US2221685A

US2221685A - Elastic fluid turbine bucket unit

Info

Publication number: US2221685A
Application number: US251574A
Authority: US
Inventors: Arthur R Smith
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1939-01-18
Filing date: 1939-01-18
Publication date: 1940-11-12
Anticipated expiration: 1957-11-12
Also published as: GB534193A

Description

w A. R. SMITH 2 2,221,685 msm FLUID TURBINE BUCKET UNIT Filed Jan. 18, 1959 I Y 2 Sheets -She et l- Inveh'torfi "Arthgdr' 1Q. rni'th,

ay ad qp Fig.5.

Hi Attornqy.

5 In the drawings,

Patented Nov. '12,

UNITED- STATES 2.321.885. ELASTIC FLUID TURBINE BUCKET UNIT Arthur R. Smith, Schenectady. N. Y.. minor to General Electric Company, a

New York corporation of Application January 1a, 1030. Serial No. 251.514

. x 2' Claims. This invention relates to elastic" fluid turbine buckets and more particularly to removabiebuck- Vets for turbines of the axial flow type.

It is desirable that buckets of axial flow elastic I 5 fluid turbines be removably attached to the rotor in order that they may be readily replaced if, necessary. Such buckets usually consist of a blade portion and an integral base portion which is fastened onto the rim of the rotor by a dove- 10 tall connection. Such a fastening arrangement is preferred because of its relatively greater strength and its comparative ease of manufac ture. Groups of buckets are usually tied together by a cover secured onto the ends of a 16 number of adjacent blades, which cover adds to the rigidity of the blades and prevents the escape of operating fluid from the working passage. Such buckets, however. are. not suitable for use in high pressure stages of turbines designed for 20 high admission pressures of operating fluid. such as, for example, of the order of 1200 pounds per square inch and higher. In such turbines, the buckets in the high pressure stages tend to be displacedin the peripheral direction. from the as. normal radial position. due to the force of the operating fluid jet acting tangentially of the retor. The buckets passing from one jet to the next are'alternately loaded and-unloaded. and the repetition of the deflection of the blade ends 80 and the accompanying distortion of the cover ultimately causes the fatigue of the latter and its resultant failure. It is an object of this invention to provide a new and improved elastic iiuid turbine bucket as unit which is adapted to be removably fastened to therotor rim and which possesses a maximum 'obstrengtb and rigidity. 1

It is a further object of this invention to provide a new and improved high pressure turbine so bucket unit particularly adaptable for use in the;

high pressure stages of the turbine and which will not be appreciably distorted when subjected to thejets of operating iiuidtherein.

In the illustrated embodiment, a .turbinebuckm a1- a consideration of emitters. benovel and my invention, attention is directed to the following description and the claims'appended thereto taken in connection with the accompanying drawings.

a base portionformedarms-a views illustrating steps in the manufacture of bucket units according to the invention: Fig. 4

is a perspective view of a portion of a bucket wheel provided with bucket units according to the invention: Fig. .5 is a cross sectional view "is taken along theline H oiFig. i; Figs. 8, 7 and 8 are enlarged diagrammatic sketches illustrating' certain operational characteristics of typical turbine bucket arrangements: and Fig. 9 is a diagrammatic-sketch illustrating certain operam tional' characteristics of bucket units accordingto the invention.

For a better understanding of the invention. the attention is first directed to Figs. 6, 'l and 8,.

which illustrate a 'typicalturblne bucket ar- 15 dovetails of the rotor rim and the bucket bases.

Such clearance, which usually varies at different parts of the dovetail between two and fifteen thousandths of an inch, is illustrated in an exaggerated manner in Figs. 6 and 'l at II and ll be,- tween the bucket base and the adjacent portions of the rotor rim. Because of minor inaccuracies of manufacture oi the co-operating dovetails parts. also because of improper positioning of the buckets'so that theabuttins faces are not r slight spaces are also likely to exist between the abutting bucket base faces. While the average spacing between each pair of adjacent. buckets may only bob! the order of one ten thousandth of an inch, it is understood that this is cumulative around the periphery oi the wheel untilat some a particular point a spacing of several thousandths of'an inch may occur. Such a bucket spacing condition is shown in an exaggerated manner in Fig. 6. atll. In-Figs. 6 and 7, the

turbine rotor is assumed to be rotating at normal 50 speed at which time-the centrifugal force. represented by'the vector Fe, maintains the buckets in the-outermost position with the shoulderll of the bucket bases seatingflrmly against the co-operatingshould'er il ,of therotor dovetail. When the buckets are moved outwardly under the centrifugal force, the spaces ii are further widened, due to the withdrawal of the wedge shaped bucket bases.

Thus it is evident that while it is intended that the buckets be substantially rigidly mounted on the wheel rim, in actual practice many of the buckets are relatively loose on the rim, contacting therewith only at the points of load transmission. A predetermined variation of the tangential bucket loading, indicated by the vector -Fo in Fig. 8, therefore, will cause a shifting of the centrifugal load distribution and physical rocking of such buckets relative to the rim. When the blades are subjected to the impact of the jets of operating fluid, particularly during light load conditions of the turbine, when the load carried by the high pressure stage buckets is greatest, the buckets may be rocked on the rim to some such position as indicated in Fig. 8. The total deflection of the blade ends is due to the cumulative effect of the deformation of the blade portions because of their relative resiliency, and the rocking movement above described. The deflection of the blade ends is accompanied by the .distortion of the cover band as indicated in Fig. 8. It must be remembered that the cover band and its fastening must also withstandthe centrifugal force and the expansive force of the operating fluid, both acting outwardly thereon. Primarily because of the continuous alternating bending stresses, the cover band ultimately becomes fatigued and fails.

In accordance with my invention, the turbine rotor is provided with bucket units formed from a single, solid piece of suitable forged steel each mostefllcient functioning thereof.

unit comprising a cover portion, a plurality of blades, and a base portion. As indicated in Figs. 1, 2 and 3, a single block of metal 25,selected for the desired characteristics as to strength, is first squared to the approximate outer dimensions of the bucket unit. It is then transversely slotted in any suitable manner, as at 26, toprovide an elastic fluid passageway therethrough of a shape corresponding to the channel normally provided between adjacent turbine blades for the The blades 21 and 28 may then be completed by machining the opposite sides of the block leaving the cover portion 29 and the base block 30 integrally bridging the ends of the blades. The base block 30 may then be machined out in a well-known manner to form the dovetail fastening slot 3| and suitably tapered for assembly onto the rotor rim as indicated in Fig. 4. Such finishing operations as the turning down of the outer surface of the cover portions may be done with the bucket units assembled on the rotor.

Although a greater number of blades may be formed in each bucket unit, I prefer to provide only two bladesggper unit for the reason that the rigidity of .the Jpucket assembly is thereby sufli-' ciently increased for successful operation under the most extreme conditions of present day turbine design. By the arrangement shown, with a common base block for each pair of blades, relative sliding or tilting between the base portions of each individual blade is positively precluded. Further, since the centrifugal force actingoutwardly on each blade is'transmitted to the rotor rim over a base width twice vas great as in the case of individual bases, the resistance to the rocking moment of the operating fluid jet is doubled. The most extreme condition of bucket 75 loading, according to present practice, will be inefiective to rock the bucket units relative to the rim against the opposing moment due to the centrifugal force. Assuming, however, that may beso loaded as to cause the rocking thereof within the limits of the spacings-between adjacent bucket units, corresponding to inherent spacings indicated at Ill/l6 and ii in Figs? 6 and '7, it will be obvious that the maximum possible angle of deflection of the upper end of each bucket unit will be but one-half as much as in the case of the individual base buckets. Even such rocking. of the bucket units'will not cause a distortion of the cover portions because the blades, being integrally united at opposite ends, must move as a unit carrying the cover portion parallel with the base block.

The cover portion will be stressed somewhat due to the force acting against blade portions under load. For withstanding this stress the cover portion is made relatively heavy, for example of the order of one-half inch for a blade length of two inches, so that the stress will be distributed substantially equally between the base block and the cover portion necessitating the compound bending of the blades, if at all, as indicated in the diagrammatic sketch, Fig. 9. At

'due to unforeseen conditions the bucket units least twice the force will be required to distort the blades in this manner than will be required if the ends were free or merely loosely connected.

As shown in Figs. 4 and 5, the notch closing bucket unit may be riveted onto the periphery of the rim by means of the pins 36 extending therethrough. The bucket unit is further secured to the ends of the adjacent bucket units by means of the keys 31 extending transversely therebetween which assist in precluding relative movement between the notch closing bucket unit and the adjacent units.

Thus, with my invention I have accomplished an improved construction of bucket units for high pressure elastic' fluid -turbine wheels. Briefly, such bucket units are made from a single piece of forged steel and eachunit has a base forming a dovetail for connection to a circumferentially extending dovetail formed by the rim of a bucket wheel. Each unit includes at least two bucket blades having ends integral with the base and other ends integral with an outer cover. The cover is relatively inflexible, that is, of a thickness of the order of 25% of the blade length with regard to blades of about 2 inch length. The units are machined in any suitable manner from a solid block of material, for example, in the manner set forth in the copending application of A. M. Reynolds, Serial No. 251,573, filed on January 18, 1939 and assigned to the same assignee as the present application.

Having described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof, I desire to have it understood that the apparatus shown is only illustrative and that the invention may be' carried out by other means.

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

1. A bucket unit for high pressure elastic fluid turbine wheels comprising a base forming a dovetail for connection with the dovetailed rim of a bucket wheel disk, two spaced bucket blades each having an end integrally formed withv the base, and a cover integrally formed with the other ends of the blades, the entire bucket unit being machined from a single piece of forged steel.

2. A high pressure elastic fluid turbine wheel comnrising a disk having a rim with circumferentially extending grooves forming a dovetail,

- and a. plurality of bucket units forming a row of bucket blades secured to the disk, each unit being machined from a single piece of forged steel and having a base forming a. dovetail connected to the dovetail of the rim, two spaced blades each having an end integral with the base and a cover integral with the other endsof the blades, the covers of adjacent units abutting each other. r

I ARTHURR. SMITH.