U.S. patent application number 11/934290 was filed with the patent office on 2008-05-08 for eletric pump.
Invention is credited to Toshiro Fujii.
Application Number | 20080107550 11/934290 |
Document ID | / |
Family ID | 39265178 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080107550 |
Kind Code |
A1 |
Fujii; Toshiro |
May 8, 2008 |
ELETRIC PUMP
Abstract
A Roots pump has a driving shaft press fitted into a driving
rotor, and a driven shaft press fitted into a driven shaft. A
driving timing gear is located between an electric motor and the
driving rotor. The driving rotor has a driving assist shaft that
projects in a direction opposite to the driving timing gear. The
specific gravity of the material of the driving assist shaft is
less than the specific gravity of the material of the driving
rotary shaft. A driven rotor has a driven assist shaft that
projects in a direction opposite to a driven timing gear. The
specific gravity of the material of the driven assist shaft is less
than the specific gravity of the material of the driven rotary
shaft.
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: |
39265178 |
Appl. No.: |
11/934290 |
Filed: |
November 2, 2007 |
Current U.S.
Class: |
417/423.1 |
Current CPC
Class: |
F04C 2240/60 20130101;
F04C 18/126 20130101; F04C 29/0078 20130101; F01C 17/02 20130101;
F04C 23/00 20130101; F04C 2240/52 20130101 |
Class at
Publication: |
417/423.1 |
International
Class: |
F04B 17/00 20060101
F04B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2006 |
JP |
2006-299057 |
Claims
1. An electric pump, comprising: an electric motor; a first rotary
shaft driven by the electric motor; a first rotor that is coupled
to and rotates integrally with the first rotary shaft, the first
rotor being formed of a material the specific gravity of which is
less than the specific gravity of the material of 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 an axial direction of the first rotary
shaft, wherein the first rotor has a first gear facing surface, a
first opposite surface, and a first assist shaft, the first gear
facing surface being an end face that faces the first timing gear
with respect to the axial direction, the first opposite surface
being an end face that is opposite to the first gear facing
surface, and the first assist shaft projecting from the first
opposite surface, wherein the first gear facing surface has a first
recess, wherein the first rotary shaft is press fitted into the
first recess so that the first rotor is coupled to the first rotary
shaft, wherein the first assist shaft is arranged to be coaxial
with the first rotary shaft, and wherein the specific gravity of
the material of the first assist shaft is less than the specific
gravity of the material of the first rotary shaft; a first bearing
rotatably supporting the first assist shaft; a second rotary shaft;
a second rotor that is coupled to and rotates integrally with the
second rotary shaft, the second rotor being formed of a material
the specific gravity of which is less than the specific gravity of
the material of the second rotary shaft; a second timing gear
provided to the second rotary shaft, wherein the first timing gear
and the second timing gear cause the second rotary shaft to rotate
synchronously with the first rotary shaft, 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, wherein
the second rotor has a second gear facing surface, a second
opposite surface, and a second assist shaft, the second gear facing
surface being an end face that faces the second timing gear with
respect to the axial direction, the second opposite surface being
an end face that is opposite to the second gear facing surface, and
the second assist shaft projecting from the second opposite
surface, wherein the second gear facing surface has a second
recess, wherein the second rotary shaft is press fitted into the
second recess so that the second rotor is coupled to the second
rotary shaft, wherein the second assist shaft is arranged to be
coaxial with the second rotary shaft, and wherein the specific
gravity of the material of the second assist shaft is less than the
specific gravity of the material of the second rotary shaft; and a
second bearing rotatably supporting the second assist shaft.
2. The electric pump according to claim 1, wherein the axial
dimension of the first recess is less than half the axial dimension
of the first rotor, and wherein the axial dimension of the second
recess is less than half the axial dimension of the second
rotor.
3. The electric pump according to claim 1, wherein the first assist
shaft is made of a material that is the same as the material of the
first rotor, and wherein the second assist shaft is made of a
material that is the same as the material of the second rotor.
4. The electric pump according to claim 3, wherein the first assist
shaft is integrally molded with the first rotor, and wherein the
second assist shaft is integrally molded with the second rotor.
5. The electric pump according to claim 1, wherein the length of a
portion of the first rotary shaft that is press fitted into the
first recess is set such that the value obtained by multiplying
transmission torque transmitted from the first rotary shaft to the
first rotor by a safety factor is equal to a fastening force
between the first recess and the first rotary shaft, wherein the
length of a portion of the second rotary shaft that is press fitted
into the second recess is set such that the value obtained by
multiplying transmission torque transmitted from the second rotary
shaft to the second rotor by the safety factor is equal to a
fastening force between the second recess and the second rotary
shaft.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an electric pump having
timing gears between an electric motor and rotors.
[0002] Japanese Laid-Open Patent Publication No. 2002-54587
discloses a screw pump that includes a driving shaft and a driven
shaft. The driving shaft extends through and supports a driving
rotor, and the driven shaft extends through and supports a driven
rotor. When an electric motor rotates the driving shaft, meshing of
the driving timing gear and the driven timing gear makes the driven
shaft rotate synchronously with the driving shaft. The driving
timing gear is located between the electric motor and the driving
rotor.
[0003] To reduce the weight of the screw pump disclosed in the
document, the axial dimension of the driving shaft and driven shaft
may be reduced. However, if the axial dimension of the driving
shaft is reduced while maintaining the structure in which the
driving shaft extends through the driving rotor, the axial
dimension of the driving rotor needs to be reduced. This reduces
the amount of fluid that is transferred by the driving rotor, which
lowers the performance of the screw pump. Likewise, if the axial
dimension of the driven shaft is reduced while maintaining the
structure in which the driven shaft extends through the driven
rotor, the axial dimension of the driven rotor needs to be
reduced.
SUMMARY OF THE INVENTION
[0004] Accordingly, it is an objective of the present invention to
provide an electric pump that is capable of reducing the weight
without changing the size and shape.
[0005] According to one aspect of the invention, an electric pump
including an electric motor and a first rotary shaft driven by the
electric motor is provided. A first rotor is coupled to and rotates
integrally with the first rotary shaft. The first rotor is formed
of a material the specific gravity of which is less than the
specific gravity of the material of the first rotary shaft. A first
timing gear is provided to the first rotary shaft. The first timing
gear is located between the electric motor and the first rotor with
respect to an axial direction of the first rotary shaft. The first
rotor has a first gear facing surface, a first opposite surface,
and a first assist shaft. The first gear facing surface is an end
face that faces the first timing gear with respect to the axial
direction. The first opposite surface is an end face that is
opposite to the first gear facing surface. The first assist shaft
projects from the first opposite surface. The first gear facing
surface has a first recess. The first rotary shaft is press fitted
into the first recess so that the first rotor is coupled to the
first rotary shaft. The first assist shaft is arranged to be
coaxial with the first rotary shaft. The specific gravity of the
material of the first assist shaft is less than the specific
gravity of the material of the first rotary shaft. A first bearing
rotatably supports the first assist shaft. The electric pump
includes a second rotary shaft and a second rotor that is coupled
to and rotates integrally with the second rotary shaft. The second
rotor is formed of a material the specific gravity of which is less
than the specific gravity of the material of the second rotary
shaft. A second timing gear is 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 second timing gear is located between the electric motor and
the second rotor with respect to an axial direction of the second
rotary shaft. The second rotor has a second gear facing surface, a
second opposite surface, and a second assist shaft. The second gear
facing surface is an end face that faces the second timing gear
with respect to the axial direction. The second opposite surface is
an end face that is opposite to the second gear facing surface. The
second assist shaft projects from the second opposite surface. The
second gear facing surface has a second recess. The second rotary
shaft is press fitted into the second recess so that the second
rotor is coupled to the second rotary shaft. The second assist
shaft is arranged to be coaxial with the second rotary shaft. The
specific gravity of the material of the second assist shaft is less
than the specific gravity of the material of the second rotary
shaft. A second bearing rotatably supports the second assist
shaft.
[0006] 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
[0007] 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:
[0008] FIG. 1 is a cross-sectional view illustrating an electric
Roots pump according to one embodiment of the present
invention;
[0009] FIG. 2 is a cross-sectional view taken along line 2-2 in
FIG. 1; and
[0010] FIG. 3 is a cross-sectional view taken along line 3-3 in
FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] 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.
[0012] 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, 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, and the
motor housing member 14 is secured to the front end of the bearing
housing member 12.
[0013] 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 motor
housing member 14 defines a motor chamber 17 that accommodates an
electric motor M, and 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 gear chamber 16 is
located at the opening of the motor housing member 14, and the
bearing housing member 12 lids the gear chamber 16. The driving
rotor 22 and the driven rotor 23 are each made of aluminum.
[0014] 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. The
driving shaft 20 and the driven shaft 21 are each made of an
iron-based material. That is, the specific gravity of the material
of the driving rotor 22 and the driven rotor 23 is less than the
specific gravity of the material of the driving shaft 20 and the
driven shaft 21.
[0015] The Roots pump 10 has five bearings, or a first bearing 31,
a second bearing 32, a third bearing 33, a fourth bearing 34, and a
fifth bearing 35. The third bearing 33 and the fifth bearing 35
rotatably support the driving shaft 20, and the fourth bearing 34
rotatably supports the driven shaft 21. The rotor housing member 11
has the first bearing 31 and the second bearing 32, the bearing
housing member 12 has the third bearing 33 and the fourth bearing
34, and the motor housing member 14 has the fifth bearing 35.
[0016] The driving shaft 20 extends from the rear toward the front
with connecting the driving rotor 22, the third bearing 33, the
driving timing gear 28, the electric motor M, and the fifth bearing
35 in this order. That is, the driving timing gear 28 is located
between the driving rotor 22 and the electric motor M with respect
to an axial direction of the driving shaft 20. The driving shaft 20
has a front end 20a supported by the fifth bearing 35 and a rear
end 20b coupled to the driving rotor 22. The driving rotor 22, the
driving timing gear 28, and the rotor of the electric motor M
rotate integrally with the driving shaft 20.
[0017] The driven shaft 21 extends from the rear toward the front
with connecting the driven rotor 23, the fourth bearing 34, and the
driven timing gear 29 in this order. That is, the driven timing
gear 29 is located between the driven rotor 23 and the electric
motor M with respect to an axial direction of the driven shaft 21.
The driven shaft 21 has a front end 21a coupled to the driven
timing gear 29 and a rear end 21b coupled to the driven rotor 23.
The driven rotor 23 and the driven timing gear 29 rotate integrally
with the driven shaft 21.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] As shown in FIG. 1, the driving rotor 22 has a driving gear
facing surface 22a, which is an end face facing the driving timing
gear 28 with respect to the axial direction of the driving shaft
20, and a driving opposite surface 22b, which is an end face
opposite to the driving gear facing surface 22a. Likewise, the
driven rotor 23 has a driven gear facing surface 23a, which is an
end face facing the driven timing gear 29 with respect to the axial
direction of the driven shaft 21, and a driven opposite surface
23b, which is an end face opposite to the driven gear facing
surface 23a. The driving gear facing surface 22a, which functions
as a first gear facing surface, is a front end face of the driving
rotor 22, and the driving opposite surface 22b, which functions as
a first opposite surface, is a rear end face of the driving rotor
22. Likewise, the driven gear facing surface 23a, which functions
as a second gear facing surface, is a front end face of the driven
rotor 23, and the driven opposite surface 23b, which functions as a
second opposite surface, is a rear end face of the driven rotor
23.
[0022] A center portion of the driving gear facing surface 22a has
a driving recess 41, which functions as a first recess. The driving
recess 41 has a circular cross section, and the axis M1 of the
driving recess 41 coincides with the axis L1 of the driving shaft
20. The rear end 20b of the driving shaft 20 is press fitted into
the driving recess 41, so that the driving rotor 22 is coupled to
and rotates integrally with the driving shaft 20. That is, the
driving shaft 20 does not extend through the driving rotor 22.
Torque of the driving shaft 20 is transmitted from the
circumferential surface of the driving shaft 20 to the
circumferential surface of the driving recess 41.
[0023] In the present embodiment, the driving shaft 20 is inserted
to the bottom of the driving recess 41. The depth, or the axial
dimension, of the driving recess 41 is less than half the axial
dimension (thickness) of the driving rotor 22. That is, the length
of a portion of the driving shaft 20 connected to the driving rotor
22 is less than the axial dimension of the driving rotor 22. The
axial dimension of a press fitted portion, which is a portion of
the driving shaft 20 that is press fitted into the driving recess
41, is set to a value that allows transmission torque to be
transmitted from the driving shaft 20 to the driving rotor 22,
while ensuring that the press fitting strength is equal to or less
than the strength of the driving shaft 20. Specifically, the length
of the portion of the driving shaft 20 that is press fitted into
the driving recess 41 is set such that the value obtained by
multiplying the transmission torque transmitted from the driving
shaft 20 to the driving rotor 22 by a safety factor is equal to the
fastening force between the driving recess 41 and the driving shaft
20. That is, the depth of the driving recess 41 is set to a value
that allows transmission torque to be transmitted from the driving
shaft 20 to the driving rotor 22, and prevents the driving shaft 20
from slipping relative to the driving rotor 22.
[0024] Likewise, a center portion of the driven gear facing surface
23a has a driven recess 42, which functions as a second recess. The
driven recess 42 has a circular cross section, and the axis M2 of
the driven recess 42 coincides with the axis L2 of the driven shaft
21. The rear end 21b of the driven shaft 21 is press fitted into
the driven recess 42, so that the driven rotor 23 is coupled to and
rotates integrally with the driven shaft 21. That is, the driven
shaft 21 does not extend through the driven rotor 23. Torque of the
driven shaft 21 is transmitted from the circumferential surface of
the driven shaft 21 to the circumferential surface of the driven
recess 42.
[0025] In the present embodiment, the driven shaft 21 is inserted
to the bottom of the driven recess 42. The depth, or the axial
dimension, of the driven recess 42 is less than half the axial
dimension (thickness) of the driven rotor 23. That is, the length
of a portion of the driven shaft 21 connected to the driven rotor
23 is less than the axial dimension of the driven rotor 23. The
axial dimension of a press fitted portion, which is portion of the
driven shaft 21 that is press fitted into the driven recess 42, is
set to a value that allows transmission torque to be transmitted
from the driven shaft 21 to the driven rotor 23, while ensuring
that the press fitting strength is equal to or less than the
strength of the driven shaft 21. Specifically, the length of the
portion of the driven shaft 21 that is press fitted into the driven
recess 42 is set such that the value obtained by multiplying the
transmission torque transmitted from the driven shaft 21 to the
driven rotor 23 by a safety factor is equal to the fastening force
between the driven recess 42 and the driven shaft 21. That is, the
depth of the driven recess 42 is set to a value that allows
transmission torque to be transmitted from the driven shaft 21 to
the driven rotor 23, and prevents the driven shaft 21 from slipping
relative to the driven rotor 23.
[0026] The driving rotor 22 has a columnar driving assist shaft 37,
which functions as a first assist shaft projecting from a center
portion of the driving opposite surface 22b in the axial direction.
The axis N1 of the driving assist shaft 37 is set to coincide with
the axis L1 of the driving shaft 20 and the axis M1 of the driving
recess 41. That is, the driving assist shaft 37, functioning as a
first shaft portion, is coaxial with the driving shaft 20. The
driving assist shaft 37 and the driving rotor 22 are formed
integrally by molding. That is, the driving assist shaft 37 is also
made of aluminum. The first bearing 31 receives the radial load of
the driving rotor 22 and a small amount of transmission torque by
rotatably supporting the driving assist shaft 37 to the rotor
housing 11. That is, the driving rotor 22 is supported at both
axial ends by the first bearing 31 and the third bearing 33.
[0027] Likewise, the driven rotor 23 has a columnar driven assist
shaft 38, which functions as a second assist shaft projecting from
a center portion of the driven opposite surface 23b in the axial
direction. The axis N2 of the driven assist shaft 38 is set to
coincide with the axis L2 of the driven shaft 21 and the axis M2 of
the driven recess 42. That is, the driven assist shaft 38,
functioning as a second shaft portion, is coaxial with the driven
shaft 21. The driven assist shaft 38 and the driven rotor 23 are
formed integrally by molding. That is, the driven assist shaft 38
is also made of aluminum. The second bearing 32 receives the radial
load of the driven rotor 23 and a small amount of transmission
torque by rotatably supporting the driven assist shaft 38 to the
rotor housing 11. That is, the driven rotor 23 is supported at both
axial ends by the second bearing 32 and the fourth bearing 34.
[0028] The load (transmission torque) applied to the driving rotor
22 by the driving shaft 20 has the greatest value in an area in the
vicinity of the driving gear facing surface 22a close to the
electric motor M, and is reduced as the distance from the electric
motor M increases. Thus, the transmission torque acting on the
driving assist shaft 37 is close to zero and is smaller than the
transmission torque acting on the driving shaft 20. Namely, the
torsion of the driving assist shaft 37 is close to zero and is
smaller than the torsion of the driving shaft 20. The load applied
to the driving assist shaft 37 is merely the sum of the small
amount of transmission torque and the radial load of the driving
rotor 22. That is, unlike the driving shaft 20, which transmits
torque from the electric motor M to the driving rotor 22, the
driving assist shaft 37 does not need to have a great stiffness. As
a result, the specific gravity of the material of the driving
assist shaft 37 is permitted to be less than the specific gravity
of the material of the driving shaft 20. Therefore, the driving
shaft 20 does not need to extend through the driving rotor 22, and
the axial dimension of the driving shaft 20 can be reduced. By
making the specific gravity of the material of the driving assist
shaft 37 smaller than the specific gravity of the material of the
driving shaft 20, the weight of the Roots pump 10 is reduced
without changing the shape and size of the Roots pump 10.
[0029] Likewise, the load (transmission torque) applied to the
driven rotor 23 by the driven shaft 21 has the greatest value in an
area in the vicinity of the driven gear facing surface 23a close to
the driven timing gear 29, and is reduced as the distance from the
driven timing gear 29 increases. Thus, the transmission torque
acting on the driven assist shaft 38 is close to zero and is
smaller than the transmission torque acting on the driven shaft 21.
Namely, the torsion of the driven assist shaft 38 is close to zero
and is smaller than the torsion of the driven shaft 21. The load
applied to the driven assist shaft 38 is merely the sum of the
small amount of transmission torque and the radial load of the
driven rotor 23. That is, unlike the driven shaft 21, which
transmits torque from the driven timing gear 29 to the driven rotor
23, the driven assist shaft 38 does not need to have a great
stiffness. As a result, the specific gravity of the material of the
driven assist shaft 38 is permitted to be less than the specific
gravity of the material of the driven shaft 21. Therefore, the
driven shaft 21 does not need to extend through the driven rotor
23, and the axial dimension of the driven shaft 21 can be reduced.
By making the specific gravity of the material of the driven assist
shaft 38 smaller than the specific gravity of the material of the
driven shaft 21, the weight of the Roots pump 10 is reduced without
changing the shape and size of the Roots pump 10.
[0030] The preferred embodiment has the following advantages.
[0031] (1) The driving shaft 20 is press fitted partway into the
driving rotor 22 along the axial direction of the driving rotor 22.
The driving rotor 22 has the driving assist shaft 37, the material
of which has a specific gravity smaller than that of the material
of the driving shaft 20. Therefore, for example, compared to a case
where the driving shaft 20 extends through the driving rotor 22,
the weight of the Roots pump 10 is reduced. That is, the weight of
the Roots pump 10 can be reduced without changing the size and the
shape of the Roots pump 10.
[0032] Likewise, the driven shaft 21 is press fitted partway into
the driven rotor 23 along the axial direction of the driven rotor
23. The driven rotor 23 has the driven assist shaft 38, the
material of which has a specific gravity smaller than that of the
material of the driven shaft 21. Therefore, the weight of the Roots
pump 10 can be reduced without changing the fluid transferring
performance.
[0033] (2) The driving assist shaft 37 is integrally molded with
the driving rotor 22. Therefore, compared to a case where the
driving assist shaft 37 is formed separately from and attached to
the driving rotor 22, the driving assist shaft 37 is prevented from
being eccentric with respect to the driving rotor 22. This reduces
vibration of the driving shaft 20. Likewise, since the driven
assist shaft 38 is integrally molded with the driven rotor 23, the
driven assist shaft 38 is prevented from being eccentric with
respect to the driven rotor 23. This reduces vibration of the
driven shaft 21.
[0034] (3) The driving assist shaft 37 and the driving rotor 22 are
formed simultaneously using a common material (aluminum). Likewise,
the driven assist shaft 38 and the driven rotor 23 are formed
simultaneously using a common material (aluminum). Therefore,
compared to, for example, a case where the driving assist shaft 37
and the driving rotor 22 are formed separately and assembled
thereafter, the manufacturing costs of the Roots pump 10 are
reduced, and the productivity is increased.
[0035] (4) The depth of the driving recess 41 is equal to or less
than half the axial dimension of the driving rotor 22, and the
depth of the driven recess 42 is equal to or less than the axial
dimension of the driven rotor 23. As the axial dimensions of the
driving shaft and the driven shaft 21 are reduced, the weight of
the Roots pump 10 is reduced.
[0036] (5) The driving recess 41 can be formed simultaneously when
the driving rotor 22 is molded. Therefore, for example, compared to
a case where the driving shaft 20 extends through the driving rotor
22 and a through hole is formed in the driving rotor 22 after the
driving rotor 22 is molded, the time required for forming the
driving rotor 22 is reduced. This increases the productivity of the
Roots pump 10. Likewise, since the driven recess 42 is formed
simultaneously with the driven rotor 23 when the driven rotor 23 is
molded, the time required for forming the driven rotor 23 is
shortened.
[0037] The above-mentioned embodiment may be modified as
follows.
[0038] The driving shaft 20 does not need to be press fitted into
the driving recess 41 to the bottom of the driving recess 41.
Likewise, the driven shaft 21 does not need to be press fitted into
the driven recess 42 to the bottom of the driven recess 42. Also,
the depth of the driving recess 41 and the depth of the driven
recess 42 may be changed.
[0039] The driving assist shaft 37 and the driving rotor 22 do not
need to be integrally molded, but may be formed separately and
assembled thereafter. Likewise, the driven assist shaft 38 and the
driven rotor 23 do not need to be integrally molded, but may be
formed separately and assembled thereafter.
[0040] The material of the driving assist shaft 37 does not need to
be the same as the material of the driving rotor 22. The driving
assist shaft 37 may be formed of any material as long as its
specific gravity is less than the specific gravity of the material
of the driving shaft 20. For example, in a case where the driving
shaft 20 is made of an iron-based material, the driving assist
shaft 37 may be formed of titanium or a synthetic resin. Likewise,
the material of the driven assist shaft 38 does not need to be the
same as the material of the driven rotor 23. The driven assist
shaft 38 may be formed of any material as long as its specific
gravity is less than the specific gravity of the material of the
driven shaft 21. For example, the material of the driven assist
shaft 38 may be titanium or a synthetic resin.
[0041] The driving rotor 22 and the driven rotor 23 may be formed
of synthetic resin. In this case, the driving shaft 20 and the
driven shaft 21 are made of a material having a specific gravity
that is grater than that of the synthetic resin.
[0042] 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.
[0043] 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.
* * * * *