SCREEN

Abstract: The present invention relates to a screening apparatus comprising an outer housing with connections for material to be treated and material treated, end pieces, a stationary screen drum (5) disposed inside the housing (1) and secured with bolts (20) at its upper end flange (14) to an intermediate ring (16) and supported via the intermediate ring (16) by the housing (1); and a rotating rotor (6) disposed inside the screen drum. The screening apparatus is characterized in that the bolt (20) heads are locked to be non-rotatable with a means (18, 22) which leans against at least one side of the bolt (20) head.

Description: SCREEN The present invention relates to a screen. In particular the invention relates to the structure of a screen used in the wood processing industry, providing an improvement in the working reliability and safety of the device. The screens used in the wood processing industry are most often apparatus surrounded by substantially cylindrical outer housing having connections at least for the fiber suspension to be supplied to the apparatus, for the accepted fraction, or the so-called accept, to be discharged from the apparatus, and for the rejected fraction, or the so-called reject. Further, the apparatus may have connections for, for example, plastics separated in the apparatus and for dilution or washing liquid to be fed into the apparatus. In most cases, the apparatus is installed in an upright position so that the axis of the substantially cylindrical housing is vertical. Then the top of the apparatus is easy to open or detach from the essentially cylindrical housing and the apparatus may easily be maintained. The shaft of the apparatus passes through the opposite end of the apparatus, i.e. usually the lower end, and rotates usually a so-called rotor although in some screens the screen cylinder itself is rotatable. Inside the cylindrical housing, there is, in addition to the rotor, also a screen or a sieve cylinder, which in most cases is cylindrical as is indicated also by its name, although a number of cone-shaped screen drums are also used. usually the screen cylinder is located outside the rotor in the radial direction. The screen cylinder is supported at its both ends to the housing of the screen via intermediate rings. The intermediate rings mentioned have been secured to the housing of the screen and the screen cylinder is secured to these intermediate rings via a securing flange at both ends. A screen of the type described is disclosed for example in U.S. patent no. 5,326,470 which also describes many different ways of securing a screen cylinder to the intermediate rings mentioned. Quite until recent years the problem has been the securing of the screen cylinder to the intermediate rings in a way to obtain maximum strength. Before the securing methods disclosed in the U.S. patent mentioned were introduced, screen cylinders were secured to intermediate rings so that axial compression stress was directed to the cylinders. If the screen cylinder was adequately robust and the application stressed the screen cylinder relatively little, no problems arose. On the other hand, when the so-called wire screen drums become more common the mechanical endurance of the screen cylinders became essentially weaker whereby the mills found in particular the compression stress to cause problems which were solved for example by the screen drum securing methods described in the U.S. patent mentioned by which the screen was subjected to tensile stress which eliminated the danger of deflection of individual screen bars. Despite the fact that one problem in the securing of a screen cylinder was eliminated a number of new problems were revealed which were previously hidden behind the more serious problems discussed above. It has been found out now that it is very difficult to screw on the screen cylinder securing bolts so tight that they do not come off gradually for example as a consequence of vibration of the screen. When conventional securing methods were used, a small axial clearance was common at the upper end of the screen drum between the screen drum flange and the intermediate ring attached to the housing of the screen which was due to the many tolerances of the manufacturing technique and often even differences in tolerances of different manufacturers. In many cases, the screen drum is not delivered by the same manufacturer who originally manufactured the screen. However, the clearance mentioned allows the bolt securing to "live" , for example according to the temperatures or other stresses, so that the securing bolts of the drum are loosened quite easily. A preferred embodiment of the invention brings a simple and advantageous solution to this problem. An essential feature of the approach according to the invention is that the securing bolts of the screen drum are locked non- turnable by a means locking several bolts at a time, preferably all the securing bolts of the screen drum. An other drawback of the securing methods based on the tensile stress of the screen drum described in the U.S. patent discussed is their complexity. In order to secure the drum in a way creating tensile stress, according to the embodiment of the patent mentioned illustrated in figures 4 to 9 and 11, the drum must be secured by bolts at its lower end and even inside the drum which means that in practise the rotor must be detached from the screen before the drum can be replaced. From the figures of the patent mentioned, only one allows bolting the screen cylinder at its upper end, only, and even then the rotor must be removed from its place in order to detach the screen cylinder because the inner diameter of the securing flange of the lower end of the screen cylinder is smaller than the outer diameter of the rotor. In other words, it seems clear that a securing method allowing tensile stress presupposes the use of a flange extending inside the screen cylinder which in turn means that the rotor must be detached for the replacement or maintenance of the screen cylinder. International patent application PCT/SE94/00013 discloses an approach in which the screen cylinder is in a way floating so that there is at both ends of the screen cylinder a radial slot and at the slot there is a groove for for example an O ring which seals the slot and at the same time supports the screen cylinder radially in its place. In the axial direction the screen drum is supported at least at one of its ends by pins extending through the drum to the body of the screen so that the drum cannot move axially or be turned. A problem is, however, that when a drum is left floating in the radial direction only supported by rubber rings, pressure pulses always vibrate the drum to some extent and even a small movement wears out both the sealing members and the pins mentioned. An approach according to an other preferred embodiment of the invention eliminates these problems. In other words, it both allows replacing the cylinder without detaching the rotor, and it supports the cylinder in its place so that pressure pulses cannot move the screen cylinder. The characteristic features of the screen according to the invention are disclosed in the appended patent claims. The screen according to the invention is described more in detail below with reference to accompanying drawing figures of which Fig. l is a general illustration of a screen used in the wood processing industry; Figs. 2a and 2b illustrate, partly in section, a screen detail according to a preferred embodiment of the invention; Fig. 2c illustrates an alternative embodiment the function of which corresponds to that of Fig. 2b; Fig. 3 illustrates an other preferred embodiment which in principle corresponds to that of Fig. 2a; Fig. 4 illustrates, partly in section, a screen detail according to an other preferred embodiment of the invention; Figs. 5a and 5b illustrate, partly in section, a screen detail according to a third preferred embodiment of the invention; Fig. 6 illustrates, partly in section, a screen detail according to a fourth preferred embodiment of the invention; Fig. 7 illustrates, partly in section, a screen detail according to a fifth preferred embodiment of the invention; Fig. 8 illustrates, partly in section, a screen detail according to a sixth preferred embodiment of the invention; and Fig. 9 illustrates, partly in section, a screen detail according to a seventh preferred embodiment of the invention. As illustrated in Fig. 1, a screen mainly comprises an outer housing l, connections arranged therein for the pulp 2 to be fed in, for the accepted fraction, the so-called accept 3, and for the rejected fraction, the so-called reject 4. Inside the outer housing, there is a screen or sieve surface 5, and in the vicinity of the surface 5 there is provided a member 6, a so-called rotor movable in relation to the surface 5. Figure 2 illustrates in detail how a screen drum is reliably secured in its place according to a preferred embodiment of the invention. The figure illustrates only the top end 12 of the screen drum, its securing flange 14, an intermediate ring 16 secured/arranged to the housing of the screen, and a so-called impurity separator 18 and the actual securing bolts 20 of the screen drum. As may be seen in the figure, the intermediate ring 16 includes in this embodiment an annular surface 162 parallel with the axis of the screen against which the guiding surface 142 of the securing flange 14 of the screen drum is tightly set. Guided by these two surfaces, the screen drum 5 is positioned centrally around the axis of the screen. Thus, the hole in the securing flange 14 for the securing bolt 20 is a little loose so as to allow the axial guiding of the screen drum 5 to take place via the surfaces 142, 162 mentioned, only. As illustrated in the figure, the screen drum 5 has been secured to the intermediate ring through its flange 14 by means of the bolts 20. In prior art apparatus, the same bolts were used to secure also the so- called impurity separator 18 which is an essentially cylindrical ring extending above the screen drum 5 and prevents very coarse impurities, such as for example stones, nuts and other such pieces, from entering the screening zone between the screen drum and the rotor 6 (see Fig. 1) . Figure 2b illustrates that the securing flange 14 of the screen drum 12 first of all has an annular guide surface 19 for the impurity separator 18 for centering the impurity separator 18 centrally around the axis of the screen. The figure also illustrates the securing flange 14 having a cylindrical recess 21 which partly opens to the cylindrical guide surface 19. The idea is that when the screen drum is secured with the bolts 20 to the intermediate ring 16, one of the sides of the head of the hexagon head bolt substantially aligns with the guide surface 19 mentioned and is parallel with its tangent so that when the impurity separator 18 is installed in its place against the guide surface 19 it locks the securing bolt 20 of screen drum 12 unrotatable in its place. In a corresponding way the guide surface of the impurity separator may be arranged outside the periphery of the screen drum securing bolts, thus obtaining a corresponding locking of the bolts . Further, in the embodiment illustrated in figure 2c, the lower part of an impurity separator 18, either the securing flange 22 of the impurity separator 18, or the lower part of the cylindrical piece itself, has been so shaped that it prevents the securing bolts 20 of the screen drum 5 from being unwound. In other words, in the securing flange 22 or at the lower end of the cylindrical piece there is a recess 23 which at least partly adapts to the form of the end of the securing bolt 20 and which locks the end of the securing bolt 20 non-turnable in its place. Naturally, if the screen does not include an impurity separator the separator may be replaced by a plain ring, which is placed by the side of or on top of the circle of bolts securing the screen drum, depending on which of the embodiments, i.e. securing methods described above is chosen, and locks all the bolts non-turnable either in the way illustrated in Figs. 2b and 2c or in some other way. Of course, it would be advantageous in most cases to be able to lock all the bolts with one single means, for example by means of an impurity separator or a particular ring, but it is possible also to provide several locking means each of which locks several bolts at a time. Preferably these means form an annular ring on top of or by the side of the bolt circle but the inventive idea covers also other applications in which several bolts are locked in their places at a time. The impurity separator 18 itself or the ring replacing it is secured in its place by separate securing bolts. Thus, although the total number of securing bolts used increases the load directed to the securing bolts of the impurity separator and the ring is negligible compared to the securing bolts 20 of the screen drum 5 and thus the tendency of these bolt to be unwound is also remarkably smaller. Figure 3 illustrates an alternative of the embodiment of Fig. 2a in which the bolt 20 of Fig. 2a has been replaced by a so-called pin bolt 20' and the means to be locked in the embodiment of Fig. 3 is a nut 200 which corresponds to the head of the bolt 20 in Fig. 2a. This means that the nut may be locked non-turnable in its place by means of the impurity separator or a corresponding means as described above. Figure 3 further illustrates a preferred but not indispensable additional alternative in which spring washers 202 have been provided in connection with the nut in the Fig. 3 embodiment, but in practise also separately, by means of which the securing flange 14 is pressed against the intermediate ring 16 of the screen. Further, it is possible that the bolt 20' is not a so- called pin bolt but an ordinary bolt with a head which has been screwed down through the intermediate ring 16 so that the end of the bolt extends above the surface of the intermediate ring like the end of the pin bolt 20' . In this case the securing is carried out just as illustrated in Fig. 3. And advantage provided by this securing way is that when the bolt has for some reason broken off by the nut side the bolt may be screwed out of the hole in a conventional way after the screen drum has been removed. During the first assembly, the head of the bolt may be secured in its place for example by two welding spots so that the bolt cannot, if snapped, be wound off and end up in the accept. The welding spot mentioned is easy to grind off when the bolt is replaced by a new one. Fig. 4 illustrates a way according to a preferred embodiment of the invention of supporting the screen drum 5 in its place without directing practically any compression stress at all to the screen drum. As already stated, previously there were two ways of securing a screen drum. The first way was to secure the screen drum with bolts at its top end, only, and to press the conical bottom end of the screen drum against the surface of the conical intermediate ring. In other words, the screen drum was set centrally in its place by utilizing compression stress and thus it was easy to remove it from its place without having to detach the rotor located inside it. The other way, which was described in the U.S. patent mentioned, avoided the compression stress but brought the drawback of having to dismount the rotor at first before the screen drum could be detached and remove . Fig. 4 shows how an intermediate ring 26 connected to the housing of the screen has been provided with a conical surface 262 opening upwards the angle of which is of the order of 5 - 25, preferably 10 - 20, and appropriately about 15 degrees. In a corresponding way the flange 28 of the lower end of the screen drum 5 has been provided with a conical surface 282 tapering downwards. An annular groove 30 has been provided in the conical surface 282 of the screen drum 5 flange 28, substantially at the center region thereof, the depth of the groove being in most of the embodiments presented preferably of the order of 3 - 10 mm and the length preferably of the order of 5 - 15 mm depending on the dimensions of the ring/rings used, and the groove in turn being preferably provided with a ring 32 made of rubber of some other corresponding flexible material and preferably affixed. The cross section of the ring 32 mentioned is preferably such that its surface 322, preferably conical, placed against the conical surface 262 of the intermediate ring 26 is relatively broad thus ensuring a good sealing between the surfaces 262 and 322. The dimensions of the conical surface 262, 282 and 322 and the ring 32 have been chosen so as to have as little axial compression stress of the screen drum 5 as possible and to provide a guiding which centers the screen drum lower end in the radial direction as well as possible. Figs. 5a and 5b illustrate a method of guiding the lower end of the screen drum according to another preferred embodiment of the invention. The starting point is practically the same as in the embodiment of Fig. 4. The only difference is that the flexible ring 42 has been affixed in this embodiment in the conical surface 262 of the intermediate ring 26 affixed in the housing of the screen. The purpose is to cause the flexible ring 42 to be bent, when the screen drum is pushed in its place, into the groove 30 provided in the conical surface 282 of the screen drum lower end flange 28 into the position illustrated in Fig. 5b, and both to seal the space between the conical surfaces 262 and 282 efficiently and to center the screen drum exactly in the correct position. Figure 6 illustrates a guiding method of the lower end of the screen drum 5 according to a third preferred embodiment of the invention. More precisely expressed, the flexible ring 52 is a ring which is mainly made of for example rubber but has for example a metal ring 524 disposed in the middle of it, which remarkably stiffens the structure of the ring 52. This kind of a ring 52 need not necessarily be affixed in the counter surfaces at all but the ring 52 stays in its place pressed by the metal ring 524, only. Figure 7 illustrates a guiding method of the lower end of a screen drum 5 according to a fourth preferred embodiment of the invention. In this embodiment, a flexible ring 62 replaces the flange of the lower end of the screen drum 5. Preferably the flexible ring 62 is placed in an annular groove (60) machined in the screen drum outer surface. In this embodiment it is advantageous to have two annular bands 624 and 626 (for example steel bands) arranged inside the flexible ring 62 as illustrated in the figure. The function of the bands 624 and 626 is to stiffen the ring 62 particularly in the radial direction so that the ring 62, firstly, stays firmly in its groove 60 and, secondly, supports the screen drum 5 centrally in its place as well as possible. Figure 8 illustrates a guiding method of the lower end of a screen drum 5 according to still a fifth preferred embodiment of the invention. In this embodiment, a ring 78 sliding in the axial direction has been provided at the lower end of the screen drum 5 the conical surface 782 of which is supported by the conical surface 262 of the intermediate ring 26. A flexible ring or several flexible rings 72, preferably large 0 rings, have been provided between the screen drum 5 and the sliding ring 78, the rings pushing the sliding ring 78 downwards and causing it to be set tightly against the conical surface 262 of the intermediate ring 26. Figure 9 illustrates a guiding method of the lower end of a screen drum 5 according to still a sixth preferred embodiment of the invention. In this embodiment, an annular groove 30 having a bottom surface 302 has been arranged in the conical surface 882 of a ring 88 at the lower end of the screen drum 5. The bottom surface 302 is conical. The surface 302 is preferably not parallel with the conical surface 882 but tapers downwards faster. The conical surface 882 in turn preferably tapers downwards a little faster than the conical surface 262 of the intermediate ring 26 although these surface may be parallel, also. The groove 30 has been provided with an O ring 82, preferably two O rings, preferably of rubber, so that when the screen drum is pushed downwards the ring/rings mentioned roll upwards along the bottom surface 302 of the groove 30 so that they are wedged between the bottom surface 302 of the groove and the conical surface 262 of the intermediate ring 26 and both seal the lower end of the screen drum 5 and center the drum exactly in its central place. An advantage provided by the conical forms, in which the coning angle of the conical surface 262 of the intermediate ring 26 is smaller than the coning angle of the lower end 28 of the screen drum or the coning angle of the bottom of the groove provided at the lower end, illustrated in most of the figures mentioned above is that there is a wedge-like space tapering towards the accept space provided between the surfaces mentioned. When considering the pressure circumstances inside the screen, i.e. the fact that the pressure is the highest inside the screen drum and the lowest in the accept space, the pressure inside the screen drum presses the member between the conical surfaces mentioned towards the more tapered end of the wedge-like space and thus the sealing ability of the member improves. In other words, it is typical of a preferred embodiment of the invention that the sealing and support of the lower end of the screen drum is taken care of by two conical members so that a space is arranged between the two surfaces which space tapers in a wedge- like fashion towards the lower pressure, and a member sealing and supporting the lower end of the screen drum has been arranged in the space mentioned. As may be understood from the above, a structure of a new type has been developed which eliminates the drawbacks and problems of prior art apparatus. However, it should be remembered that only a few most preferred embodiments of the structure according to the invention have been presented above which in no way intend to limit the scope of protection of the invention from what has been defined in the appended patent claims.

Claims: We claim : 1. A screen mainly comprising an outer housing (l) ; connections (2, 3, 4) arranged therein for material to be treated and for material treated; end pieces,- a stationary screen drum (5) disposed inside the housing (1) and secured with bolts (20) at its upper end flange (14) to an intermediate ring (16) and supported at its lower end by an intermediate ring (26) and supported via the intermediate rings (16, 26) by the housing (l) ; and a rotating rotor (6) disposed inside the screen drum; characterized in that the lower end of the screen drum (5) rest via a flexible member (32, 42, 52, 62, 72, 82) on the intermediate ring (26) . 2. A screen as claimed in claim l, characterized in that the inner edge of the intermediate ring (26) has the form of a conical surface (262) opening upwards which the flexible member (32, 42, 52, 62, 82) leans on. 3. A screen as claimed in claim 1 or 2, characterized in that there is a flange (28, 78, 88) at the lower end of the screen drum (5) the outer surface of which has the form of a cone (282, 782, 882) tapering downwards. 4. A screen as claimed in claim 3, characterized in that there is an annular groove (30) in the conical surface (282, 882) into which the flexible member (32, 42, 52) is placed. 5. A screen as claimed in claim 1, characterized in that the flexible member (62) is placed directly between the screen drum (5) and the conical surface (262) of the intermediate ring (26) without a flange at the lower end of the screen drum (5) . 6. A screen as claimed in claim 5, characterized in that the flexible member (62) is provided with at least one support ring (624, 626) to stiffen the ring. 7. A screen as claimed in claim 5, characterized in that there is an annular groove (60) at the lower end of the screen drum (5) provided for the flexible member (62) . 8. A screen as claimed in claim l, characterized in that there is a flange (78) sliding axially provided at the lower end of the screen drum (5) , the flange leaning directly on the conical surface (262) of the intermediate ring (26) and on the screen drum (5) at least via one flexible member (72) . 9. A screen as claimed in claim 4, characterized in that the bottom surface (302) of the annular groove (30) is conical . 10. A screen mainly comprising an outer housing (1) ; connections (2, 3, 4) arranged therein for material to be treated and for material treated; end pieces; a stationary screen drum (5) disposed inside the housing (l) and secured with bolts (20, 20') at its upper end flange (14) to an intermediate ring (16) and supported via the intermediate ring (16) by the housing (1) ; and a rotating rotor (6) disposed inside the screen drum; characterized in that the bolts (20) or the nuts (200) used in connection with the bolts (20') have been locked unrotatable with a means (18, 22) leaning against at least one side of the head of the bolt (20) or the nut (200) . 11. A screen as claimed in claim 10, characterized in that there are recesses (23) provided in the means (18, 22) the form of which essentially conforms to the form of the head of the bolts (20) or the nuts (200) and which lean against at least one side of the bolt (20) head or the nut (200) . 12. A screen as claimed in claim 10 or 11, characterized in that the means mentioned is an impurity separator (18) the lower edge of which or the guiding surface of the securing flange leans against a side of the bolt (20) head or the nut (200) mentioned or in the lower edge of which the recesses (23) mentioned have been provided. 13. A screen as claimed in claim 10 or li, characterized in that the means mentioned is a ring provided particularly for this purpose against the edge of which the side of the bolt (20) head or the nut mentioned leans or in which the recesses have been provided.

Inventors: LJOKKOI RISTO, LJOKKOI, RISTO
Applicants: AHLSTROM MACHINERY OY, LJOKKOI RISTO, LJOKKOI, RISTO
Application year: 1997
Main classification: D21D5
IPC codes: D21D5