Patent Publication Number: US-10323520-B2

Title: Platform cooling arrangement in a turbine rotor blade

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure is directed to a cooling arrangement and method of cooling a turbine rotor blade. More particularly, the present disclosure is directed to a cooling arrangement and method of cooling a platform region of a turbine rotor blade. 
     BACKGROUND OF THE DISCLOSURE 
     Certain components, such as gas turbine components operate at high temperatures and under harsh conditions. Cooling passages may be formed in gas turbine components to help circulate coolant for extending the service life of these components. However, incorporating cooling passages, such as by casting, is expensive. 
     BRIEF DESCRIPTION OF THE DISCLOSURE 
     In an exemplary embodiment, a platform cooling arrangement in a turbine rotor blade has a platform at an interface between an airfoil and a root. The rotor blade includes an interior cooling passage formed therein that extends from a connection with a coolant source at the root to at least the approximate radial height of the platform. In operation, the interior cooling passage includes a high-pressure coolant region in fluid communication with a corresponding high-pressure coolant region of the platform, the high-pressure coolant region of the platform extending to a low-pressure coolant region of the platform at least one of a pressure side slashface and a suction side slashface. The platform cooling arrangement includes a platform slot formed through at least one of the pressure side slashface and the suction side slashface, the platform slot being in fluid communication with the high-pressure coolant region of the turbine rotor blade. The platform cooling arrangement further provides an insert inserted in the platform slot, the insert having a blind channel extending inside the insert from a predetermined location of the insert, the insert aligns with the platform slot to fluidly connect the channel to the high-pressure coolant region at the predetermined location. The platform cooling arrangement further provides at least one passage in fluid communication with the channel and an exterior region of the turbine rotor blade. 
     In another exemplary embodiment, a method of creating a platform cooling arrangement for a turbine rotor blade having a platform at an interface between an airfoil and a root. The rotor blade includes an interior cooling passage formed therein that extends from a connection with a coolant source at the root to at least the approximate radial height of the platform. In operation, the interior cooling passage includes a high-pressure coolant region in fluid communication with a corresponding high-pressure coolant region of the platform, the high-pressure coolant region of the platform extending to a low-pressure coolant region of the platform at least one of a pressure side slashface and a suction side slashface. The method includes the steps of forming a platform slot through at least one of the pressure side slashface and the suction side slashface, the platform slot being in fluid communication with the high-pressure coolant region of the turbine rotor blade. The method further includes forming an insert that includes a blind channel extending inside of the insert from a predetermined location of the insert. The method further includes installing the insert within the platform slot such that the insert aligns with the platform slot to fluidly connect the channel to the high-pressure region at the predetermined location. The method further includes forming at least one passage in fluid communication with the channel and an exterior surface of the turbine rotor blade. 
     Other features and advantages of the present disclosure will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of an exemplary turbine rotor blade in which embodiments of the present disclosure may be employed. 
         FIG. 2  illustrates an underside view of a turbine rotor blade in which embodiments of the present disclosure may be used. 
         FIG. 3  illustrates a sectional view of neighboring turbine rotor blades having a cooling system according to conventional design. 
         FIG. 4  illustrates a cross-sectional view of a turbine rotor blade having a platform with interior cooling channels according to conventional design. 
         FIG. 5  illustrates a top view of a turbine rotor blade having a platform with interior cooling channels according to an alternative conventional design. 
         FIG. 6  illustrates a perspective view of a turbine rotor blade and platform insert in disassembled state according to an exemplary embodiment of the present disclosure. 
         FIG. 7  illustrates a top perspective view of the platform with partial cross-sectional view of the turbine rotor blade and platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 8  illustrates a cross-sectional view of the platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 9  illustrates a top perspective view of the channel of the platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 10  illustrates a cross-sectional view of the channel of the platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 11  illustrates a cross-sectional view of the channel of the platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 12  illustrates an upper perspective partial cutaway view of the turbine rotor blade and platform insert according to an exemplary embodiment of the present disclosure. 
         FIG. 13  illustrates a sectional view of neighboring turbine rotor blades having a cooling system according to an exemplary embodiment of the present disclosure. 
         FIG. 14  illustrates a partial enlarged view of one neighboring turbine rotor blades taken from region  14  of  FIG. 13  according to an exemplary embodiment of the present disclosure. 
     
    
    
     Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Provided is a platform cooling arrangement  101  ( FIG. 1 ) and method of creating a platform cooling arrangement for a turbine rotor blade  100 . The platform cooling arrangement  101  and method of creating a platform cooling arrangement includes utilizing a platform slot  134  ( FIG. 6 ) formed through at least one of the pressure side slashface  126  ( FIG. 4 ) and the suction side slashface  122  ( FIG. 4 ) of a platform at an interface between an airfoil  102  ( FIG. 1 ) and a root  104  of the turbine rotor blade  100 , the platform slot  134  ( FIG. 7 ) being in fluid communication with the high-pressure coolant region  116  ( FIG. 7 ) of the turbine rotor blade  100 . An insert  130  ( FIG. 7 ) is inserted in the platform slot  134 , the insert having a blind channel  140  ( FIG. 7 ) extending inside the insert from a predetermined location of the insert. The insert  130  aligns with the platform slot  134  to fluidly connect the channel  140  to the high-pressure coolant region  116  at the predetermined location to provide cooling for the turbine rotor blade at reduced cost. 
     Referring to  FIGS. 1 and 2 , turbine rotor blades  100  generally include an airfoil portion or airfoil  102  and a root portion or root  104 . The airfoil  102  may be described as having a convex suction face  105  and a concave pressure face  106 . The airfoil  102  further may be described as having a leading edge  107 , which is the forward edge, and a trailing edge  108 , which is the aft edge. The root  104  may be described as having structure (which, as shown, typically includes a dovetail  109 ) for affixing the blade  100  to the rotor shaft, a platform  110  from which the airfoil  102  extends, and a shank  112 , which includes the structure between the dovetail  109  and the platform  110 . 
     As illustrated, the platform  110  may be substantially planar. (Note that “planar,” as used herein, means approximately or substantially in the shape of a plane. For example, one of ordinary skill in the art will appreciate that platforms may be configured to have an outboard surface that is slightly curved and convex, with the curvature corresponding to the circumference of the turbine at the radial location of the rotor blades. As used herein, this type of platform shape is deemed planar, as the radius of curvature is sufficiently great to give the platform a flat appearance.) More specifically, the platform  110  may have a planar topside  113 , which, as shown in  FIG. 1 , may include an axially and circumferentially extending flat surface  115 . As shown in  FIG. 2 , the platform  110  may have a planar underside  114 , which may also include an axially and circumferentially extending flat surface  118 . The topside  113  and the bottom side  114  of the platform  110  may be formed such that each is substantially parallel to the other. As depicted, it will be appreciated that the platform  110  typically has a thin radial profile, i.e., there is a relatively short radial distance between the topside  113  and the bottom side  114  of the platform  110 . 
     In general, the platform  110  is employed on turbine rotor blades  100  to form the inner flow path boundary of the hot gas path section of the gas turbine. The platform  110  further provides structural support for the airfoil  102 . In operation, the rotational velocity of the turbine induces mechanical loading that creates highly stressed regions along the platform  110  which, when coupled with high temperatures, ultimately cause the formation of operational defects, such as oxidation, creep, low-cycle fatigue cracking, and others. These defects, of course, negatively impact the useful life of the rotor blade  100 . It will be appreciated that these harsh operating conditions, i.e., exposure to extreme temperatures of the hot gas path and mechanical loading associated with the rotating blades, create considerable challenges in designing durable, long-lasting rotor blade platforms  110  which both perform well and are cost-effective to manufacture. 
     One common solution to make the platform region  110  more durable is to cool it with a flow of compressed air or other coolant during operation, and a variety of these type of platform designs are known. However, as one of ordinary skill in the art will appreciate, the platform region  110  presents certain design challenges that make it difficult to cool in this manner. In significant part, this is due to the awkward geometry of this region, in that, as described, the platform  110  is a periphery component that resides away from the central core of the rotor blade and typically is designed to have a structurally sound, but thin radial thickness. 
     To circulate coolant, rotor blades  100  typically include one or more hollow cooling passages  116  (see  FIGS. 3, 4 and 5 ) which, at minimum, extend radially through the core of the blade  100 , including through the root  104  and the airfoil  102 . As described in more detail below, to increase the exchange of heat, such cooling passages  116  may be formed having a serpentine path that winds through the central regions of the blade  100 , though other configurations are possible. In operation, a coolant may enter the central cooling passages via one or more inlets  117  formed in the inboard portion of the root  104 . The coolant may circulate through the blade  100  and exit through outlets (not shown) formed on the airfoil and/or via one or more outlets (not shown) formed in the root  104 . The coolant may be pressurized, and, for example, may include pressurized air, pressurized air mixed with water, steam, and the like. In many cases, the coolant is compressed air that is diverted from the compressor of the engine, though other sources are possible. As discussed in more detail below, these cooling passages typically include a high-pressure coolant region and a low-pressure coolant region. The high-pressure coolant region typically corresponds to an upstream portion of the cooling passage that has a higher coolant pressure, whereas the low-pressure coolant region corresponds to a downstream portion having a relatively lower coolant pressure. 
     In some cases, the coolant may be directed from the cooling passages  116  into a cavity  119  formed between the shanks  112  and platforms  110  of adjacent rotor blades  100 . From there, the coolant may be used to cool the platform region  110  of the blade, a conventional design of which is presented in  FIG. 3 . This type of design typically extracts air from one of the cooling passages  116  and uses the air to pressurize the cavity  119  formed between the shanks  112 /platforms  110 . Once pressurized, this cavity  119  then supplies coolant to cooling channels that extend through the platforms  110 . After traversing the platform  110 , the cooling air may exit the cavity through film cooling holes formed in the topside  113  of the platform  110 . 
     It will be appreciated, however, that this type of conventional design has several disadvantages. First, the cooling circuit is not self-contained in one part, as the cooling circuit is only formed after two neighboring rotor blades  100  are assembled. This adds a great degree of difficulty and complexity to installation and pre-installation flow testing. A second disadvantage is that the integrity of the cavity  119  formed between adjacent rotor blades  100  is dependent on how well the perimeter of the cavity  119  is sealed. Inadequate sealing may result in inadequate platform cooling and/or wasted cooling air. A third disadvantage is the inherent risk that hot gas path gases may be ingested into the cavity  119  or the platform itself  110 . This may occur if the cavity  119  is not maintained at a sufficiently high pressure during operation. If the pressure of the cavity  119  falls below the pressure within the hot gas path, hot gases will be ingested into the shank cavity  119  or the platform  110  itself, which typically damages these components as they were not designed to endure exposure to the hot gas-path conditions. 
       FIGS. 4 and 5  illustrate another type of conventional design for platform cooling. In this case, the cooling circuit is contained within the rotor blade  100  and does not involve the shank cavity  119 , as depicted. Cooling air is extracted from one of the cooling passages  116  that extend through the core of the blade  110  and directed aft through cooling channels  120  formed within the platform  110  (i.e., “platform cooling channels  120 ”). As shown by the several arrows, the cooling air flows through the platform cooling channels  120  and exits through outlets in the aft edge  121  of the platform  110  or from outlets disposed along the suction side edge  122 . (Note that in describing or referring to the edges or faces of the rectangular platform  110 , each may be delineated based upon its location in relation to the suction face  105  and pressure face  106  of the airfoil  102  and/or the forward and aft directions of the engine once the blade  100  is installed. As such, as one of ordinary skill in the art will appreciate, the platform may include an aft edge  121 , a suction side edge  122 , a forward edge  124 , and a pressure side edge  126 , as indicated in  FIGS. 3 and 4 . In addition, the suction side edge  122  and the pressure side edge  126  also are commonly referred to as “slashfaces” and the narrow cavity formed therebetween once neighboring rotor blades  100  are installed may be referred to as a “slashface cavity”.) 
     It will be appreciated that the conventional designs of  FIGS. 4 and 5  have an advantage over the design of  FIG. 3  in that they are not affected by variations in assembly or installation conditions. However, conventional designs of this nature have several limitations or drawbacks. First, as illustrated, only a single circuit is provided on each side of the airfoil  102  and, thus, there is the disadvantage of having limited control of the amount of cooling air used at different locations in the platform  110 . Second, conventional designs of this type have a coverage area that is generally limited. While the serpentine path of  FIG. 5  is an improvement in terms of coverage over  FIG. 4 , there are still dead areas within the platform  110  that remain uncooled. Third, to obtain better coverage with intricately formed platform cooling channels  120 , manufacturing costs increase dramatically, particularly if the cooling channels having shapes that require a casting process to form. Fourth, these conventional designs typically dump coolant into the hot gas path after usage and before the coolant is completely exhausted, which negatively affects the efficiency of the engine. Fifth, conventional designs of this nature generally have little flexibility. That is, the channels  120  are formed as an integral part of the platform  110  and provide little or no opportunity to change their function or configuration as operating conditions vary. In addition, these types of conventional designs are difficult to repair or refurbish. 
     It will be appreciated that turbine blades that are cooled via the internal circulation of a coolant typically include an interior cooling passage  116  that extends radially outward from the root, through the platform region, and into the airfoil, as described above in relation to several conventional cooling designs. It will be appreciated that certain embodiments of the present disclosure may be used in conjunction with conventional coolant passages to enhance or enable efficient active platform cooling, and the present disclosure is discussed in connection with a common design: an interior cooling passage  116  having a winding or serpentine configuration. The serpentine path is typically configured to allow a one-way flow of coolant and includes features that promote the exchange of heat between the coolant and the surrounding rotor blade  100 . In operation, a pressurized coolant, which typically is compressed air bled from the compressor (though other types of coolant, such as steam, also may be used with embodiments of the present disclosure), is supplied to the interior cooling passage  116  through a connection formed through the root  104 . The pressure drives the coolant through the interior cooling passage  116 , and the coolant convects heat from the surrounding walls. 
     As the coolant moves through the cooling passage  116 , it will be appreciated that it loses pressure, with the coolant in the upstream portions of the interior cooling passage  116  having a higher pressure than coolant in downstream portions. As discussed in more detail below, this pressure differential may be used to drive coolant across or through cooling passages formed in the platform. It will be appreciated that the present disclosure may be used in rotor blades  100  having internal cooling passages of different configurations and is not limited to interior cooling passages having a serpentine form. Accordingly, as used herein, the term “interior cooling passage” or “cooling passage” is meant to include any passage or hollow channel through which coolant may be circulated in the rotor blade. As provided herein, the interior cooling passage  116  of the present disclosure extends to at least to the approximate radial height of the platform  116 , and may include at least one region of relatively higher coolant pressure (which, hereinafter, is referred to as a “region of high pressure” and, in some cases, may be an upstream section within a serpentine passage) and at least one region of relatively lower coolant pressure (which, hereinafter, is referred to as a “region of low pressure” and, relative to the region of high pressure, may be a downstream section within a serpentine passage). 
     In general, the various designs of conventional internal cooling passages  116  are effective at providing active cooling to certain regions within the rotor blade  100 . However, as one of ordinary skill in the art will appreciate, the platform region proves more challenging. This is due, at least in part, to the platform&#39;s awkward geometry—i.e., its narrow radial height and the manner in which it juts away from the core or main body of the rotor blade  100 . However, given its exposures to the extreme temperatures of hot gas path and high mechanical loading, the cooling requirements of the platform are considerable. As described above, conventional platform cooling designs are ineffective because they fail to address the particular challenges of the region, are inefficient with their usage of coolant, and/or are costly to fabricate. 
       FIGS. 6 through 11  provide several views of exemplary embodiments of the present disclosure. Referring to  FIG. 6 , a perspective view of a turbine rotor blade  100  and an insert  130  according to an embodiment of the present disclosure is provided. As shown, the present disclosure generally includes an insert  130  that is installed within a turbine rotor blade  100 . More specifically, the platform  110  of the rotor blade  100  may include a platform slot  134  that is formed so that the insert  130  fits therein. In one particularly suitable embodiment, as shown, the platform slot  134  may be positioned in the pressure side edge or slashface  126 , though other locations along the other edges of the platform  110  are also possible, such as the suction side slashface  122 . The platform slot  134  may have a generally rectangular shaped mouth, and may be described as including an outboard surface or ceiling  135  and an inboard surface or floor  136 . As shown, the mouth may be configured such that it is relatively thin in the radial direction and relatively wide in the axial direction. It will be appreciated that, from the mouth, the platform slot  134  extends circumferentially into the platform  110 , thereby forming a cavity therein. 
     The platform insert  130  may have a planar, thin, disk-like/plate shape and may be configured such that it fits within the platform slot  134  and, generally, has a similar profile (i.e., the vantage point of  FIG. 7 ) as the platform slot  134 . 
     The shape of the platform slot  134  may vary. In a particularly suitable embodiment, as more clearly shown in  FIGS. 6 and 7 , the platform slot  134  may extend circumferentially from the pressure side slashface or edge  126 . It will be appreciated that the platform slot  134 , in this particularly suitable embodiment, narrows as it extends from the pressure side slashface  126  toward the center of the platform  110 . The narrowing may generally correspond to the curved profile that is formed at the junction of the airfoil pressure face  106  and the platform  110 . As such, in profile (i.e., the shape from the vantage point of  FIG. 7 ), the platform slot  134  may have a curved back or inner wall that relates closely to the curved profile of the airfoil pressure face  106 . It should be apparent to those skilled in the art that other configurations of the platform slot  134  also may be employed. However, it will be appreciated that the embodiments of  FIGS. 6 through 11  effectively address the cooling requirements for a large coverage area, which includes some of the more difficult areas within the platform  110  to cool. Those of ordinary skill in the art will appreciate that other performance advantages and efficiencies are possible. 
     Referring back to  FIG. 7 , once insert  130  is aligned, inserted and then secured, such as by brazing once the insert is fully inserted inside of platform slot  134 , a passage  146  in fluid communication with cooling passage  116  of turbine rotor blade  100  is aligned with an opening  148  at a predetermined location  137  ( FIG. 8 ) of insert  130  that is in fluid communication with a blind channel  140 . In one embodiment, insert  130  is permanently secured in platform slot  134 . In one embodiment, insert  130  is non-permanently secured in platform slot  134 . As a result, during operation, coolant in cooling passage  116  is urged to flow via passage  146 , through opening  148  at predetermined location  137  of insert  130  into channel  140  which has a lower pressure compared to cooling passage  116 . As further shown in  FIG. 7 , insert  130  includes a surface  133  that is opposite opening  148  at predetermined location  137 . In one embodiment, surface  133  is substantially flush or coincident with pressure side edge or slashface  126  such that coolant can flow from channel  140  through passages  150  toward a slashface of a platform  110  of an adjacent blade  100 . In one embodiment, a tab or protrusion  128  extends outwardly from surface  133  to assist with inserting insert  130  inside of platform slot  134 , with surface  133  being recessed relative to slashface  126 . In one embodiment, a tab or protrusion  128  may also be functionally related to a corresponding slashface of the neighboring blade for securing insert  130  and position in platform slot  134 . In one embodiment, tab or protrusion  128  may not be functionally related to a corresponding slashface of the neighboring blade. 
       FIG. 8  shows blind channel  140  formed inside of insert  130 . The term “blind” means that only one end of the channel positioned inside of the insert extends to an opening formed in an exterior surface of the insert. For example, as shown, channel  140  extends to opening  148  at predetermined location of  137 . In one embodiment, a plurality of channels (not shown) may be formed in the insert, with each channel extending to a different cooling passage  116  or to different portions of the same cooling passage  116 . In one embodiment, each channel of the plurality of channels may operate separately of another channel, and none of the channels intersect. In one embodiment, at least a portion of one channel may intersect with another channel. 
     As further shown in  FIG. 8 , channel  140  includes at least one passage  141  that extends through surface  131  of insert  130 , such as by a corresponding opening  142  formed through surface  131  which is opposite surface  132  facing a build plate  138  onto which the insert is formed or manufactured. During manufacturing of insert  130 , such as by a suitable additive manufacturing process, passageway(s)  141  are formed in insert  130  to permit the removal of “loose” material, such as residual unfused powder from the additive manufacturing process, such as by introducing a pressurized fluid to predetermined location  137 . Such residual material may otherwise obstruct flow through channel  140  and degrade cooling performance. A monitoring process, which may include an x-ray, may be used to determine if channel obstruction remains. Additive manufacturing processes include, but are not limited to, direct metal laser melting, direct metal laser sintering, selective laser sintering, direct metal laser sintering, laser engineered net shaping, selective laser sintering, selective laser melting, electron beam welding, used deposition modeling or a combination thereof. 
       FIG. 8  further shows passages  150  formed through surface  133  in fluid communication with channel  140 , surface  133  being opposite predetermined location  137  of the channel, for providing cooling along slashface  126 . 
       FIGS. 9 and 10  show embodiments of cross-sections taken substantially perpendicular to the longitudinal direction in which the channel  140  ( FIG. 8 ) extends. As shown in  FIG. 9 , channel  140  resembles a generally rectangular profile, while as shown in  FIG. 10 , the channel resembles a teardrop profile. It is to be appreciated that the channel may resemble other profiles. As further shown, channel  140  includes flow modification features  144  to enhance cooling performance, such as by altering flow characteristics, e.g., creating turbulent flow. As shown in  FIG. 9 , flow modification features  144  may be formed along a portion of a periphery of channel  140 . Flow modification features  144  may protrude into the channel or may be recessed into the peripheral wall of the channel. As shown in  FIG. 10 , flow modification features  144  may be continuously formed along the entire periphery of at least a portion of channel  140 . In one embodiment, at least a portion of flow modification features  144  may extend generally perpendicular to the longitudinal length of the channel  140 . In one embodiment, at least a portion of flow modification features may extend anywhere between generally perpendicular to the longitudinal length of channel  140  and generally parallel to longitudinal length channel  140 . It is to be appreciated that in one embodiment, the size, shape and profile of channels, including flow modification features may remain generally uniform, i.e. uniform cross-section, and that in other embodiments, one or more portions of the channel may have differences in at least one of size, shape, and profile of channels compared to other portions of the channel. In one embodiment, during manufacture of insert  130 , at least a portion of channel  140  is constructed to resemble a teardrop profile, with profile changing during the manufacturing process, i.e., deformation of the profile of the channel to resemble a generally circular profile. It is to be understood that other profiles may be utilized that made to form into other predetermined channel profiles. 
       FIG. 11  shows flow modification features  144  forming a lattice along at least a portion of the longitudinal length of channel  140  with the flow modification features intersecting each other. In one embodiment, the lattice may be formed via pre-determined additive manufacturing process algorithms. In one embodiment, the flow modification features  144  form a pattern, such as an X-shaped pattern. In one embodiment, the flow modification features  144  form a predetermined arrangement that is not a repeating pattern. In one embodiment, the flow modification features  144  do not intersect. In one embodiment, the flow modification features  144  are generally straight. In one embodiment, the flow modification features  144  are curved. In one embodiment, the flow modification features  144  comprise a single member extending between different points along the periphery of the channel. In one embodiment, the flow modification features  144  may vary in width. The arrangement of flow modification features depends upon the application and channel parameters, including shape, size, pressure drop, etc., for optimizing cooling of the turbine rotor blade  100 . 
       FIG. 12  shows an exemplary arrangement of cooling passages  150  formed in insert  130  and cooling passages  139  extending through both insert  130  and platform  110  in fluid communication with channel  140 . Cooling passages  139  are in direct fluid communication or fluid communication with an exterior region of blade  100  such as an exterior surface of airfoil  102  for providing enhanced cooling to the blade. 
       FIG. 13  and  FIG. 14 , which is an enlarged partial view taken along region  14  of  FIG. 13 , collectively show an exemplary flow path for providing cooling air to platform  110 . Cooling air from cooling passage  116  flows through an aperture formed in blade  100  into cavity  119 . By virtue of an aperture  152  formed through underside  114  of platform  110  and facing surface of insert  130 , cooling air in cavity  119  is in fluid communication with and flows into channel  140 , thereby providing cooling to the platform before exiting the insert via passages  150  as previously discussed. In one embodiment, aperture  152  is formed in platform  110  and insert  130  after the insert has been installed in platform slot  134 . In one embodiment, corresponding portions of aperture  152  are formed in each of platform  110  and insert  130  prior to insertion of the insert in platform slot  134 , with the corresponding portions of aperture  152  being aligned with each other. 
     While the disclosure has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.