FI119942B - Process for manufacturing a screen cylinder and a screen cylinder - Google Patents

Description

A method of making a screen cylinder and a screen cylinder

The present invention relates to a process for making screening cylinders and a screening cylinder which is particularly suitable for sorting, filtering, fractionating or screening cellulosic pulp suspensions and similar suspensions of the pulp and papermaking industry. More particularly, the present invention relates to screening apparatuses comprising a plurality of screening yarns disposed in parallel with each other, wherein the plurality of screening yarns form a screen surface per sortable pulp suspension and wherein adjacent yarns form screen passages through which the main portion of the pulp suspension flows.

For example, EP-A1-0929714 discloses a sorting device in which the screening yarns are downstream of integral transverse slots in integral support elements, support rings or support bars.

In known screening apparatuses of this type, the supporting elements supporting the screening yarns are uniform bars of rectangular or circular cross-section and are typically located perpendicular to the screening yarns. In addition, the aforementioned EP-A1-0929714 discloses a wire strainer having a support ring in the form of a U-shaped bar to which screened wires are attached to mechanically actuated recesses transverse to the support bar.

The screening threads are usually attached to the support rods by welding, which has many drawbacks such as material unevenness, thermal stresses, and chamfering. The heat caused by welding often causes the wires to twist and • change in the width of the gap between adjacent wires. This makes it difficult to keep • · ·: *. · The mesh openings perfectly aligned, which reduces the efficiency of the screen. While today the desired width of the · · · screen hole can be as small as 0.1 mm, only extremely small distortions (if any) are accepted.

*: · 30 • · · · Heat stress and deburring can also cause malfunctions due to the load applied to the screening device by the user. The fatigue load can be either continuous or intermittent. Freeze-dried fibers may also become entangled in the suspension, causing the strainer or filter to gradually become clogged, or to form so-called "straps", which are highly detrimental to the user's process.

US Patent Nos. 5,090,721 and 5,094,360 also suggest, in the prescribed manner, "wedge-shaped" screening yarns for attachment to recesses in the support rods, which are also "wedge-shaped." The screening yarns can be locked in place by bending the support rods into rings. , and the wedge-shaped attachment coupled with the locking feature has received many suggestions well known in the art, i.e., bonding, soldering, welding, etc. have been proposed to secure the wedge-shaped attachment.

Due to, among other things, the above-mentioned drawbacks, the quality of the sorting is low, mechanical weaknesses occur or manufacturing costs are increased (for example, the wedge-shaped recess must be exactly the right size for locking to occur). the method used for its preparation.

However, considering that when using a wedge-shaped solution on the support bars or support rings, the spacing between adjacent screening yarns remains substantially constant, the possibility of finding a reliable and easy way to lock the screening yarns into the wedge-shaped recesses should be explored. The recess has a wedge-shaped shape, that is, mechanically provided either completely within the support element or the ring or rod, or mechanically provided that the rock is open on one side of the support element or the rod or ring, • · · · '· 1 in only one direction relative to its axis. In other words, the wedge structure · · ·: 1. '25 either locks the wire substantially tightly or allows the wire to slide into the wedge in the direction of the wire. Thus, it is clear that the wedge structure prevents the movement of the wire in the direction of the pressure pulses due to the sorting.

• · · · · · · · · ·

It is therefore also an object of the present invention to provide an easy-to-fabricate # e1: 1 30 and assembled screen cylinder, in which the screening yarns are not subjected to thermal stresses.

• · · · · · · · · ·

It is also an object of the present invention to provide a screening cylinder having screening slots • · φ φ #. that is, the screen openings are exactly the same size and uniform.

It is a further object of the present invention to provide a more sophisticated method of manufacturing a screen cylinder, whereby uniform screen openings, i.e., good tolerances are obtained, allowing very small-width gaps to be manufactured.

It is a further object of the present invention to provide a more advanced sih-5 distillation cylinder with a minimum of chamfering or other protruding elements that cause fiber to accumulate on the upstream surfaces of the support rods.

One of the basic ideas of the present invention is that at least one of the support elements or support rings can move relative to the other support element or support ring 10 so that the movement of the support elements or support rings locks the screen yarns in place.

According to a preferred embodiment of the invention, one of the support elements of the screen cylinder comprises two parts, i.e. two rings, at least one of which is pivotally circumferentially relative to the other, after which the rings are attached to each other as a support structure. Two support rings are also used to attach the screening yarns to prevent the yarns from moving in all directions.

According to another preferred embodiment of the invention, the support elements are individual annular pieces arranged at an axial distance from one another. At least one of the support rings is rotated circumferentially relative to the other support ring so that the screening yarns of the screen cylinder are slightly inclined and thereby locked into place in the recesses or grooves of the support rings.

In accordance with yet another preferred embodiment of the invention, at least one of the support elements of the screen cylinder is formed by two rings. The first ring has wedge-shaped or tail-shaped recesses or recesses into which the yarns are to be placed, and the second • · ·: V ring (which may not have recesses or recesses) is moved at least partially radially, · · · '· ../ · that the screening yarns are pressed against one wall of the cavity or recess.

The second ring may be moved either mechanically or first by heating and expanding the ring 2 and then allowing it to cool and thereby shrinking against the screening yarns: 3.

«·· ·

A process for making a screen cylinder, which screen screen cylinder consists of a number of screen wires. and, essentially, circular ring support elements, consisting of 3 ·· ♦ * ... 1 35 of the following steps a) arranging transverse grooves / recesses in at least a portion of said rings, 4 b) arranging said rings parallel to the rings, c) d) moving at least one of said rings relative to the second ring 5 to lock the screening yarns into said transverse grooves / recesses, and e) securing the rings in place to lock the screening yarns.

A screen cylinder consisting essentially of circular support elements, said support elements being in the form of a support ring, at least a portion of which said support rings 10 have transverse grooves / recesses and at least a plurality of screening yarns between which at least one of said support rings must be movable during the assembly of the screen cylinder to lock said screen yarns into said transverse recesses / grooves of said rings.

Other features of the invention will be apparent from the appended claims.

The method of making the screen cylinder and the screen cylinder are explained in more detail in accordance with the accompanying drawings, in which:

Fig. 1 schematically illustrates a prior art wire screen cylinder, Fig. 2 schematically illustrates several different embodiments of wedge structures arranged by machining prior art support elements, for example * · t • · *. Figures 3a and 3b illustrate a preferred embodiment of a support element wire assembly according to the present invention, Figures 4a and 4b show a support element wire assembly according to the present invention. * two other preferred embodiments of the method,

Figures 5a and 5b show an advantageous fastening of the support element-wire assembly of the present invention.

Figures 6a and 6b show another preferred embodiment of a support element-wire assembly according to the present invention, · ·: ***; Figures 7a and 7b show a support element wire according to the present invention. *. and yet another preferred embodiment of the method, and Fig. 8 shows a further preferred embodiment of a support element-wire assembly according to the present invention.

5

Fig. 1 schematically shows a prior art wire screen cylinder. The screening cylinder 1 consists essentially of axial screening yarns 10, so-called wedge beams (originally resembling a wedge), which are attached to the supporting elements 20. The most common type of wedge screening cylinder is a from the inside of the distiller to the outside. Also known as inflow key wire cylinders are known. The distance between adjacent yarns 10 defines screening slots 15. Depending on the purpose of the screening cylinder 1, the slit width is generally about 0.1-0.3 mm. A plurality of substantially circular support elements 10 are arranged along the length of the screening yarns, with the axial distance between the support elements being again about 20-200 mm, again depending on the size and purpose of the screen cylinder 1. The screening cylinder is often made such that the screening yarns 10 are attached to the support rods before the screen is bent into a cylindrical shape, whereupon the support bars form the support elements after bending, or the screening yarns can be attached to the support elements after being bent into circular rings.

A common way of attaching the screening yarns to the support elements is to use substantially transverse recesses or grooves of the support elements 20 into which the screening yarns 10 are placed. Fig. 2 shows some alternative forms of the so-called wedge-shaped or tail-shaped recess 30 in the support element 20 or the support bar. There are also a few alternatives for grooves in the support element. The recesses and grooves have common features. The first is that the recess / groove 30 is most commonly made mechanically • · ·: · 'at a right angle to the rod or element. And secondly, as the pictures show, the basic idea of the wedge-shaped recess 30 Qa is of course also the groove) is to keep the screening wire in the recess so that the screening wire cannot move '* * in the same direction as the axis of the thread, i.e. at right angles to the support element • »·: V den. In other words, a so-called shape lock is used. Of course, the axial movement of the screening wire is also not a desirable feature, but it can be used in the manufacture of a screen cylinder. In other words, unless locking the threads into the recesses. * · * 30 is used to attach the yarns to the recess, according to US Patent No. 5,090,721, the support rods can be easily bent into circular support members 20, after which the yarns are inserted into recesses 30. should be as close as possible to the cross-section of the sieve yarn. Next, to prevent the yarns from moving axially, the yarns can be welded, glued or soldered to the support bar, or deformed to prevent all movement. However, • · 6 all of the attachment methods shown are complicated, may cause chips that collect fibers, or otherwise not ideal for their intended use.

Figure 3 shows a preferred embodiment of the present invention on an enlarged scale. It should be noted that the images are very schematic and the proportions therein do not correspond to the actual dimensions of the screen cylinder and the support element. It should be noted that the width of the screening wire 10, i.e. the horizontal dimension in the figures, is 2 to 5 mm in a conventional mass screen and the depth (vertical dimension) of the support element 20 is 5 to 15 mm. It should also be noted that for the sake of clarity, only one screening yarn 1010 is shown, despite the fact that sorting between screening yarns would require multiple screening yarns for each image. The pictures are therefore exemplary. A preferred method of manufacturing the screen cylinder according to the invention is that the circular support elements 20 having wedge-shaped or tail-shaped or similar grooves are attached to the assembly bracket. At least one of the screen cylinder support members 20 is formed by two rings, the first ring 24 being fastened to the holder and the second ring 22 resting freely on the other. Next, the screening yarns 10 are pushed through the recesses / grooves in the support elements 20 and, naturally, through the recesses / gaps 30 in the first and second rings (24,22). Advantageously, but not necessarily (see Figures 6a and 6b), the recesses or grooves are identical in all the elements / rings. Once all the screening yarns 10 have been inserted into the recesses / grooves 30 in the support elements 20, the screening yarns are secured so that they cannot move in the direction of their axis as well. This is done by rotating the second ring 22 of the tongue element 20 consisting of two rings in a circumferential direction relative to the first ring 24. The circumferential movement is of a natural nature. 1 25 is negligible because it is limited by the clearance between the screen wire 10 and the walls 26 of the recess / groove 30, but by the movement of the second ring 22 all the screening threads 20 M · · · are fastened or locked into place in the recesses / grooves. After locking the screen threads 10, the first and second rings 24 and 22, respectively, are secured to each other by welding, brazing, gluing, riveting or using screws or nuts.

·: · 30 ····

It is natural to make a sieve cylinder in other ways as well. For example, it is possible to attach sieve yarns of a certain length to the assembly rack, the length of which corresponds to the sieve yarns of the target cylinder. The support elements 20 would be sequentially added such that the screen yarns 10 would pass through the recesses / grooves 30 in the support elements and leave a desired distance between two adjacent support elements 20. Once all the support elements 20 have been installed, the one (s) consisting of the two rings 7 is pivoted as above to lock the screening yarns into the recesses / grooves 30.

Figure 4a shows another preferred embodiment of the present invention in a situation where the screening thread is already tightened, essentially as shown in Figure 3b. The only difference in the embodiment shown in Fig. 4a is the shape of the wall 26 of the recess 30 of the second support ring 22. A portion 28 of the wall 26 of the recess 30 is shaped, for example sharpened, so that it penetrates the screening wire slightly. When the second ring 22 is pivoted, the portion 28 of the wall 26 engages the screen wire 10 and locks the screen wire 10 more securely into the support element 20.

Figure 4b shows another alternative of the same embodiment. Each of the first and second rings 24 and 22, respectively, have screening portions 28 penetrating the screening thread 10.

15

It should also be noted that the "sharpened" wall portion 28 does not necessarily correspond to images 4a and 4b, i.e. is substantially centered on the rings 22, but the recess / groove 30 may have an inclined wall such that the sharpened wall portion coincides with the recess / groove. 30 with an edge 20 between the wall and at least one sidewall of the ring.

Figures 5a and 5b show a preferred fastening of two rings 22 and 24 to each other by means of a screw • · · • · *. Figure 5a shows the location of the two rings 22 and 24 in a situation where the screening threads can be mounted in the recesses / grooves 30. It has been shown that the first ring ·· ·: *. * 25 24 has a threaded hole 32, and the second ring 22 has a hole 34. The holes are not in the same alignment * · ** · at the screening thread installation step. Figure 5b illustrates a situation in which the second ring 22 is rotated to the right so that the screen threads are locked between the recesses / grooves in the two rings. The holes 32 and 34 are now aligned, and the rings 22 and 24 can be secured so that the screening yarns 10 are firmly seated by the rings 22 and 24.

• · · · *

• • I

• · • · • · ·

Figures 5a and 5b also show, as an additional feature, that is, not as an essential feature, how a second ring 22 may have a conical seat 36 in its bore 36. The conical chuck may be used to tighten the screening yarns 10 into the recesses · · · 35 into the grooves 30 provided that the screw has a corresponding conical portion. In this case, the hole 34 in the second ring 22 could be slightly larger 8 than the threaded hole 32 in the first ring 24. In this case, the screw used to secure the rings together can be easily screwed into the hole 32 until the conical top of the screw When the screw, actually the screws, since use-5 amp screws are needed to attach the rings to each other, further tightening, the conical parts of the screw and the hole force the second ring 22 to move to the right and lock the screening threads 10 in the grooves. When the screws have been tightened all the way, the desired tension of the screen threads in the recesses / grooves is achieved.

10 As stated above, the two rings of the support element can also be bonded to each other by welding, whereby the screening threads must be sufficiently tightened between the rings and then the rings are welded together. The same applies when soldering, gluing or riveting the rings. None of the above attachment methods attach the rings to the screen yarns themselves.

15

Another way to secure the rings to each other is to use a wedge between the two support rings. The support surface of each support ring must have a substantially radial notch on its surface. The position and size of the notches should be such that, when the wedge is inserted into the notch, the rings move relative to one another such that the screening yarns lock between the recesses / grooves in the rings. After the wedge is inserted into the notch, it can be secured, for example, by welding, soldering, gluing or riveting.

Figs. 6a and 6b show another embodiment of the invention. In Fig. 6a, the two rings 22 'and 24' are shown separately, and in Fig. 6b, the two rings are shown attached to one another, i.e. a second ring 22 'on the first ring 24'. The figures show how the grooves 30 ', just like the recesses, can be M · * *. · In outer circumference wider than the diameter of the screening wire. In this case, the grooves will hold the screen wire • · ·:: only when the rings 22 'and 24' are set in the locked position.

7a and 7b show another embodiment of the invention. Until now, all ···· embodiments have been referred to as a circumferentially movable locking ring. However, the locking can also be implemented in another way. Figures 7a and 7b illustrate • ·. suggest that the locking ring 22 'could be movable at least partially in the radial direction. Here- • · . in this case, only the first ring 24 'has recesses / grooves into which • · · 35 screening yarns 10 are to be mounted. The tightening of the 22 ”ring is accomplished as shown in Figure 7a. One possibility is to arrange radially extending parts 9 near the ends of the ring 22 through which the ends of the ring can be pulled together and tightened, for example, by a bolt or screw. Another way of securing the ring 22 "in place is by welding, i.e. after the ring 22" has been tightened, and thus also the screening threads are locked, the ring 22 "is welded to another ring 24". Of course, other fastening methods such as gluing, soldering, riveting or securing with bolts and screws are also possible.

Figure 7b shows another way of securing the ring 22 "in place after the ring 22" has been used to lock the screen yarns 10. On the outside of both the 24 "ring and the locking ring 10 22" is provided a rim 25 which holds the rings tightly together and a 22 "ring against the screening yarns 10. One preferred way of mounting the rim 25 is to warm the rim 25 and install it and allow the rim 25 to shrink and secure the rings 22 "and 24" as described above.

Another alternative way of attaching one of the support rings to the recesses or grooves is to arrange a second ring in connection with said first ring. The first ring has wedge-shaped or tail-shaped recesses or grooves where the threads are to be placed. The second ring, which may not have grooves, but may still be present, is first heated and expanded, after which the second ring is placed over the first ring so that while the second ring gradually cools, it slides along the first ring and locks the screening yarns. in the recesses / grooves of the first ring. Of course, there may be more than one such pair of rings that lock the screening threads in place in each of the screen cylinders.

··························••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Contrary to the above embodiments, in which ·· ·: locking of the screen yarns is accomplished by two opposing support elements · · · which are attached to each other in almost all embodiments, this embodiment presents spacer rings spaced 30 apart. However, the basic starting point remains the same. The screening yarns 110 are inserted into the grooves of the support rings 122, 124. But the axial distance between the present adjacent support rings • · · 122, 124 is 20-200mm. When all the screening yarns are inserted into the recesses / grooves 124, at least one of the support rings, preferably closer to the mount ring 124, is ring 122 (at least one of the support rings * · .. * 35 is attached to the holder, move circumferentially) or rotate circumferentially about the axis of the rings at a predetermined distance relative to the preceding ring such that the threads 110 are positioned at a slight angle of inclination. At the same time, any gaps between the walls of the support ring gaps and the wires are closed. A preferred embodiment of the screen cylinder assembly is that when all the pivotable rings which are pivotable are pivoted a certain amount, 5 additional pivot rings are placed radially outside the existing pivot rings (e.g. with shrink connection), and finally the top and bottom , with bolts extending through the holes passing through the additional / auxiliary support ring.

In addition to providing easy and reliable attachment of the screening yarns to the support rings, this embodiment offers other advantages. For example, by appropriately arranging the spacing between the screen yarns and the recess / groove walls of the support rings, it is possible to determine the width of the screen slot. But by using support rings with different recesses / grooves, the width of the screen gaps 15 of a single screen cylinder can be varied. For example, by providing slightly larger recesses / grooves in the upper rings of the screen cylinder and slightly smaller recesses / grooves in the lower rings of the screen cylinder, the inclination angle of the screening yarns at the top of the cylinder Similarly, the screening cylinder 20 may include a plurality of axial portions wherein the inclination of the yarns and the width of the screening slot are individual.

·· ·: .1 It should also be understood that at least some of the support rings may be made of ·· ·: 1. · all or at least part of the material other than steel. For example, either ·· ·: V 25, movable or fixed, shown in Figures 4a and 4b, or in any of the figures, with the support element *: **! ring, could be made to some extent from a flexible material that would conform to the wire width tolerances of the · · · · 1. • bomb. Such flexible materials include rubber, hard rubber and plastics to name but a few.

··· 30 Note that several inexpensive and exemplary implementations are listed above

«• M

: ***: Shapes for locking screening yarns into recesses / grooves in support elements • · · | . by adjacent support rings relative to each other. In addition, some pre- • •. A notable embodiment for attaching two adjacent rings to each other. to provide a support element for locking the screen yarns. Thus, it is clear that all locking methods can be used with all fastening methods and vice versa. Thus, it is possible to use the method shown in Figures 5a and 5b to engage the rings 11 and lock the screen yarns simultaneously with the embodiment shown in Figures 7a and 7b with the sole exception that the holes in both rings are not deflected circumferentially (as shown in Figure 5). It follows that it will be understood that the foregoing detailed examples are in no way limiting the present invention, the scope and scope of which are defined by the appended claims.

· · » ··· ♦ ·•••••••••••••••••

Claims (22)

A method of manufacturing a screen cylinder, said screen cylinder comprising a plurality of screen trees (10, 110) and supporting members (20, 122, 124) consisting of substantially circular rings (22, 24; 22 ', 24'; 22 ''). 24 "; 122, 124), the method comprising the steps of: a) providing at least a portion of said rings (22, 24; 22 ', 24'; 24"; 122, 124) with transverse locking / recesses (30); B) said rings (22, 24; 22 ', 24'; 24 "; 122, 124) are arranged side by side; (c) the screen trees are displaced longitudinally through the rings (22, 24; 22 ', 24'; 24" ; 122, 124) transverse recesses / rafters (30), d) at least one of said rings (24; 24 '; 24 "; 124) is moved relative to another ring (22; 22'; 22"; 122) for the purpose of reading the screen trees in said transverse recesses / recesses (30), and e) the rings (22, 24; 22 ', 24'; 22 ", 24"; 122, 124) are attached at their locations to read the screen trees. . Method according to claim 1, characterized in that at least one of said rings (24; 24 '; 124) is moved circumferentially relative to another ring (22; 22'; 122) in step d) to read the screen trees (10, 110) in said transverse barriers / recesses (30). Method according to claim 1, characterized in that at least one of said rings (24 ") is moved substantially in the radial direction relative to another ring (22") in step d) to read the screen trees (10, 110). in said transverse locking grooves (30) at least in said second ring (22 "). Method according to claim 3, characterized in that the ring is first heated in step d) and then the ring is cooled and shrunk in order to read the screen trees in said transverse recesses / rafters. Method according to claim 1 or 3, characterized in that the rings (22, 24; 22 ', 24'; 22 ", 24") are attached to each other in step e) by welding, soldering, riveting or gluing. Method according to claim 1 or 3, characterized in that the rings (22, 24; 22 ', 24'; 22 ", 24") are fastened to each other in step e) by means of a bolt or screw. Method according to claim 1, characterized in that the rings (22, 24; 22 ', 24') are moved circumferentially in step e) by means of a wedge and the rings (22, 24; 22 ', 24') are attached to each other by welding, soldering, riveting or gluing, or by means of screws or bolts. Method according to any one of claims 1-3, characterized in that pairs of sharks 10 (32, 34) spaced apart are used for moving the rings (22, 24; 22 ', 24'; 22 "). 24 ") and for attaching the rings (22, 24; 22 ', 24'; 22", 24 ") to each other. Θ. Method according to claim 1, characterized in that an axial spacing is left between said support rings (122, 124), the movement of a support ring (122) circumferentially relative to another support ring (124) causing the screen trees (110) to adjust in position. an angle of inclination relative to the axis of the screen cylinder. Method according to claim 9, characterized in that the screening slot is adjustable by moving a support ring (122) circumferentially relative to another support ring (124). Method according to Claim 9, characterized in that said screen trees (110) are arranged at different angles of inclination in the different parts of the screen cylinder in the axial direction. 25 12. Cylinder cylinder consisting of substantially circular support members (20), which support members are in the form of support rings (22, 24; 22 ', 24'; 22 ", 24"; 122, 124), of which support rings (22, 24; 22 ', 24'; 24 "; 122, 124) at least a portion is provided with transverse locking grooves (30) and at least a plurality of screen trees (10, 110), between which a slot is formed when said screen trees (10, 110) is displaced longitudinally through said transverse barriers / depressions (30), characterized in that at least one of said support rings (22, 24; 22 ', 24'; 22 ", 24"; 122, 124) is movable in the step of assembling the screen cylinder for the purpose of reading the screen threads (10, 110) in the transverse recesses / grooves (30 "of the rings (22, 24; 22 ', 24'; 22", 24 "; 122, 124) ). Screen cylinder according to claim 12, characterized in that both support rings (22, 24; 22 ', 24'; 122, 124) have transverse recesses / latches (30) for the screen trees. Screen cylinder according to claim 12, characterized in that only one support ring (22 ") 5 has transverse recesses / latches (30) for the screen trees (10). Screen cylinder according to any one of the preceding claims, characterized in that the cutter tone tone of one of said support rings (22, 24; 22 ', 24'; 24 "; 122, 124) is provided with transverse recesses / latches (30) for screen trees. said transverse recesses / rafters (30) have a wall (26) and said wall (26) has a ground edge (28) which penetrates the screen trees (10, 110). Screen cylinder according to any one of the preceding claims, characterized in that said support rings (22, 24; 22 ', 24'; 24 "; 122, 124) have heels (32, 34) for attaching the rings to each other. Screen cylinder according to claim 16, characterized in that said heel (32, 34) is circumferentially displaced from each other in the step of assembling the screen cylinder. Screen cylinder according to Claim 16, characterized in that said heel (32, 34) is displaced in the radial direction from each other in the step of assembling the screen cylinder. Screen cylinder according to claim 12, characterized in that said support rings (122, 124) are arranged at an axial distance from one another. Screen cylinder according to claim 19, characterized in that said styling trees (110) are inclined in relation to the axis of the screen cylinder. Screen cylinder according to claim 20, characterized in that said screen trees (110) are at different inclination angles in the different parts of the screen cylinder. Screen cylinder according to claim 12, characterized in that at least one of said supporting members consists of two support rings, the first ring having transverse recesses or rafters for screen trees, and the second ring being mounted on the first ring and shrink-fit outer screen rows. for loading the strainers in said transverse recesses / spars.