Abstract:
Prior to instrumenting a turbine wheel for component and developmental testing, a protective coating is applied to the turbine wheel so that sensors can be welded to the coating rather than to the base material of the turbine wheel. But it is important to prevent the dovetail slots, which are critical to the usable life of the turbine wheel, from being coated. Plugs are provided that can be inserted into the dovetail slots prior to applying the coating. Each plug is shaped to match the shape profile of the dovetail slot. The plug prevents critical areas from being coated, removes the need for post processing, and allows a single coating to be applied.

Description:
The present invention relates generally to turbo machines. In particular, one or more aspects of the present invention relate to method and apparatus to apply protective coating to gas turbine wheels. 
     BACKGROUND OF THE INVENTION 
     Turbines generally include a rotor comprised of a plurality of rotor turbine wheels, each of which mounts a plurality of circumferentially-arranged buckets. Each bucket includes an airfoil, a platform, a shank and a dovetail, the dovetail being received in mating dovetail slot in the turbine wheel. The airfoils project into a hot gas path downstream of the turbine combustors and convert kinetic energy into rotational, mechanical energy. 
     Often, a protective coating is applied to the turbine wheel for various purposes. For example, the turbine wheel can be instrumented for component and developmental testing (CDT). In CDT, sensors or instruments are attached to the turbine wheel—often by resistance welding the sensors to the turbine wheel. Rather than resistance welding the sensors directly to the turbine wheel itself, a nickel-chromium (NiCr) coating can be applied to the turbine wheel using a plasma spray for example. The sensors then can be welded to the protective coating. In this way, the turbine wheel can be instrumented without inducing or creating stress risers into the base/parent material of the turbine wheel. 
     However, it is necessary to prevent the dovetail slots from being coated. The slots, which are critical to the usable life of the turbine wheel, are machined to a precisely shaped profile and surface finish. Complementarily shaped dovetails (also precisely machined) of the buckets are mated with the slots for assembly of the turbine. Due in large part to the precise machining of the dovetails and slots, the usable life of the turbine would be compromised if the slots are coated. The coating can be removed, but the removal process generally requires an abrasive device, which disturbs the surface finish. Any disturbance of the dovetail surface can decrease the usable life of the turbine wheel and negate any applied metal treatments such as shotpeen. 
     Prior attempts to prevent the slots from being coated included using high temperature adhesive tapes to mask off the dovetail slots and other critical areas. This is a labor intensive and a time consuming process. Also, the tapes can create sharp edges that can result in coating chipping and flaking which requires extensive detail and blending post processing to remove such defects. In addition, the plasma spray is applied at high pressures, such as at 90 PSI. This can cause the tape to lift allowing overspray to come in contact with the dovetail surface. 
     Thus, it is desirable to provide a method and a device to apply protective coating with a greater control of pattern definition, coating surface finish, and to eliminate or vastly reduce incidences of process damage and the necessary re-work that follows such incidences. 
     BRIEF SUMMARY OF THE INVENTION 
     A non-limiting aspect of the present invention relates to a dovetail plug adapted to be inserted into a dovetail slot of a turbine wheel. The plug comprises an insertion part and a protrusion part. The insertion part is shaped to be axially inserted into the dovetail slot from a turbine wheel face to a predetermined insertion depth when the plug is fully inserted into the turbine wheel, and the protrusion part is shaped to axially protrude from the turbine wheel face when the plug is fully inserted into the turbine wheel. The protrusion part comprises a blast portion connected to the insertion part, and a shadow portion on outside of the blast portion. The shadow portion is such that a first contour of the shadow portion is defined at the turbine wheel face and a second contour of the shadow portion is defined at a predetermined protrusion distance from the turbine wheel face. The second contour is outside of the first contour. A shadow surface is a surface of the shadow portion between the first and second contours, and a shadow angle formed between the shadow surface and the turbine wheel face is less than a right angle. 
     Another non-limiting aspect of the present invention relates to a method of forming a dovetail plug to be inserted into a dovetail slot of a turbine wheel. The method comprises forming an insertion part in a shape to be axially inserted into the dovetail slot from a turbine wheel face to a predetermined insertion depth when the plug is fully inserted into the turbine wheel. The method also comprises forming a protrusion part in a shape to axially protrude from the turbine wheel face when the plug is fully inserted into the turbine wheel. The step of forming the protrusion part comprises forming a blast portion connected to the insertion part and forming a shadow portion on outside of the blast portion. The shadow portion is formed such that a first contour of the shadow portion is defined at the turbine wheel face and a second contour of the shadow portion is defined at a predetermined protrusion distance from the turbine wheel face. The second contour is outside of the first contour. A shadow surface is a surface of the shadow portion between the first and second contours, and a shadow angle formed between the shadow surface and the turbine wheel face is less than a right angle. 
     Another non-limiting aspect of the present invention relates to a method of applying protective coating to a turbine wheel. The method comprises inserting plugs into dovetail slots of a turbine wheel, and subsequently applying the protective coating on the turbine wheel. Each plug inserted into the dovetail slots comprises an insertion part and a protrusion part. The insertion part is shaped to be axially inserted into the dovetail slot from a turbine wheel face to a predetermined insertion depth when the plug is fully inserted into the turbine wheel, and the protrusion part is shaped to axially protrude from the turbine wheel face when the plug is fully inserted into the turbine wheel. The protrusion part comprises a blast portion connected to the insertion part, and a shadow portion on outside of the blast portion. The shadow portion is such that a first contour of the shadow portion is defined at the turbine wheel face and a second contour of the shadow portion is defined at a predetermined protrusion distance from the turbine wheel face. The second contour is outside of the first contour. A shadow surface is a surface of the shadow portion between the first and second contours, and a shadow angle formed between the shadow surface and the turbine wheel face is less than a right angle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the present invention will be better understood through the following detailed description of non-limiting example embodiments in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example turbine wheel with a plurality of plugs inserted into corresponding dovetail slots; 
         FIG. 2  illustrates a perspective view of a plug inserted into a turbine wheel; 
         FIG. 3  illustrates a more detailed perspective view of a plug inserted into a dovetail slot of a turbine wheel; 
         FIG. 4  illustrates an axial view of a plug according to a non-limiting embodiment of the present invention; 
         FIG. 5  illustrates a circumferential view of a cross-section of the plug illustrated in  FIG. 4  along a line ‘j’-‘j’; 
         FIG. 6  illustrates a detailed view of a circled portion in  FIG. 5 ; 
         FIG. 7  illustrates a circumferential view of a cross-section of the plug illustrated in  FIG. 4  along a line ‘jj’-‘jj’; 
         FIG. 8  illustrates a detailed view of a circled portion in  FIG. 7 ; 
         FIG. 9  illustrates a radial view of a cross-section of the plug illustrated in  FIG. 4  along a line ‘jjj’-‘jjj’; 
         FIG. 10  illustrates a detailed view of a circled portion in  FIG. 9 ; 
         FIG. 11  illustrates perspective views of a plug according to a non-limiting embodiment of the present invention; 
         FIG. 12  illustrates a non-limiting example flow chart of a method to form a plug; 
         FIG. 13  illustrates a non-limiting example flow chart of a method to form a protrusion part of a plug; and 
         FIG. 14  illustrates a non-limiting example flow chart of a method to apply protective coating on a turbine wheel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Novel plug for use when applying a protective coating on a turbine wheel is described. Methods of forming as well as using the plug are also described. 
       FIG. 1  illustrates an example turbine wheel  10  with a plurality of plugs inserted  20  into corresponding dovetail slots.  FIG. 1  is an axial view of the wheel  10  towards the turbine wheel face  110 .  FIG. 2  illustrates a perspective view of a plug  20  inserted into the turbine wheel  10 , and  FIG. 3  illustrates a more detailed view of the inserted plug  20  and corresponds to the circled portion in  FIG. 2 . In this particular embodiment, the plug  20  is shaped to match the contour of the dovetail slots  120 . When fully inserted, the plug  20  covers at least a part of the circumferential surface  130  of the turbine wheel  10 . 
     As seen in  FIG. 5 , when the plug  20  is fully inserted, the plug  20  is shaped such that a part of the plug  20  still protrudes a distance ‘a’ axially from the turbine wheel face  110 , and is referred to as the protrusion part  22  in this document. The part of the plug  20  that is inserted to the predetermined depth ‘d’ is referred to as the insertion part  24 . Thus, the insertion part  24  can be said to be shaped to be axially inserted into the dovetail slot  120  from the turbine wheel face  110  to the predetermined insertion depth ‘d’ when the plug  20  is fully inserted into the turbine wheel  10 . 
       FIG. 4  is an axial view of the plug  20  as indicated by reference coordinate direction arrows R (radial), Z (circumferential), and A (axial). In this figure, the axial coordinate reference ‘A’ is circled to indicate that the axial direction is into the page. In particular,  FIG. 4  is an axial view of the protrusion part  22 . As seen, the protrusion part  22  includes a central blast portion  210  and a shadow portion  230  on the outside of the blast portion  210 . The shadow portion  230  is bounded by the first contour  232  (long dashed line) and a second contour  234  (solid line). The first contour  232  would not necessarily be visible when viewing the protrusion part  22 . It is drawn in  FIG. 4  to demarcate the different portions of the plug  20  for explanatory purposes. As seen, the second contour  234  is outside of the first contour  232 . Distance ‘b’ between the first and second contours  232 ,  234  represents a width of the shadow portion  230 . 
     Before proceeding further, the following should be noted. For explanatory purposes, the plug  20  is described being comprised of the protrusion and insertion parts  22 ,  24  and the protrusion part  22  itself is described as including various portions, the separation of the plug  20  into various parts and portions is for ease of explanation. But it is fully envisioned that the parts and portions of the actual plug  20 , at least in one aspect, are integrally formed as one piece, for example, through a molding process. 
       FIG. 5  illustrates a circumferential view of the plug  20  as indicated by reference coordinate direction arrows in which circumferential reference direction Z is circled. In particular,  FIG. 5  is a view of a cross-section of the plug  20  taken along a line a line ‘j’-‘j’ in  FIG. 4 .  FIG. 6  is a detailed view of the circled portion in  FIG. 5 . As seen in these figures, the first contour  232  is a contour of the shadow portion  230  at the turbine wheel face  110 , and the second contour  234  is a contour of the shadow portion  230  at a predetermined protrusion distance from the turbine wheel face  110 . As noted above, the second contour  234  is outside of the first contour  232  when viewed axially. 
     The surface of the shadow portion  230  between the first and second contours  232  and  234  is referred to as the shadow surface  236 , which forms a shadow angle α with the turbine wheel face  110  as seen in  FIG. 6 . In one embodiment, it is preferred that the shadow angle α be less than 90°, i.e., be less than a right angle. 
     The shadow angle α being less than the right angle is beneficial for at least the following reason. When the protective coating is sprayed, the shadow portion  230  prevents protective coating with sharp edges, i.e., abrupt changes in coating thickness, from being formed. Instead, coatings with gradual thickness transitions are formed in between the shadow surface  236  and the turbine wheel face  110 . This removes the need for post processing to profile the protective coating. In addition, because the gradual thickness transitions are possible, a single coating of sufficient thickness may be applied rather than the traditional method of applying multiple coats. This saves both time and money. 
     It should be noted that the predetermined protrusion distance of the second contour  234  need not be all the way at the thickness ‘a’ of the protrusion part  22 . The second contour  234  need only be defined at some distance away from the turbine wheel face  110 , even if less than ‘a’, so that the shadow surface  236  forms the proper angle α with the turbine wheel face  110 . Any combination of the predetermined distance protrusion distance of the second contour  234 , the thickness ‘b’ of the shadow portion  230 , and the shadow angle α may be adjusted depending on the circumstances. For the remainder of this document, it is assumed that the second contour  234  is the contour of the shadow portion  230  at distance ‘a’ for convenience. 
     Preferably, the shape profile of the plug  20  is consistent throughout so that the protection from the coating process can be consistently maintained. This can be achieved by shaping the plug  20  to have various characteristics. As an example, it is preferred that the angle α be substantially constant over an entirety of the shadow surface  236 . 
       FIG. 7  illustrates a circumferential view of another cross-section of the plug  20 , this time along a line ‘jj’-‘jj’ in  FIG. 4 , and  FIG. 8  is a detailed view of the circled portion in  FIG. 7 . While  FIG. 6  illustrates a cross section of the plug  20  near a center thereof,  FIG. 7  illustrates a cross section of the plug  20  near an end thereof. Nonetheless, as seen in  FIG. 7 , the shadow portion  230  is formed such that the shadow surface  236  forms a shadow angle that is substantially the same angle α as in  FIGS. 5 and 6 . In addition, the width ‘b’ of the shadow portion  230 , the predetermined protrusion distance of the second contour  234 , and a distance ‘c’ from the dovetail slot edge  125  to the first contour  232  are substantially the same as in  FIGS. 7 and 8 . 
       FIG. 9  illustrates a radial view of a cross-section of the plug illustrated in  FIG. 4  along a line ‘jjj’-‘jjj’, and  FIG. 10  is a detailed view of the circled portion in  FIG. 9 . Again, it is seen that the shadow portion  230  is formed such that the shadow angle α, the width ‘b’, the predetermined protrusion distance of the second contour  234 , and the distance ‘c’ are substantially the same as in  FIGS. 5 ,  6 ,  7  and  8 . 
     It suffices to say that when possible, some or all of the predetermined protrusion distance of the second contour  234 , the width ‘b’ of the shadow portion  230 , the distance ‘c’, and the shadow angle α are preferred to be substantially constant throughout.  FIG. 11  illustrates perspective views of the plug  20 . Note that throughout the plug  20 , consistent shape profile is maintained. 
     It is also preferred that the shape of the dovetail slots  120  be followed so that as much of the surface of the turbine wheel  110  can be protected. Regarding the insertion part  24 , it is indicated above that the insertion part  24  is shaped to be axially inserted into the dovetail slot  120 . Referring back to  FIG. 4 , reference numeral  215  represents a contour of the insertion part  24 . It is preferred that the insertion part contour  215  match the contour of the dovetail slot  120  along at least a part of the predetermined insertion depth ‘d’. In  FIG. 11 , it is seen that the insertion part contour  215  is shaped to match the contour of the dovetail slot  120  along an entirety of the predetermined insertion depth ‘d’. 
     As seen in  FIG. 3 , reference numeral  125  represents an edge the contour of the dovetail slot  120  at the turbine wheel face  110 . In an embodiment, the first contour  232  is at or outside the dovetail slot edge  125 . In  FIG. 4 , the first contour  232  is shown to be outside the insertion part contour  215 , which in turn coincides with the dovetail slot edge  125 . Thus,  FIG. 4  is an example of the first contour  232  being outside of the dovetail slot edge  125 . 
     While not shown, it can also be that the first contour  232  and the dovetail slot edge  125  match, i.e., the distance ‘c’ can be zero. But as long as the first contour  232  is at or outside the dovetail slot edge  125 , the dovetail slot  120  will not be coated. It is also preferable that the second contour  234  follow the outline of the dovetail slot edge  125 . That is, an offset from the dovetail slot edge  125  to the second contour  234  (distance ‘b’ plus ‘c’) is preferred to be substantially constant. 
     Some engineering requirements dictate that an area of the turbine wheel face  110  near the slot edge  125 , the so-called critical area, not be coated. Typically, these are high stress areas. Any damage or surface finish to such areas causes cracks to develop which in turn can leads to a failure in the dovetail slot allowing the “bucket”, i.e., turbine blade to liberate from the gas turbine causing catastrophic failure. 
     The plug  20  in  FIG. 4  includes a protection portion  220  in between the blast and shadow portions  210 ,  230 . In this instance, it is assumed that the critical area is an area of the turbo turbine wheel face  110  within a critical distance ‘c’ from the dovetail slot edge  125 . The first contour  232  is then outside of the insertion part contour  215  and is at least the critical distance ‘c’ from the dovetail slot edge  125 . The protection portion  220  in this embodiment is shaped to cover the critical area of the turbine wheel face  110 , which is the area from the dovetail slot edge  125  to the first contour  232  when the plug  20  is fully inserted into the turbine wheel  10 . In  FIGS. 5-10 , the critical distance ‘c’ is more clearly illustrated. 
     Preferably, an offset from the dovetail slot edge  125  to the first contour  232  is substantially constant. That is, the first contour  232  should follow the outline of the dovetail slot edge  125 . This offset should be at least the critical distance ‘c’ and most preferably at ‘c’. This allows the maximum area of the turbine wheel face  110  to be protected while still meeting critical area requirement. This is a vast improvement over the conventional adhesive tape method in which it is difficult, and most certainly impracticable, to shape the tapes to match the shape of the dovetail slots  120 . Also, the offset from the first contour  232  to the second contour  234  should be substantially constant, again to provide nice coating transitions. 
     Generally, if critical areas are required, then the first contour  232  is outside the dovetail slot edge  125 , preferably at a constant distance ‘c’. But on the other hand, if there is no critical area requirement, then the protection portion  220  need not be provided. If the protection portion  220  is not provided, then the first contour  232  can coincide with the dovetail slot edge  125 . This again maximizes the area of the turbine wheel  110  being protected while at the same time, preventing the dovetail slot  120  from being coated. 
     In  FIGS. 4 ,  5  and  7 , it is seen that the plug  20  includes a flange part  26  connected to the insertion part  24  and to the protrusion part  22 . The flange part  26  is shaped such that when the plug  20  is fully inserted into the turbine wheel  10 , at least a part of the turbine wheel surface  130  along the predetermined insertion depth. The flange part  26  is at a height ‘h’ above the turbine wheel surface  130  when inserted. 
       FIG. 12  illustrates a non-limiting example flow chart of a method  1200  to form the plug  20 . In step  1210 , the insertion part  24  of the plug  20  is formed in a shape to be axially inserted into the dovetail slot from a turbine wheel face to a predetermined insertion depth when the plug is fully inserted into the turbine wheel. In step  1220 , the protrusion part  22  is formed in a shape to axially protrude from the turbine wheel face when the plug is fully inserted into the turbine wheel. 
       FIG. 13  illustrates an example method to implement step  1220 . In step  1310 , the blast portion  210  is formed to be connected to the insertion part  24 , the protection portion  220  is formed in step  1320 , and the shadow portion  230  is formed in step  1330 . If the protection portion  220  is not necessary, then only the steps  1310  and  1330  can be performed. As discussed above, the shadow portion  230  is formed such that the shadow angle formed between the shadow surface  236  and the turbine wheel face  110  is less than 90°. Other details of forming the plug  20  is straight forward from the detailed description of the plug  20  provided above with reference to  FIGS. 4-10 . 
       FIG. 14  illustrates a non-limiting example flow chart of a method  1400  to apply protective coating on the turbine wheel. In step  1410 , the inventive plugs  20  as described above are inserted into the dovetail slots  120  of the turbine wheel  10 . Subsequently, the protective coating is applied on the turbine wheel in step  1420 . 
     Recall that due to the advantageous features of the plugs  20 , there is no need to perform post processing to profile the protective coating. Also, in step  1420 , a single coating may be applied. That is, multiple coating is not necessary. 
     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.