U.S. patent number RE34,229 [Application Number 07/426,569] was granted by the patent office on 1993-04-20 for cylindrical permanent magnet and method of manufacturing.
This patent grant is currently assigned to Hitachi Metals, Ltd.. Invention is credited to Noriyuki Noda, Yoshihiro Noguchi, Motoharu Shimizu.
United States Patent |
RE34,229 |
Shimizu , et al. |
April 20, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Cylindrical permanent magnet and method of manufacturing
Abstract
A cylindrical permanent magnet suitable for use as the rotor
magnet of a stepping motor. The magnet is a sintered cylindrical
permanent magnet having a composition expressed by MO.nFe.sub.2
O.sub.3, where M represents Ba, Sr, Pb or mixture thereof, while n
represents a value of 5 to 6. The sintered cylindrical permanent
magnet is provided on its surface with multipolar anisotropy of
more than 8 (eight) magnetic poles. Disclosed also is a method of
producing the cylindrical permanent magnet.
Inventors: |
Shimizu; Motoharu (Kumayaga,
JP), Noda; Noriyuki (Fukaya, JP), Noguchi;
Yoshihiro (Fukaya, JP) |
Assignee: |
Hitachi Metals, Ltd.
(JP)
|
Family
ID: |
27027104 |
Appl.
No.: |
07/426,569 |
Filed: |
October 6, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
446322 |
Dec 2, 1982 |
04547758 |
Oct 15, 1985 |
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Current U.S.
Class: |
335/302;
264/DIG.58; 310/156.45 |
Current CPC
Class: |
H01F
7/021 (20130101); H02K 1/2733 (20130101); H02K
15/03 (20130101) |
Current International
Class: |
H01F
7/02 (20060101); H02K 1/27 (20060101); H02K
15/03 (20060101); H01F 007/02 () |
Field of
Search: |
;335/284,302,303
;252/62.63 ;264/DIG.58 ;310/156 |
References Cited
[Referenced By]
U.S. Patent Documents
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3114715 |
December 1963 |
Brockman et al. |
4057606 |
November 1977 |
Kobayashi et al. |
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Foreign Patent Documents
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50-56594 |
|
May 1975 |
|
JP |
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56-74907 |
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Jun 1981 |
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JP |
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57-37803 |
|
Mar 1982 |
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JP |
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57-128909 |
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Aug 1982 |
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JP |
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57-130407 |
|
Aug 1982 |
|
JP |
|
130407 |
|
Dec 1982 |
|
JP |
|
Primary Examiner: Harris; George
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner
Claims
What is claimed is:
1. A cylindrical permanent magnet, comprising:
a sintered material having a composition expressed by MO.nFe.sub.2
O.sub.3, where M represents Ba, Sr, Pb or a mixture thereof, and n
is a value of 5 to 6,
the magnet being characterized by a surface multipolar anisotropy
of more than eight poles imparted to the cylindrical portion of the
magnet, and
the magnet being further characterized by having an inside diameter
D.sub.1 and an outside diameter D.sub.2 of the cylindrical portion
which satisfy the condition:
where P is the number of poles imparted to the cylindrical portion
of the magnet, and K.sub.1 is a constant defined by
.[.1.5.]..varies..Iadd.1.76.Iaddend..ltoreq.K.sub.1
.ltoreq.2.5.
2. A cylindrical permanent magnet according to claim 1 wherein
D.sub.1 /D.sub.2 .ltoreq.0.65.
3. A method of producing a cylindrical permanent magnet comprising
the steps of forming a cylindrical green body from a material
consisting essentially of MO.nFe.sub.2 O.sub.3, where M represents
Ba, Sr or Pb and n being a value of 5 to 6, and containing 14 to
20% of water, subjecting the green body to multi-polar magnetic
fields to impart a surface multipolar anisotropy of more than 8
(eight) poles, and then sintering the body.
4. A method of producing a cylindrical permanent magnet according
to claim 3, wherein the inside diameter D.sub.1 and the outside
diameter D.sub.2 of the cylindrical portion of the sintered body
are selected to meet the following condition:
where, K.sub.1 representing a constant value not smaller than 1.5,
said sintered body being magnetized at its surface to develop a
multiplicity of magnetic poles.
5. A method of producing a cylindrical permanent magnet according
to claim 4, wherein said constant value K.sub.1 is selected to meet
the condition of 1.5.ltoreq.K.sub.1 .ltoreq.2.5.
Description
BACKGROUND OF THE INVENTION
The present invention broadly relates to a magnet suitable for use
as the rotor magnet of a stepping motor and, more particularly, to
a cylindrical permanent magnet having a surface multipolar
anisotropy.
In order to effect a multipolar magnetization on the surface of a
permanent magnet, it is essential that the permanent magnet has a
high coercive force and that the reversible magnetic permeability
is around 1. To meet these demands, hitherto, ferrite magnets such
as barium ferrite magnet, strontium ferrite magnet and so forth
have been used as the permanent magnet for multipolar
magnetization. Materials of such permanent magnets are
stoichiometrically expressed by the MO.multidot.6Fe.sub.2 O.sub.3,
where M represents an element such as Ba, Sr, Pb or their mixtures
with or without bivalent metal such as Ca. Actually, however, the
MO content is somewhat excessive so that the materials of such
permanent magnets are expressed by MO.multidot.nFe.sub.2 O.sub.3,
where n represents a value of 5 to 6.
Various cylindrical permanent magnets have been proposed and used
hitherto, such as isotropic ferrite magnet, ring anisotropic
ferrite magnet, and plastic magnets formed by dispersing ferrite
magnetic powder in a matrix such as synthetic rubber, synthetic
resin or natural rubber.
These known permanent magnets, however, suffer from various
disadvantages as follows. Namely, the isotropic ferric magnet,
which is usually produced by compacting, cannot provide
satisfactory magnetic properties, since magnetic rubber is oriented
in direction at random. The ring anisotropic magnet 11 as shown in
FIG. 1 has a radial particle orientation as shown by arrows B. The
magnetization after the sintering, however, is conducted by means
of a magnetizing yoke A composed of a yoke member 1 formed of
ferromagnetic material and coils 2 as shown in FIG. 2. Therefore,
the direction of particle orientation and the direction of line of
magnetic force indicated by arrows C locally discord with each
other. Thus, the ring anisotropic magnet 11 cannot provide
effective particle orientation. Such ring anisotropic ferrite
magnet is described in U.S. Pat. Nos. 3,114,715 and 4,057,606. In
the plastic permanent magnet 12 as shown in FIG. 3, the particle
orientation coincides with the directions of lines of magnetic
force indicated by arrow C. This plastic magnet 12, however, cannot
provide sufficient amount of magnetic flux because of small
residual magnetic flux density. Such plastic magnet is described in
Japanese Laid-Open Patent Publication No. 57-130407.
In addition, the conventional cylindrical permanent magnet could
not avert from the problem of large inertia force which is caused
inevitably by increased wall thickness for obtaining a high
magnetic properties.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a multipolar
surface magnetized magnet having a high surface magnetic flux
density.
Another object of the invention is to provide a permanent magnet
which exhibits a high holding torque when used as the rotor magnet
of a stepping motor.
To these ends, according to the invention, there is provided a
sintered cylindrical permanent magnet having a magneto-plumbite
type crystalline structure expressed by MO.multidot.nFe.sub.2
O.sub.3, where M represents Ba, Sr, 15 Pb or their mixture and n
being a value of 5 to 6, the magnet being provided in its
cylindrical surface with multipolar anisotropy.
The advantage of the invention is remarkable particularly when more
than 8 (eight) poles are provided on the outer peripheral surface
of the cylindrical body of the magnet.
The term "surface anisotropy" in this specification is used to mean
a state in which magnetic poles of different polarities actually
exist or are formable on the same surface of the permanent magnet
such as cylindrical outer peripheral surface, and easy axes of
magnetization of ferrite particles, which have magnetoplumbite
crystalline structure having unidirectional magnetic anisotropy,
are substantially aligned with the lines, normally arcuate lines,
interconnecting the magnetic poles through the body of the
magnet.
The invention aims also at providing a cylindrical permanent magnet
in which the particle orientation for attaining effective amount of
magnetic flux is achieved in advance to the sintering thereby to
impart the surface multipolar anisotropy while attaining high
magnetic properties and a reduced inertia force.
These and other objects, features and advantages of the invention
will become clear from the following description of the preferred
embodiments taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the particle orientation in a conventional cylindrical
permanent magnet;
FIG. 2 is an enlarged view of a part of permanent magnet and
magnetizing yoke;
FIG. 3 is an illustration of the directions of lines of magnetic
force in a permanent magnet;
FIG. 4 shows surface magnetic flux density and inertia force in
relation to the wall thickness as observed in a cylindrical
permanent magnet; and
FIG. 5 shows the surface magnetic flux density and inertia moment
in relation to a change in the inside diameter of the cylindrical
permanent magnet.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Generally, the multipolar cylindrical magnet for use as the rotor
of a motor is required to satisfy both of the demands for high
magnetic properties and low inertia force. These two demands,
however, are generally incompatible with each other. If the wall
thickness (t) of the cylindrical permanent magnet is increased for
attaining a high surface magnetic flux density Bo, the inertia
force I is increased undesirably, as will be understood from FIG.
4. To the contrary, if the wall thickness (t) is reduced to satisfy
the requirement for smaller inertia force, the surface magnetic
flux density Bo is decreased unfavourably. Accordingly, it is
necessary to optimize the size of the magnet so as to obtain
sufficiently small inertia force without being accompanied by
substantial reduction in the surface magnetic flux density Bo.
According to the invention, the inside and outside diameters of the
cylindrical permanent magnet are selected to meet the following
condition.
Where, D.sub.1 represents the inside diameter of magnet, D.sub.2
represents the outside diameter of permanent magnet, P represents
the number of magnetic poles and K.sub.1 represents a constant.
In the aforementioned formula, the inside to outside diameter ratio
T ranges between 0 and 1.0. As the ratio gets closer to 1, the wall
thickness (t) becomes smaller so that the surface magnetic flux
density B.sub.O gets smaller. To the contrary, as the ratio T
approaches 0 (zero), the wall thickness (t) gets larger to make the
magnet resemble a pillar or solid cylinder, resulting in an
increased inertia force I.
The right side of the aforementioned formula suggests that an
increase in number of magnetic poles P causes an increase in the
inside to outside diameter ratio T resulting in a reduced wall
thickness (t). On the other hand, the wall thickness (t) is
increased as the number P of magnetic poles is decreased. It is
thus possible to obtain the optimum inside and outside diameters,
i.e. the diameter ratio T, also on consideration of the number P of
magnetic poles.
An explanation will be made hereinunder as to how the formula
mentioned before is derived. As the outer peripheral surface of the
cylindrical permanent magnet is magnetized to develop magnetic
poles of a number P, the circumferential distance between adjacent
magnetic poles, i.e. the distance along the outer peripheral
surface of the cylindrical magnet between the centers of adjacent
magnetic poles of different polarities, ties, is expressed by
.pi.D.sub.2 /P. The penetration depth of the magnetic flux is then
expressed by K.sub.1 .pi.D.sub.2 /P, where the constant K.sub.1 is
determined experimentally as explained later. Since the thickness
of the wall portion not penetrated by the magnetic flux does not
constitute an essential part of the cylindrical permanent magnet,
the effective inside diameter D.sub.1 of the magnet is expressed as
follows.
The following formula is derived by diving both sides of the above
formula by D.sub.2.
The optimum ratio T between the inside diameter and the outside
diameter of the cylindrical permanent magnet of the invention is
determined by this formula.
An explanation will be made hereinunder as to an example of
experimental determination of the constant value K.sub.1, with
specific reference to FIG. 5. Namely, FIG. 5 shows how the surface
magnetic flux density B.sub.o and the inertia moment I are changed
in accordance with changes in the inside diameter D.sub.1 and the
constant value K.sub.1, in a cylindrical permanent magnet having an
outside diameter D.sub.2 of 26 mm and 24 (twenty four) magnetic
poles in total. From this Figure, it will be seen that a
specifically high surface magnetic density is obtained when the
constant K.sub.1 takes a value not smaller than 1.5. Considering
that the smaller inertia force ensures higher performance of rotor
magnet, the constant value K.sub.1 is selected to meet the
condition of 1.5.ltoreq.K.sub.1 .ltoreq.2.5.
The cylindrical permanent magnet of the invention is formed from a
material containing, in addition to the major constituents
mentioned before, a suitable amount of additives for imparting a
self-supporting force, as well as 14 to 20% of water for permitting
the rotation of particles when the material is placed under the
influence of a magnetic field. Then, the formed body is placed
under the influence of the magnetic field to orientate the
particles in the direction of lines of magnetic force, and is then
sintered to become the permanent magnet as the final product. The
magnet, if desired, is machined to a final shape.
The water content is selected to range between 14 and 20% because
any water content below 14% makes the compacted body too hard to
permit smooth orientation of particles for attaining desired
anisotropy, resulting in an imperfect magnetic properties, while a
water content in excess of 20% seriously deteriorates the
self-supporting force of the compacted body to make the forming
materially impossible.
To apply pulse magnetic fields to the compacted body, the coil of
the yoke may be connected to an instantaneous D.C. power supply,
wherein an A.C. power source is used as an input and the A.C.
current is rectified and raised to a predetermined D.C. voltage to
charge a group of capacitors which effect discharge through
thyristors.
With respect to the magnitude of the pulse magnetic field, a
magnetic field of over about 10,000 Oersted is enough to accomplish
this invention. Not only one but a combination of two pulse
magnetic fields may be applied.
An experimental example of the invention will be described
hereinunder.
(Experiment)
A green body having an outside diameter, inside diameter and axial
length of 33 mm, 23 mm and 30 mm, respectively, was formed by a
compressing machine from a material consisting of powdered Sr
ferrite (SrO.multidot.5.multidot.6Fe.sub.2 O.sub.3) containing 18%
of water. The green body was inserted into a multipolar magnetizing
yoke A as shown in FIG. 2 and was subjected to a magnetic field of
more than 3,000 Oe. The green body was then, sintered at
1200.degree. C. following 24 hour drying. The sintered body was
machined to have an outside diameter, inside diameter and axial
length of 26 mm, 18 mm and 20 mm, respectively. The body 3 was then
magnetized by a multipolar magnetizing yoke having a shape similar
to the magnetizing yoke A shown in FIG. 2, and the surface magnetic
flux density B.sub.O was measured, the result of which is shown in
Table 1 below together with the values obtained with a conventional
isotropic magnet, ring anisotropic magnet and a plastic magnet by
way of reference.
TABLE 1 ______________________________________ Surface magnetic
flux density B., (G) ______________________________________
isotropic magnet 500 ring anisotropic magnet 1000 plastic magnet
1000 magnet of invention 1400
______________________________________
As will be understood from Table 1 above, the permanent magnet of
the invention has attained about 40% increase in the surface
magnetic flux density as compared with conventional magnets. The
cylindrical permanent magnet of the invention used in this
experiment had an outside diameter, inside diameter and axial
length of 26 mm, 18 mm and 20 mm, respectively, and had 24 (twenty
four) magnetic poles in total. In addition, the constant value
K.sub.1, is selected to range between 1.5 and 2.5 in view of the
curve shown in FIG. 5, so that the size of the permanent magnet is
optimized to provide a reduced inertia force without being
accompanied by substantial reduction in the surface magnetic flux
density.
Using the permanent magnets shown in Table 1 as the rotors of the
stepping motor, an experiment was conducted by applying a DC
voltage of 12 V to confirm the holding torques produced by these
magnets, the result of which is shown in Table 2 below together
with the magnetic properties of these permanent magnets.
TABLE 2 ______________________________________ holding torque
magnetic properties (g .multidot. cm) Br (G) Hc (Oe)
______________________________________ isotropic magnet 660 2000
1700 ring anisotropic 720 3200 2700 magnet plastic magnet 700 3000
2500 magnet of invention 900 2600 3000
______________________________________
Since the magnetic properties were all measured in the radial
direction, the magnet of the invention exhibits quite a low
residual induction Br of 2600 Gauss. This magnet, however, exhibits
quite a high holding torque.
As has been described, the cylindrical permanent magnet of the
invention is provided with the surface multipolar anisotropy prior
to the sintering so that the particles are oriented in the
magnetizing direction. The sintered permanent magnet of the
invention, therefore, has an extremely high surface magnetic flux
density which in turn permits a high magnetic properties. In
addition, since the size of the magnet is optimized in view of the
formula mentioned before, the weight and, hence, the inertia force
of the magnet is decreased advantageously as compared with
conventional magnets, without being accompanied by any substantial
decrease in the magnetic properties. The reduced weight permits
also an economical use of the material.
Although a specific embodiment applied to a rotor magnet of
stepping motor has been explained, the described embodiment is not
exclusive and various changes and modifications are possible
without departing from the scope of the invention. For instance,
the cylindrical permanent magnet of the invention may have an
increased axial length so that it may be used in a copying machine
incorporating a magnetic brush development for latent images.
* * * * *