U.S. patent application number 11/666980 was filed with the patent office on 2007-12-27 for vacuum pump impeller.
Invention is credited to Wolfgang Giebmanns.
Application Number | 20070297907 11/666980 |
Document ID | / |
Family ID | 35645593 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070297907 |
Kind Code |
A1 |
Giebmanns; Wolfgang |
December 27, 2007 |
Vacuum Pump Impeller
Abstract
The invention relates to a vacuum pump impeller (10) which
comprises a shaft (14) made of steel, and a one-piece rotor (12)
supported by the shaft (14) and made of a material different from
the steel of the shaft. The shaft (14) comprises an axial cavity
(22), and the rotor (12) comprises an axial projection (16) seated
in the cavity (22) in positive and/or non-positive engagement
therewith.
Inventors: |
Giebmanns; Wolfgang;
(Erfstadt, DE) |
Correspondence
Address: |
FAY SHARPE LLP
1100 SUPERIOR AVENUE, SEVENTH FLOOR
CLEVELAND
OH
44114
US
|
Family ID: |
35645593 |
Appl. No.: |
11/666980 |
Filed: |
October 10, 2005 |
PCT Filed: |
October 10, 2005 |
PCT NO: |
PCT/EP05/55413 |
371 Date: |
May 2, 2007 |
Current U.S.
Class: |
416/204R |
Current CPC
Class: |
Y02T 50/60 20130101;
F05D 2300/171 20130101; F05D 2230/21 20130101; F05D 2300/122
20130101; F04D 29/266 20130101; F05D 2230/25 20130101; F01D 5/025
20130101; F04D 29/023 20130101; Y02T 50/671 20130101 |
Class at
Publication: |
416/204.00R |
International
Class: |
F04D 29/20 20060101
F04D029/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2004 |
DE |
10 2004 053 289.3 |
Claims
1. A vacuum pump impeller comprising: a shaft made of steel, and a
one-piece rotor supported by the shaft and made of a material
different from the steel of the shaft, shaft defining an axial
cavity and the rotor including an axial projection seated in the
cavity in positive and/or non-positive engagement therewith.
2. The vacuum pump impeller according to claim 1, wherein the rotor
projection and the shaft comprise, within the cavity respectively
shaped portions, the shaped portions effecting a positive
engagement with each other in the axial direction and the
circumferential direction.
3. The vacuum pump impeller according to claim, wherein the rotor
is a cast part having its projection cast into the shaft
cavity.
4. The vacuum pump impeller according to claim 2, wherein said
shaped portions are formed as grooves and bars.
5. The vacuum pump impeller according to claim 1, wherein on a
rotor-side end, the wall thickness of the shaft continuously
decreases towards an opening of the cavity.
6. The vacuum pump impeller according to claim 1, wherein the rotor
is supported by a single shaft.
7. The vacuum pump impeller according to claim 1, wherein the rotor
is formed of a light metal or a plastic material.
8. The vacuum pump impeller according to claim 7, wherein the rotor
material is aluminum.
9. A method for producing a vacuum pump impeller according to claim
1 by casting, comprising the method steps of: inserting a shaft
having an axial shaft cavity into a rotor mold, filling in the
liquid rotor molding material into the rotor mold and into the
shaft cavity, and removing the vacuum pump impeller from the mold
after the rotor has set.
10. A method for producing a vacuum pump impeller according to
claim 1 by forging, comprising the method steps of: inserting a
shaft having an axial shaft cavity into a drop-forge die for the
rotor, forging red-hot rotor forging material into the rotor
drop-forge die and into the shaft cavity, and removing the vacuum
pump impeller from the drop-forge die.
11. A vacuum pump impeller comprising: a shaft defining an axial
cavity in at least one end thereof; a rotor constructed of a
material which expands radially outward relative to the steel shaft
when drawing a vacuum, the rotor including an axial projection
which extends into the shaft axial cavity such that when drawing
the vacuum, the rotor axial projection expands into a tighter
relationship with shift to avoid slippage.
12. The vacuum pump impeller according to claim 11, wherein a
thickness of a shaft wall surrounding the axial cavity thins toward
the one end that receives the rotor axial projection such that a
stiffness of the shaft decreases toward the rotor to reduce fatigue
and a risk of the rotor breaking from the shaft.
13. The vacuum pump impeller according to claim 11, further
including: mating ridges and grooves defined in an interior of a
peripheral wall of the shaft surrounding the axial shaft cavity and
the rotor axial projection.
14. The vacuum pump impeller according to claim 11, wherein the
rotor axial projection is one of cast or forged into the shaft
axial cavity.
15. The vacuum pump impeller according to claim 11, wherein the
shaft is constructed of steel and the rotor is constructed of
aluminum.
Description
[0001] The invention relates to a vacuum pump impeller comprising a
steel shaft and a one-piece rotor held by the shaft and made from a
material different from the shaft steel, and to a method for
producing a vacuum pump impeller.
[0002] Vacuum pump impellers are known in various variants, e.g. as
screw impellers, turbo impellers, rolling-piston impellers,
side-channel impellers, inter alia. Vacuum pump impellers can be
configured for cantilevered arrangement, i.e. the impeller is
supported only on one axial end so that the rotor is arranged in
overhung position. Impellers are partially operated at high
rotational speeds so that high radial forces can occur. For these
reasons, when designing the impellers, efforts are made to keep the
weight particularly of the rotor as small as possible and to
provide the shaft with the highest possible strength and rigidity.
In practice, this is realized by attaching the rotor on the outer
side of the steel shaft, and by forming the rotor of a material
having a particularly light weight, e.g. aluminium. Since, due to
high temperature and high centrifugal forces, the rotor may in
operation may expand more than the shaft, a permanent and
free-from-play fixing of the rotor to the shaft can be effected
only with difficulty and in a complicated manner, e.g. by welding,
soldering, bonding, axially biased toothed engagement on the end
sides, tie rods etc.
[0003] In view of the above, it is an object of the invention to
provide a vacuum pump impeller and a method for producing the same
which allow for a simple and permanent fixation of the rotor the
shaft.
[0004] According to the invention, the above object is achieved by
the features of claim 1 and 9, respectively.
[0005] In the vacuum pump impeller of the invention, the shaft is
provided with a cavity on one axial end thereof, and the rotor is
provided with a corresponding axial projection seated in positive
and/or non-positive engagement in the axial cavity of the shaft.
Thus, the rotor is not fixed to the outer side of the shaft but is
fixed substantially on the inner side of a sleeve-like portion of
the hollow steel shaft. Under the effect of the centrifugal forces
and the introduction of heat into the rotor as occurring during
operation, the rotor projection seated in the shaft can expand more
than the hollow shaft. In operation and particularly in case of
high rotational speeds, the connection between the rotor and the
shaft will thus become stronger. This effect obviates the need for
complex connection arrangements involving the use of tie rods etc.,
as well as the need for holes in the shaft and the rotor so that,
by the resultant homogenization of the force flows, breakage of the
shaft or the rotor is avoided. Further still, by the omission of
complex fastening arrangements, the weight of the impeller is kept
low, so that--particularly in case of high rotational speeds at
which e.g. turbo impellers are operated--the bearing support can be
simplified. Thus, with the inventive impeller, higher rotational
speeds and/or a lower weight and a reduced constructional size can
be realized.
[0006] Preferably, the rotor projection and the shaft each
comprise, within the cavity, suitably shaped portions for effecting
a positive engagement with each other in the axial direction and/or
the circumferential direction. The provision of a positive
connection results in a safe and easily established connection
between rotor and shaft.
[0007] According to a preferred embodiment, the rotor is a cast
piece whose projection is integrally cast in the shaft cavity.
Prior to casting the rotor, the shaft is inserted, by its portion
including the cavity, into a casting box, and then the liquid rotor
material is injected into the rotor casting box, with the liquid
rotor material flowing into the shaft cavity. In this manner, the
shaped portions in the cavity of the shaft are transferred to the
rotor projection already when casting the rotor. No further
production step for fixing the rotor to the shaft will be required.
Thereby, in turn, the weight is reduced and causes of possible
imbalances are avoided.
[0008] Preferably, the above shaped portions are configured as
grooves and bars. The grooves and bars can be arranged in axial and
circumferential directions; however, they can be arranged also in
other directions. The exact shape and orientation of the bars and
grooves will depend, inter alia, on the thermal expansion behavior
of the materials used for the rotor and the shaft, the rotational
speeds and centrifugal forces occurring during operation, and other
limiting conditions.
[0009] According to a preferred embodiment, it is provided that the
wall thickness of the shaft on the rotor-side end of the shaft
decreases toward the opening of the cavity, i.e. the wall thickness
of the shaft casing continuously decreases towards the opening so
that the stiffness of the shaft end becomes weaker towards the
opening of the cavity and the axial cavity opening is relatively
elastic in radial directions. Thereby, relatively large and/or
sudden changes of the moments of inertia of the shaft are avoided,
which otherwise would cause high bending and/or torsional stresses
and thus--particularly in case of massive variations in
stress--could result in premature fatigue with generation of
fissures. The danger of breakage of the rotor projection in this
region is considerably reduced, so that a correspondingly small
dimensioning and thus a reduction of weight of the rotor projection
can be realized. The region of decreasing wall thickness as
compared to the overall length of the shaft can be less than 1/10
but should at least 3 mm long.
[0010] In principle, the vacuum pump impeller can comprise two
shafts, each of them arranged at a respective axial end of the
rotor. Preferably, however, only one shaft is provided, holding an
axial end of the rotor. In this manner, there is obtained a vacuum
pump impeller adapted for cantilevered support; in such a vacuum
pump impeller, the reduction in weight which can be realized by the
inventive construction is of particular advantage.
[0011] Preferably, the rotor material is a light metal or a plastic
material. If the rotor is to be produced as a cast piece, the rotor
material must have a melting temperature which allows for the rotor
material to be poured into the shaft cavity without causing damage
to the steel shaft. Apart of light metal, also plastic or
fiber-reinforced plastic can be used for the rotor.
[0012] According to a method for producing a vacuum pump impeller
comprising a rotor with an axial projection and further comprising
a steel shaft having a corresponding cavity formed therein, the
following method steps are provided: [0013] inserting the shaft
having the axial cavity into a rotor mold, [0014] filling in the
liquid rotor molding material into the rotor mold and into the
shaft cavity, and [0015] removing the vacuum pump impeller from the
mold after the impeller has been cooled.
[0016] Using the above described manufacturing method, upon
provision of suitably shaped portions in the shaft cavity, a
positive connection can be established between the shaft and the
rotor. In this manner, no further components will be needed for
effecting a positive connection between the rotor and the shaft.
The method is relatively simple and thus inexpensive.
[0017] According to an alternative method for producing a vacuum
pump impeller, the shaft comprising an axial cavity is laid into a
rotor drop-forge die; the red-hot rotor forging material is forged
into the drop-forge die for the rotor and into the shaft cavity;
and finally the vacuum pump impeller is removed from the drop-forge
die.
[0018] When applying this method, advantages similar to those
mentioned in connection with the molding method are obtained.
[0019] Several embodiments of the invention will be explained in
greater detail hereunder with reference to the drawings.
[0020] In the drawings, the following is shown:
[0021] FIG. 1 is a longitudinal sectional view of a first
embodiment of a vacuum pump impeller comprising a rotor which has
been cast into the shaft and then machined,
[0022] FIG. 2 is a longitudinal sectional view of the shaft of the
vacuum pump impeller,
[0023] FIG. 3 is a longitudinal sectional view of second embodiment
of a vacuum pump impeller,
[0024] FIG. 4 is a longitudinal sectional view of third embodiment
of a vacuum pump impeller,
[0025] FIG. 5 is a longitudinal sectional view of fourth embodiment
of a vacuum pump impeller.
[0026] Illustrated in FIG. 1 is a vacuum pump impeller 10 to be
used as one of the two impellers of a screw vacuum pump. Impeller
10 substantially consists of two parts, notably a one-piece rotor
12 made of aluminum and a one-pieced steel shaft 14 formed as a
hollow shaft throughout its length. The interrupted line in FIG. 1
schematically indicates the contour of shaft 14' and of rotor 12'
presented by these components immediately after casting and prior
to machining.
[0027] Rotor 12 comprises two parts in its longitudinal direction,
notably a projection 16 and an active part 18 which on its radially
outer side has a helical structure 20.
[0028] Shaft 14 is throughout its length provided with a slightly
conical and/or cylindrical cavity 22 having the projection 16 of
rotor 12 cast thereinto with positive engagement between the cavity
and the projection. Internally of shaft cavity 22, there are
arranged longitudinal bars 24 and transverse bars 26 constituting
said shaped portions, engaging corresponding longitudinal grooves
28 and transverse grooves 30 of projection 16.
[0029] On the rotor-side end 32 of shaft 14, the wall thickness of
the hollow shaft continuously decreases towards the opening 34 of
the cavity so that also the stiffness of shaft 14 in this region
decreases towards the rotor-side shaft end 32. The shaft end 32 can
be axially toothed, as illustrated, so as to be able to transmit
large torques from shaft 14 to rotor 12 in case that the positive
connections via the transverse bars and grooves 24,26,28,30 are not
sufficient for this purpose. The shaft end 32 both on its outer
side and on its inner side is inclined by about 5.degree. relative
to the axial line. In combination with the transverse bars and
grooves 24,26,28,30 of shaft 14 and rotor 12--which bars and
grooves have inclined flanks when viewed in cross section--this
arrangement will compensate for thermal expansion effects caused by
the differing thermal expansion effects of the two different
materials of which the rotor 12 and the shaft 14 are made. In this
manner, the connection will always reliably free of play.
[0030] For manufacture of the vacuum pump impeller 10, a blank of
shaft 14' as illustrated in FIG. 4 is first laid into a molding
box; the molding box is closed and aluminum as a rotor material is
filled into the molding box while still in a liquid state. In the
process, the liquid aluminum will also flow into the shaft cavity
22 and thus assume an outer shape which is complementary to the
shaft-side bars 24,26. After cooling, rotor 12' and shaft 14' are
removed from the molding box and supplied to a machining process
which will give the rotor and the shaft their final shape on their
outer and inner sides, as illustrated in FIG. 1.
[0031] In this manufacturing method, all shaped elements which are
provided to take up forces and torques can be generated by casting
technology. The use of a cast shaft will advantageously obviate the
need for additional machining. Further, the casting method makes it
possible to already shape all elements with cast radii which
thus--because of reduced notch effect--are useful to obtain a good
connection between the aluminum rotor 12 and the steel shaft 14.
During the casting of rotor 12, the hollow shaft 14 can serve as a
cooling iron by which a well-aimed cooling process of the rotor and
thus increased non-porosity and better cohesion in the rotor
material can be obtained.
[0032] By way of alternative to the above described casting method,
the impeller can also be produced by a forging method carried out
in analogous manner.
[0033] Illustrated in FIG. 3 is a second embodiment of a vacuum
pump impeller 50 comprising a radial compressor rotor 52, shown by
interrupted lines, and a hollow shaft 54. The conical rotor
projection 56 is cast into the conical shaft cavity 58. The
positive connection between rotor 52 and shaft 54 is effected by
bars and grooves in the longitudinal and circumferential
directions. The rotor blank 52' machined into the rotor 52 is
represented by continuous lines.
[0034] In FIG. 4, there is again illustrated a radial compressor
rotor 60 whose rotor 62, by means of high-precision casting and,
inter alia, by wax melting, already after the casting presents
blades 66 of which only the outer contour has to be machined. Shaft
67 includes a hollow portion 67 which does not extend throughout
the length of the shaft but covers only about a third of the shaft
length. In the region of hollow portion 67, a conical or
cylindrical axial cavity 65 is provided, with a conical or
cylindrical axial projection 63 of rotor 62 seated therein. The
positive engagement between rotor 62 and shaft 64 is obtained by at
least one eccentric rotor pin 68 seated in a corresponding number
of eccentric recesses 69 of shaft 64. To avoid imbalances caused by
the different materials of the rotor and the shaft, the pins have
to be arranged in a manner effecting the best possible mass
equilibrium, e.g. by means of two pins arranged at a relative
displacement by 180.degree. or three pins arranged at a relative
displacement by 120.degree.. In FIG. 4, only one pin is shown for
ease of illustration. The shaft blank 64' and the rotor blank are
shown by continuous lines, and the rotor 62 completed by machining
as well as the machined shaft 64 are shown by interrupted
lines.
[0035] FIG. 5 shows a vacuum pump impeller 80 which comprises a
diagonal compressor rotor 82 and a shaft 84 provided with an axial
cavity 86 extending along only about a third of the axial length of
shaft 84. The shaft cavity 86 has a corresponding projection 88 of
rotor 82 seated therein. The unworked rotor 82' and the unworked
shaft 84' are shown by continuous lines, and the machined rotor 82
as well as the machined shaft 84 are shown by interrupted
lines.
[0036] When subjected to high mechanical and/or thermal stresses,
cast rotors may reach the limits of their stability so that other
methods and materials have to be contemplated. The rotor 82 of
impeller 80 according to FIG. 5 is a forged part which consists
e.g. of aluminum and has to be hot-forged in a drop forge wherein
the shaft 84 with its cavity 86 has been laid. The stability of the
rotor is improved not only by the forging behavior but also by a
radially toothed shaft collar 90 which is effective to reduce
tensions caused by centrifugal forces and can be used to compensate
for imbalances by removal of material and/or placement of
compensation weights such as e.g. balancing screws. Additionally,
for enhancing the positive connection, a collar 92 can be formed
which together with a corresponding counterpart groove 94 will
provide for axial guidance.
[0037] The corrosion resistance of the aluminum rotor can basically
be improved by eloxadizing or hard-anodizing.
* * * * *