryer fabric. In addition to a turn roll, the fiber web is controlled by a runnability component 14, which is placed in a pocket space 15 formed by successive dryer cylinders and the turn roll (Figure lb) . The turn roll has a suction connection at least at one end and the turn roll is aspirating over the entire circular sector. In other words, the turn roll is vacuumed. Figure lb also depicts an opening gap 16 defined by the dryer cylinder 11 and the dryer fabric 13 and, correspondingly, a closing gap 17 formed by the dryer fabric 13 and the turn roll 12. In addition to the runnability component 14, the fiber web and particularly the threading tail 18 are controlled with air blows 19, for example. In both cases, air packed in the closing gap 17 creates an overpressure, which tends to detach both the tail and the fiber web from the surface of the dryer fabric 13 at the turn roll 12. Access of air to the closing gap 17 is here illustrated with a straight line arrow. A grooved and perforated turn roll creates a suction effect, which, however, essentially weakens due to clogging of suction holes and grooves.

The invention is thus related to a grooved and perforated turn roll 12 of a fiber web machine. The turn roll 12 includes a shell 20 and grooves 22 formed on the surface 21 of the shell 20. Figure 2a depicts a part of the shell 20 shown in the machine direction. The shell 20 has suction holes 23 extending through the shell 20 (Figure 3a) . More specifically, suction holes 23 extending through the shell 20 are provided on the bottom 24 of the groove, in the inward area of the edges 27. A vacuum generated within the turn roll extends via the suction holes through the dryer fabric up to the fiber web. Even at its minimum value, the vacuum removes the overpressure produced by the closing gap. In the invention, the open area AR of the suction hole 23 is at least two times larger than the cross-sectional area Au of the groove 22. The open area AR of the suction hole 23 is illustrated particularly in Figure 3b. In other words, the area of the suction hole is as large as or larger than the sum of the cross-sectional areas of air flows arriving to the suction hole from two directions along the groove; that is, the cross-sectional area of the groove extending over the suction hole summed up on both sides of the suction hole. In other words, the area of the suction hole is as large as or larger than the second multiple of the cross-sectional area Au of the groove 22. This ensures an efficient flow in the grooves and simultaneously prevents formation of a bottleneck in the suction hole. The area of the suction hole is determined at its narrowest point, thus ignoring any countersinking .

More specifically, the open area AR of the suction hole 23 is 2.01 to 4.0, more preferably 2.8 to 3.6 times the cross- sectional area Au of the groove 22. In this way, air fitting in the grooves can be reliably removed and the suction effect spreads in the most efficient way possible. In practice, this has been achieved by changing the drill pattern, which has larger holes at longer intervals than heretofore. In addition, groove dimensions and the groove spacing have been changed. In Figure 2b, the cross-sectional area Au of the groove 22 is illustrated with two striped areas. Figures 2a - 3b do not show the fabric that closes the groove during production from the top of the fiber web by traveling around a part of the turn roll. A hole/groove ratio smaller than described above would probably also function better than the current one. Thus, advantages according to the invention would be achieved with a suction hole 1.8 to 1.99 times larger than the groove.

The drill pattern has additionally been changed so that suction holes 23 are only present in every second groove 22 (Figure 2a) . Tests have surprisingly shown that air also moves laterally inside the dryer fabric. Thus, the suction effect spreads from perforated grooves to unperforated grooves as well. Thus, when entering the closing gap, excessive air passes from unperforated grooves to perforated grooves and therethrough into the turn roll. Therefore, the number of holes can be reduced to compensate for the known greater need of increased air amount required by larger holes. In practice, with the new drill pattern, however, a more uniform and greater vacuum than heretofore is achieved with a smaller amount of air. The drill pattern also includes a phase reversal of suction holes, which further equalizes the distribution of the suction effect over the area of the turn roll. Figures 2a and 4a also show a tail threading area 26 at the end of the turn roll with suction holes 23 in each groove. In the invention, the distance K between the grooves 22 is 2 to 4 times the diameter D of the suction hole 23.

Thus, the neck between the grooves is notably wider than the groove, the neck thus providing a good support surface for the fabric and simultaneously separating the vacuum effects of the grooves from each other. The suction hole of Figure 4a illustrated with a dashed line shows that each groove has suction holes in the edge areas, although in different places, as shown in Figure 2a.

Figure 3a illustrates the dimensions and profiles of the groove 22 in more detail. According to the invention, the bottom 24 of the groove 22 has at least one radius of curvature R, the center of curvature 28 of which is outside the groove 22. In other words, the center of curvature is above the surface of the turn roll, i.e., on the surface that is opposite to the groove. Thus, the cross section of the groove is a segment, keeping the area of the groove moderate. In this way, the size of the suction hole is well sufficient to remove the air passing along the groove underneath the fabric. At the same time, the groove becomes smooth and without corners, avoiding thus soiling of the groove. Correspondingly, a smooth groove is easy to clean. In other words, according to the invention, the groove is cornerless. In addition, a chamfer 25 is provided between the edge of the groove 22 and the surface 21 of the shell 20. The chamfer equalizes air flow and reduces dryer fabric wear. In other words, chamfers prevent fabric wear while the vacuum tends to bend the fabric into the groove. Both the smooth profile and the chamfer can be produced in one work step, such as profile turning. In this way, the curvature of the groove and edge chamfers can be provided in a single machining step. The bottom can also be curved for a partial distance deviating from the radius of curvature. Thus, the volume of a groove machined with a curved profile can be increased by removing more material from the roll surface. Especially in a wide groove, a straight section can be provided between the groove flanks of a circular arc shape, whereupon the center part of the bottom becomes flat for a small part. In this case, too, the groove is cornerless and curved flanks are formed on the bottom of a shallow groove. Thus, the invention has a groove without walls, lacking prior art radial straight wall surfaces .

Generally, the radius of curvature R is 0.5 to 0.7 times the width L of the groove 22. Thus, a groove shallower than its width is formed, which is shallow and cornerless. So the groove is easy to machine and the groove is kept clean. Advantageously, the center of curvature 28 is at an equal distance from the edges 27 of the groove 22. The groove thus becomes symmetrical and the air flow remains undisturbed. The center of curvature 28 is placed so far from the roll surface that the bottom extends from edge to edge. Thus, the groove according to the invention is without prior art walls.

In the invention, the width L of the groove 22 is 1.0 to 1.3 times the diameter D of the suction hole 23. This enables maximum utilization of the groove width. In addition, the suction effect is efficiently distributed over the entire groove area. It has been possible to reduce the cross- sectional area by decreasing the groove depth. Generally, the depth S of the groove 22 is 0.2 to 0.4 times the width L of the groove 22. In other words, the groove is clearly wider than its depth. Groove dimensions are selected so that two times the cross-sectional area of the groove does not exceed the surface area of the suction hole.

During testing, a suction hole was used with a diameter of eight millimeters, a groove width of nine millimeters, a radius of curvature of 5.3 millimeters and a groove depth of

2.5 millimeters. In addition, suction holes were spaced less closely than heretofore. The diameter of the suction groove is preferably between 6.5 and 10 mm. Correspondingly, according to the example, the radius of curvature is between

4.5 and 6 mm, the groove width is between 7 and 10 mm and the groove depth is between 1.8 and 3.8 mm. Now, a turn roll with a diameter of 1500 mm had ten suction holes at an equal spacing around the shell perimeter. Departing from the normal practice, suction holes were only made in every second groove. In addition, the groove profile was shallow and gentle. The depth of the groove was 2.5 mm with a width of 9 mm. At the same time, the groove spacing and the groove width increased, which increased the openness of the turn roll to 0.23%. Nevertheless, the number of grooves reduced by 20%. With the new dimensioning, the cross-sectional area of the groove was 20% smaller than currently and the sum of the cross-sectional areas of the grooves was about a half of the current one. At the same time, the diameter of the suction hole was almost double. Thus, surprisingly, a fiber web can be well kept on the outer surface of a dryer fabric with a smaller amount of suction air, since a suction hole larger than before reduces losses. In practice, the number of suction holes was up to 70% and the number of grooves up to 30% smaller than in a known turn roll. Thus, the drilling of suction holes and groove machining take clearly less time than heretofore. The machining time can even be halved by machining several grooves simultaneously.

Based on the tests, the runnability of the turn roll was good and tail threading operated flawlessly. In addition, air leak through the grooves on the ascending side of the turn roll was insignificant. This was at least partly due to grooves shallower than in known designs. Based on the test results, the new turn roll consumes less energy than heretofore, although the suction effect is bigger and the vacuum level is higher than before.

A turn roll according to the invention carries improvement to the soiling problem of grooves and suction holes. In other words, the turn roll remains clean and clogging of suction holes is avoided. The consumption of energy is reduced at the same time. Furthermore, turn roll manufacturing is faster and raw material costs decrease. In practice, the runnability of the fiber web machine and tail threading also improve so that tail threading takes less time than before and web breaks are reduced. Thanks to the lower groove depth and the notch effect of a shallower and more gently sloping groove, the shell thickness can also be reduced. A smooth and shallow groove is easy to clean. Furthermore, a suction hole notably larger than heretofore can be quickly cleaned. A thinner shell accelerates drilling, decreases the shell weight and reduces the material quantity. At the same time, the depth of the suction holes is reduced, which means slower soiling speed. As the weight of the turn roll decreases, a drive motor with smaller dimensions can be used.