Vacuum pump

Details Vacuum pump Vacuum pump

1999-10-26

2001-11-13

Verdier, Christopher (Department: 3745)

Rotary kinetic fluid motors or pumps

Runner has spirally arranged blade or fluid passage

Extending along runner axis

C415S073000, C415S075000, C415S090000

Reexamination Certificate

active

06315517

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vacuum pump for evacuating a vacuum chamber, and more particularly to an improvement of the vacuum pump in which a rotor arranged inside has a threaded portion on the surface thereof.
2. Description of the Related Art
Vacuum pumps are used, for example, as vacuum producing apparatus for exhausting gas within a chamber of semiconductor manufacturing equipment, so at to discharge a process gas supplied to the chamber for processing the semiconductor devices therein.
FIG. 5
shows the overall structure of the vacuum pump. In
FIG. 5
, reference numeral
101
denotes a casing in which a gas inlet port
102
and a gas outlet port
103
are formed. A rotor
104
is housed in the casing
101
. Formed on this rotor
104
are a plurality of rotor blades
105
projecting outwardly in a radial direction toward the inner circumferential wall of the casing
101
, and a threaded portion
108
disposed below the rotor blades
105
and having spiral thread grooves formed therein.
A plurality of stator blades
106
and a stator
109
are attached to the inner circumferential wall of the casing
101
facing the rotor blades
105
and the threaded portion
108
, respectively. The rotor
104
is rotated by a motor
107
housed in the casing
101
, which causes the rotor blades
105
and the threaded portion
108
to rotate at a high-speed relative to the stator blades
106
and the stator
109
, respectively.
A stator blade
106
and a stator
109
are attached onto the inner circumferential wall of the casing
101
while facing with the rotor blade
105
and the threaded portion
108
, respectively. The rotor
104
is rotated by a motor
107
housed in the casing
101
, which causes the rotor blade
105
and the threaded portion
108
to rotate at a high-speed relative to the stator blade
106
and the stator
109
, respectively.
The rotor
104
is fixedly provided with a rotor shaft
112
and is rotatably floated by magnetic force produced by an axial electromagnet
113
and a radial electromagnet
114
. Further, touch down bearings
115
and
116
are provided in an outer member of the rotor shaft
112
so as to come in contact with the rotor shaft
112
and to rotatably support the same in the case where the rotor shaft
112
is floated, but is not supported through magnetic force by the electromagnets
113
and
114
.
However, a conventional vacuum pump constructed as described above has structural defects. As shown in
FIG. 6
, a terminal end face, which is located on the downstream gas suction side (lower end in the drawing), of a thread
108
a
in the threaded portion
108
is formed so as to be identical with the end face of the rotor
104
on the downstream gas suction side (lower end in the drawing). A thread groove
108
b
is formed axially between two adjacent lines of thread
108
a
, which is formed by machining with an edge tool to have a sharply gouged bottom corner. Such structure causes the centrifugal force upon rotation of the rotor
104
to tend to concentrate stress on the bottom corner of the thread groove
108
b.
In particular, a bottom corner C (see
FIG. 7
) of the thread groove
108
b
, located at a terminal B of the thread
108
a
in
FIG. 6
, is at a location at which the edge tool is pulled out upon completing the machining. Accordingly, a notch is liable to be produced due to imbalance in machining resistance. For this reason, the bottom corner C is liable to start a crack to eventually damage the rotor
104
with the centrifugal force upon rotation.
To solve the above-mentioned problem, there is known a vacuum pump having a rotor in which the spiral thread is provided on the surface of the rotor so as to project with a thread groove that is axially formed between two adjacent lines of the thread. In this vacuum pump, the position of the terminal end face of the thread on the downstream gas suction side is shifted so that it becomes shorter than the end face of the rotor on the downstream gas suction side, and a recessed R portion is formed at the root of the terminal end face of the thread on the downstream gas suction side.
FIG. 8
is a side view of the end portion of the rotor. In
FIG. 8
, reference numeral
4
denotes the rotor of the vacuum pump. A spiral thread
4
a
is formed projectingly on the surface of this rotor
4
so that a thread groove
4
b
is formed axially between two adjacent lines of the thread
4
a
. At the lower end portion of the rotor
4
in the drawing (the end portion on the downstream gas suction side), the position of a terminal face
40
of the thread
4
a
on the downstream gas suction side is shifted so as to be shorter by a length H than the position of an end face
41
of the rotor
4
on the downstream gas suction side.
For this reason, at a bottom corner of the thread groove
4
b
at the end portion of the downstream gas suction side of the thread
4
a
, even if a notch is produced when an edge tool is pulled out upon completing the machining, due to imbalance in machining resistance, if, thereafter, the end portion of the thread
4
a
on the downstream gas suction side is scraped by H so that the downstream gas suction side terminal face
40
of the thread
4
a
is shifted to position to reach short of the downstream gas suction side end face
41
of the rotor
4
, it is capable of scraping off the notch, too, caused by the imbalance of machining resistance, thereby being capable of, unlike conventional pumps, preventing formation of a crack developed from the notch, which may cause damage to the rotor with the centrifugal force upon rotation.
In the vacuum pump with such a structure, the downstream gas suction side terminal face of the thread is shifted so as to reach short of the end face of the rotor on the downstream gas suction side. Therefore, even if a notch is produced at the bottom corner of the thread groove at the downstream gas suction side terminal face of the thread by an edge tool pulled out upon completing the machining, due to imbalance in machining resistance, if, thereafter, the end portion of the thread on the downstream gas suction side is scraped a little so that the downstream gas suction side terminal face of the thread is shifted to reach short of the downstream gas suction side end face of the rotor, it is capable of scraping off the notch, too, caused by the imbalance of machining resistance, and further, by finishing the root of the downstream gas suction side terminal of the thread into a shape of a recess R, it is capable of preventing the concentration of stress on the root, thereby being capable of preventing damage to the rotor due to a crack developed from the notch by the centrifugal force upon rotation.
However, even this device, having the thread near the rotor end, is not free from a problem of concentration of bending stress at the thread terminal, which is stress concentration on the thread root caused by a difference in thickness between the portions with thread and without thread.
SUMMARY OF THE INVENTION
The present invention has been made to solve such problems.
To attain the above object, according to an object of the present invention, there is provided a vacuum pump having a rotor with a spiral thread projecting on its surface, wherein an axial length of the thread is shorter than that of the rotor and the terminal portion of the thread slopes down or inward toward the surface of the thread groove.
The junction face between the terminal portion of the slope of the thread and the surface of the thread groove forms a recessed R portion.
The vacuum pump with the structure described above can abate stress concentration on the thread root caused by a difference in thickness between the portions with and without the thread, especially at the thread terminal portion on which bending stress is concentrated, and can prevent the damage to the rotor.


REFERENCES:
patent: 2957423 (1960-10-01), Audemar
patent: 3667276 (1972-06-01), Moodey
patent: 3884451 (1975-05-01), Stenmark et al.
patent: 3913897 (1975

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