Bearing apparatus

A bearing apparatus comprises a housing formed of a non-magnetizable material and having a bottom portion, a fluid lubricating type radial bearing device provided coaxial with the housing and having a magnetic fluid sealing section and a radial bearing section, a thrust bearing device provided at the bottom portion of the housing, and a rotating shaft of a permeable material rotatably supported by the radial bearing device and thr thrust bearing device to extend through the radial bearing device. The magnetic fluid sealing section is disposed on the opening side of the housing with a predetermined spacing away from the radial bearing section whereby a space is provided between the magnetic fluid sealing section and a liquid surface of fluid lubricant filled in the radial bearing section.

BACKGROUND OF THE INVENTION 
This invention relates to a bearing apparatus which uses a fluid lubricant 
such as a lubricating oil, a magnetic fluid and, the like, and more 
particularly, to a bearing apparatus suitable for use in a polygon mirror 
drive motor for laser beam printers or a motor for magnetic disk 
equipments, VTR and the like, which motor is required to exhibit a high 
rotational accuracy with a minimum of shaft whirling. 
Recently, high speeds and high accuracy rotation with less shaft whirling 
are required of drive motors of the above type in the aspect of highly 
minute images and highly densed memory. In particular, a clean bearing 
apparatus adapted for "high accuracy rotation" and free from oil 
contamination are desired in laser beam printers and magnetic disk 
equipment. 
With respect to such demands, problems relating to rotational accuracy and 
to contamination of main machines are directly related to performances of 
a bearing apparatus. In this respect, conventional ball bearings have 
limitations on high speeds and accuracy of rotation as a result of the 
accuracy in working rolling members and inner and outer races. For this 
reason, fluid-lubricated plain bearings have been employed as bearings 
which are effective in high speed rotation and enabling "high accuracy 
rotation", and various improvements have been made on such plain bearings 
for use in motors of the above type. 
In a plain bearing, a fluid lubricant film formed on sliding surfaces upon 
rotation of a shaft maintains a shaft and a bearing in non-contact 
condition to rotatably support the shaft, and oil or gas is used as a 
lubricant. Unlike a gas bearing, a plain bearing making use of a 
lubricating oil is not expected to involve low torque, but provides an oil 
film of high rigidity which attains "high accuracy rotation" with less 
shaft whirling. Accordingly, such plain bearing can be designed to be of a 
small diameter as compared with a gas bearing, thus enabling a realization 
of a compact motor. However, in a plain bearing making use of a 
lubricating oil, oil leakage is always problematic, and dispersion of oil 
during high speed rotation poses a problem in the practical use of polygon 
mirror drive motors and magnetic disk spindle drive motors. 
To cope with this problem, a magnetic fluid bearing has been proposed for 
use in drive motors of the above type including permanent magnet and a 
magnetic fluid with the magnetic fluid having a sealing function and 
providing lubrication. The magnetic fluid is formed by treating magnetic 
powders with a surface active agent and dispersing the same in a base oil. 
There are two types of magnetic fluid bearings classified in a basic 
construction. One type of magnetic fluid bearing retains a magnetic fluid 
on sliding bearing surfaces by magnetizing the same by a 
cylindrical-shaped permanent magnet, as disclosed in, for example, 
Japanese Patent Unexamined Publications Nos. 55-139559 and 59-147117. The 
other type of magnetic fluid bearing has a permanent magnet arranged at an 
end of the bearing and the permanent magnet and a permeable rotating shaft 
constitute a magnetic fluid sealing to have a magnetic fluid filled in a 
bearing section for lubrication, as disclosed in, for example, Japanese 
Patent Unexamined Publications Nos. 61-270520 and 60-88223. These two 
types of magnetic fluid bearings are intended for the prevention of 
dispersion of a magnetic fluid by magnetizing and providing the same with 
a sealing function, and are designed to increase magnetic flux density for 
sealing. In these prior bearings, a magnetic fluid is disadvantageously 
dispersed due to increased centrifugal forces upon rotation of high speeds 
if an amount of magnetic fluid as retained is slightly more than as 
required. Even if an amount of magnetic fluid as retained is appropriate, 
the magnetic fluid tends to be dispersed in the range of high rotational 
speeds due to cubical expansion produced by temperature rise since the 
magnetic fluid has a considerably large, thermal expansion coefficient as 
compared with constituent materials of the bearing, as described in the 
monthly magazine "Tribology" page 20, March 1988. In addition, there is a 
possibility of deterioration of a magnetic fluid when used at high 
temperatures. That is, as described in "Tribology" page 15, a surface 
active agent tends to separate from the magnetic powders when exposed to 
high temperatures, so that the magnetic powders will cohere to mar 
dispersion. In particular, in the case of a plain bearing, an eccentric 
load produced by centrifugal forces are applied on sliding surfaces to 
increase viscous shearing stresses on the lubricant film, unlike a 
magnetic fluid sealing. In addition, as rotational speeds are increased, a 
shear velocity becomes high to increase the density of generation of heat 
due to viscous friction, so that the surface active agent is separated 
from the magnetic powders to cause deterioration of the bearing 
performance. Therefore, thermal measures such as cooling are necessary in 
case a magnetic fluid is used as a sealing or lubricating fluid in high 
speeds. 
SUMMARY OF THE INVENTION 
It is an object of the invention to solve the above-mentioned problems of 
the prior art. 
It is another object of the invention to provide a bearing apparatus which 
is effective in rotation of high speeds and high accuracy and is improved 
in durability and sealing performance. In accordance with advantageous 
features of the present invention, a magnetic fluid sealing which 
constitutes a bearing apparatus is spaced away from a bearing section to 
provide therebetween a space which accommodates a cubical expansion of a 
lubricant and maintains a constant amount of a magnetic fluid in a 
magnetic fluid sealing section to prevent dispersion of the magnetic fluid 
upon rotation of high speeds. In addition, deterioration of performance 
due to temperature rise accompanied by viscous shearing of the magnetic 
fluid is prevented by providing a mechanism for circulating the magnetic 
fluid through making use of shaft rotation. 
The present invention provides a bearing apparatus comprising a housing 
formed of a non-magnetizable material and having a bottom portion, a fluid 
lubricating type radial bearing means provided coaxial with housing and 
having a magnetic fluid sealing section and a radial bearing section, with 
a thrust bearing means provided at the bottom portion of said housing. A 
rotating shaft of a permeable material is rotatably supported by radial 
bearing means and thrust bearing means to extend through the radial 
bearing means, and the magnetic fluid sealing section is disposed on the 
opening side of the housing with a predetermined spacing away from the 
radial bearing section whereby a space is provided between the magnetic 
fluid sealing section and a liquid surface of fluid lubricant filled in 
the radial bearing section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a bearing apparatus 10 includes a bearing housing 14, 
a magnetic fluid sealing section generally designated by the reference 
numeral 2, a first radial bearing 6, a second radial bearing 7, a thrust 
bearing 8 and a fluid lubricant 11 which completely fills one end of the 
bearing apparatus 10. The magnetic fluid sealing section 2 includes a 
permanent magnet 3, a pole piece 4, a cover 5 and a magnetic fluid 12 with 
the permanent magnet 3, pole piece 4 and cover 5 being coaxial with the 
bearing housing 14. The magnetic fluid sealing section 2 is securedly 
mounted on an open end of the bearing housing 14 so as to be spaced from 
the first radial bearing 6 by a distance e. The fluid lubricant 11 for the 
first and second radial bearings 6,7 and the thrust bearing 8 may be the 
same as the magnetic fluid 12 for the magnetic fluid sealing section 2 or 
a magnetic fluid of a different composition or a lubricant such as a 
lubricating oil. 
Passages a, b, c and d for the fluid lubricant are defined by the bearing 
housing 14, the first and second radial bearings 6, 7 and a rotating shaft 
1. In FIG. 2, three axial passages c disposed around the radial bearing 6 
or 7. However, one passage c may suffice. Instead of forming such passage 
c by cutting the outer periphery of the radial bearing, it may be 
constituted by a groove or grooves formed in the bearing housing 14. 
As shown in FIG. 3, fluid lubricant 11 circulates about the radial bearing 
6 in the bearing apparatus 10 according to the first embodiment of the 
invention, and, when the fluid lubricant 11 revolves upon rotation of the 
rotating shaft 1, the axial length of the passage b on one end of the 
radial bearing 6 becomes smaller than that of the passage d on the other 
end of the radial bearing 6. Accordingly, centrifugal forces applied to 
the fluid lubricant are larger in the passage d than in the passage b, so 
that the fluid lubricant 11 flows in a direction shown by an arrow in FIG. 
3. As the rotating shaft 1 rotates with a small amount of eccentricity S 
(FIG. 4) relative to the radial bearing 6, a bearing clearance on the end 
surface of the radial bearing 6, that is, in the passage b is divergent in 
shape on the lefthand side of a point P (FIG. 4) where the fluid lubricant 
flows axially. Also, as the bearing clearance is convergent in shape on 
the righthand side of the point P in a clockwise direction, the fluid 
lubricant tends to flow axially toward the passage b but it generally 
flows in the passage a in a direction from the passage b to the passage d 
since the centrifugal forces are large in the passage d as compared with 
the forces in the passage b. Such flow is realized by the provision of the 
passages a, b, c and d. Accordingly, the fluid lubricant having been 
subjected to viscous shearing in the passage a to generate heat flows to 
the end surface of the radial bearing and circulates while discharging 
heat to the bearing housing 14 to be cooled. Thus, according to the 
invention, the fluid lubricant is not subjected to repeated viscous 
shearing in the bearing clearance or the passage a, so that a surface 
active agent will not separate from the magnetic powders, which are 
treated with the agent, for thermal reasons. 
Referring now to FIG. 5, there is a circulating flow of the fluid lubricant 
in the thrust bearing 8 of the bearing apparatus according to the first 
embodiment of the invention. The thrust bearing 8 is formed at its central 
portion with a hole 29 and in the peripheral portion thereof with a 
plurality of holes 30. The central portion of the thrust bearing 8 faces 
the end surface of the rotating shaft 1. As the rotating shaft 1 rotates, 
a lubricant film f is formed on the sliding surface in the central portion 
of the thrust bearing 8, and the fluid lubricant flows radially outwardly. 
The fluid lubricant 11 circulates through the holes 29 and 30 in a 
direction shown by the arrows in FIG. 5. Thus, the fluid lubricant 11 is 
not subjected to repeated viscous shearing in an area between the end 
surface of the rotating shaft 1 and the central portion of the thrust 
bearing 8, but it circulates in the above-mentioned manner to improve 
lubrication on the sliding surface or the central portion of the thrust 
bearing 8. Without such hole 29, the sliding surface of the rotating shaft 
1 is not supplied with any fluid lubricant, so that rotation of high 
speeds will cause poor lubrication to be resulted in seizure and wear. 
In FIG. 6, a magnetic fluid sealing 2 includes a magnetic circuit m shown 
by dotted lines formed by the permanent magnet 3 which is magnetized to be 
axially oriented, with the pole piece 4 being closely contacted by the 
permanent magnet, and the permeable rotating shaft 1. Magnetic flux 
density is increased in an area between the pole piece 4 and the rotating 
shaft 1, in which area a magnetic fluid 12 is retained to provide a 
magnetic fluid sealing. Also, a non-magnetizable disk or cover 5 is 
provided so that the magnetic fluid 12 will not flow out of the bearing 
housing 14 even when it is dispersed out of the magnetic fluid sealing 
section. The cover 5 is non-magnetizable so as not to define any magnetic 
circuit in a gap g between the cover 5 and the rotating shaft 1, so that 
even if the magnetic fluid 12 retained at the inner periphery of the pole 
piece 4 is dispersed, the fluid will be caught by the sealing portion of 
the pole piece 4 to maintain a regenerative function of the magnetic fluid 
sealing. Accordingly, if the gap g between the rotating shaft 1 and the 
cover 5 is set as small as possible, reduction of the magnetic fluid 12 
caused by vaporization can be suppressed to eliminate environmental 
contamination produced by oil vapor even in long term use. In the present 
bearing apparatus, it is possible to suppress reduction of the magnetic 
fluid caused by vaporization by setting the size of the gap g equivalent 
to the bearing clearance, that is, the passage a or smaller than the gap g 
around the magnetic fluid sealing section, that is, the gap g between the 
rotating shaft 1 and the pole piece 4. Since there is a possibility of 
injuring the rotating shaft 1 due to oscillations and the like when the 
gap g is set equivalent to the bearing clearance, such injury to the 
rotating shaft 1 can be prevented by employing a lubricating copper alloy 
softer than the material of the rotating shaft 1 or a sliding material 
such as Teflon and nylon to form the cover. 
In the present bearing apparatus, cubical expansion of the fluid lubricant 
11 produced when the fluid lubricant 11 is subjected to thermal expansion 
caused by generation of heat due to viscous friction is accommodated by 
the space e. Thus the pressure within the space e is somewhat increased 
because a compressive gas is contained in the space e. However, such 
pressure is not raised to affect the sealing performance, so that 
dispersion of the magnetic fluid is avoided unlike the prior art. In 
addition, the provision of such space e enables making use of a 
lubricating oil for the magnetic fluid 12 and adjusting an amount of the 
fluid lubricant 11 in the permissible range determined by the space e 
without having to strictly control an amount of the magnetic fluid 12 
retained by the gap g. This constitution is advantageous in terms of cost 
in the field of mass-produced motors of this type. 
Referring to FIG. 7, there is partly shown a bearing apparatus according to 
a second embodiment of the invention, in which a hole 30a constituting a 
circulating mechanism of a thrust bearing 8a is formed in a bearing 
housing 14a. 
Referring to FIG. 8, a magnetic fluid sealing section 2b with a permanent 
magnet 3b has a shape in an axial cross-section without any pole piece. 
The magnetic fluid sealing section 2b functions in a manner equivalent to 
that shown in FIG. 6 while it is superior to the latter in that it has a 
less number of elements than the latter. 
Referring now to FIGS. 9 and 10, a groove (16 or 17) is formed in the 
rotating shaft 1 or a radial bearing (6c or 7c) to promote the circulating 
function of a circulating mechanism. 
Referring to FIG. 11, bearing apparatus 10d includes a rotating shaft 1d 
rotatably supported by a single radial bearing 6d and a disk-shaped thrust 
bearing 8d. The rotating shaft 1d securedly mounts thereon a rotor 15 for 
a polygon mirror, disk, cylinder and the like, so that when the rotor 15 
rotates, an air flow in a direction shown by an arrow is produced due to 
an action of fan between the cover 5 and the rotor 15. Without the cover 
5, such air flow would tend to vaporize the magnetic fluid 12. However, 
such reduction of the magnetic fluid 12 caused by vaporization can be 
suppressed by making the gap g between the rotating shaft 1d and the cover 
5 small. 
FIG. 12 shows a polygon mirror drive motor for laser printers wherein the 
motor comprises a motor rotor 117 formed of a permanent magnet and secured 
to a rotor 115, a motor stator 118 secured to a motor housing 119, a 
polygon mirror 120 securedly mounted on the rotor 115 by a mirror retainer 
121, and a rotating shaft 101 press fitted into the rotor 115 and 
rotatably supported by a bearing apparatus. The bearing apparatus 
comprises a magnetic fluid sealing 102, radial bearings 106, 107, a thrust 
bearing 108 and a bearing housing 114. A predetermined amount of magnetic 
fluid 111 is filled in the bearing apparatus as a fluid lubricant to 
submerge the end surface of the upper radial bearing 106. A body of 
magnetic fluid 112 is provided by lifting the rotating shaft 101 having 
been inserted into the bearing housing so as to cause a portion of the 
magnetic fluid 111 adhering to the rotating shaft 101 to be retained in a 
gap between the magnetic fluid sealing 102 and the rotating shaft 101. 
The polygon mirror drive motor thus constituted operates in the following 
manner. When the rotor 115 is driven at high speeds, the magnetic fluid 
111 circulates in a flow passage on the sliding surfaces of the radial 
bearings 106, 107 and the thrust bearing 108 to ensure lubrication and 
cooling for stable rotation in a long term. Even when the rotation of the 
rotor 115 causes a temperature rise in the magnetic fluid 111 and hence a 
cubical expansion thereof, the magnetic fluid 111 will not overflow or 
disperse to contaminate the polygon mirror since the radial bearing 106 
and the magnetic fluid sealing 102 are spaced away from each other to 
provide a space e. In the bearing apparatus of the invention, the magnetic 
fluid 111 performs a circulating flow to be improved in cooling, by which 
a surface active agent will not separate from magnetic powders treated 
therewith. Thus, the properties of the magnetic fluid 111 is maintained 
stable over a long period of time. Accordingly, when the bearing apparatus 
of the invention is applied on a polygon mirror drive motor, a rotor is 
rotatably supported by an oil film of high rigidity in a sliding bearing, 
so that shaft whirling is all but eliminated and rotation of high accuracy 
is maintained while a polygon mirror is free from contamination due to 
dispersion of a fluid lubricant. In this manner, the polygon mirror drive 
motor can be made reliable through application of the invention. 
According to the invention, a magnetic fluid having a high saturation 
magnetization (that is, a large proportion of magnetic powders in a base 
oil) and a high viscosity is retained in a magnetic fluid sealing section, 
and a fluid lubricant of low viscosity or a magnetic fluid is filled in a 
bearing section, whereby bearing loss is reduced and cubical expansion is 
suppressed by a lowering of temperature rise to improve reliability of a 
magnetic fluid sealing. In addition, a circulating mechanism making use of 
rotation of a rotating shaft cooperates with a space between a magnetic 
fluid sealing and a bearing section to contribute to improvement of 
sealing performance and prevent deterioration of performance of a magnetic 
fluid sealing. 
In the invention, the provision of a space between a bearing section and a 
magnetic fluid sealing section eliminates the need of strictly adjusting 
an amount of fluid lubricant filled into the bearing section to be very 
effective in reduction of cost in mass production. Furthermore, a fluid 
lubricant different from a magnetic fluid retained by a magnetic fluid 
sealing section can be filled in a bearing section, so that it is possible 
to use a fluid lubricant which meets a desired operating condition. In 
addition, a circulating mechanism for a fluid lubricant can prevent 
deterioration of performance of a magnetic fluid due to temperature rise 
even when a magnetic fluid is used for a fluid lubricant. Thus a bearing 
apparatus having durability and high reliability can be provided by the 
invention. 
In a magnetic fluid sealing section, a non-magnetizable disk is provided on 
an open end of a bearing housing, and a gap defined between the disk and a 
rotating shaft is set to have the same size as that of a bearing 
clearance, whereby reduction of a magnetic fluid due to vaporization can 
be suppressed in the bearing apparatus to make the magnetic fluid durable. 
As described above, when the bearing apparatus of the invention is applied 
on a polygon mirror drive motor and a spindle drive motor for magnetic 
disk equipments, which motors are required of "high accuracy rotation" and 
cleanliness, it is possible to realize durability, high reliability and 
reduction in production cost.