Abstract:
A compressor forward stub shaft comprising a plurality of axially spaced, annular rows of dovetail grooves, at least a first and a second of the plurality of rows having dovetail slots coated in part with an anti-wear coating.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to rotating machine technology, and specifically, to the mounting of compressor blades in rotor dovetail slots. 
   Typically, wear coatings are applied to the dovetail portions of compressor blades in order to reduce compressive stresses and wear between the blades and the compressor wheel dovetail slots. Such coatings have been applied to the blade dovetails primarily due to the fact that coatings are easily applied here. In practice, for example, the blade itself is masked, and there is a direct line of sight for spraying the coating on the blade dovetail and if curing is required, the blades can be easily handled and moved through an oven. Some typical wear coatings, however, such as MoS2 and other generally similar coatings, are not compatible with a typical steel C450 alloy used for the blades. In fact, coatings applied to C450 alloy material can degrade the corrosion fatigue resistance of that material. 
   Accordingly, there remains a need to provide wear resistance between compressor blades and compressor wheel dovetails without jeopardizing the corrosion fatigue resistance of the blade. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In the exemplary embodiment of this invention, the wear coating is applied directly onto the compressor wheel dovetail slots in order to minimize, if not eliminate, potential corrosive conditions that could develop between the coatings and the blade material used in certain gas turbines. More specifically, in one exemplary embodiment, an Alumazite-ZD coating is applied directly to the dovetail slots in stages  1  and  2  of the compressor forward stub shaft, where the above noted problem has been identified. This is a particularly advantageous solution since the coating itself is a conventional coating used to reduce wear and crush stresses, and no redesign of any component parts is necessary. 
   Accordingly, in one aspect, the present invention relates to a compressor forward stub shaft comprising a plurality of axially spaced, annular rows of dovetail grooves, at least a first and a second of the plurality of rows having dovetail slots coated in part with a wear-resistant coating. 
   In another aspect, the present invention relates to a turbine compressor shaft having a plurality of dovetail slots formed about a periphery of the shaft supporting at least one annular row of blades, each dovetail slot supporting a blade having an airfoil portion and a dovetail mounting portion received in the dovetail slot wherein a portion of each dovetail slot has an anti-wear coating applied thereto. 
   In still another aspect, the invention relates to a turbine compressor shaft having a plurality of dovetail slots formed about a periphery of the shaft supporting at least one annular row of blades, each dovetail slot supporting a blade having an airfoil portion and a dovetail mounting portion received in the dovetail slot, wherein each dovetail slot includes a pair of inwardly projecting tangs and a pair of outwardly directed grooves connected by substantially flat transition surfaces; and further wherein a thermoplastic aluminum pigmented coating is applied to said transition surfaces. 
   In a further aspect, the compressor shaft supports at least three annular rows of blades. 
   The invention will now be described in connection with the drawings identified below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a conventional compressor forward stub shaft; 
       FIG. 2  is a perspective view of a conventional compressor blade; 
       FIG. 3  is an enlarged partial detail of a first compressor stage, taken from  FIG. 1 , and showing a coating applied to the dovetail slot in accordance with an exemplary embodiment of the invention; and 
       FIG. 4  is a simplified partial end view of a dovetail slot indicating surface areas where a coating has been applied. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a conventional compressor forward stub shaft  10  formed with six integral, annular rows of dovetail slots  12 ,  14 ,  16 ,  18 ,  20  and  22 , each slot  24  configured to support a compressor blade having a mating dovetail portion. Each row of blades represents a stage of the compressor stub shaft and, with respect to this invention, it is the first two rows or stages  12  and  14  that are of particular interest. 
   A typical first stage compressor blade  26  is shown in  FIG. 2 . The blade includes an airfoil  28 , platform  30  and dovetail or dovetail mounting portion  32 , shaped to be received in a corresponding dovetail slot  24  ( FIG. 1 ). In the past, a wear-resistant coating (shown generally in phantom) has been applied to the surfaces  34  (one shown) on opposite sides of the dovetail  32 . As already noted above, however, certain coatings, such as Alumazite ZD, is not compatible with C450 steel alloy material used for the blades  26 . As a result, the corrosion and fatigue resistance of the blade can be degraded. 
   Turning to  FIGS. 3 and 4 , the first stage  12  of the forward stub shaft  10  is shown partially, but in greater detail. Each dovetail slot  24  is formed with a pair of inwardly directed tangs  36  and a pair of outwardly directed grooves  38 , connected by a flat base surface  40 . Note that the first dovetail slot  24  in  FIG. 3  has been sectioned through the radial centerline of the slot for ease of understanding the location of the wear-resistant coating. In the exemplary embodiment, an anti-wear coating  41  applied over the full axial length of a substantially flat surface  42  that serves as a transition between the convex radius of the dovetail tang  36  and the concave radius of the dovetail groove  38 . It will be appreciated that there are two such laterally opposed transition surfaces  42  in each dovetail slot  24  (see  FIG. 4 ), and that each slot  24  about the full 360° extent of the row of slots is similarly coated. 
   In the exemplary embodiment, for a stub shaft composed of a NiCrMoV alloy, a thermoplastic aluminum-pigmented coating is applied to a thickness of between 0.0008 to 0.0018 inch. One such coating is commercially available under the name Alumazite ZD, manufactured by Tiodize Co., Inc. This coating (or similar suitable coating) prevents galvanic and environmental oxidation, and is compatible with the NiCrMoV dovetail material. The coating may be applied by conventional spray techniques, recognizing that tooling must be adapted to access the underside of the dovetail tangs. 
   Test results to date confirm that coating the dovetail slots  24  is a feasible technique for avoiding the previously experienced degradation of corrosion fatigue resistance properties of the blade dovetails  32  as described above. For example, an Alumazite ZD-coated NiCrMoV material was exposed to salt fog for 405 hours, and there was no corrosion under the coating with only minor attack at the root where the coating was cut with a knife to expose the material. Wear tests were also conducted between coated NiCrMoV and GT-450, demonstrating no evidence of pitting after 5000 cycles and low friction. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.