US3730641A - Centrifugal pumps - Google Patents

Abstract

A multi-stage submersible centrifugal pump comprising a drive shaft having a plurality of plastic impellers mounted thereon for rotation therewith. Such impellers have hubs mutually engageable in stacked relation. A plurality of diffusers in stacked relation, one for each impeller, is provided with a diffuser plate having an annular embossment coaxially disposed with respect to a central opening in the plate. This annular embossment is engageable with similar annular embossment on the impeller which coaxially encircles the inlet opening therein, both embossments having radially extending sealing faces which are engageable with each other thereby to prevent recirculation of water between stages. The impeller hubs as well as the engageable annular embossments are formed of particular plastic materials, the two annular embossments being of different plastic materials which are non-fusible, lubricous, and have ''''cold flow'''' properties which permit deformation of at least one of the embossments during pump operation. Particular plastic materials and dimensions are so selected that ordinary manufacturing tolerances may be compensated for by reason of the ''''cold flow'''' properties of the plastics whereby in one case deformation of the engaged plastic hubs may occur to provide sealing engagement of the two annular embossments, or alternatively deformation of one of the annular embossments engaged with the other may occur to provide thrust-transmitting engagement of the hubs.

Description

Uite States atent 1 Gordon CENTRIFUGAL PUMPS Donald L. Gordon, South Shores, Incl.

[73] Assignee: Flint & Walling, line, Kendallville,

Ind.

[22] Filed: Mar. 10, 1972 [21] Appl. No.: 233,570

[75] Inventor: Kimmellss [52] 1.1.5. Cl ..4l5/l99 A,415/501,4l5/214 [51] lint. Cl ..F04d 29/44, F04d 13/08, F04d 7/00 [58] Field of Search ..415/214, 199 A, 140,

[56] References Cited UNITED STATES PATENTS Primary Examiner-Henry F. Raduazo Attorney-Harold B. Hood et al.

[4 1 May 1, 1973 [57] ABSTRACT A multi-stage submersible centrifugal pump comprising a drive shaft having a plurality of plastic impellers mounted thereon for rotation therewith. Such impellers have hubs mutually engageable in stacked rela- 'embossment is engageable with similar annular em- 7 bossment on the impeller which coaxially encircles the inlet opening therein, both embossments having radially extending sealing faces which are engageable with each other thereby to prevent recirculation of water between stages. The impeller hubs as well as the engageable annular embossments are formed of particular plastic materials, the two annular embossments being of different plastic materials which are non-fusible, lubricous, and have cold flow properties which permit deformation of at least one of the embossments during pump operation. Particular plastic materials and dimensions are so selected that ordinary manufacturing tolerances may be compensated for by reason of the cold flow properties of the plastics whereby in one case deformation of the engaged plastic hubs may occur to provide sealing engagement of the two annular embossments, or alternatively deformation of one of the annular embossments engaged with the other may occur to provide thrust-transmitting engagement of the hubs.

9 Claims, 11 Drawing Figures PATENIED W5 3130x341 SHEET 1 BF 4 FIG] CENTRIFUGAL PUMPS BACKGROUND OF THE INVENTION Multi-stage centrifugal pumps conventionally mount the impellers on the shaft in such a manner that the axial thrust of each impeller is transmitted to the shaft. A single thrust bearing is provided for taking up the entire axial thrust on the shaft. In one common design, the impellers are provided with hubs having stacked engagement, the thrust of the impellers being transmitted through the hubs to the shaft and finally to a thrust bearing. When a large number of pump stages are used, the total axial thrust on the shaft becomes substantial. When all of the impellers are connected to the main drive shaft to transmit the axial impeller thrust to the shaft, it is necessary accurately to space the impellers relative to the respective pump diffuser casing so that the impellers are not pressed axially against the respec tive diffuser casing. Commonly, the impellers are mounted on the shaft with the adjacent impeller hubs in abutting engagement to space the impellers relative to the diffuser casings. This arrangement requires accurate machining of the lengths of the impeller hubs to provide the proper spacing. It has been commonly understood that because of the axial pressures applied to the impellers during operation, the use of plastics has been limited. Furthermore, because of the close tolerance requirements on the hub lengths, seals for preventing recirculation have been provided by means of closely spaced juxtaposed cylindrical and radial surfaces on the impeller and diffuser, respectively. Sand and abrasives may become lodged between such cylindrical surfaces thereby causing rapid wear and disruption of the seal.

SUMMARY OF THE INVENTION In accordance with the broader aspects of this invention, certain problems inherent in prior art structures have been overcome with reference to the structures employing radial sealing surfaces, the maintenance of spacing tolerances therebetween is critical, but is minimized by this invention. In a different structure, problems in connection with sand or abrasives becoming lodged between the cylindrical sealing surfaces which thereby results in either stalling the pump or producing excessive wear of the sealing surfaces are avoided.

In this connection, there is provided a multi-stage submersible centrifugal pump comprising a drive shaft, a plurality of impellers each having a hub, means for mounting said hubs on said shaft in stacked relation for rotation therewith, said impellers each having a discshaped body portion provided with radial passages therethrough, one side of the body portion having a central inlet opening communicating with the radially inner ends of said passages, the other side of the body portion connecting with the hub thereof. A plurality of diffusers in contiguous stacked relation are provided, one for each impeller. Each diffuser includes a cup having a flat bottom provided with a journal opening which receives for rotation the central hub of the respective impeller. The bottom is provided with a plurality of ports adjacent to the periphery and a plurality of radial diffuser vanes on the side opposite the impeller body portion. A diffuser plate is mounted on the cup for defining with said bottom a chamber which encloses the impeller, the diffuser plate having a central opening larger than the central hub. A first annular embossment is provided with a radially extending sealing face coaxially disposed on the diffuser plate, an annular portion on the impeller side adjacent to the embossment being provided also with a radially extending sealing face engageable with the sealing face of the embossment. Means are provided for holding the diffuser and plate stationary with respect to the impellers, and other means are provided for absorbing the thrust of said impeller hubs in one direction. The annular embossment and the annular portion are formed of dissimilar, nonfusible, lubricous plastic materials having predetermined cold flow properties. The central hubs are also of plastic material of predetermined cold flow" properties, the aforesaid cold flow" properties being such as to permit deformation of the engaged plastic hubs to provide sealing engagement of the annular embossment with the annular portion or alternatively deformation of the annular embossment engaged with said annular portion to provide thrust-transmitting engagement of said hubs. Thus, normal manufacturing tolerances may be compensated for by normal operation of the pump in which the back pressure of the water being pumped acts on the various parts to produce the aforesaid deformation whereby positive seals against recirculation are provided as well as maximum wear life of all the engaged parts.

It is an object of this invention to provide a centrifugal pump having an improved arrangement for limiting recirculation.

It is another object of this invention to provide a pump having long wear life, improved seals between the impeller and diffuser, and which is self-adjusting to compensate for manufacturing tolerances in a manner that provides the aforesaid seal. 7

It is yet another object of this invention to provide a pump wherein a unique plastic seal for preventing recirculation also serves to minimize the possibilities of the pump stalling due to sand or other abrasives becoming lodged between the sealing inner faces.

1 A still further object of this invention is to provide a pump which is the ultimate in structural simplicity, is economical to construct and maintain, and furthermore, is reliable in operation over relatively long periods of time.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partly sectioned longitudinally, of one embodiment of this invention;

FIG. 2 is a bottom view of one of the diffuser cups; FIG. 3 is a cross-section taken substantially along section line 3--3 of FIG. 2;

FIG. 4 is a view of the opposite side of the diffuser; FIG. 5 is a plan view of the diffuser plate;

FIG. 6 is a cross-section taken substantially along the section line 66 of FIG.

FIG. 7 is a fragmentary, sectional view of another embodiment of the diffuser plate;

FIG. 8 is a fragmentary sectional view of the diffuser plate assembled to the diffuser cup;

FIG. 9 is a fragmentary cross-sectional view of an impeller;

FIG. 10 is a fragmentary cross-section of one of the two molded parts of the impeller; and

FIG. 11 is a bottom view of the molded part of FIG. 10 showing the vane structure.

The drawings are substantially to scale.

Referring to the drawings, the improved pump of this invention, shown as being embodied in a submersibletype pump includes a drive motor 20, having an output shaft 24, impeller shaft 22 of hexagonal cross-section is coupled to the motor shaft 24 for rotation therewith by means of a sleeve coupler 26 splined to the shaft 24 and connected to the shaft 22 by means of a companion hexagonal bore 28. A thrust bearing (not shown) of conventional construction is coupled between the shaft 24 and the motor frame (also not shown) to absorb thrust from the abutted shafts 22 and 24 exerted in a rightward direction as shown in FIG. 1. The upper (left) end of the shaft 22 is journaled in a bearing 30 mounted on the frame part 32 having at the upper end a conventional pipe coupling 34. To the frame part 32 is threaded the conventional cylindrical shell 36 which is also threaded at its lower end to the casing 38 having enlarged water inlet openings 40 around the circumference thereof. Conventionally, a screen 42 encircles the casing 38.

Within the shell 36 is mounted a plurality of stacked pumping stages, each stage consisting of a centrifugal impeller 44, a diffuser 46, and a diffuser plate 48. Since these parts for the several stages are identical in construction, like numerals are used to indicate like parts.

The impellers 44 are generally disc-shaped and formed of a plastic material having a constituency explained in more detail later on. Each impeller 44 has a central hub 50 provided with a hexagonal bore 52 for a sliding fit with the shaft 22. The opposite ends 54 of hub 50 are flat and define planes normal to the axis of the hub as well as the shaft 22.

As shown more clearly in FIGS. 9, l0, and 11, the impeller 44 is formed of two molded parts, one of the parts as shown in FIGS. 10 and 11 being composed of the hub 50, a radially extending supporting disc 56 which carries on one face thereof a plurality of centrifuging vanes 58 of conventional configuration. The vanes 58 are formed to define a flat plane 60, a disc or shroud 62 (FIG. 9) being flatly secured to this surface for completing the impeller structure. While adhesives may be used for securing the shroud 62 to the vane surfaces 60, for the particular plastic materials used for these two parts, ultrasonic welding is preferred.

The shroud 62 has a water inlet opening 64 of larger diameter than and coaxial with the hub 50, and on the outer surface a coaxial annular embossment or sealing pad 66 having a radially extending sealing surface 68 that defines a plane normal to the axis of the hub 50.

As shown more clearly in FIG. 1, the hubs 50 are stacked in abutting superposed relation on a shaft 22, a suitable properly dimensioned spacer 70 also received on the shaft 22 being interposed between the lowermost hub 50 and the coupling 26. Considering for the moment the hub 50 which directly engages the spacer 70, it will be noted that the sealing surface 68 engages a flat, annular, radial surface 72 on the upper end of the frame casing 38. This upper casing end has an opening 74 coaxial with respect to the shaft 22 for the flow of water to the first impeller 44.

Each diffuser 46 is essentially a single molded part and is shown more clearly in FIGS. 2, 3, 4 and 8. Generally, the diffuser is cup-shaped, having a cylindrical shell portion 76 and an essentially flat bottom 78 provided on one side with the usual radial vanes 80 which define passages therebetween which terminate in a plurality of circumferentially spaced openings 82 at the perimeter of the bottom 78. A central circular opening 84 fitted with a metal bushing in the bottom 78 is sized to provide a running fit with a hub 50 as shown more clearly in FIG. 8. The surface 86 of the plate 78 opposite the vanes 80 is generally radially flat except in the perimetral portion inclined ramps 88 are provided which has the thinnest portion 90 adjacent to an opening 82 and the thickest portion 92 distally therefrom. This diffuser construction is conventional. The opposite rims of the diffuser shell 76 define planes which are parallel and normal to the axis of the shaft 22 in the assembled pump.

A diffuser plate 48 (FIGS. 5, 6, and 8) is molded of plastic as a separate part. This plate 48 for one stage is positioned to overlie the diffuser vanes 80 therebelow to enclose the diffuser passageways. The plate periphery is rabbeted as shown to provide locating shoulders94 and radial abutment surfaces 96 whereby the plate 48 is located both radially and axially with respect to the stacked assembly. An enlarged central circular opening 98 in the plate 48 is defined by an annular coaxial embossment or wear ring 100 having a flat sealing face 102 in a plane normal to the axis of the shaft 22. This face 102 as shown more clearly in FIG. 1 is coaxially engaged by the surface 68 of the adjacent impeller, to provide a water seal.

The impeller 44 as well as the diffuser 46 are preferably fabricated of the same plastic material, the constituency of this being described more fully later on.

A slightly different embodiment of the diffuser plate 48 is shown in FIG. 7, wherein an annular ridge or crush ring 104 upstands coaxially from the surface 102. Depending upon design preferences, the surface 102 may be truly flat and radial or in the alternative have the crush ring 104 thereon.

As shown in FIG. 1, the pump stages are stacked with the impeller hubs 50 in superposed abutting engagement and the surfaces 68 and 102 engaged as shown. Similarly, all diffuser parts are stacked in abutting engagement, the lowermost diffuser cup abutting against and fitting into a companion locating groove 106 in the upper end of the frame casing 38.

In operation, the pump is commonly in an upright position, the coupling 34 being uppermost. With the motor 20 operating, the rotating shaft 22 causes rotation of the impellers 44. Water is drawn through the screen 42, the openings 40, and the inlet 74 into the first impeller 44 via its opening 64. Water is centrifuged outwardly to pass through the openings 82 into the diffuser vane passages between vanes 80 to the central opening 98 in the plate 48. The water from there flows into the next impeller 44 for continued forceful movement through the succeeding pump stages.

The impeller 44 next adjacent to the casing 38, has annular surface 68 engaging an annular portion of the surface 72 to seal against water leakage or recirculation. The two engaged surfaces 68 and 102 also provide a seal against recirculation. By reason of the water pressure exerted on the individual impellers during operation, each impeller is forced downwardly causing intimate rubbing engagement of the two surfaces 68 and 102, the surface 102 being stationary and the surface 68 rotating.

The impeller 44 and the diffuser plate 48 are molded of different plastics which are mutually non-fusing and serve as a bearing couple to each other, lubricous and deformable due to inherent cold flow. A typical material for the diffuser plate is Delrin (a trademark of the duPont Co.) a polyformaldehyde plastic as disclosed in US. Pat. No. 2,768,994 and later patents; and for the impeller, and especially the sealing pad 68, is Noryl (a trademark of the General Electric Co.), a modified polyphenylene oxide plastic, as described in 1970-1971 Modern Plastics Encyclopedia, page 168 et seq., also disclosed in US. Pat. No. 3,383,435. A plastic that may be used instead of Delrin is Arnite, a trademark and a product of Algemene Kunstzijde, Unie N.V. (A.K.U. Holland) commonly referred to as modified polyethylene terephthalate, a poly-condensation product of ethylene glycol and terephthalic acid, a thermoplastic polyester. Arnite plastic is more specifically described in a publication entitled ARNITE, The Thermoplastic Polyester, l/250/e, Apr. 1967, Verenigd Plastic, Verkoopkantoor N.V., Plasttrading, Holland. Alternatively, one of the sealing pad 68 or wear ring 100 may be of metal such as stainless steel so long as it is not fusible with the plastic with which it is engaged as aforesaid.

0f importance is the fact that both of these materials possess limited cold flow," the diffuser plate 48, hence the wear ring 100, cold flowing more readily than the impeller 44, and more particularly, the sealing pad 66.

Dimensions are significant in the preferred embodiment shown, the radial thickness of the wear ring 100 being smaller than that of the sealing pad 66 by the ratio of about 1 to 2. Also, it is desirable that the wear ring have a longer axial dimension than the pad 66 as shown by a factor of about 3 to 1.

It is important that the hubs 50 also be of suitable dimensions substantially as shown such that the cold flow" characteristics of the plastic may be utilized as will now be explained.

During normal pump operation, it is desired that the surfaces 68 and 102 be in rubbing contact. Plastic materials that are non-fusing to each other must be used for these interfaces so as to prevent damage thereto should the pump lose its prime and run dry. The materials above-mentioned have been found to be nonfusible. Also, it is desirable that these materials have a high degree of lubricity so as to minimize wear and provide compatible bearing surfaces. Further, it is important that these plastic materials possess cold flow" properties which will permit run in and positive sealing at the interfaces 68 and 102 after a period of normal pump operation. This is best explained as follows.

Assuming in a newly assembled pump that all of the interfaces 68 and 102 in the various stages are respectively engaged (or ring 104 with face 68 if it is used) but that the hubs 50 have slight clearances therebetween (0.00l inch) after a period of normal pump operation, the water pressure exerted on the impellers causing rubbing contact of the interfaces 68 and 102 will result in "cold flow of the wear rings 100 (and crush rings 104) such that the axial dimensions thereof .become shorter thereby eventually permitting mutual engagement of the hubs 50. Once this occurs, the thrust on the impellers is transmitted via the spacer 70, coupler 26, shaft 22, and shaft 24 to the thrust bearing in the motor frame. Positive sealing contact at the interfaces 68 and 102 is thereby assured preventing recirculation of the water. The load imposed by the head of water pressure is absorbed by both the engaged hubs 50 and the interfaces 68 and 102 thereby providing an effective seal having a long wear life. The crush rings 104 facilitate this deformation and establishment of the seal, because of the minimal radial thickness.

thereof.

0f utmost importance as regards the seal at the interfaces 68 and 102 is the fact that sand and abrasives will be for the most part precluded from entering therebetween, because they are in rubbing engagement. In prior art structures, due to slight clearances between the cylindrical or radial sealing surfaces,-sand or abrasives have entered therebetween causing rapid wear and leakagef An alternative run in" condition is realized in this invention which involves mutual engagement of all the hubs 50 but slight spacing between the interfaces 68 and 102. Under this circumstance, the downward force I exerted by the head of water pressure during pump operation on the impellers causes the plastic material in the hubs 50 to cold flow" and thereby to shorten axially until the interfaces 68 and 102 establish rubbing contact. The positive se'al at the interfaces thereby is formed which continues for the normal life of the pump cold flow properties will vary depending upon the pump design and the water pressures at which the pump will be operated. In the preferred design herein disclosed, the dimensions and materials specified have provided reliable, efficient pump performance with little or no interference therewith due to sand or abrasives in the water.

Recapitulating, the non-fusibiility of the engaged parts is of major importance as it permits the parts to become intimately engaged in stacked configuration, the creepage or cold flow of the material ensuring the seal. Dimensions of the interfaced parts are important to ensure the necessary deformation and creepage which ultimately provides the seal against recirculation. The wear ring is of a radial thickness that ensures deformation thereof. The sealing pad 66 is relatively flat and solid so as to be less yielding.

After a period of run in, zero clearances between the engaged sealing interfaces results which thereby positively prevents recirculation of fluid and quite importantly prevents sand and abrasives from entering therebetween.

In a practical embodiment of this invention, dimensionally the length of the hub 50 is desirably 0.00l inch less than the sum of the diffuser cup 76 and diffuser plate 48 dimensions. This provides a slight clearance between the hubs 50, but permits engagement of the interfaces 68 and 102 to provide the face seals.

Suitable dimensions for a pump capable of delivering five gallons of water per minute at peak efficiency are as follows, it being understood that these dimensions are given by way of example only and not by way of limitation.

PARTS INCHES Length of hub 50 0.795 Hub 50 outer diameter 0.746 Hub 50 inner diameter 0.375 hex. Radial thickness of wear ring 100 0.137 Radial thickness of sealing pad 66 0.488 Axial thickness of sealing pad 66 0.057 Dimension X (FIG. 8) 0.857 Dimension Y (FIG. 9) 0.933 Dimension Z (FIG. 8) 0.796

Significant is the fact that plastic parts are in rubbing contact with each other. Simplicity and economy are realized since only two basic materials are required for the parts of the pump stages, these materials being primarily plastic, no metal inserts or parts being required for the hubs or sealing faces. Highly efficient scaling is secured by the engagement of plastic, radial faces in sharp contrast with prior art structures which utilize spaced sealing faces of both the cylindrical and radial design.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

l. A multi-stage submersible centrifugal pump comprising a drive shaft, a plurality of impellers each having a hub, means for mounting said hubs on said shaft in stacked relation for rotation therewith, said impellers each having a disc-shaped body portion provided with radial passages therethrough, one side of said body portion having a central inlet opening communicating with the inner ends of said passages, the other side of said body portion connecting with the hub thereof, a plurality of diffusers in contiguous stacked relation one for each of said impellers; each diffuser comprising a cup having a bottom provided with ajournal opening which receives for rotation the hub of the respective impeller, said bottom having a plurality of ports adjacent to the periphery and a plurality of radial diffuser vanes on the side opposite the impeller body portion, a diffuser plate mounted on the cup for defining with said bottom a chamber which encloses said impeller, said diffuser plate having a central opening larger than said hub; a

first annular embossment having a radially extending sealing face coaxially disposed on one of said diffuser plates and one side of said impeller adjacent to the central opening thereof for sealing engagement with a radiall extending sealing face of an annular or tion on the ot er thereo one contiguous diffuser aving its cup and radial vanes engaged with the opposite side of the same diffuser plate for defining diffuser passages communicating with the central opening of said plate; a stage of said pump including an assembled impeller, diffuser and plate, as aforesaid, there being a plurality of said stages with the cups and plates alternated and superposed for providing a rigid stacked structure, means for holding said diffusers and plates stationary with respect to said impellers, means for absorbing the thrust of said impeller hubs in one direction; said annular embossment and said annular portion being of dissimilar, non-fusible, lubricous materials, one such material being plastic, having predetermined cold flow properties, said hubs also being of plastic material of predetermined cold flow" properties, the

aforesaid cold flow properties being such as to permit deformation of the engaged plastic hubs to provide sealing engagement of the annular embossment with said annular portion or alternatively deformation of said annular embossment engaged with said annular portion to provide thrust-transmitting engagement of said hubs.

2. The pump of claim 1 in which said annular embossment has a predetermined radial thickness as will permit the aforementioned deformation thereof to provide said sealing engagement.

3. The pump of claim 2 in which said annular portion is also an embossment but of greater radial dimension than said first embossment.

4. The pump of claim 3 in which the plastic material of said first embossment cold flows more readily than that of the second embossment.

5. The pump of claim 3 in which each said impeller hub has flat end surfaces normal to the axis thereof, said first embossment being an integral part of said plate which is formed of the same plasticmaterial as said first embossment, each said hub being an integral part of said other side of said body portion which is formed of the same plastic material as said hub.

6. The pump of claim 5 in which the engageable interfaces of said embossments contact only in a plane that is radial, there being no axial overlapping of said embossments.

7. The pump of claim 6 in which said diffuser plate is of polyformaldehyde plastic and said impeller embossment is of modified phenylene oxide plastic.

8. The pump of claim 6 in which the first embossment is a part of said diffuser plate and said second embossment of said impeller.

9. The pump of claim 1 in which said annular embossment has an annular crush ring on said sealing face which is linelike in radial depth to facilitate deformation thereof upon engagement with said annular portion.

Claims (9)

1. A multi-stage submersible centrifugal pump comprising a drive shaft, a plurality of impellers each having a hub, means for mounting said hubs on said shaft in stacked relation for rotation therewith, said impellers each having a disc-shaped body portion provided with radial passages therethrough, one side of said body portion having a central inlet opening communicating with the inner ends of said passages, the other side of said body portion connecting with the hub thereof, a plurality of diffusers in contiguous stacked relation one for each of said impellers; each diffuser comprising a cup having a bottom provided with a journal opening which receives for rotation the hub of the respective impeller, said bottom having a plurality of ports adjacent to the periphery and a plurality of radial diffuser vanes on the side opposite the impeller body portion, a diffuser plate mounted on the cup for defining with said bottom a chamber which encloses said impeller, said diffuser plate having a central opening larger than said hub; a first annular embossment having a radially extending sealing face coaxially disposed on one of said diffuser plates and one side of said impeller adjacent to the central opening thereof for sealing engagement with a radially extending sealing face of an annular portion on the other thereof; one contiguous diffuser having its cup and radial vanes engaged with the opposite side of the same diffuser plate for defining diffuser passages communicating with the central opening of said plate; a stage of said pump including an assembled impeller, diffuser and plate, as aforesaid, there being a plurality of said stages with the cups and plates alternated and superposed for providing a rigid stacked structure, means for holding said diffusers and plates stationary with respect to said impellers, means for absorbing the thrust of said impeller hubs in one direction; said annular embossment and said annular portion being of dissimilar, non-fusible, lubricous materials, one such material being plastic, having predetermined ''''cold flow'''' properties, said hubs also being of plastic material of predetermined ''''cold flow'''' properties, the aforesaid ''''cold flow'''' properties being such as to permit deformation of the engaged plastic hubs to provide sealing engagement of the annular embossment with said annular portion or alternatively deformation of said annular embossment engaged with said annular portion to provide thrust-transmitting engagement of said hubs.

2. The pump of claim 1 in which said annular embossment has a predetermined radial thickness as will permit the aforementioned deformation thereof to provide said sealing engagement.

3. The pump of claim 2 in which said annular portion is also an embossment but of greater radial dimension than said first embossment.

4. The pump of claim 3 in which the plastic material of said first embossment cold flows more readily than that of the second embossment.

5. The pump of claim 3 in which each said impeller hub has flat end surfaces normal to the axis thereof, said first embossment being an integral part of said plate which is formed of the same plastic material as said first embossment, each said hub being an integral part of said other side of said body portion which is formed of the same plastic material as said hub.

6. The pump of claim 5 in which the engageable interfaces of said embossments contact only in a plane that is radial, there being no axial overlapping of said embossments.

7. The pump of claim 6 in which said diffuser plate is of polyformaldehyde plastic anD said impeller embossment is of modified phenylene oxide plastic.

8. The pump of claim 6 in which the first embossment is a part of said diffuser plate and said second embossment of said impeller.

9. The pump of claim 1 in which said annular embossment has an annular crush ring on said sealing face which is linelike in radial depth to facilitate deformation thereof upon engagement with said annular portion.

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