Recording and/or reproducing apparatus for controlling thermal expansion problems in disk drives

A drive for a recording disk includes a drive motor and a head transport mounted on a chassis which has a coefficient of expansion equal to that of the disk to reduce tracking errors caused by differential expansion of the chassis and the disk due to environmental changes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention This device relates generally to a recording 
and/or reproducing apparatus for magnetic disks which include polymeric 
sheets stretched over disk-like base plates. 
2. Description of the Prior Art 
A relatively new medium for mass data storage is known as a stretched 
surface recording (SSR) disk wherein at least one thin polymeric film is 
stretched across and attached to a circular base plate which is inflexible 
compared to the film. 
A recording and/or reproducing apparatus (drive) for such a disk includes a 
drive motor for rotating the disk and a transfer mechanism for moving the 
recording head, all secured to a chassis. When the disk is mounted on the 
drive and driven by the drive motor, the head in proximity to the magnetic 
sheet is moved in the radial direction of the disk to transfer information 
to and from the disk. 
However, when data transfer is performed by the above-mentioned drive, 
tracking errors are often produced due to changes of temperature. 
The base plate of the magnetic disk is molded from synthetic resin, whereas 
the chassis of the drive is made of metal, so that expansion and 
contraction of the chassis due to temperature or humidity changes are 
smaller than those of the base plate. Hence, positional errors are 
produced between the track on the magnetic sheet which is secured to the 
base plate, and the head held by the transfer mechanism which is secured 
to the chassis. 
As the tracks of the magnetic disk tend to become denser so as to enable 
the disk to perform high density recording, a complex servo-mechanism will 
become necessary to overcome tracking errors produced by environmental 
changes. 
SUMMARY OF THE INVENTION 
Tracking errors due to environmental changes are reduced or eliminated by 
providing an improved recording and/or reproducing apparatus including a 
drive motor, at least one magnetic head and transfer mechanism for moving 
the head with respect and in proximity to a magnetic disk comprising a 
disk-like base plate molded from synthetic resin and at least one magnetic 
sheet stretched over at least one side of and in parallel with the base 
plate, wherein at least both of said drive motor and said transfer 
mechanism are secured to a chassis, and said chassis is made of a material 
having substantially the same coefficient of thermal expansion as the base 
plate of said magnetic disk.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A magnetic disk 1 comprises a base plate 2 and magnetic sheets 3. The 
disk-like base plate 2 is molded of synthetic resin, has an inner annular 
projection 4, an outer annular projection 5 and flat portions 6. Further, 
an insertion hole 7 for the shaft 13a of a drive motor 13 is provided at 
the center of the base plate 2. 
The inner perimeter and the outer perimeter of each magnetic sheet 3 are 
respectively fixed to the inner annular projection 4 and the outer annular 
projection 5 on each surface of the base plate 2 with adhesives, by 
welding or the like, so that the magnetic sheets 3 are stretched over the 
base plate 2 in parallel therewith to provide gaps 8 between the magnetic 
sheets 3 and the flat portions 6 of the base plate 2. 
Resins suitable for base plate 2 include thermoplastics such as polyester, 
polycarbonate, polysulfone and polyphenylene sulfide, and thermosets such 
as epoxy, polyester and polyurethane. The preferred resin is an amorphous 
plastic, polyetherimide. It has high rigidity and is excellent in heat 
resistance and dimensional stability. The coefficient of thermal expansion 
of polyetherimide is 47-56.times.10.sup.-6 in./in./.degree. C. and 
21-25.times.10.sup.-6 in./in./.degree. C. when 20% fiber glass and 20% 
mica are incorporated. 
In a recording and/or reproducing apparatus 11 for the above magnetic disk 
1, the drive motor 13 for rotating the magnetic disk 1, and a transfer 
mechanism 16 for moving a pair of upper and lower heads 14 and 15 are 
mounted on a chassis 12. 
The transfer mechanism 16 comprises a movable table 18 guided by a guide 
shaft 17 which is supported by the chassis 12, the movable table 18 being 
moved in the radial direction of the magnetic disk 1 (in direction of 
arrow a); a stepping motor 19 for moving the heads 14 and 15; and a steel 
belt 20 connected to the movable table 18 and a motor pulley 19a which is 
secured to the motor 19. The heads 14 and 15 are respectively held by a 
pair of upper and lower arms 21 and 22 which are leaf springs or the like. 
The chassis 12, on which the drive motor 13 and the transfer mechanism 16 
are mounted, is made of a material having substantially the same 
coefficient of thermal expansion as the synthetic resin used for the base 
plate 2 of the magnetic disk 1. In the preferred embodiment, the chassis 
12 is made of the same synthetic resin, polyetherimide, as the base plate 
2. 
To mount the magnetic disk 1 on the recording and/or reproducing apparatus 
11, the shaft 13a of the drive motor 13 is first fitted into the insertion 
hole 7 of the magnetic disk 1, and then the heads 14 and 15 are brought 
into proximity with the respective upper and lower magnetic sheets 3 of 
the magnetic disk 1. 
When the magnetic disk 1 is rotated at high speed by the drive motor 13, 
the movable table 18 is moved in the direction of arrow a by the motor 19 
of the transfer mechanism 16. As the result, the heads 14 and 15 are moved 
in the radial direction of the magnetic disk 1. Thus, information transfer 
is performed with respect to the magnetic disk 1. 
Since the chassis 12 of the apparatus 11 is made of the same synthetic 
resin as the base plate 2 of the magnetic disk 1, the expansion and 
contraction of the chassis 12 and the base plate 2 due to changes in 
temperature are equal. Hence, positional errors between the tracks on the 
magnetic sheet 3 and the heads 14 and 15 on the transfer mechanism 16 are 
reduced. 
Having described an illustrative embodiment of the invention with reference 
to the accompanying drawing, it is understood that the invention is not 
limited to that precise embodiment, and that various changes and 
modifications may be effected therein. For example, the chassis 12 may be 
made of metal or the like, if the coefficient of thermal expansion of such 
metal or the like is substantially equal to that of synthetic resin of the 
base plate 2.