Rotor for rotary electrical machinery

A rotor for rotary electrical machinery comprising a permanent-magnet member made of a mixture having ferromagnetic material powder and a binder resin for chief ingredients, and a shaft, in which a recess whose projected contour on a plane orthogonally intersecting the axial line of the permanent-magnet member is of a non-circular shape is provided at least an end face of the permanent-magnet member, and a shaft is press-fit into a metallic bush fitted to the recess, thereby ensuring high coaxiality tolerance between the permanent-magnet member and the shaft, easy assembly and low manufacturing cost.

BACKGROUND OF THE INVENTION 
This invention relates generally to a rotor for rotary electrical 
machinery, including stepping motors, and more particularly to a rotor for 
rotary electrical machinery using as a constituent element a so-called 
bonded magnet made of a mixture having ferromagnetic material powder and a 
binder resin for chief ingredients. 
DESCRIPTION OF THE PRIOR ART 
Conventional rotors for rotary electrical machinery using as a constituent 
element a bonded magnet are usually of a construction shown in FIGS. 1 and 
2. FIG. 1 is a perspective view, and FIG. 2 is a longitudinal sectional 
view of a conventional type of rotor for rotary electrical machinery. In 
FIGS. 1 and 2, numeral 1 indicates a permanent-magnet member formed of a 
mixture of ferrite powder and a binding resin into a bottomed hollow 
cylindrical shape. Numeral 2 indicates a boss to which a shaft 3 is 
concentrically fixed. On the outer circumferential surface of the 
permanent-magnet member 1 provided are a plurality of magnetic poles (not 
shown) extending axially. By rotatably supporting the rotor in a stator on 
which a wire is wound, an electric motor or generator is formed. 
An efficient means for forming a rotor of the aforementioned construction 
is molding the permanent-magnet member 1 and the shaft 3 integrally by 
injection molding. That is, a shaft 3 is placed at a predetermined 
location in a molding metal die, and a mixture of ferrite powder and a 
binder resin is charged into the mold to integrally mold the shaft 3 with 
the boss 2. In this process, it is generally practiced that a criss-cross 
or axially parallel knurling pattern is provided in advance on the outer 
periphery of the shaft 3 where the shaft 3 is integrally molded with the 
boss 2, or so-called D-cutting (the cross-section of the shaft is machined 
into a D shape) is performed, that is, a flat part 4 is provided on part 
of the outer circumferential surface of the shaft 3, to ensure a firm 
grip, or prevent the slipping, axial displacement or falling-off of the 
shaft 3 due to the difference in thermal expansion coefficients of the 
binder resin and the shaft 3 after molding. 
By manufacturing a rotor in the aforementioned way, the unwanted slipping, 
axial displacement or falling-off of the shaft 3 can be prevented, but the 
conventional construction of rotors for rotary electrical machinery has 
the following problems. 
The shaft 3 and the permanent-magnet member 1 should preferably be 
perfectly coaxial, and axial misalignment, if any, should be reduced to 
the minimum. Too large an axial misalignment between the shaft 3 and the 
permanent-magnet member 1 would make the gap between the rotor and the 
stator uneven, leading to the deteriorated performance of the rotary 
electrical machinery. In practice, however, it is extremely difficult to 
completely eliminate the aforementioned axial misalignment, that is, to 
obtain perfect coaxiality. 
To mold the permanent-magnet member 1 and the shaft 3 integrally by 
injection molding, as shown in FIGS. 1 and 2, the shaft 3 must be placed 
in advance in a molding metal mold. In doing so, a certain gap is needed 
between a shaft insert hole and the shaft 3 in the metal mold, and this 
gap cannot be eliminated. 
When the shaft 3 is placed in the molding metal mold, therefore, the shaft 
3 can deviate to any one direction in the inside surface of the shaft 
insert hole, or held in an inclined state with respect to the 
predetermined axial line. Even when the shaft 3 is perfectly aligned with 
the axial line, as a compound forming the permanent-magnet member 1 is 
injected or poured into the molding metal mold, the shaft 3 can be 
deviated or inclined from the predetermined axial line by the pressure of 
the compound. 
When the gap between the shaft insert hole and shaft 3 in the molding metal 
mold is made extremely small to prevent the misalignment or inclination of 
the shaft 3, it becomes troublesome to place the shaft 3 in the molding 
metal mold, and extract moldings from the molding metal mold. This reduces 
molding efficiency substantially. 
Furthermore, if specifications of the shaft 3 is changed, the molding metal 
mold must be replaced with a new one even when the permanent-magnet member 
1 is the same in shape and in size. This results in increased mold 
manufacturing cost, and requires additional work for mold replacement. 
This lowers the ratio of the molding of the rotor proper to the entire 
molding work, including tooling, leading to increased cost. The 
aforementioned machining, such as knurling, is needed to prevent the shaft 
3 from slipping, displacement and falling, increasing machining cost. In 
addition, the need for using soft materials to make this machining easy 
inevitably reduces mechanical strength. 
SUMMARY OF THE INVENTION 
This invention is intended to overcome the problems inherent to the prior 
art described above. It is an object of this invention to provide a rotor 
for rotary electrical machinery that can accomplish high coaxiality 
between the permanentmagnet member and the shaft, good workability and low 
manufacturing cost.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
FIG. 3 is a perspective view illustrating the essential part of an 
embodiment of this invention, and FIG. 4 is an exploded perspective view 
showing components of an embodiment of this invention shown in FIG. 3. 
Like parts are indicated by like numerals shown in FIGS. 1 and 2. In FIGS. 
3 and 4, numeral 5 refers to recesses provided coaxially on both end faces 
of the permanent-magnet member 1. The recesses 5 are formed into a square 
shape in the projected contour on a plane orthogonally intersecting the 
axial line of the permanent-magnet member 1. Numeral 6 refers to a 
through-hole formed coaxially with the permanent-magnet member 1 into an 
inside diameter slightly larger than the outside diameter of the shaft 3, 
or an inside diameter having a certain press-fit allowance. The 
permanent-magnet member 1 of this type can be formed by injection molding, 
for example. 
Numeral 7 refers to a metallic bush made of sheet metal, for example, and 
formed by pressing or punching means into a square shape in outside 
contour, and the size of the metallic bush 7 corresponds to the recess 5, 
with a hole 8 coaxially provided at the center thereof. The inside 
diameter of the hole 8 is made slightly smaller than the outside diameter 
of the shaft 3 so that resistance to falling of the shaft 3, when 
press-fitted, as will be described later, can be maintained at over 20 
kgf, for example. The shaft 3 is formed into a substantially equal outside 
diameter along the overall length. 
With the aforementioned construction, the rotor is assembled by 
press-fitting the metallic bushes 7 into the recesses 5 on both end faces 
of the permanent-magnet member 1, then press-fitting the shaft 3 into the 
hole 8. Since the outside contour of the metallic bushes 7 is formed in 
such a fashion as to correspond to that of the recess 5, coaxiality 
between the permanent-magnet member 1 and the hole 8 can be maintained by 
press-fitting the shaft 3 into the recess 5. As the shaft 3 and the hole 8 
are fixedly fitted to each other by press-fitting, the slipping, axial 
misalignment or falling of the shaft 3 can be prevented. 
FIG. 5 is a longitudinal sectional view illustrating the essential part of 
another embodiment of this invention, FIG. 6 is a left-side view of the 
same, and FIG. 7 a right-side view of the same. Like parts are indicated 
by like reference numerals shown in FIGS. 1, 2, 3 and 4. In FIGS. 5 
through 7, the permanent-magnet member 1 is formed into a bottomed hollow 
cylindrical shape and has ribs 9 therein. The construction of this 
embodiment is the same as the embodiment shown in FIGS. 3 and 4, except 
that the recesses 5 are provided on the bottom end face of the 
permanent-magnet member 1 and on the end face of the boss 2. Consequently, 
the method of assembly, the maintenance of coaxiality between the 
permanent-magnet member 1 and the shaft 3, and prevention of the slipping, 
axial misalignment or falling of the shaft 3 are also the same as in the 
embodiment shown in FIGS. 3 and 4. 
In this embodiment, description has been made about the use of ferrite 
powder as the most commonly used materials for the permanent-magnet 
material. Needless to say, known ferromagnetic materials other than 
ferrite, such as Sm-Co or Nd-Fe-B and other rare-earth magnet materials 
having excellent magnetic properties may be used as ferromagnetic material 
powder. Furthermore, known resin materials, such as nylon, may be used as 
the binder resin. Injection molding has been described in this embodiment 
as the means for molding so-called bonded magnets, but other molding means 
may be used. This invention may be applied to anisotropic bonded magnets 
manufactured in a magnetic field. The metallic bush and the recess may be 
provided at least an end face of the permanent-magnet member, and the 
shape of them may not be limited to a square shape, but rectangular, 
triangular, polygonal, elliptical and other geometric shapes other than a 
non-circular shape can be used so long as the slipping of the shaft can be 
prevented. 
This invention having the aforementioned construction and operation can 
accomplish the following effects. 
(1) Since the permanent-magnet member can be molded separately, molding 
efficiency can be increased and the coaxiality of the permanent-magnet 
member with respect to the shaft can be improved substantially. 
(2) Molding metal molds need not be changed even for different lengths of 
the shaft. This can substantially improve productivity in the short 
production run system in which a small quantity of a wide variety of 
products are manufactured. 
(3) The shaft can be formed into an equal diameter, and the knurling or 
D-cutting of the shaft as practiced in the prior art is eliminated. This 
leads to reduced machining cost. 
(4) Since no additional machining of the shaft is needed, the material of 
the shaft can be selected freely, and even high-strength materials can be 
used. 
(5) The shaft can be fixedly fitted to the permanent-magnet material by 
press-fitting the shaft into the non-circular metallic bush. The slipping, 
axial displacement or falling of the shaft can be prevented.