U.S. patent application number 11/679710 was filed with the patent office on 2007-07-26 for radially anisotropic ring magnets and method of manufacture.
Invention is credited to Mitsuo Kawabata, Takehisa Minowa, Koji SATO.
Application Number | 20070171017 11/679710 |
Document ID | / |
Family ID | 31972641 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070171017 |
Kind Code |
A1 |
SATO; Koji ; et al. |
July 26, 2007 |
RADIALLY ANISOTROPIC RING MAGNETS AND METHOD OF MANUFACTURE
Abstract
A radially anisotropic ring magnet endowed with good magnetic
characteristics and having throughout the magnet an angle of 80 to
100.degree. between a center axis thereof and a radial anisotropy
imparting direction is manufactured by a pressing operation.
Inventors: |
SATO; Koji; (Takefu-shi,
JP) ; Kawabata; Mitsuo; (Takefu-shi, JP) ;
Minowa; Takehisa; (Takefu-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
31972641 |
Appl. No.: |
11/679710 |
Filed: |
February 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10526012 |
Feb 25, 2005 |
7201809 |
|
|
PCT/JP03/10844 |
Aug 27, 2003 |
|
|
|
11679710 |
Feb 27, 2007 |
|
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Current U.S.
Class: |
335/296 |
Current CPC
Class: |
H02K 15/03 20130101;
H01F 7/021 20130101; H01F 41/028 20130101 |
Class at
Publication: |
335/296 |
International
Class: |
H01F 3/00 20060101
H01F003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2002 |
JP |
2002-250657 |
Claims
1. A radially anisotropic ring magnet characterized by having
throughout the magnet an angle of 80 to 100.degree. between a
center axis thereof and a radial anisotropy imparting
direction.
2. The radially anisotropic ring magnet of claim 1 which is
characterized by having, on a plane perpendicular to the center
axis thereof, a magnet powder average degree of orientation with
respect to the radial direction of at least 80%.
3. The radially anisotropic ring magnet of claim 1 or 2 which is
characterized by having a length in the direction of the center
axis and an inside diameter such that the length divided by the
inside diameter is at least 0.5.
Description
[0001] This application is a Divisional of co-pending application
Ser. No. 10/526,012 filed on Feb. 25, 2005 and for which priority
is claimed under 35 U.S.C. .sctn. 120. application Ser. No.
10/526,012 is the national phase of PCT International Application
No. PCT/JP2003/010844 filed on Aug. 27, 2003 under 35 U.S.C. .sctn.
371. The entire contents of each of the above-identified
applications are hereby incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to radially anisotropic ring
magnets and to a method of manufacturing such magnets.
BACKGROUND ART
[0003] Anisotropic magnets produced by milling crystalline,
magnetically anisotropic materials such as ferrites or rare-earth
alloys and pressing the milled material in a specific magnetic
field are widely used in speakers, motors, measuring instruments
and other electrical devices. Of these, because magnets with
anisotropy in the radial direction in particular are endowed with
excellent magnetic properties, are freely magnetizable and require
no reinforcement to fix the magnet in place as in the case of
segment magnets, they are used in AC servomotors, DC brushless
motors and other related applications. The trend in recent years
toward higher motor performance has brought with it a demand for
elongated radially anisotropic magnets.
[0004] Magnets having a radial orientation are manufactured by
vertical pressing in a vertical magnetic field or by backward
extrusion. Vertical pressing in a vertical magnetic field is
characterized by applying opposing magnetic fields through the core
of a mold in the pressing direction so as to obtain a radial
orientation. That is, as shown in FIG. 1, a magnet powder 8 packed
into a mold cavity is radially oriented by means of a magnetic
circuit in which magnetic fields generated by orienting magnetic
field-generating coils 2 are applied toward each other through
cores 4 and 5, pass from the cores through a die 3, and circulate
back through the press frame 1. Also shown in FIG. 1 are a top
punch 6 and a bottom punch 7.
[0005] Thus, in this vertical magnetic field-generating
vertical-compacting press, the magnetic fields generated by the
coils create a magnetic path consisting of the cores, the die and
the press frame. To reduce magnetic field leakage loss, a
ferromagnet, and primarily a ferrous metal, is used as the material
making up the portions of the press that form the magnetic path.
The strength of the magnet powder-orienting magnetic field is set
by the following parameters. The core diameter (magnet powder
packing inside diameter) is represented below as B, the die
diameter (magnet powder packing outside diameter) as A, and the
magnet powder packing height as L. Magnetic fluxes which have
passed through the top and bottom cores meet from opposite
directions at the core center and move on into the die. The amount
of magnetic flux that passes through the core is determined by the
saturation flux density of the core. The saturation magnetic flux
density in an iron core is about 20 kG. Therefore, the strength of
the orienting magnetic field at the magnet powder packing inside
and outside diameters is obtained by dividing the magnetic flux
which has passed through the top and bottom cores by, respectively,
the inside surface area and outside surface area of the region in
which the magnet powder is packed, as follows:
2.pi.(B/2).sup.220/(.pi.BL)=10B/L (inner periphery);
2.pi.(B/2).sup.220/(.pi.AL)=10B.sup.2/(AL) (outer periphery).
Because the magnetic field is smaller at the outer periphery than
at the inner periphery, to obtain good orientation in all areas of
the packed magnet powder, a magnetic field of at least 10 kOe is
required at the outer periphery. As a result, 10B.sup.2/(AL)=10,
and so L=B.sup.2/A. Given that the height of the powder compact is
about one-half the height of the packed powder and is reduced
further during sintering to about 80%, the magnet ultimately
obtained has a very small height. Because core saturation
determines in this way the strength of the orienting magnetic
field, the size (i.e., height) of the magnet that can be oriented
is dependent on the core shape. Manufacturing cylindrical magnets
that are elongated in the axial direction has thus been difficult.
In particular, it has been possible to manufacture small-diameter
cylindrical magnets only to very short lengths.
[0006] The backward extrusion process for manufacturing radially
oriented magnets is not conclusive to the production of low-cost
magnets because it requires the use of large equipment and has a
poor yield.
[0007] Thus, regardless of which method is used, radially
anisotropic magnets are difficult to manufacture. The inability to
achieve the low-cost, large-volume production of such magnets has
in turn made motors that use radially anisotropic magnets very
expensive to manufacture.
[0008] Recently, owing to a strong desire to manufacturers for
lower material and assembly costs, there has been an urgent need to
improve the productivity and ease of assembly for radially
anisotropic ring magnets as well. On top of this, product
miniaturization and labor-saving trends have also created a desire
for higher magnet performance. It is believed that elongated
radially anisotropic ring magnets can satisfy such requirements by
manufacturers. Here, "elongated" is used to refer to ring magnets
whose length is greater than the inside diameter.
[0009] When such a magnet is achieved by stacking a plurality of
short magnets, a number of problems arise. That is, the magnet and
the motor core are bonded together with an adhesive or by the
magnetic forces of attraction between the magnet and the
ferromagnetic motor core. However, when the adhesive fails, because
the force of attraction between the magnets is greater than the
force of attraction between the magnets and the core, the north
poles and south poles on adjacent magnets bond to each other. As a
result, the motor ceases to function. Moreover, even when the
adhesive has not failed, the forces that try to pull the magnetic
north and south poles toward each other create shear stresses on
the adhesive that encourage it to fail. By contrast, in a one-piece
magnet, such forces do not arise. Even should the adhesive happen
to fail, because the magnet and the ferromagnetic motor core are
mutually attracted by magnetic forces, they do not separate.
[0010] Radially anisotropic ring magnets are manufactured by
vertical pressing in a vertical magnetic field as shown in FIG. 1,
yet this conventional process is only capable of producing short
magnets. A method for producing radial magnets which are elongated
bodies of integral construction is disclosed in JP-A 2-281721.
however, this prior-art publication describes a multi-stage molding
process in which a starting powder that has been filled into a die
cavity is magnetically oriented and pressed to form a compact. The
compact is transferred to a non-magnetic portion of the die, and
the cavity in the magnetic portion of the die that opens up as a
result is filled with more starting powder, which is then pressed.
The resulting compact is likewise transferred downward. Powder feed
and pressing are repeated a desired number of times in this way to
obtain an overall compact having a large dimension L in the axial
direction of the ring (referred to hereinafter as the
"length").
[0011] Radially anisotropic ring magnets of substantial length can
indeed be manufactured by multi-stage molding. However, this
process involves repeatedly feeding and pressing powder, causing
joints to form in the powder compact. In addition, the long molding
time required to produce a single multilayer powder compact makes
such a process unsuitable for mass production. Moreover, the load
applied during pressing of the compact is constant, and so sintered
bodies obtained from the resulting compacts of uniform density tend
to develop cracks at the joints in the powder compact. JP-A
10-55929 discloses a way to reduce crack formation at joints in the
powder compact by setting the density of the compact during
multi-stage molding to a value of at least 3.1 g/cm.sup.3 in the
case of Nd--Fe--B-based magnets, and carrying out a final pressing
operation (the compact obtained by final pressing being called
herein the "final compact") such as to result in a compact density
at least 0.2 g/cm.sup.3 higher than the density of the compacts
obtained up to that point (referred to herein as "preliminary
compacts").
[0012] However, this method requires strict pressure control.
Moreover, because the condition of the magnet powder varies
considerably depending on the particle size and particle size
distribution of the magnet powder and the type and amount of
binder, the optimal pressure differs each time, making the pressing
conditions difficult to set. In addition, if the preliminary
compacts have a low density, they are subject to the influence of
the magnetic field during the second and subsequent pressing
operations, resulting in poor magnetic properties. If the final
compact has a low density, cracks form at the joints. On the other
hand, a final compact with too high a density will result in
disruption of the orientation during final pressing. It is thus
exceedingly difficult to manufacture by the foregoing process
elongated radially anisotropic ring magnets in such a way as to
achieve both good magnetic characteristics and a good yield.
DISCLOSURE OF THE INVENTION
[0013] One object of the present invention is to provide radially
anisotropic ring magnets which are endowed with good magnetic
characteristics. Another object of the invention is to provide a
method of manufacturing such radially anisotropic ring magnets by
vertical pressing in a horizontal magnetic field.
[0014] Accordingly, the invention provides the following radially
anisotropic ring magnet and method of manufacture. [0015] (1) A
radially anisotropic ring magnet characterized by having throughout
the magnet an angle of 80 to 100.degree. between a center axis
thereof and a radial anisotropy imparting direction. [0016] (2) The
radially anisotropic ring magnet of (1) above which is
characterized by having, on a plane perpendicular to the center
axis thereof, a magnet powder average degree of orientation with
respect to the radial direction of at least 80%. [0017] (3) The
radially anisotropic ring magnet of (1) or (2) above which is
characterized by having a length in the direction of the center
axis and an inside diameter such that the length divided by the
inside diameter is at least 0.5. [0018] (4) A method of
manufacturing radially anisotropic ring magnets in which a magnet
powder packed into a cavity in a cylindrical magnet-forming mold
having a core composed at least in part of a ferromagnetic material
with a saturation magnetic flux density of at least 5 kG is pressed
under the application of an orienting magnetic field by a
horizontal magnetic field vertical compacting process; the method
being characterized by carrying out at least one of the following
operations (i) to (v):
[0019] (i) rotate the magnet powder a given angle in the
circumferential direction of the mold during application of the
magnetic field.
[0020] (ii) rotate the magnet powder a given angle in the
circumferential direction of the mold following application of the
magnetic field, then again apply a magnetic field,
[0021] (iii) rotate a magnetic field-generating coil a given angle
in the circumferential direction of the mold with respect to the
magnet powder during application of the magnetic field,
[0022] (iv) rotate a magnetic field-generating coil a given angle
in the circumferential direction of the mold with respect to the
magnet powder following application of the magnetic field, then
again apply a magnetic field,
[0023] (v) use a plurality of coil pairs to first apply a magnetic
field with one coil pair, then apply a magnetic field with the
other coil pair
[0024] so as to apply to the magnet a magnetic field from a
plurality of directions rather than one direction and thereby
manufacture in a pressing operation a radially anisotropic ring
magnet having throughout the magnet an angle of 80 to 100.degree.
between a center axis thereof and a radial anisotropy imparting
direction. [0025] (5) The method of manufacturing radially
anisotropic ring magnets according to (4) above which is
characterized in that, if the packed magnet powder is rotated, such
rotation is effected by rotating at least the core, die or punch of
the mold in the circumferential direction thereof. [0026] (6) The
method of manufacturing radially anisotropic ring magnets according
to (4) above which is characterized in that, if the packed magnet
powder is rotated after application of a magnetic field, the
ferromagnetic core and the magnet powder have remanent
magnetization values of at least 50 G and the magnet powder is
rotated by rotating the core in the circumferential direction.
[0027] (7) The method of manufacturing radially anisotropic ring
magnets according to any one of (4) to (6) above which is
characterized in that the magnetic field generated during vertical
pressing within a horizontal magnetic field is from 0.5 to 10 kOe.
[0028] (8) The method of manufacturing radially anisotropic ring
magnets according to any one of (4) to (7) above which is
characterized in that the magnetic field generated by the
horizontal magnetic field-generating vertical-compacting press just
before or during pressing is from 0.5 to 3 kOe. [0029] (9) The
method of manufacturing radially anisotropic ring magnets according
to any one of (4) to (8) above which is characterized in that,
after applying a magnetic field one or more times, the magnet
powder is rotated 60 to 120.degree.+n.times.180.degree. (where n is
an integer .gtoreq.0) under the application of a coil-generated
magnetic field of at least 0 but less than 0.5 kOe, the latter
magnetic field being from 1/20 to 1/3 as large as the magnetic
field previously applied, and the magnet powder is pressed during
or after said application.
[0030] The present invention enables the low-cost, large-volume
supply of radially anisotropic ring magnets which have an excellent
performance and are easy to work with in assembly operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 shows a prior-art vertical magnetic field-generating
vertical-compacting press used to manufacture radially anisotropic
cylindrical magnets. FIG. 1(a) is a longisectional view, and FIG.
1(b) is a cross-sectional view taken along line A-A' in FIG.
1(a).
[0032] FIG. 2 is a diagram showing the angles of various radial
anisotropy imparting directions with respect to the center axis of
a ring magnet.
[0033] FIG. 3 shows an example of a horizontal magnetic
field-generating vertical-compacting press used to manufacture
cylindrical magnets. FIG. 3(a) is a plan view, and FIG. 3(b) is a
longisectional view.
[0034] FIG. 4 schematically shows the magnetic lines of force when
a magnetic field is generated by a horizontal magnetic
field-generating vertical-compacting press during the production of
a cylindrical magnet. FIG. 4(a) shows a press according to the
present invention, and FIG. 4(b) shows a prior-art press.
[0035] FIG. 5 shows a rotary horizontal magnetic field-generating
vertical-compacting press used for manufacturing cylindrical
magnets.
[0036] FIG. 6 is a schematic view of a cylindrical magnet being
magnetized with a magnetizer.
[0037] FIG. 7 is a plan view of a three-phase motor assembled from
a cylindrical magnet subjected to multipolar magnetization in a
sextupole configuration and nine stator teeth.
[0038] FIG. 8 shows the surface magnetic flux density when a
Nd--Fe--B-based cylindrical magnet manufactured in accordance with
the present invention using a horizontal magnetic field-generating
vertical-compacting press was subjected to sextupolar
magnetization.
[0039] FIG. 9 shows the surface magnetic flux density when a
Nd--Fe--B-based cylindrical magnet manufactured using a prior-art
horizontal magnetic field-generating vertical-compacting press was
subjected to sextupolar magnetization.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] The invention is described more fully below. The description
that follows relates primarily to Nd--Fe--B-based cylindrical
sintered magnets. However, it is not limited only to
Nd--Fe--B-based magnets, and applies as well the manufacture of
ferrite magnets, Sm--Co-based rare-earth magnets and various types
of bonded magnets.
[0041] The radially anisotropic ring magnets of the invention are
preferably manufactured by carrying out a pressing operation in a
magnetic field that is shifted just prior to pressing. Moreover, as
shown in FIG. 2, the inventive ring magnet has throughout it an
angle of 80 to 100.degree. between the center axis of the magnet
and a radial anisotropy imparting direction. Preferably, the
inventive magnet has, on a plane perpendicular to the center axis
thereof, a magnet powder average degree of orientation with respect
to the radial direction of at least 80%. It is also preferable for
the inventive magnet to have a length in the direction of the
center axis and an inside diameter such that the length divided by
the inside diameter is at least 0.5.
[0042] As the angle between the center axis of the ring magnet and
the radial anisotropy imparting direction departs further from a
range of 80 to 100.degree., only the cosine component of the
magnetic flux generated by the radially anisotropic ring magnet
ends up contributing to the rotational force in a motor, resulting
in a smaller motor torque. Hence, the angle between the center axis
of the ring magnet and the radial anisotropy imparting direction
must be within a range of 80 to 100.degree.. In addition, most
practical applications for radially anisotropic ring magnets are
electric motors such as AC servomotors and DC brush motors. When a
radially anisotropic ring magnet is used in a motor, skew is
imparted to the magnet or stator to counteract cogging. If the
angle between the center axis of the ring magnet and the radial
anisotropy imparting direction falls outside a range of 80 to
100.degree., the effectiveness of skewing diminishes. This tendency
is especially pronounced in cases where the angle between the
center axis of the ring magnet and the radial anisotropy imparting
direction departs substantially from 80 to 100.degree. at the ends
of the radially anisotropic ring magnet in the length direction
thereof. When skew is imparted, there are places on the magnet
where the ends and the center portion are of opposite polarity; the
ratio of magnetic flux at the north and south poles changes
linearly and gradually, thus reducing cogging. However, at the ends
of the magnet, the angle between the center axis of the ring magnet
and the radial anisotropy imparting direction departs substantially
from 80 to 100.degree.; hence, the magnetic flux at the ends which
have a polarity opposite to the polarity at the center of the
magnet becomes small.
[0043] Magnets in which the departure from an angle of 100.degree.
at the ends thereof is particularly large arise in the following
manufacturing process. Radially anisotropic ring magnets have
hitherto been produced by vertical pressing in a vertical magnetic
field as shown in FIG. 1. However, as noted above, conventional
methods are only able to produce short ring magnets. In ring
magnets produced by a multi-stage compacting process, separation
occurs at joints within the resulting magnet, and disturbances in
the magnetic poles arise. Moreover, the magnet may break up into
sections; because surface treatment at the separation planes is
impossible, this leads to corrosion. When orientation is carried
out using the vertical magnetic field-generating
vertical-compacting press shown in FIG. 1, if a magnetic field
stronger than the saturation magnetization of the core is applied
in order to achieve a greater magnet length, following core
saturation, the lines of magnetic force from the top punch magnetic
field-generating coil and the bottom punch magnetic
field-generating coil meet from opposite directions without passing
through the core, and generate a magnetic field in the radial
direction. However, the angle between the center axis of the core
and the radial anisotropy imparting direction departs significantly
from 80 to 100.degree., a tendency which increases near the top and
bottom punches. As a result, the angle between the center axis of
the ring magnet and the radial anisotropy imparting direction
becomes small at the ends of the magnet, making this process
unsuitable for the manufacture of radially anisotropic ring
magnets.
[0044] It is therefore critical for the ring magnet to have
throughout an angle between the center axis thereof and the radial
anisotropy imparting direction of from 80.degree. to
100.degree..
[0045] The degree of orientation f of a magnet is computed as
follows.
f=Br/[Is.times.{.rho./.rho..sub.0.times.(1-.alpha.)}.sup.2/3] In
the above formula, Br represents the remanent flux density, Is
stands for the saturation magnetization, .rho. is the density of
the sintered body, .rho..sub.0 is the theoretical density, and
.alpha. is the volumetric ratio of the nonmagnetic phase.
[0046] At a low degree of orientation, the magnetic flux generated
by the magnet is low and the motor torque is small. Moreover, the
magnetizability may suffer. Because motor magnetization is often
carried out using the motor rotor, a decline in magnetizability can
be a serious problem. Accordingly, in the radially anisotropic ring
magnets of the invention, the magnet powder has an average degree
of orientation of preferably at least 80%, and most preferably 80
to 100%.
[0047] For ease of handling in assembly operations, it is
preferable that the ring magnet have a length in the direction of
the center axis and an inside diameter such that the length divided
by the inside diameter (length/inside diameter) is at least 0.5,
and preferably from 0.5 to 50.
[0048] Such radially anisotropic ring magnets are preferably
manufactured using the process of vertical pressing in a horizontal
magnetic field described below. FIG. 3 shows a horizontal magnetic
field-generating vertical-compressing press for carrying out
orientation in a magnetic field during pressing of a cylindrical
magnet. This diagram illustrates in particular a horizontal
magnetic field-generating vertical-compressing press for making
motor magnets. As in FIG. 1, the diagram shows a press frame 1,
orienting magnetic field-generating coils 2, a die 3, and a core
5a. Also shown are a top punch 6, a bottom punch 7, a packed magnet
powder 8, and pole pieces 9.
[0049] In the practice of the invention, at least part and
preferably all of the core 5a is composed of a ferromagnet having a
saturation magnetic flux density of at least 5 kG, preferably 5 to
24 kG, and most preferably 10 to 24 kG. Examples of suitable core
materials include ferromagnets prepared using ferrous materials,
cobalt-based materials, or alloys thereof.
[0050] By using a ferromagnet having a saturation flux density of
at least 5 kG in the core, when an orienting magnetic field is
applied to the magnet powder, the magnetic flux tries to enter the
ferromagnet perpendicularly, creating lines of magnetic force that
are nearly radial. Thus, as shown in FIG. 4a, the direction of the
magnetic field in the region packed with magnet powder can be made
to approach a radial orientation. By contrast, in the prior art,
the overall core 5b is made of a material which is either
nonmagnetic or has a saturation magnetic flux density comparable to
that of the magnet powder. In this case, as shown in FIG. 4b, the
lines of magnetic force are mutually parallel; in the diagram,
although the lines of force do extend in the radial direction near
the center, toward the top and bottom sides they merely extend in
the direction of the orienting magnetic field generated by the
coils. Even when the core is made of a ferromagnet, if it has a
saturation flux density of less than 5 kG, it is readily saturated.
In such cases, in spite of the use of a ferromagnetic core, the
magnetic field will be in a state close that shown in FIG. 4b. In
addition, at a saturation flux density of less than 5 kG, the core
has the same saturation flux density as the packed magnet powder
(saturation flux density of packed magnet powder=saturation flux
density of magnet.times.packing density) and the direction of
magnetic flux within the packed magnet powder and the ferromagnetic
core becomes the same as the direction of the magnetic field
generated by the coils.
[0051] The use of a ferromagnet having a saturation flux density of
at least 5 kG as part of the core provides effects similar to those
described above and is thus acceptable, although it is preferable
for the entire core to be made of a ferromagnet.
[0052] However, simply forming the core material of a ferromagnet
does not in and of itself result in a radial orientation in
directions close to perpendicular to the direction of the orienting
magnetic field generated by the coils. When a ferromagnet is
present in a magnetic field, because the magnetic flux is drawn to
the ferromagnet in such a way as to try to enter the ferromagnet
perpendicularly, the magnetic flux density rises at surfaces of the
ferromagnet lying in the direction of the magnetic field and falls
at surfaces perpendicular to the magnetic field. Therefore, when a
ferromagnet core is placed within the mold, the packed magnet
powder is well-oriented by the strong magnetic field at surfaces of
the ferromagnet core which are parallel to the direction of the
magnetic field, but is not oriented very much at surfaces of the
core perpendicular to the magnetic field. To compensate for this,
the magnet powder is rotated relative to the magnetic field
generated by the coils, either during or after application of the
field, so as to place incompletely oriented areas in positions that
are parallel to the magnetic field and thus subject to a higher
flux density in order to reorient them. This enables a good magnet
to be achieved. Relative rotation of the magnet powder, either
after application of the magnetic field or in a magnetic field that
is no more than one-third the initially applied field, is even more
preferable. Although the areas of the magnet powder that are
initially oriented in this way may be put in positions that are
perpendicular to the applied magnetic field in subsequent
orientation, because the magnetic flux density at such positions is
small, the good initial orientation is not disturbed to any
significant degree.
[0053] The method of rotating the magnet powder relative to the
magnetic field generated by the coil involves carrying out at least
one of operations (i) to (v) below, either once or a plurality of
times after changing the magnetic field each time: [0054] (i)
rotate the magnet powder a given angle in the circumferential
direction of the mold during application of the magnetic field;
[0055] (ii) rotate the magnet powder a given angle in the
circumferential direction of the mold following application of the
magnetic field, then again apply a magnetic field; [0056] (iii)
rotate a magnetic field-generating coil a given angle in the
circumferential direction of the mold with respect to the magnet
powder during application of the magnetic field; [0057] (iv) rotate
a magnetic field-generating coil a given angle in the
circumferential direction of the mold with respect to the magnet
powder following application of the magnetic field, then again
apply a magnetic field; [0058] (v) use a plurality of coil pairs to
first apply a magnetic field with one coil pair, then apply a
magnetic field with the other pair of coil.
[0059] So long as the packed magnet powder is rotatable relative to
the direction of the coil-generated magnetic field in the manner
shown in FIG. 5, such rotation may be effected by rotating the
orienting field-generating coils 2, the core 5a, the die 3 or the
top and bottom punches 6 and 7. In those cases in particular where
the packed magnet powder is rotated following application of the
magnetic field, if the ferromagnetic core and the magnet powder are
provided. with a remanent magnetization of at least 50 G, and
preferably at least 100 G, forces of magnetic attraction will arise
between the magnet powder and the ferromagnetic core, enabling
rotation of the magnet powder to be effected merely by rotating the
ferromagnetic core.
[0060] Because using a plurality of coil pairs to first apply a
magnetic field in one direction then apply a magnetic field in
another direction is substantially the same as rotating the
magnetic field direction and the magnet powder relative to each
other, this method may also be employed to achieve the same
effect.
[0061] When rotation is carried out before magnetic field
application just prior to the pressing operation, the magnetic
field applied following rotation is small. Thus, apply a large
magnetic field during rotation will prevent the final application
of a magnetic field following rotation from having an observable
effect. For this reason, the strength of the magnetic field applied
during rotation is preferably 0 to 0.5 kOe, and more preferably 0.3
kOe or less. Rotation in the absence of a magnetic field is
typically preferred. Because those sites in the magnet powder which
are disturbed by the application of a magnetic field prior to
rotation are at positions perpendicular to the direction of the
magnetic field prior to rotation, the angle of rotation for
alleviating disturbances at these sites is preferably from 60 to
120.degree.+n+180.degree. (where n is an integer .gtoreq.0), and
more preferably 90.degree.+n.times.180.degree. (where n is an
integer .gtoreq.0).+-.10.degree.. The angle of rotation is
typically 90.degree.+n.times.180.degree. (where n is an integer
.gtoreq.0). If a strong magnetic field is applied prior to
rotation, this results in a large deviation from a radial
orientation in the direction perpendicular to the applied magnetic
field direction. Hence, unless the strength of the magnetic field
applied following rotation also is made larger than in cases where
the magnetic field prior to rotation is weak, the disruption in
orientation is not alleviated. Yet, if the magnetic field applied
following rotation is too strong, the resulting orientation will
deviate from a radial orientation in the direction perpendicular to
the magnetic field direction. Hence, the magnetic field applied
following rotation is preferably from 1/20 to 1/3, and most
preferably from 1/10 to 1/3, as large as the magnetic field applied
before rotation.
[0062] Here, when the magnetic field generated in a horizontal
magnetic field-generating vertical-compacting press is large, the
core 5a in FIG. 4a becomes saturated and assumes a state close to
that shown in FIG. 4b. That is, the orienting magnetic field
imparts a nearly parallel orientation rather than a radial
orientation. Hence, it is preferable for the magnetic field to have
a strength of not more than 10 kOe. When a ferromagnetic core is
used, the magnetic flux concentrates in the core, creating a
magnetic field which, in the vicinity of the core, is larger than
the magnetic field generated by the coils. However, if the
orienting magnetic field is too small, a magnetic field sufficient
for orientation will not be achieved even in the vicinity of the
core. Accordingly, an applied magnetic field strength of at least
0.5 kOe is preferred. As just noted, due to concentration of the
magnetic flux in the vicinity of the ferromagnet, the magnetic
field here becomes larger. Therefore, the phrase "magnetic field
generated by the horizontal magnetic field-generating
vertical-compacting press" refers herein to the magnetic field in
places at a sufficient remove from the ferromagnet, or to magnetic
field values measured in the absence of the ferromagnetic core.
[0063] Rotating the magnet powder relative to the direction of the
magnetic field generated by the coils enables incompletely oriented
areas to be re-oriented by the strong magnetic field in the
magnetic field direction. Although initially oriented areas may end
up in areas perpendicular to the magnetic field at the time of the
subsequent orientation, as has already been explained, because the
magnetic flux density in such areas is low, the good initial
orientation is not disrupted to any significant degree. However, if
the magnetic field generated is relatively large, localized
disruption does sometimes occur. In such cases, just prior to the
pressing operation, by rotating the magnet powder about 90.degree.
relative to the direction of the coil-generated magnetic field
without applying a magnetic field, then applying a magnetic field
smaller than that applied during pressing, preferably a magnetic
field of 0.3 to 3 kOe, and subsequently pressing the powder,
reorientation can be effected only in the magnetic field direction,
enabling a more complete radial orientation to be achieved. If the
magnetic field generated by the horizontal magnetic
field-generating vertical compacting press prior to the pressing
operation exceeds 3 kOe, as noted above, the application of a
magnetic field of this size subjects areas that already have a good
orientation to an unnecessary magnetic field, which is undesirable.
On the other hand, a magnetic field generated by the press which is
less than 0.5 kOe is too weak to improve orientation. Hence, a
magnetic field within a range of 0.5 to 3 kOe is preferred.
[0064] Moreover, in working the present invention, it is desirable
to impart orientation a number of times. Decreasing the magnetic
field strength in a plurality of stages is advantageous. It is
especially preferable to impart orientation three times. Carrying
out such orientation up to five times is advantageous for achieving
good magnetic characteristics.
[0065] Aside from the above-described conditions, the radially
anisotropic ring magnet of the invention can be obtained by an
otherwise ordinary vertical pressing process in a horizontal
magnetic field which includes applying an orienting magnetic field
to the magnet powder, compacting the powder in a pressure range of
50 to 2,000 kg/cm.sup.2, and firing the pressed compact in an inert
gas at 1,000 to 1,200.degree. C. The sintered body is then
subjected to such operations as aging treatment and machining to
give a sintered magnet. The invention may enable magnets of the
required axial length to be obtained by a single powder feeding
operation and a single pressing operation, although several
pressing operation may be employed.
[0066] The magnet powder used in the process of the invention is
not subject to any particular limitation. The inventive process is
especially well-suited to the manufacture of Nd--Fe--B-based
cylindrical magnets, but can also be effectively used to
manufacture ferrite magnets, Sm--Co-based rare-earth magnets and
various types of bonded magnets. In each of these cases, pressing
is preferably carried out using an alloy powder having an average
particle size of 0.1 to 10 .mu.m, and especially 1 to 8 .mu.m.
EXAMPLE
[0067] Examples of the invention and comparative examples are given
below to illustrate the invention, and are not intended to limit
the scope thereof.
Examples and Comparative Examples
[0068] Neodymium, dysprosium, iron, cobalt and M (where M stands
for aluminum, silicon or copper), each having a purity of 99.7 wt
%, and boron of 99.5 wt % purity were melted and cast in a vacuum
melting furnace to produce ingots composed of a
Nd.sub.2Fe.sub.14B-based magnet alloy
(Nd.sub.31.5Dy.sub.2Fe.sub.62Co.sub.3B.sub.1Cu.sub.0.2Al.sub.0.3Si.-
sub.1; subscripts indicated percent by weight). The ingot was
crushed with a jaw crusher, then reduced to an average particle
size of 3.5 .mu.m in a jet mill using a stream of nitrogen. The
resulting powder was molded in the horizontal magnetic
field-generating vertical-compacting press shown in FIG. 3 about a
ferromagnet core (S50C) having a saturation magnetic flux density
of 20 kG.
[0069] In Example 1, the magnet powder was oriented in a
coil-generated magnetic field of 4 kOe, following which the coils
were rotated 90.degree. and the powder was compacted under an
orienting magnetic field of 1 kOe and a pressure of 500
kgf/cm.sup.3. The mold used at this time had an outside diameter of
30 mm, an inside diameter of 17 mm, and a cavity 60 mm deep. The
packing density of the magnet powder was 33%. The powder compact
was sintered in argon at 1,090.degree. C. for one hour, following
which the sintered body was heat-treated at 490.degree. C. for one
hour. The resulting radial magnet had an outside diameter of 26 mm,
an inside diameter of 19 mm and a length of 27 mm (length/inside
diameter=1.4). A sample measuring 2 mm on a side was cut in the
magnetic field direction from the center portion of the magnet, and
the magnetic properties of the sample were measured using a
vibrating sample magnetometer (VSM). The results were as follows:
remanent flux density (Br)=12.1 kG, coercivity (iHc)=15 kOe, degree
of orientation=89%. The angle formed between the center axis of the
ring magnet and the radial anisotropy imparting direction was
87.degree. at the longitudinal center, 91.degree. at 3 mm from the
top face and 89.degree. at 3 mm from the bottom face of the
magnet.
[0070] In Example 2, the same type of mold and magnet powder were
used as in Example 1. The packing density of the magnet powder was
32%. The powder was oriented in a coil-generated magnetic field of
4 kOe, following which the side, core and punches were rotated
90.degree. and the powder was compacted under an orienting magnetic
field of 1.5 kOe and a pressure of 500 kgf/cm.sup.2. The powder
compact was sintered in argon at 1,090.degree. C. for one hour,
then the sintered body was heat-treated at 490.degree. C. for one
hour. The resulting radial magnet had an outside diameter of 26 mm,
an inside diameter of 19 mm and a length of 27 mm (length/inside
diameter=1.4). A sample measuring 2 mm on a side was cut in the
magnetic field direction from the center portion of the magnet, and
the magnetic properties were measured with a VSM. The results were
as follows: Br=12.0 kG, iHc=15 kOe, degree of orientation=88%.
[0071] In Example 3, the same type of mold and magnet powder were
used as in Example 1. The packing density of the magnet powder was
32%. The powder was oriented in a coil-generated magnetic field of
4.5 kOe, following which the core, which had a remanent
magnetization at the tip of 0.2 kG, was rotated 90.degree.. The
remanent magnetization of the magnet powder at this time was 600 G.
The powder was compacted under an orienting magnet field of 0.7 kOe
and a pressure of 500 kgf/cm.sup.2. The powder compact was sintered
in argon at 1,090.degree. C. for one hour, following which the
sintered body was heat-treated at 490.degree. C. for one hour. The
resulting radial magnet had an outside diameter of 26 mm, an inside
diameter of 19 mm and a length of 27 mm (length/inside
diameter=1.4). A sample measuring 2 mm on a side was cut in the
magnetic field direction from the center portion of the magnet, and
the magnetic properties were measured with a VSM. The results were
as follows: Br=11.9 kG, iHc=15 kOe, degree of orientation=87%.
[0072] The magnets obtained in Examples 1, 2 and 3 were
subsequently machined, giving cylindrical magnets having an outside
diameter of 25 mm, an inside diameter of 20 mm and a length of 25
mm.
[0073] These cylindrical magnets were skew magnetized (sextupole
configuration, 20.degree.) using the magnetizer shown in FIG. 6. In
each case, a motor was then built in which the resulting magnetized
magnet was installed within a stator of the same height as the
magnet and having the construction shown in FIG. 7.
[0074] FIGS. 6 and 7 show a cylindrical magnet 11, a magnetizer 20,
magnetizer pole teeth 21, a magnetizer coil 22, a three-phase motor
30, stator teeth 31, and a coil 32.
[0075] The motor obtained in Example 1 was rotated at 5,000 rpm and
the induced electromotive force was measured. In addition, the
degree of torque ripple with rotation of the same motor at 5 rpm
was measured with a torque transducer. Similar measurements were
carried out in the other examples. Table 1 shows the maximum
absolute value for the induced electromotive force in each example,
and also the difference between the maximum and minimum torque
ripple.
[0076] In Example 4, using the same horizontal magnetic
field-generating vertical-compacting press in which the coils can
be rotated as in Example 1, orientation was carried out with
90.degree. rotation in a 20 kOe magnetic field. This was followed
by 90.degree. rotation in the absence of a magnetic field, after
which the powder was compacted under a pressure of 500 kgf/cm.sup.2
while subjecting the powder to orientation once again in a 1.5 kOe
magnetic field. The powder compact was sintered in argon at
1,090.degree. C. for one hour, following which the sintered body
was heat-treated at 490.degree. C. for one hour. The resulting
radial magnet had an outside diameter of 26 mm, an inside diameter
of 19 mm and a length of 27 mm (length/inside diameter=1.4). A
sample measuring 2 mm on a side was cut in the magnetic field
direction from the center portion of the magnet, and the magnetic
properties were measured with a VSM. The results were as follows:
Br=12.0 kG, iHc=15 kOe, degree of orientation=88%. The magnet was
machined to the same shape as in Example 1, and the motor
characteristics were measured.
[0077] In Comparative Example 1, use was made of a vertical
magnetic field-generating vertical-compacting mold. The mold shape
and core material were the same as in Example 1, but the die
material was SKD11 having a saturation magnetic flux density of 15
kG. The packing density of the magnet powder was 33%, and opposing
30 kOe pulsed magnetic fields were applied from top and bottom
coils. The powder was subsequently compacted under a pressure of
500 kgf/cm.sup.2. The powder compact was sintered in argon at
1,090.degree. C. for one hour, then heat-treated at 490.degree. C.
for one hour. The resulting radial magnet had an outside diameter
of 27 mm and an inside diameter of 19.5 mm at the top and bottom
thereof, an outside diameter of 26 mm and an inside diameter of
18.7 mm at the center, and a length of 27 mm. The average value for
the length/inside diameter ratio was 1.35. A sample measuring 2 mm
on a side was cut in the magnetic field direction from the center
portion of the magnet, and the magnetic properties were measured
with a VSM. The results were as follows: Br=11.8 kG, iHc=15 kOe,
degree of orientation=87%. At a distance of 3 mm from the top and
bottom faces of the magnet, the angle formed between the center
axis of the ring magnet and the radial anisotropy imparting
direction was 120.degree. at 3 mm from the top face and 60.degree.
at 3 mm from the bottom face of the magnet. The magnet was machined
to the same shape as in Example 1, and the same motor
characteristics of the magnet were measured as in Example 1.
[0078] In Comparative Example 2, use was made of a vertical
magnetic field-generating vertical-compacting mold. The mold shape
and core material were the same as in Example 1, but the die
material was SKD11 having a saturation magnetic flux density of 15
kG. The packing density of the magnet powder was 28%, and opposing
3 kOe pulsed magnetic fields were applied from top and bottom
coils. The powder was subsequently compacted under a pressure of
300 kgf/cm.sup.2. The powder compact was sintered in argon at
1,090.degree. C. for one hour, then heat-treated at 490.degree. C.
for one hour. The resulting radial magnet had an outside diameter
of 25.8 mm, an inside diameter of 19.5 mm, and a length of 27 mm.
The average value for the length/inside diameter ratio was 1.4. A
sample measuring 2 mm on a side was cut in the magnetic field
direction from the center portion of the magnet, and the magnetic
properties were measured with a VSM. The results were as follows:
Br=9.5 kG, iHc=15 kOe, degree of orientation=70%. The magnet was
machined to the same shape as in Example 1, and the motor
characteristics were measured.
[0079] In Comparative Example 3, the magnet powder was oriented in
a 4 kOe magnetic field under the same compacting conditions as in
Example 1, but the subsequent procedure differed. That is, the
magnet powder was then compacted under a pressure of 500
kgf/cm.sup.2 in the magnetic field in this state without rotation.
Next, the powder compact was sintered in argon at 1,090.degree. C.
for one hour, after which it was heat-treated at 490.degree. C. for
one hour. The resulting radial magnet had an outside diameter of 26
mm, an inside diameter of 19 mm and a length of 27 mm
(length/inside diameter=1.4). A sample measuring 2 mm on a side was
cut in the magnetic field direction from the center portion of the
magnet, and the magnetic properties were measured with a VSM. The
results were as follows: Br=12.3 kG, iHc=15 kOe, degree of
orientation=90%. Separately another sample measuring 2 mm on a side
was cut from the center portion of the magnet in a direction
shifted 90.degree. on a plane perpendicular to the ring center axis
from the magnetic field direction, and the magnetic properties were
measured, with the results: Br=2.5 kG, iHc=15.8 kOe, degree of
orientation=18%. The magnet was machined to the same shape as in
Example 1, and the motor characteristics were measured.
[0080] The results from the examples and comparative examples are
given in Table 1. TABLE-US-00001 TABLE 1 Induced electromotive
force (effective value) Torque ripple (mV/rpm) (mNm) Example 1 15.7
6.7 Example 2 15.8 6.7 Example 3 15.6 6.6 Example 4 15.3 6.5
Comparative Example 1 13.2 8.4 Comparative Example 2 9.5 5.9
Comparative Example 3 11.8 6.3
[0081] It is apparent from Table 1 that the induced electromotive
force, which corresponds to torque, is much larger in the examples
according to the invention than in the comparative examples. This
demonstrates that the method of the invention is an excellent way
to manufacture magnets for motors.
[0082] FIG. 8 shows the surface magnetic flux measured for the
rotor magnet obtained in Example 1 according to the invention, and
FIG. 9 shows the surface magnetic flux measured for the rotor
magnet obtained in Comparative Example 3. In Example 1, each pole
is homogeneous and has a large surface area relative to Comparative
Example 3. Hence, in the example according to the invention, a
large magnetic field can be uniformly generated.
[0083] Through the invention, there can be obtained radially
anisotropic ring magnets which are endowed with good magnetic
characteristics.
* * * * *