WO2000006906A1

WO2000006906A1 - Scroll-type vacuum pump

Info

Publication number: WO2000006906A1
Application number: PCT/US1999/009200
Authority: WO
Inventors: Anthony G. Liepert
Original assignee: Varian, Inc.
Priority date: 1998-07-30
Filing date: 1999-04-28
Publication date: 2000-02-10

Abstract

A scroll-type vacuum pumping apparatus is provided with one or more base seals. The apparatus includes a scroll blade set having an inlet and outlet, and an eccentric drive operatively coupled to the scroll blade set. The scroll blade set includes a first scroll member and a second scroll member, each having a base surface, and a scroll blade extending from the base surface. The eccentric drive produces orbiting movement of one of the scroll blades relative to the other scroll blade so as to cause interblade pockets between the scroll blades to move toward the outlet. A base seal is disposed on the base surface between adjacent turns of the corresponding scroll blade. The blade tip of the opposing scroll blade contacts the base seal and slides relative to the base seal during orbiting movement. One or both of the scroll members may be provided with a base seal.

Description

SCROLL-TYPE VACUUM PUMP

FIELD OF THE INVENTION

This invention relates to scroll-type vacuum pumps and, more particularly, to seal configurations which permit the scroll-type vacuum pump to operate across a relatively large pressure differential.

BACKGROUND OF THE INVENTION

Scroll pumps are disclosed in U.S. Patent No. 801,182 issued in 1905 to Creux. In a scroll pump, a movable spiral blade orbits with respect to a fixed spiral blade within a housing. The configuration of the scroll blades and their relative motion traps one or more volumes or "pockets" of a fluid between the blades and moves the fluid through the pump. The Creux patent describes using the energy of steam to drive the blades to produce rotary power output. Most applications, however, apply rotary power to pump a fluid through the device. Oil-lubricated scroll pumps are widely used as refrigerant compressors. Other applications include expanders, which operate in reverse from a compressor, and vacuum pumps. To date, scroll pumps have not been widely adopted for use as vacuum pumps, mainly because the cost of manufacture for a scroll pump is significantly higher than for a comparably sized oil-lubricated vane pump. Scroll pumps must satisfy a number of often conflicting design objectives. The scroll blades must be configured to interact with each other so that their relative motion defines the pockets that transport, and often compress, the fluid within the pockets. The blades must therefore move relative to each other, with seals formed between adjacent turns. In vacuum pumping, the vacuum level achievable by the pump is often limited by the tendency of high pressure gas at the outlet to flow backward toward the lower pressure inlet and to leak through the sliding seals to the inlet. The effectiveness and durability of the scroll blade seals are important determinants of performance and reliability.

Seals for scroll-type apparatus, including a seal element backed by an elastomeric member, are disclosed in U.S. Patent No. 3,994,636 issued November 30, 1976 to McCullough et al. A seal configuration including a sealing strip biased by a silicone rubber tube is disclosed in U.S. Patent No. 4,883,413 issued November 28, 1989 to Perevuznik et al. A seal arrangement for a scroll-type vacuum pump, including a seal element and an elastomer seal loading bladder which may be pressurized, is disclosed in U.S. Patent No. 5,366,358 issued November 22, 1994 to Grenci et al. A scroll-type pump having a seal configuration, including a seal member and a backup member of a soft porous material, is disclosed in U.S. Patent No. 5,258,046 issued November 2, 1993 to Haga et al. Additional seal configurations for scroll-type apparatus are disclosed in U.S. Patent No. 4,730,375 issued March 15, 1988 to Nakamura et al. Prior art tip seals typically include a seal element that forms a sliding seal and an energizer element that forces the seal element against an opposing surface.

Seals critically affect the performance and reliability of scroll pumps. The seal must provide adequate sealing for long periods of time, for example greater than 9,000 hours, with little wear, minimal friction, and over a range of operating temperatures and pressures. In prior art scroll pumps, an axially-compliant tip seal has been mounted in a groove machined into the top edge, or tip, of the scroll blade. The seal slides on the recessed area between turns of the opposing scroll blade and limits leakage across the top of the scroll blade. The tip seal groove at the tip of the scroll blade typically has a width of about 0.1 inch and a depth of about 0.1 inch. Given the small width and the considerable length of the seal groove in a scroll-type vacuum pump, machining of the groove requires considerable time and expense. If the scroll blades are cast of plastic or aluminum, the tip seal groove adds complexity and cost to the mold tooling. Another disadvantage of prior art tip seals is that the walls constraining the tip seals occupy significant radial space within the pump. Although the width of the tip seal is relatively small, the spiral scroll blades may have multiple complete turns in a scroll-type vacuum pump. Additional scroll blade wall width is required to define the tip seal groove. This wall width is typically 0.025 inch on each side of the tip seal. On a scroll set with four complete turns, for example, a total of about 0.5 inch of the pump diameter is taken up by these walls. In general, the manufacturing cost of the pump can be reduced by reducing its diameter.

Accordingly, there is a need for improved seal configurations for scroll-type vacuum pumps. SUMMARY OF THE INVENTION

According to a first aspect of the invention, vacuum pumping apparatus is provided. The vacuum pumping apparatus comprises a scroll blade set having an inlet and an outlet, and an eccentric drive operatively coupled to the scroll blade set. The scroll blade set comprises a first scroll member having a first base surface and a first scroll blade extending from the first base surface, and a second scroll member having a second base surface and a second scroll blade extending from the second base surface. The first and second scroll blades are nested together to define one or more interblade pockets and have first and second blade tips, respectively. The eccentric drive produces orbiting movement of one of the scroll blades relative to the other scroll blade so as to cause the interblade pockets to move toward the outlet. The vacuum pumping apparatus further comprises a base seal disposed on the first base surface between adjacent turns of the first scroll blade. The blade tip of the second scroll blade contacts the base seal and slides relative to the base seal during orbiting movement.

The vacuum pumping apparatus may further comprise a second base seal disposed on the second base surface between adjacent turns of the second scroll blade. The blade tip of the first scroll blade contacts the second base seal and slides relative to the second base seal during orbiting movement.

In another embodiment, the vacuum pumping apparatus further comprises a tip seal on the blade tip of the first scroll blade. The tip seal contacts the second base surface and slides relative to the second base surface during orbiting movement. In this embodiment, the first scroll member may be fabricated of a plastic material.

The edges of the blade tip that contacts the base seal may be chamfered or rounded to facilitate sliding during orbiting movement.

The base seal may comprise a seal element for contacting the blade tip of the opposing scroll blade and an energizer element affixed to the seal element. In a preferred embodiment, the seal element comprises an ultra high molecular weight polyethylene, and the energizer element comprises a foam.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: FIG. 1 is a cross-sectional view of an example of a scroll-type vacuum pump incorporating the base seal of the invention;

FIG. 2 is an enlarged partial cross-sectional view of a scroll blade set and base seals in accordance with the invention; FIG. 3 is an enlarged partial cross-sectional view of a scroll blade set showing an alternate seal configuration in accordance with the invention; and

FIG. 4 shows an example of a base seal configuration in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of a scroll-type vacuum pump incorporating the present invention is shown in FIG. 1. A dry, two-stage vacuum pump is shown. A gas, typically air, is evacuated from a vacuum chamber or other equipment (not shown) connected to a vacuum inlet 12 of the pump. A housing 14 includes a housing member 14a that encloses and in part defines a first scroll pump stage 18 and a housing member 14b that encloses and in part defines a second scroll pump stage 30. Each scroll pump stage includes at least one scroll blade set. An outlet port 16 is formed in the second stage housing.

The first scroll pump stage 18 is located within the housing and is connected to vacuum inlet 12 via a header 21 and one or more intake slots (not shown). Scroll pump stage 18 may be formed by one or more sets of nested spiral-shaped scroll blades. A first stage scroll blade set includes a stationary blade 19 and an orbiting blade 20. Scroll blade 19 is preferably formed integrally with housing member 14a to facilitate thermal transfer and to increase the mechanical rigidity and durability of the pump. Scroll blade 20 is preferably formed integrally with an orbiting plate 22. Scroll blades 19 and 20 extend axially toward each other and are nested together to form interblade pockets 24. Orbiting motion of plate 22 relative to scroll blade 20 produces a scroll-type pumping action of the gas entering the scroll blades through vacuum inlet 12.

Gas exits scroll pump stage 18 at its outer periphery and enters an annular inlet region of second scroll pump stage 30. Second scroll pump stage 30 has a second stage scroll blade set including a stationary scroll blade 32 and an orbiting scroll blade 31. Scroll blade 32 is preferably formed integrally with housing member 14b, and scroll blade 31 is preferably formed integrally with orbiting plate 22. Scroll blades 31 and 32 extend axially toward each other and are nested together to form interblade pockets 36. The scroll blades of the first and second pump stages may have different blade heights and different numbers of turns to achieve a desired pump performance. As scroll blade 20 orbits relative to scroll blade 19 and scroll blade 31 orbits relative to scroll blade 32, the interblade pockets formed between the respective scroll blades move from the inlet of each scroll pump stage toward the outlet and pump gas from the inlet to the outlet.

An eccentric drive 40 for pump stages 18 and 30 is powered by a motor 42. The eccentric drive 40 produces orbiting movement of plate 22 with respect to an axis of rotation 44 of motor 42. Additional details regarding the construction and operation of scroll-type vacuum pumps are known to those skilled in the art and are given, for example, in U.S. Patent No. 5,616,015, assigned to the Assignee of the present invention, which is hereby incorporated by reference.

In accordance with a feature of the present invention, the scroll-type vacuum pump is provided with a novel seal configuration. In order to provide an acceptable compression ratio, it is necessary to provide seals between the scroll blades of each scroll blade set. The seal blocks gas leakage across the tip of the scroll blade. The scroll pump of FIG. 1 includes base seals. In first scroll pump stage 18, a base seal 50 on a base surface of orbiting plate 22 makes sliding, sealed contact with the tip of scroll blade 19, and a base seal 52 on a base surface of housing member 14a makes sliding contact with the tip of scroll blade 20. Similarly, in second scroll pump stage 30, a base seal 60 on a base surface of orbiting plate 22 makes sliding contact with the tip of scroll blade 32, and a base seal 62 on a base surface of housing member 14b makes sliding contact with the tip of scroll blade 31. The base seals 50, 52, 60 and 62 are described in detail below. A partial cross-sectional view of first scroll pump stage 18 is shown in FIG. 2. Like elements in FIGs. 1 and 2 have the same reference numerals. Housing member 14a includes a base surface 70 between adjacent, spaced-apart turns of scroll blade 19. Similarly, orbiting plate 22 includes a base surface 72 between adjacent, spaced-apart turns of scroll blade 20. The base surfaces 70 and 72 are located at the bases of recesses defined between adjacent turns of the respective scroll blades. Base seal 52 is disposed on base surface 70 of housing member 14a, and base seal 50 is disposed on base surface 72 of orbiting plate 22. The seals 50 and 52 may be secured on base surfaces 72 and 70, respectively, by any suitable method, including for example adhesive, friction and/or mechanical fasteners such as screws, rivets or pins.

The base seals are contacted by the tip of the scroll blade on the opposing member of the scroll blade set to form a seal. Thus tip 20a of scroll blade 20 contacts base seal 52, and tip 19a of scroll blade 19 contacts base seal 50. As orbiting plate 22 orbits relative to housing member 14a, each of the scroll blade tips slides on the opposing base seal, thereby maintaining a sealed relationship between the scroll blades of each scroll blade set. The seals 50 and 52 can have a wide variety of configurations. In a preferred embodiment, each seal includes a seal element 90 and an energizer element 92. The seal element establishes a sealed, sliding contact with the tip of the opposing scroll blade. The energizer element forces the seal element into contact with the scroll blade tip. The energizer element may be affixed to the seal element, typically by an adhesive, to form a unitary base seal. Preferably, the seal element is fabricated of an ultra high molecular weight (UHMW) polyethylene. The polyethylene may be filled with dry lubricants such as polytetrafluoroethylene, molybdenum disulfide or graphite. The energizer element may be fabricated of a urethane or silicone foam such as a microcellular urethane foam manufactured by Poron Corporation. By way of example, the seal element 90 may be about 0.045 inch thick, and the overall base seal may be about 0.1 inch thick. The seal element may be attached to the energizer element by an acrylic or silicone contact adhesive. The foam energizer element provides axial compliance to compensate for manufacturing tolerances, wear and thermal expansion of the orbiting scroll blade relative to the stationary housing. The UHMW polyethylene seal element provides a long- wearing, self-lubricating surface. The seal element is also relatively stiff to bending deflection, so that the base seal remains essentially flat under light loading. The desired contact pressure between the base seal and the opposing scroll blade tip is preferably less than 5 pounds per square inch (psi) and more preferably less than 1 psi.

Because the base seal covers the base surface between adjacent turns of the scroll blade, the required area of seal material for a given vacuum pump is necessarily greater than that used in prior art tip seals. However, the greater area is economically feasible when the seal element and the energizer element are fabricated of the low cost materials described above.

The scroll members, including housing members 14a and 14b, orbiting plate 22 and scroll blades 19, 20, 31 and 32, may be machined aluminum that is hard anodized

(hardcoated). After hardcoating, the scroll blade tips may be lapped to a 16 microinch surface finish. The edges of the scroll blade tips 20a and 19a may be chamfered and are preferably slightly rounded to allow smooth sliding on the respective base seals. In other embodiments, the scroll members may be machined, cast or formed from a wide variety of materials such as plastic, aluminum, iron and powdered metal.

The seal element 90 may be fabricated of a variety of self-lubricating synthetic resins, such as filled polytetrafluoroethylene or polyimide. Furthermore, the seal element 90 may be fabricated of a thin, hard metal such as chrome or nickel plated steel. In this case, the scroll blade tips are fabricated of a material that is self-lubricating and has long- wearing properties.

Examples of such materials include plastics such as glass reinforced nylon and polyethylene.

The energizer element 92 can have a wide variety of forms, such as a porous polytetrafluoroethylene paste, mechanical springs, a pneumatic bladder, etc.

The scroll-type vacuum pump utilizing one or more base seals as described above provides a number of advantages in comparison with prior art scroll-type vacuum pumps. The pump diameter can be reduced because the requirement for walls defining the tip seal grooves in the scroll blades is eliminated. Thus, the thickness of the scroll blades can be reduced to that required for maintaining rigidity and reliable operation. A reduced diameter pump produces a savings in material costs and provides a pump that is smaller and lighter than the prior art pumps.

In addition, the machining of the tip seal groove is eliminated. A typical scroll-type vacuum pump may have about 240 inches of tip seal groove. Given the relatively small width of the tip seal groove, a feed rate of 15 inches per minute is recommended. With a machine usage cost of $1.00 per minute, a savings of $16.00 in machining time per pump is achieved. Since the scroll blade tip contacts only a small fraction of the available seal area at any given crankshaft position, seal life is considerably extended. Peak temperatures due to frictional heating on the seal surface are also reduced, since the frictional heating is distributed over a larger area in comparison with prior art tip seals.

The counterface surface which contacts the base seal can be polished more easily than the seal counterface in prior art scroll-type vacuum pumps. A consistent good surface finish on the seal counterface of 12 to 16 microinches is known to minimize the wear rates of some seal element materials, such as filled polytetrafluoroethylenes. However, in prior art scroll- type vacuum pumps which utilize tip seals, it is difficult to modify the seal counterface surface finish because the counterface surface is located between turns of the scroll blade. This problem is aggravated when machined aluminum parts are hard anodized for durability, because the hard anodizing process approximately doubles the original surface finish irregularities. In accordance with the present invention, the seal counterface surface is the scroll blade tip and is readily accessible. The surface finish on the scroll blade tip can easily be improved through a variety of well-known techniques such as sanding, buffing, lapping, etching, burnishing, etc.

A partial cross-sectional view of a scroll-type vacuum pump in accordance with a second embodiment of the invention is shown in FIG. 3. A first scroll pump stage 100 includes a housing member 102 having an integrally-formed scroll blade 104 and an orbiting plate 106 having an integrally-formed scroll blade 108. Scroll blades 104 and 108 extend axially toward each other and are nested together to define interblade pockets 110. A second scroll pump stage 120 includes a housing member 122 having an integrally-formed scroll blade 124 and orbiting plate 106 having an integrally-formed scroll blade 126. Scroll blades 124 and 126 extend axially toward each other and are nested together to define interblade pockets 130.

In the embodiment of FIG. 3, a base seal 140 is positioned on base surface 142 of orbiting plate 106 between adjacent turns of scroll blade 108. In addition, a base seal 146 is positioned on base surface 148 of orbiting plate 106 between adjacent turns of scroll blade 126. Base seals 140 and 146 may be configured as described above in connection with FIGs. 1 and 2.

In contrast to the configuration of FIGS. 1 and 2, scroll blade 108 is provided with a tip seal 150, and scroll blade 126 is provided with a tip seal 152. Tip seals 150 and 152 are positioned in grooves in the tips of the respective scroll blades 108 and 126. Tip seal 150 contacts a base surface 156 of housing member 102 between adjacent turns of scroll blade 104, and tip seal 152 contacts a base surface 158 of housing member 122 between adjacent turns of scroll blade 124. A variety of tip seal configurations are known in the art. The configuration of FIG. 3 provides certain advantages in the construction of a scroll- type vacuum pump. It may be observed that the orbiting plate 106 does not make direct contact with either of the housing members 102 or 122. In each case, orbiting plate 106 is separated from the respective housing member by a base seal or a tip seal. Thus, the orbiting plate 106 is not required to be constructed of a material that has good wear properties. A variety of injection moldable thermoplastics can be used for the orbiting plate without wear concerns. In a preferred embodiment, orbiting plate 106 is fabricated of a plastic material, and housing members 102 and 122 are fabricated of aluminum.

It may be noted that both the base seals 140 and 146 and the tip seals 150 and 152 are secured to orbiting plate 106. In each seal, frictional heating produced by the seal sliding relative to the opposing counterface surface is generated in the respective housing members 102 and 122 and can be conducted away from the orbiting plate 106. This is desirable because it is relatively difficult to cool the orbiting plate 106, especially in a vacuum pump where convection is greatly diminished. Since the base seals and the tip seals have very poor thermal conduction as compared to the metal housing members 102 and 122, most of the frictional heat generated at the seals is conducted through the housing members to ambient. The temperature rise of the orbiting plate is therefore reduced. This is desirable since the yield strength of plastic decreases and creep increases as temperature increases.

Each stage of the scroll pump shown in FIG. 1 and described above includes a stationary scroll blade and an orbiting scroll blade. In other scroll pump configurations, one scroll blade orbits relative to the other, and both scroll blades rotate about an axis of rotation. The invention is applicable to both configurations. It will be understood that the base seal of the present invention may be utilized in a two-stage scroll-type vacuum pump, as shown in FIG. 1 and described above, may be utilized in a single stage scroll-type vacuum pump, or may be utilized in any other scroll-type apparatus.

An example of the base seal in accordance with the invention is shown in FIG. 4, as viewed along the of axis of rotation of the drive motor. A base seal 180 has the general outline of the base surface on which it is mounted in the scroll pump. Base seal 180 is provided with a spiral groove 182 that corresponds to the scroll blade of the corresponding scroll member. Spiral groove 182 is preferably cut using a water jet process, but may be formed by laser cutting or stamping. The base seal 180 is secured in place against the base surface of the scroll member, with the scroll blade extending through spiral groove 182.

The scroll-type vacuum pump of FIG. 1 utilizes four base seals. It will be understood that one or more of the base seals may be replaced with a tip seal or other seal type within the scope of the invention. In general, the invention is directed to a scroll-type apparatus having one or more base seals.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS;

1. A vacuum pumping apparatus comprising: a scroll blade set having an inlet and outlet, said scroll blade set comprising a first scroll member having a first base surface and a first scroll blade extending from said first base surface, and a second scroll member having a second base surface and a second scroll blade extending from said second base surface, said first and second scroll blades being nested together to define one or more interblade pockets and having first and second blade tips, respectively; an eccentric drive operatively coupled to said scroll blade set for producing orbiting movement of one of said scroll blades relative to the other of said scroll blades so as to cause said one or more interblade pockets to move toward said outlet; and a first base seal disposed on said first base surface between adjacent turns of said first scroll blade, wherein the blade tip of said second scroll blade contacts said first base seal and slides relative to said first base seal during said orbiting movement.

2. The vacuum pumping apparatus as defined in claim 1 , further comprising a second base seal disposed on said second base surface between adjacent turns of said second scroll blade, wherein the blade tip of said first scroll blade contacts said second base seal and slides relative to said second base seal during said orbiting movement.

3. The vacuum pumping apparatus as defined in claim 1 , further comprising a tip seal on the blade tip of said first scroll blade, wherein said tip seal contacts said second base surface and slides relative to the second base surface during said orbiting movement.

4. The vacuum pumping apparatus as defined in claim 3, wherein said first scroll member is fabricated of a plastic material.

5. The vacuum pumping apparatus as defined in claim 4, wherein said first scroll member is an orbiting member and said second scroll member is a stationary member.

6. The vacuum pumping apparatus as defined in claim 1 , wherein said first base seal comprises a seal element for contacting the blade tip of said second scroll blade and an energizer element affixed to said seal element.

7. The vacuum pumping apparatus as defined in claim 6, wherein said seal element comprises an ultra high molecular weight polyethylene.

8. The vacuum pumping apparatus as defined in claim 6, wherein said energizer element comprises a foam.

9. The vacuum pumping apparatus as defined in claim 1, wherein edges of the blade tip of said second scroll blade are chamfered.

10. The vacuum pumping apparatus as defined in claim 1, wherein edges of the blade tip of said second scroll blade are rounded.

11. The vacuum pumping apparatus as defined in claim 1 , wherein said blade tip of said second scroll blade contacts said first base seal with a pressure less than about 5 pounds per square inch.

12. A scroll blade assembly for a scroll-type device, comprising: a scroll blade set having an inlet and an outlet, said scroll blade set comprising a first scroll member having a first base surface and a first scroll blade extending from said first base surface, and a second scroll member having a second base surface and a second scroll blade extending from said second base surface, said first and second scroll blades being nested together to define one or more interblade pockets and having first and second blade tips, respectively; and a first base seal disposed on said first base surface between adjacent turns of said first scroll blade, wherein the blade tip of said second scroll blade contacts said first base seal and slides relative to said first base seal during orbiting movement of said first scroll blade relative to said second scroll blade.

13. The scroll blade assembly as defined in claim 12, further comprising a second base seal disposed on said second base surface between adjacent turns of said second scroll blade, wherein the blade tip of said first scroll blade contacts said second base seal and slides relative to said second base seal during said orbiting movement.

14. The scroll blade assembly as defined in claim 12, further comprising a tip seal on the blade tip of said first scroll blade, wherein said tip seal contacts said second base surface and slides relative to said second base surface during said orbiting movement.

15. The scroll blade assembly as defined in claim 14, wherein said first scroll member is fabricated of a plastic material.

16. The scroll blade assembly as defined in claim 12, wherein said first base seal comprises a seal element that contacts the blade tip of said second scroll blade and an energizer element affixed to said seal element.

17. The scroll blade assembly as defined in claim 16, wherein said seal element comprises an ultra high molecular weight polyethylene and wherein said energizer element comprises a foam.

18. A scroll blade assembly for a scroll-type device, comprising: a scroll member having a base surface and a scroll blade extending from said base surface; and a base seal disposed on said base surface between adjacent turns of said scroll blade.

19. The scroll blade assembly as defined in claim 18, wherein said base seal comprises a seal element for contacting a blade tip of an opposing scroll blade and an energizer element affixed to said seal element.