Patent Publication Number: US-8523525-B2

Title: Snubber assembly for turbine blades

Description:
This invention was made with U.S. Government support under Contract Number DE-FC26-05NT42644 awarded by the U.S. Department of Energy. The U.S. Government has certain rights to this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a snubber assembly for turbine blades in a turbine engine, and, more particularly, to a snubber assembly that reduces circumferential loading imparted on sidewalls of the turbine blades during operation of the turbine engine. 
     BACKGROUND OF THE INVENTION 
     A turbomachine, such as a steam or gas turbine is driven by a hot working gas flowing between rotor blades arranged along the circumference of a rotor so as to form an annular blade arrangement, and energy is transmitted from the hot working gas to a rotor shaft through the rotor blades. As the capacity of electric power plants increases, the volume of flow through industrial turbine engines has increased more and more and the operating conditions (e.g., operating temperature and pressure) have become increasingly severe. Further, the rotor blades have increased in size to harness more of the energy in the working gas to improve efficiency. A result of all the above is an increased level of stresses (such as thermal, vibratory, bending, centrifugal, contact and torsional) to which the rotor blades are subjected. 
     In order to limit vibrational stresses in the blades, various structures may be provided to the blades to form a cooperating structure between blades that serves to dampen the vibrations generated during rotation of the rotor. For example, mid-span snubber structures, such as cylindrical standoffs, may be provided extending from mid-span locations on the blades for engagement with each other. Two mid-span snubber structures are located at the same height on either side of a blade with their respective contact surfaces pointing in opposite directions. The snubber contact surfaces on adjacent blades are separated by a small space when the blades are stationary. However, when the blades rotate at full load and untwist under the effect of the centrifugal forces, snubber surfaces on adjacent blades come in contact with each other to dampen vibrations by friction at the contacting snubber surfaces. A disadvantage of snubber damping is that the large bending stresses associated with large diameter blades typically necessitates larger snubber structures for mechanical stability to avoid outward bending of the snubber structure, resulting in increased bending stresses on the blade surfaces supporting the snubber. Specifically, the bending stresses of the snubber structures are transferred to the respective blade pressure and suction sidewalls, which can cause damage to the sidewalls, resulting in repair or replacement of the blades. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a snubber assembly is provided. The snubber assembly is associated with a rotatable turbine blade in a turbine engine, the turbine blade including a pressure sidewall and a suction sidewall opposed from the pressure wall. The snubber assembly comprises a first snubber structure associated with the pressure sidewall of the turbine blade, a second snubber structure associated with the suction sidewall of the turbine blade, and a support structure. The support structure extends through the blade and is rigidly coupled at a first end portion thereof to the first snubber structure and at a second end portion thereof to the second snubber structure. Centrifugal loads exerted by the first and second snubber structures caused by rotation thereof during operation of the engine are at least partially transferred to the support structure, such that centrifugal loads exerted on the pressure and suctions sidewalls of the turbine blade by the first and second snubber structures are reduced. 
     In accordance with another aspect of the invention, a method is provided of affixing a snubber assembly to a rotatable turbine blade of a turbine engine. The turbine blade includes a pressure sidewall and a suction sidewall opposed from the pressure sidewall and has a bore formed therein extending from the pressure sidewall through the turbine blade to the suction sidewall. A support structure is inserted into the bore in the turbine blade such that a first end portion of the support structure extends outwardly from the turbine blade pressure sidewall and a second end portion of the support structure extends outwardly from the turbine blade suction sidewall. The support structure is secured to the turbine blade within the bore. A first snubber structure is coupled to the first end portion of the support structure. A second snubber structure is coupled to the second end portion of the support structure. Centrifugal loads exerted by the first and second snubber structures caused by rotation thereof during operation of the engine are at least partially transferred to the support structure such that centrifugal loads exerted on the pressure and suctions sidewalls of the turbine blade by the first and second snubber structures are reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is a partial end view of a rotor, as viewed in an axial flow direction, taken in a plane perpendicular to an axis of rotation and showing an embodiment of the invention; 
         FIG. 2  is view taken on the plane indicated by the line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a view similar to that of  FIG. 2  wherein a snubber assembly according an embodiment of the invention has been removed; 
         FIG. 4  is a view of the snubber assembly removed from the turbine blade of  FIG. 3 ; 
         FIG. 5  is a view taken on the plane indicated by the line  5 - 5  in  FIG. 4 ; and 
         FIG. 6  is a flow chart illustrating exemplary steps for affixing a snubber assembly to a turbine blade according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
     Referring to  FIG. 1 , a section of a rotor  10  is illustrated for use in a turbomachine (not shown), such as for use in a gas or steam turbine engine. The rotor  10  comprises a rotor disc  12  and a plurality of blades  14 , illustrated herein as a first blade  14   a  and an adjacent second blade  14   b . The blades  14   a ,  14   b  comprise radially elongated structures extending from a blade root  16  engaged with the rotor disc  12 , to a blade tip  18 . Each of the blades  14   a ,  14   b  includes a pressure sidewall  20  and a suction sidewall  22  opposed form the pressure sidewall  20 . Each of the blades  14   a ,  14   b  further includes a snubber assembly  24  located mid-span between the blade root  16  and the blade tip  18  of each of the blades  14   a ,  14   b.    
     The snubber assembly  24  associated with the first blade  14   a  will now be described, it being understood that the snubber assemblies  24  of the other blades  14  are substantially identical to the snubber assembly  24  described herein. As most clearly shown in  FIG. 4 , the snubber assembly  24  comprises a first snubber structure  26 , a second snubber structure  28 , and a support structure  30 . The first and second snubber structures  26 ,  28  may comprise a nickel based alloy, such as, for example, CM247-DS or PWA1483. The support structure  30  may also comprise a nickel based alloy, such as, for example, INCONEL 718 (INCONEL is a registered trademark of Special Metals Corporation, located in New Hartford, N.Y.) It is noted that the material selected for the first and second snubber structures  26 ,  28  preferably has good oxidation, corrosion, and/or creep resistance and the material selected for the support structure  30  is preferably a high strength material. It is also noted that it may be preferable to form both the first and second snubber structures  26 ,  28  and the blade  14   a  from the same/similar material, but to form the support structure  30  from a different material than the first and second snubber structures  26 ,  28  and the blade  14   a . Hence, the material properties of these components can be closely matched to the requirements of the respective components. For example, since the support structure  30  is not directly exposed to the high temperature gases flowing through the engine, it need not have as good of oxidation, corrosion, and/or creep resistance as the first and second snubber structures  26 ,  28  and the blade  14   a , which are directly exposed to the high temperature gases flowing through the engine. Moreover, since bending loads are transferred to the support structure  30 , as will be discussed herein, the support structure  30  is preferably formed from a high strength material. 
     Referring back to  FIG. 1 , the first snubber structure  26  is associated with and extends outwardly from the pressure sidewall  20  of the first blade  14   a  toward the suction sidewall  22  of the second blade  14   b . As shown in  FIGS. 1 and 2 , the first snubber structure  26  includes a base portion  31  that is abutted against a first fillet  32 , which first fillet  32  in the embodiment shown is integral with the pressure sidewall  20  of the first blade  14   a . The first fillet  32  may act as a landing area for receiving the base portion  31  of the first snubber structure  26  during the assembly of the snubber assembly  24 , as will be discussed in greater detail herein. In a preferred embodiment, the base portion  31  is in contact with but not affixed to the fillet  32 , although the base portion  31  could be affixed to the fillet  32  if desired. 
     As shown in  FIGS. 1 and 2 , the first snubber structure  26  is a tapered cylindrical-shaped member having an outer diameter D 1  that decreases as the first snubber structure  26  extends away from the pressure sidewall  20 , although it is understood that the first snubber structure  26  could have a generally constant outer diameter D 1  and could have other shapes as desired, such as, for example, elliptical, airfoil-shaped, etc. 
     An end portion  34  of the first snubber structure  26  in the embodiment shown defines a first angled surface  34   a . The first angled surface  34   a  is spaced from a corresponding second angled surface  64   a  of a second snubber structure  28  of the adjacent second blade  14   b , such that a first space S 1  is formed therebetween, see  FIG. 1 . As will be described below, during operation of the engine, as the blades  14  rotate they are “untwisted” slightly, such that the first angled surface  34   a  of the snubber assembly  24  of the first blade  14   a  comes into contact with the second angled surface  64   a  of the snubber assembly  24  of the second blade  14   b.    
     As shown in  FIG. 4 , the first snubber structure  26  includes an inner wall  40  that defines a hollow interior portion  42 . The support structure  30  is received within the hollow interior portion  42  and affixed to the inner wall  40  as will be described in detail herein. The hollow interior portion  42  extends from the open end of the base portion  31  to an inner endwall  44  of the first snubber structure  26  that is located proximate to the end portion  34  thereof. It is noted that the inner endwall  44  could be located closer to the first blade  14   a  if desired, depending on the length of the support structure  30 . 
     Referring to  FIG. 4 , the end portion  34  of the first snubber structure  26  includes a cooling fluid exit aperture  46  formed therein. The aperture  46  allows cooling fluid located in a first gap G 1 , described below, to escape out of the first snubber structure  26 . The cooling fluid may be provided into the first gap G 1  from the support structure  30 , which support structure  30  may receive the cooling fluid from an interior cooling fluid channel  48  located within the first blade  14   a , see  FIG. 1 . Additional details in connection with the cooling fluid in the support structure  30  will be discussed in detail herein. It is noted that the location and number of cooling fluid exit apertures  46  formed in the first snubber structure  26  may vary as desired. 
     Referring to  FIG. 2 , the first snubber structure  26  includes antirotation structure  50 , illustrated herein as an antirotation tab that extends outwardly from the base portion  31  toward the pressure sidewall  20  of the first blade  14   a . The antirotation structure  50  is received in a corresponding indentation  52  formed in the fillet  32  (see also  FIG. 3 ) such that the first snubber structure  26  is prevented from rotating with respect to the first blade  14   a  during operation of the engine. 
     Referring back to  FIG. 1 , the second snubber structure  28  is associated with and extends outwardly from the suction sidewall  22  of the first blade  14   a  toward the pressure sidewall (not shown) of an adjacent blade (not shown). As shown in  FIGS. 1 and 2 , the second snubber structure  28  includes a base portion  60  that is abutted against a second fillet  62 , which second fillet  62  in the embodiment shown is integral with the suction sidewall  22  of the first blade  14   a . The second fillet  62  may act as a landing area for receiving the base portion  60  of the second snubber structure  28  during the assembly of the snubber assembly  24 , as will be discussed in greater detail herein. In the preferred embodiment, the base portion  60  is in contact with but not affixed to the fillet  62 , although the base portion  60  could be affixed to the fillet  62  if desired. 
     As shown in  FIGS. 1 and 2 , the second snubber structure  28  is a tapered cylindrical-shaped member having an outer diameter D 2  that decreases as the second snubber structure  28  extends away from the suction sidewall  22 , although it is understood that the second snubber structure  28  could have a generally constant outer diameter D 2  and could have other shapes as desired, such as, for example, elliptical, airfoil-shaped, etc. 
     An end portion  64  of the second snubber structure  28  in the embodiment shown defines a second angled surface  64   a , which second angled surface  64   a  is spaced from a corresponding first angled surface (not shown) of an adjacent snubber structure (not shown) of an adjacent blade (not shown) such that a second space (similar to the first space S 1  discussed above) is formed therebetween. 
     As shown in  FIG. 4 , the second snubber structure  28  includes an inner wall  70  that defines a hollow interior portion  72 . The support structure  30  is received within the hollow interior portion  72  and affixed to the inner wall  70  as will be described in detail herein. The hollow interior portion  72  extends from the open end of the base portion  60  to an inner endwall  74  of the second snubber structure  28  that is located proximate to the end portion  64  thereof. It is noted that the inner endwall  74  could be located closer to the first blade  14   a  if desired, depending on the length of the support structure  30 . 
     Referring to  FIG. 4 , the end portion  64  of the second snubber structure  28  includes a cooling fluid exit aperture  76  formed therein. The aperture  76  allows cooling fluid located in a second gap G 2 , described below, to escape out of the second snubber structure  28 . The cooling fluid may be provided into the second gap G 2  from the support structure  30 , which support structure  30  may receive the cooling fluid from the interior cooling fluid channel  48  located within the first blade  14   a , as noted above. It is noted that the location and number of cooling fluid exit apertures  76  formed in the second snubber structure  28  may vary as desired. 
     As shown in  FIG. 2 , the second snubber structure  28  includes antirotation structure  80 , illustrated herein as an antirotation tab that extends outwardly from the base portion  60  toward the suction sidewall  22  of the first blade  14   a . The antirotation structure  80  is received in a corresponding indentation  82  formed in the fillet  62  (see also  FIG. 3 ) such that the second snubber structure  28  is prevented from rotating with respect to the first blade  14   a  during operation of the engine. 
     Referring to  FIGS. 1 ,  2 ,  4 , and  5 , the support structure  30  comprises a generally cylindrical-shaped body member  88  having first and second tapered end portions  90 ,  92  and an intermediate portion  93  located between the first and second end portions  90 ,  92 . As shown in  FIG. 5 , the body member  88  is defined by a generally cylindrical, outer wall  94  and a web member  96  that extends within the outer wall  94  to divide a hollow interior portion  98  of the body member  88 . The web member  96  acts as an I-beam structure to provide structural rigidity to the support structure  30 . As shown in  FIGS. 1 ,  2 ,  4 , and  5 , the web member  96  extends in the radial direction, which improves load bearing of the support structure  30 . In particular, the web member  96  and the hollow interior portion  98  provide a stiff and light support structure  30 , which is used to bear centrifugal loads of the blade  14   a  during operation of the engine, as will be described in detail herein. 
     The intermediate portion  93  extends through a bore  95  formed in the blade  14   a  (see  FIGS. 1-3 ), which bore  95  is formed through the blade  14   a  from the pressure sidewall  20  to the suction sidewall  22 . The intermediate portion  93  is structurally coupled to the blade  14   a , such as, for example, by shrink fitting the intermediate portion  93  of the support structure  30  into the bore  95  of the blade  14   a , as will be described in detail herein. As shown in  FIG. 2 , an outer diameter D 3  of the intermediate portion  93  is substantially the same size as the bore  95  formed in the turbine blade  14   a.    
     The hollow interior portion  98  of the body member  88  acts as a flow path for cooling fluid that enters the support structure  30  through one or more cooling fluid holes  100  (see  FIGS. 2 ,  4 , and  5 ) that are formed in the body member  88 . The holes  100  provide fluid communication between respective passageways  48 A that branch off from the interior cooling fluid channel  48  located within the first blade  14   a  and the hollow interior portion  98  of the body member  88 . Specifically, the cooling fluid enters the interior cooling fluid channel  48  located within the first blade  14   a  and flows into the hollow interior portion  98  of the body member  88  through the passageways  48 A and the holes  100 , which holes  100  are aligned with the passageways  48 A during assembly of the snubber assembly  24 . The cooling fluid flowing within the hollow interior portion  98  of the body member  88  provides cooling to the support structure  30 . 
     The end portions  90 ,  92  of the support structure  30  define respective openings  90 A and  92 A (see  FIG. 4 ) so as to allow the cooling fluid in the hollow interior portion  98  of the body member  88  to flow out of the support structure  30  into the respective hollow interior portions  42 ,  72 , where the cooling fluid can provide cooling to the first and second snubber structures  26 ,  28 . 
     The first end portion  90  of the support structure  30  is received in the hollow interior portion  42  of the first snubber structure  26  and is coupled to the inner wall  40 , such as by brazing or otherwise bonded, as will be discussed in greater detail herein. As shown in  FIGS. 1 ,  2 , and  4 , the first end portion  90  is located in the hollow interior portion  42  of the first snubber structure  26  such that the first gap G 1  is formed between a first end surface  104  of the support structure  30  and the endwall  44  of the first snubber structure  26 , which endwall  44  and the first end surface  104  of the support structure  30  face one another. The first gap G 1  provides a flow path for the cooling fluid in the hollow interior portion  98  of the support structure  30  to the cooling fluid exit aperture  46  formed in the first snubber structure  26  so as to allow the cooling fluid to flow out of the snubber assembly  24 . 
     The second end portion  92  of the support structure  30  is received in the hollow interior portion  72  of the second snubber structure  28  and is coupled to the inner wall  70 , such as by brazing or otherwise bonded, as will be discussed in greater detail herein. As shown in  FIGS. 1 ,  2 , and  4 , the second end portion  92  is located in the hollow interior portion  72  of the second snubber structure  28  such that the second gap G 2  is formed between a second end surface  106  of the support structure  30  and the endwall  74  of the second snubber structure  28 , which endwall  74  and the second end surface  106  of the support structure  30  face one another. The second gap G 2  provides a flow path for the cooling fluid in the hollow interior portion  98  of the support structure  30  to the cooling fluid exit aperture  76  formed in the second snubber structure  28  so as to allow the cooling fluid to flow out of the snubber assembly  24 . 
     During operation of the engine, centrifugal forces are exerted on the first and second snubber structures  26 ,  28  as a result of the rotation of the rotor  10 . These centrifugal forces cause the blades  14  to “untwist”, which causes the first and second angled surfaces  34   a ,  64   a  of the respective snubber structures  26 ,  28  to move toward each other to engage each other with a damping force. It should be noted that it is desirable to configure the snubber structures  26 ,  28  to produce a damping force that is sufficient to produce damping at the interface between the snubber structures  26 ,  28  to control blade vibration. 
     As noted above, the damping forces create bending stresses, which, in prior art engines, are transferred from snubber structures to the blade pressure and suction sidewalls. However, according to aspects of the present invention, the majority of these bending stresses are transferred from the snubber structures  26 ,  28  to the support structure  30  and not to the blade pressure and suction sidewalls  20 ,  22 , such that stresses exerted on the blade pressure and suction sidewalls  20 ,  22  are reduced. 
     Specifically, since the snubber structures  26 ,  28  are directly coupled to the support structure  30 , the bending stresses exerted thereby are transferred from the snubber structures  26 ,  28  to the support structure  30  via the coupling of the support structure end portions  90 ,  92  to the inner walls  40 ,  70  of the respective snubber structures  26 ,  28 . Thus, damage to the blades  14  as a result of bending stresses from the snubber structures  26 ,  28  is believed to be reduced, and a lifespan of the blades  14  is believed to be increased by the snubber assemblies  24 . It is noted that, in the case of damage to or destruction of one or more of the components of the snubber assembly  24 , the damaged portion(s) can be removed and replaced without requiring replacement of the entire blade  14 . 
     Referring now to  FIG. 6 , a method  150  is illustrated for affixing a snubber assembly, such as the snubber assembly  24  described above with reference to  FIGS. 1-5 , to a turbine blade having a bore formed therein, such as the blade  14   a  with the bore  95  discussed above. 
     At step  152 , the outer diameter D 3  of the intermediate portion  93  of the support structure  30  is sized to be substantially the same size as the bore  95  in the turbine blade  14   a . The outer diameter D 3  of the intermediate portion  93  of the support structure  30  may be sized, for example, by grinding the outer wall  94  of the support structure  30  down to the correct diameter D 3 , e.g., by centerless grinding the intermediate portion  93 . 
     After the outer diameter D 3  of the of the intermediate portion  93  of the support structure  30  is sized at step  152 , the support structure  30  is cooled at step  154  to temporarily reduce the diameter D 3  of the intermediate portion  93  of the support structure  30 , such that the support structure  30  can be inserted into the bore  95  formed in the turbine blade  14   a . As one example, the support structure  30  may be disposed in liquid nitrogen to cool the support structure  30  down to a temperature of about −300° Fahrenheit. 
     Once the outer diameter D 3  of the support structure  30  is reduced by cooling at step  154 , the support structure  30  is inserted into the bore  95  in the turbine blade  14   a  at step  156 . The support structure  30  is inserted into the bore  95  in the turbine blade  14   a  such that the first end portion  90  of the support structure  30  extends outwardly from the turbine blade pressure sidewall  20  and the second end portion  92  of the support structure  30  extends outwardly from the turbine blade suction sidewall  22 . Also, if cooling of the snubber assembly  24  is desired during engine operation, the support structure  30  may be inserted into the bore  95  in the turbine blade  14   a  such that holes  100  of the support structure  30  are aligned with passageways  48 A that branch off from the interior cooling fluid channel  48  located within the blade  14   a . Thus, cooling fluid provided to the interior cooling fluid channel  48  located within the blade  14   a  may flow into the hollow interior portion  98  of the support structure  30  to provide cooling to the snubber assembly  24  as discussed above. 
     It should be noted that, prior to insertion of the support structure  30  into the bore  95  at step  156 , the support structure  30  may be turned to reduce at least a portion of the diameters D 1  and D 2  of the first and second end portions  90 ,  92  sufficiently to form a braze gap between the first and second end portions  90 ,  92  and the respective first and second snubber structures  24 ,  26  for receiving a brazing material. 
     The support structure  30  is then secured to the turbine blade  14   a  within the bore  95  at step  158 . Securing the support structure  30  to the turbine blade  14   a  may comprise, for example, heating the support structure  30  such that the outer diameter D 3  thereof expands. Upon the expansion of the diameter D 3  of the support structure  30 , the outer wall  94  thereof engages the turbine blade  14   a  to secure the support structure  30  to the turbine blade  14   a , such that the support structure  30  is shrink fitted into the bore  95  of the turbine blade  14   a . Heating the support structure  30  may comprise, for example, exposing the turbine blade  14   a  and the support structure  30  to the atmosphere and allowing the support structure  30  to heat up to atmospheric temperature. It is noted that the outer diameter D 3  of the support structure  30  may expand to the size of the bore  95  quite rapidly after the transition from cooling to heating, e.g., about 5-10 seconds, so it is desirable to insert the support structure  30  into the bore  95  quickly after the transition from cooling to heating. It is also noted that the support structure  30  could be heated up by inserting the turbine blade  14   a  and the support structure  30  into a heating device, such as a furnace. 
     At step  160 , the first snubber structure  26  is coupled to the first end portion  90  of the support structure  30 . Coupling the first snubber structure  26  to the first end portion  90  of the support structure  30  may comprise, for example locating a first brazing material  200  (see  FIG. 4 ) in the hollow interior portion  42  of the first snubber structure  26  and/or on the first end portion  90  of the support structure  30  outside of the turbine blade  14   a , and applying heat to melt the first brazing material  200 . Upon a cooling of the first brazing material  200  it couples the first snubber structure  26  to the first end portion  90  of the support structure  30 . 
     At step  162 , which may be performed at the same time as step  160  or subsequent to or before step  160 , the second snubber structure  28  is coupled to the second end portion  92  of the support structure  30 . Coupling the second snubber structure  28  to the second end portion  92  of the support structure  30  may comprise, for example locating a second brazing material  202  (see  FIG. 4 ) in the hollow interior portion  72  of the second snubber structure  28  and/or on the second end portion  92  of the support structure  30  outside of the turbine blade  14   a , and applying heat to melt the second brazing material  202 . Upon a cooling of the second brazing material  202  it couples the second snubber structure  28  to the second end portion  92  of the support structure  30 . 
     In accordance with another embodiment, it may be desirable to couple one of the first or the second snubber structures  26 ,  28  to the support structure  30  before the support structure  30  is cooled at step  154 . In this embodiment, the first or the second snubber structure  26 ,  28  coupled to the support structure  30  may be cooled at step  154  along with the support structure  30 . Hence, when the support structure  30  is inserted into the bore  95  in the turbine blade  14   a  at step  156 , the first or second snubber structure  26 ,  28  may act as a stop when the support structure  30  is inserted into the bore  95  the appropriate amount, i.e., the base portion  31  or  60  of the respective snubber structure  26  or  28  will contact the corresponding fillet  32 ,  62 , such that the support structure  30  is not inserted too far through the bore  95 . 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.