U.S. patent application number 11/989920 was filed with the patent office on 2010-06-24 for vacuum pump.
Invention is credited to Peter Hugh Birch, Nigel Paul Schofield.
Application Number | 20100158728 11/989920 |
Document ID | / |
Family ID | 34983964 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100158728 |
Kind Code |
A1 |
Schofield; Nigel Paul ; et
al. |
June 24, 2010 |
Vacuum pump
Abstract
A multistage vacuum pump comprises a stator housing a multistage
rotor assembly, each stage comprising inter-meshing Roots rotor
components, wherein the tip radius of the rotor components at an
inlet stage of the pump is larger than the tip radius of the rotor
components at an exhaust stage of the pump.
Inventors: |
Schofield; Nigel Paul; (West
Sussex, GB) ; Birch; Peter Hugh; (West Sussex,
GB) |
Correspondence
Address: |
Edwards Vacuum, Inc.
2041 MISSION COLLEGE BOULEVARD, SUITE 260
SANTA CLARA
CA
95054
US
|
Family ID: |
34983964 |
Appl. No.: |
11/989920 |
Filed: |
July 18, 2006 |
PCT Filed: |
July 18, 2006 |
PCT NO: |
PCT/GB2006/002679 |
371 Date: |
February 1, 2008 |
Current U.S.
Class: |
418/9 |
Current CPC
Class: |
F04C 18/084 20130101;
F04C 23/001 20130101; F04C 18/126 20130101; F04C 2240/20
20130101 |
Class at
Publication: |
418/9 |
International
Class: |
F04C 23/00 20060101
F04C023/00; F04C 18/12 20060101 F04C018/12; F04C 18/08 20060101
F04C018/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2005 |
GB |
0515905.8 |
Claims
1. A multistage vacuum pump comprising a stator housing a
multistage rotor assembly, each stage comprising intermeshing Roots
rotor components, wherein the tip radius of the rotor components at
an inlet stage of the pump is larger than the tip radius of the
rotor components at an exhaust stage of the pump.
2. The vacuum pump according to claim 1 wherein the tip radius of
the exhaust stage rotor components is at least 15% smaller than the
tip radius of the inlet stage rotor components.
3. The vacuum pump according to claim 1 or claim 2 wherein the tip
radius of the exhaust stage rotor components is at least 20%
smaller than the tip radius of the inlet stage rotor
components.
4. The vacuum pump according to claim 3 wherein the pump comprises
a first number of pumping stages each comprising rotor components
of a first tip radius, and a second number of pumping stages each
comprising rotor components of a second tip radius smaller than the
first tip radius.
5. The vacuum pump according to claim 4 wherein each of the first
and second numbers of pumping stages comprises a plurality of
pumping stages.
6. The vacuum pump according to claim 5 comprising a one-way valve
located between the first plurality of pumping stages and the
second plurality of pumping stages for exhausting from the stator
gas at a pressure above atmospheric pressure.
7. The vacuum pump according to claim 6 wherein each of the rotor
components comprises a plurality of lobes, and wherein the rotor
components at the inlet stage of the pump have the same number of
lobes as the rotor components at the exhaust stage of the pump.
8. The vacuum pump according to claim 7 wherein each of the rotor
components has between two and five lobes.
9. The vacuum pump according to claim 8 wherein each of the rotor
components has three lobes.
10. The vacuum pump according to claim 9 wherein each stage
comprises rotor components having different profiles.
11. The vacuum pump according to claim 10 wherein one of the rotor
components of a stage comprises pockets for receiving the lobes of
the other rotor component of that stage.
12. The vacuum pump according to claim 11 wherein the rotor
assembly comprises two intermeshing sets of Roots rotor components,
each set being mounted on a respective shaft for rotation relative
to the stator.
13. The vacuum pump according to claim 12 wherein the meshing
clearance between the rotor components at the inlet stage of the
pump is greater than the meshing clearance between the rotor
components at the exhaust stage of the pump.
14. The vacuum pump according to claim 2 wherein the pump comprises
a first number of pumping stages each comprising rotor components
of a first tip radius, and a second number of pumping stages each
comprising rotor components of a second tip radius smaller than the
first tip radius.
15. The vacuum pump according to claim 3 wherein the pump comprises
a first number of pumping stages each comprising rotor components
of a first tip radius, and a second number of pumping stages each
comprising rotor components of a second tip radius smaller than the
first tip radius.
16. The vacuum pump according to claim 5 comprising a one-way valve
located between the first plurality of pumping stages and the
second plurality of pumping stages for exhausting from the stator
gas at a pressure above atmospheric pressure.
17. The vacuum pump according to claim 4 comprising a one-way valve
located between the first plurality of pumping stages and the
second plurality of pumping stages for exhausting from the stator
gas at a pressure above atmospheric pressure.
18. The vacuum pump according to claim 14 wherein each of the rotor
components comprises a plurality of lobes, and wherein the rotor
components at the inlet stage of the pump have the same number of
lobes as the rotor components at the exhaust stage of the pump.
19. The vacuum pump according to claim 4 wherein each of the rotor
components comprises a plurality of lobes, and wherein the rotor
components at the inlet stage of the pump have the same number of
lobes as the rotor components at the exhaust stage of the pump.
20. The vacuum pump according to claim 17 wherein each of the rotor
components comprises a plurality of lobes, and wherein the rotor
components at the inlet stage of the pump have the same number of
lobes as the rotor components at the exhaust stage of the pump.
21. The vacuum pump according to claim 20 wherein one of the rotor
components of a stage comprises pockets for receiving the lobes of
the other rotor component of that stage.
22. The vacuum pump according to claim 21 wherein the rotor
assembly comprises two intermeshing sets of Roots rotor components,
each set being mounted on a respective shaft for rotation relative
to the stator.
23. The vacuum pump according to claim 22 wherein the meshing
clearance between the rotor components at the inlet stage of the
pump is greater than the meshing clearance between the rotor
components at the exhaust stage of the pump.
24. The vacuum pump according to claim 5 wherein the rotor assembly
comprises two intermeshing sets of Roots rotor components, each set
being mounted on a respective shaft for rotation relative to the
stator.
25. The vacuum pump according to claim 24 wherein the meshing
clearance between the rotor components at the inlet stage of the
pump is greater than the meshing clearance between the rotor
components at the exhaust stage of the pump.
Description
[0001] The present invention relates to a vacuum pump, and in
particular to a multistage Roots vacuum pump.
[0002] A multistage Roots pump generally comprises a pair of shafts
each supporting plurality of rotor components within a housing
providing a stator component for the pump. The stator comprises a
gas inlet, a gas outlet and a plurality of pumping chambers, with
adjacent pumping chambers being separated by a transverse wall. A
gas flow duct connects a chamber outlet from one pumping chamber to
a chamber inlet of the adjacent, downstream pumping chamber.
[0003] Each pumping chamber houses a pair of lobed Roots rotor
components to provide a pumping stage of the pump. The rotor
components are housed with the pumping chamber such that there is a
small clearance between the rotor components and between each rotor
component and an inner wall of the pumping chamber.
[0004] As the rotors do not come into contact with each other or
with the pump housing, a multistage Roots pump can be operated at
high rotational speeds up to 12,000 rpm or even higher. With
rotation of the shafts, the rotor components of each pair are
rotated in opposite directions at high speed to draw gas through
the chamber inlet and transport the gas through the pumping chamber
without internal compression to the chamber outlet. The gas thus
passes through each of the pumping, chambers before being exhaust
from the gas outlet of the housing.
[0005] The energy required to transport the gas through the pumping
chambers is dependent, amongst others, on the volume of the pumping
chambers and the downstream pressure acting on the gas as it is
transported through the pumping chamber. In order to compress the
gas as it passes through the multistage pump, and thereby generate
a vacuum at the inlet of the housing, and reduce energy
consumption, it is known to progressively reduce the width of the
pumping chambers from the inlet stage to the exhaust stage, and
thereby progressively reduce the volume of the pumping chambers.
The ratio between the volume of the inlet stage of the pump and the
volume of the outlet stage of the pump, commonly referred to as the
"volume ratio" of the pump, thus determines both the power
consumption of the pump and the size of the vacuum which can be
generated at the inlet of the housing.
[0006] By reducing the width of the pumping stages, the thickness
of the rotor components must decrease progressively from the inlet
to the outlet of the pump. Whilst this tends not to be a problem at
low volume ratios, for example up to 5:1, at higher ratios the
rotor components of the exhaust stage can become very thin. For
example, for a pump having rotor components of 30 mm thickness at
the inlet stage, a rotor thickness of 1.5 mm would be required at
the exhaust stage to achieve a volume ratio of 20:1. This can make
machining and mounting of the rotor components very difficult.
Furthermore, due to the varying thermal expansions between the
rotor components and the stator from the inlet stage to the exhaust
stage, it can be difficult to maintain small clearances between the
rotor components and the stator, particularly at the exhaust stage
where the rotor components are thin, and this can significantly
reduce the pumping efficiency of the pump.
[0007] It is an aim of at least the preferred embodiment of the
present invention to seek to solve these and other problems.
[0008] The present invention provides a multistage vacuum pump
comprising a stator housing a multistage rotor assembly, each stage
comprising intermeshing Roots rotor components, wherein the tip
radius of the rotor components at an inlet stage of the pump is
larger than the tip radius of the rotor components at an exhaust
stage of the pump.
[0009] By providing a pump where the tip radius of the exhaust
stage rotor components is smaller than the tip radius of the inlet
stage rotor components, a pump having a relatively high volume
ratio of at least 10:1, more preferably of at least 15:1 can be
achieved without having to reduce the thickness of the rotor
components at the exhaust stage to the extent described above. For
example, where the inlet stage rotor components have a thickness of
around 30 mm, a pump having a relatively high volume ratio can be
achieved with exhaust stage rotor components having a thickness of
around 5 mm.
[0010] The pump may comprise a first plurality of pumping stages
each comprising rotor components of a first tip radius, and a
second plurality of pumping stages each to comprising rotor
components of a second tip radius smaller than the first tip
radius. For example, each of the first and second plurality of
pumping stages may comprise at least two pumping stages.
Alternatively, the tip radius of the rotor components may
progressively decrease from the inlet stage of the pump to the
exhaust stage of the pump. Therefore, in more general terms the
pump may comprise a first number (one or more) pumping stages each
comprising rotor components of a first tip radius, and a second
number (one or more) of pumping stages each comprising rotor
components of a second tip radius smaller than the first tip
radius.
[0011] To allow the pump to operate at maximum nominal speed during
roughing, that is, when a chamber attached to an inlet of the pump
is evacuated from atmospheric pressure, a pressure relief valve may
be located between the first plurality of pumping stages and the
second plurality of pumping stages for selectively exhausting gas
from the pump. The pressure relief valve is preferably configured
to automatically close when the pressure of gas at the valve inlet
falls below atmospheric pressure, at which point the second
plurality of pumping stages become effective in further reducing
the pressure at the inlet of the pump and enhancing the net pumping
speed.
[0012] Each of the rotor components preferably comprises a
plurality of lobes, with the inlet stage rotor components
preferably having the same number of lobes as the exhaust stage
rotor components. The rotor components of a stage may have the same
profile, or different profiles. For example, one of the rotor
components of a stage may have sockets for receiving the lobes of
the other rotor component of that stage.
[0013] The rotor assembly preferably comprises two intermeshing
sets of Roots rotor components, each set being mounted on a
respective shaft for rotation relative to the stator.
Alternatively, each set of rotor components may be integral with
the shaft, with the stator being provided by two stator "half
shells" that are assembled once the shafts have been mounted within
one of the half shells.
[0014] The meshing clearance between the rotor components at the
inlet stage of the pump is preferably greater, most preferably
between 10 and 30% greater, than the meshing clearance between the
rotor components at the exhaust stage of the pump. The rotor
components at the inlet stage of the pump may be used to "time" the
rotors to gears connecting the shafts so that the shafts are
rotated synchronously but in opposite directions. The larger
meshing clearance between the rotor components at the inlet stage
of the pump can thus facilitate the assembly of the pump, whilst
the smaller meshing clearance between the rotor components at the
exhaust stage of the pump can maintain the ultimate power
consumption and pressure at acceptable levels.
[0015] Preferred features of the present invention will now be
described with reference to the accompanying drawing, in which
[0016] FIG. 1 illustrates a multistage vacuum pump comprising two
sets of intermeshing rotor components.
[0017] FIG. 2 illustrates a set of rotor components of the pump of
FIG. 1;
[0018] FIG. 3 illustrates the profiles of the rotor components of
an inlet stage of the pump of FIG. 1; and
[0019] FIG. 4 illustrates the profiles of the rotor components of
an exhaust stage of the pump of FIG. 1.
[0020] With reference first to FIG. 1, a multi-stage vacuum pump 10
comprises a stator 12 housing a multistage rotor assembly 14. The
stator 12 comprises a plurality of transverse wails 16 which divide
the stator 12 into a plurality of pumping chambers. In this
example, the stator 12 is divided into five pumping stages,
although the stator 12 may be divided into any number of pumping
stages required to provide the pump 10 with the desired pumping
capacity.
[0021] The rotor assembly 14 comprises two intermeshing sets of
lobed Roots rotor components 18, 20, 22, 24, 26, each set being
mounted on a respective shaft 28, 30. Each shaft 28, 30 is
supported by bearings for rotation relative to the stator 12. The
shafts 28, 30 are mounted within the stator 12 so that each pumping
15, chamber houses a pair of intermeshing rotor components, which
together provide a stage of the pump 10. One of the shafts 28 is
driven by a motor 32 connected to one end of that shaft 28. The
other shaft 30 is connected to that shaft 28 by means of meshed
timing gears 34 so that the shafts 28, 30 are rotated synchronously
but in opposite directions within the stator 12.
[0022] A pump inlet 36 communicates directly with the inlet pumping
stage, which comprises rotor components 18, 18' and pump outlet 38
communicates directly with the exhaust pumping stage, which
comprises rotor components 26, 26'. Gas passageways 40, 42, 44, 46,
48 are provided within the pump 10 to permit the passage
therethrough of pumped gas from the inlet 36 to the outlet 38.
[0023] In order to achieve a reduced pressure at the inlet 36 of
the pump 10, the volume of the pumping chambers defined within the
stator 12 progressively decreases from the inlet pumping stage to
the exhaust pumping stage. In this example, the reduction in the
volume of the first three pumping chambers is achieved by
progressively reducing the thickness of the pumping chambers, and
the reduction in the volume of the last two pumping chambers is
achieved both by progressively reducing the thickness of the
pumping chambers and by reducing the diameter of the pumping
chambers in comparison to the first three pumping chambers.
[0024] The sets of rotor components are profiled in order to
maintain small clearances between the walls of the pumping chambers
and the surfaces of the rotor components. One of the sets of rotor
components is illustrated in more detail in FIG. 2. The thickness t
of the rotor components progressively decreases from a thickness
t.sub.1 of the inlet stage rotor component 18 to a thickness
t.sub.2 of the exhaust stage rotor component 26.
[0025] The rotor components are divided into a plurality of numbers
of rotor components, each number comprising one or more rotor
components of a particular tip radius, that is, the maximum
distance d between the outer profile of the rotor component and the
centre of the rotor component. In the illustrated example, the
rotor components are divided into a first plurality of rotor
components 50 having a tip radius d.sub.1 and a second plurality of
rotor components 52 having a tip radius d.sub.2, where d.sub.2 is
smaller than d.sub.1, preferably at least 15% smaller than d.sub.1,
more preferably at least 20% smaller than d.sub.1. For the example
illustrated in FIGS. 1 and 2, the first plurality of rotor
components 50 comprises the three rotor components 18, 20, 22
proximate the inlet 36 of the pump 10, and the second plurality of
rotor components 52 comprising the two rotor components 24, 26
proximate the outlet 38 of the pump 10.
[0026] The number and size of the pumping stages may be varied
according to the required pumping capacity. For example, a six
stage vacuum pump may comprises three rotor components of tip
radius d.sub.1 and three rotor components of tip radius d.sub.2, or
three rotor components of tip radius d.sub.1, two rotor components
of tip radius d.sub.2, and one rotor component of tip radius
d.sub.3, where d.sub.1>d.sub.2>d.sub.3.
[0027] Each of the rotor components 18, 20, 22, 24, 26 may comprise
the same number of lobes. As illustrated in FIGS. 3 and 4, each of
the rotor components comprises three lobes 60, although the rotor
components may have any number of lobes, for example between two
and five lobes. The lobes may have any desired curved profile. For
example, as illustrated in FIG. 3, one of the rotor components 18;
26 of a stage may comprise sockets 62 for receiving the lobes of
the other rotor components 18', 26' of that stage.
[0028] By reducing the tip radius of at least the exhaust stage
rotor component, the required reduction of the thickness of the
exhaust stage pumping component to achieve a relatively high volume
ratio is less than that required if the tip radius of the exhaust
stage pumping component was the same as that of the inlet stage
rotor component. For example, if the tip radius was held at a
constant value, the thickness of the exhaust stage rotor component
would need to around 5% that of the inlet stage rotor component to
achieve a volume ratio of 20:1. If, however, the tip radius of the
exhaust stage pumping component was between 15 and 20% smaller than
that of the inlet stage rotor component, the thickness of the
exhaust stage rotor component would only need to around 10-15% that
of the inlet stage rotor component to achieve the same volume
ratio, thereby facilitating machining and mounting of the exhaust
stage pumping components.
[0029] The meshing clearance between the rotor components 18, 18'
at the inlet stage of the pump 10 is preferably greater, most
preferably between 10 and 30% greater, than the meshing clearance
between the rotor components 26, 26' at the exhaust stage of the
pump 10. The rotor components 18, 18' at the inlet stage of the
pump may be used to "time" the rotors to the gears 34, and so the
larger meshing clearance between the inlet stage rotor components
18, 18' can thus facilitate the assembly of the pump 10. The
smaller meshing clearance between the exhaust stage rotor
components 26, 26' can maintain the ultimate power consumption and
pressure at acceptable levels, the extra clearance between the
inlet stage rotor components 18, 18' having a negligible effect on
ultimate power and pressure, and on peak volumetric pumping
speed.
* * * * *