Rotary head assembly with pump-out type spiral grooves for more stable tape running

In a rotary head assembly in which a rotary member (14) provided with a magnetic head (23) in the gap between the upper and the lower fixed cylinders (17 and 18) having nearly equal diameters and arranged coaxially with each other is turned, use is made of a structure such that the upper and the lower fixed cylinders (17 and 18) are secured in place and supported by a fixed shaft arranged coaxially therewith, whereby more stable magnetic tape running has been realized, while achieving ease of assembling with high accuracy and reduction of power consumption.

FIELD OF THE INVENTION 
The present invention relates to a rotary head assembly of video tape 
recorder (hereinafter called VTR) and particularly pertains to a rotary 
head assembly of a type having its upper and lower cylinders secured in 
place on a fixed central shaft, which leads itself to ease of assembling, 
reduced power consumption and further stabler magnetic tape running. 
BACKGROUND OF THE INVENTION 
There are available various types of construction for a rotary head 
assembly of a VTR. For example, a basic type is constructed of two 
cylindrical bodies (hereinafter called cylinder) having nearly equal ODs 
which are arranged coaxially with each other. 
FIG. 1 shows an example of the structure of a conventional rotary head 
assembly. Referring to FIG. 1, 1 denotes a rotary shaft, which is 
rotatably supported by bearings 2 and 3 which is rotationally driven by a 
driving motor (not shown in this figure). Magnetic head 4 is secured on 
the rotary shaft 1 through a member 5, to be rotated at a specified speed. 
Upper fixed cylinder 6 is bound coaxially to lower fixed cylinder 8 
through a member 7 and the lower fixed cylinder 8 is, in turn, secured in 
position on a base of the VTR. Numerals 9 and 10 designate collars or 
sleeves and 11 a bearing retaining ring. The structure of the conventional 
rotary head assembly such as shown in FIG. 1 has defects as described 
hereunder: 
(1) It is difficult to align the axial center of the upper fixed cylinder 
and that of the lower fixed cylinder at a desired accuracy. 
(2) In the small rotary head assembly which is employed in a 8 mm type 
portable video camera having a camera and video tape recorder combined as 
a single unit, its cylinder diameter is reduced to about 2/3, and the 
magnetic tape width to 1/2, as compared with a conventional video rotary 
head assembly utilizing a 1/2 inch width magnetic tape, but its rotational 
speed is still the same as the conventional speed of 1,800 rpm. For this 
reason, in order to have an adequate running distance of the magnetic head 
on the magnetic tape per one TV picture frame, the angle that the running 
surface of the magnetic head makes with the running direction of the 
magnetic tape needs to be reduced from that in the conventional assembly. 
To effect this, the angle at which the magnetic tape is wound on the 
cylinders, which is approx. 190.degree. in the case of the conventional 
1/2 inch tape must be increased to approx. 220.degree. in the 8 mm type 
camera. This makes impractical the upper cylinder fixing method such as 
shown in FIG. 1. 
SUMMARY OF THE INVENTION 
According to this invention, a rotary head assembly is provided having a 
member with a magnetic head which is turned in a gap between an upper and 
a lower fixed or stationary cylinder having nearly equal diameters 
arranged coaxially with each other and a magnetic tape which is helically 
wound around or runs on the peripheral surfaces of the aforementioned 
upper and lower fixed cylinders, the upper fixed cylinder being fixedly 
attached to a central shaft coaxially arranged therewith, for ease of 
assembling with high accuracy, reduced power consumption and more stable 
magnetic tape running.

THE MOST PREFERABLE EMBODIMENT OF THIS INVENTION 
FIGS. 2 and 3 present an embodiment of this invention. FIG. 2 is a 
sectional view of a rotary head assembly embodying this invention and FIG. 
3 is a bottom view of its upper fixed cylinder. 
Referring to FIG. 2, 12 denotes a fixed shaft having a central axis 
extending in an axial direction, 13 bearings, 14 a rotary member, 15 and 
16 magnet and coil composing a direct drive motor which rotationally drive 
the rotary member 14 and on the respective lower axial end surface and the 
upper axial end surface of the upper fixed or stationary cylinder 17 and 
the lower fixed or stationary cylinder 18, which are coaxial with each 
other, pump-out type spiral grooves 19 and 20 are provided. The rotary 
member has a disc portion 14 which is rotated with a minute clearance "s" 
between the upper and lower surfaces thereof with each of the end surfaces 
of the upper and the lower fixed cylinders. The pattern of the spiral 
groove is formed radially and spirally on the bottom surface of the upper 
fixed cylinder with ID T.sub.1 and OD T.sub.2, as roughly shown in FIG. 3. 
In this figure, the parts shown with cross-hatching indicate the groove 
parts 21, while the other parts 22 represent the land parts. In FIG. 2, 23 
designates a magnetic head, 24 a guide which regulates the running 
position of the magnetic tape in its width direction, 25 a bearing 
retaining ring, and 26 and 27 vent holes for communicating between the 
interior and exterior sides of the rotary head assembly. The arrow mark a 
in FIG. 3 indicates the turning direction of the rotary member 14. 
In the following, there is set out an explanation of the operation of the 
rotary head assembly composed as hereinabove described: 
As the rotary member 14 is rotationally driven by means of the magnet 15 
and the coil 16, the air inside the rotary head assembly is discharged 
through minute clearances a between the upper and the lower fixed 
cylinders 17 and 18 and the rotary member 14 by the hydrodynamic action of 
the spiral grooves 19 and 20. The air discharged outwardly gives the 
magnetic tape a buoying force, forming a uniform air film all over the 
part of the peripheral surface of the upper and lower cylinders of the 
rotary head assembly on which the magnetic tape slides, whereby the 
sliding contact between the magnetic tape and the rotary head assembly is 
averted and stable and smooth tape running is realized. 
In the embodiment of FIG. 2, the spiral grooves are provided on the end 
surfaces of the upper and the lower fixed cylinders, but the spiral 
grooves may be provided on the end surfaces of the upper and the lower 
fixed cylinders which are formed conical. This structure is especially 
effective in increasing the effective area of the spiral groove. 
As described in the foregoing, according to this embodiment, the upper 
fixed cylinder is secured in place and supported on a fixed shaft 
coaxially with the lower fixed cylinder and the rotary member on which the 
magnetic head is mounted is turned. In this way, following improvements 
are achieved, as compared with the conventional assembly shown in FIG. 1: 
(1) Since the upper cylinder is secured in place and supported by the fixed 
central shaft, coaxiality of the upper and the lower fixed cylinders is 
readily ensured and with high accuracy, in assembling them. 
(2) The angle at which the magnetic tape is wound on the cylinder may be 
increased, resulting in an advantage in miniaturization of cylinders. 
Actually, even when the angle at which the magnetic tape is wound on the 
cylinders is larger than about 220.degree., as in the rotary head assembly 
of 8 mm type portable video camera, for example, employment of the upper 
cylinder fixing system is feasible by use of the structure shown in FIG. 
3. 
(3) By taking advantage of the air discharging effect of the pump-out type 
spiral grooves, the magnetic tape is buoyed up from the outer peripheral 
surfaces of the upper and the lower fixed cylinders, whereby more stable 
tape running may be realized, while preventing wear of the magnetic tape 
and cylinders. 
INDUSTRIAL APPLICABILITY 
As hereinabove described, the upper fixed cylinder is secured in place and 
supported by a fixed shaft coaxially arranged therewith and a rotary 
member provided with a magnetic head which is turned coaxially with the 
fixed shaft is arranged in the gap between the upper and the lower fixed 
cylinders, with pump-out type spiral grooves provided on the lower end 
surface of the upper fixed cylinder and the upper end surface of the lower 
fixed cylinder, whereby more stable tape running may be realized, while 
achieving ease of assembling with high accuracy and reduction of power 
consumption.