Magnetic disk cartridge

A magnetic disk cartridge includes a magnetic disk, a cassette shell encasing the magnetic disk, and a disk-shaped center core which supports the magnetic disk and is adapted for rotation by a turntable equipped with an annular chucking magnet. A magnetic head access opening is formed in one side surface of the cassette shell, a center portion of the center core is formed with a center hole for engagement by a center locating pin of the turntable, and a peripheral portion of the center core is formed with a step-like annular chucking region whose level differs from that at the mouth of the center hole. The chucking region is adapted to make surface contact with the annular chucking magnet. The structure enables the chucking magnet to strongly attract and reliably secure the center core, thereby preventing slippage during high-speed rotation, and to prevent rotational vibration owing to unbalanced loading.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic disk cartridge and, more 
particularly, to the structure of a center core for supporting the center 
portion of the magnetic disk of a magnetic disk cartridge having a 
rotatable magnetic disk encased in a cassette shell. 
2. Description of the Related Art 
A magnetic disk cartridge comprises a magnetic disk for magnetically 
recording analog and/or digital signals and which is rotatably encased in 
a thin cassette shell. The cassette shell is formed in the shape of a flat 
prism (substantially a hexahedron) with a top surface slightly larger than 
the magnetic disk, a bottom surface having an opening through which a 
center core supporting the center of the magnetic disk is exposed and 
narrow side surfaces extending between the outer peripheral edges of the 
top and bottom surfaces. The magnetic disk cartridge is provided with a 
magnetic head access opening for insertion/withdrawal of magnetic heads 
used for magnetic recording and reproducing so as to enable the magnetic 
heads to be brought into contact with, or close to, the opposite surfaces 
of the magnetic disk from the exterior. The magnetic head access opening 
is equipped with a shutter member for preventing invasion of dust and the 
like when the magnetic disk cartridge is not in use. 
The magnetic head access opening of the prior art magnetic disk cartridge 
is a slender opening of substantially rectangular shape formed in the top 
and bottom surfaces of the cassette shell to extend in the radial 
direction of the magnetic disk. 
However, it is not absolutely necessary for the magnetic head access 
opening to be formed in the top and bottom surfaces of the cassette shell 
to extend in the radial direction of the magnetic disk. Specifically, it 
can be provided at any location so long as it enables the magnetic heads 
to be brought in contact with, or close to, the opposite surfaces of the 
magnetic disk over the full radius thereof from the exterior. The opening 
is preferably as small as possible so as to minimize invasion of dust into 
the cartridge. The ability to fabricate smaller magnetic heads has 
afforded greater freedom in the positioning and sizing of the magnetic 
head access opening and has even made it possible to realize the desired 
small magnetic head access opening by providing the opening only in a side 
surface rather than in the top and/or bottom surface. This can be achieved 
by forming a narrow side surface of the cassette shell with a magnetic 
head access opening of a size enabling access to the opposite surfaces of 
the magnetic disk from the exterior and inserting/withdrawing thin, 
arm-shaped pickups having tiny magnetic heads attached at their tips 
through this hole to conduct recording and reproduction while moving the 
pickups in the radial direction of the magnetic disk. 
Such a magnetic disk cartridge having the magnetic head access opening 
formed in one side surface can accommodate a high-density, high-capacity 
magnetic disk. Since such a magnetic disk requires a high recording 
density and a high data transfer rate, it is adapted to conduct recording 
and reproducing while being rotated at high speed (several thousand rpm). 
The center portion of the magnetic disk of the conventional thin magnetic 
disk cartridge is also supported by a center core made of ferromagnetic 
sheet material and the center core is rotated by a turntable which secures 
it in place by magnetic attraction. In order to prevent eccentric 
rotation, the center core is formed with a polygonal hole at its center 
and with a separate polygonal hole at a peripheral portion. 
A magnetic disk cartridge whose magnetic disk is rotated at high speed 
cannot tolerate rotational offset owing to slippage occurring between the 
turntable and the magnetic disk at the start or in the course of rotation 
and requires the magnetic disk to be rotated in a single plane (with a 
high degree of flatness). 
Prevention of slippage by the method adopted in the conventional magnetic 
disk cartridge, i.e., by providing a polygonal hole at a location apart 
from the center, is not practical because the presence of the hole causes 
a load imbalance in the rotational plane and results in rotational 
vibration during high-speed rotation. If this were the only point to be 
considered, it would be preferable to suppress vibration of a magnetic 
disk cartridge that rotates at high speed by providing its center core 
with only one hole at the center. In this case, however, the transmission 
of torque from the turntable to the center core relies solely on chucking 
utilizing the attractive force of a chucking magnet and, as pointed out 
above, the likelihood of slippage occurring between the turntable and the 
center core at the start or in the course of rotation is increased. 
In order to maintain flatness and reduce modulation in a conventional 
magnetic disk cartridge during rotation, it is possible to adopt a known 
structure, such as described in JU-A 61-24877, in which the contact 
surface of the center core is depressed toward the center hole. Since this 
structure results in tangential contact between the center core and the 
magnet, however, it is liable to experience rotational offset (slippage) 
owing to insufficient magnetic attraction if applied to a high-speed 
magnetic disk cartridge. 
SUMMARY OF THE INVENTION 
The object of this invention is to overcome the aforesaid problems by 
providing a magnetic disk cartridge wherein the center core of the 
magnetic disk can be chucked by the chucking magnet of a turntable to 
enable high-speed rotation of the center core without rotational slippage 
and with minimal vibration. 
For achieving this object, the invention provides a magnetic disk cartridge 
comprising a magnetic disk, a cassette shell encasing the magnetic disk to 
be rotatable therein, and a disk-shaped center core fabricated from sheet 
material which supports the center of the magnetic disk and is exposed 
through an open portion in the bottom surface of the cassette shell for 
rotation by a turntable equipped at an outer peripheral portion thereof 
with an annular chucking magnet for magnetically chucking the center core, 
wherein the cassette shell is formed as a flat prism with a top surface 
slightly larger than the magnetic disk, the bottom surface of 
approximately the same shape as, and lying parallel to, the top surface, 
and narrow side surfaces extending between outer peripheral edges of the 
top and bottom surfaces, a magnetic head access opening is formed in one 
side surface of the cassette shell for enabling magnetic reproducing and 
recording heads to be brought in contact with, or close to, opposite 
surfaces of the magnetic disk from the exterior, a center portion of the 
center core is formed with a center hole for engagement by a center 
locating pin of the turntable, and a peripheral portion of the center core 
is formed with a step-like annular chucking region whose level differs 
from that at a mouth of the center hole and which is adapted to make 
surface contact with the annular chucking magnet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The magnetic disk cartridge of the present invention will hereinbelow be 
described with reference to the accompanying drawings. FIGS. 1 to 3 show a 
magnetic disk cartridge which is an embodiment of the invention. The 
magnetic disk cartridge is shown in perspective view from the top with its 
shutter member open in FIG. 1 and from the bottom with its shutter member 
closed in FIG. 2. FIG. 3 is a sectional view showing the magnetic disk of 
the magnetic disk cartridge magnetically chucked to a turntable. 
The magnetic disk cartridge 1 of this embodiment comprises a cassette shell 
2 encasing a magnetic disk 3 so as to be freely rotatable therein. The 
cassette shell 2 is formed of a pair of top and bottom cassette shell 
halves made of molded plastic. The cassette shell 2 is formed 
substantially as a flat hexahedron having an approximately square top 
surface 21 slightly larger than the magnetic disk 3 encased therein, a 
bottom surface 22 of substantially the same shape as the top surface 21 
and lying parallel thereto, and narrow side surfaces 23 extending between 
the outer peripheral edges of the top surface 21 and the bottom surface 
22. 
The center portion of the magnetic disk 3 (recording medium) is supported 
by a disk-shaped center core 31 pressed from ferromagnetic sheet material. 
The center core 31 faces into a circular opening 2a in the bottom surface 
22 of the cassette shell 2 (see FIG. 2). 
When the magnetic disk cartridge is loaded in a magnetic recording and 
reproducing device, the center core 31 is chucked by a turntable 12, as 
shown in FIG. 3, and rotated at high speed. The turntable 12 is formed at 
the center of rotation with a locating pin 13 having a tapered tip and at 
an outer peripheral portion with an annular chucking magnet 14. 
The center core 31 is formed at a center portion with a center hole 31a for 
engagement by the locating pin 13 of the turntable 12 and at an outer 
peripheral portion with an annular, step-like chucking region 31b whose 
level differs from that at the mouth of the center hole 31a. The chucking 
region 31b is adapted to make surface-to-surface contact with the annular 
chucking magnet 14 at the outer peripheral region of the turntable 12. 
A magnetic head access opening 6 enabling magnetic recording and 
reproducing heads to be brought in contact with, or close to, the opposite 
surfaces of the magnetic disk 3 from the exterior is formed in one side 
surface 4 among the four side surfaces 23 of the cassette shell 2. 
A slidable shutter member 8 for opening and closing the magnetic head 
access opening 6 is provided on the cassette shell 2. The shutter member 8 
is constituted as a sectionally u-shaped metal sheet member comprising, as 
interconnected, an upper wing 81 extending along the top surface 21, a 
shutter section 82 extending along the one side surface 4 and a lower wing 
83 extending along the bottom surface 22. The shutter member 8 moves along 
these surfaces. The shutter section 82 is provided with a shutter window 
82a. It can therefore open and close the magnetic head access opening 6 
depending on whether the magnetic disk cartridge 1 is in storage or 
operation. 
On the other hand, portions of the top surface 21, the side surface 4 and 
the bottom surface 22 of the cassette shell 2 adjacent to the magnetic 
head access opening 6 are formed with a shallow recess 9 for retaining and 
guiding the upper wing 81, the shutter section 82 and the lower wing 83 of 
the shutter member 8. 
The shutter member 8 is constantly biased in the direction of closing the 
magnetic head access opening 6 by a shutter closing spring (a torsion 
spring not shown in the drawings). The shutter closing spring is inserted 
into the cassette shell 2 through a spring insertion opening 4a formed at 
one end portion of the side surface 4 of the cassette shell 2, a first end 
portion thereof engages with a spring engaging portion 82b provided at a 
lateral end portion of the shutter section 82 of the shutter member 8, and 
the other end portion thereof engages with the inner surface of the 
cassette shell. The spring engaging portion 82b of the shutter member 8 
passes through a slide groove 4b formed in the side surface 4 of the 
cassette shell 2. The lower wing 83 of the shutter member 8 is formed at 
two locations with projecting guide claws 83a, 83a that engage with a 
guide groove 22a formed in the bottom surface 22 of the cassette shell 2 
to extend in the sliding direction of the shutter member 8, thereby 
retaining the shutter member 8 on the cassette shell 2. 
The magnetic disk cartridge 1 of this embodiment is used in the same manner 
as the prior-art magnetic disk cartridge. Specifically, it is loaded into 
the magnetic recording and reproducing device (hereinafter called simply 
"device") in the state shown in FIG. 2, i.e., with the magnetic head 
access opening 6 closed by the shutter member 8. At this time, an 
operating member provided in the device responds to the loading operation 
by sliding the shutter member 8 to the open position shown in FIG. 1. 
Then, when the operator performs a prescribed operation for removing the 
magnetic disk cartridge 1 from the device, for instance, when the operator 
presses a prescribed operating button of the device, the shutter member 8 
is closed to the position shown in FIG. 2. 
Since various types of device side mechanisms are available for conducting 
the opening and closing operations of the shutter member 8 and such a 
mechanism can easily be constituted as in conventional devices, a detailed 
explanation of such a mechanism will not be given here. 
When the magnetic disk cartridge 1 is loaded in a device of the foregoing 
type, the device responds by moving the turntable 12 toward the center 
core 31 exposed at the bottom surface 22 of the cassette shell 2. 
Specifically, the tip of the locating pin 13 of the turntable 12 is 
engaged with center hole 31a of the center core 31 to regulate the 
position of the center of the center core 31 and, consequently, that of 
the magnetic disk 3, while the chucking surface of the chucking magnet 14 
at the outer periphery of the turntable 12 is brought into 
surface-to-surface contact with the chucking region 31b of the center core 
31 so as to strongly attract and secure the center core 31 at an annular 
region thereof. As a result, the turntable 12 can rotate the magnetic disk 
3 with little or no slippage (offset) in the direction of rotation. 
Since the invention provides the magnetic head access opening of the 
magnetic disk cartridge in one side surface of the cassette shell, the 
magnetic head access opening can be made small to minimize invasion of 
dust and the like into the interior of the cartridge. Moreover the center 
core of the magnetic disk cartridge is formed with a hole at only at its 
center. Owing to this center hole, the center of rotation of the magnetic 
disk can be regulated by engagement of the locating pin of the turntable 
with the center hole, while the absence of any other hole ensures 
excellent rotational balance, thereby enabling the magnetic disk to be 
rotated at high speed with minimal rotational vibration and 
high-performance recording and reproducing to be conducted with little 
modulation. 
In addition, turntable torque is transmitted to the center core through an 
annular chucking magnet which is provided at an outer peripheral region of 
the turntable and magnetically chucks an annular chucking region of the 
center core. Since the torque is therefore transmitted at high efficiency 
to enable obtainment of a large driving force, the magnetic disk can be 
rotated without occurrence of slippage between the turntable and the 
center core at the start or in the course of rotation. This enables 
excellent recording and reproduction to be conducted by use of magnetic 
heads. Further, since the chucking region of the center core is formed as 
an annular, step-like region on a different level from that at the mouth 
of the center hole, contact of the chucking magnet with portions it should 
not contact is positively avoided, the center core can be reliably seated 
by magnetic chucking, and a large chucking force can be secured through 
surface-to-surface contact. 
The stepped configuration defining the mouth of the center hole and the 
chucking region of the center core at different levels is also 
advantageous in that it increases the rigidity of the center core, thereby 
minimizing deformation under exposure to external force, and enables the 
prescribed shape to be maintained with high precision after fabrication. 
Since the chucking region therefore has the high flatness required for 
broad surface-to-surface contact with the chucking magnet, it is strongly 
chucked by the chucking magnet to prevent rotational offset with high 
reliability.