U.S. patent application number 11/068925 was filed with the patent office on 2005-09-08 for power tool.
This patent application is currently assigned to Hitachi Koki Co., Ltd.. Invention is credited to Nishikawa, Tomomasa, Toukairin, Junichi.
Application Number | 20050196273 11/068925 |
Document ID | / |
Family ID | 34879842 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050196273 |
Kind Code |
A1 |
Nishikawa, Tomomasa ; et
al. |
September 8, 2005 |
Power tool
Abstract
Blades are provided on a fan body such that an outer diameter d2
of the fan body is in a range of 45 mm.ltoreq.d2.ltoreq.50 mm, an
axial height h1 at a substantially intermediate position of the
blade is in a range of 0.2.ltoreq.h1/d2.ltoreq.0.3 with respect to
the outer diameter of the fan body, and an axial height h2 at an
outer peripheral edge of the blade is in a range of
0.12.ltoreq.h2/d2.ltoreq.0.17 with respect to the outer diameter d2
of the fan body.
Inventors: |
Nishikawa, Tomomasa;
(Ibaraki, JP) ; Toukairin, Junichi; (Ibaraki,
JP) |
Correspondence
Address: |
MCGINN & GIBB, PLLC
8321 OLD COURTHOUSE ROAD
SUITE 200
VIENNA
VA
22182-3817
US
|
Assignee: |
Hitachi Koki Co., Ltd.
Tokyo
JP
|
Family ID: |
34879842 |
Appl. No.: |
11/068925 |
Filed: |
March 2, 2005 |
Current U.S.
Class: |
415/206 |
Current CPC
Class: |
F04D 29/281 20130101;
H02K 7/145 20130101; B25F 5/008 20130101; F04D 29/30 20130101; H02K
9/06 20130101; H02K 2213/03 20130101 |
Class at
Publication: |
415/206 |
International
Class: |
F01D 005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2004 |
JP |
P2004-060035 |
Claims
What is claimed is:
1. A power tool comprising: a housing in which an inlet port for
introducing air and an outlet port for exhausting the air are
formed; a motor having a rotor and a stator, which is accommodated
in the housing; and a centrifugal fan capable of rotating with the
rotor, which is fixed coaxially to the rotor, the centrifugal fan
including; a disc-shaped fan body; and a plurality of blades
capable of flowing the air along an axial direction of the rotor
radially and outwardly of the fan body, the blades extending from a
predetermined position in a radial direction of the fan body to an
outer peripheral edge of the fan body, and the blades formed at
predetermined pitches along a circumferential direction of the fan
body; a first channel formed between the stator and the housing;
and a second channel formed between the stator and the rotor,
wherein a value S0 is defined by a sectional area which is smallest
in the first channel and the second channel among cross sections
which are perpendicular to an axial direction of the rotor and are
arranged in an axial direction of the rotor, the value S0 is in a
range of 350 mm.sup.2.ltoreq.S.sub.0.ltor- eq.650 mm.sup.2, an
outer diameter d2 of the fan body is in a range of 45
mm.ltoreq.d.sub.2.ltoreq.50 mm , and a height h1 in the axial
direction of the blade at a position where the blade is highest
with respect to the outer diameter of the fan body is in a range of
0.2.ltoreq.h.sub.1/d.sub.- 2.ltoreq.0.3.
2. The power tool according to claim 1, wherein the height h1 in
the axial direction of the blade at the position where the blade is
highest is in a range of 0.25.ltoreq.h.sub.1/d.sub.2.ltoreq.0.3
with respect to the outer diameter d2 of the fan body.
3. The power tool according to claim 2, wherein a height h2 in the
axial direction of the blade at an outer peripheral edge of the
blade is in a range of 0.12.ltoreq.h.sub.2/d.sub.2.ltoreq.0.17 with
respect to the outer diameter d2 of the fan body.
4. The power tool according to claim 1, wherein a number of the
blades, n, is in a range of 23.ltoreq.n.ltoreq.30.
5. The power tool according to claim 4, wherein the number of the
blades, n, is in a range of 25.ltoreq.n.ltoreq.28.
6. The power tool according to claim 1, wherein a first area S1 is
defined by product of both a distance L1 and a height h1, where the
distance L1 is defined along a circumferential direction of the fan
body and between opposing portions of mutually adjacent blades
where the blades are highest, and the height h1 of the blades is
defined in an axial direction of the blades where the blades are
highest, an inside diameter d1 is defined by a distance between a
pair of the blades located in a same diametrical direction of the
fan body where the pair of blades are highest, and a second area S2
is defined by a product of a distance L2 and a height h2, where the
distance L2 is defined along a circumferential direction of the fan
body and between opposing portions of the mutually adjacent blades
at outer peripheral edges thereof, and the height h2 is defined in
an axial direction of the blade at the outer peripheral edge
thereof, d2 is defined by an outer diameter of the fan body, and
S1, S2, d1, and d2 are arranged to satisfy a relationship of
S.sub.1.multidot.d.sub.1=(1.+-.0.3)S.sub.2.multidot.d.sub.2.
7. The power tool according to claim 1, wherein the blades are
configured by an inward portion extending from the predetermined
radial position to the position where the blades are highest and an
outward portion extending from the position where the blades are
highest to the outer peripheral edge, a direction extending toward
the outward portion of the blades is inclined by a first
predetermined angle .alpha..sub.1 in an opposite direction to a
rotating direction of the fan with respect to a straight line
connecting a center of the fan and an outer peripheral edge of the
outward portion of the blades a direction extending toward the
inward portion of the blades is inclined by a second predetermined
angle .alpha..sub.2 in the opposite direction to the rotating
direction of the fan with respect to a straight line connecting the
center of the fan and the predetermined radial position, the first
predetermined angle .alpha..sub.1 is in a range of
30.degree..ltoreq..alpha..sub.1.ltoreq.50.- degree., and the second
predetermined angle .alpha..sub.2 is in a range of
0.degree..ltoreq..alpha..sub.2.ltoreq.10.degree..
8. The power tool according to claim 6, wherein the first
predetermined angle .alpha..sub.1 is in a range of
35.degree..ltoreq..alpha..sub.1.ltor- eq.45.degree., and the second
predetermined angle .alpha..sub.2 is in a range of
2.5.degree..ltoreq..alpha..sub.2.ltoreq.7.5.degree..
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a power tool having a
centrifugal fan for cooling a motor.
[0003] 2. Description of the Related Art
[0004] Because of demand for compact size, high output, and low
noise of power tools, there has been an increasing need for
development of motor cooling fans which are compact and have a high
cooling capacity and low noise as well as peripheral components of
the fan. Accordingly, optimization of the configuration of blades
has been proposed for the purpose of increasing the air volume and
reducing the noise (e.g., refer to JP-A-10-153194).
[0005] As shown in FIGS. 10 and 11, a plurality of blades 222 of a
centrifugal fan 220 disclosed in JP-A-10-153194 are provided in
such a manner as to extend from a predetermined radial position to
an outer rim on one surface side of the fan body 221, to be
arranged at predetermined pitches along a circumferential direction
of the fan body 221, and to project in an axial direction of an
unillustrated drive shaft from the one surface side.
[0006] If the centrifugal fan 220 is rotated, energy is imparted to
air by the centrifugal action, and the air passes from an inlet
portion at inner ends of the blades 222 through air passages formed
by the blades 222 and a fan guide 211, and is exhausted radially
outwardly from an outlet portion at outer peripheral portions of
the blades 222.
[0007] Here, the centrifugal fan 220 is constructed such that the
product of a diameter D1 of inner ends of the pair of blades 222
located in the same diametrical direction, a projecting length H1
at the inner end of the blade 222, and an interval L1 in the
circumferential direction of the fan body 221 between the mutually
opposing portions at the inner ends of the blades 222, and the
product of a diameter D2 of the fan body 221, a projecting length
H2 at the outer periphery of the blade 222, and an interval L2 in
the circumferential direction of the fan body 221 between the
mutually opposing portions at the outer edges of the blades 222,
become substantially equal. Namely, by ensuring such that
D1.times.H1.times.L1.congruent.D2.times.H2.times.L2, air flows
smoothly and the noise reduction effect is exhibited.
SUMMARY OF THE INVENTION
[0008] However, the demand for the low noise of power tools is
becoming increasingly high, and there is a need to optimize the
configuration of the centrifugal fan and make the noise smaller. In
general, it is said that the noise of a fluid is proportional to
about the sixth power of the flow velocity. In the case of the
centrifugal fan, if the number of revolutions is the same, if the
outer diameter of the fan is made small, the flow velocity becomes
small, so that if the outer diameter of the fan is made small, it
is possible to make the noise small. Also, although if the outer
diameter of the fan is made small, the flow rate becomes small, but
by making the blade height of the fan high, it is possible to
compensate for the flow rate by increasing the amount of air to
which the centrifugal force is imparted. However, if the blade
height is made high, a need arises to form deeper slots in portions
of a mold of the fan corresponding to the blades. Since processing
becomes difficult such as due to the fact that an end mill is made
liable to run out during the formation of the slots, a problem
arises in that the manufacturing cost becomes substantially
high.
[0009] It is an object of the invention to provide a power tool
capable of setting an optimum blade height with respect to the fan
diameter of the centrifugal fan, and realize low noise and an
increase in the air volume at an appropriate manufacturing
cost.
[0010] According to one aspect of the invention, there is provided
with a power tool including: a housing in which an inlet port for
introducing air and an outlet port for exhausting the air are
formed; a motor having a rotor and a stator, which is accommodated
in the housing; and a centrifugal fan capable of rotating with the
rotor, which is fixed coaxially to the rotor, the centrifugal fan
including; a disc-shaped fan body; and a plurality of blades
capable of flowing the air along an axial direction of the rotor
radially and outwardly of the fan body, the blades extending from a
predetermined position in a radial direction of the fan body to an
outer peripheral edge of the fan body, and the blades formed at
predetermined pitches along a circumferential direction of the fan
body; a first channel formed between the stator and the housing;
and a second channel formed between the stator and the rotor,
wherein a value S0 is defined by a sectional area which is smallest
in the first channel and the second channel among cross sections
which are perpendicular to an axial direction of the rotor and are
arranged in an axial direction of the rotor, the value S0 is in a
range of 350 mm.sup.2.ltoreq.S.sub.0.ltor- eq.650 mm.sup.2, an
outer diameter d2 of the fan body is in a range of 45
mm.ltoreq.d.sub.2.ltoreq.50 mm, and a height h1 in the axial
direction of the blade at a position where the blade is highest
with respect to the outer diameter of the fan body is in a range of
0.2.ltoreq.h.sub.1/d.sub.- 2.ltoreq.0.3.
[0011] By thus configuration, the outer diameter d2 of the fan body
is in a range of 45 mm.ltoreq.d.sub.2.ltoreq.50 mm, and the height
h1 in the axial direction of the blade at the position where the
blade is highest is in a range of
0.2.ltoreq.h.sub.1/d.sub.2.ltoreq.0.3 with respect to the outer
diameter of the fan body. Therefore, it is possible to realize low
noise and an increase in the air volume at an appropriate
manufacturing cost.
[0012] According to another aspect of the invention, the height h1
in the axial direction of the blade at the position where the blade
is highest is in a range of 0.25.ltoreq.h.sub.1/d.sub.2.ltoreq.0.3
with respect to the outer diameter d2 of the fan body.
[0013] By thus configuration, the height h1 in the axial direction
of the blade at the position where the blade is highest is in a
range of 0.25.ltoreq.h.sub.1/d.sub.2.ltoreq.0.3 with respect to the
outer diameter d2 of the fan body. Therefore, it is possible to
realize low noise and an increase in the air volume at a more
appropriate manufacturing cost.
[0014] According to another aspect of the invention, a height h2 in
the axial direction of the blade at an outer peripheral edge of the
blade is in a range of 0.12.ltoreq.h.sub.2/d.sub.2.ltoreq.0.17 with
respect to the outer diameter d2 of the fan body.
[0015] By thus configuration, the height h2 in the axial direction
of the blade at an outer peripheral edge of the blade is in a range
of 0.12.ltoreq.h.sub.2/d.sub.2.ltoreq.0.17 with respect to the
outer diameter d2 of the fan body. Therefore, it is possible to
realize a centrifugal fan which is capable of generating a large
air volume and is low-noise.
[0016] According another aspect of the invention, a number of the
blades, n, is in a range of 23.ltoreq.n.ltoreq.30.
[0017] By thus configuration, since the number of the blades, n, is
in a range of 23.ltoreq.n.ltoreq.30, vortices which cause noise are
substantially not produced, and it is possible to ensure the
passage of air sufficiently. Hence, it is possible to lower the
noise while securing a sufficient air volume.
[0018] According to another aspect of the invention, the number of
the blades, n, is in a range of 25.ltoreq.n.ltoreq.28.
[0019] By thus configuration, since the number of the blades, n, is
in a range of 25.ltoreq.n.ltoreq.28, it becomes possible to further
lower the noise while securing a more sufficient air volume.
[0020] According to another aspect of the invention, a first area
S1 is defined by product of both a distance L1 and a height h1,
where the distance L1 is defined along a circumferential direction
of the fan body and between opposing portions of mutually adjacent
blades where the blades are highest, and the height h1 of the
blades is defined in an axial direction of the blades where the
blades are highest, an inside diameter d1 is defined by a distance
between a pair of the blades located in a same diametrical
direction of the fan body where the pair of blades are highest, and
a second area S2 is defined by a product of a distance L2 and a
height h2, where the distance L2 is defined along a circumferential
direction of the fan body and between opposing portions of the
mutually adjacent blades at outer peripheral edges thereof, and the
height h2 is defined in an axial direction of the blade at the
outer peripheral edge thereof, d2 is defined by an outer diameter
of the fan body, and S1, S2, d1, and d2 are arranged to satisfy a
relationship of
S.sub.1.multidot.d.sub.1=(1.+-.0.3)S.sub.2.multidot.d.sub.2.
[0021] By thus configuration, since an arrangement is provided so
as to satisfy a relationship of
S.sub.1.multidot.d.sub.1=(1.+-.0.3)S.sub.2.mult- idot.d.sub.2, the
air flow between mutually adjacent blades is difficult to be
disturbed, making it possible to lower the noise.
[0022] According to another aspect of the invention, the blades are
configured by an inward portion extending from the predetermined
radial position to the position where the blades are highest and an
outward portion extending from the position where the blades are
highest to the outer peripheral edge, a direction extending toward
the outward portion of the blades is inclined by a first
predetermined angle .alpha..sub.1 in an opposite direction to a
rotating direction of the fan with respect to a straight line
connecting a center of the fan and an outer peripheral edge of the
outward portion of the blades, a direction extending toward the
inward portion of the blades is inclined by a second predetermined
angle .alpha..sub.2 in the opposite direction to the rotating
direction of the fan with respect to a straight line connecting the
center of the fan and the predetermined radial position, the first
predetermined angle .alpha..sub.1 is in a range of
30.degree..ltoreq..alpha..sub.1.ltoreq.50.- degree., and the second
predetermined angle .alpha..sub.2 is in a range of
0.degree..ltoreq..alpha..sub.2.ltoreq.10.degree..
[0023] By thus configuration, since the first predetermined angle
.alpha.1 is in a range of
30.degree..ltoreq..alpha..sub.1.ltoreq.50.degree., the velocity of
air in the vicinity of the outer peripheral edge of the centrifugal
fan can be set to an appropriate velocity, and it is possible to
realize low noise while maintaining a sufficient air volume. In
addition, since the second predetermined angle .alpha.2 is in a
range of 0.degree..ltoreq..alpha..sub.2.ltoreq.10.degree., it is
possible to alleviate a stress occurring at a root of each blade,
thereby making it possible to prevent the destruction of the
blades. In addition, it is possible to suppress the occurrence of
turbulence which causes the noise.
[0024] According to another aspect of the invention, the first
predetermined angle .alpha..sub.1 is in a range of
35.degree..ltoreq..alpha..sub.1.ltoreq.45.degree., and the second
predetermined angle .alpha..sub.2 is in a range of
2.5.degree..ltoreq..alpha..sub.2.ltoreq.7.5.degree..
[0025] Since the first predetermined angle .alpha.1 is in a range
of 35.degree..ltoreq..alpha.1.ltoreq.45.degree., and the second
predetermined angle .alpha.2 is in a range of
2.5.degree..ltoreq..alpha.2- .ltoreq.7.5.degree., it is possible to
realize lower noise while maintaining a more sufficient air
volume.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a cross-sectional view of a power tool according
to one embodiment of the invention which is applied to a
grinder;
[0027] FIG. 2 is a cross-sectional view taken along line II-II in
FIG. 1;
[0028] FIG. 3 is a cross-sectional view of a centrifugal fan
provided in the power tool;
[0029] FIG. 4 is a front elevational view of the centrifugal fan
provided in the power tool;
[0030] FIG. 5 is a diagram illustrating changes in the flow rate of
air in a case where a pressure difference is applied between an
inlet and an outlet of air;
[0031] FIG. 6 is a diagram illustrating changes in the flow rate of
air in a case where a first blade height ratio is varied;
[0032] FIG. 7 is a diagram illustrating relationships between, on
the one hand, respective combinations of the first blade height and
a second blade height and, on the other hand, a noise ratio and an
air volume;
[0033] FIG. 8 is a diagram illustrating the relationship between
the number of blades and an air volume ratio;
[0034] FIG. 9 is a cross-sectional view of a modification of the
centrifugal fan provided in the power tool;
[0035] FIG. 10 is a front elevational view of a conventional
centrifugal fan; and
[0036] FIG. 11 is a cross-sectional view of the conventional
centrifugal fan.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Referring to FIG. 1, a description will be given of a power
tool according to one embodiment of the invention is applied to a
grinder (disc grinder).
[0038] FIG. 1 shows an overall structure of a disc grinder 1. If it
is assumed that the left-hand side in the drawing is a front end, a
resin-made handle portion 2, a resin-made motor housing 3, an
aluminum alloy-made gear cover 4 are consecutively connected in
that order from the rear side, thereby constituting a housing.
Spaces defined in the respective interiors of the handle portion 2,
the motor housing 3, and the gear housing 4 communicate with each
other. A power supply cable 5 is attached to the handle portion 2,
and a switch mechanism 6 is incorporated therein. The switch
mechanism 6 is provided with a lever 2A which can be operated by a
user. The power supply cable 5 connects the switch mechanism 6 to
an external power source (not shown), and the connection and
disconnection between the switch mechanism 6 and the power source
are changed over. In addition, a first air inlet 2a is formed at a
rear end portion of the handle portion 2, and unillustrated second
and third air inlets are formed at a front end portion thereof.
[0039] A motor 9 having a rotor 7 and a stator 8 is accommodated in
the motor housing 3, and the rotor 7 has a drive shaft 10 axially
therewith. A fan guide 11 is fixed to the motor housing 3 in front
of the motor 9.
[0040] Inside the gear housing 4 and in front of the fan guide 11,
a centrifugal fan 20 is fixed to the drive shaft 10 concentrically
therewith and is provided rotatably with the drive shaft 10. A
first air outlet 4a, a second air outlet 4b, and an unillustrated
third air outlet are formed in the gear cover 4 at positions
located radially outwardly of the centrifugal fan 20. In addition,
a power transmission mechanism including a pinion gear 12 fixed to
one end of the drive shaft 10 and a gear 14 fixed to an output
shaft 13, i.e., an output portion, is disposed inside the gear
housing 4. The pinion gear 12 meshes with the gear 14 to transmit
the rotation of the rotor 7 to the output shaft 13. A grinding
wheel 15 is fixed to the output shaft 13.
[0041] Referring next to FIG. 2, a description will be given of the
internal structure of the motor housing 3. FIG. 2 is a
cross-sectional view taken along line II-II of FIG. 1. As described
above, the motor 9 has the rotor 7 and the stator 8. The stator 8
is fixedly held in the motor housing 3, and a hollow portion 8a for
loosely inserting the rotor 7 is formed in the stator 8. Further, a
plurality of first air channels 3a are respectively defined by the
motor housing 3 and the stator 8, and a plurality of second air
channels 3b are respectively defined by the stator 8 and the rotor
7.
[0042] Next, a description will be given of the operation of the
disc grinder 1. By pressing the lever 2A against the handle portion
2, an electric current is supplied from the unillustrated external
power source to the motor 9 to rotate the rotor 7. The drive shaft
10 also rotates together with the rotation of the rotor 7, and the
rotation is transmitted to the output shaft 13 and the grinding
wheel 15 by the pinion gear 12 and the gear 14. As the rotating
grinding wheel 15 is pressed against a workpiece, the grinding
operation is carried out.
[0043] At this time, air flows in a built-up pressure space 20a,
which will be described later, as shown by arrows c1, by the
rotation of the centrifugal fan 20 fixed to the stator 7, and the
pressure drops on the inner diameter side of the centrifugal fan
20, while the pressure becomes high on the outer diameter side
thereof. Consequently, air is introduced, as shown by arrows a1,
a2, and a3, through the first air inlet 2a and the unillustrated
second and third air inlets in the tail cover 2. Next, the air
flows through the first air channel 3a and the second air channel
3b, as shown by arrows b1 and b2, to cool the motor 9.
Subsequently, the air flows through the built-up pressure space
20a, as shown by the arrows c1, and flows out to the outside from
the first air outlet 4a, the second air outlet 4b, and the
unillustrated third air outlet, as shown by arrows e1, e2, and
e3.
[0044] Next, referring to FIG. 3, a description will be given of
the structure of the centrifugal fan 20. FIG. 3 is a
cross-sectional view of the centrifugal fan 20, and FIG. 4 is a
front elevational view, taken from a direction IV in FIG. 3, of the
centrifugal fan 20. The centrifugal fan 20 has a fan body 21 and a
plurality of blades 22 provided integrally with the fan body 21 and
projecting in the axial direction of the fan body 21, and rotates
in the direction indicated by arrow A (FIG. 4). The fan body 21 is
disc-shaped and is made up of a hub 21A having a rotor
fitting/inserting hole 21a for engaging with the drive shaft 10, as
well as a main plate 21B The plurality of blades 22 extend from a
predetermined radial position B on the fan body 21 to an outer
peripheral edge thereof and are formed at predetermined pitches
along the circumferential direction of the fan body 21, in order to
allow the air flowing along the axial direction of the rotor 7 to
flow radially outwardly of the fan body 21.
[0045] As shown in FIG. 4, the blades 22 are inclined in an
opposite direction to a rotating direction A with respect to a
direction extending radially outwardly from the predetermined
radial position B on the fan body 21. Each of the blades 22 is made
up of an inward portion 22A extending from the predetermined radial
position B to a substantially intermediate position C and an
outward portion 22B extending from the substantially intermediate
position C to the outer peripheral edge. The inward portion 22A has
a configuration in which its axial height becomes gradually higher
toward its radially outward side. On the other hand, the outward
portion 22B has a configuration in which its axial height becomes
gradually lower toward the radially outward side. In addition, the
built-up pressure space 20a where the air flows is defined by the
mutually adjacent blades 22, and part of the built-up pressure
space 20a is opposed to the fan guide 11 (FIG. 1).
[0046] Here, it is assumed that a distance between the
substantially intermediate positions C of the pair of blades 22
located on mutually opposite sides in the same diametrical
direction (hereafter referred to as the fan inside diameter) is d1,
that the diameter of the fan body 21 (hereafter referred to the fan
outer diameter) is d2, that the axial height at the substantially
intermediate position C of the blade 22 (hereafter referred to as
the blade inward height) is h1, and that the axial height at the
peripheral edge of the outward portion 22B of the blade 22
(hereafter referred to as the blade outward height) is h2. Further,
it is assumed that an angle formed by a direction in which the
outward portion 22B extends and a straight line connecting the
center of the centrifugal fan 20 and the outer peripheral edge of
the outward portion 22B is .alpha.1, and that an angle formed by a
direction in which the inward portion 22A extends and a straight
line connecting the center of the centrifugal fan 20 and the
predetermined radial position B is .alpha.2. Furthermore, it is
assumed that the distance along the circumferential direction of
the centrifugal fan 20 between mutually opposing portions at the
substantially intermediate positions C of the mutually adjacent
blades 22 (hereafter referred to as the distance between the
substantially intermediate positions C) is L1, and that the
distance along the circumferential direction of the centrifugal fan
20 between the mutually opposing portions at the peripheral edges
of the mutually adjacent blades 22 (hereafter referred to as the
distance between the outer peripheral edges of the blades) is
L2.
[0047] In this embodiment, settings are provided such that the fan
inside diameter d1=35 mm; the fan outer diameter d2=48 mm; the
blade inward height h1=13 mm; and the blade outward height h2=7 mm.
Meanwhile, the dimensions of a conventional centrifugal fan were
such that the fan inside diameter d1'=33 mm; the fan outer diameter
d2'=52 mm; the blade inward height h1'=9 mm; and the blade outward
height h2'=3.5 mm. In addition, settings are provided such that
.alpha.1=40.degree., .alpha.2=5.degree., and the number of the
blades 22 is set to 27. It should be noted that since there are
measurement errors and variations in dimensional values, the symbol
`=` when expressing a dimensional value will be used as
substantially meaning `.congruent..`
[0048] Next, a description will be given of the reason for changing
the fan outer diameter d2 from the conventional 52 mm to 48. The
selection of the fan outer diameter d2 is based on the property
that the relationship between the sound pressure p [pa] of the
noise generated by a fluid and the flow velocity v [m/sec]
generally becomes
p.varies.v.sup.6.
[0049] In the centrifugal fan, the relationship between the fan
outer diameter d2 and the flow velocity v at the fan outlet
generally becomes
d2.varies.v.
[0050] Therefore, if these are combined, we have
p.varies.d2.sup.6.
[0051] In addition, the relationship between the conventional fan
outer diameter d2' and the sound pressure p' similarly becomes
P'.varies.d2'.sup.6.
[0052] Accordingly, we have
(p/P').varies.(d2/d2').sup.6.
[0053] Namely, the selection of the fan outer diameter d2 is made
on the basis of the property that if the fan outer diameter d2 is
made small, the flow velocity v becomes small in proportion to it,
and the sound pressure p becomes small substantially in proportion
to its 6th power. In this embodiment,
(p/P').varies.(48/52).sup.6.congruent.0.62.
[0054] As such, theoretically, the sound pressure p becomes smaller
than the conventional level by approximately 0.62-fold. In
addition, as a result of conducting experiments, it was found that
in contrast to the fact that a conventional noise value was
approximately 81 dB, the noise value in this embodiment was
approximately 77.7 dB and became smaller by approximately 3.5 dB.
In addition, if the fan outer diameter d2 is in the range of 45
mm.ltoreq.d2.ltoreq.50 mm, a substantially similar effect can be
obtained.
[0055] It should be noted that the sizes of the centrifugal fan 20
in accordance with this embodiment and the conventional centrifugal
fan are so designed as to satisfy a formula (1) which will be
described later and is for smoothening the flow of air in the
centrifugal fan and reducing noise. Accordingly, the fact that the
noise value became smaller by approximately 3.5 dB is derived only
from the effect of changing the fan outer diameter d2 to 48 mm.
[0056] Next, a description will be given of the reason for
providing the setting as the blade inward height h1=13 mm with
respect to the fan outer diameter d2=48 mm. If we take a look at
the ratio between the blade inward height h1 and the fan outer
diameter d2 (hereafter referred to as the first blade height
ratio), h1/d2, we note that h1/d2=0.27, which is set to be larger
than the conventional h1'/d2'=9/52.congruent.0.17. The reason for
this is to make the fan outer diameter d2 small so as to compensate
for a decline in the flow rate attributable to the fact that the
flow velocity became small.
[0057] Hereafter, a description will be given of factors affecting
the flow rate. The pressure difference P [Pa] between an inlet and
an outlet of the built-up pressure space 20a, which is an air flow
capacity necessary for the occurrence of a flow rate Q
[m.sup.3/min] in the channel from the inlet (the first air inlet 2a
and the unillustrated second and third inlets) in the air channel
to the outlet (the first air outlet 4a, the second air outlet 4b,
and the unillustrated third air outlet), can be expressed by the
following formula:
P=aQ.sup.2,
[0058] where a is a coefficient of channel resistance. In addition,
the above formula can be rewritten as
Q={square root}(P/a).
[0059] Namely, the factors affecting the flow rate Q are the
pressure difference P and the coefficient of channel resistance a.
Hereafter, a description will be given of the coefficient of
channel resistance a. The coefficient of channel resistance a is a
characteristic value which is determined by the configuration of
the channel. It is known that the value of the coefficient of
channel resistance a is substantially determined by the size of a
sectional area (which is set to S0) of the narrowest portion in the
channel.
[0060] The minimum sectional area S0 in the cooling channel of the
disc grinder 1 in this embodiment is a value in which the sectional
area is smallest in the first air channel 3a and the second air
channel 3b among cross sections which are perpendicular to the
axial direction of the rotor 7 and are arranged in the axial
direction of the rotor 7 (hereafter referred to as the in-motor
channel sectional area: S0). The sectional area of the first air
channel 3a is naturally determined from the demand for a compact
size for making the outer diameter of the motor housing 3 as small
as possible and from the outer diameter of the stator 8 required
for obtaining desired power. As for the second air channel 3b as
well, the sectional area is naturally determined from the need to
efficiently convert the magnetic force into torque.
[0061] As a result of investigating the in-motor channel sectional
area S0 with respect to general portable power tools, it was found
such that 350 mm.sup.2.ltoreq.S0.ltoreq.650 mm.sup.2 or
thereabouts. FIG. 5 shows results of investigation into changes in
the flow rate Q [mm.sup.2] with respect to these in-motor channel
sectional areas in cases where the pressure difference between the
air inlet and outlet (pressure difference) P [Pa] was produced. The
curve X in FIG. 5 was the result in the case of S0=350 mm.sup.2,
and the curve Y was the result in the case of S0=650 mm.sup.2. In
addition, the coefficient of flow resistance a in the curve X was
approximately 3000, and the coefficient of flow resistance a in the
curve Y was approximately 2000. The coefficient of flow resistance
a in the portable power tool of 350 mm.sup.2.ltoreq.S0.ltoreq.650
mm.sup.2 is 2000.ltoreq.a.ltoreq.3000 or thereabouts.
[0062] Hereafter, a description will be given of the air flow
capacity of the fan. A factor exerting a large influence on the air
flow capacity of the fan is the first blade height ratio. FIG. 6
shows results of investigation into changes in the flow rate Q with
respect to the portable power tools of 350
mm.sup.2.ltoreq.S0.ltoreq.650 mm.sup.2 in cases where the fan outer
diameter was varied in the range of 45 mm.ltoreq.d2.ltoreq.50 mm
and the first blade height ratio (h1/d2) was varied. In the range
of h1/d2.ltoreq.0.2 in terms of the first blade height ratio, the
flow rate increases substantially in proportion to an increase in
the first blade height ratio. This is because, in this region,
since the channel resistance ratio is sufficiently small with
respect to the air flow capacity of the fan, air easily flows
through the blades 22, and the flow in the vicinity of the fan is
smooth.
[0063] In addition, in the region of 0.2.ltoreq.h1/d2.ltoreq.0.3,
the rate of increase in the flow rate becomes small, and in the
range of 0.3.ltoreq.h1/d2, the flow rate practically ceases to
increase. This is because in the range of 0.3.ltoreq.h1/d2, since
the channel resistance ratio with respect to the capacity of the
fan is excessively large, air becomes difficult to flow through the
blades 22, and the energy of the fan is used to agitate the
surrounding air, and because fine bubbles are produced between the
blades 22. Namely, in these power tools, if the first blade height
ratio h1/d2 is in the range of 0.2.ltoreq.h1/d2.ltoreq- .0.3, the
blade inward height h1 can be set to a proper height in terms of
the flow rate and the manufacturing cost.
[0064] More preferably, if the first blade height ratio h1/d2 is
set in the range of 0.25.ltoreq.h1/d2.ltoreq.0.3, the blade inward
height h1 can be set to a more appropriate height, and if the first
blade height ratio h1/d2 is set such that h1/d2=0.27, the blade
inward height h1 can be set to a most appropriate height. It should
be noted that in this embodiment, since the fan outer diameter d2
is 48 mm,
h1/d2=0.27.
[0065] Hence, we have
h1=0.27.times.48.congruent.13 mm,
[0066] so that the blade inward height h1 is set as h1=13 mm.
[0067] From the above, in a power tool having a cooling channel for
cooling a motor by a centrifugal fan and having the in-motor
channel sectional area S0 in the range of 350
mm.sup.2.ltoreq.S0.ltoreq.650 mm.sup.2, it is possible realize low
noise and an increase in the air volume at an appropriate
manufacturing cost by setting the fan outer diameter in the range
of 45 mm.ltoreq.d2.ltoreq.50 mm and the blade inward height h1 in
the range of 0.2.ltoreq.h1/d2.ltoreq.0.3.
[0068] Next, a description will be given of the reason for setting
the fan inside diameter d1 to 35 mm and the blade outward height h2
to 7 mm. If it is assumed that an area indicated by the product of
the distance between the substantially intermediate positions C,
L1, and the blade inward height h1 is an inlet portion area S1
(=L1.times.h1), and that an area indicated by the product of the
distance between the outer peripheral edges of the blades, L2, and
the blade outward height h2 is an outlet portion area S2
(=L2.times.h2), the configuration of the blade 22 is designed so as
to satisfy the following relationship:
S1.multidot.d1=.mu..multidot.S2.multidot.d2
0.7.ltoreq..mu..ltoreq.1.3 (1)
[0069] By so doing, the ratio between the velocity in the radial
direction of air between the blades 22 and the velocity in the
rotating direction thereof becomes equal between the inlet portion
(the substantially intermediate position C of the blade 22) and the
outlet portion (the outer peripheral edge of the blade 22), whereby
the airflow is made difficult to be disturbed, and the noise can be
reduced.
[0070] Accordingly, it suffices if the design is made in this
embodiment as well so as to satisfy the relationship of Formula
(1). Specifically, from Formula (1) (in this embodiment, a setting
is provided such that .mu.=1), we have
(.pi..multidot.d1/n).multidot.h1.multidot.d1=(.pi..multidot.d2/n).multidot-
.h2.multidot.d2.
[0071] Further, we have
d1.sup.2.multidot.h1=d2.sup.2.multidot.h2 (2)
[0072] Here, since in this embodiment d2=48 mm, and h1=13 mm,
Formula (2) can be rewritten as
d1.sup.2.multidot.13=48.sup.2.multidot.h2.
[0073] The values of d1=35 mm and h2=7 mm are set so as to satisfy
this relationship.
[0074] On the basis of the above, the inventors conducted
experiments by varying the first blade height ratio h1/d2 and a
ratio of the blade outward height h2 to the fan outer diameter d2
(hereafter referred to as the second blade height ratio, h1/d2) in
predetermined ranges. FIG. 7 shows results of conducting the
experiments by combining the second blade height ratio and the
first blade height ratio to (10.0%, 22.0%), (14.5%, 27.0%), and
(20.0%, 32.0%), respectively. The ordinate on the left denotes the
noise ratio which is a value obtained by dividing the obtained
noise value by a predetermined noise value. The ordinate on the
right denotes the air volume [m.sup.2/min].
[0075] From FIG. 7, it can be understood that in the region where
the second blade height ratio and the first blade height ratio are
less than (12.0%, 25.0%), the noise is small, but it is impossible
to obtain a sufficient air volume for cooling the motor 9, and that
in the region exceeding (17.0%, 30.0%), a sufficient air volume can
be obtained, but the noise becomes large. Accordingly, if the
second blade height ratio is in the range of 12.0 to 17.0% and the
first blade height ratio is in the range of 25.0 to 30.0%, it is
possible to lower the noise while maintaining a sufficient air
volume. More preferably, if the second blade height ratio and the
first blade height ratio are (14.5%, 27.0%) or in their vicinities,
it is possible to realize a centrifugal fan 20 which is capable of
producing a large air volume and is low-noise.
[0076] Next, a description will be given of the reason for setting
the number of the blades 22 to 27. Hereafter, the number of the
blades 22 is assumed to be n. As a result of investigating changes
in the air volume in cases where the number of the blades 22, n,
was changed concerning the centrifugal fan 20 having the fan outer
diameter d2 of 45 mm.ltoreq.d2.ltoreq.50 mm, the air volume did not
change substantially, and a tendency such as the one shown in FIG.
8 was generally shown in the respective cases. The ordinate on the
left denotes the air volume ratio which is a value obtained by
dividing an air volume value obtained for each number of blades by
an air volume value persisting in the case where the number of
blades is 27. As can be seen from FIG. 8, the largest air volume
was obtained when n=27, and in the range of 23.ltoreq.n.ltoreq.30
in terms of the number of blades 22, a very large drop in the air
volume was not noted in comparison with n=27. In the range of
n<23, since the number of the blades 22, n, is small relative to
the fan outer diameter d2, the distance between the adjacent blades
22 becomes large in the vicinity of the outer diameter portion of
the centrifugal fan 20. For this reason, the air flow through the
blades 22 becomes disturbed, and the air volume declines.
[0077] Meanwhile, in the range of n>30, since the number of the
blades 22, n, is large relative to the fan outer diameter d2, the
interval between the adjacent blades 22 becomes narrow in the
vicinity of the inside diameter portion of the centrifugal fan 20.
For this reason, air becomes difficult to flow in between the
blades 22, so that the air volume declines. For the reasons
described above, with the centrifugal fan 20 whose outer diameter
d2 is in the range of 45 mm.ltoreq.d2.ltoreq.50 mm, if the number
of the blades 22, n, is set in the range of 23.ltoreq.n.ltoreq.30,
it becomes possible to lower the noise while securing a sufficient
air volume. More preferably, if the number of the blades 22, n, is
set in the range of 25.ltoreq.n.ltoreq.28, it becomes possible to
further lower the noise while securing a sufficient air volume.
Furthermore, if n=27, since it becomes possible to lower the noise
most while securing a sufficient air volume, the number of the
blades 22 is set to 27 in this embodiment.
[0078] Next, a description will be given of the reason for setting
.alpha.1=40.degree., .alpha.2=5.degree.. The inventors examined
changes in the noise and air volume by varying the angles of
.alpha.1 and .alpha.2. As a result, it was found that it is
possible to obtain a sufficient air volume and realize low noise in
the ranges of 30.degree..ltoreq..alpha.1.ltoreq.50.degree. and
0.degree..ltoreq..alpha.- 2.ltoreq.10.degree..
[0079] The reason for this is that if .alpha.1 is less than
30.degree., the flow velocity of air becomes fast in the vicinity
of the outer peripheral edge of the centrifugal fan 20, which
causes noise, whereas if .alpha.1 exceeds 50.degree., the flow
velocity of air in the vicinity of the outer peripheral edge of the
centrifugal fan 20 becomes slow to the contrary, and a sufficient
air volume cannot be obtained. Also, if .alpha.2 is less than
0.degree. or exceeds 10.degree., a large stress easily occurs at
the root of the van 22, or turbulence easily occurs, so that it is
not desirable. If .alpha.2 is in the range of
0.degree..ltoreq..alpha.2.ltoreq.10.degree., the turbulence can be
suppressed.
[0080] Furthermore, it was found that in the ranges of
35.degree..ltoreq..alpha.1.ltoreq.40.degree. and
2.5.degree..ltoreq..alph- a.2.ltoreq.7.5.degree., it is possible to
obtain a more sufficient air volume and lower noise, and that if
.alpha.1=40.degree. and .alpha.2=5.degree., it is possible to
obtain a largest air volume and reduce the noise to a lowest
level.
[0081] The power tool is not limited to the above-described
embodiment, and various modifications and improvements are possible
within the scope of the claims. For example, the centrifugal fan 20
in terms of its configuration may be formed such that, as in a
centrifugal fan 120 shown in FIG. 9, a fan body 121 is not
disk-shaped, but tapered in such a manner as to be inclined in an
opposite direction to the direction in which its blades 122 are
projectingly provided. In addition, although the ridge line from
the substantially intermediate position C to the outer peripheral
edge of the blade 22 has been described as being rectilinear, the
invention is not limited to the same, and the ridge line may be
formed in the shape of a circular arc or the like as in the
centrifugal fan 120. Furthermore, the power tool is not limited to
a hammer drill and a disc grinder, and is applicable to a cutter, a
screwdriver, and the like.
* * * * *