WO2009112071A1

WO2009112071A1 - Metal belt with pre-stressed coating

Info

Publication number: WO2009112071A1
Application number: PCT/EP2008/052927
Authority: WO
Inventors: Riku Pihko; Voli-Matti Vainio; Erkki Kiiski
Original assignee: Metso Paper, Inc.
Priority date: 2008-03-12
Filing date: 2008-03-12
Publication date: 2009-09-17
Also published as: AT507957A3; AT507957A2; DE112008003802T5; AT507957B1

Abstract

The invention relates to a belt 100 made from metal for use in a paper- or board making machine and a method for manufacturing such a belt. The belt has a plate shaped belt body 10 adapted to be formed into an endless loop, and has a coating 101, 102 formed on at least one surface of the belt, wherein the coating is formed from a different material than the belt body 10. The belt is characterized in that stress in the coating 101, 102 is different from stress in the coated belt body 10, for example, the stress in the coating 101 is compressive stress, while the stress in the belt body 10 is tensile stress, when the coated side of the belt is the outside surface side of a belt loop formed form this belt. Alternatively or additionally, the stress in the coating 102 is tensile stress and the stress in the belt body 10 is compressive stress, when the coated side of the belt body is the inside surface side of a belt loop formed form this belt. The invention further provides a method for manufacturing such a belt in which method the belt body 10 is bent with a radius and/or in a direction different from the bending of the belt 100 when the belt is in use in form of said loop in the paper- or board making machine, and the coating 101, 102 is formed on at least one surface of the belt body while the belt body 10 is in the bent state.

Description

Metal belt with pre-stressed coating

The invention relates to belts made from metal and used in fiber web machines including paper- or board making machines and finishing machines, as well as to a method for making such belts. Metal belts exhibit properties such as smoothness, stiffness, heat conduction and the like making the use of these belts in paper making or board making particularly advantageous.

Metal belts are often used in cooperation with doctor blades which are in sliding contact with the belt causing wear of the belt. It is therefore required to improve wear characteristics and wear resistance of the respective belts. It is known from the prior art to provide hard coatings, especially hard chromium plating, on the belt to provide a hard and comparatively wear resistant surface.

However, it was found that there are several other difficulties involved in the improvement of the metal belt. On the one hand, the hard chromium plating as one typical example for a hard coating, the adhesion properties between the so coated metal belt and the web being conveyed on this belt are unsatisfying. On the other hand, the service life or durability of the hard coated metal belts were found to be short.

Studies have shown that the hard chromium plated belt exhibits a lot of initial cracks in the metal belt surface and/or the coating. When the belt is in operation, the belt is bent many million times, when the running direction of the belt changes. It may well be assumed that repeatedly bending the belt promotes the progress of the small initial cracks into the metal belt which finally causes the belt to break.

Another problem with the prior art coatings is seen in that delamination or local peeling off of coatings is observed, especially when the metal belt undergoes significant temperature changes during its run, so that thermal expansion phenomena - e.g. of a quickly shrinking metal belt - may cause such coming off of the coating from the belt body.

In view of the above, it is the object of the invention to suggest a wear resistant metal belt which has a high durability, wears evenly and shows a low adhesion to a web, and a method for making such a belt.

With regard to the belt, the object is solved with a belt according to claim 1, and with regard to the method, the object is solved with a method according to claim 15.

It was found that a belt with a particular stress distribution in the metal belt and its hard coating is resistant against the progress of initial cracks, so that the service life of the belt is significantly increased. On the other hand, the advantageous properties of known hard coatings with respect to wear resistance can be maintained.

It was found that the pre-stressed coating can be processed further by e.g. opening initial cracks in the coating with a corroding agent and subsequently filling the so obtained cavities in the coating with a low adhesion material, thereby providing a metal belt having a wear resistant and low adhesion surface. It was further found that this hard coating even improves the fatigue resistance of the metal belt as compared to an uncoated metal belt. Further improvements have shown advantages when the inside surface of a belt loop is covered with a suitable coating too, which coating also fulfils a predetermined stress distribution in the coating and the belt body. For example elastic (and/or soft) coatings are particularly suitable for being provided at the inside of the belt, and such coatings are considered useful to encapsulate the metal belt body to improve e.g. corrosion resistance of the metal belt.

Summarily, the invention provides a belt made from metal for use in a fiber web machine including paper- or board making machine and finishing machine, wherein the belt has a plate shaped belt body adapted to be formed into an endless loop and a coating formed on at least one surface of the belt. The coating is formed from a different material than the belt body, and stress in the coating is different from stress in the coated belt body.

Advantageously, the stress in the coating is compressive stress and/or the stress in the belt body is tensile stress. This distribution of stress in the metal belt can be selected when the coating is provided on the outside surface of a loop formed by the belt. In this case there is a further advantage, when the coating is a wear resistant coating, in particular, the coating may be a hard chromium plating.

In an advantageous modification, the coating may carry a low adhesion material having low adhesion to a paper or board web, which material may comprise a fluoroplastic material, which is filled into cavities in the coating. In case the inner loop surface of the belt loop is to be covered with a coating, it is advantageous when the stress in the coating is tensile stress and/or the stress in the belt body is compressive stress. Advantageously, the coating is a flexible coating or is a diamond or diamond-like coating.

The method according to the invention manufactures a belt made from metal for use in a fiber web machine including paper- or board making machine and finishing machine, wherein the method includes a step of providing a plate shaped belt body adapted to be formed into an endless loop. Then, the belt body is bent with a radius and/or in a direction different from the bending of the belt when the belt is in use in form of the aforementioned loop in the paper- or board making machine, and a coating on at least one surface of the belt body is formed from a different material than the belt body while the belt body is maintained in the bent state, so that a coating is formed in which stress in the coating is different from stress in the coated belt body.

Depending on the desired stress distribution in the belt body and the coating, the belt body may be bent with a smaller radius of curvature than in operation in the machine, and the coating may then be applied to the convex surface of the bent belt body, which surface is an outside surface of the belt loop. Alternatively, when a different or opposite stress distribution is to be realized, the belt body is bent in opposite direction than in operation in the machine, and the coating is applied to the concave surface of the bent belt body, which surface is an inside surface of the belt loop. As a device for bending the belt body and applying a coating on the outside surface of the belt loop -which belt loop may be formed before or after coating- a roller may be used over which the belt body is bent, wherein the roller has the radius of curvature of the bending of the belt body as its radius. By this, compressive stress in the coating and/or tensile stress in the belt body can be obtained, when the belt is in operation.

Alternatively, a device may be used for putting the method into practice in which device the belt body is bent by applying vacuum on the side opposite the side to be coated forcing the belt body into a bent shape. That is, the inner or concave surface of the bent belt body can be coated using such a device, which is one solution to allow access to the inner surface of the curvature when the belt body is held in bent state. By this, tensile stress in the coating and/or compressive stress in the belt body can be obtained, when the belt is in operation.

It is noted that all coating methods may be used, in particular thermal spaying, HVOF, HVAF, electrochemical coating chrome coating, brush coating, chemical coating, nickel coating, immersion coating, deposition coating, welding coating, laser coating CVD, PVD, DLC, ALD, diamond and diamond-like coating and hybrid coating alone or in combination.

The method may further comprise steps of preparation of the metal belt surface (s) for the coating (s) to be applied, such steps include surface roughening, surface cleaning, surface disinfection, surface pickling, application of primers or the like. Also, the surface may be finished after coating by grinding, polishing etc. The invention will hereinafter be explained by way of practical embodiments shown in the drawings. It is clear that the practical embodiments are non-restrictive examples of putting the claimed invention into practice, so that modifications thereof may be made within the field of knowledge of those skilled in the art.

In the drawings: Fig. 1 is a schematic view of a metal belt coated on its both sides in operation on a roller of a paper making machine;

Fig. 2 are largely magnified schematic drawings of a small piece of a belt coated on one side for explaining principles of the invention;

Fig. 3 is schematic view of a device useable for manufacturing a coated belt having a coating at its inside surface with regard to a curvature of the belt in operation . Fig. 4 shows an exemplary arrangement for carrying out the invention; and

Fig. 5 shows a largely magnified portion of a belt.

In Fig. 1 there is shown a part of a belt 100 which has coating 101 and 102. Although not shown, it is understood that the belt 100 is formed as an endless loop which passes over at least two roller or other means in which the belt changes its running direction. In Fig. 1 there is shown the passage of the belt 100 over a roller 2 where the belt changes its running direction from upper right to lower left into horizontal left to right movement. Only for explanatory purpose, a piece of a web W is indicated which is to be carried on the belt 100. The belt 100 has a belt body 10 with two coatings, wherein the coating 101 covering the outside surface of the belt loop and getting in contact with the web W will hereinafter be named outside or outer coating 101, while the coating 102 on the opposite side surface, the inside surface of the belt loop will be entitled inside or inner coating 102.

The metal belt 100 of Fig. 1 is slung around the roller 2 to about 160° and is bent with a radius r2 when following the roller surface of the roller having a diameter of two times r2. It is noted that such a metal belt can be made of several materials according to particular requirements in the process. For the present explanation, of the invention a stainless steel metal sheet belt body having thickness of 0.6 to 1.2 mm is used which is formed and welded to form the endless loop.

The metal belt and the coatings as shown in Fig. 1 are drawn extremely thick for the sake of drawing clarity only, in reality the thickness of the coatings (typically 5 to 100 micrometers) would vanish compared to the diameter of roller 2. It is noted that r2 represents the minimum or smallest bending radius the belt 100 has to pass when operating in the machine.

In the left drawing A) of Fig. 2 there is shown a metal belt body 10 with a coating 101 forming an outer coating with respect to belt loop is formed. Due to the size of the drawings, belt body 10 appears to be flat but is bent with a curvature or bending radius of r being smaller than r2, i.e. smaller than the smallest bending radius the belt passes when operating in the machine. Due to the bending, one can roughly expect the profile P of stress to occur in the bent belt body 10. In that state, the coating 101 is applied to the belt body 10, i.e. there is the stress distribution profile P within the belt body whereas the coating 101 is free from stress.

In the right drawing B) of Fig. 2 there is shown the operating state of the belt, i.e. when it is bent with the bending radius r2 (for example when passing a roller having radius r2) . It is noted that for the reasons of dimensions of the elements, the belt body 10 is drawn flat in drawing B) although it is bent with the radius r2.

In the bent state of drawing B) , one can assume a stress profile P2 which is only schematically shown in Fig. 2. Further, assuming that the thickness tc of the coating 101 is much smaller than the thickness tb of the belt body 10, it is found that the stress profile P2 in the belt body is more flat and that there develops a compressive stress indicated as a single arrow F in the coating 101. Since radius r2 is the minimum bending radius in the run of the belt 100, there will always be a compressive stress within the coating, which compressive stress even increases where the belt runs actually flat.

With the permanently acting compressive stress in the coating 101, it was found that natural initial cracks (not shown) in the coating will not progress, since there is no considerable force acting in a direction to further open the cracks; by contrast, the cracks are rather pushed to close, thereby inhibiting crack progress into the metal belt material.

In addition, it was found that a hard coating arranged in the manner described above improves the fatigue strength of the metal belt as such. Accordingly, stronger bending is acceptable and smaller roller diameters can be provided in the paper machine, making the machine more compact and the roller (s) smaller and less expensive.

With regard to the formation of the coating 102 of Fig.l, i.e. the coating on the inside surface of the belt loop, an exemplary device in which such a coating can be formed to be under tensile stress, when the belt is in use, is shown in Fig. 3. A trough 3 is provided to have an open front with suitably formed edges over which the portion of the belt body 10 to be coated may slide (from right to left in Fig. 3) in at least substantially air tightly sealed manner. A vacuum pump 5 is provided which is used to remove air from the inside of the trough 3, so that a pressure difference between the two surfaces of the belt body forces the belt body 10 to bend and form a curvature. Preferably, the trough 3 and the pressures inside and outside the trough are designed and selected such that the radius of the curvature (the bending radius) is smaller than any bending radius the belt will be bent when in operation in the machine. Then, only a schematically shown coating equipment 4 applies the desired coating onto the belt body so as to form the inside surface coating 102 of the belt loop. Since the belt body 10 is bent more when the coating is applied than afterwards when the belt is in operation, it can be understood that there is tensile stress in the coating when the belt is flattened. For the sake of brevity, it is considered sufficient that the explanations of this effect given with reference to Fig. 2 - although the result is the opposite (tensile stress instead of compressive stress in the coating) - apply mutatis mutandis here. Fig. 4 shows a possible arrangement of several devices for forming the outside coating (coating 101 in Fig. 1) on a belt body 10. The belt body 10 passes over a roller having radius r which is smaller than any bending radius of the belt 100 when in operation in the paper machine. Three working stations are schematically shown in Fig. 4, wherein 501 is a surface disinfection and cleaning station; 502 is a coating station; and 503 is a surface finishing (grinding and/or polishing etc.) station. The belt body 10 is continuously passed through the arrangement of devices, and so the desired pre-stressed coating is formed. It is noted that, alternatively, the roller may also be a lower turning point of the belt, so that a mirrored arrangement of the devices as compared to Fig. 4 is obtained, depending on the selected coating method. Also, several rollers may be used over which the belt passes successively, at which rollers one or more several working stations for surface treatment of the belt are provided. Furthermore, a combined arrangement of one or more rollers and one or more troughs for coating the respective side of the belt may be provided in series for subsequent surface treatment processes.

As for the coatings on the inside or the outside surfaces of the loop, i.e. coating 101 and 102 in Fig. 1, can be made by the coating methods hot spraying HVOF, HVAF, electrochemical coating (e.g. hard chromium plating), chrome coating, brush coating, immersion coating, deposition coating, spread coating, chemical coating or plating, nickel plating, dip coating, weld coating, laser coating, coagulating bath coating, laser coating, physical gas phase coating. Also thin coating may be used like CVD, PVD, DLC, ALD, Diarc method, hybrid coatings. Of course, combinations of these methods may be used as well. Besides, surface roughening and/or the application of adhesion primer may be used to further improve the connection between the belt body and the coating.

Fig. 5 shows a magnified portion of a belt 100 having a coating 101. The coating 101 has initial cracks 111 which usually occur by themselves, when e.g. hard chromium plating and other hard coatings are applied to the metal belt body. In order to improve the adhesion properties of such belts to a web to be conveyed, treated and separated from such a belt, the initial cracks 111 may be filled with a low adhesion material, like fluoroplastic materials or the like. Here, due the pre-stressed coating, progression of the initial cracks is reduced or avoided. Therefore, widening the cracks e.g. by etching or other chemical treatment is possible so as to increase the share of low adhesion material in the coating without adversely affecting the durability of the belt.

An example explaining the belt material's behaviour in stress tests by way of testing samples shows the differences and effects in a belt material according to the invention (sample B) compared to a belt material to which the invention had not been applied (sample A) .

Two samples, size of 50 x 4 mm and thickness of 1,25 mm, were made of a thin piece of metal belt basic material which was of cold forged austenite stainless steel. Both samples were coated with Chrome metal by electrochemical plating. During coating, the basic material of the first sample A was laid horizontally without any bending whereas the basic material for the second sample B was laid curved so that the surface to be coated had a convex shape. After coating and when laid flat, the second sample thus had a residual compressive stress on its surface. The interfacial crack of the samples was studied by a bending fatigue test using a universal testing equipment MTS four point bend test device available from MTS Systems Corporation, MN, USA. Under a constant force corresponding a tensile stress of 770 MPa on the outer surface of the sample, bending cycles were performed for each sample. The selected stress level corresponds well the conditions under which the belt is exposed during operation in a fiber web machine. At suitable intervals, the sample was removed from the MTS device and its behaviour was studied by a microscope. The aim was to record the cycle number at which the very first initial stage cracks were born in the coating. When the crack was grown large enough to allow the sample to bend 1,3 mm, which was considered as a failure point, the test was finished and a duration number was recorded.

Sample A is an example of the prior art. No indication of cracks was seen in sample A after 10 000 cycles. After

40 000 cycles the first cracks were detected. At the end of the test, the cracks formed a grid-type pattern on the surface. Sample B is an example of the invention. No indication of cracks was seen after 40 000 cycles. It was only after 100 000 cycles when the first cracks were detected. At the end of the test, the cracks were travelling straightforward across the sample.

Test results for each sample are summarized in the below table.

Table 1 - Duration number in four point bending

As can be seen in the above table, the durability performance of the belt material sample B of the invention is superior over the prior art belt material sample A. Thus, it can be concluded that the belt of the invention stands more than 100 000 bending cycles to failure at the maximum stress level of 770 MPa, measured by MTS four point bending tester. When coated under tension, a residual compressive stress is generated to the belt surface and fatigue resistance of the basic material is improved more than with a similar coating, but not being bended. Therefore, the expected service life of the belt of the invention is significantly improved.

Claims

1. A belt (100) made from metal for use in a paper- or board making machine, said belt having a plate shaped belt body (10) adapted to be formed into an endless loop, a coating (101, 102) formed on at least one surface of the belt, said coating being formed from a different material than said belt body (10), characterized in that stress in the coating (101, 102) is different from stress in the coated belt body (10), so that the coated belt body withstands at least 80 000 cycles of a fatigue bending test generating a testing tensile stress of 700 to 800 MPa on at least one outer surface of the coated belt body before cracks occur in said coating.

2. A belt according to claim 1, wherein said belt is coated on one surface thereof and the stress in the coating (101) is compressive stress, wherein the testing tensile stress is generated on an uncoated surface of the belt .

3. A belt according to claim 2, wherein the stress in the belt body (10) is tensile stress.

4. A belt according to any one of claims 1 to 3, wherein the coating (101) is provided on the outside surface of a loop formed by said belt (100) .

5. A belt according to any one of claims 1 to 4, wherein the coating (101) is a wear resistant coating.

6. A belt according to any one of claims 1 to 5, wherein the coating (101) is a hard chromium plating.

7. A belt according to any one of claims 1 to 6, wherein the coating (101) carries a low adhesion material (112) having low adhesion to a paper or board web.

8. A belt according to claim 7, wherein the low adhesion material (112) comprises a fluoroplastic material.

9. A belt according to claims 7 or 8, wherein the coating

(101) has cavities (111) filled with said low adhesion material (112) .

10. A belt according to claim 1, wherein said belt is coated on one surface thereof and the stress in the coating (102) is tensile stress, wherein the testing tensile stress is generated on a coated surface of the belt.

11. A belt according to claim 10, wherein the stress in the belt body (10) is compressive stress.

12. A belt according to any one of claims 1, 10 and 11, wherein the coating (102) is provided on the inside surface of a loop formed by said belt (100) .

13. A belt according to any one of claims 1, 10, 11 and 12, wherein the coating (102) is a flexible coating.

14. A belt according to claim 13, wherein the coating

(102) is a diamond or diamond-like coating.

15. A method for manufacturing a belt (100) made from metal for use in a paper- or board making machine, comprising the steps of: providing a plate shaped belt body (10) adapted to be formed into an endless loop, bending said belt body (10) with a radius and/or in a direction different from the bending of the belt (100) when the belt is in use in form of said loop in the paper- or board making machine, and forming a coating (101, 102) on at least one surface of the belt body (10) from a different material than said belt body (10) while the belt body (10) is in said bent state, so that a coating (101, 102) is formed in which stress in the coating (101, 102) is different from stress in the coated belt body (10) .

16. A method according to claim 15, wherein the belt body (10) is bent with a smaller radius (r) of curvature than in operation in the machine, and the coating (101) is applied to the convex surface of the bent belt body (10), which surface is an outside surface of the belt loop.

17. A method according to claim 15, wherein the belt body (10) is bent in opposite direction than in operation in the machine, and the coating (102) is applied to the concave surface of the bent belt body (10), which surface is an inside surface of the belt loop.

18. A method according to claim 16, wherein the belt body (10) is bent over a roller (2) having the radius of curvature (r) as its radius.

19. A method according to claim 17, wherein the belt body (10) is bent by applying vacuum on the side opposite the side to be coated forcing the belt into a bent shape.

20. A method according to any one of claims 15 to 19, wherein the coating method comprises at least one of the following methods: thermal spaying, HVOF, HVAF, electrochemical coating, chrome coating, brush coating, chemical coating, nickel plating, spread coating, plating, dip coating, coagulation bath coating, physical gas phase coating, immersion coating, deposition coating, welding coating, laser coating, CVD, PVD, DLC, ALD, diamond and diamond- like coating and hybrid coating.