WO2009000267A2

WO2009000267A2 - A rotating object and a method of balancing a rotating object

Info

Publication number: WO2009000267A2
Application number: PCT/DK2008/000231
Authority: WO
Inventors: Bo Gillesberg; Søren ERIKSEN
Original assignee: Danfoss A/S
Priority date: 2007-06-22
Filing date: 2008-06-19
Publication date: 2008-12-31
Also published as: WO2009000267A3

Abstract

A method of balancing a coated rotating object (1) comprising forming at least one balancing zone (2), each balancing zone (2) providing an interface having corrosion properties being at least substantially identical to the corrosion properties of the coating material. The rotating object (1) is balanced by adding and/or removing material at position(s) corresponding to the balancing zone(s) (2). Thereby the corrosion properties provided by the coating are not compromised during balancing of the rotating object (1). Furthermore, a coated rotating object (1) having balancing zone(s) (2) is disclosed.

Description

A ROTATING OBJECT AND A METHOD OF BALANCING A ROTATING OBJECT

FIELD OF THE INVENTION

The present invention relates to a rotating object, e.g. a rotor, such as a centrifuge or a turbine, and to a method of balancing such a rotating object. More particularly, the present invention relates to a method of balancing a rotating object which has been provided with a coating, while maintaining the properties provided by the coating.

BACKGROUND OF THE INVENTION

When an object rotates at a high rotational speed, it is necessary that the object is properly balanced in order to prevent damage to the object or to parts connected thereto. It is known in the art to balance rotating objects by removing or adding material at various parts of the object. Examples of such balancing methods can, e.g., be found in US 5,206,988 and in US 5,011 ,374.

US 5,206,988 discloses a centrifuge rotor having a balancing ring integrally formed with and raised from the upper surface thereof. The rotor is manufactured by removing material from the ring to balance the rotor in the upper plane. However, in some applications it is desirable to provide a coating to a rotor. For instance, if the rotor is expected to operate under harsh conditions, such as in a corrosive environment, it may be desirable to apply a coating of a corrosion resistant material in order to improve the corrosion resistant properties of the rotor. If the rotor is coated after the balancing has been performed, then there is a risk that an imbalance is introduced due to an uneven coating layer. On the other hand, if the rotor is coated prior to performing the balancing, then there is a risk that the removal of material during the balancing process damages the coating, especially be penetrating it where material is removed, thereby weakening the coating, possibly creating pinholes. Furthermore, it will not be possible to adjust the rotor in case imbalance occurs as a result of wear or distortion during operation, and it may therefore be necessary to discard the rotor before it is actually worn out. Finally, adding weights to balance the rotor is often not practical since the weight must be attached securely to the rotor without compromising the coating, i.e. without using screw holes, and without welding.

US 5,011 ,374 discloses a method and apparatus for balancing the rotor in the low pressure stage of the turbine in a jet engine. The method comprises attaching at least one balancing clip to the shroud of the rotor. The balancing clip comprises a clip body formed with a forward hook portion adapted to extend over the forward rail of the shroud and an aft tab which is crimped over the aft rail of the shroud onto its outer surface so that the clip body is retained against the interior face or diameter of the shroud. The balance clip is formed of sheet material whose weight can be varied by altering the thickness or width of the clip body, or by drilling holes in the clip body to remove material therefrom. In the case that it is desirable or necessary to apply a coating to the rotor, the problems described above would also occur when using the method and apparatus of US 5,011 ,374.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a method of balancing a coated rotating object while maintaining the properties provided by the coating.

It is a further object of the invention to provide a method of balancing a coated rotating object, the method being suitable for adjusting imbalances occurring as a result of wear or distortion during operation. It is an even further object of the invention to provide a coated rotating object which can be balanced without compromising the coating layer.

It is an even further object of the invention to provide a balanced rotating object which is suitable for operation in corrosive environments.

According to a first aspect of the invention the above and other objects are fulfilled by providing a method of balancing a rotating object, the method comprising the steps of:

- providing a rotating object,

- forming at least one balancing zone on the rotating object,

- providing a coating to the rotating object, and

- balancing the rotating object by adding and/or removing material at position(s) corresponding to the balancing zone(s),

wherein each balancing zone provides an interface having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

The coating provided to the rotating object may advantageously be a corrosive resistant coating, i.e. the coating material may advantageously be of a kind which prevents, or at least considerably reduces damage to the object due to corrosion. In this case the coating layer allows the rotor to operate under harsh conditions, such as in a corrosive environment, such as in an acid, a base, ion containing environments, such as chloride, etc. However, in order for the coating to be able to provide efficient protection against corrosion, it is imperative that the coating layer remains substantially intact, preferably completely intact. In particular, it is important that no pinholes occur in the coating.

The step of providing a coating may be performed in any suitable manner known per se in the art, such as by means of an electrochemical deposition technique, e.g. the technique described in WO 02/068729, or by means of a chemical vapour deposition technique, e.g. the technique described in WO 02/068007. Alternatively, an electrolytic surface coating technique may be used, e.g. the technique described in EP 0 578 605.

The balancing zones are parts of the rotating object where material is added or removed in order to balance the object. Each balancing zone provides an interface having corrosion resistant properties being at least substantially identical to the corrosion resistant properties of the coating material. In this context the interface is a part of the balancing zone which is readily accessible, e.g. for removing or adding material from/to the balancing zone. Thus, this should be interpreted to mean that, at least at the interface, a balancing zone provides substantially the same protection against corrosion as the coating layer does. Accordingly, in the case that coating material is removed from the rotating object at a position corresponding to an interface of a balancing zone, the corrosion resistant properties of the object are maintained, even if the coating layer is damaged. Thereby it is possible to remove material, e.g. by grinding, cutting, drilling, etc. or to add material, e.g. by screwing or welding material onto the rotating object, without compromising the corrosion resistant properties of the object. This is very advantageous.

The method may further comprise the step of pre-balancing the rotating object prior to providing a coating to the rotating object. According to this embodiment, the object is at least roughly balanced before the coating is applied. After the coating has been applied, the balancing is adjusted or fine tuned in order to obtain an exactly balanced rotating object. The fact that the pre-balancing is performed minimises the balancing needed after the coating has been applied. This is advantageous because it minimises the amount of coating material which it may be necessary to remove during balancing.

The step of forming at least one balancing zone may comprise attaching at least one surplus weight at one or more balancing zones, said surplus weight(s) being made from a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material. A surplus weight adds weight to the rotating object at the position where it is attached. Preferably, each surplus weight is or forms part of the interface provided by the corresponding balancing zone. Accordingly, if coating material is subsequently removed from a position corresponding to a surplus weight, the corrosion resistant properties of the rotating object will not be compromised, even if the coating is damaged. The surplus weight(s) may, e.g., be in the form of balancing clips.

The surplus weight(s) may be made from the same material as the coating material. Alternatively, the surplus weight(s) may be made from a material which is different from the coating material, as long as the corrosion resistant properties of the material of the surplus weight(s) is substantially identical to, or at least comparable to, the corrosion resistant properties of the coating material, i.e. as long as an uncoated surplus weight provides substantially the same protection against corrosion as the coating provides.

The step of attaching at least one surplus weight may comprise welding material onto a surface part of the rotating object. This may, e.g., be obtained by welding relatively thick sheet material or a wire onto the surface part. This allows post coating balancing of the rotating object by grinding, cutting, drilling or combinations thereof in areas covered by the material welded onto the surface.

As an alternative, the step of attaching at least one surplus weight may comprise welding a plug into a pre-drilled hole in the rotating object. This embodiment also allows post coating balancing of the rotating object by grinding, cutting, drilling or combinations thereof in areas where a plug has been welded into a pre-drilled hole. Furthermore, this embodiment allows the surface of the coated and balanced object to be substantially flat or smooth, contrary to the embodiments described above where material is added to a surface part of the rotating object. Finally, this embodiment allows drilling into the plug, e.g. in order to mount one or more balancing weights after applying the coating.

As another alternative, the step of attaching at least one surplus weight may comprise depositing material onto one or more selected parts of a surface of the rotating object. Suitable deposition techniques include, but are not limited to, spray deposition technoques, e.g. plasma or flame spraying. This results in relatively thick coating layers having thicknesses in the millimetre range. The deposition may be performed either before or after the step of providing a coating.

As mentioned above, the step of balancing the rotating object may comprise removing material at the balancing zone(s) by means of grinding, cutting and/or drilling.

Alternatively to adding surplus weights, the step of forming at least one balancing zone may comprise the step of providing one or more mounting holes in such a manner that coating material is allowed to enter the mounting hole(s), and the step of balancing the rotating object may comprise mounting one or more balancing weights in the one or more mounting holes, the balancing weights being made from, or at least being coated with, a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

Preferably, the mounting holes are oversized as compared to the balancing weights in order to accommodate the coating material as well as a balancing weight in each mounting hole. The mounting hole(s) may advantageously be threaded, thereby allowing a balancing weight to be mounted directly in a mounting hole by means of a mating thread formed on the balancing weight. The balancing weight(s) may, in this case, be in the form of (a) screw(s). Alternatively or additionally, the balancing weight(s) may be attached by means of other attachment methods, e.g. based on holes, rivets, expansion fasteners, etc.

The balancing weights may be made from the same material as the coating material, or they may at least be coated with the same material in order to provide a surface of each balancing weight having corrosion properties being identical to the corrosion properties of the coated surface of the rotating object.

The coating material may have enhanced corrosion resistant properties as compared to a base material of the rotating object. According to this embodiment, the coating provides protection against corrosion to the rotating object, i.e. the coated object is more resistant towards corrosion than the uncoated object.

The coating material may be a refractory metal or an alloy of a refractory metal. The coating material may, in this case, preferably be tantalum or an alloy of tantalum. Alternatively, it may be any other suitable refractory metal, such as tungsten, molybdenum, niobium or rhenium, and/or an alloy of any of these refractory metals. As an alternative, the coating material may be a reactive metal or an alloy of a reactive metal. A reactive metal is a metal which combines with oxygen to form very stable oxides. In this case, the coating material may, e.g., be titanium or zirconium, and/or an alloy of any of these reactive metals.

Refractory metals as well as reactive metals are known to be corrosion resistant. It is therefore preferred to use a coating material containing a refractory metal and/or a reactive metal.

According to a second aspect of the invention, the above and other objects are fulfilled by providing a rotating object comprising:

- a coating covering at least part of an outer surface of the rotating object, and

- at least one balancing zone, each balancing zone providing an interface having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

It should be noted that a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa.

The rotating object could, e.g., be a pump impeller, an agitator, a centrifuge, a rotating part of a turbine, or any other kind of object being adapted to perform rotational movements, preferably relatively fast rotational movements.

The rotating object according to the second aspect of the invention is preferably manufactured using the method according to the first aspect of the invention. Thus, the rotating object according to the second aspect of the invention can be balanced without compromising the corrosion resistant properties provided by the coating. Accordingly, the rotating object may operate under harsh or hostile conditions, such as in a corrosive environment, as described above.

At least one balancing zone may have at least one surplus weight attached thereto, said surplus weight(s) being made from a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material. The surplus weight(s) may be made from the same material as the coating material, as described above.

Alternatively, at least one balancing zone may be provided with one or more mounting holes, interior parts of said mounting hole(s) being provided with a layer of coating material, and each mounting hole may be adapted to receive a balancing weight. This has also been described above.

The coating material may have enhanced corrosion resistant properties as compared to a base material of the rotating object and/or the coating material may be a refractory metal or an alloy of a refractory metal, such as tantalum or an alloy of tantalum.

According to a first example, the rotating object may be a tantalum coated pump impeller. In this case the uncoated impeller is balanced in a pre- balancing step, e.g. by grinding, to a given tolerance. Subsequently, 3 mm thick 10x10 mm tantalum coupons are welded to a flat backside of the impeller at six positions. Then a 50±20 μm tantalum coating is applied by means of a chemical vapour deposition (CVD) process. Finally, the coated impeller is balanced by grinding material off the coupons. According to a second example, the rotating object may be a tantalum coated agitator. In this case the uncoated agitator may optionally be balanced in a pre-balancing step, e.g. by grinding. Subsequently, 10 mm diameter holes are drilled through the disk at six positions, and tantalum plugs are welded into each of the holes. Then a 100±20 μm tantalum coating is applied by means of a molten salt electrodeposition technique. Finally, the coated agitator is balanced by drilling into the tantalum plugs using a 7 mm drill. According to this example, the coating acts as corrosion barrier, but at the same time the coating may assist in firmly attaching the plugs into the holes by providing a 'gluing effect'.

Accordingly, it may not be necessary to weld the plugs into the holes prior to providing the coating. This may result in a larger resistance against loosing the plugs due to vibrations.

According to a third example, the rotating object may be a tantalum coated object. In this case the uncoated object is balanced in a pre-balancing step, e.g. by grinding. Subsequently, M6 threaded holes are made to 50 μm oversize, i.e. M6.1 , at six positions. Then a 30±10 //m tantalum coating is applied by a chemical vapour deposition (CVD) technique. Finally, the coated object is balanced by inserting solid tantalum screws into the threaded holes. The lengths of the screws are adjusted by cutting or grinding to balance the object.

According to a fourth example, the rotating object may be a tantalum coated object. In this case the uncoated object is balanced in a pre- balancing step, e.g. by grinding. Subsequently. M10 threaded holes are made to 50 μm oversize, i.e. M 10.1 , at six positions. Then a 30±10 μm tantalum coating is applied by means of a chemical vapour deposition (CVD) technique. Finally, the coated object is balanced by inserting prefabricated tantalum coated screws into the threaded holes. The screws are made in increments of 0.2 g. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1a is a top view of a rotating object according to an embodiment of the invention,

Fig. 1b is a cross sectional view of a rotating object according to an embodiment of the invention, prior to forming balancing zones on the object,

Figs. 2a-2c illustrate various steps of a method according to a first embodiment of the invention,

Figs. 3a-3d illustrate various steps of a method according to a second embodiment of the invention, and

Figs. 4a-4c illustrate various steps of a method according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1a is a top view of a rotating object 1 according to an embodiment of the invention. The rotating object 1 is provided with six balancing zones 2. The balancing zones 2 may be in the form of surplus weights or holes or recesses as described above.

Fig. 1b is a cross sectional view of a rotating object 1 according to an embodiment of the invention, e.g. the embodiment shown in Fig. 1a. In Fig. 1 b balancing zones have not yet been formed on the rotating object. Figs. 2a-2c are cross sectional views of a rotating object 1. Figs. 2a-2c illustrate various steps of a method according to a first embodiment of the invention.

Fig. 2a is a cross sectional view of the rotating object 1 of Fig. 1b. However, in Fig. 2a surplus weights 3 have been attached on a surface part of the rotating object 1. The surplus weights 3 may be in the form of material, such as sheet material or a wire, which has been welded onto the surface, in the form of material which has been deposited onto the surface part, or it may be in any other suitable form.

Fig. 2b shows the rotating object 1 of Fig. 2a. However, in Fig. 2b a substantially uniform coating 4 has been applied to the rotating object 1. Since the surplus weights 3 were attached to the surface part of the rotating object 1 before the coating 4 was applied, the coating 4 also covers the surplus weights 3. This is clearly seen in Fig. 2b.

The surplus weights 3 are made from a material having corrosion properties which are at least substantially identical to the corrosion properties of the coating material, as described above. Preferably, the surplus weights 3 are made from the same material as the coating 4. The surplus weights 3 constitute balancing zones of the rotating object 1 , and the upper surface of each surplus weight 3 defines an interface of the corresponding balancing zone in the sense defined above.

Fig. 2c is a cross sectional view of the rotating object 1 of Figs. 2a and 2b. However, in Fig. 2c the rotating object 1 has been balanced by removing material from a position corresponding to the position of surplus weight 3a. It is clear from Fig. 2c that this has the consequence that the thickness of the coating 4 at this position is considerably reduced. However, since the surplus weight 3a is made from a material having corrosion properties being substantially identical to the corrosion properties of the coating material, the overall corrosion resistance of the rotating object 1 is not compromised. It is even possible to remove the entire coating 4 at this position and grinding into the surplus weight 3a without compromising the overall corrosion resistance of the rotating object 1.

Figs. 3a-3d are cross sectional views of a rotating object 1. Figs. 3a-3d illustrate various steps of a method according to a second embodiment of the invention.

Fig. 3a is a cross sectional view of the rotating object 1 of Fig. 1b. However, in Fig. 3a a number of holes 5 have been drilled in the rotating object 1. Two holes 5 are shown in Fig. 3a. Each of the holes 5 defines a balancing zone of the rotating object 1.

Fig. 3b shows the rotating object 1 of Fig. 3a. However, in Fig. 3b a plug 6 has been mounted in each of the holes 5.

Fig. 3c shows the rotating object 1 of Figs. 3a and 3b. However, in Fig. 3c a substantially uniform coating 4 has been applied to the rotating object 1. Since the plugs 6 were mounted in the holes 5 before the coating 4 was applied, the coating 4 also covers the plugs 6. This is clearly seen in Fig. 3c.

The plugs 6 are made from a material having corrosion properties which are at least substantially identical to the corrosion properties of the coating material, as described above. Preferably, the plugs 6 are made from the same material as the coating 4. The holes 5 and the plugs 6 in combination constitute balancing zones of the rotating object 1 , and the upper surface of each plug 6 defines an interface of the corresponding balancing zone in the sense defined above. Fig. 3d shows the rotating object 1 of Figs. 3a-3c. However, in Fig. 3d a mounting hole 7 has been drilled at a position corresponding to the position of plug 6a. The mounting hole 7 may, e.g., be used for mounting weights in order to add weight at the corresponding position. As an alternative, the mounting hole 7 may be left as shown in Fig. 3d. In this case the rotating object 1 has been balanced by removing material, similar to the situation described above with reference to Figs. 2a-2c. It is clear from Fig. 3d that the mounting hole 7 completely penetrates the coating 4 and even extends far into the plug 6a. However, since the plug 6a is made from a material having corrosion properties being substantially identical to the corrosion properties of the coating material, the overall corrosion resistance of the rotating object 1 is not compromised.

Figs. 4a-4c are cross sectional views of a rotating object 1. Figs. 4a-4c illustrate various steps of a method according to a third embodiment of the invention.

Fig. 4a is a cross sectional view of the rotating object 1 of Fig. 1b. However, as it is the case in Fig. 3a, a number of holes 5 have been drilled in the rotating object 1 shown in Fig. 4a. Two of the holes 5 are shown in Fig. 4a. Each of the holes 5 defines a balancing zone of the rotating object 1.

Fig. 4b shows the rotating object 1 of Fig. 4aFig. 4b shows the rotating object 1 of Fig. 4a. However, in Fig. 4b a substantially uniform coating 4 has been applied to the rotating object 1. It is clear from Fig. 4b that the coating 4 has also been applied to interior parts of the holes 5.

Fig. 4c shows the rotating object 1 of Figs. 4a and 4b. However, in Fig. 4c the rotating object 1 has been balanced by mounting balancing weights 8 in the coated holes 5. It can be seen that balancing weight 8a is smaller than balancing weight 8b. This is in order to illustrate that the rotating object 1 is balanced by mounting balancing weights 8 with various masses in appropriate mounting holes 5.

The balancing weights 8 are made from a material having corrosion properties being substantially identical to the corrosion properties of the coating material. Furthermore, since the inner parts of the holes 5 have been coated, the balancing weights 8 are mounted directly on a coated surface, and the coated inner parts of the holes 5 may be regarded as an interface as defined above. Accordingly, mounting the balancing weights 8 in the holes 5 does not compromise the overall corrosion resistance of the rotating object 1.

Claims

1. A method of balancing a rotating object (1 ), the method comprising the steps of:

- providing a rotating object (1 ),

- forming at least one balancing zone (2) on the rotating object (1 ),

- providing a coating (4) to the rotating object (1 ), and

- balancing the rotating object (1 ) by adding and/or removing material at position(s) corresponding to the balancing zone(s) (2),

wherein each balancing zone (2) provides an interface having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

2. A method according to claim 1 , further comprising the step of pre- balancing the rotating object (1 ) prior to providing a coating (4) to the rotating object (1 ).

3. A method according to claim 1 or 2, wherein the step of forming at least one balancing zone (2) comprises attaching at least one surplus weight (3) at one or more balancing zones (2), said surplus weight(s) (3) being made from a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

4. A method according to claim 3, wherein the surplus weight(s) (3) is/are made from the same material as the coating material.

5. A method according to claim 3 or 4, wherein the step of attaching at least one surplus weight (3) comprises welding material onto a surface part of the rotating object (1 ).

6. A method according to claim 3 or 4, wherein the step of attaching at least one surplus weight (3) comprises welding a plug (6) into a pre-drilled hole (5) in the rotating object (1 ).

7. A method according to claim 3 or 4, wherein the step of attaching at least one surplus weight (3) comprises depositing material onto one or more selected parts of a surface of the rotating object (1).

8. A method according to any of the preceding claims, wherein the step of balancing the rotating object (1 ) comprises removing material at the balancing zone(s) (2) by means of grinding, cutting and/or drilling.

9. A method according to claim 1 or 2, wherein the step of forming at least one balancing zone (2) comprises the step of providing one or more mounting holes (5) in such a manner that coating material is allowed to enter the mounting hole(s) (5), and wherein the step of balancing the rotating object (1 ) comprises mounting one or more balancing weights (8) in the one or more mounting holes (5), the balancing weights (8) being made from a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

10. A method according to claim 9, wherein the balancing weights (8) are made from the same material as the coating material.

11. A method according to any of the preceding claims, wherein the coating material has enhanced corrosion resistant properties as compared to a base material of the rotating object (1 ).

12. A method according to any of the preceding claims, wherein the coating material is a refractory metal or an alloy of a refractory metal.

13. A method according to claim 12, wherein the coating material is tantalum or an alloy of tantalum.

14. A method according to any of claims 1-11 , wherein the coating material is a reactive metal or an alloy of a reactive metal.

15. A rotating object (1 ) comprising:

- a coating (4) covering at least part of an outer surface of the rotating object (1 ), and

- at least one balancing zone (2), each balancing zone (2) providing an interface having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

16. A rotating object (1) according to claim 15, wherein at least one balancing zone (2) has at least one surplus weight (3) attached thereto, said surplus weight(s) (3) being made from a material having corrosion properties being at least substantially identical to the corrosion properties of the coating material.

17. A rotating object (1 ) according to claim 16, wherein the surplus weight(s) (3) is/are made from the same material as the coating material.

18. A rotating object (1 ) according to claim 15, wherein at least one balancing zone (2) is provided with one or more mounting holes (5), interior parts of said mounting hole(s) (5) being provided with a layer of coating material, and each mounting hole (5) being adapted to receive a balancing weight (8).

19. A rotating object (1) according to any of claims 15-18, wherein the coating material has enhanced corrosion resistant properties as compared to a base material of the rotating object (1 ).

20. A rotating object (1) according to any of claims 15-19, wherein the coating material is a refractory metal or an alloy of a refractory metal.

21. A rotating object (1 ) according to claim 20, wherein the coating material is tantalum or an alloy of tantalum.