Fluid bearing device

A fluid bearing device including a shaft, a sleeve rotatably supported by the shaft, a thrust bearing plate supported by the sleeve, a radial groove formed on one of an outer periphery of the shaft and an inner periphery of the sleeve, and a spiral groove formed on one of one end face of the shaft and one face of the thrust bearing plate such that the radial groove retains oil, while the spiral groove retains grease containing thickener and base oil identical, in compositions and viscosity, with the oil of the radial groove.

BACKGROUND OF THE INVENTION 
The present invention generally relates to bearings and more particularly, 
to a fluid bearing device employing lubricant, which is constituted by a 
thrust bearing member and a radial bearing member provided independently 
of each other. 
One example of prior art fluid bearing devices, which is applied to a 
rotating head apparatus (hereinbelow, referred to as a "VTR cylinder") of 
a video tape recorder, is shown in FIGS. 1 to 3. In FIGS. 1 to 3, a fixed 
shaft 1 is press fitted into a central bore of a lower cylinder 2. A disk 
3 is rotatably fitted around the fixed shaft 2. A thrust bearing plate 4 
is mounted on an upper face of the disk 3, while an upper cylinder 5 is 
secured to a shoulder portion of the disk 3. Furthermore, a magnetic head 
6 is fixed to a lower face of the upper cylinder 5. A rotary member 7 of a 
rotary transformer is secured to a lower portion of the disk 3 so as to 
transmit, through the rotating magnetic head 6, to a fixed member 8 of the 
rotary transformer video signals delivered from a magnetic tape. The fixed 
member 8 of the rotary transformer, which receives the video signals of 
the magnetic tape, is fixed to the lower cylinder 2 so as to confront the 
rotary member 7 of the rotary transformer. Thus, a rotary unit 10 is 
constituted by the disk 3, thrust bearing plate 4, upper cylinder 5, 
magnetic head 6 and rotary member 7. 
Meanwhile, a C-shaped retaining ring 9 shown in FIG. 3 is fitted around an 
upper portion of the fixed shaft 1 so as to prevent the rotary unit 10 
from being removed from the fixed shaft 1. Furthermore, an armature magnet 
11 of a direct drive type motor 13 is secured to a lower portion of the 
disk 3. A fixed coil unit 12 is mounted on the lower cylinder 2 so as to 
confront the armature magnet 11 such that the direct drive type motor 13 
is secured to a lower portion of the disk 3. A fixed coil unit 12 is 
mounted on the lower cylinder 2 so as to confront the armature magnet 11 
such that the direct drive type motor 13 is constituted by the armature 
magnet 11 and the fixed coil unit 12. The fixed coil unit 12 is at all 
times attracted at a force of 600 to 900 grams by the armature magnet 11. 
Moreover, radial grooves 14A and 14B are, respectively, formed at the upper 
portion and a central portion of the fixed shaft 1 by etching, etc. An end 
face 1A of the fixed shaft 1 is subjected to precision machining in 
flatness and perpendicularity to an axis of the fixed shaft 1. A 
single-row spiral groove 15 shown in FIG. 2 is formed on a central lower 
face of the thrust bearing plate 4 by etching, etc. so as to confront the 
end face 1A. Either an identical oil or an identical grease is supplied, 
as lubricant of the known fluid bearing device, into the radial grooves 
14A and 14B and the spiral groove 15. Accordingly, when the rotary unit 10 
is rotated by the motor 13, a pressure is generated by a pumping effect of 
the grooves 14A, 14B and 15, so that rigidity of the oil film increases 
and thus, the rotary unit 10 rotates relative to the fixed shaft 1 with a 
bearing clearance of the oil film being positively defined therebetween. 
It is to be noted that the known fluid bearing device is constituted by a 
radial bearing member 16 and a thrust bearing member 17. The radial 
bearing member 16 of the known fluid bearing device is constituted by the 
fixed shaft 1, the radial grooves 14A and 14B and the disk 3, while the 
thrust bearing member 17 of the known fluid bearing device is constituted 
by the thrust bearing plate 4, the end face 1A and the spiral groove 15. 
However, in the case where an identical oil or an identical grease is 
supplied into the radial grooves 14A and 14B and the spiral groove 15 as 
described above, the prior art fluid bearing device has the following 
three drawbacks. 
Firstly, in the case where oil is used as the lubricant, a frictional 
bearing torque of the known fluid bearing device is required to be lowered 
to a small value in order to decrease power consumption of the motor 13 
acting as a driving source. A load capacity Pr of the radial bearing 
member 16 and a frictional bearing torque Mr of the known fluid bearing 
device are, respectively, given by the following equations (1) and (2): 
EQU Pr.varies.R.sup.2 .multidot.B.sup.2 ( 1) 
EQU Mr.varies.R.sup.3 .multidot.B (2) 
where: 
R=radius of fixed shaft 1, and 
B=length of known fluid bearing device. 
It will be readily understood from the above equations (1) and (2) that 
when a value of (R/B) is decreased, the frictional bearing torque Mr can 
be decreased while fixing the load capacity Pr at a constant value. 
Meanwhile, in order to lower a production cost of the known fluid bearing 
device through reduction of the number of its components, it has been 
usually so arranged as described above that the thrust bearing member 17 
is constituted by the thrust bearing plate 4 and the end face 1A. Thus, 
when the radius R of the fixed shaft 1 is decreased, an area of the end 
face 1A of the fixed shaft 1 becomes excessively small. Furthermore, since 
oils usually have viscosities lower than those of greases and become 
further less viscous at high temperatures, a sufficient bearing clearance 
of the oil film is not defined at the thrust bearing member 17 and thus, 
the thrust bearing plate 4 and the end face 1A are subjected to wear 
through contact therebetween. 
Secondly, in the case where grease is used as the lubricant, the grease has 
such inferior temperature characteristics compared with a low-viscosity 
oil that the grease has an extremely high viscosity at low temperatures as 
shown in FIG. 4. Meanwhile, the radial bearing member 16 is usually 
subjected to not less than 80% of the frictional bearing torque Mr. Thus, 
in the case where the grease is used for the radial bearing member 16, the 
frictional bearing torque Mr increases at low temperatures, so that a 
torque generated by the motor 13 becomes insufficient and thus, it becomes 
impossible to rotate the motor 13 at a predetermined number of 
revolutions. 
Thirdly, in view of the disadvantages of the above first and second cases, 
oil is used for the radial bearing member 16, while grease is used only 
for the thrust bearing member 17. However, in this case, when the known 
fluid bearing device is operated at high temperatures of about 80.degree. 
C. or more for a long time, a portion of base oil in the grease oozes out 
of the grease so as to be mixed with the oil of the radial bearing member 
16 such that viscosity of the oil of the radial bearing member 16 changes 
or lubricating property of the oil changes extremely due to deterioration 
of effects of the additives, thereby impairing reliability of the known 
fluid bearing device. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide an 
improved fluid bearing device which has a small frictional bearing torque 
and is highly reliable in actual use, with substantial elimination of the 
disadvantages inherent in conventional fluid bearing devices of this kind. 
In accomplishing these and other objects according to one preferred 
embodiment of the present invention, there is provided an improved fluid 
bearing device comprising: a shaft; a sleeve which is rotatably supported 
by said shaft; a thrust bearing plate which is supported by said sleeve 
such that one face of said thrust bearing plate is brought into contact 
with one end face of said shaft; a radial groove for producing a dynamic 
pressure, which is formed on one of an outer periphery of said shaft and 
an inner periphery of said sleeve; and a spiral groove which is formed on 
one of said end face of said shaft and said face of said thrust bearing 
plate; said radial groove retaining therein oil, while said spiral groove 
retains therein grease containing base oil and thickener, with said base 
oil being identical, in compositions and viscosity, with said oil retained 
in said radial groove. 
In accordance with the present invention, it becomes possible to obtain a 
highly reliable fluid bearing device having excellent temperature 
characteristics.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIGS. 5 and 6, a fluid 
bearing device K1 according to a first embodiment of the present 
invention, which is applied to a VTR cylinder of a video tape recorder. In 
FIGS. 5 and 6, a fixed shaft 18 having an end face 18A is press fitted 
into a central bore of a lower cylinder 19. A disk 20 having a sleeve 
portion 20A is rotatably fitted around the fixed shaft 18. The disk 20 
further has an opening 20B formed at an upper portion thereof. A thrust 
bearing plate 21 is mounted on an upper face of the disk 20, while an 
upper cylinder 22 is secured to a shoulder portion of the disk 20. 
Furthermore, a magnetic head 23 is secured to a lower face of the upper 
cylinder 22. A rotary member 24 of a rotary transformer is secured to a 
lower portion of the disk 20 so as to transmit, though the rotating 
magnetic head 23, to a fixed member 25 of the rotary transformer video 
signals delivered from a magnetic tape. The fixed member 25, which 
receives the video signals of the magnetic tape, is fixed to the lower 
cylinder 19 so as to confront the rotary member 24 of the rotary 
transformer. 
Meanwhile, an armature magnet 29 of a direct drive type motor 31 is secured 
to a lower portion of the disk 20. A fixed coil unit 30 is mounted on the 
lower cylinder 19 so as to confront the armature magnet 29 such that the 
direct drive type motor 31 is constituted by the armature magnet 29 and 
the fixed coil unit 30. The fixed coil unit 30 is at all times attracted 
at a force of 600 to 900 grams by the armature magnet 29. Thus, a rotary 
unit 28 is constituted by the disk 20, thrust bearing unit 21, upper 
cylinder 22, magnetic head 23, rotary member 24 and armature magnet 29. A 
C-shaped retaining ring 26 is fitted around an upper portion of the fixed 
shaft 18 so as to prevent the rotary unit 28 from being removed from the 
fixed shaft 18. 
Furthermore, radial grooves 32A and 32B are, respectively, formed at the 
upper portion and a central portion of the fixed shaft 18 by etching, etc. 
The end face 18A of the fixed shaft 18 is subjected to precision machining 
in flatness and perpendicularity to an axis of the fixed shaft 18. A 
single-row spiral groove 33 is formed on a central lower face of the 
thrust bearing plate 21 by etching, etc. so as to confront the end face 
18A. 
It should be noted that the fluid bearing device K1 is constituted by a 
radial bearing member 34 and a thrust bearing member 35. The radial 
bearing member 34 is constituted by the fixed shaft 18, disk 20 and radial 
grooves 32A and 32B, while the thrust bearing member 35 is constituted by 
the end face 18A, thrust bearing plate 21 and spiral groove 33. It should 
be further noted that grease 36 and oil 37 are supplied, as lubricant of 
the fluid bearing device K1, independently of each other into the thrust 
bearing member 35 and the radial bearing member 34, respectively. 
Hereinbelow, the lubricant of the fluid bearing device will be described. 
The oil 37 of the radial bearing member 34 is obtained by adding a small 
amount of additive to one of such oils having excellent temperature 
characteristics as diester, polyol ester, alpha-olefin, mineral oil, etc. 
Meanwhile, the grease 36 of the thrust bearing member 35 is obtained by 
adding one of such thickeners as stearic acid, oleic acid, lithium 
stearate, etc. to base oil. The oil 37 containing the additive is as it is 
employed as the base oil of the grease 36 such that the base oil of the 
grease 36 is identical, in at least viscosity and compositions, with the 
oil 37. It is to be noted here that the present invention is characterized 
in that the oil 37 is identical with the base oil of the grease 36. 
In this embodiment of the present invention, dimer, trimer or tetramer of 
decene-1 which is a kind of alpha-olefin was employed as the oil 37 of the 
radial bearing member 34. Meanwhile, the grease 36 of the thrust bearing 
member 35 was obtained by adding lithium stearate (thickener) to the oil 
37 prepared as described above. 
In the case where a fluid bearing device of this kind is applied to the VTR 
cylinder or the like, the fluid bearing device should be operated at 
temperatures ranging widely from about -10.degree. C. to about 80.degree. 
C. or more. Thus, the fluid bearing device is required to have such 
contrary characteristics as a small frictional bearing torque at low 
temperatures and high reliability at high temperatures. In the fluid 
bearing device K1, since the oil 37 having excellent temperature 
characteristics is used for the radial bearing member 34 subjected to a 
majority of a frictional bearing torque of the fluid bearing device K1, 
the radial bearing member 34 functions excellently. Meanwhile, the grease 
36, which has a viscosity far higher than that of the oil 37 even at high 
temperatures, is used for the thrust bearing member 35 having a small 
bearing area, it becomes, needless to say, possible to positively ensure 
lubricating property and reliability of the thrust bearing member 35. 
Furthermore, in the fluid bearing device K1, the oil 37 of the radial 
bearing member 34 is identical with the base oil of the grease 36 of the 
thrust bearing member 35. Accordingly, even if the fluid bearing device K1 
is operated at high temperatures for a long time and thus, a small amount 
of the base oil in the grease 36 oozes out of the grease 36 so as to be 
mixed with the oil 37 of the radial bearing member 34, such undesirable 
phenomena as change of viscosity of the oil 37, deterioration of 
lubricating property of the oil 37, etc. do not take place, whereby it 
becomes possible to ensure proper performances and reliability of the 
fluid bearing device K1. 
The fluid bearing device K1 further includes a ring 27 fitted around the 
fixed shaft 18 and adjacent to the end face 18A such that the ring 27 is 
disposed between the retaining ring 26 and the thrust bearing plate 21. A 
fluid bearing device of this kind is required to be resistant against 
vibrations during its transport for a long time, drop impact forces during 
its use, etc. When the fluid bearing device is subjected to such heavy 
vibrations or large drop impact forces, the rotary unit 28 is vibrated 
vertically as shown by the arrow in FIG. 6 and thus, a portion of the 
grease 36 of the thrust bearing member 35 is scattered therearound, 
thereby resulting in possible lack of the grease 36 in the thrust bearing 
member 35. However, the fluid bearing device K1 is provided with the ring 
27 such that a clearance between the thrust bearing plate 21 and the ring 
27 is set at 0.1 to 0.5 mm. Thus, when the rotary unit 28 is rotated, most 
of the scattered portion of the grease 36 scattered by pumping effect of 
the spiral groove 33 of the thrust bearing plate 21 can be again returned 
to a bearing clearance of the thrust bearing member 35, so that such a 
phenomenon as lack of the grease 36 in the thrust bearing member 35 does 
not take place. Moreover, the ring 27 prevents the grease 36 from being 
mixed with the oil 37 of the radial bearing member 34 with the result that 
the frictional bearing torque of the fluid bearing device K1 increases. 
As shown in FIG. 6, since the ring 27 has a transport groove 27A formed on 
one face thereof adjacent to the thrust bearing plate 21 and the spiral 
groove 33 is formed so large as to be approximately identical, in size, 
with the ring 27, the scattered grease can be rapidly collected into the 
bearing clearance of the thrust bearing member 35. 
Referring to FIGS. 7 and 8, there is shown a fluid bearing device K2 
according to a second embodiment of the present invention. The fluid 
bearing device K2 includes a C-shaped ring 38 in place of the retaining 
ring 26 and the ring 27 of the fluid bearing device K1. The ring 38 is 
formed with a split extending in a radial direction thereof. The ring 38 
is further formed with a plurality of transport grooves 38A for collecting 
grease scattered upon drive of the rotary unit 28, a retainer portion 38B 
engageable with a circumferential groove 18B of the fixed shaft 18, and a 
pair of holes 38C provided at both sides of the slit and adjacent to the 
slit, respectively. The retainer portion 38B is brought into engagement 
with the circumferential groove 18B of the fixed shaft 18 so as to prevent 
the rotary unit 28 from being removed from the fixed shaft 18. The holes 
38C are used for assembly and disassembly of the fluid bearing device K2. 
The spiral groove 33 is formed so large as to collect grease thereinto. 
Referring to FIG. 9, there is shown a fluid bearing device K3 according to 
a third embodiment of the present invention. The fluid bearing device K3 
includes a ring 40 and a fixed shaft 41. Furthermore, the thrust bearing 
plate 21 is formed with a spiral groove 39. The ring 40 is held in contact 
with the thrust bearing plate 21 between the disk 20 and the fixed shaft 
41 so as to be rotated together with the disk 20. Thus, even if the fluid 
bearing device K3 is subjected to vibrations, etc., it becomes possible to 
substantially prevent the grease 36 from scattering out of the thrust 
bearing member 35 and the spiral groove 39 collects the grease 36 to a 
central portion of the thrust bearing member 35. Moreover, since a helical 
groove 41C is formed on either an outer periphery of an upper portion of 
the fixed shaft 41 or an inner periphery of the ring 40, the scattered 
grease can be more positively collected into the thrust bearing member 35. 
Meanwhile, a vent hole 20C is formed on the disk 20 so as to prevent the 
oil 37 from being forced out of the radial bearing member 34. 
It is to be noted here that in the case where the ring 27, rotary unit 28 
and ring 40 are made of elastic synthetic resin such as polyether sulphone 
(PES), polybutylene terephthalate (PBT), etc., the fluid bearing device of 
the present invention becomes excellent in oil resistance and creep 
resistance. 
Referring further to FIG. 10, there is shown a fluid bearing device K4 
according to a fourth embodiment of the present invention. The fluid 
bearing device K4 includes a shaft 42, a lower cylinder 43 having a sleeve 
portion 43A and the ring 38 of the fluid bearing device K2. The shaft 42 
is formed with radial grooves 42A and 42B. In the fluid bearing device K4, 
it is so arranged that the shaft 42 is rotated. 
Since other constructions of the fluid bearing devices K2, K3 and K4 are 
similar to those of the fluid bearing device K1, detailed description 
thereof is abbreviated for the sake of brevity. Furthermore, although the 
end face 18A of the fixed shaft 18 is formed flat in the above embodiments 
of the present invention, it can be also so arranged that the end face 18A 
is formed into a shape of a sphere or trapezoid. 
As is clear from the foregoing description, in accordance with the present 
invention, since oil is used for the radial bearing member while grease 
containing the oil of the radial bearing member as its base oil is used 
for the thrust bearing member, it becomes possible to obtain a highly 
reliable fluid bearing device functioning excellently at high 
temperatures. Furthermore, in accordance with the present invention, the 
fluid bearing device produces such remarkably practical effects as 
resistance against vibrations and drop impact forces. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.