COATING REMOVAL METHOD AND GUIDE FOR USE THEREWITH

Abstract

The present invention relates to a method for removing a coating from a defined area of a high pressure turbine part comprising the steps of: securing a guide to said turbine part, said guide both exposing said defined area while protecting adjacent areas; repeatedly inserting and withdrawing a brush through an opening in said guide which directs said brush to impinge said defined area and remove said coating. The invention also relates to a guide used in the aforementioned method.

Description

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates generally methods for manufacturing such parts which permit selective inclusion of coatings where desired on the one hand, but removal of such coatings at certain portions where they might interfere with the mechanical fitting and functioning of the part, such as at dovetail joints of a high pressure turbine engine.

(ii) Description of Related Art

High pressure turbines (“HPT” s), such as those used in aircraft engines, as well as other dynamoelectric machines, such as generators used with gas and steam turbines, typically employ parts which are mechanically joined using dovetails. In aircraft engines, it is common to employ dovetail joints on turbine rotor disks. In generators, dovetails are typically located at the top of the radial slots, and may be used in conjunction with slides, wedges and ripple springs that aid in retaining windings.

To protect HPT's and other machine parts subject to extremes of temperature and pressure, it is common to “paint” onto the parts anti-corrosion protective coatings. Typical anti-corrosion coatings include ceramic coatings such as Alseal 700 or Alseal 589 Tan, manufactured by Coatings for Industry, Inc. of Souderton, Pa. Typically, the anti-corrosion coatings are applied to the parts prior to their assemblage. However, in joining component parts together to form a mechanical assembly, there are certain pressure face areas where the presence of the anti-corrosion coating is detrimental in terms of the closeness of the tolerances. For example, the presence of the anti-corrosive coating deleteriously affects dovetail joints in HPTs, where direct contact between the component parts provides a better connection.

In seeking to resolve this issue, it was initially contemplated to modify the coating process so as to not apply coating to the pressure face areas of the article. However, it was found to be too difficult to control the application process so as to both sufficiently coat those areas where the corrosion-resistant coating was desired while, at the same time, sufficiently masking the pressure face areas to prevent application of the coating thereon. Techniques for removing the coating such as high pressure waterjet stripping and milling the coating from the pressure face areas were found ineffective in situations requiring precision and selectivity in coating removal.

SUMMARY OF THE INVENTION

The present invention addresses the above described problem in that it applies the corrosion-resistant coating to those portions of the part where it is needed for protecting the part from high temperature and pressure environments while, at the same time, providing little or no corrosion-resistant coating on the pressure face areas of the part. As a result of having no corrosion-resistant coating at pressure face areas of the parts, junction areas between the parts exhibit better tolerances.

In a more specific embodiment, the part is an HPT rotor disk such as those employed in aircraft engines and the pressure face area is a dovetail joint. However, it will be appreciated that the method of the invention is applicable to any HPT part, such as those employed in either aviation or power generation.

In a first aspect, the present invention relates to a method for removing a coating from a defined area of a high pressure turbine part comprising the steps of:

• • securing a guide to the turbine part, wherein the guide both exposes the defined area while protecting adjacent areas; and
  • repeatedly inserting and withdrawing a brush through an opening in the guide which directs the brush to impinge the defined area and remove the coating.

In a second aspect, the present invention relates to a method for removing a corrosion-resistant coating from the dovetail slot of an HPT rotor comprising the steps of:

• • securing a guide to the dovetail slot by inserting and wedging the guide inside the dovetail slot including the defined coating removal area; and
  • repeatedly inserting and withdrawing a brush through an opening in the guide which directs the brush to impinge the defined area of the dovetail and remove the coating.

In a third aspect, the present invention relates to a guide comprising a contoured side surface adapted to interface with the teeth of a dovetail joint of a rotor; a top surface having a lip adapted to hold the guide in place against one side of the rotor and at least one hole positioned over a guide channel, wherein the guide channel includes at least one opening that exposes a surface of the dovetail joint when the guide is fitted to the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a High Pressure Turbine Disk including a dovetail slot having a guide inserted therein.

FIG. 2 depicts the guide, showing a series of holes for passage of the brush.

FIG. 3 depicts a brush at the point of insertion into one of the holes of the guide inserted into the dovetail slot of the high pressure turbine disk.

FIG. 4 depicts the brush attached to the machinery which controls its insertion into and deletion from one of the holes in the guide inserted in the dovetail of the high pressure turbine disk.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method of the present invention is generally applicable to the removal of deposits from a defined area on a component, such as an anti-corrosion coating from a dovetail pressure face on turbine rotor disk in a jet engine. The process is particularly effective in removing anti corrosion coating deposited on an HPT disk, in which the coating is removed from a specifically defined area such as the pressure faces of the dovetail slots. This method is effective at maintaining close tolerances for the perimeter of the defined area without disturbing the remaining coating adjacent to the defined area.

The anti-corrosion coatings suitable for application to HPT's are generally known in the art. They generally include inorganic ceramic binders which permit the coatings to endure the extreme temperatures and environments that exist in stationary and aviation gas turbines. Commercially available coatings suitable for use in the present invention include ALSEAL 700 and ALSEAL 598 Tan, or similar paint-like materials.

As depicted in FIG. 1, the invention relates to the treatment of the HPT disk 1, which includes around its circumference a number of dovetail slots 2. To carry out the method of the invention, a guide 3 is inserted and wedged into the dovetail slot 2 including six pressure faces.

An embodiment of the guide 3 is depicted in greater detail in FIG. 2. FIG. 2 shows three of the six openings corresponding to the pressure faces of a HPT turbine, labeled 5a, 5b, and 5c. The openings allow a brush to impinge upon only that area of the pressure face where removal of the coating is desired. The guide 3 further includes six guide holes, 6a, 6b, 6c, 6d, 6e and 6f. As shown in FIG. 3, the guide holes 6a-f are used to guide the brush 4 to engage the surface of one of the six pressure faces of the HPT disk 1. When properly inserted and wedged into the dovetail slot 2, the guide 3 defines a coating removal area, exposing only the defined area while protecting the adjacent areas.

The guide also controls the degree of impingement and the defined area. The defined area, as discussed above, relates to that area from which it is desired to remove the coating, such as dovetail pressure faces. The guide 3 provides a physical covering over those areas of the surface where it is desired to retain the coating.

The guide 3 needs to be strong enough so as not to break down when exposed to the physical stresses of the reciprocating action of the brush strokes, as well as strong enough to maintain its integrity so as to protect those areas of a component, such as an HPT, to prevent coating removal. Generally both metals, as well as plastics, such as polycarbonates, have been found to be suitable in this regard, though other plastics could be employed. The guide can be manufactured by use of 3-d printing methods known in the art or by other methods. It will be appreciated that the geometry of the guide can be tailored to any surface of a component where it is desired to remove a selected portion of the coating thereof in accordance with the method of the present invention. The guide 3 is typically inserted and wedged securely into the piece to be treated, such as the dovetail slot 2 containing the coating removal area.

The brush 4 is depicted at FIG. 3 at the point of insertion into the guide 3. The geometry of the brush 4 generally relates to the size of opening in the guide 2 that the brush 4 reciprocates through on entering and withdrawing from the guide orifices 6a-6f The diameter has to be small enough to permit entry and withdrawal of the brush 4 into and out from the guide orifices 6a-6f while at the same time being large enough to exert a certain amount of force on the coatings so as to be able to abrade all or substantially all of the coating from the selected area. Generally, the diameter of the brush preferably ranges from 2 to 5 mm and more preferably about 2.5 mm. The brushes themselves generally include a central shaft 7 having bristles 8 about its circumference. In the embodiment shown in FIG. 3, the bristles 8 form a spiral pattern about the central shaft 7.

The bristles 8 can be made of any materials with suitable strength to withstand the reciprocating insertion and withdrawing from the orifices as well as suitably abrading qualities to remove coating from the device upon impingement with that coating. The material also needs to have suitable flexibility so that the bristles can bend in the orifice as they impinge on the selected area for the coating removal. Persons skilled in the art will understand how to choose metals with sufficient strength and flexibility to provide suitable abrading of a surface in the context of the present invention. A particularly preferred metal for the process is stainless steel for both the bristles and the central shaft.

As depicted in FIGS. 3 and 4, the central shaft 7 can be attached to a chuck 9 which firmly houses and holds the central shaft and creates the reciprocating up and down motion to insert and withdraw the brush 4 into and out of the orifices 6a-6f of the guide 2, thereby achieving the desired abrading at the exposed areas of the device to achieve removal of all or substantially all of the coating therefrom. The controlled method utilizes the guide 2 to force the brush to impinge the coating removal area to a controlled amount such 0.015 inch. The method strokes the brush in and out through the guide 2 at a controlled rate of speed ranging between 20 and 60 inches per minute and preferably at about 40 inches per minute. In a preferred embodiment, the method indexes a cylindrical brush a controlled amount after every stroke cycle to provide even brush wear, e.g., by rotating the brush after each cycle. An exemplary brush index is about 36 degrees after each stroke cycle, though it will be appreciated that this can be higher or lower.

In a preferred embodiment, the method tracks the stroke cycles to provide the proper number of strokes per coating removal area to completely remove the coating from the area. The method also preferably tracks the brush usage to a predetermined brush life cycle. For example, the method advantageously employs as a control a CNC machine such as a standard CNC milling machine. The method also works manually, though a more automated method is preferred.

By virtue of the method of the present invention, the integrity of the surrounding adjacent coating, as evidenced by highly magnified metal, is maintained [Any illustration of this we could include in the application?]. On the other hand, the method effectively removes all or substantially all of the coating from selected areas.

EXAMPLE

A guide was designed by use of a CNC machine to fit into the dovetail joints on a turbine rotor disk as depicted in FIG. 1. The guide is made of polycarbonate and includes six orifices adapted to receive a brush. Each orifice corresponds to a dovetail section which includes one or more coatings to be removed by the brush. In the embodiment illustrated, each orifice was 1-2 mm in diameter, preferably 1.5 mm, and the length of the dovetail section on the turbine rotor, corresponding to the length of a corresponding section on the guide, was 30-40 mm in length, preferably 35 mm. The brush was made of stainless steel, and included a bristle portion being at least as large as the length of the guide. The shaft itself had diameter less than that of the orifice and the bristles had diameter slightly greater than that of the orifice and at least large enough to make contact with the surface of the dovetail section when inserted into the orifice. The brush portion was formed as a spiral about the axis of the shaft. In operation, the machine reciprocated the brush into and out of each orifice at a rate of 40 inches/minute, while the shaft was rotated 36° after each insertion to ensure even wear of the brush. Inspection of the dovetails on the turbine rotor disk after completion of the process revealed that the corrosion-resistant paint coating had indeed been removed from the defined area, whereas the guide had effectively protected the coating in the areas protected by the guide. The result obtained was better as compared to where the defined areas were protected before spraying the corrosion-resistant paint coating onto the part, which was unexpected.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1. A method for removing a coating from a defined area of a high pressure turbine part comprising the steps of:

securing a guide to said turbine part, said guide both exposing said defined area while protecting adjacent areas;

repeatedly inserting and withdrawing a brush through an opening in said guide which directs said brush to impinge said defined area and remove said coating.

2. The method of claim 1, wherein said coating a corrosion-resistant coating.

3. The method of claim 2, comprising painting said corrosion-resistant coating onto said high pressure turbine to form the coating.

4. The method of claim 1, wherein said defined area is a dovetail slot.

5. The method of claim 4, comprising securing said guide in said dovetail slot by inserting and wedging said guide inside the dovetail slot containing the defined coating removal area.

6. The method of claim 1, wherein said guide comprises plastic or metal.

7. The method of claim 1, wherein said brush comprises stainless steel bristles.

8. The method of claim 7, wherein said bristles have a diameter between 1 and 3 mm.

9. The method of claim 8, wherein said bristles have a diameter of 2.5 mm.

10. The method of claim 7, wherein a defined length of said brush bristles impinge on said defined area.

11. The method of claim 10, wherein said defined length is 0.3 to 0.5 mm.

12. The method of claim 12, wherein said defined length is 0.38 mm.

13. The method of claim 1, comprising stroking the brush in and out through the guide at a rate of speed between 30 and 50 inches per minute.

14. The method of claim 14, wherein said brush is cylindrical and indexed by a controlled amount after every stroke.

15. The method of claim 15, wherein the brush is indexed by 36 degrees after each stroke.

16. The method of claim 2, wherein the corrosion-resistant coating is a ceramic coating.

17. The method of claim 1, wherein said high pressure turbine part is a rotor disk.

18. A guide for removing a coating from a surface of a dovetail joint of a rotor comprising: a contoured side surface adapted to interface with the teeth of a dovetail joint of a rotor; a top surface having a lip adapted to hold the guide in place against one side of the rotor and at least one hole positioned over a guide channel, wherein the guide channel includes at least one opening that exposes a surface of the dovetail joint when the guide is fitted to the rotor.

19. The guide of claim 18, wherein the side surface of the guide includes six cavities adapted to interface with the six teeth in the dovetail joint of the rotor and six guide channels corresponding to each tooth of the rotor.

20. The guide of claim 18, wherein the guide includes a cutout portion of the top surface.