Patent Application
20130233790
1. Technical Field
The present invention relates to a method of manufacturing a screen cylinder, and a screen cylinder that is particularly suitable for screening, filtering, fractionating, or sorting cellulose pulp suspensions of the pulp and paper making industry, or other similar suspensions. The present invention is especially suitable for the manufacture of screening devices of the type comprising either a plurality of screen wires positioned at a small spacing parallel to each other, the plurality of screen wires forming a screening surface facing the pulp suspension to be screened and adjacent wires forming screening openings in the form of slots therebetween allowing an accept portion of the pulp suspension to flow therethrough or a perforated sheet metal mat. The present invention discusses in particular the manufacture of the connecting rings of a screen cylinder, i.e. rings via which the screen cylinder is connected to the surrounding screen cylinder housing or to another screen cylinder in cases where the screening member is formed of more than one screen cylinder section positioned axially one on top of the other. Thus a connecting ring may be positioned at either the mid portion of a screening member or at an end thereof. In the latter case the connecting ring is most often called an end ring.
2. Description of Related Art
The pressure screens (as a general example, U.S. Pat. No. 4,634,521) used in the wood processing industry most often comprise a stationary screen basket or screen cage or screen cylinder within a substantially cylindrical outer housing having an inlet for suspension to be screened, and outlets for both the accept and the reject. Other inlets and outlets may also be provided, for example, inlet/s for dilution liquid to be fed into the apparatus, and an outlet for light rejects, most often plastics separated in the apparatus. In most cases, the apparatus is installed in an upright position so that the axis of the substantially cylindrical housing is substantially vertical. Then the top cover of the apparatus is easy to open or detach from the essentially cylindrical housing whereby the maintenance of the apparatus is easy. However, screening apparatus having an inclined or horizontally positioned axis or shaft exist, too. The shaft of the apparatus passes through the end of the apparatus opposite to the top cover, i.e. normally through the lower end, and usually rotates a 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, although a few cone-shaped screen baskets have also been proposed. Usually the screen cylinder is located in the radial direction outside the rotor, though also other constructions exist. The screen cylinder is supported to the housing at both of its ends, and sometimes also at its mid-portion, via support flanges. The support flanges are, when it is a question of an outflow screen i.e. the accept passing the screen cylinder outwardly, most often integral parts of the housing. The screen housing is most often substantially cylindrical, is often made by casting and has at least two support flanges extending radially inwardly from its inner surface. The screen housing may be made of rolled steel plate, too, and in that case the support flanges are welded on the inside of the housing. The screen cylinder may also be built of at least two parts or sections, which are connected to each other at their ends by means of said connecting rings. The screen cylinder is connected to the support flanges at least at both of its ends via so-called end rings, or connecting rings.
The connecting rings have been, naturally, used in cases of both milled or drilled screen cylinders and so-called wedge wire screen cylinders. In milled screen cylinders the screen surface is made of sheet metal in which sorting slots have been machined. In drilled screen cylinders the screening openings are round holes. The surface facing the fiber suspension to be treated i.e. the so called screening surface has in many cases been provided with grooves or ribs for creating turbulence in front of the screening openings to enhance the screening. The so-called wedge wire screen cylinders are such that the sheet metal screen of the milled or drilled screen cylinders is replaced with a screen made of profiled wires arranged side by side leaving a gap, so called screening slot therebetween. The profiled wires, which are often called wedge wires, are most often manufactured of steel by rolling and/or drawing. The cross section of the wires is such that the final screening surface resembles the milled cylinder. In other words, the wires often have on their screening surface, i.e. on the side facing the fiber suspension to be screened, also known as the “feed side”, a longitudinal ridge so that the screening slot left between two adjacent wires is positioned substantially at the bottom of the groove left between the ridges of two adjacent wires. The profiled wires are normally arranged in the axial direction of the screen cylinder, though also spirally or circumferentially arranged wires have been suggested.
The wires of a wedge wire screen cylinders have been attached to support elements that are arranged on the faces of the wires opposite to the screening surface such that a cylindrical screening element is formed. The milled or drilled sheet metal screening elements are structurally so weak that they need additional support elements to maintain their round shape and mechanical integrity. Thus in both screen types the support elements are most often circular rings, which are attached in radial planes at regular intervals on the screen element or screen cylinder surface at the accepts side of the cylinder (the side opposite the aforementioned “feed side”) along the entire length of the cylinder. The cross section of a support element is normally rectangular, though also other shapes have been suggested, including U-shape, round and triangular shapes.
As to the support elements there is a couple of ways to manufacture such. One way is to manufacture such from a metal bar, which is dimensioned in an appropriate manner, by bending the bar to circular form, and welding the ends together. The bending and welding may be done either before or after fastening the screen wires to the support elements when it is a question of the manufacture of a wedge wire screen element or cylinder. Another way is to prefabricate the support elements in the form of circular rings, for instance by cutting the rings from suitable metal plate. When manufacturing a milled screen element, the support rings are normally fastened to the screen element after the element has been rolled into cylindrical shape.
Coming back to the connecting rings, or in a more limited sense end rings, called also as top and bottom rings, they are normally of somewhat sturdier structure than the support elements and support rings, as they need to transfer the various loads subjected to the screen cylinder during screening to the screen housing.
There are at least four different ways used for fastening or arranging the screen cylinder in the screen housing, two different ways for the top end and two for the bottom end. In the following, only top and bottom rings are discussed, though the discussion applies also to the connecting rings arranged on mid-portion of the screen cylinder. The connecting rings are treated most often in a similar manner as the bottom rings.
The first way is to arrange the screen cylinder, in a way, floating at its bottom end. In other words, both the bottom end ring of the screen cylinder and the cooperating support flange have axially extending cylindrical surfaces facing each other. Since there has to be a gap between the surfaces for the installation of the screen cylinder within the support flange, usually the end ring of the screen cylinder is provided with a circumferential groove and an O-ring placed in the groove for sealing the gap. The rotation of the cylinder along with the rotor is normally prevented by bolting the cylinder to the support flange at its opposite end. Sometimes specific pins have been arranged to run from the screen cylinder end ring/rings to the support flange to prevent the rotation.
A second way of fastening or supporting the bottom end of a screen cylinder to the bottom support flange is to arrange a conical seating surface at the inner rim of the lower support flange. The conical seating surface opens upwardly, and is coaxial with the axis of the rotor. The lower end ring or the bottom ring of the screen cylinder is provided with a corresponding conical seating surface so that the screen cylinder is positioned centrally in relation to the rotor of the screening apparatus. The mid-part of the screen cylinder may be supported by means of a similar connecting ring/support flange combination to the screen housing.
A first way to attach the upper end of the screen cylinder to the support flange is to bolt the upper flange-like connecting ring onto the support flange.
A second way of fastening the upper end of the screen cylinder to the support flange is to use a separate clamping or centering ring at the top end of the screen cylinder. In other words, the actual end ring at the upper end of the screen cylinder is an upwardly tapering ring whereby, in an ideal situation, the ring is not only similar to, but identical with the bottom ring. The actual fastening of the screen cylinder to the housing takes place by using a centering ring that has a corresponding tapering surface and another surface acting in cooperation with a counter surface in the support flange. Thus by bolting the centering ring on the support flange between the support flange and the end ring centers the screen cylinder in place. As an example of such fastening U.S. Pat. No. 4,634,521 may be mentioned.
The connecting rings have been manufactured thus far from steel, as steel has been the easiest material to machine and to join the various parts together. This has been especially true when the screen cylinders were made of sheet metal as welding the end rings on a steel cylinder offered an easy, cheap and reliable way of fastening the two parts together. Now that the wedge wire screen cylinders have become predominant in the market, the fastening of an end ring or a connecting ring to the wedge wire screen cylinder has proven to be problematic. Welding that has been used in fastening the end rings to machined sheet metal cylinders is not the best possible way of fastening anything to wedge wire screen surfaces. The reason is that, in addition to changing the properties of the metal material of the screen wires due to high temperature, the welding of the wires to the end ring results in a relatively stiff fastening, which greatly limits the flexibility of a screen cylinder resulting from either flexing due to the forces applied in screening or because of small differences in the fit between the connecting rings and the intermediate rings. Wedge wire screen cylinders are to some extent flexible and the wires have a slight tendency to move either alone or in groups especially when the screen cylinder is attached to the support flanges. Thus the interface between the welded connecting ring and wedge wire mat is a potential location for a screen failure. The minor movements in the screen cylinder and/or stress caused by the installation of the screen cylinder may break the stiff fastening or the screen wires close to it, especially as the metal material of the screen wires has been hardened during welding.
Yet another problem relating to welding the connecting rings onto the wedge wires is the decrease in the diameter of the connecting ring after the welding. It is a characteristic feature of welding that the diameter of a welded circular ring is smaller after welding than before. This results in that the screen cylinders will be somewhat barrel-shaped after the connecting or end rings have been welded onto the ends of wedge wires unless the shrinkage of the connecting ring after the welding has been taken into account when originally manufacturing the connecting or end rings. Thus the manufacture of a connecting or end ring is a somewhat cumbersome and challenging task.
However, prior art knows screening or sorting baskets or sieves having end or connecting rings that are at least partially made of non-weldable material, whereby the welding of the rings to the ends of the screen wires do not pose a problem. Such end or connecting rings are made at least partially of thermoset plastic or composite materials. For instance EP-A1-0512873 or U.S. Pat. No. 5,255,791 discusses a sieve made by winding U-shaped sheet metal screening element strips spirally, and welding the strips along their edges together to create a screen cylinder. The end ring, which must form a plane perpendicular to the axis of the cylinder or basket, is made in a mold into which the end of the cylinder is inserted. The mold is a hollow substantially U-shaped circular sheet metal channel, whose cross-section corresponds to that of the end ring of a screen cylinder. In other words, the mold gives the connecting or end ring its final shape and size. Furthermore, the mold has at the bottom thereof upwardly extending cores for arranging openings through the end ring. The openings are utilized in the installation of the screen cylinder in the sorter i.e. for fastening the screen cylinder to the support flange by means of bolts. The screen cylinder-end ring combination is manufactured such that the U-shaped mold is installed on a horizontal table; the sieve cylinder is introduced therein vertically and maintained suspended in this position at a predetermined level in the mold. Then the mold is filled with a hardenable plastic or thermoset plastic or composite material. After a certain setting time the screen cylinder equipped with an end or connecting ring is removed from the mold, and the operation may be started over again for the opposite end of the cylinder. Naturally, if the end rings are not similar at both ends of the screen cylinder, the same mold cannot be used.
EP-A1-1161308 discusses a screen cylinder basically similar to the EP-A1-0512873 except that the screen cylinder has been provided with special locking anchors for increasing the contact surface between the screen cylinder and the moulded end ring material. The end rings are made of polymer, fiberglass or composite material.
In other words, the prior art documents referred above discuss the manufacture of a screen cylinder having an end ring being made of plastic or composite materials only. However, in spite of the fact that the idea of full plastic or composite end ring is attractive, it is, in practice, not applicable. Firstly, as soon as the end ring has to be used for bolting or otherwise fastening the screen cylinder to the support flange by using fasteners running in the end ring material and subjecting the end ring material to different mechanical loads, such an applicable material that is reliable enough is hard, if not impossible, to find. And secondly even if such a material is found, it may easily prove to be too expensive as the competing steel is in any case relatively cheap.
FI-A1-20041567 discusses a wedge wire screen cylinder having an end ring made of composite material/s and having preferably several different layers. A starting point of the invention discussed in the FI-document is that at least one support ring, i.e. the one closest to an axial end of a screen cylinder is arranged as a part of the end or connecting ring of the invention. Thus the end or connecting ring is formed by embedding the at least one support ring in the composite material. The FI-document also discusses an end ring embedding at least partially two support rings having reinforcing rings thereon such that the composite material is applied between the reinforcing rings forming the body part of the end ring. Thus the end ring of prior art may have its both radial “flange surfaces” covered with metal.
As already discussed above, a problem with the present wedge wire screen cylinders having end rings made of steel is the joining of a somewhat flexible wedge wire mat to a rigid end ring. Since the wedge wire mat is currently welded to the end rings, the heat of welding changes the steel properties whereby the part of the wedge wires subjected to the heaviest load is the weakest to withstand the applied stresses. The wedge wires made of steel, when subjected to dynamic loading, typically break down through crack growth. The internal stresses and the changes in the physical properties in the so-called heat-affecting zone (HAZ) caused by welding render the elements made of steel liable to crack growth.
DE-A1-10 2004 047949 discusses a fractionation or screening device with a fractionation or screening structure and a bearing or support for mounting the fractionation or screening structure on a rigid machine base. The bearing or support has greater compliance than the fractionation or screening structure itself.
The above problems, among others, have been attempted to be solved with the screen cylinder of FI-A1-20041567 discussing a preferably layered, composite end ring. However, the manufacture of such a layered end or connecting ring is very expensive and complicated, as it requires several different materials that have different requirements. Also the manufacture of the end ring is very time consuming and labour demanding as each layer has to be given some time to solidify prior to applying the next layer. However, the layered end ring, even if it may have both of its radially extending faces covered with metal, has the same problem as that made of a single composite. In other words, the solidified composite material cannot normally be subjected to such mechanical loads that are involved in fastening the screen cylinder by means of its end ring to the support flange on the screen apparatus. In other words, if the composite material is supposed to be provided with holes used for attaching the screen cylinder by means of bolts to a support flange, the loads subjected to the end ring via the fastening bolts easily start cracking the solidified composite material resulting in weakening of the attachment and the end ring structure and the creation of minor composite particulates that probably enter the pulp to be screened though there is a small possibility that the particulates may enter the accepts, too. And, the composite end ring proves to be even more problematic in case the end ring should be provided with threaded holes for eyebolts that are used in the installation and removal of the screen cylinder. The threaded holes probably are able to carry the load in the installation phase of the screen cylinder, but in the removal stage the screen cylinder may be for one or another reason stuck in its place whereby a substantial force is needed for its removal. Thereby there is a clear risk that the fastening of one or more of the eyebolts break, and the removal of the screen cylinder gets extremely complicated.
An object of the present invention is to offer a solution to at least some of the above-discussed problems. The solution is to use in the manufacture of at least one of the connecting rings or end rings a combination of a massive ring, preferably of metal, and a plastic material fastening the screen wires to the ring. This kind of a fastening is reliable and still allows some relative internal motion between the screen cylinder components whereby the loads subjected to the screen element may be easily controlled.
Another object of the present invention is to solve the problem concerning the attachment of the ends of the screen wires to the end ring in a reliable manner while still ensuring some flexibility.
A further object of the present invention is to ensure that the installation and removal of the screen cylinder from the screen housing may be performed without any unnecessary risks.
At least some of the objects of the present invention are met with the method of manufacturing a screen cylinder, said method comprising the steps of manufacturing a round screen element of one of screen wires fastened to support elements and a sheet metal member fastened to support elements; said screen element having a screening surface, an opposite surface including the support elements, and screening openings in the form of one of screening slots left between adjacent screen wires, and milled or drilled perforations in the sheet metal member; said round screen element having two axial ends, manufacturing a massive circular ring (for fastening to an end of said round screen element, said massive circular ring having a radial end surface, fastening said round screen element to said massive circular ring, said end surface of said massive circular ring forming at least partially an end surface of the screen cylinder, wherein said fastening is performed by placing said massive circular ring on a working surface such that said end surface of said massive circular ring lies against the working surface, positioning said round screen element coaxially with said massive circular ring above said working surface, placing a barrier against the screening surface such that the barrier extends from said working surface at least up to the level of the lowermost support element, applying non-composite polymeric material in a generally circular cavity formed by at least the barrier, and the massive circular ring such that at least the lowermost support element is arranged into contact with said non-composite polymeric material, and allowing the non-composite polymeric material to harden, whereby the massive circular ring is attached to the round screen element and to its lowermost support element by means of the hardened non-composite polymeric material.
In a similar manner at least some of the objects of the present invention are met with a screen cylinder comprising a round screen element and two end rings; the round screen element being formed at least one of a number of screen wires attached to support elements, and a sheet metal member attached to support elements; the round screen element having a screening surface, an opposite surface including the support elements, and screening openings extending through said screen element in the form of one of screening slots left between adjacent screen wires, and milled or drilled perforations in the sheet metal member; the end rings being fastened to said screen element at an axial end area thereof, wherein at least one of said end rings is formed by fastening a massive circular ring said end area of said screen element and to at least one support element by means of a non-composite polymeric material cast between the circular ring, the screen element and the at least one support element.
Other characteristic features of the method and the screen cylinder of the present invention will become apparent from the appended dependent claims.
The present invention, when solving at least some of the above-mentioned problems, also brings about a number of advantages, of which a few are listed as follows:
However, it should be understood that the listed advantages are only optional, whereby one or more of the advantages may be obtained depending on the way the invention is put into practice.
In the following, the present invention is explained in more detail in reference to the accompanying drawings, of which
In
The prior art screen cylinder 10 of
The screen cylinder 10 further comprises end or connecting rings 16 and 18 at both axial ends of the cylinder 10. The screen cylinder 10 is aligned, fastened and supported by means of the end or connecting rings 16, 18 to support flanges attached to the housing of the screening apparatus. An option is to attach the upper end of a screen cylinder by means of its end ring 16 to an upper support flange by means of bolts, and support i.e. push the lower end ring of the lower end of a screen cylinder against a conical surface in the lower support flange. The conical surfaces of the lower support flange and the lower end ring cooperate for sealing the lower end of the screen cylinder i.e. for preventing the screening accepts from leaking to the rejects or vice versa and ensure that the screen cylinder is coaxial with the rotor axis.
There are several options for fastening the end rings to the wedge wires. Sometimes the end rings have been shrink-fitted on the support rings closest to the ends of the screen wires, and, optionally, thereafter welded to the support ring and/or the screen wires. Some prior art screen cylinders have also been provided with a circumferential groove into which the ends of the screen wires have been installed prior to welding. Sometimes the ends of the screen wires have just been welded on and against the inner surface of the end ring. As an alternative to the screen cylinder shown in
Thus the second boundary area (IIB) may be either outside or inside the perimeter formed by the screen element 30. The main task of the molded portion 282 is to fasten the massive ring 284 to both the screen wires 12 and the support ring 14. Another task the molded portion 282 may have, depending on the material used, is to form a dampening layer between the screen wires 12 and the massive ring 284. The shape and size of the ring 284 is, on the one hand, such that it together with the screen wires 12 and the support ring 14 forms an applicable substantially annular cavity for the molded portion 282, and on the other hand, such that the screen cylinder 10 may be supported and/or fastened centrally within the screening apparatus. In other words, the dimensions of the second boundary area (IIB) of the massive circular ring 284 have to match the mating dimensions of the fastening, supporting and/or aligning elements of the screening apparatus. Also, the massive circular ring 284 may have openings or threaded holes as shown by reference numeral 34 for pins or bolts used for aligning and/or fastening the screen cylinder 10 to, for instance, the support flange supported by the housing of the screening apparatus. The circular ring 284 may also be provided with threaded holes, which are in use only when installing or removing the screen cylinder from the screen housing. In other words, eye bolts or other lifting means are fastened to said threaded holes.
At this stage it has to be understood that normally, but not necessarily always, the end ring having openings for fastening bolts is used at the end of the screen cylinder closer to the openable cover of the screening apparatus i.e. the upper end of a screen cylinder in a vertical screening apparatus. Thus, in a sense,
In accordance with the embodiment of
As an additional and also independently useable embodiment of the present invention the massive ring 484 has at its first boundary area (IB) another, axially outside the axial ends of the screen wires 10 extending flange portion 488. This is, thus, an optional structure to the one illustrated in
The dimensions or the surface/surfaces of the massive circular ring 484 facing the screen element 30 may be formed rather freely.
As to the manufacture of the end ring for the screen cylinder shown in
As shown in
In accordance with a further preferred embodiment of the present invention the massive ring is provided with anchor means, for instance additional sheet metal pieces or metal bars extending into the cavity between the circular ring and the screen element to increase the surface area for the non-composite polymeric material to adhere. Further, as already discussed earlier the circular ring may be provided with holes for bolts used for fastening the screen cylinder to the fastening means, like for instance to the support flange. However, if the holes of the circular ring open into a cavity that is supposed to be molded with the non-composite polymeric material, the holes should be provided, before molding, with cores extending into and through the cavity the non-composite polymeric material is poured in. After the non-composite polymeric material has hardened the cores are removed whereby the end or connecting ring has holes/openings for fastening bolts. Naturally another option is to drill corresponding holes through both the massive circular ring and the non-composite polymeric material after the non-composite polymeric material has solidified. However, when arranging the holes to extend in the non-composite material it is important to ensure that the thickness of the massive circular ring, preferably of steel, at the hole is sufficient for carrying all the loads subjected to the attachment of the screen cylinder i.e. so that substantially no load is subjected to the non-composite material. A possible way of protecting the non-composite material from even compressive loads is to arrange a sleeve-like metal insert to extend through the non-composite material as an extension of the hole in the massive ring so that when the hole is used for fastening the screen cylinder to the support flange the compressive forces created by the fastening bolts is carried by the insert and the massive ring together.
It has to be noted that, in the above specification, it has been taken for granted that the screen wires extend either all the way from the end surface of the screen cylinders up to the opposite end surface thereof or, in accordance with
This far the above specification has only discussed the end rings combined with mere support elements 14, 14′, 44 and 54. However, already when discussing prior art screen cylinders in connection with
The presence of a reinforcing ring on the support element closest to an end of a screen cylinder does not change the construction of the end ring of the present invention. In other words, both the support element and the reinforcing ring thereon may be at least partially embedded in the end ring, like shown with mere support elements in
Firstly, it is possible that the reinforcing shell does not extend to the entire length of the screen cylinder, not at least at the end of the cylinder applying the present invention. It is, for instance, possible that the reinforcing shell is arranged to support the second support element counted from the end of the screen cylinder whereby the first support element is embedded in the end ring and receives the required additional reinforcement from the end ring. Naturally if both the first and second support elements are part of the end ring, the reinforcing shell is arranged to reinforce the support elements from the third support element onwards.
Secondly, it is also possible that the reinforcing shell is arranged to extend closer to the end of the screen cylinder whereby the end of the reinforcing shell is either embedded in the end ring material or at least has a common interface therewith. In such a case the plastic end ring material is used to fasten the reinforcing shell to the support element or ring it, at least partially, embraces. In fact, as shown in
Yet another option is to extend the reinforcing shell even closer to the axial end of the screen cylinder. In such a case the end part of the shell is provided with openings, either similar to the rest of the shell or specifically designed ones, via which the outside of the reinforcing shell is in flow communication with the cavity between the shell and the screen wires. Now when manufacturing the end ring the massive circular ring is arranged on a working surface, the screen mat together with reinforcing shell is positioned centrally within the circular ring, the barrier is placed against the screening surface and the non-composite polymeric end ring material is applied, preferably poured, in the cavity within the circular ring. The openings in the reinforcing shell allow the non-composite polymeric material to fill the space between the barrier and the massive circular rings, just like in any other embodiment discussed earlier in the specification. Also, it should be understood that all the positioning options discussed in connection with the end of the screen cylinder or wedge wires apply to the positioning of the end rim of the reinforcing shell. In other words, the rim may extend to the axial end surface of the end ring of the screen cylinder or be positioned at a distance thereof.
As to the material aspects of the invention the screen wires, its support rings as well as the milled screen cylinder are made of steel. However, the rest of the components of the screen cylinder of the present invention i.e. the massive circular ring and the non-composite polymeric material have several options. Nowadays, the circular ring is most often made of metal, preferably steel, but it is also a worthwhile option that may get more and more popular in the future to manufacture the circular rings of plastics and/or composite materials. It is, for instance, possible to cut the circular ring from a plastic sheet having a suitable thickness, or to mold such a ring separately.
The operating environment of a screen cylinder at a pulp or paper mill sets a number of requirements for the end ring material. The loads may be divided in three categories. Chemical loading, thermal loading and mechanical loading.
The chemical loading is based on the use of very different i.e. both alkaline and acidic chemicals in the pulp and paper production. Most typical chemicals are hydrogen peroxide, sodium hydroxide, sulphuric acid, sodium dithionite, sodium sulfide, sodium borohydride, lignin residues, hemicelluloses and inorganic chemicals.
Thermal loading is based, on the one hand, on the storage or transportation phase of a screen cylinder when the temperature may be well below 0 degrees centigrade, sometimes easily down to −35 degrees. On the other hand, the temperature in the operating conditions at a pulp mill range normally between +50 and +80 degrees, but sometimes may rise up to about +125 degrees centigrade. Though such temperatures do not form any challenge to metal materials, they set quite hard prerequisites for non-metallic materials.
Mechanical loading is, in a way, the easiest to handle, as the load levels do not grow very high. There are tensile stresses, sometimes compressive stresses, as well as torsion, and vibration, but the stress levels in each case are rather easily manageable by non-metallic materials, too.
Thus, in brief, the non-metallic i.e. non-composite polymeric material used for fastening the massive annular ring to the screen element must
Some thermoset plastics and castable polyurethanes are materials that are preferably applied for the above described purpose, and have the required properties. Thermosetting polymers are materials that are usually liquid prior to curing and designed to be molded into their final form. Curing process transforms the resin into a plastic by a cross-linking process. The thermoset material cannot be melted or re-shaped after it is cured.
Thermoset plastics, such as polyesters, vinyl esters, and epoxies, are typically used as matrix materials of reinforced plastics and as unreinforced they are rather hard and brittle. Vinyl esters have the best chemical resistance, but their mechanical properties are not as good as those of epoxies. Chemical resistance of epoxies is good and their mechanical properties are excellent. Epoxies are also available as toughened grades. The performed tests have shown that by using appropriate additives (including both liquid and particulate components) the material properties of polyesters, vinyl esters and epoxies may be improved to match those required by the present invention.
However, certain polyurethanes that are also based on thermoset materials have proven to be very attractive alternatives for the above discussed thermoset plastics. For instance polyurethane cast elastomers are suitable materials for the end ring application of the present invention. They have good wear resistance and are widely used in various wear-related applications. Good adhesion to metals can be achieved, and their chemical resistance is good. Just like with thermoset plastics, also the properties of polyurethane cast elastomers may be improved by using appropriate additives (including both liquid and particulate components).
In addition to thermoset materials, also thermoplastic materials, like for instance PTFE, have been considered for the above described purpose. In view of the mere material requirements there are a number of thermoplastics that meet the demands of the operational environment. However, there are practical factors that complicate the use of thermoplastics. For instance, the thermoplastics are not normally castable by low pressure process but are applied by for instance extrusion, which requires special tooling and therefore substantial investments. Additionally, since the price of thermoplastics is comparable to that of thermoset materials, no significant savings (countering the additional investments) can be gained by the use of thermoplastics.
It should be understood that the above is only an exemplary description of a novel and inventive method of manufacturing a screen cylinder and a screen cylinder. The above should not be understood as limiting the invention by any means but the entire scope of the invention is defined by the appended claims only. From the above description it should be understood that separate features of the inventive screen cylinder or end or connecting ring may be used in connection with other separate features even if such a combination has not been specifically shown in the description or in the drawings. In other words, for instance various features discussed in connection with wedge wire screen elements or screen cylinders may be applied in connection with milled or drilled sheet metal members or screen cylinders, too. It should also be understood that the above specification when talking about a screen cylinder means a unit comprising the screening element having the screen wires or a perforated (provided with milled holes or slots) sheet metal member or cylinder attached to support rings and end rings at both axial ends thereof. Thus, the above definition covers also cylindrical screen sections of such screen baskets that are made of several cylindrical sections attached one on top of the other. Thus the end rings of screen cylinders may be used as the intermediate connecting rings of such a multi-section screen basket.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20106231 | Nov 2010 | FI | national |
This application is a U.S. national phase application of International Application No. PCT/FI2011/051025 filed on 22 Nov. 2011, which designated the U.S., and which claims priority to Finnish Patent Application No. 20106231 filed on 23 Nov. 2010. The contents of each of these applications are hereby incorporated by reference in their entirety in this application.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2011/051025 | 11/22/2011 | WO | 00 | 5/23/2013 |
