U.S. patent application number 12/087701 was filed with the patent office on 2009-06-11 for vacuum pump.
Invention is credited to Thomas Dreifert, Hans-Rochus Gross.
Application Number | 20090148284 12/087701 |
Document ID | / |
Family ID | 38189966 |
Filed Date | 2009-06-11 |
United States Patent
Application |
20090148284 |
Kind Code |
A1 |
Dreifert; Thomas ; et
al. |
June 11, 2009 |
Vacuum Pump
Abstract
A vacuum pump for producing a vacuum in a space which is to be
evacuated comprises a housing (10) having a cylindrical inner space
(22). An expeller (26) is arranged eccentrically in the inner space
(22). A helical sealing element (34) is provided between the
expeller (26) and an inner wall (38) of the housing (10) for
forming at least one crescent-shaped conveying space. The housing
(10) or the expeller (26) are connected to a drive device (44) for
producing a relative rotary movement between the housing (10) and
the expeller (26). The vacuum pump is configured as a dry-running
vacuum pump.
Inventors: |
Dreifert; Thomas; (Kerpen,
DE) ; Gross; Hans-Rochus; (Bergisch Gladbach,
DE) |
Correspondence
Address: |
Fay Sharpe LLP
1228 Euclid Avenue, 5th Floor, The Halle Building
Cleveland
OH
44115
US
|
Family ID: |
38189966 |
Appl. No.: |
12/087701 |
Filed: |
January 11, 2007 |
PCT Filed: |
January 11, 2007 |
PCT NO: |
PCT/EP2007/050250 |
371 Date: |
October 7, 2008 |
Current U.S.
Class: |
415/230 |
Current CPC
Class: |
F05C 2201/0442 20130101;
F04C 18/107 20130101 |
Class at
Publication: |
415/230 |
International
Class: |
F04D 29/10 20060101
F04D029/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2006 |
DE |
10 2006 001 733.1 |
Claims
1. A vacuum pump for producing a vacuum in a space, comprising a
housing having a cylindrical inner space, an expeller arranged
eccentrically in the inner space, a helical sealing element
arranged between the expeller and an inner wall of the housing for
forming at least one crescent-shaped conveying space, and a drive
device, connected to the housing or the expeller for producing a
relative rotary movement between the housing and the expeller, the
vacuum pump being configured as a dry-running vacuum pump.
2. The vacuum pump according to claim 1, wherein the expeller is
supported concentrically.
3. The vacuum pump according to claim 1, wherein the dry-running
operation is realized by suitable material pairings between the
sealing element and a sliding surface with good sliding properties,
particularly by use of material pairings of PTFE and/or PEEK and/or
smoothed anodized aluminum.
4. The vacuum pump according to claim 1, wherein an inner wall of
the inner space has a roughness Rz of 0.1 to 5, preferably of 0.5
to 2.
5. The vacuum pump according to claim 1, wherein the sealing
element is held in a groove for radial displacement therein, said
groove being arranged in the housing or in the expeller.
6. The vacuum pump according to claim 5, wherein said groove is
coated.
7. The vacuum pump according to claim 1, characterized by a cooling
element, connected to the housing, for cooling the housing inner
wall.
8. The vacuum pump according to claim 1, wherein said cooling
element comprises cooling ribs arranged on an outer side of the
housing.
9. The vacuum pump according to claim 1, wherein bearings are
arranged for rotatable support of the housing or the expeller.
10. The vacuum pump according to claim 1, wherein the expeller is
supported in an overhung manner.
11. The vacuum pump according to claim 1, wherein the expeller is
provided with inner bearings.
12. The vacuum pump according to claim 11, wherein the expeller
comprises a hollow space having arranged therein a drive shaft
connected to the expeller.
13. The vacuum pump according to claim 12, wherein said hollow
space has provided therein a hollow axis surrounding the drive
shaft and serving to accommodate the bearings.
14. The vacuum pump according to claim 1, wherein the rotational
number of the housing or of the expeller is delimited to 500 l/min,
preferably to 200 l/min.
15. A vacuum pump comprising: a cylindrical rotor having a rotor
central axis, the cylindrical rotor defining a helical channel
along an outer surface thereof; a sealing element received in the
helical channel; a housing defining a cylindrical chamber having
housing central axis; a bearing assembly which mounts the
cylindrical rotor and the housing for relative rotational movement
around an axis of rotation, the axis of rotation being aligned with
the rotor axis and radially offset from the housing axis.
Description
BACKGROUND
[0001] The invention relates to a vacuum pump for producing a
vacuum in a space.
[0002] Vacuum pumps are used for evacuation of spaces in which
processes are performed which would be critically affected
particularly by particles. Such processes are performed especially
in semiconductor production, in vapor-deposition processes, in
chemical processes etc. To avoid an intrusion of lubricant
substances from the vacuum pump into the space which is to be
evacuated, it is required to use dry-running pumps.
[0003] There are already known, for instance, rotary slide valve
vacuum pumps of the dry-running type which, however, suffer from
the disadvantage of high relative speeds occurring on the
frictional components, i.e. between the slide and the inner wall of
the pumping chamber. Since high relative speeds are inevitable for
acceptable volume flows, the lifespan of rotary slide valve vacuum
pumps is relatively short.
[0004] A further variety of vacuum pumps of the dry-running type is
a scroll pump. Scroll pumps comprise one stationary component and
one orbiting component, which components are provided with spirally
shaped conveying means arranged in mutual engagement. Scroll pumps,
however, are expensive in manufacture and need frequent maintenance
work in order to safeguard a reliable continuous operation.
[0005] Piston vacuum pumps of the dry-running type have the
disadvantage that the production costs are high and, further, their
constructional volume is large. In such pumps, also the noise
development and the occurring vibrations are disadvantageous.
[0006] From US 2005/0163632, there is known a dry-running vacuum
pump comprising an eccentrically supported expeller. A helical
sealing element is arranged in a spirally shaped groove of the
cylindrical expeller. Internally of the hollow cylinder, a crank is
arranged eccentrically to the rotational axis of the cylinder. The
system consisting of the rotating crank and the orbiting expeller
is arranged in a cylindrical housing. The vacuum pump described in
US 2005/0163632 is well-suited for high conveying capacities but
has a complicated design.
SUMMARY
[0007] It is an object of the invention to provide a vacuum pump
which can be reliably operated in an economical manner also in dry
running conditions, particularly also in case of a low suction
capacity.
[0008] The vacuum pump of the invention comprises a housing having
a cylindrical inner space. Within said inner space, an expeller is
arranged eccentrically. Between the expeller and the inner wall of
the housing, a helical sealing element is arranged for forming at
least one crescent-shaped conveying space. The expeller and/or the
housing are directly connected to a drive device wherein, depending
on the given design, either the housing or the expeller is
preferably stationary. Due to said eccentricity, a simple
rotational movement of the housing or the expeller will effect a
conveyance, e.g. of a gas, from a to-be-evacuated space through an
inlet of the vacuum pump to an outlet of the vacuum pump.
[0009] An aspect of the invention in this regard resides in that
the expeller is supported centrally, i.e. that the shaft of the
expeller which, depending on whether or not the expeller is driven,
forms the drive shaft or the support shaft--is arranged
concentrically to the cylindrical expeller. Thus, the expeller will
not perform an orbiting movement but a rotating movement. According
to the invention, the expeller is supported concentrically.
[0010] According to the invention, the vacuum pump is configured as
a dry-running vacuum pump. The vacuum pump of the invention offers
the particular advantage of being a vacuum pump of a simple
constructional design wherein only the housing or the expeller has
to be driven. This can be realized by a simple connection of the
expeller or the housing to an electric motor, which connection is
realized e.g. via a belt drive or an intermediate transmission, or
also directly. The vacuum pump of the invention is particularly
suited as a slow-running vacuum pump which has rotational numbers
of preferably less than 300 l/min and most preferably 200 l/min.
Because of the configuration and the geometry of the vacuum pump of
the invention, a good conveying flow can be obtained in spite of
the low rotational numbers.
[0011] To be able to operate the inventive dry-running vacuum pump
in a reliable manner and with easy maintenance, it is of particular
advantage to realize a suitable material pairing between the
helical sealing element and the sliding surface having the sealing
element sliding thereon. Depending on the design of the vacuum
pump, the sealing element, if provided in a vacuum pump with
outer-friction sealing element, will slide on the inner wall of the
cylindrical inner space of the housing in cases where the sealing
element is connected to the expeller for common rotation therewith.
In a vacuum pump with inner-friction sealing element, the sealing
element is connected to the housing for common rotation therewith
and will side on an outer surface of the cylindrical expeller.
Particularly preferred material pairings between the sealing
element and a sliding surface, i.e. between the inner wall of the
housing and the outer surface of the expeller, consist of a sealing
element made of a PTFE compound or PEEK in combination with a
sliding surface of smoothed, particularly anodized aluminum.
Further, the sealing element can be made of spring steel,
particularly coated spring steel. The coating preferably comprises
any one of PTFE compounds (PTFE with fillers), PEEK and PEEK
compounds, PI (polyimides) and PI compounds, PPS (poly-phenylene
sulfide) and PPS compounds or epoxy resins with fillers. Plastics
highly resistive to mechanical and thermal stresses are generally
of good use. For such a tribological system, relative speeds of up
to 5 m/s can be realized. Suitable materials for the sliding
surface are, apart from the anodized and thereby preferably
hardened aluminum, grey cast iron, spheroidal cast iron, annealed
cast iron, as well as steels, particularly stainless steels. These
materials are preferably coated with wear-protection layer,
particularly CrN or TiN. For reducing the friction between the
sealing element and the sliding surface, it is further advantageous
to provide coatings or sliding lacquers. These can be ceramic
coatings as well as TiN, CrN, AICrN and CrC.
[0012] In respect to the above, the roughness R.sub.z of the
sliding surface is preferably lower than 5, more preferably lower
than 2 and most preferably lower than 0.5.
[0013] Due to the eccentricity between the expeller and the
cylindrical inner space of the housing, the sealing element is
arranged in a groove and is radially displaceable therein. Said
groove is formed either in the expeller or in the housing. Although
the relative speeds within the sealing element and a groove wall
are distinctly lower, it would be advantageous to provide also the
inner wall of the groove with a coating, for instance, thus
reducing the friction.
[0014] The width and the depth of the groove are selected to the
effect that, in the operative condition of the vacuum pump, there
is guaranteed a good radial displacement of the sealing element,
one the one hand, and the gap between the sealing element and the
groove is as small as possible, on the other hand, in order to
avoid leakage flows which would reduce the pumping capacity of the
vacuum pump. In this respect, it is to be considered, particularly
in dependence on the given selection of materials, that the
expansion coefficients and therefore also the thermal expansion of
the constructional components, may differ from each other. This
holds true particularly for a sealing element made of plastic.
[0015] According to a particularly preferred embodiment of the
invention, a cooling element is provided for dissipation of the
heat generated at the sliding surface of the sealing element. This
allows for a considerable reduction of the wear development,
particularly of the sealing element. In a vacuum pump of the type
with inner sliding movement, wherein the sealing element is
arranged to slide on the inner wall of the cylindrical inner space
of the housing, the cooling element, preferably provided in the
form of cooling ribs, is preferably connected to the housing. For
safeguarding a good cooling effect, it is possible to direct an air
flow against the cooling ribs. This onflow can be controlled in
dependence on the temperature so that the vacuum pump will quickly
reach its operating temperature at the beginning of the operation
and then will be operated within a narrow temperature range. This
particularly offers the advantage that the sealing element can be
produced from a material having a high thermal expansion
coefficient and thus will only with relatively narrow temperature
ranges be capable to guarantee a reliable sealing effect along with
good sliding properties.
[0016] In a vacuum pump having a sealing element for inner sliding
movement, it is particularly advantageous if the expeller is
cooled. For this purpose, one can provide e.g. a hollow expeller
wherein, also in this case, cooling ribs can be arranged in the
hollow space, with air flowing around the cooling ribs for
cooling.
[0017] With particular preference, the support of the housing or
the expeller can be provided on the outlet side. In this case, the
supports are arranged in such a manner that no connection exists to
the underpressure side of the vacuum pump. Thereby, it is
safeguarded that no lubricant from the bearings which are formed
particularly as roller bearings, will enter the space that is to be
evacuated. Particularly preferred is an overhung support of the
expeller. A further improvement of the overhung support of the
expeller, which is effective to reduce the tilting moments acting
on the bearings, resides in the preferred configurations of
internally arranged bearings. For instance, it is possible to
provide the expeller in the form of a hollow cylinder and to
arrange the drive shaft for the expeller within the hollow
cylinder, with the drive shaft connected to the cylinder. To make
it possible that the drive shaft arranged in the cavity is
supported internally and the supports are nonetheless arranged on
the outlet side, the drive shaft can be surrounded by a hollow axis
which, if required, is arranged within the cavity of the expeller.
The hollow axis is stationary and carries the bearings for support
of the drive shaft.
[0018] The helical sealing element preferably comprises a plurality
of windings or turns. In correspondence thereto, also the spirally
shaped groove comprises a plurality of thread turns. To keep the
power input of the vacuum pump as low as possible and--while at the
same time maintaining a small construction space--to keep the
compression temperatures low, an internal compression is to be
preferred. Such an internal compression of the medium which is to
be conveyed can be obtained in that the pitch of the groove or
sealing element is reduced in the conveying direction. Compression
ratios of 1/3 to 1/10 between the internal compression and the
external compression are preferred. Preferably, a bypass valve is
provided for preventing an over-compression in the compression
region.
[0019] A mathematical analysis of the kinematics of the inventive
vacuum pump shows that the relative speed will influence, to the
extent of its third power, the maximum obtainable sectional
performance. Said relative speed is the relative speed between the
sealing element and the sliding element. The maximum obtainable
relative speed substantially depends on the material pairing, the
surface quality and the temperatures occurring at the contact
sites.
[0020] The number of rotations, on the other hand, has a reciprocal
quadratic effect on the possible sectional performance when the
maximum relative speed is kept constant. Thus, it is possible to
realize pumps which in comparison with known vacuum pumps have
extraordinarily low rotational numbers of less than 500 l/min or
200 l/min. while still reaching a high suction performance. Due to
the inventive constructional design of the vacuum pump, it is thus
possible to reach a high suction performance at low rotational
numbers. Because of the required low rotational numbers, the vacuum
pump can be designed as a dry-running vacuum pump. Higher
rotational numbers would cause maximum relative speeds at the
contact sites that would result in faster wear effects. Thus, using
the inventive dry-running vacuum pump, there can be obtained
suction performances of about 0.1 m.sup.3/h-30 m.sup.3/h. Further,
the simple configuration of the vacuum pump allows for an
inexpensive construction. Due to the low number of constructional
components, maintenance will be very convenient. Further, a long
useful life will be guaranteed, particularly due to the low
rotational numbers. Still further, the demands posed to the impact
resistance are low. A further advantage resides in the occurrence
of merely slight vibrations and the low noise development.
[0021] According to a preferred embodiment of the invention, the
sealing element is installed with a radial bias. This will entail a
good sealing effect. Further, this provision will bring about a
balancing of the thermal expansion and allow for convenient
replaceability. A radial bias can be generated e.g. by an
elastomeric material at the bottom of the groove. Further, the
sealing element can be tangentially tensioned by providing a
spring.
[0022] The following is a detailed description of an embodiment of
the invention with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the drawings:
[0024] FIG. 1 is a schematic lateral view of a first preferred
embodiment of a vacuum pump;
[0025] FIG. 2 is a schematic lateral view of a second preferred
embodiment of a vacuum pump;
[0026] FIG. 3 is a schematic perspective view of a vacuum pump with
electromotoric drive.
DETAILED DESCRIPTION
[0027] According to a first preferred embodiment of the invention
(FIG. 1), the vacuum pump comprises a stationary housing 10.
Housing 10 is closed by an inlet cover 12 and an outlet cover 14.
Inlet cover 12 includes an inlet 16 connected to the space which is
to be evacuated. Outlet cover 14 is provided with a connection 18
for discharge of the gases suctioned from the space which is to be
evacuated. Thus, the gas will move from the to-be-evacuated space
in the direction of arrow 20 via inlet 16 into a cylindrical inner
space 22 of housing 10 and will leave said space in the direction
of arrow 24 via said outlet connection 18 arranged in outlet cover
14.
[0028] Internally of said cylindrical space 22, a cylindrical
expeller 26 is located. Expeller 26 is arranged centrically to a
shaft 28. Thus, the central axis 30 of the expeller corresponds to
the central axis of shaft 28. Shaft 28 is arranged eccentrically in
the cylindrical inner space 22. Consequently, the central axis 32
of inner space 22 has an eccentricity e relative to the central
axis 30 of expeller 26 and shaft 28, respectively. In the
illustrated exemplary embodiment, shaft 28 is guided through the
cover 50 fastened to inlet cover 12.
[0029] Expeller 26 is provided with a helically or spirally shaped
sealing element 34. The sealing element 34 is arranged in a
corresponding helically or spirally shaped groove 36. Sealing
element 34 is connected to expeller 26 for common rotation
therewith and for radial displacement relative thereto. By rotation
of expeller 26, sealing element 34 will be rotated relative to an
inner wall 38 of housing 10. At the same time, expeller 26 will be
radially displaced, resulting in the conveyance of gas in the
direction of arrows 20,24. The conveyance of gas is performed in
that crescent-shaped conveying spaces, formed between adjacent
helical passages of sealing element 34, will convey the medium
towards the right-hand side in FIG. 1. The distance between
adjacent helical passages of sealing element 34 can decrease in the
direction of conveyance so that the medium will be compressed or
condensed.
[0030] In the exemplary embodiment illustrated in FIG. 1, the
relative movement takes place between the inner wall 38 of inner
space 22 and an outer face 40 of sealing element 34. This movement
causes both the housing 10 and the sealing element 34 to heat up.
Therefore, it is imperative in this embodiment that the housing 10
has a good thermal conductivity and is provided with cooling ribs
42 on the outside. Further, for enhancing the cooling effect, an
air flow can be directed towards the cooling ribs 42.
[0031] The shaft 28 connected to expeller 26 is connected to a
drive means 44 (FIG. 3) which preferably consists of an electric
motor. In the illustrated exemplary embodiment, the support of the
shaft is preferably provided by bearings 46,48, preferably formed
as roller bearings, which are arranged in the inlet cover 12 and
the outlet cover 14, respectively. Said bearings 46,48 are held by
covers 50 and are preferably formed as roller bearings, and further
are greased and are sealed or provided with lids. Particularly on
the outlet side, it is of advantage to arrange a sealing 52 with
good sealing properties between inlet cover 12 and shaft 28 so as
avoid a gas exchange between the surroundings and the space which
is to be evacuated. Such an occurrence would be disadvantageous
particularly if the space to be evacuated is a space wherein
semiconductor processes or the like are carried out. The contacting
shaft sealings can be of the dry-running or of the lubricated type.
In case of small pressure differences, also gap sealings can be
provided. Also the inlet-side bearing 48 can be separated from the
process medium in the pumping chamber by a contacting shaft
sealing.
[0032] Further, it is possible to arrange the sealing element 34 in
a groove formed in housing 10; in this case, the generated relative
speeds will occur between the sealing element and an outer side of
expeller 26. The dissipation of the heat caused by such an
arrangement will however be more difficult. For heat dissipation,
one could use e.g. a hollow expeller.
[0033] As a further option, there could be provided a stationary
expeller and a rotating housing. In this case, the bore in the
housing would be arranged eccentrically to the rotational axis of
the rotation of the housing.
[0034] In connection with a further preferred embodiment (FIG. 2),
similar or identical components to those in FIG. 1 are provided
with the same reference numerals.
[0035] Housing 10 is configured in correspondence with the
embodiment shown in FIG. 1 and is closed by housing covers 12,14.
Housing 10 again comprises a cylindrical opening accommodating an
expeller 54. In the same manner as depicted in FIG. 1, expeller 54
has a cylindrical outer contour wherein grooves 36 have a helical
sealing element 34 arranged therein so that the functioning of the
second embodiment (FIG. 2) corresponds to that of the embodiment of
FIG. 1.
[0036] In order to avoid the occurrence of a connection between the
bearings of expeller 54 and the to-be-evacuated space and the inlet
opening 16, respectively, the bearings in the illustrated
embodiment are arranged within the expeller 54. For this purpose,
expeller 54 is formed with a cylindrical opening 60 connecting to a
cylindrical opening 62 having a smaller diameter. The cylindrical
opening 60,62 is arranged with rotational symmetry relative to
expeller 54 and thus is located on the central axis of expeller 54.
Provided in concentric arrangement opposite to opening 62 is a
cylindrical opening 64, said openings 62,64 being connected to each
other via a further concentric opening.
[0037] A drive shaft 66 formed with a plurality of steps is
arranged in the cylindrical openings 60,62 and comprises a threaded
pin 68 extending into opening 64. To connect the shaft 66 to the
expeller 54 for common rotation therewith, threaded pin 68 is fixed
in position by means of a nut 70, and there is provided an
adjusting spring 72. Within the cylindrical hollow space 60 of
expeller 54, the bearings 56,58 are arranged on the outer side of
shaft 66. Further, the cylindrical opening 60 has arranged therein
a hollow axis 74 carrying the bearings 56,58. Hollow axis 64 is
connected to cover 14 for common rotation therewith. For position
definition, bearing 56 is biased by means of a spring 76. Bearing
58 is held by a cover 78.
[0038] Since the bearings are arranged internally and on the outlet
side, respectively, no connection exists to the space which is to
be evacuated, thereby precluding the possibility that, due to the
existing underpressure, lubricant could be sucked out from the
bearings and could possibly intrude into the space which is to be
evacuated.
[0039] Apart from the supports as depicted in FIGS. 1-2 for the
expellers 26,54, the expellers can also be supported in an overhung
manner externally of the housing.
[0040] The two-sided support illustrated in FIG. 1 is advantageous
particularly if a double-flow system is provided. In a double-flow
vacuum pump, the suctional intake is performed by a centrally
arranged suction nozzle; then, using a suitably configured
expeller, the gas will be conveyed in both directions towards the
side and be laterally discharged from the two outlet openings. An
expeller of this type comprises two sealing elements which again
have a helical shape and will transport the medium from the central
inlet nozzle respectively to the outside. In a double-flow system
with two-sided support, the bearings are arranged respectively on
the outlet side, thus avoiding also in this case that lubricant of
the bearings can intrude into the space which is to be
evacuated.
[0041] Depending on the arrangement of the bearings, the bearings
are sealed towards the conveying space by shaft bearings or other
sealing elements.
[0042] The perspective view of FIG. 3 shows a fully assembled
vacuum pump comprising a drive element 44 formed as an electric
motor. The Figure clearly shows the housing 10 provided with the
circumferentially distributed, substantially radially arranged
cooling ribs 42. Further shown are the inlet 16 and the outlet 18.
The shaft 28 fastened to expeller 26 is connected to electric motor
44 via a belt drive 82.
[0043] The two components are arranged on a base plate 80. The
arrangement depicted in FIG. 3 is an extremely compact and thus
space-saving arrangement.
[0044] Concerning the kinematics of the various possible
embodiments of the vacuum pump of the invention, it is to be noted
that the rotational axis preferably is the axis of the component
comprising the groove. Both the bore and the expeller are always
circular in cross section. The central line of the cylindrical
expeller corresponds to the central line of the axis, particularly
the rotational axis of the expeller. Preferably, in the various
embodiments, the bottom of the groove is always concentric with the
component comprising the groove.
[0045] A sealing effect can also be obtained by providing a
decrease of the pitch of the groove in the conveying direction.
Further, also a two-stepped configuration of the vacuum pump is
possible.
[0046] The invention has been described with reference to the
preferred embodiments. Modifications and alterations may occur to
others upon reading and understanding the preceding detailed
description. It is intended that the invention be construed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
* * * * *