U.S. patent application number 11/934279 was filed with the patent office on 2008-05-08 for electric pump.
Invention is credited to Toshiro Fujii.
Application Number | 20080107552 11/934279 |
Document ID | / |
Family ID | 39265179 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080107552 |
Kind Code |
A1 |
Fujii; Toshiro |
May 8, 2008 |
ELECTRIC PUMP
Abstract
A driving timing gear is located between an electric motor and a
driving rotor with respect to an axial direction of a driving
shaft. A first bearing is located on the same side of the driving
rotor as the driving timing gear. The driving shaft has a rotor
attaching end. The rotor attaching end is a free end that is
located on the same side of the first bearing as the driving rotor.
A weight is at a portion of the driving rotor that is close to the
driving timing gear with respect to the axial direction of the
driving shaft. Another weight is located at a portion of a driven
rotor that is closer to a driven timing gear.
Inventors: |
Fujii; Toshiro; (Kariya-shi,
JP) |
Correspondence
Address: |
KNOBLE, YOSHIDA & DUNLEAVY
EIGHT PENN CENTER
SUITE 1350, 1628 JOHN F KENNEDY BLVD
PHILADELPHIA
PA
19103
US
|
Family ID: |
39265179 |
Appl. No.: |
11/934279 |
Filed: |
November 2, 2007 |
Current U.S.
Class: |
417/423.12 |
Current CPC
Class: |
F04C 2/126 20130101;
F04C 2240/51 20130101; F04C 15/0042 20130101; F04C 11/008 20130101;
F04C 2240/20 20130101 |
Class at
Publication: |
417/423.12 |
International
Class: |
F04B 17/00 20060101
F04B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2006 |
JP |
2006-299056 |
Claims
1. An electric pump, comprising: an electric motor; a first rotary
shaft driven by the electric motor; a first rotor fixed to the
first rotary shaft; a first timing gear provided to the first
rotary shaft, the first timing gear being located between the
electric motor and the first rotor with respect to the axial
direction of the first rotary shaft; a first bearing supporting the
first rotary shaft, the first bearing being located on the same
side of the first rotor as the first timing gear, wherein the first
rotary shaft has a free end that is located on the same side of the
first bearing as the first rotor; a first weight provided to the
first rotor so as to rotate integrally with the first rotor,
wherein the material of the first weight has a specific gravity
greater than that of the material of the first rotor, and the first
weight is located at a portion of the first rotor that is close to
the first timing gear with respect to an axial direction of the
first rotary shaft; a second rotary shaft; a second rotor fixed to
the second rotary shaft; a second timing gear provided to the
second rotary shaft, the first timing gear and the second timing
gear causing the second rotary shaft to rotate synchronously with
the first rotary shaft, and the second timing gear being located
between the electric motor and the second rotor with respect to an
axial direction of the second rotary shaft; a second bearing
supporting the second rotary shaft, the second bearing being
located on the same side of the second rotor as the second timing
gear, wherein the second rotary shaft has a free end that is
located on the same side of the second bearing as the second rotor;
and a second weight provided to the second rotor so as to rotate
integrally with the second rotor, wherein the material of the
second weight has a specific gravity greater than that of the
material of the second rotor, and the second weight is located at a
portion of the second rotor that is close to the second timing gear
with respect to the axial direction of the second rotary shaft.
2. The electric pump according to claim 1, wherein the first rotor
and the second rotor are Roots type, each of the first rotor and
the second rotor having a plurality of lobes arranged at equal
angular intervals along a circumferential direction of the first
rotary shaft and along a circumferential direction of the second
rotary shaft, respectively, and a plurality of recesses each
located between an adjacent pair of the lobes, wherein the first
rotor has a first through hole extending along the axial direction,
the first weight being arranged in the first through hole, and
wherein the second rotor has a second through hole extending along
the axial direction, the second weight being arranged in the second
through hole.
3. The electric pump according to claim 2, wherein the first rotor
and the second rotor are each a two-lobe type, wherein the two
lobes of the first rotor are located on opposite sides of the first
rotary shaft, each lobe having the first through hole, and wherein
the two lobes of the second rotor are located on opposite sides of
the second rotary shaft, each lobe having the second through
hole.
4. The electric pump according to claim 1, wherein the first weight
is located on an end face of the first rotor that faces the first
timing gear, and wherein the second weight is located on an end
face of the second rotor that faces the second timing gear.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an electric pump in which a
bearing is located between a driving rotor and a driving timing
gear, and a free end of a driving shaft is located on a side
opposite to the driving timing gear with respect to the driving
rotor.
[0002] As shown in FIG. 5, a Roots pump 90 disclosed in Japanese
Laid-Open Patent Publication No. 2006-214380 includes a driving
shaft 91 and a driven shaft 96. The driving shaft 91 has a rotor
attaching end 91a to which a driving rotor 93 is attached. The
driven shaft 96 has a rotor attaching end 96a to which a driven
rotor 97 is attached. When rotation of an electric motor M is
transmitted to the driven timing gear 92b through a driving timing
gear 92a, the driven shaft 96 rotates in synchronization with the
driving shaft 91. As a result, the Roots pump 90 draws and
discharges fluid. The driving timing gear 92a is located between
the electric motor M and the driving rotor 93.
[0003] The Roots pump 90 has three bearings, or a first bearing 94,
a second bearing 95, and a third bearing 98. The first bearing 94
rotatably supports the driving shaft 91 between the driving timing
gear 92a and the driving rotor 93. The second bearing 95 rotatably
supports the driven shaft 96 between the driven timing gear 92b and
the driven rotor 97. The driving shaft 91 can be considered as a
cantilever that is supported by the first bearing 94 in a
cantilever manner, and the rotor attaching end 91a of the driving
shaft 91 is a free end. The driven shaft 96 can be considered as a
cantilever that is supported by the second bearing 95 in a
cantilever manner, and the rotor attaching end 96a of the driven
shaft 96 is a free end. The third bearing 98 rotatably supports a
motor attaching end 91b of the driving shaft 91.
[0004] The center of gravity G1 of the driving rotor 93 is located
at the center of the driving rotor 93 in the axial direction, that
is, at a center of thickness of the driving rotor 93. The center of
gravity G2 of the driven rotor 97 is located at the center of the
driven rotor 97 in the axial direction, that is, at a center of
thickness of the driven rotor 97.
[0005] The weight of the driving rotor 93 can bend the driving
shaft 91 in relation to the first bearing 94, and the weight of the
driven rotor 97 can bend the driven shaft 96 in relation to the
second bearing 95. During the operation of the Roots pump 90, the
driving shaft 91 and the driven shaft 96 undergo precession due to
the clearance within the first bearing 94 and the clearance within
the second bearing 95, which can produce vibration and noise. For
example, the driving rotor 93 and the driven rotor 97 contact each
other and produce noise.
SUMMARY OF THE INVENTION
[0006] Accordingly, it is an objective of the present invention to
provide an electric pump that reduces vibration of rotary
shafts.
[0007] In accordance with one aspect of the present invention, an
electric pump that includes an electric motor and a first rotary
shaft driven by the electric motor is provided. The electric pump
includes a first rotor fixed to the first rotary shaft, a first
timing gear provided to the first rotary shaft, and a first bearing
supporting the first rotary shaft. The first bearing is located on
the same side of the first rotor as the first timing gear. The
first rotary shaft has a free end that is located on the same side
of the first bearing as the first rotor. A first weight is provided
to the first rotor so as to rotate integrally with the first rotor.
The material of the first weight has a specific gravity greater
than that of the material of the first rotor. The first weight is
located at a position of the first rotor that is close to the first
timing gear with respect to the axial direction of the first rotary
shaft. The electric pump includes a second rotary shaft, a second
rotor fixed to the second rotary shaft, and a second timing gear
provided to the second rotary shaft. The first timing gear and the
second timing gear cause the second rotary shaft to rotate
synchronously with the first rotary shaft. The first timing gear is
located between the electric motor and the first rotor with respect
to the axial direction of the first rotary shaft. The second timing
gear is located between the electric motor and the second rotor
with respect to the axial direction of the second rotary shaft. A
second bearing supports the second rotary shaft. The second bearing
is located on the same side of the second rotor as the second
timing gear. The second rotary shaft has a free end that is located
on the same side of the second bearing as the second rotor. A
second weight is provided to the second rotor so as to rotate
integrally with the second rotor. The material of the second weight
has a specific gravity greater than that of the material of the
second rotor. The second weight is located at a position of the
second rotor that is close to the second timing gear with respect
to the axial direction of the second rotary shaft.
[0008] Other aspects and advantages of the invention will become
apparent from the following description, taken in conjunction with
the accompanying drawings, illustrating by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The features of the present invention that are believed to
be novel are set forth with particularity in the appended claims.
The invention, together with objects and advantages thereof, may
best be understood by reference to the following description of the
presently preferred embodiments together with the accompanying
drawings in which:
[0010] FIG. 1 is a cross-sectional view illustrating an electric
Roots pump according to one embodiment of the present
invention;
[0011] FIG. 2 is a cross-sectional view taken along line 2-2 in
FIG. 1;
[0012] FIG. 3 is a cross-sectional view taken along line 3-3 in
FIG. 1;
[0013] FIGS. 4A and 4B are partial cross-sectional views each
illustrating an electric Roots pump according to a modified
embodiment; and
[0014] FIG. 5 is a cross-sectional view illustrating a prior art
Roots pump.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] FIGS. 1 to 3 show one embodiment of the present invention.
FIG. 1 shows an electric pump according to the present embodiment,
which is a Roots pump 10. Arrow Y of FIG. 1 represents a direction
from the rear toward the front of the Roots pump 10.
[0016] As shown in FIG. 1, a pump housing 10a, which is the housing
of the Roots pump 10, includes a rotor housing member 11, a bearing
housing member 12, a gear housing member 13, and a motor housing
member 14 arranged in this order from the rear toward the front.
The bearing housing member 12 is secured to the front end of the
rotor housing member 11, the gear housing member 13 is secured to
the front end of the bearing housing member 12, and the motor
housing member 14 is secured to the front end of the gear housing
member 13.
[0017] The rotor housing member 11 defines a pump chamber 15 that
accommodates a first rotor, which is a driving rotor 22, and a
second rotor, which is a driven rotor 23, and the bearing housing
member 12 covers the opening of the pump chamber 15. The gear
housing member 13 defines a gear chamber 16 that accommodates a
first timing gear, which is a driving timing gear 28, and a second
timing gear, which is a driven timing gear 29. The bearing housing
member 12 lids the gear chamber 16. The motor housing member 14
defines a motor chamber 17 that accommodates an electric motor M.
The gear housing member 13 lids the motor chamber 17.
[0018] The pump housing 10a accommodates a first rotary shaft,
which is a driving shaft 20, and a second rotary shaft, which is a
driven shaft 21. The driving shaft 20 and the driven shaft 21
extend parallel to each other in a direction along arrow Y.
[0019] The Roots pump 10 has four bearings, that is, a first
bearing 31, a second bearing 32, a third bearing 33, and a fourth
bearing 34. The first bearing 31 and the third bearing 33 rotatably
support the driving shaft 20, and the second bearing 32 and the
fourth bearing 34 rotatably support the driven shaft 21. The
bearing housing member 12 has the first bearing 31 and the second
bearing 32, the motor housing member 14 has the third bearing 33,
and the gear housing member 13 has the fourth bearing 34.
[0020] The driving shaft 20 extends from the rear toward the front
through the driving rotor 22, a shaft sealing member 35a, the first
bearing 31, the driving timing gear 28, a shaft sealing member 35b,
the electric motor M, and the third bearing 33 in this order. The
driving rotor 22, the driving timing gear 28, and the rotor of the
electric motor M rotate integrally with the driving shaft 20.
[0021] The driven shaft 21 extends from the rear toward the front
through the driven rotor 23, a shaft sealing member 35c, the second
bearing 32, the driven timing gear 29, and the fourth bearing 34 in
this order. The driven rotor 23 and the driven timing gear 29
rotate integrally with the driven shaft 21.
[0022] As shown in FIGS. 2 and 3, the driving rotor 22 and the
driven rotor 23 are each a two-lobe type Roots rotor. A cross
section of each of the rotors 22, 23 perpendicular to the axial
direction is shaped like a gourd. The driving rotor 22 has a pair
of driving lobes 24 extending radially outward from the driving
shaft 20 in opposite directions. Also, two driving recesses 25 are
formed between the driving lobes 24. Likewise, the driven rotor 23
has a pair of driven lobes 26 extending radially outward from the
driven shaft 21 in opposite directions. Also, two driven recesses
27 are formed between the driven lobes 26. That is, a pair of the
driving lobes 24 are arranged in the circumferential direction at
an equal interval, and the driven lobes 26 are also arranged in
circumferential direction at an equal interval.
[0023] The outer surface of the driving rotor 22, the outer surface
of the driven rotor 23, and the inner surface of the rotor housing
member 11 define the pump chamber 15. As shown in FIG. 3, the rotor
housing member 11 has a suction port 18 for drawing fluid into the
pump chamber 15, and a discharge port 19 for discharging fluid from
the pump chamber 15.
[0024] The driving timing gear 28 and the driven timing gear 29
form pair of timing gears meshed with each other. When the electric
motor M rotates the driving shaft 20, the rotation of the driving
shaft 20 is transmitted from the driving timing gear 28 to the
driven timing gear 29, so that the driven shaft 21 rotates
synchronously with the driving shaft 20. As a result, the driving
rotor 22 and the driven rotor 23 rotate in opposite directions, so
that fluid is drawn into the pump chamber 15 through the suction
port 18, and is discharged to the outside through the discharge
port 19.
[0025] The relative positions of the components will now be
described.
[0026] The rear end of the driving shaft 20 is referred to as a
rotor attaching end 20a, to which the driving rotor 22 is attached.
The front end of the driving shaft 20 is rotatably supported by the
third bearing 33. The rear end of the driven shaft 21 is referred
to as a rotor attaching end 21a, to which the driven rotor 23 is
attached. The front end of the driven shaft 21 is rotatably
supported by the fourth bearing 34.
[0027] The driving timing gear 28 is located between the electric
motor M and the driving rotor 22 with respect to the axial
direction of the driving shaft 20. The driven timing gear 29 is
located between the electric motor M and the driven rotor 23 with
respect to the axial direction of the driven shaft 21. The first
bearing 31 is located between the driving rotor 22 and the driving
timing gear 28, and the second bearing 32 is located between the
driven rotor 23 and the driven timing gear 29. That is, the driving
shaft 20 can be considered as a cantilever that is supported by the
first bearing 31 in a cantilever manner, and the rotor attaching
end 20a is a free end of the driving shaft 20. Likewise, the driven
shaft 21 can be considered as a cantilever that is supported by the
second bearing 32 in a cantilever manner, and the rotor attaching
end 21a is a free end of the driven shaft 21. The weight of the
driving rotor 22 can be considered as load acting on the driving
shaft 20, which is a cantilever. Similarly, the weight of the
driven rotor 23 can be considered as load acting on the driven
shaft 21, which is a cantilever.
[0028] In other words, the first bearing 31 supports the driving
shaft 20 at a side corresponding to the driving timing gear 28 with
respect to the driving rotor 22, thus the driving shaft 20 is
supported by the first bearing 31 in a cantilever manner. Further,
the second bearing 32 supports the driven shaft 21 at a side
corresponding to the driven timing gear 29 with respect to the
driven rotor 23, thus the driven shaft 21 is supported by the
second bearing 32 in a cantilever manner.
[0029] The driving rotor 22 has a bearing facing surface 22a that
faces the first bearing 31 and a free end surface 22b opposite to
the bearing facing surface 22a. The free end surface 22b is located
on the same plane as the end face of the rotor attaching end 20a
and faces the rear wall of the rotor housing 11. Likewise, the
driven rotor 23 has a bearing facing surface 23a that faces the
second bearing 32 and a free end surface 23b opposite to the
bearing facing surface 23a. The free end surface 23b is located on
the same plane as the end face of the rotor attaching end 21a and
faces the rear wall of the rotor housing 11.
[0030] The driving rotor 22 and the driven rotor 23 will now be
described.
[0031] As shown in FIGS. 1 to 3, the driving rotor 22 has a pair of
first through holes, which are through holes 50 each axially
extending through one of the driving lobes 24. A pair of the
through holes 50, which are each formed as a circular hole, are
located at opposite positions to each other with respect to the
driving shaft 20. Each through hole 50 is located in a radial
center of the corresponding driving lobe 24. The center of gravity
G1 of the driving rotor 22 is located at the center of the driving
rotor 22 in the axial direction.
[0032] A disc-shaped first weight, which is a weight 60, is press
fitted into each through hole 50. As shown in FIGS. 1 and 2, each
weight 60 is located between the center of gravity G1 of the
driving rotor 22 and a bearing facing surface 22a. In other words,
each weight 60 is located on a side of the center of gravity G1 of
the driving rotor 22 that corresponds to the first bearing 31, that
is, on a side that corresponds to the driving timing gear 28. That
is, the weight 60, which serves as a first weight, is located in a
portion of the driving rotor 22 that is close to the driving timing
gear 28 with respect to the axial direction. As a result, the
center of gravity J1 of the combined unit of the driving rotor 22
and the weight 60 is shifted toward the first bearing 31 from the
center of gravity G1 of the single driving rotor 22. As shown in
FIGS. 1 and 3, the part of the through hole 50 that is rearward of
the weight 60, for example, remains hollow.
[0033] To reliably suppress vibration of the driving shaft 20, the
axial measurement and the mass of the weight 60 are adjusted to
adjust the position of the center of gravity J1.
[0034] Likewise, the driven rotor 23 has a pair of second through
holes, which are through holes 51 each axially extending through
one of the driven lobes 26. A pair of the through holes 51, which
are each formed as a circular hole, are located at opposite
positions to each other with respect to the driven shaft 21. Each
through hole 51 is located in a radial center of the corresponding
driven lobe 26. The center of gravity G2 of the driven rotor 23 is
located at the center of the single driven rotor 23 in the axial
direction.
[0035] A disc-shaped second weight, which is a weight 60, is press
fitted into each through hole 51. Each weight 60 is located between
the center of gravity G2 of the driven rotor 23 and a bearing
facing surface 23a. In other words, each weight 60 is located on a
side of the center of gravity G2 of the driven rotor 23 that
corresponds to the second bearing 32, that is, on a side that
corresponds to the driven timing gear 29. That is, the weight 60,
which serves as a second weight, is located in a portion of the
driven rotor 23 that is close to the driven timing gear 29 with
respect to the axial direction. As a result, the center of gravity
J2 of the combined unit of the driven rotor 23 and the weight 60 is
shifted toward the second bearing 32 from the center of gravity G2
of the single driven rotor 23.
[0036] To reliably suppress vibration of the driven shaft 21, the
axial measurement and the mass of the weight 60 are adjusted to
adjust the position of the center of gravity J2.
[0037] For example, the weight 60 is made of an iron based
material, and the driving rotor 22 and the driven rotor 23 are made
of an aluminum based material such that the specific gravity of the
weight 60 is greater than those of the driving rotor 22 and the
driven rotor 23.
[0038] The preferred embodiment has the following advantages.
[0039] (1) The weights 60 are attached to portions of the driving
rotor 22 that are close to the driving timing gear 28. Therefore,
the center of gravity J1 of the combined unit of the driving rotor
22 and the weight 60 is shifted toward the first bearing 31 from
the center of gravity G1 of the single driving rotor 22. As a
result, precession of the driving rotor 22 is suppressed, and
vibration and noise of the driving rotor 22 are suppressed
accordingly. Particularly, vibration and noise during high speed
operation of the driving rotor 22 are suppressed.
[0040] Likewise, the weights 60 are attached to portions of the
driven rotor 23 that are close to the driven timing gear 29.
Therefore, the center of gravity J2 of the combined unit of the
driven rotor 23 and the weight 60 is shifted toward the second
bearing 32 from the center of gravity G2 of the single driven rotor
23. As a result, precession of the driven rotor 23 is suppressed,
and vibration and noise of the driven rotor 23 are suppressed
accordingly. Particularly, vibration and noise during high speed
operation of the driven rotor 23 are suppressed.
[0041] (2) Since vibration of the driving shaft 20 and the driven
shaft 21 are suppressed, the power loss of the electric motor M is
reduced. Thus, the performance of the Roots pump 10 at high speeds
is improved, and the size and the weigh of the electric motor M and
the Roots pump 10 can be reduced.
[0042] (3) The driving rotor 22 and the driven rotor 23 each have
the through holes 50, 51 for accommodating the weights 60. That is,
portions of the driving rotor 22 and the driven rotor 23 are
removed so that the weights are reduced. This allows the size of
the electric motor M to be reduced. The size and the weight of the
Roots pump 10 are therefore further reduced.
[0043] (4) The weights 60, which are symmetrical with respect to
the driving shaft 20, are provided in a pair of the driving lobes
24, and the weights 60, which are symmetrical with respect to the
driven shaft 21, are provided in a pair of the driven lobes 26.
This, for example, prevents the weights 60 from decentering the
driving rotor 22 and the driven rotor 23. Thus, accurate
synchronizing timing of the driving rotor 22 and the driven rotor
23 is easily maintained.
[0044] (5) The rotor attaching end 20a of the driving shaft 20 and
the rotor attaching end 21a of the driven shaft 21 are free ends.
Thus, the Roots pump 10 has a reduced number of bearings for
rotatably supporting the driving shaft 20 and the driven shaft 21.
Accordingly, the number of shaft sealing members for sealing the
driving shaft 20 and the driven shaft 21 is reduced. Therefore, the
power loss due to shaft sealing members is reduced.
[0045] (6) The greater the distance between the center of gravity
G1 of the driving rotor 22 and the first bearing 31, and the
greater the distance between the center of gravity G2 of the driven
rotor 23 from the second bearing 32, the more intense the
precession of the driving shaft 20 and the driven shaft 21 is
likely to become. In the present embodiment, the weights 60 cause
the centers of gravity J1, J2 to be shifted toward the first
bearing 31 and the second bearing 32 with respect to the centers of
gravity G1, G2. The present embodiment is therefore suitable for a
Roots pump 10 having elongated driving rotor 22 and driven rotor
23.
[0046] (7) Vibration and noise of the driving shaft 20 are
suppressed by simply forming the through holes 50 in the driving
rotor 22, and press fitting the weights 60 in the through holes 50.
Also, vibration and noise of the driven shaft 21 are suppressed by
simply forming the through holes 51 in the driven rotor 23, and
press fitting the weights 60 in the through holes 51. Further,
vibration and noise of the driving shaft 20 and the driven shaft 21
are suppressed by a simple operation by adjusting the mass of the
weights 60.
[0047] The above-mentioned embodiment may be modified as
follows.
[0048] As shown in FIG. 4A, a plate-like weight 61 may be bonded to
the bearing facing surface 22a of the driving rotor 22, and
likewise, another thin plate-like weight 61 may be bonded to the
bearing facing surface 23a of the driven rotor 23. These weights 61
may be each sized to expand over the entirety of the corresponding
one of the bearing facing surfaces 22a, 23a. In this case, compared
to the case in which the weights 60 are press fitted into the
through hole 50, 51, the mass of each weight 61 is adjusted more
freely. Also, since the through holes 50, 51 can be omitted from
the driving rotor 22 and the driven rotor 23, the driving rotor 22
and the driven rotor 23 do not need to be subject to post-machining
for forming the through holes 50, 51. The production efficiency of
the driving rotor 22 and the driven rotor 23 is thus easily
improved.
[0049] The configuration shown in FIG. 4B may be employed. In this
configuration, the through holes 50 are omitted, and attachment
recesses 52 are formed on the bearing facing surface 22a of the
driving rotor 22. Disc-shaped weights 62 are press fitted into or
adhered to the attachment recesses 52. Likewise, the through holes
51 may be omitted, and attachment recesses 52 may be formed on the
bearing facing surface 23a of the driven rotor 23, and disc-shaped
weights 62 may be press fitted into or adhered to the attachment
recesses 52. The attachment recesses 52 are formed simultaneously
with the driving rotor 22 and the driven rotor 23 when molding the
rotors 22, 23. As in the above described modification, the driving
rotor 22 and the driven rotor 23 do not need to be subject to
post-machining for forming the through holes 50, 51. The production
efficiency of the driving rotor 22 and the driven rotor 23 is thus
easily improved.
[0050] The driving rotor 22 and the driven rotor 23 may be formed
of resin. In this case, the weights 60 are made of a metal material
having a specific gravity greater than that of the resin. For
example, the weights 60 are made of an iron-based material.
[0051] The weights 60 may be embedded in the driving rotor 22 and
the driven rotor 23 through casting.
[0052] As long as the driving rotor 22 and the driven rotor 23 are
Roots rotors each having two or more lobes, the rotors 22, 23 may
each have three lobes. When the driving rotor 22 and the driven
rotor 23 each have an odd number of lobes, weights 60 are
preferably provided in all of the driving lobes 24 and the driven
lobes 26.
[0053] The driving shaft 20 may have a plurality of driving rotors
22, and the driven shaft 21 may have a plurality of driven rotors
23. That is, the present invention is not limited to a single-stage
Roots pump section, but may be embodied in a multi-stage Roots
pump.
[0054] The driving rotor 22 and the driven rotor 23 do not need to
be Roots rotors, but may be screw rotors. That is, the electric
pump may be an electric screw pump.
* * * * *