U.S. patent application number 09/104171 was filed with the patent office on 2001-08-23 for turbo-molecular pump.
This patent application is currently assigned to EBARA CORPORATION. Invention is credited to IKEGAMI, TETSUMA, KAWASAKI, HIROYUKI, MIYAMOTO, MATSUTARO.
Application Number | 20010016160 09/104171 |
Document ID | / |
Family ID | 26367314 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016160 |
Kind Code |
A1 |
IKEGAMI, TETSUMA ; et
al. |
August 23, 2001 |
TURBO-MOLECULAR PUMP
Abstract
A turbo-molecular pump of high safety and reliability has been
developed so that if an abnormal condition should develop on the
rotor structure, it will not lead to damage to the stator or pump
casing to cause loss of vacuum in a vacuum processing system. The
turbo-molecular pump comprises a pump casing housing a stator and a
rotor therein, a vane pumping section and/or a groove pumping
section comprised by the stator and the rotor, and a constriction
releasing structure for releasing constriction of at least a part
of the stator when an abnormal torque is applied to the stator by
the rotor.
Inventors: |
IKEGAMI, TETSUMA;
(YOKOHAMA-SHI, JP) ; MIYAMOTO, MATSUTARO;
(CHIGASAKI-SHI, JP) ; KAWASAKI, HIROYUKI;
(CHIGASAKI-SHI, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
EBARA CORPORATION
TOKYO
JP
|
Family ID: |
26367314 |
Appl. No.: |
09/104171 |
Filed: |
June 25, 1998 |
Current U.S.
Class: |
415/90 ;
415/173.1 |
Current CPC
Class: |
F04D 29/522 20130101;
F04D 19/042 20130101; F04D 27/008 20130101 |
Class at
Publication: |
415/90 ;
415/173.1 |
International
Class: |
F01D 001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 1997 |
JP |
187681/1997 |
Jan 27, 1998 |
JP |
29160/1998 |
Claims
What is claimed is:
1. A turbo-molecular pump comprising: a pump casing housing a
stator and a rotor therein; a vane pumping section and/or a groove
pumping section comprised by said stator and said rotor; and a
constriction releasing structure for releasing constriction of at
least a part of said stator when an abnormal torque is applied to
said stator by said rotor.
2. A turbo-molecular pump according to claim 1, wherein said
constriction releasing structure is provided in a fixation
structure for fixing said stator to said pump casing.
3. A turbo-molecular pump according to claim 1, wherein said stator
is comprised by a plurality of stator elements, said constriction
releasing structure being provided in a fixation structure for
mutually fixing said stator elements.
4. A turbo-molecular pump according to claim 1, wherein said
constriction releasing structure comprises a fragile section
provided on at least a part of said stator.
5. A turbo-molecular pump according to claim 1, wherein said vane
pumping section comprises a plurality of stator vanes, said
constriction releasing structure being constructed to release
constriction of said stator vanes.
6. A turbo-molecular pump according to claim 5, wherein said vane
pumping section comprises layered stator vane spacers for fixing
said stator vanes, said constriction releasing structure being
constructed to release constriction of said stator vane
spacers.
7. A turbo-molecular pump according to claim 6, wherein said
constriction releasing structure comprises a space radially outside
of said stator vane spacer for allowing said stator vane spacer to
withdraw therein.
8. A turbo-molecular pump according to claim 6, wherein said
constriction releasing structure comprises a receiving space
radially outside of said stator vane spacer capable of receiving
said stator vane spacer.
9. A turbo-molecular pump according to claim 3, wherein said
constriction releasing structure comprises a strength adjusted
fastening device for mutually fixing said stator elements.
10. A turbo-molecular pump according to claim 1, wherein said
groove pumping section comprises a groove pumping section spacer
fixed to said stator, said constriction releasing structure being
constructed to release constriction of said groove pumping section
spacer to said stator.
11. A turbo-molecular pump according to claim 10, wherein said
constriction releasing structure comprises a strength adjusted
fastening device for mutually fixing said groove pumping section
spacer to said stator.
12. A turbo-molecular pump according to claim 10, wherein said
groove pumping section spacer is fixed at one end thereof to said
stator.
13. A turbo-molecular pump according to claim 12, wherein said
groove pumping section spacer comprises a cylindrical body and a
flange section provided at one end of said cylindrical body, and a
fragile section is provided on said groove pumping section spacer
at an area between said cylindrical body and said flange
section.
14. A turbo-molecular pump comprising: a pump casing housing a
stator and a rotor therein; a vane pumping section and/or a groove
pumping section comprised by said stator and said rotor; and a
friction reducing structure provided in at least a part of a space
between said stator and said pump casing.
15. A turbo-molecular pump according to claim 14, wherein said
friction reducing structure comprises a mechanical bearing.
16. A turbo-molecular pump according to claim 15, wherein said
mechanical bearing comprises an inner sleeve and an outer sleeve,
said inner sleeve having a larger thickness than said outer
sleeve.
17. A turbo-molecular pump according to claim 14, wherein said
friction reducing structure comprises a friction reducing member
made of a material having a low friction coefficient.
18. A turbo-molecular pump comprising: a pump casing housing a
stator and a rotor therein; a vane pumping section and/or a groove
pumping section comprised by said stator and said rotor; and an
impact absorbing structure provided in at least a part of a space
between said stator and said pump casing.
19. A turbo-molecular pump according to claim 18, wherein said
impact absorbing structure comprises a composite structure of a
high impact absorbing characteristic member and a high rigidity
member.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a turbo-molecular pump for
evacuating gas by using a high speed rotor.
[0003] 2. Description of the Related Art
[0004] An example of a conventional turbo-molecular pump is shown
in FIG. 13. The pump is comprised by a cylindrical pump casing 14
housing a vane pumping section L.sub.1 and a groove pumping section
L.sub.2 which are constituted by a rotor (rotation member) R and a
stator (stationary member) S. The bottom portion of the pump casing
14 is covered by a base section 15 which is provided with an
exhaust port 15a. The top portion of the pump casing 14 is provided
with a flange section 14a for coupling the pump to an apparatus or
a piping to be evacuated. The stator S comprises a stator cylinder
section 16, fixed sections of the vane pumping section L.sub.1 the
groove pumping section L.sub.2.
[0005] The rotor R is comprised by a rotor cylinder section 12
attached to a main shaft 10 which is inserted into the stator
cylinder section 16. Between the main shaft 10 and the stator
cylinder section 16 are constructed a drive motor 18, an upper
radial bearing 20 and a lower radial bearing 22 disposed on the
upper and lower sides of the drive motor 18 respectively. Under the
main shaft 10, there is an axial bearing 24 having a target disk
24a at the bottom end of the main shaft 10 and an upper and a lower
electromagnets 24b on the stator side. In this configuration, a
high speed rotation of the rotor R is supported under a five
coordinate active control system.
[0006] Rotor vanes 30 are provided integrally with the upper
external surface of the rotor cylinder section 12 to form an
impeller, and on the inside of the casing 14, stator vanes 32 are
provided in such a way to alternately interweave with the rotor
vanes 30. These vane members constitute the vane pumping section
L.sub.1 carries out gas evacuation by cooperative action of the
high speed rotor vanes 30 and the stator vanes 32. Below the vane
pumping section L.sub.1, the groove pumping section L.sub.2 is
provided. The groove pumping section L.sub.2 is comprised by a
spiral groove section 34 having spiral grooves 34a on the outer
surface of the bottom end of the rotor cylinder section 12, and a
spiral groove section spacer 36 surrounding the spiral groove
section 34 of the stator S. The gas evacuation action of the groove
pumping section L.sub.2 is due to the dragging effect of the spiral
grooves 34a against gases.
[0007] By providing the groove pumping section L.sub.2 at
downstream of the vane pumping section L.sub.1, a wide-range
turbo-molecular pump can be constructed so as to enable evacuation
over a wide range of gas flow rates using one pumping unit. In this
example, the spiral grooves of the groove pumping section L.sub.2
are provided on the rotor side of the pump structure, but some
pumps have the spiral grooves formed on the stator side of the pump
structure.
[0008] Such turbo-molecular pumps are assembled as follows.
Firstly, the groove pumping section spacer 36 is attached by
coupling the lower surface of the step 36a to the protruded ring
section 15b formed on the base section 15. Next, the rotor R is
fixed in some position, and the stator vanes 32, which are normally
split into two half sections, are clamped around to interweave
between the rotor vanes 30. This is followed by placing a stator
vane spacer 38, having steps on its top and bottom regions, on top
of the clamped rotor vane 30. This assembling step is repeated for
each rotor vane 30 to complete the assembly of the stator vanes 32
around the rotor R.
[0009] Lastly, the pump casing 14 is attached by sliding it around
the layered stator vane structure and fixing the flange 14b to the
base of the stator S by fasteners such as bolts, thereby pressing
the top stator vane spacer 38 firmly against the stepped surface
14c on the inside surface of the casing 14 and binding the entire
layered assembly and the groove pumping section spacer 36. It can
be understood from this assembly structure that the peripheries of
each of the stator vanes 32 are pressed together by stator vane
spacers 38 located above and below, and similarly the groove
pumping section spacer 36 is pressed down by the lowermost stator
vane 32, stator vane spacer 38 and the protrusion section 15b of
the base section 15, so that the axially applied pressing force
prevents induced rotation of the stator vanes 32 and the groove
pumping section spacer 36 with the rotor R in the circumferential
direction.
[0010] Also, though not shown in the drawing, sometimes the groove
pumping section spacer 36 is fastened to the stator cylinder
section 16 of the stator S by bolts to assure the fixation.
[0011] In such turbo-molecular pumps, operational difficulties are
sometimes encountered, such as abnormal rotation caused by
eccentricity of rotor R, and they may be accompanied by damaging of
the rotor vanes 30. In such a case, the stator structure can also
be subjected to significant circumferential or radial force by the
rotor R and its debris, which may impact on not only the stator
vanes 32 but the stator vane spacers 38 and the groove pumping
section spacer 36.
[0012] These abnormal operating conditions can cause not only
deformation of the stator vanes 32 and spacers 36, 38, but can
cause fracture of casing 14 and stator cylinder section 16, or
damage to their joints or severing of vacuum connections attached
to the pump. Such damage and severing to any parts of the stator S
cause breakage of vacuum in the whole processing system connected
to and evacuated by the pump not only to damage the system
facilities and in-process goods, but also to lead to accidental
release of gases in the system to outside environment.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a
turbo-molecular pump of high safety and reliability so that if an
abnormal condition should develop on the rotor structure, it will
not lead to damage to the stator or pump casing to cause loss of
vacuum in a vacuum processing system.
[0014] The object has been achieved in a turbo-molecular pump
comprising: a pump casing housing a stator and a rotor therein; a
vane pumping section and/or a groove pumping section comprised by
the stator and the rotor; and a constriction releasing structure
for releasing constriction of at least a part of the stator when an
abnormal torque is applied to the stator by the rotor.
[0015] Accordingly, when an abnormal torque is applied to a stator
side of the pump structure due to some abnormal condition
developing in the rotor structure, the constriction releasing
structure acts to loosen the stator structure so that the rotation
energy of the rotor is absorbed and transmission of torque to the
pump casing is prevented and damage to pump casing and vacuum
connection can be avoided. The constriction releasing structure is
normally provided on the stator side of the pump structure, i.e.,
fixed vanes and structures for fixing the groove pumping section
spacer to the pump casing.
[0016] The stator may be comprised by a plurality of stator
elements, and the constriction releasing structure may be provided
in a fixation structure for mutually fixing the stator
elements.
[0017] The constriction releasing structure may be a fragile
section provided on a stator side of the pump structure.
Accordingly, the rotation energy of the rotor is absorbed by
fracture of the fragile section, thereby reducing the effects of
abnormal torque on the pump casing.
[0018] Stator element may be provided with a flange section for
their fixation, and the fragile section may be formed in the flange
section. Accordingly, transmission of abnormal torque to the pump
casing is prevented by fracture along the fragile section in the
groove pumping section in the stator which can be readily deformed
outward.
[0019] In another aspect of the invention, the turbo-molecular pump
comprises: a pump casing housing a stator and a rotor therein; a
vane pumping section and/or a groove pumping section comprised by
the stator and the rotor; and a friction reducing structure
provided in at least a part of a space between the stator and the
pump casing. Accordingly, friction between the stator and the pump
casing is reduced, and it is more difficult to transmit rotational
torque on the stator to the pump casing, thereby preventing
abnormal torque to be transmitted to the casing. For example, in
addition to an inherently low friction material such as
polytetrafluoroethylene, low-friction structures comprised by ball
bearings or rod bearings may also be used.
[0020] In another aspect of the invention, the turbo-molecular pump
comprises: a pump casing housing a stator and a rotor therein; a
vane pumping section and/or a groove pumping section comprised by
the stator and the rotor; and an impact absorbing structure
provided in at least a part of a space between the stator and the
pump casing. In this type of pump, because impact transmitted from
the rotor to the stator is absorbed by the impact absorbing
structure, it is possible to prevent abnormal torque to be
transmitted to the pump casing. Such impact absorbing structure can
be comprised by relatively soft metallic materials, polymeric
materials or a mixture thereof. Additionally, by combining such
materials with a relatively tough material, a composite material
may be used to combine an impact absorbing function and shape
retaining function.
[0021] The stator of a cylindrical shape to comprise the groove
pumping section may be secured to the pump casing in such a way
that, the stator is attached firmly at an exhaust end of the groove
pumping section, but at an intake end of the groove pumping
section, a stator wall is attached to the pump casing so as to
leave a clearance between self and the pump casing. Accordingly,
the bottom end of the stator comprising the groove pumping section
which can be readily deformed outward is separated from the casing
so that transmission of abnormal torque to the pump casing can be
prevented.
[0022] The friction reducing structure may be comprised by a
mechanical bearing sleeve means having an inner sleeve and an outer
sleeve wherein an inner sleeve thickness is larger than an outer
sleeve thickness. Accordingly, by increasing the toughness of the
inner bearing member, the bearing device can perform its friction
reducing function without losing its rotational capability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross sectional view of a turbo-molecular pump
in a first embodiment;
[0024] FIG. 2 is a plan view of a stator vane spacer used in the
uppermost stage and the lowermost stage of the vane pumping section
shown in FIG. 1;
[0025] FIG. 3 is a cross sectional view of a turbo-molecular pump
in a second embodiment;
[0026] FIG. 4 is a cross sectional view through a plane A-A in FIG.
3;
[0027] FIG. 5 is a cross sectional view of a turbo-molecular pump
in a third embodiment;
[0028] FIG. 6 is a plan view of a rotor vane spacer shown in FIG.
5;
[0029] FIG. 7 is a cross sectional view through a plane B-B in FIG.
6;
[0030] FIG. 8 is a cross sectional view of a turbo-molecular pump
in a fourth embodiment;
[0031] FIG. 9 is a cross sectional view of a variation of the pump
shown in FIG. 8;
[0032] FIG. 10 is a cross sectional view of another variation of
the pump shown in FIG. 8;
[0033] FIG. 11 is a cross sectional view of a turbo-molecular pump
in a fifth embodiment;
[0034] FIG. 12A is a cross sectional view of a turbo-molecular pump
in a sixth embodiment;
[0035] FIG. 12B is a cross sectional view of another configuration
of the impact absorbing structure; and
[0036] FIG. 13 is a cross sectional view of a conventional
turbo-molecular pump.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] In the following, preferred embodiments will be presented
with reference to the drawings.
[0038] FIGS. 1 and 2 relate to the first embodiment of the
turbo-molecular pump. The present pump shares some common
structural features with the conventional pump shown in FIG. 13,
such as vane pumping section L.sub.1 comprised by alternating rotor
vanes 30 and the stator vanes 32, the groove pumping section
L.sub.2 having spiral groove section 34 and groove pumping section
spacer 36. As well, the pump casing 14 is used to press down the
stator vanes 32, stator vane spacers 38 and the groove pumping
section spacer 36. Therefore, an overall illustration of this
embodiment is omitted.
[0039] In the present pump is constructed so that, when abnormal
torque is applied to the stator vane due to abnormal conditions
developing in any rotor components, a part of the stator vane
spacers 38 is able to move radially outward. This is achieved by
having the uppermost vane spacer 38a and the lowermost vane spacer
38b each of which is comprised by vane spacer halves 40. The inner
surface of the casing 14 has grooves 42, 44 extending all around
its circumference at corresponding heights with that of the outer
surfaces of the uppermost and lowermost vane spacers 38a, 38b. The
width of the grooves 42, 44 is slightly larger than the thickness
of the stator vane spacers 38a, 38b.
[0040] During the normal operation of such a pump, no large torque
will be applied to either the stator vanes 32 or the stator vane
spacers 38 in the circumferential or radial direction, and the
assembly, consisting of stator vanes 32 and stator vane spacers 38,
retain their positions because of mutual friction therebetween.
Stator vane spacers 38a, 38b retain their ring shape, and hold
individual stator vanes 32 in contact with the associated stator
vane spacers 38.
[0041] If an abnormal condition should develop in the rotation of
the rotor R or if the rotor R should break for whatever reason, and
either or both of the stator vane spacers 38a, 38b are subjected to
a large force acting in circumferential or radial direction, stator
vane spacers 38a, 38b are pushed outwards, and the upper and lower
split spacers 40 are separated into half pieces and the half pieces
enter into the grooves 42, 44. In this condition, other stator vane
spacers 38 become loose and rotatable because of the release of
constrict in an axial direction. This causes the stator vanes 32
and the stator vane spacers 38 to be dragged with the rotor R, and
causes the rotation energy of the rotor R to be gradually
dissipated, and the rotor R eventually stops. Because of the
release of an axial constrict of the stator vanes 32 and stator
vane spacers 38 against the casing 14, damage to casing 14 or to
connection to external facility is not produced.
[0042] In the embodiment presented above, the uppermost and the
lowermost stator vane spacers 38a, 38b are made into split rings,
but either one of the split type spacer is enough for the purpose
of invention, and also, any one or more of the spacers 38 disposed
in the mid-section of the rotor R can be selected as the split type
spacer. It is also possible to split the spacers into more than two
pieces.
[0043] FIGS. 3 and 4 show a second embodiment of the
turbo-molecular pump according to the invention. This pump is also
constructed so that the axial constrict of the stator vane 32 is
released at an early stage of the onset of abnormal condition. As
shown in FIG. 4, a plurality of support pins 46 are provided
equally spaced in the circumferential direction in a space between
the vanes 32c of the uppermost stator vane 32a. Similar support
pins 48 are also provided in a space between the vanes 32c of the
lowermost stator vanes 32.
[0044] With reference to FIG. 3, the support pins 46 are fitted
between the step surface 14c of the casing 14 and the uppermost
stator vane spacer 38c as a "support rod". The length of the pins
is chosen to be slightly greater than the thickness of the
uppermost stator vane 32a. Similarly support pin 48 is fitted
between the groove pumping section spacer 36 and the lowermost
stator vane spacer 38d and its length is made slightly larger than
the thickness of the lowermost stator vane 32b. Therefore, a
clearance T.sub.1 is formed between the uppermost stator vane 32a
and the step surface 14c and a clearance T.sub.2 is formed between
the lowermost stator vane spacer 38d and the lowermost stator vane
32b.
[0045] These support pins 46, 48 are made in such a way that,
during normal operation of the pump, they are sufficient in their
strength and number to support the stator vane spacer 38 in place,
and if some abnormal condition should develop, such as twist of the
rotor R or torque on the stator S by the rotor R, then the pins can
be readily broken. Also, the sizes of the clearance T.sub.1,
T.sub.2 are chosen to be in a range of about 50-100 mm such that,
during normal operation, the stator vanes 32a do not experience any
slack.
[0046] Such a pump operates as follows. During normal operation,
the pump will remain in the condition illustrated in FIG. 3, but if
the rotor R should break or experience abnormal rotation to cause
some twist or torque to be developed between the stator S and the
rotor R, the support pins 46, 48 will either fall down or break.
This causes the clearances T.sub.1, T.sub.2 to be spread among the
stator vanes 32 and stator vane spacers 38, thereby the assembly
becomes loose and releases the axial constricting force which had
been exerted on the assembly. The result is that the stator vane
spacers 38 become rotatable with the impeller, and reduces the
chances of torque being transmitted to the casing components,
thereby preventing damage to the pump. Although top and bottom pins
46, 48 are provided in this embodiment, it is permissible to
provide such pins at either end of the vane pumping section
L.sub.1.
[0047] FIGS. 5 to 7 show a third embodiment of the turbo-molecular
pump according to the invention. In this pump, all the stator vane
spacers 50, excepting the uppermost stator vane spacer, are
provided with a series of threaded holes 50a and bolt holes 50b
alternately distributed in a circumferential direction so that a
shear bolt 52 can be inserted through a bolt hole 50b of an upper
stator vane spacer 50 to be fastened into a threaded holes 50a of a
lower stator vane spacer 50 so as to assemble all the stator vane
spacers 50 to each other. The lowermost stator vane spacer 50 is
fixed to the top of the groove pumping section spacer 54 also by
shear bolts 52.
[0048] The strength of the shear bolts 52 is selected such that,
when abnormal torque is transmitted to the spacer 50 due to
breaking of the rotor R or abnormal rotation, they will fracture.
The bolt strength is determined either by selecting the material or
diameter, or by providing a notch on the shear bolts 52.
[0049] Groove pumping section spacer 54 in the groove pumping
section L.sub.2 is fixed to the base section 15 of the stator S by
inserting shear bolt 56 through a bolt receiving slit 55 and
screwing the shear bolt 56 into the base section 15. The strength
of the bolt 56 is selected so that it will break when torque of a
certain magnitude is transmitted to the spacer 54.
[0050] In this embodiment, the inside corners of the protrusion 17a
which supports the bottom end of the groove pumping section spacer
54 are chamfered, and the height H of the contact surface 17b
contacting the bottom end of the groove pumping section spacer 54
is made shorter than the case shown in FIG. 13. Also, a friction
reducing device is provided in the form of a cylinder-shaped
low-friction sleeve 58 which is made of a low friction material
disposed in the space formed between the spacers 50, 54 and the
casing 14.
[0051] Such a pump operates as follows. When abnormal torque acts
on the stator vane spacers 50 or groove pumping section spacer 54,
the shear bolts 52, 56 fastening the stator vane spacers 50 and
groove pumping section spacer 54 to the stator S are fractured,
thus releasing the axial compression to enable the stationary
members to rotate with the impeller. This causes the energy of the
rotor R to be dissipated, and lowers the torque transmitted from
the rotor R to the stator S, thus preventing damage to the stator
S.
[0052] Also, because the friction reducing devices 58 is provided
in the space between the casing 14 and the stator vane spacers
50/groove pumping section spacer 54, frictional force resulting
between the casing 14 and stator vane spacers 50/groove pumping
section spacer 54 is reduced. Also, because the contact area
between the base section 15 and the groove pumping section spacer
54 is made small, the force transmitted to the stator S is further
reduced. The purpose of providing a circumferential groove 42
opposite the outer edge of the uppermost stator vane spacer 38 has
been explained in the first embodiment.
[0053] FIG. 8 shows a fourth embodiment of the pump according to
the invention. The casing 14 in this case is made of an intake-side
casing 14A and an exhaust-side casing 14B, which are attached to
form a complete casing 14. Stator vane spacers 50 in the vane
pumping section L.sub.1 are axially fixed layer by layer by using
shear bolts 52 as in the previous embodiment.
[0054] The exhaust side casing 14B has a step surface 60 at the top
end, and the groove pumping section spacer 54 has a flange section
54a, so that the groove pumping section spacer 54 is attached to
the exhaust-side casing 14B by fastening the step surface 60 to the
flange section 54a by bolts 56. The strength of the bolts 56 is
selected such that they will break at a given torque. In this
embodiment also, cylinder-shaped friction reducing sleeves 58a, 58b
are provided in the spaces between the stator vanes 50 and the
intake-side casing 14A on the one hand, and the groove pumping
section spacer 54 and the exhaust-side casing 14B. The
turbo-molecular pump of this embodiment provides the same
protective effects described above.
[0055] FIG. 9 shows a variation of the fourth embodiment shown in
FIG. 8. Groove pumping section spacer 54 in the groove pumping
section of this pump is attached by bolting the top flange section
54a to the step surface 60 at the top end of the exhaust-side
casing 14B as in the previous embodiment. Friction reducing sleeves
58a, 58b are provided in the spaces formed in the intake-side
casing 14A and likewise in the exhaust-side casing 14B. In the
previous embodiment, the bottom end of the groove pumping section
spacer 54 contacted the inside surface of the base section 15 to
produce the circumferential constricting force, but in this
embodiment, there is a clearance T.sub.3 between the outer
periphery of the bottom end of the spacer 54 and the inner edge of
the base section 15 of the stator S so that the groove pumping
section spacer 54 is not restrained directly by the casing. The
reason is as follows.
[0056] For those turbo-molecular pumps that have vane pumping
section L.sub.1 and the groove pumping section L.sub.2 made into an
integral unit, damage to the rotor R is most likely to occur at the
bottom end of the groove pumping section. Firstly, this is because
the top end of the spiral groove section 34 is constrained by the
vane pumping section L.sub.1, but the bottom end is not restrained,
therefore, the elastic deformation caused by the mass of the high
speed rotor R is greater towards the bottom side of the pump unit.
Secondly, the bottom section of the spiral groove section 34 is
subjected to a high pressure process gases used in semiconductor
device manufacturing, making this section susceptible to corrosion,
and consequently this section is vulnerable to cracks by stresses
resulting from elastic deformation.
[0057] When the groove pumping section spacer 54 is deformed
outward in a pump unit having its bottom end of the groove pumping
section spacer 54 fixed to or contacting the casing 14B, as shown
in FIG. 8, the contact section will resist the deformation and the
circumferential stress is transmitted directly to the casing. In
contrast, in this variation of the pump, there is a clearance
T.sub.3 provided between the bottom end of the groove pumping
section spacer 54 and the casing 14B, so that a small degree of
elastic deformation is not sufficient to make them contact, and the
spacer 54 can rotate while sliding by way of the friction reducing
sleeve 58b, thereby dissipating the rotational energy.
[0058] FIG. 10 shows a further variation of the pump shown in FIG.
8, and includes a fragile section 72 comprised by a notched
fracturing groove section 70 extending in the circumferential
direction provided at the boundary between the groove pumping
section spacer 54 and the flange section 54a for relieving the
stress by fracturing. This variation of the fourth embodiment
provides constriction release by breaking at the fragile section 72
along the fracturing groove section 70 when an abnormal torque
exceeding a threshold value is applied to the groove pumping
section spacer 54, leading the main section of the groove pumping
section spacer 54 to be separated from the flange section 54a. In
this condition, the groove pumping section spacer 54 rotates with
the rotor R along the low friction sleeve 58b to gradually
dissipate its rotational energy.
[0059] FIG. 11 shows a fifth embodiment of the pump comprised by a
split casing 14 having an intake-side casing 14A and an
exhaust-side casing 14B, and a ball bearing devices (friction
reducing structure) 80a, 80b, respectively, between the stator vane
spacers 50 and the intake-side casing 14A on the one hand, and
between the groove pumping section spacer 50 and the exhaust-side
casing 14B. These ball bearing devices 80a, 80b are comprised by
inner sleeves 82a, 82b and outer sleeves 84a, 84b with bearing
balls therebetween. The inner sleeves 82a, 82b are made thicker,
and therefore, stronger than the outer sleeves 84a, 84b.
[0060] Protective mechanism of this embodiment is as follows.
Because the inner sleeves 82a, 82b are made stronger than the outer
sleeves 84a, 84b, if abnormal conditions develop on the rotor
components of the rotor R or its debris impact upon the stator S to
apply high local stresses to the stator S, the inner sleeves 82a,
82b are able to withstand the stresses so that the ball bearing
device 80 can continue to operate relatively undisturbed. It should
be noted that the outer sleeves 84a, 84b are supported by the
casings 14A, 14B so that the deformation is small and their traces
of revolution will remain essentially intact even though they are
thinner.
[0061] It is permissible to use rollers in stead of balls in the
bearing device, and in this case also, the inner sleeves should be
made thicker than the outer sleeves to achieve the same effect as
above.
[0062] FIG. 12A shows a sixth embodiment which is an improvement in
the pump structure presented in FIG. 11. In this pump unit, the
groove pumping section L.sub.2 is provided with an impact absorbing
member (impact absorbing structure) 86 between the groove pumping
section spacer 54 and the ball bearing device 80b. Suitable
material for the impact absorbing member 86 are soft metals,
polymeric materials or their composite materials. By providing an
impact absorbing material between the stator S and pump casing 14,
stress transmission from the stator S to the casing 14 can be
prevented to avoid damaging the casing 14 or to the vacuum
processing system. By using both the friction reducing structure
such as ball bearing device 80b and the impact absorbing structure,
even greater advantages may be obtained.
[0063] FIG. 12B shows a composite structure of an impact absorbing
member 86 made of a tough material such as stainless steel, and an
impact absorbing member 90 made of a soft but high impact absorbing
material, thus providing both impact absorbing function and shape
retaining function.
[0064] It should be noted that, in the foregoing embodiments, the
application of damage prevention to turbo-molecular pump was
represented by those pumps having a vane pumping section L.sub.1
and groove pumping section L.sub.2. However, depending on the
nature of the processing facilities under consideration, the damage
prevention structure can be applied to those pumps having only the
vane pumping section L.sub.1 or only the groove pumping section
L.sub.2. For those wide-range pumps having both pumping sections
L.sub.1 and L.sub.2, it is understandable that the damage
prevention structure can be provided only on one of the two pumping
sections. It is equally understandable that a combination of any of
the embodied structures can be combined in any suitable combination
to either or both pumping sections L.sub.1 and L.sub.2.
* * * * *