Shaft support assembly for use in a polygon mirror drive motor

A shaft support assembly particularly well suited for supporting the high speed rotor in a laser polygon mirror drive motor assembly. The rotor has conically tapered ends. These ends are supported by conical plain bearings. At least one of the bearings is adjustable with respect to the shaft and the bearing so that the gap between the shaft and bearing can be easily adjusted.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a shaft support assembly for use in a polygon 
mirror drive motor for laser beam printers or a motor for magnetic disk 
equipment, VTR and the like. Laser polygon mirror drive motor assemblies 
present a singular set of shaft support conditions. The high speed motor 
must exhibit a high rotational accuracy with a minimum of shaft whirling; 
this is particularly true in modern laser printers where high speed and 
highly minute images and very dense memory are involved. The shaft support 
conditions are, however, quite stable. The rotor reaches its high 
operating speed almost instantaneously and operates at a constant speed. 
2. Description of the Prior Art 
It is known that problems relating to rotational accuracy and to 
contamination of the overall machine are directly related to performance 
of the shaft support assembly used to support the high speed rotor. The 
speed and accuracy of rotation of conventional rolling element bearings 
are limited by the accuracy of the working rolling members and inner and 
outer races. The high rotational speeds also dramatically reduce rolling 
element life. Consequently, rolling element bearings are not suitable for 
such applications. Instead, fluid-lubricated plain bearings have been 
employed in shaft support assemblies for laser polygon mirror drive 
motors. Such bearings are known to be effective in applications requiring 
high speed rotation and "high accuracy rotation". Of course, various 
improvements must be made in such plain bearing shaft support assemblies 
for use in mirror drive motors of the above type. 
In a plain bearing shaft support assembly, a fluid lubricant film is formed 
between sliding surfaces upon rotation of the shaft or rotor. Unlike a gas 
bearing, a plain bearing is not limited to low load applications. Indeed, 
if a lubricating oil is used, an oil film of high rigidity which attains 
"high accuracy rotation" and high load capacity can be obtained. 
Accordingly, such plain bearings can be designed to be of a shorter length 
as compared with a gas bearing, thus enabling a realization of a compact 
motor. 
There are, however, certain problems presented by the use of plain bearings 
in shaft support assemblies in polygon mirror drive motors. For example, 
in a plain bearing making use of a lubricating oil, oil leakage is always 
a problem, and dispersion of oil during high speed rotation poses a 
problem in the practical use of polygon mirror drive motors. Another 
problem is that plain bearings typically provide thrust or radial support, 
but not both. Finally, in applications demanding accurate shaft 
positioning the clearances in plain bearings must be accurately 
maintained. Manufacturing to the close tolerances required for such 
applications is expensive--if it is possible at all. Moreover, adjustment 
is particularly difficult since adjustment of one clearance can affect 
another clearance. 
To cope with the problem of oil leakage, a magnetic fluid bearing has been 
proposed for use in polygon mirror drive motors. Such bearings include a 
permanent magnet and a magnetic fluid. The magnetic fluid serves two 
functions: sealing and providing lubrication. The magnetic fluid can be 
formed by treating magnetic powders with a surface active agent and 
dispersing the same in a base oil. 
As noted in U.S. Pat. No. 4,938,611 to Nii et al. there are two basic types 
of magnetic fluid bearings. One type of magnetic fluid bearing retains a 
magnetic fluid on sliding bearing surfaces by magnetizing the same by a 
cylindrical-shaped permanent magnet. 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. 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. 
There are, however, problems associated with such bearing constructions. 
These problems are discussed in detail in the aforementioned U.S. Pat. No. 
4,938,611 to Nii et al. 
According to Nii et al., these problems can be addressed by providing a 
magnetic fluid sealing which constitutes a bearing apparatus 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, a 
mechanism for circulating the magnetic fluid through making use of shaft 
rotation is provided to prevent deterioration of performance due to 
temperature rise accompanied by viscous shearing of the magnetic fluid. 
The shaft support assembly of Nii et al. includes a housing formed of a 
non-magnetizable material and having a bottom portion, a fluid lubricating 
type radial bearing means coaxially located in the 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 surface of fluid lubricant filled in the radial bearing 
section. 
While the shaft support assembly disclosed by Nii et al. might solve the 
oil leakage problem experienced with previous polygon mirror motor support 
bearings, it fails to address the other two problems mentioned above--the 
need for close tolerances to permit precise alignment of the shaft and the 
need to provide simultaneously adjustable radial and thrust supports. As a 
result, the shaft support arrangement disclosed by Nii et al. is 
unnecessarily complex and expensive to construct. Thus, there remains a 
need for a simple reliable inexpensive easily adjustable bearing assembly 
for supporting a high speed shaft of the type used to support polygon 
mirrors. 
SUMMARY OF THE INVENTION 
The present invention relates to a polygon mirror drive motor assembly 
which includes an adjustable plain bearing which supports the high speed 
rotor on which the polygon mirror is supported. This assembly overcomes 
the problems experienced in conventional assemblies by providing a plain 
bearing assembly which offers both combined radial and thrust support and 
easy adjustment. With the assembly of the present invention, support in 
both the radial and thrust directions is simultaneously adjusted so that, 
at assembly, any slack in the system can be easily and quickly removed. 
This simplifies assembly and permits the use of component parts 
manufactured to less exacting tolerances. 
The present invention resides, in part, in designing a workable plain 
conical bearing system. The advantages of such a system are apparent: one 
step adjustment and combined radial thrust support to name two. The 
greatest difficulty in getting a simple continuous conical surface 
combined radial/thrust bearing to work has been the need for precise 
adjustment and tolerances. The relationship between fluid stiffness and 
the gap between the bearing and rotor is critical in a simple continuous 
conical combined radial/thrust bearing. When the thickness of the fluid 
film--determined by the gap--is properly adjusted, the film has exactly 
the stiffness needed to support load applied by the rotor. The gap must 
therefore be adjusted such that under normal operating conditions the 
fluid film--which has a known stiffness--is thick enough to provide 
support. By providing a conical bearing at each end of the shaft, the 
hydrodynamic forces balance one another so long as the proper gap is 
maintained. A static system with fixed gaps is possible in certain 
applications, e.g., polygon mirror drive motors, because the loads are 
substantially constant. 
Shaft support assemblies which include radial/thrust bearing arrangements 
include a shaft having a conical portion often referred to as a runner. 
The runner may be formed as a part of the shaft or formed separately and 
rotatably secured to the shaft. The bearing has a continuous conical 
surface which is similar, but not complimentary to the runner's surface 
since a complimentary surface would tend to seize. Generally, the bearing 
surface has a slightly greater diameter than the runner. 
A hydrodynamic fluid is located between the surface of the shaft runner and 
the bearing pad surface. The fluid has a calculable fluid film stiffness 
or spring characteristic. This fluid film stiffness acts in opposition to 
the load applied by the rotor in operation. When the load applied by the 
rotor varies during operation, the system should include a spring or the 
like to vary the position of the bearing surface to accommodate variations 
in load. In a stable system such as a laser polygon mirror drive motor 
assembly, however, the position of the conical bearing surface may be 
fixed, if it can be accurately positioned. 
When the shaft is at rest the bearing contacts the shaft runner. Because 
the bearing and runner have different shapes, this contact occurs along a 
single line (if the cone angles are equal) or discrete points. Fluid, 
preferably either air or liquid or another lubricant such as oil, fills 
the remaining space between the runner and bearing. As the shaft begins to 
rotate, the pressure and stiffness of the fluid increases. Under normal 
operating conditions, the fluid film has a calculable stiffness when the 
shaft is at rest. As the shaft speed approaches normal operating 
conditions, the fluid film stiffness increases until the shaft is lifted 
out of contact with the bearing. If the opposed conical bearings are 
accurately positioned, an equilibrium position will be reached. At 
equilibrium, the fluid stiffness acting on each end of the shaft exactly 
opposes the rotor loads and the bearing is spaced from the shaft runner 
and the rotor is supported on a fluid film. 
The polygon mirror drive motor assembly of the present invention preferably 
includes a motor casing, a high speed rotor or a shaft supported within 
the motor casing, a motor located within the motor casing for driving the 
rotor and a polygon mirror secured to the rotor such that the polygon 
mirror is adapted to be driven with the rotor by the motor. In accordance 
with the present invention, the high speed rotor has two axial ends and 
each axial end is conically tapered. The respective tapered ends of the 
rotor are supported by bearing assemblies. 
Each of these bearing assemblies include a conical bearing for supporting 
the conical end of the high speed rotor or shaft. The conical bearing is 
secured within a bearing housing. The conical bearing may be formed 
integrally with a portion or all of the bearing housing, if desired. The 
interior of the bearing housing is sealed except for an opening through 
which the conical end of the high speed rotor passes. The gap between the 
bearing housing and the periphery of the high speed rotor is sealed by a 
magnetic seal. The magnetic seal may be of any conventional form. The 
bearing housings are preferably substantially cylindrical and received in 
complimentary cylindrical openings formed in the motor casing. A locking 
set screw or some other means is provided to selectively fix the bearing 
housing in the motor casing once a desired position is found. 
In accordance with the present invention, the outer cylindrical surface of 
at least one of the bearing housings, and preferably both, are provided 
with means such as fine threads to enable precise adjustment of the 
bearing housings. Likewise, the interior surface of the cylindrical 
opening formed in the motor casing is provided with complimentary fine 
threads such that the bearing housing can be threaded into the motor 
casing. This allows the position of the two bearing housings to be 
precisely set. If only one of the two housings is provided with an 
adjusting means such as the threads, the other housing is first locked 
into position. The position of the other, adjustable, housing is used for 
all fine adjustment. Once the precise position of the housing is set, the 
position of the housing relative to the casing is fixed through the use of 
a locking set screw or the like. 
Because both the high speed rotor and the casing have fixed dimensions, 
clearances between the conical end of the high speed rotor and the conical 
support surface of the bearings within each bearing housing can be 
adjusted by simply adjusting the position of one of the bearing housings. 
Further, since the conical bearing provides both the radial and thrust 
support for the high speed rotor, this one simple mechanical adjustment 
results in simultaneous adjustment of the radial and thrust supports on 
both ends of the shaft. Moreover, the bearing components can be 
manufactured to less exacting tolerances because the bearing assembly can 
be quickly adjusted to compensate for any manufacturing errors or loose 
tolerances. Thus, the present invention overcomes the greatest difficulty 
in getting a simple continuous conical surface bearing to work--the need 
for precise adjustment and tolerances. 
The simple fixed geometry conical bearing of the present invention works in 
laser polygon mirror drive motor assemblies because the conical supports 
at each end of the shaft work together and because of the stable operating 
conditions, high speed and quick start up of such assemblies. Other 
variables in a plain bearing system such as the stiffness of the magnetic 
fluid and the shape and size of the various components are all fixed. The 
requirement for precise tolerances is, of course, solved by the present 
invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
The operating principles of the double conical plain bearing assembly of 
the present invention can be understood with reference to FIGS. 1 and 2. 
These figures show a simplified combined radial/thrust support assembly in 
which a conically shaped runner 5r is formed at each end of a shaft 5 and 
a pair of coaxially aligned bearing 3 each having a conical face are 
mounted in a housing 1 to support the runners 5r at each end. The shaft 5 
is thus supported in both the radial and thrust directions. The bearings 3 
for purposes of the illustration, are simple continuous conical surfaces. 
The bearings 3 are typically tapered at the same angle as the runner 5r 
but are slightly larger so that, at rest, the bearing and runner are 
eccentric and a wedge shaped space is formed between them. When the 
surfaces contact, they contact along a single line with a converging wedge 
shaped space extending from each side of the line of contact. The bearings 
3 are precisely spaced from the ends of the shaft 5. To facilitate this, 
at least one of the bearings 3 is provided with threads to allow the 
bearing to be threaded into and out of the housing 1. Rotating the bearing 
in the directions indicated by arrows leads to axial displacement of the 
bearing 3 relative to the housing 1, the shaft 5 and the other bearing 3. 
Ultimately, the clearance between the shaft 5 and both conical bearings is 
changed by this one simple adjustment. 
At rest, the bearings 3 contact the conical surfaces of the shaft runner 
5r. The bearing surfaces and conical surfaces are pressed against one 
another by gravity. As the shaft 5 begins to rotate, the stiffness of the 
hydrodynamic fluid increases until the stiffness of the fluid exceeds the 
gravity force which causes contact between the shaft and bearing surfaces. 
At that point, the fluid forces the surfaces apart until an equilibrium is 
reached and the shaft runner 5r and shaft 5 are supported on a film of 
pressurized fluid. At equilibrium, the axial stiffness at each end of the 
shaft is balanced so that shaft position is maintained with a high degree 
of accuracy. The equilibrium is maintained as long as operation conditions 
are stable. In a laser polygon mirror drive motor, the high speed 
equilibrium state is reached almost immediately at start up and is 
maintained throughout operation. 
There are several constraints faced when designing a conical bearing 
support system. For instance, once the fluid to be used is known, the 
stiffness characteristics of that fluid are fixed since they are physical 
characteristics. In such a case, the balance depends entirely on setting 
an appropriate clearance between the shaft and bearings. The clearance is 
preferably adjusted manually and fixed once the proper setting is found. 
If the operating conditions vary, however, an automatic adjustment means 
should be provided. Automatic adjustment can also be used as an 
alternative to manual adjustment, if desired. As described in applicant's 
previous application Ser. No. 07/685,148 filed Apr. 15, 1991, such an 
automatic adjustment can be provided by a spring such as a Belleville 
(initially coned) spring, a spring washer and an elastomeric cushion or a 
beam-like support structure. In this way a very simple and reliable 
combined radial and thrust bearing arrangement can be provided. 
FIG. 3 shows a polygon mirror drive motor assembly for laser printers or 
the like. The assembly includes a casing 10 which is preferably 
constructed of aluminum or the like. A motor 20 is secured to the casing. 
The motor 20 includes a motor rotor 21 formed of a permanent magnet 22 
secured to a rotor shaft 23 and a motor stator 24 secured to the motor 
casing 10. A polygon mirror 27 is securely mounted on the rotor 23 by a 
mirror retainer 28. An opening 12 is formed in the motor casing 10. The 
polygon mirror 27 is arranged such that a portion of the mirror 27 extends 
through the opening 12. In this way, the mirror can be used to reflect the 
laser beam in a laser printer or the like as is known. 
As shown in FIG. 3, the axial ends 23r of the rotor 23 are conically 
tapered and rotatably supported by a pair of coaxially aligned bearing 
assemblies 30. In the illustrated embodiment, each bearing assembly 30 
includes a bearing housing 31 having a substantially cylindrical outer 
periphery. As discussed below, the interior of the bearing housing should 
be fluid tight. In the illustrated embodiment, this is accomplished by 
closing one end with an aluminum cap 32. The aluminum cap is secured to 
the bearing housing 31 by cap bolts 34. An O-ring 35 provides a fluid 
tight seal between the aluminum cap 32 and the bearing housing 31. A 
filler screw 33 or similar device to allow access to the fluid tight 
interior of the housing is provided. By removing this filler screw 33 
magnetic oil can be inserted into the sealed housing. The end of the 
housing opposite the cap is open to allow the tapered end of the high 
speed rotor 23 to pass into the housing and to be supported on a conical 
bearing 36 which is secured to the housing 31. In the illustrated 
embodiment, a conical bearing 36 is actually formed integrally with the 
housing 31. A magnetic seal 37 is mounted in the housing 31 and secured in 
place by a snap ring 38. The magnetic seal may be of any known 
construction. 
At least one of the two, and preferably both, bearing housings 31 is 
provided with a fine thread 31t formed on the outer periphery for reasons 
which will be discussed hereinafter. 
As discussed below, the bearing housing must be fixed in place once it is 
properly positioned with respect to the motor casing 10. Accordingly, in 
the illustrated embodiment a locking set screw 17 is provided in the motor 
casing 10 such that it can be screwed down to fix the bearing housing 31 
into position. Naturally, any other means for releasing and locking the 
bearing housing with respect to the motor casing could be used for this 
purpose. 
The conical bearing 36 is a plain bearing having a tapered surface. The 
angle of the taper of the conical bearing surface should be equal to the 
angle of taper of the end of the high speed rotor 23. The diameter of the 
bearing surface should, however, be slightly greater than the diameter of 
the conical shaft end. A supply of magnetic oil 50 is provided in the 
sealed interior of the housing 30. 
In operation, the clearance between the conical bearing 36 and each tapered 
end of the high speed rotor can be precisely adjusted with ease by simply 
threading one or both of the housings into or out of the motor casing. 
Because of the conical shape of the bearing 36 and the complimentary 
conical shape of the ends of the high speed rotor 23, such adjustment 
causes simultaneous radial and axial or thrust positioning of the high 
speed rotor. Moreover, both ends of the rotor are adjusted by simply 
adjusting the housing 30 at one end. Thus, the present invention allows 
one-step adjustment, a feature which is very difficult in systems where 
the radial and thrust adjustment are separately undertaken. 
Another important operational feature of the present invention is that once 
the bearing is precisely aligned in the position which gives optimum 
operation, the entire system is fixed into position so that good operation 
is maintained. The system is designed such that there is very little wear 
because when the high speed rotor is rotating there is no contact between 
the rotor and the conical bearing surface. Instead, the rotor is supported 
on a film of magnetic oil. Likewise, because of the provision of a 
magnetic seal which operates by providing a sealing with a layer of oil 
instead of with frictional contact, the system operates such that the high 
speed rotor is entirely supported by fluids. Additionally, because there 
is no slack in the system the position of the high speed rotor is 
accurately maintained. 
FIG. 4 shows another laser polygon mirror drive motor assembly according to 
the present invention. This assembly is similar to that of FIG. 3 and 
similar components have similar reference numerals. The assembly of FIG. 4 
is, however, more suited to retrofit applications, whereas the assembly of 
FIG. 3 is intended to be manufactured as original equipment. Briefly, the 
assembly includes motor casing 10, which is preferably constructed over 
aluminum or the like. The casing of this embodiment includes separate end 
casing portions 110 formed with threaded bearing receiving openings. This 
type of assembly might be useful in retrofitting an existing casing to 
receive the bearing housings of the present invention. The end casing 
portions 110 are secured to the casing 10 by cup bolts 134 or the like. 
A motor 20 is secured to the casing 10. The motor 20 includes a motor rotor 
21 formed of a permanent magnet 22 and a rotor shaft 23 and a motor stator 
24 secured to the motor casing 10. A polygon mirror 27 is rotatably 
secured to the rotor shaft 23 by a mirror retainer 28 or the like. An 
opening 12 is formed in the motor casing so that the polygon mirror 27 is 
visible from the outside of the casing 10 or extends outside the casing 
10, as desired. 
In the embodiment of FIG. 3, the axial ends of the rotor 23 are conically 
tapered and rotatably supported by a pair of coaxial bearing assemblies 
30. In the embodiment of FIG. 4, however, the conical taper is provided by 
adding a separate conical end piece 123 to the rotor 23. Such a 
construction might be useful in retrofitting an existing rotor for use in 
the shaft support assembly of the present invention. The assembly of FIG. 
4 also differs from the assembly of FIG. 3 in that the conical bearing 136 
of FIG. 4 is formed separately from the bearing housing 30 and a sealing 
plug 137 is provided to maintain the conical bearing 136 in position and 
an O-ring 35 to provide sealing against the bearing housing 31. Unlike 
embodiment of FIG. 3, a second O-ring 135 is provided in the sealing plug 
137. This O-ring 135 seals against the bearing housing 31 and the aluminum 
cap 32. If desired, the O-ring 135 can also act as an elastomeric cushion 
spring. This offers the possibility of using a spring load to 
automatically adjust bearing clearances in addition to or in lieu of 
manual adjustment. Finally, in the embodiment of FIG. 4 locking screws 138 
are used to retain the magnetic seal assembly 37 in place instead of a 
snap ring as used in FIG. 3. The embodiment of FIG. 4 is otherwise similar 
to the embodiment of FIG. 3 and operates in essentially the same manner. 
As mentioned above, FIG. 4 illustrates a construction which can be designed 
to have automatic clearance adjustment in addition to or as an alternative 
to manual adjustment. Specifically, since the conical bearing 136 is 
formed separate from the bearing housing 31, the conical bearing can slide 
axially within the housing. As shown in FIG. 4 the bearing 136 can not, 
however, slide closer to the rotor 23 because the housing 31 includes a 
flange which restricts axial movement of the bearing 136. Axial movement 
in the opposite direction is similarly restricted by the sealing plug 137 
and the cap 32. If there are no clearances between the bearing 136, the 
sealing plug 137 and the cap 32, then the bearing is axially fixed within 
the housing. On the other hand, if there is some axial clearance between 
those components, the bearing 136 will be axially movable within the 
housing. In such a case, automatic adjustment can be achieved by 
appropriately providing a spring within the clearance to oppose axial 
movement of the bearing 136. In the embodiment of FIG. 4 the spring could 
be an elastic cushion such as the O-ring 135 pressing against the cap 32. 
Of course, other springs such as a spring washer or coil spring could be 
used. 
As explained in co-pending application Ser. No. 07/685,148 with such a 
construction it is possible to provide a self adjusting bearing 
construction in which the bearing surface is in contact with the shaft 
surface when the shaft is at rest, but the two surfaces are forced apart 
by a pressurized fluid film when the shaft rotates under normal operating 
conditions. This is achieved by designing the bearings such that the force 
tending to push the shaft and bearing surfaces together is less than the 
counteracting stiffness of the fluid under normal operating conditions. At 
rest, the surface of the bearing 136 is in contact with the conical 
surface of the shaft runner 123. The two surfaces are pressed against one 
another by the force of the O-ring spring cushion 135 and gravity force. 
As the shaft 23 begins to rotate, the stiffness of the hydrodynamic fluid 
increases until the stiffness of the fluid exceeds the force of the spring 
135 acting to push the surface of the bearing 136 into contact with the 
surface of the shaft runner 123. At that point, the fluid forces the 
surfaces apart against the bias of the spring 135 and any additional 
forces until an equilibrium is reached and the shaft runner 123 and shaft 
23 are supported on a film of pressurized fluid. 
The advantage of such a self adjusting system is that there is not need to 
maintain close tolerances since the fluid itself balances with the spring 
force and the other forces to assure proper spacing between the surface of 
the bearing 136 and the surface of the shaft runner 123. In arranging for 
an operable balance of spring force versus fluid film stiffness, there are 
several constraints. For instance, once the fluid to be used is known, the 
stiffness characteristics of that fluid are fixed since they are physical 
characteristics. In such a case, the balance must be provided for by 
selecting an appropriate spring stiffness. The spring force can be 
provided by any known spring such as a Belleville (initially coned) 
spring, a spring washer or an elastomeric cushion as shown. Regardless of 
the specific type of spring selected, the spring can be designed using 
known principles to have the necessary spring characteristic to operate as 
described above. In this way, a very simple and reliable combined radial 
and thrust bearing arrangement can be provided. 
The polygon mirror drive motors constructed as described above operate in 
the following manner. When the rotor 23 is driven at high speeds, when the 
bearing apparatus of the invention is applied on a polygon mirror drive 
motor, the rotor is rotatably supported by an oil film of high rigidity on 
a conical 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.