US2278041A - Turbine blade shroud - Google Patents

Description

March 31, 1942. R. c. ALLEN v TURBINE BLADE SHROUD original Filed ot. 25, 1939 Patented Mar. 31, 1942 crisp ATENT OFFICE TURBINE BLADE SHRO-UD Original application October 23, 1939, Serial No.

Divided and this application August 24, 1940, Serial No. 354,048

Claims.

This invention relates to tur-bine blading and more particularly to an improved shroud construction for the moving blades of elastic uid turbines.

The shroud structures are preferably attached to the blade tips in the manner fully disclosed and claimed in my copending application Serial No. 300,708, led October 23, 1939, of which this application is a division.

The bending stresses set up in shroud members by centrifugal force become excessive at relatively high rotative speeds and as a result the types of shrouding hitherto used frequently fail when the tip speed of the blades approaches or exceeds 1100 feet per second. The adjacent edges of the individual shrouds or of the shroud strips, Where strips of shrouding of suicient length to span several blades are used, are usually secured together by a soldered, brazed or welded bond, and due to the nature of this bond, in those structures in which the individual shrouds or strips are secured to the blades by riveting or upsetting, to looseness between the vshroud members and blades, the bond between adjacent shroud members usually fails first. Damage to the turbine is, in many instances, attributable to the failure of the bond between adjacent shroud members :and to the action of centrifugal force in bending the then free ends of the shrouds radially outward, thereby causing the free end of the shroud to contact a stationary part of the machine.

The difficulties heretofore mentioned are also experienced in those structures in which the free ends of the individual shrouds or of the shroud strips are not secured together. Attempts to overcome the aforementioned difficulty by using shroud strips channel shaped in crosssection and of suiiicient length to span several blades and by increasing the radial thickness of shroud material have not been successful in that the shrouds buckle or otherwise distort sufciently to contact a stationary part of the turbine or if the radial thickness is increased suiiciently to prevent distortion, the increased mass results in a large centrifugal moment which causes destructive bending and breaking of the shroud adjacent its point of connection to the blade.

It is therefore an object of this invention to provide an improved shroud construction for turbine blades which is capable of withstanding the bending stresses produced by centrifugal force without failure or distortion.

The invention accordingly consists of the various features of construction, combinations of elements and arrangements of parts as more fully pointed out in the appended claims and in the detailed description, in which:

Fig. 1 is a Vertical section taken on line I--I of Fig. 2 illustrating a portion of a circumferential blade roW embodying the invention;

Fig. 2 is a plan View of the portion of the circumferential blade row shown in Fig. 1;

Fig. 3 is a vertical section taken on line III-III of Fig. 2;

Fig. 4 illustrates a segmental blade group mounted in a jigadapted to hold the blades in their proper cooperative relationship; and

Fig. 5 is a plan View of a portion of a circumferential blade roW illustrating a modified construction.

The invention, reference being had to the drawing, is shown as applied to a circumferential row of low pressure reaction blading of which only a portion of the blade row I is shown in the interest of simplicity. The blade row I which is formed by blade structures 2 each having a body portion 3 presenting oppositely disposed fluid confining surfaces 4, adepending serrated root portion 6 which is only shown in Fig. 4 in the interest of simplicity, lashing members 'I which are welded to oppositely disposed portions of the fluid confining surfaces 4, and a shroud member 8 welded to the top or tip of the body portion 3. The blade structures 2 are secured in .the spindle or rotor (not shown) by means of blade retaining grooves, the cross-sectional configuration of which conforms with that of the root portion 6 shown in Fig. 4. The manner of securing the blade structures to the turbine spindle or rotor forms no part of this invention and in this connection it should be understood that the blades may be provided with any desirable form of retaining construction desired and that the root structure 6 shown in Fig. 4 merely illustrates one of several practical constructions that might be used.

The individual shroud members 3, which are preferably shaped from a non-air hardening stainless steel material, each have oppositely disposed ends or edges I3, a substantially smooth bottom surface I4, a top surface I 6 provided with a plurality (preferably three) of upstanding and approximately parallel extending spaced flanges II. The height of each of said anges I 'I is a maximum approximately midway between the said oppositely disposed ends or edges I3 and progressively decreases toward each of said edges, The members 8 are so dimensioned that when the member is secured to the top of a blade with the blade top coacting with the portion of the member' approximately midway between said opposite edges and blades provided with these members are assembled in their proper cooperative relationship to form a circumferential blade row, said flanges extend circumferentially of and radially outward with respect to said row, said opposite edges or ends on the members on adjacent blades are disposed in welding proximity, as best shown in Figs. l and 2, and said bottom surfaces on adjacent members provide in effect a continuous fluid conning surface. The underside of the individual shroud members is preferably Welded to the tops of the individual blades,

as shown in Figs. 1, 3 and 4, with a non-air hardening stainless steel weld material.

The shroud members 8 are preferably formed in two complementary parts by a stamping operation and these parts are tack welded or otherwise secured together to form the unitary shroud4 structure illustrated. This construction provides in effect a double central flange as best shown in Fig. 3 which greatly reduces the stresses set up by the centrifugal bending moment. However, a feature of much greater importance is the cantilever bridge conguration of the flanges I'I when seen in side elevation (see Figs. l and 4) as this construction greatly reduces the mass of the unsupported portions of the shroud members which materially decreases the bending moment produced by centrifugal force and permits the use of flanges having sufficient strength to resist the centrifugal bending moment. Shroud members having the characterizing features previously described can obviously be formed or shaped in numerous ways different from the described stamping a'nd welding operation which merely suggests one practical mode of construction.

The blade structures 2 are formed by placing i the blade per se in a suitable jig or other means operative to hold the blade in a position permitting the bracing means (the shroud and/or the lashing means) to be held in their proper cooperative relationship with respect to the blade. The bracing means is then welded to the blade preferably using a non-air hardening stainless steel weld material as previously described. If both lashing and shrouding members are to be welded to the blade, to form the type of blade .i

structure illustrated in the drawing, it is preferable, although not necessary, to first weld the lashing member 'I to the fluid confining surfaces 4. A suitable means for holding the blades so that the bracing means can be welded thereto as described will be obvious to any one skilled in the art and a detailed disclosure of a suitable means for this purpose is not deemed essential for a complete understanding of this invention.

The blade structures thus formed are then assembled in their proper cooperative relationship on a turbine spindle or rotor to form a circumferential blade row with the edges I3 of the shroud members and the free ends of the lashing members on adjacent blades disposed in welding proximity. The edges of the shroud members 3 and the free ends of the lashing members I on adjacent blades are then welded together, preferably with a non-air hardening stainless steel weld material which will produce a ductile and reliable bond between the adjacent end portions of the shroud and lashing members on adjacent blades. In this connection, it has been found that a more stable structure is obtained by welding together the shroud and lashing members on a predetermined number of blades rather than by Welding together the shroud and lashing members on all of the blades forming the circumferential blade row. This construction is clearly illustrated in Fig. 2 in which blade structures are welded together in groups of three.

Instead of assembling the individual blade structures on a turbine spindle or rotor to form a circumferential blade row prior to the Welding together of the edges and free ends of the shroud and lashing members, respectively, on adjacent blade structures, a plurality of these blade structures may be assembled in their proper cooperative relationship and the edges and/or the free ends of the shrouds and lashing members on adjacent blades welded together to form a segmental blade group adapted to be mounted as a unit on a turbine spindle or rotor to form a portion of a circumferential blade row. Fig. 4 illustrates a jig I8 provided with a plurality of grooves I I conforming in shape, size and arrangement to the blade retaining grooves in the turbine spindle or rotor (not shown) in which the blades are to be mounted so as to retain the blade structures mounted thereon in the same cooperative relationship that exists when the blade structures are mounted in the turbine rotor or spindle to form a circumferential blade row. Obviously, the jig I8 may be provided with a different number of grooves than that shown or an entirely different type of jig can be employed if it is so desired. The only essential in this connection is that a jig be provided which is operative to hold a predetermined number of blade structures in their proper cooperative relationship so that the end portions or edges of the shroud and lashing members on adjacent blades are disposed in welding proximity and can be welded together to form a segmental blade group. The segmental blade group thus formed is then removed from the jig and mounted on the turbine rotor or spindle to form a portion of a circumferential blade row. The manner of mounting segmental blade groups on a turbine spindle having axially extending retaining grooves similar to the groovesshown in the jig I8 is well known in the art as are also various forms of jigs for holding a plurality of blade structures in their proper cooperative relationship. Consequently, a more detailed disclosure in this respect is deemed unnecessary for a complete understanding of the invention.

Fig. 5 illustrates a modified construction in that each shroud member 8 is made of sufficient length to span two blades and the height of the flanges I'I which is a minimum at the opposite ends or edges I3 of the shroud member, progressively increases to a maximum and then decreases to a minimum throughout successive ciicumferentially extending portions each of which is approximately equal to one-half the tip pitch of the blades the shroud member is adapted to span. These flanges also provide a cantilever bridge type construction when viewed in side elevation which is similar in appearance to that provided by two of the adjacent blade structures seen in Figs. 1 and 4. The procedure to be followed in connection with this embodiment of the invention, assuming that the blades are to be provided with lashing members as previously described, is to first attach the lashing member 'I to the fluid confining surfaces 4, as described in connection with Figs. l, 3 and 4, then either first weld the free ends of the lashing members together or rst place and hold the shroud member 8 on the tops of the blades so that the top or tip portion of two adjacent blades coacts with those portions of the underside of the shroud member lying beneath the point at which the height of the flanges il is a maximum and weld the shroud member to the top or tip portions of the blades, then remove the unitary blade group thus formed from the jig, then mount the unitary blade group on the turbine rotor to form a portion of a circumferential blade row. The free ends of the shroud and lashing members on two or more adjacent groups may if desired be welded together preferably with a non-air hardening stainless steel weld material to form a circumferential blade row having a plurality of reinforced segmental blade groups consisting of two or more of the unitary blade groups formed in said jig in the manner just described. If desired, each shroud member 8 can be readily made of suicient length to span three or more blades which, of course, would necessitate that the unitary blade groups be formed by using the desired or predetermined number of blades.

The shroud construction herein disclosed is of general application and it should be understood that the shrouding may be attached to the blade tips in any desired or preferred manner, that the lashing members are not an essential feature of this invention and that it is not desired to limit the invention to the exact details of construction herein shown and described, as modications within the scope of the appended claims may appear to persons skilled in the art.

It is claimed and desired to secure by Letters Patent:

1. A shroud structure for individual turbine blades comprising a member having oppositely disposed edges, a substantially smooth bottom surface, a top surface presenting an upstanding coextensive flange the height of which is a maximum ata point approximately midway between said oppositely disposed edges, and progressively decreases towards each of said edges, and dimensions such that when the member are secured to a blade with the top of the blade coacting with a portion of the member approximately midway between said oppositely disposed edges and blades provided with these members are assembled on a turbine element to form a blade row, said flanges will extend circumferentially of and radially out- Ward with respect to the blade row, said opposite edges on adjacent members will' be disposed in Welding proximity and said bottom surface on adjacent members will coact and form in effect a continuous fluid confining surface.

2. A shroud structure for individual turbine blades comprising a member having oppositely disposed edges, a substantially smooth bottom surface, a top surface provided with a plurality of similar upstanding and approximately parallelly extending spaced flanges the height of each of which is a maximum at a point approximately midway between said opposite disposed edges and progressively decreases towards .each of said edges, and dimensions such that when the said member is secured to a blade with the top of the blade coacting with a portion of the member approximately midway between said oppositely disposed edges and blades provided with these members are assembled on a rotary turbine element to form a circumferential blade row, said flanges will extend circumferentially of and radially outward with respect to the blade row, said opposite edges on adjacent members will be disposed in Welding proximity, and said bottom surfaces on adjacent members will coaot and form in effect a continuous uid confining surface.

3. A shroud structure for individual turbine blades comprising a member having oppositely disposed edges, a substantially smooth bottom surface, a top surface provided with similar up-v standing and approximately parallelly extending side anges and an intermediate double ange the height of each of which is a maximum at a point approximately midway between said oppositely disposed edges and progressively decreases towards each of said edges and dimensions such that when the member is secured to a blade with the top of the blade'coacting with a portion of the member approximately midway between said oppositely disposed edges and blades provided with these members are assembled on a rotary turbine element to form a circumferential blade row, said flanges will extend circumferentially of and radially outward with respect to the blade row, said opposite edges on adjacent members will be disposed in welding proximity, and said bottom surfaces on adjacent members will coact and form in effect a continuous fluid confining surface.

4. A shroud structure adapted to be secured to and span the tips of a plurality of turbine blades constituting in effect a segmental blade group comprising a member approximately equal in length to the tip pitch of the blades multiplied by the number of blades in a segmental group, said member having a top surface presenting an upstanding ange coextensive in length with the length of said member, the height of which is a minimum adjacent the oppositely disposed ends of said member and then progressively increases to a maximum and decreases to minimum sequentially throughout successive longitudinally extending portions of the member which portions are equal in length to one-half the said tip pitch of the blades and dimensions such that when the member is secured to a plurality'of blades constituting in effect a segmental blade group with the top portions of the blades coacting with those portions of the member below the points at which the height of said flange is a maximum, said ange will extend circumferentially of and radially outward with respect to said group.

5. A shroud structure adapted to be secured to and span the tips of a plurality of turbine blades constituting in effect a segmental blade group comprising a member approximately equal in length to the tip pitch of the blades multiplied by the number of blades in a segmental group, said member having a top surface provided with a plurality of similar upstanding and approximately parallelly extending flanges the height of each of which is a minimum adjacent the oppositely disposed ends of said member and then progressively increases to a maximum and decreases to a minimum sequentially throughout successive longitudinally extending portions of the member, which portions are equal in length to one-half the said tip pitch of the blades, and dimensions such that when the member is secured to a plurality of blades constituting in effect

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