US3756738A

US3756738A - Centrifugal pump with differential thermal expansion relief means

Info

Publication number: US3756738A
Application number: US00191863A
Authority: US
Inventors: W Lee
Original assignee: Clarkson Industries Inc
Current assignee: Rexnord Industries LLC
Priority date: 1971-10-22
Filing date: 1971-10-22
Publication date: 1973-09-04
Anticipated expiration: 1990-09-04
Also published as: GB1347616A; DE2249626A1; IT965255B; FR2157515A5

Abstract

A centrifugal blower having one or more impellers mounted on a shaft for rotation therewith with the impellers and shaft being made of materials having different thermal coefficients of expansion and the impeller hubs being provided with means for relieving compressive stresses generated by differential expansion. The stress relieving means comprises disk-shaped channels formed in the impeller hubs to permit the central portions of the impeller hubs to be resiliently deformed to relieve the compressive stresses, thereby avoiding shaft distortion which produces vibration of the blower.

Description

111 3,756,738 Sept. 4, 1973 United States Patent (1 1 Lee [ CENTRIFUGAL PUMP WITH DIFFERENTIAL THERMAL EXPANSION RELIEF MEANS [75] Inventor: Wilfred J. Lee, East Syracuse, N. Y.

[73] Assignee: Clarkson Industries, Inc., New York, Raduazo Att0rneyG0rdon K. Lister 22 Filed: Oct. 22, 1971 21 App1.No.: 191,863

ABSTRACT A centrifugal blower having one or more impellers mounted on a shaft for rotation therewith with the impellers and shaft being made of materials having different thermal coefficients of expansion and the impeller hubs being provided with means for relieving compressive stresses generated by differential expansion. The stress relieving means comprises disk-shaped channels formed in the impeller hubs to permit the central portions of the impeller hubs to be resiliently deformed to relieve the compressive stresses, thereby avoiding shaft distortion which produces vibration of the blower.

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FIG.2

CENTRIFUGAL PUMP WITH DIFFERENTIAL THERMAL EXPANSION RELIEF MEANS This invention relates to centrifugal pumps, and more particularly, to a means for relieving axial stresses due to differential thermal expansion of the impellers of such a pump and the drive shaft on which they are mounted for rotation. While the present invention is adapted to be used in all types of centrifugal pumps including liquid pumps, centrifugal blowers, and centrifugal compressors, it will, for cnvenience, be described as applied to a typical centrifugal blower.

Multi-stage centrifugal blowers generally comprise a housing with inlet and outlet ports and having a drive shaft mounted for horizontal rotation in bearings supported in the housing. Mounted on and secured to the drive shaft for rotation therewith there are a series of impellers having radial passages therein which cooperate with transfer conduits formed in the housing to transfer the pumped fluid from stage to stage of the blower. The hub portions of the impellers may directly abut one another or may be separated by spacer rings which maintain a predetermined spacing between the impellers. The impellers, either with or without spacing rings therebetween, are locked together against axial movement relative to the drive shaft by suitable locking means such as, for example, locking nuts on the drive shaft.

It is a common practice to make the impellers and drive shaft of different materials. For example, the drive shaft may be made of steel to achieve the desired structural strength and rigidity whereas the impellers may be made of aluminum to reduce their weight. When in operation the blower generates heat due to compression of the gas, recirculation and disk friction. Since aluminum has a larger coefficient of thermal expansion than steel, the increased temperature during operation of the blower tends to generate a compressive stress in the hub portions of the impellers.

Due to imperfections in the machining operations, the confronting surfaces of adjacent impeller hubs are rarely, if ever, precisely perpendicular to the axis of the drive shaft on which they are mounted. As a result, the compressive stresses within and between the impeller hubs tend to bend the shaft slightly and the resulting distortion of the shaft produces vibrations at the operating speed of the blower and in extreme cases can cause mechanical failure of the blower.

It is accordingly an object of the present invention to provide a centrifugal pump including a simple and effective means for relieving compressive stresses that may be generated in the hub portions of the impellers of the pump during operation thereof. It is another object of the invention to provide a multi-stage centrifugal blower including means for eliminating vibration of the rotor thereof due to deformation of the drive shaft caused by axial stresses generated in' the hub portions of the impellers of the rotor. It is still another object of the invention to provide meansfor making a portion of the hub of at least one impeller of such a compressor resiliently movable to accommodate or relieve axial stresses generated in the hub portions of the impellers. Other objects of the invention will be in part obvious and in part pointed out hereafter.

The objects and advantages of the present invention may best be understood and appreciated by reference to the accompanying drawing which incorporates a preferred embodiment of the invention and wherein:

FIG. 1 is a side view of a 3-stage centrifugal blower, partially in section, to show the arrangement of the impellers on the drive shaft and the pressure-relieving channels in the hubs of the impellers; and,

FIG. 2 is an enlarged section of one of the impeller hubs further showing the pressure relieving channel therein.

Referring to the drawing, the numeral 10 generally designates a conventional 3-stage centrifugal blower having an inlet 12, an outlet 14 and a horizontal drive shaft 16 driven by a suitable prime mover (not shown). In FIG. 1 the lower portion of the blower casing is broken away to show the arrangement of the impellers and transfer conduits therein. Mounted on shaft 16 are three impellers, namely, a first stage impeller 18, a second stage impeller 20 and a third stage impeller 22. The impeller 18 is secured to shaft 16 for rotation therewith by a key 24.

Impellers

20 and 22 are similarly keyed to the shaft 16 but the keys are notshown in FIG. 1. The impellers 18 to 22 have the usual

radial passages

18a, 20a and 22a formed therein and hub portions 18b, 20b and 22b adjacent to the shaft 16.

As shown in FIG. 1, the hubs of the impellers are separated by spacer rings which are slideably mounted on shaft 16. More particularly, there is a spacer ring 26 that abuts the left-hand or front face of the hub of impeller 22 and right-hand or rear face of the hub of impeller 20. Similarly, there is a spacer 28 that abuts the I front face of the hub of impeller 20 and the rear face of the hub of impeller l8. A third spacer ring 30 abuts at one end the front face of the hub of impeller 18 and at its other end abuts a shoulder 32 formed by an enlarged portion 34 of the shaft 16. The spacers and impeller hubs are locked against axial movement in relation to shaft 16 by the shoulder 32 at one end of the space-hub assembly and by a lock nut 36 threaded onto shaft 16 at the other end of the assembly. As is known in the art, the impeller hubs are in some cases so designed that they abut one another without the intervening spacer rings.

The structure so far described is conventional as is the general operation of the blower. The flow of air through the blower is indicated by the arrows in FIG. 1. Upon rotation of the shaft 16 and

impellers

18, 20 and 22, air or other pumped fluid is drawn through inlet 12 to the radial passage 18a of first stage impeller 18 wherein it is forced outwardly and compressed by centrifugal force to the peripheral portion of the casing.

I The air discharged from passage 18a is conducted by a transfer conduit 38 formed in the blower casing to the inlet of radial passage 20a of impeller 20. The air is further compressed as it passes through the passage 20a of second stage impeller 20 and is then conducted by transfer conduit 40 to the inlet of radial passage 22a of third stage impeller 22. After being further compressed in passage 220, the air flows through a conduit 42 to the blower output 14.

As indicated above, it is customary to make the shaft 16 and impellers 18, 20' and 22 of different materials. For example, the shaft may be made of steel and the impellers of cast aiuminum or bronze which have different thermal expansion coefiicients. During operation of the blower heat is generated by compression of the gas and other factors. As the shaft and impellers heat up, axial compressive stresses are generated in the hubs which tend to distort the drive shaft and produces undesirable vibrations of the rotor assembly. In accordance with the present invention these stresses are relieved by making a portion of the hub of at least one of the impellers resiliently deformable.

Referring to FIG. 2 of the drawing, which is a greatly enlarged view of the hub b of impeller 20, there is formed at the internal periphery of the hub a diskshaped channel 44 that extends radially outward into the hub 20b. The channel 44 is located relatively close to the rear face 46 of the hub. Thus when the portion of the hub near shaft 16 is subjected to compressive stresses of the type referred to above, the portion48 of the hub between channel 44 and the rear face 46 of the hub is resiliently deformable to relieve such stresses and shaft distortion due to such stresses is avoided.

In some cases, as for example when the differential thermal expansion of the impeller hubs and the shaft is relatively small, only one of the impellers need be provided with a stress-relieving channel. On the other hand, when the conditions of operation of the blower are such that the differential thermal expansion of the impellers and shaft is relatively large, it is desirable that each of the impellers be provided with a stress-relieving channel as indicated in FIG. 1.

From the foregoing description it should be apparent that the present invention provides a structure capable of achieving the objects set forth at the beginning of the present specification. As axial compressive stresses build up in the impeller hubs and spacer rings due to differential thermal expansion, the channels 44 permit the central rear portions of the impellers to be resiliently deflected inwardly to relieve such stresses. Thus shaft distortion and the consequent undesirable vibration of the rotor are avoided.

Also, as pointed out above, as a result of machining imperfections, the faces of the impeller hubs may not be precisely perpendicular to the axis of the shaft and stresses tending to bend the shaft may be generated as a result of these machining imperfections. Such bending stresses may occur even when there is no differential thermal expansion of the impeller assembly and shaft. Such bending stresses are also achieved by the channels formed in the impeller hubs in accordance with the present invention.

It will of course be understood that the shaft and impellers can be made of metals other than those specifically disclosed and the present structure will provide a stress-relieving function to eliminate or substantially reduce the undesirable effects of differential thermal expansion of such other metals.

It is, of course, to be understood that the foregoing description is intended to be illustrative only and that various modifications may be made in the specific structure described without departing from the spirit of the invention as defined in the appended claims.

I claim:

l. A centrifugal blower comprising in combination a housing, a rotatable shaft mounted in said housing, a plurality of impellers within said housing having hubs mounted on and secured to said shaft for rotation therewith and locking means for locking said impeller hubs in fixed relation to one another and holding them against axial motion in respect to said shaft, said impellers being made of a material having a greater thermal coefficient of expansion than said shaft, at least one of said impeller hubs having an internal peripheral channel formed therein adjacent to said shaft to form a resilient deformable hub portion whereby axial stresses generated by differential thermal expansion of said shaft and hubs and consequent bending stresses generated in the hub portions of the impellers are relieved.

2. A centrifugal blower comprising in combination a housing, a rotatable shaft mounted in said housing, a plurality of impellers within said housing having hubs mounted on and secured to said shaft for rotation therewith and locking means for locking said impeller hubs in fixed relation to one another and holding them against axial motion in respect to said shaft, said impellers being made of a material having a greater thermal coefficient of expansion than said shaft, said impeller hubs having front faces with inlet ports formed therein and rear faces substantially perpendicular to said shaft, at least one of said hubs having a disk-shaped internal peripheral channel formed therein close to the rear face of said hub whereby the portion of said hub between said channel and the rear face of the said hub is resiliently and axially deformed to relieve axial and bending stresses generated in the hub portions of the impellers by differential thermal expansion of said hub portions and said shaft.

3. A centrifugal blower according to claim 2 wherein each of said impeller hubs is provided with one of said channels.

4. A centrifugal blower according to claim 2 wherein said channel extends radially into said hub in a direction substantially perpendicular to the rotational axis of said shaft.

5. A centrifugal blower according to claim 2 wherein spacer rings are mounted on said shaft between said impeller hubs to maintain a predetermined spacing between said hubs.

6. A centrifugal blower comprising in combination a housing, a plurality of impellers within said housing having hubs mounted on and secured to said shaft for rotation therewith, spacer rings mounted on said shaft between said impellers for maintaining a predetermined spacing between said impeller hubs and locking means on said shaft for locking said impeller hubs and spacer rings together and holding them against axial motion in respect to said shaft, said impellers being made of a material having a greater thermal coefficient of expansion than said shaft, said impeller hubs having front faces with inlet ports formed therein and rear faces substantially perpendicular to said shaft, said hubs having internal peripheral channels formed therein close to the rear faces of said hubs, whereby the portion of said hubs between said channels and the rear faces of said hubs are resiliently and axially deformed to relieve axial and bending stresses generated in the hub portions of the impellers by differential thermal expansion of said hub portions and said shaft.

7. A centrifugal blower according to claim 2 wherein said peripheral channel extends radially into its hub to a point axially opposite the inlet port of said hub.

Claims

1. A centrifugal blower comprising in combination a housing, a rotatable shaft mounted in said housing, a plurality of impellers within said housing having hubs mounted on and secured to said shaft for rotation therewith and locking means for locking said impeller hubs in fixed relation to one another and holding them against axial motion in respect to said shaft, said impellers being made of a material having a greater thermal coefficient of expansion than said shaft, at least one of said impeller hubs having an internal peripheral channel formed therein adjacent to said shaft to form a resilient deformable hub portion whereby axial stresses generated by differential thermal expansion of said shaft and hubs and consequent bending stresses generated in the hub portions of the impellers are relieved.