US3043562A - Combination sealing and restraining member for long-shank turbo-machine buckets - Google Patents

Description

July 10, 1962 F. H. VAN NE ET AL 3,043,562

COMBINATION SEALING AND R RAINING MEMBER FOR LONG-SHANK TURBO-MACHINE BUCKETS 2 Sheets-Sheet 1 Filed April 10, 1961 INVENTORSZ FRANCIS H. VAN NEST, GEORGE R. FUSNER, 40424,; 8. g

THEIR ATTORNEY.

NGSHANK TURBO-MACHINE 2 Sheets-Sheet 2 y 1962 F. H. VAN NEST ETAL 3, 3,

COMBINATION SEALING AND RESTRAIN MEMBER FOR LO KETS Filed April 10, 1961 F |G.4 @YI INVENTORSI FRANCIS NNEST,

' GEORGE SNER' BY @M' ATTORNEY.

United States Patent ()fifice rinse Patented .iuiy to, ram

corporation of New York Filed Apr. 10, 1961, Ser. No. 101,998 1 Claim. (Cl. 25377) This invention relates to an improved construction for a turbo-machine rotor utilizing long-shank buckets and more particularly it relates to an improved cover plate arrangement which covers the spaces between bucket shanks and which also locks the buckets in position axially and restrains them against vibration.

Severe stress concentrations are encountered in the base or dovetail portion of a turbine bucket which is used to secure the turbine bucket to the rim of a rotor wheel. This difiiculty is accentuated in machines such as gas turbines, where the extreme temperatures near the rim greatly reduce the strength of the wheel and bucket material. Since the temperature increases toward the outside of the wheel, for example, on the order of 200 F. per inch, the allowable stress on the highly stressed bucket bases may be increased by moving them radially inward on the wheel to a cooler location. Fluid flow considerations, however, require that the platform or radially inner flow-defining wall of the annulus remain at a larger diameter. In order to compromise between these two conflicting design requirements, longshank buckets have been utilized, which consist of buckets whose platform portions and root portions have been radially separated and connected by a shank portion carefully designed so as not to have stress-raising discontinuities thereon.

In order to reduce the weight concentrated at the rim, the shank is generally of reduced cross-section, which means that there are gaps between adjacent shanks, through which leakage of the motive fluid could take place. Therefore, some means must be provided to cover up these gaps so as to substantially block gas flow axially between shanks.

It has been suggested that separate cover plates might be provided for sealing off the spaces between shanks and that such cover plates can be prevented from dropping radially inward by bending a tab on the radially inner part of the cover plate into a suitable circumferential groove. plate must have its tabs bent in this manner and failure of one tab can mean that the corresponding cover plate is free to drop off.

One of the areas of greatest concern in long-shank turbine bucket design is the stress concentration encountered in the dovetail base portion where the leverage due to the added "length of the bucket introduces the possibility of cracks in the bucket base. With long shank buckets, if it were possible to mechanically couple or lock the buckets together in a group at the bucket platform, then any bending will be restrained by the bucket platforms rather than by the dovetail bucket bases. It is much easier to design the bucket platforms and the portion where the bucket joins the bucket platform to withstand the bending than it is to so design the highly stressed bucket bases. Also the forces are much less due to the shorter moment arm. Therefore it would be desirable to lock the blade platforms together so as to reduce the stress on the bucket bases.

Any provision for locking the bucket platforms together must not interfere with the differential thermal expansion of the various parts of the assembly. The thermal gradient from the vane portion or working por- With this proposed construction, every cover f tion of the bucket inward along the shanks and to the rotor wheel is very pronounced in a gas turbine. Therefore as the buckets and cover plates expand at varying rates, any provision for locking the buckets together at the platforms must also accommodate this relative movement.

In addition to blocking the flow of motive fluid axially between bucket shanks, some types of construction require that a rotating seal be provided between each bucket wheel and the adjacent stationary nozzle-bearing diaphragm which redirects the motive fluid between successive turbine stages. Rubbing of such seals can occur and if this takes place, it is desirable to have an easily replaceable structure for providing new seals.

Accordingly one object of the present invention is to provide an improved cover plate assembly for a longshank bucket wheel for preventing gas leakage between bucket shanks, with special means for locking the buckets in place both tangentially and axially.

Another object of theinvention is to provide a cover plate for a long-shank bucket which provides an easily replaceable rotating seal surface to prevent leakage between the rotor and an adjacent diaphragm.

Still another object is to provide an improved cover plate which restrains the platforms of adjacent turbine buckets tangentially with respect to one another near the platform radius at high speed, but which is easily removable when the rotor is at a standstill.

Another object of the invention is to provide a means to couple the individual blades in a group at a location removed from the dovetail bases so as to reduce the stresses on the bases due to vibration of the blades.

A more specific object of the invention is to provide an easily removable cover plate assembly for a rotor using long-shank buckets which provides all of the aforementioned functions of preventing gas flow axially between the shanks, providing replaceable mating surfaces for the diaphragm seal, locking the buckets axially at the root and tangentially at the bucket platform radius, and aiding in damping of the bucket vibrations.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is an enlarged detail view looking in the direction of the rotor axis and showing the wheel rim and lower portion of a number of buckets;

FIG. 6 is a sectional view taken circumferentially as indicated by arrows VI-VI of FIG. 4, and showing the method of insertion of the modified cover plate; and

FIG. 7 is a perspective view of the modified cover plate.

Generally stated, the invention is practiced by providing a cover plate whose radially outermost edge bears at circumferentially separated points on the undersides of adjacent bucket platforms and whose radially lower edge is held in interrupted radial slots defined in axially extending lugs on either the Wheel rim or the bucket base. The cover plate is inserted generally axially and has recesses in its lower edge designed to avoid the lug. It is then moved one half bucket pitch circumferentially to seal the space between bucket shanks. Axially extending sealing portions on the cover plate provide a substantially continuous rotating sealing surface cooperating with the adjacent diaphragm.

Referring now to FIG. 1 of the drawings, a plurality of turbine buckets, one of 'which is indicated generally as 1, comprise vane portions 2,. platform portions 3,

bases 4, and shank portions 5. It will be seen that each shank portion 5 serves to connect a base 4 with a platform 3 to transmit the load between vane 2 and base 4. A portion of a rotor rim 6 is seen to include dovetail slots 7 plate 9 is held at-its lower or radially innermost edges in I two adjacent lugs 10, 11 extending axially from the ends of the bucket dovetail by means of slots 10a, 11a in the lug. The outer edge of plate 9 is retained in grooves 312 which are so formed in the undersides adjacent bucket platforms 3 to constitute a continuous circumferential groove. It is to be noted that the radius of the outer edge 9aof cover plate 9 is greater than that of the radius of groove 3a so that the cover plate -9 bears against the bottom of groove 3:: in two circumferentially separated points shown as 12, 13. Point 12 is located near the circumferential center of the platform of one bucket while point 13 is located near the center of the adjacent bucket.

A better understanding of the disposition of cover plate 9may be had by reference to the enlarged crosssection view of FIG. 2. There the cover plate 9 on one side of the rim 6 is shown to be held in opposed grooves 3a, 11a. The axial thickness ofplate 9 is slightly less than that of grooves 3a, 11a, as indicated at 3b, 11b, to allow plate 9 to shift radially outward under centrifugal force. The groove 3a is a groove which opens'radially inward and is cut in the underside of an axially extending portion 30 on the blade platform 3. Lug 11, as previously mentioned, is an axially extending lug on the fir tree portion of the bucket and is preferably formed integral therewith, although it could be separately atof inserting cover plate 9.

FIG. 3 is a perspective drawing illustrated more clearly the shape of cover plate 9. There are the cover plate is seen to comprise a radial web 15 of substantial thickness to withstand the buckling tendency which will be imposed by centrifugal force on the cover plate. Extending outward from radial web 15 are two arcuate flanges 16, 17 which form close clearances with portions of the adjacent diaphragm to provide awheel space seal. I These flanges 16,17 are formed on an arc to join with similar adjacent cover plate flanges. Flanges 16, 17 constitute easily replaceable sealing portions simply by substituting a new 'cover plate 9' in the event that flanges 16, 17 become damaged or worn. The top edge 18 of the radial web 15 may either bev straight or arcuate, but if arcuate, it must be of a greater radius than the arc formed by grooves 3a in thelower surface of the platforms, so that the pressure exerted by cover plate 9 on platforms 3 will be concentrated at the ends of the top edge 12, 13.

A central recess 19, whose circumferential dimension is slightly greater than the circumferential dimension of lugs 10, 11 is defined in the lower edge of the radial web 15of the cover plate 9, so that the bottom edge of cover plate 9 can he slipped over the lug. The lower edge of radial web 9' is also provided with: slots 20', 20a which isolate bendable tabs 21, 22 respectively. These tabs 21,

tached. The phantom outline 14 illustrates the method 4 22 can be bent to lock cover plate9 against circumferential movement in the grooves 3a, 11a. One such tab 22 is illustrated in its bent or locking position in FIG. 2.

It only remains to note that the remaining straight tabs 23, 24- on the other side of slots 20, 20m from tabs 21, 22 respectively are the portions of the radial web which are axially restrained in lug slots 11a after the cover plates 9 are in final position. The circumferential dimension of each straight tab 23, 24 should be approximately half the circumferential width of the lugs 10, 11.

Another modification of the invention may be seen by reference to FIGS. 4-7 of the drawings. The construction illustrated here is slightly different from the preceding described construction in that the slots holding the inner edge of the cover plates are on the rotor rim rather than on the bucket roots. The plate must be modified accordingly with this construction. Either the modification described in FIGS. l-3 or that described in connection with FIGS. 4-7 may be suitable depending on the size of the machine, shape of the buckets, etc.

FIG. 4 illustrates a large number of buckets such as 25. Each bucket 25 has a vane 26, platform 27, shank 28, and base 29. Between the shanks 28 are wide gaps 31 which are covered by cover plates 32. Each cover plate 32 bears at its radially outermost corners against the adjacent buckets as seen at 33, 34. The construction thus far is the same as that described in connection with a bucket 25 is shown taken along lines VI--VI of FIG.-

4 which also includes a portion of the tooth 35 of the rim. There it will be seen that a cover plate 32 is held in place between opposing radial grooves 27a, 35a cut in the bucket platform 27 and the axially extending rim portion 35 respectively. The phantom outline 36 of cover plate 32 shows the method of inserting the cover plate. The perspective view of FIG. 7 indicates cover plate 32 to comprise a radial web 37, axially extending arcuate flanges 38, 39. The lower'edge of web 37 is removed on either bottom corner to provide cutouts 40, 41, and slots 42, 43 divide the remainder of the lower edge into bendable tabs 44, 45 and straight central portion 46. Analogous to the previous construction, the circumferential length of either of the bottom corner cutouts 40, 41'must be at least half that of the circumferential dimension of rim tooth 35 and also the central straight portion 46 should be approximately equal to the width of tooth 35 so that when tabs 44, 45 are bent, they will hold cover plate 32 in its proper circumferential location. This is illustrated by reference to FIG. 6 where a tab 45 is shown in its bent position. It will also be observed from FIG. 6 that there is a cover plate 32 on either side of the rotor wheel. This is not shown in FIG. 2, but it is understood that the first modification described also utilizes cover plates 9 on either side of the rotor wheel. Improved sealing and locking characteristics are derived when there is a cover plate on either side of the Wheel.

The method of assembly of the modification described in FIGS. 1-3 will now be described. First, the buckets 1 are assembled on rim 6 by sliding the buckets axially on to the wheel rim in succession, the bases 4 mating in the slots 7 on the rim. Next, the cover plates 9 are inserted. To do this, they are placed approximately one-half bucket pitch from their ultimate location and the top edge 18 is tilted forward and inserted in groove 3a. This position is indicated by phantom lines 14 in FIG.

2. Next the lower edge of cover plate 9 is moved axially inward with the lug ll passing through the cutout 19. Each such cover plate is inserted in this manner until they are all in place. Next the entire group of plates is moved circumferentially one-half a bucket pitch as indicated by the arrows in FIGS. 1 and 5. Although these FIGS. 1 and 5 seem to indicate that only one cover plate is moved circumferentially at a time, this is merely done to indicate the two different circumferential positions of a cover plate.

The assembly of the cover plates 32 in the modification described in FIGS. 4-7 takes place in a similar fashion. After the buckets 25 have been inserted in the dovetail slots, cover plates 32 are inserted one-half a pitch length from their final positions. In this case, however, the cutout portions 40, 41 will just clear the interfering portions of two adjacent teeth 35. Then the cover plates 32 are moved circumferentially one-half a pitch length as indicated by the arrow in FIG. 5.

FIG. 4 may be taken as illustrative of either modification in order to discuss an important point. It will be noted that the tabs 44, 45 (similarly tabs 21, 22) are not bent upon every cover plate. FIG. 4 illustrates that the tabs are bent up only at points 48 on one cover plate and at points 49 on a widely circumferentially separated cover plate. This means that cover plates shown by the reference numerals 5b, 51 are circumferentially locked in place, whereas the intervening cover plates 52, 53, 32, 54 and 55 may move circumferentially to some extent to a degree permitted by the circumferential spacing between plates.

The operation of the cover plates will now be described. Again taking FIG. 4 as illustrative of either modification, it will be apparent that the gap 31 between shanks 28 is effectively blocked off by the central portion of radial rib 37 on cover plate 32. Where there is a cover plate on the opposite side of the wheel, added security of the .seal is accomplished.

Another important sealing function of the cover plate 32 is through the use of axially extending arcuate sealing flanges 38, 39 which are aligned circumferentially with the flanges on adjacent cover plates and which form an effective continuous sealing ring cooperating with portions of an adjacent diaphragm and forming close clearances therewith to prevent leakage in a radial direction between the diaphragm and the wheel. In the event that the flanges 38, 39 are damaged due to axial movement of the rotor against the diaphragm, the segmented construction causes them to be relatively inexpensive and easily replaceable.

It will also be observed that the cover plates 32 lock the buckets 25 against axial disengagement from the rotor, whether the rotor is at a standstill or rotating. Such a provision is necessary, since substantial thrust is exerted on the bucket vanes. This axial locking is provided by the central portion 46 at the lower edge of the cover plate which is disposed in slot 35 in the rim. Since the cover plate 32 must move axially with blade 25 by virtue of its being located at its upper end in groove 27a, central portion 46 prevents disengagement in either axial direction. Further security against axial disengagement is provided by a similar cover plate on the opposite side of the wheel.

A very important feature of the improved bucket cover is the provision for locking or restraining one bucket with respect to one another tangentially by a force applied at the location of the platform radius. As is well known, torque produced by a given force increases with the moment arm. Since each bucket 25 is separated from its root portion 29 by a slender shank 28, it has the ability to flex in a tangential direction. If it were attempted to prevent this flexing by applying a restraining force at a location near the root 29, considerable holding force would be necessary. In accordance with the invention, the location for locking one blade with respect to the other in a tangential direction is moved outward to the blade platforms, at the locations indicated by points 33, 34. As will be apparent from FIGS. 2 and 6, centrifugal force on the cover plates will cause them to move slightly radially outward and bear against the bottom of the grooves 27a in the platforms (FIG. 6). The are forming the outer edge of the cover plates is less pronounced than the arcuate platform grooves, causing the cover plates to hear at their outer corner points as seen at 33, 34 in FIG. 4. On rotors of large radius, the outer edge of one cover plate could be straight instead of arcuate. This causes the centrifugally outward force exerted by the cover plate to be substantially equally distributed between points 33, 34 cansing a resistance to tangential movement of adjacent buckets by virtue of the friction existing between the outer edge of the cover plates and the bottom of the platform groove. With this arrangement, the blades are substantially locked by friction forces with respect to one another tangentially at a location substantially removed from the bucket dovetail portion. The frictional holding force which has thus been achieved by use of the invention is sufiiciently great that in some cases it has been possible to eliminate conventional tie-wires between buckets.

It will also be observed that with this construction the vanes 26 will tend to flex individually about points 33, 34 rather than about the root portions 29, as previously was the case. Thus the natural frequency of vibration of the bucket is increased. For this reason, also, the cover plates 32 aid in damping vibrations of the blades since there is a tendency for the complete bucket to flex about two separate fixed points or nodes, i.e., the platform and the root. Hence the construction shown aids in damping undesirable blade frequencies.

Thus the novel construction of the improved cover plate described above causes it to afford a very simple mechanism for accomplishing a great number of useful and necessary functions. These functions include: sealing the space between bucket shanks, locating the buckets both tangentially with respect to one another and axially with respect to the rotor rim, providing easily replaceable sealing surfaces for mating with an adjacent diaphragm, and aiding in blade damping. The cover plate performs all of these functions, and yet is of very simple construction and is easily replaceable by means of the unique manner of axially inserting the cover plate and moving it one-half bucket pitch to lock it in position.

The invention, of course, is not limited to the specific shape shown as long as it performs its functions in a similar manner. Either modification shown in the drawings may be preferable depending upon the other characteristics of the rotor and still other modifications will occur to those skilled in the art.

Accordingly, it is desired to cover in the appended claims such modifications as fall within the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

In combination, a rotor wheel having a rim defining circumferentially spaced axial dovetail slots, a circumferential row of buckets disposed on the rim, each of said buckets having a base disposed in one of said slots, a platform portion radially spaced from the rim, and a connecting radial'shank portion, said shanks defining spaces therebetween, a plurality of cover plates each including a radial wall extending circumferentially from one bucket shank to the next adjacent bucket shank and radially sub stantially from bucket base to bucket platform, whereby said radial wall impedes the flow of motive fluid between bucket shanks, said bucket platforms having axially extending portions defining arcuate grooves in their undersides cooperating to define an inwardly facing circumferential groove, a plurality of circumferentially spaced lugs defined by alternating bucket base and rotor rim portions, said lugs together defining an interrupted outwardly facing circumferential groove radially inward from said platform arcnate grooves, said cover plates being disposed attheir radially outermost edge in the platform grooves and at their" radially innermost edge in the lug grooves so as to be both radially and circumferentially moveable therein, the radially inner portion of each cover plate also defining at least'one cutout arranged to pass axially over a said lug when the cover plate is substan: tially one-half a bucket pitch from its final position, at least one bendable tab on said cover plate arranged to be bent to prevent circumferential movement of the cover plate, each of said cover plates including at least one arcuate flange extending axially from said cover plate radial wall, whereby the flanges on adjacent cover plates cooperate to define a substantially continuous sealing ring, the

References Cited in the file of this patent UNITED STATES PATENTS.

Wilkinson July 17, 1956 Hart et a1.- ..v Feb. 23, 196Q mm" d

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