US2441432A

US2441432A - High-speed rotor

Info

Publication number: US2441432A
Application number: US635023A
Authority: US
Inventors: Hugh P Mcgee
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1945-12-14
Filing date: 1945-12-14
Publication date: 1948-05-11
Anticipated expiration: 1965-05-11
Also published as: GB655880A

Description

May 11,1948. H, p. McGEE 2,441,432

HIGH SPEED ROTOR Filed Dec. 14, 1945 'Figl.

In nt orz Hugh Mc Gee, by

His Attorney- Patented May 11, 1948 HIGH-SPEED ROTOR Hugh 1'. -McGec, Beading, Masn, anignor to General Electric Company. a corporation-oi New York Application December 14, 1945. Serial No. 635,023

' 6 Claims. 1

My invention relates to the construction of high speed rotors adapted to be mounted on a shaft. and includes a bushing arrangement for the shaft bore of such rotors. The invention is particularly adapted to compressor impellers made of light metals such as magnesium or aluminum alloy.

An object of the invention is to provide an improved method for mounting a high speed rotor on a shaft.

Another object is to provide an improved bushing arrangement for the shaft bore of a high speed rotor, which is simple, easy and cheap to manufacture, readily disassembled when a new bushing is required, and which increases the bursting strength of the rotor by the elimination of stress-concentrating rivet holes.

Other objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a cross section of a centrifugal compressor impeller with a bushing arranged in accordance with my invention: Fig. 2 is an exaggerated diagrammatic representation of the shape the impeller of Fig. 1 tends to assume when rotating at high speeds; Fig. 3 illustrates a modified bushing arrangement incorporating my invention; and Fig. 4 and Fig. 5 represent still further modifications of the structures of Fig. 1 and Fig. 3, respectively.

The present invention constitutes an improvement over the impeller bushing arrangement shown in United States patent to S. R. Puller 1,771,349, issued July 22, 1930. This prior art Puifer structure was comparatively complex and necessitated the drilling of very long. small diameter rivet holes entirel through the impeller. This manufacturing operation has become increasingly difficult as impellers have increased in axial length, as in the "mixed flow" type .of impeller and others.

In Fig. 1 my invention is disclosed as applied to a centrifugal impeller including a hub portion and a web portion, with circumferentially spaced. radially arranged blades I secured to the web in a manner which is well known to the art. An axial bore 2 is provided through the impeller to permit mounting it on a suitable shaft 3. For convenience in describing my invention, the impeller body will be considered to consist of a first hub portion 4 and a second combined hub and web portion 1, which portions may be considered to be go I have discovered, and verified by test, that separated by an imaginary plane represented by dotted line 28, the significance of which will appear hereinafter.

Annular hub portion 4 has a cross section area with a center of gravity located approximately at the point 5. It will be observed that this portion of the impeller body has a. comparatively small mass and radius of gyration, the latter. being substantially the distance from the point 5 to the axis 8. The remainder of the impeller body forms the hub and web portion 1 having a cross section with a center of gravity represented by point 8. It will be seen that this second portion includes a material part of the hub portion of the impeller and practically all of that portion commonly referred to as the web. It will also be observed that the combined hub and web portion 1 has a mass and radius of gyration appreciably greater than does the hub portion 4.

when arotor of the general configuration described above and shown in the drawings is rotated at extremely high speed about the axis 6', a characteristic type of deformation takes place, which will be seen by reference to Fig. 2. In this figure, the dotted lines 9 indicate the original location of the hub and web portions I and i when the rotor is at rest. It has been found that when rotating at very high speeds, such a body tends to take the shape indicated by the full lines in Fig. 2. It should be understood that in the drawing the deformations are greatly exaggerated for the purpose or illustration. In the case of an aluminum alloy impeller seventeen inches in tip diameter, such as those used as supercharger blowers for internal combustion engines, I have observed a rotational speed of 26,000 R. P. M. to produce a permanent deformation l0 of the order of .010 inch.

The exaggerated representation of bore portion 4' shown in Fig. 2 of. course represents what would happen if there were no driving shaft inserted in the bore 2. with a shaft assembled as in Fig. 1, the deflection of bore portions 4' and 1' shown in Fig. 2 cannot actually take place. Instead, the centriiugal forces acting on the impeller at high speed tend to compress the bore portion 4' against the bushing, while the other bore portion 1' tends to expand away from the bushing.

The reason for this interesting phenomenon is at present believed to be as follows. .The centrifugal force on a solid particle may be represented by the formula where m is the mass of the particle. 22 is the instantaneous linear velocity of the particle relative to the axis, and r is the distance of the particle from the axis. As is well known to those skilled in the art of high speed rotor design, centrifugal forces set up in operation normally tend to make all parts of the rotor expand during operation. However, because of the very material difference between the forces generated in the hub and web portion 1 and those generated in the hub portion 4, the portion 1 tends to "expand" relatively more than does the portion 4. Any actual difierential expansion between the two portions is resisted because of the fact that the portion 4 is secured to, or formed integral with, the

portion '1. The effect of these differential centrifugal forces is to deflect the cross section of the impeller in the manner represented in Fig. 2. The tendency of the portion I to expand is so superior to the corresponding tendency of the portion 4 that, by reason of the connection between the two portions, part 4 actually tends to contract, while part I expands. It has been found in practice that with an impeller suchas those used in engine superchargers and having a tip diameter of seven inches and a shaft bore of two inches diameter, the air inlet end portion 4' of the bore may, after high speeding at 32,000 R, P. lVL, be found to have decreased in diameter, as indicated at H, by an amount of the order of .0005 inch, while the opposite end of the bore increased an amount [2 of the order of .006 inch. The above figures are from an actual test. The observed Values'will of course vary with details of design and nature of the material.

and then assembling it with'suitable clearances. When the bushing returns to ambient temperature, it will of course expand into tight frictional engagement with the impeller. The latter process The plane represented by dotted line 26 may be considered to be the neutral surface separating that part of the bore which tends to contract from that portion which tends to expand.

Depending on the characteristics of the material from which the impeller is made, its geometrical shape, and the speed at which it is operated, the centrifugal forces may or may not produce permanent deformations like those described above. However, even though no permanent deformation is produced, it is clear that the impeller must have, during high speed operation, the same tendency to deflect in the manner described above and illustrated in Fig. 2.

My invention includes an improved bushing arrangement for mounting a high speed rotor on its driving shaft, which makes use of the above-described phenomenon to simplify the bushing by the elimination of all positive driving connections between the bushing and rotor.

Referring again to Fig. 1, it will be seen that the impeller is provided with a bushing represented generally at l3 comprising a cylindrical sleeve [4 having adjacent one end a radially extending flange I 5. At the outer circumference of flange I 5 is a circumferential portion l6 defining an inner annular shoulder l'i.- The hub portion of the impeller is provided with an axially extending portion i8 surrounding the shaft bore 2 andforming a circumferential shoulder arranged to be tightly engaged by shoulder II of circumferential portion H3. The bushing l3 may be assembled to the impeller by a pressing operation, or by shrinking the bushing by refrigerating it may be preferable when assembling steel bushings to impellers made from light metals such as aluminum or magnesium alloys.

as the cotter-pin 22, may be used to prevent rela- I tive rotation of nut 20 on the reduced threaded shaft portion 23.

It will be readily apparent to those skilled in the art that impeller nut 20 may be used as a clamping device to force the impeller into proper position on the bushing I 3, with portion l8 engaging surface IT as shown in Fig. 1. Alternatively, the bushing and impeller may be assembled with a press or shrink fit as suggested above, the impeller nut 20 then being used merely as a safety device to prevent separation of the impeller from the bushing, and of the bushing from the shaft.

It will now be observed that one end portion of sleeve l4 engages bore portion 4" so as to prevent the contraction II represented in Fig. 2; while the flange I5, I 6 engages the impeller at surface i? so as to prevent the expansion I2 of bore portion 1' represented in Fig. 2. It will now be appreciated that the effect of centrifugal force on the rotor at high speed is to contract hub portion 4, thereby increasing the friction force between bore portion 4' and the rotor supporting member; while at the same time the expansion tendency of web and hub portion 1 results also in increased friction force at the surface 11. Thus the total friction force available for transmitting torque between shaft and impeller increases with an increase in speed. I have found that this increase in friction force is capable of providing the sole driving connection between impeller and shaft at all operating conditions. Therefore, my invention permits the elimination of all positive connections such as rivets, splines, keys, etc., between bushing I 3 and the impeller. Bushing l3 may be keyed to shaft 3 by a standard key 21. It

' will be obvious that other suitable means, such as ing blades i seem to act somewhat as stiffening ribs tending to hold body portions 4 and I in their original positions relative to one another, thereby increasing the tendency for portion 4 to contract as portion 1 expands.

Fig. 3 illustrates a modification of the bushing arrangement of Fig. 1. In this form, the impeller hub and web portion 1 is recessed and the bushing i3 of Fig. 1 is replaced by a plain cylindrical bushing l4 and a separate reinforcing ring 25, both of which may be shrunk or pressed into position. The recess in the back of the impeller may be dimensioned so that the end surfaces of bushing l4 and of hub portion l8 lie in a plane common to the back surface 24 of portion I. The arrangement of the reinforcing ring 25 is substantially as disclosed in Patent 2,392, l ,58 issued January 15, 1946, on an application Serial No, 478,334, filed March '8, 1943, in the name of Kenton D. McMahan.

Fig. 4 illustrates a modification in which the separate bushing of Fig. 1 is eliminated and the impeller is supporteddirectly on shaft 3, with flange II, it formed integral'with the shaft.

In the modification of Fig- 5, the impeller is likewise mounted directly on shaft I, with a reinforcing ring 25 being used. as in Fig. 8.

It will be observed that when the separate rin 25 is used, as in Figs. 8 and 5, increased friction between hub portion 18 and ring 25 will not increase the torque-transmitting capacity of the connection between impeller and shaft, In this case, the ring 25 merely serves toretain hub portion l8 in frictional engagement with the bushing or shaft, as the case may be; while the friction force on bore portion 4' does increase with increasing speed, as described above. I

It will-be apparent that the shaft, bushings, and reinforcing rings may be made of any suitable material, such as high tensile strength steel. My invention is particularly applicable to the construction of centrifugal compressor impellers made of lightweight alloys of aluminum or magnesium, which have a comparatively low yield strength and therefore are particularly subject to the type of deformation represented in Fig. 2.

It will be obvious that the bushing of Fig. 1 may be constructed with portions l5, l8 arranged in a recess in the back of the impeller, as shown in Figs. 3 and 5.

While I have described my invention as applied to a centrifugal compressor impeller, it will be understood that it is also applicable to other high speed rotors subject to extreme centrifugal forces in operation, such as axial flow compressors and turbine rotors of both axial and radial flow types.

By my discovery of the interesting phenomenon described herein, I have been enabled to provide a new, greatly improved arrangement for mounting high speed rotors on a driving shaft. My improved arrangement eliminates diflicult and costly manufacturing operations, permits easy assembly and disassembly of the rotor on its shaft. and ready replacement of damaged impeller bushings. Furthermore, it has been found that the bursting strength of rotors incorporating my invention is very materially improved, by reason'of elimination of the stress-concentrating rivet holes.

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

1. A high speed rotor including a first central hub portion defining a shaft bore and having a inner circumferential surface frictionally engaging the outer surface of an annular shoulder defined by said second hub portion.

2. A high speed rotor including a first central hub portion defining a shaft bore and having a comparatively small mass and radius of gyration, a second hub portion secured to and axially adjacent the first hub portion and having a radially 6 extending web-portion. the combinerrsecondimb and web portion having a mass and radius of gyration substantially greater than the first hub portion, a rotor supporting and driving member with asmoqth outer surface and of circular cross section frictionallyengaging the inner surface of said shaft bore, and a reinforcing ring. member radially spaced from and concentric with said supporting and driving member and having an inner circumferential surface frictionally engaging the outer surface of an annular shoulder defined by said second hub portion.

3. A high speed rotor including a first central hub portion defining a shaft bore and having a comparatively small mass and radius of gyration, a second hub portion secured to and axially adjacent the first husb portion and having a radially extending web portion, the combined second hub and web portion having a mass and radius of gyration substantially greater than the first hub portion, a plurality of circumferentially spaced radially extending blades secured to the outer circumferential surface of said first hub portion and to the web portion, a rotor supporting and driving member with a smooth substantially cylindrical outer surface frictionally engaging the inner surface of said shaft bore, and a reinforcing ring member radially spaced from'and concentric with said supporting and driving member and having an inner circumferential surface frictionally engaging the outer surface of an annular shoulder defined by said second hub portion.

4. A high speed ro'tor including a first portion having a comparatively small'mass and radius ofgyration, a second portion axially adjacent and secured to the first portion and having an appreciably greater mass and radius of gyration, the central part of both portions defining an axial shaft bore, a rotor supporting and driving member frictionally engaging the inner surface of the shaft bore and having adjacent one end of the bore a radially extending flange provided at its outer circumference with an annular shoulder extending axially towards the second rotor portion and defining an inner circumferential surface frictionally engaging the outer surface of an annular shoulder formed on the second rotor pornular shoulder formed on said second rotor portion at a location axially spaced from said first portion.

6. A high speed rotor subject to elastic deformation under centrifugal stresses in operation, comprising a first rotor portion having a comparatively small mass and radius of gyration, a second rotor portion secured to and axially adjacent the first portion and having a substantially greater mass and radius of gyration, the central part of both portions defining a bore for mounting the rotor on a supporting shaft, a one-piece bushing for the shaft bore including a sleeve with a smooth substantially cylindrical outer surface frictionally engaging the inner surface 01' the shaft bore, a radially extending flange secured to REFERENCES CITED the end of the sleeve remote from the first rotor The 10110W1n8 r ferences are of record in the portion, and an annular shoulder at the outer file of this P tent:

circumierence of the flange, said shoulder extend- 5 UNITED ing axially from the flange toward the second STATES PATENTS rotor portion and having an inner circumteren- Number Name Date tial surface frictionally engaging the outer sur- 875-183 Dec. 31, 1907 face of a circumferential shoulder formed on the 1,771,349 P1111561 July 22, 1930 second rotor portlon.. 10 1373353 Dahlfltmnd 18. 3 1932 HUGH p MCGEE 2,392,858 McMahan Jan. 15, 1946