JPH0820620B2 - Rotating polygon mirror device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A rotary polygon mirror device used for scanning a laser beam in a laser beam printer, in which a polygon mirror is fixed for the purpose of improving reliability. A rotary polygonal mirror device comprising a rotating body and a casing for accommodating the rotating body and a polygonal mirror therein, wherein an annular permanent magnet magnetized in a thickness direction and the annular permanent magnet sandwiching the annular permanent magnet. A fixed magnetic circuit portion fixed to the casing by an annular yoke projecting toward the inner peripheral side, a central annular yoke portion and upper and lower sides of the fixed magnetic circuit portion, and protruding outward from the central annular yoke portion. And a rotating magnetic circuit portion fixed to the rotating body by the upper and lower annular yoke portions, and the fixed magnetic circuit portion and the rotating magnetic circuit are fixed when the rotating body is displaced in the axial direction. A magnetic thrust bearing that urges the upper and lower annular yoke portions so as to face the annular yoke by a restoring force generated between the rotor and the road portion, and floats and supports the rotating body in the frust direction, Moreover, the end side of the rotating body serves as a piston, and this is fitted into a fixed cylinder with a minute gap, and the effect of the compressibility of the air in the cylinder and the viscous resistance acting on the air passing through the minute gap. It is configured to have a vibration damping unit that generates a vibration damping force by synergistic effects.

[Industrial applications]

The present invention relates to a rotary polygon mirror device used for scanning a laser beam in a laser beam printer.

The rotary polygon mirror device rotates at high speed to scan a laser beam, and its thrust bearing is required to have high reliability.

[Conventional technology]

Conventionally, a dynamic pressure air bearing has been applied to the thrust bearing of the rotary polygon mirror device, but when the rotary shaft is started and stopped, the rotor and the bearing come into contact with each other, causing friction, and the starting torque becomes large. was there. There is a rotary polygon mirror device 1 shown in FIG. 8 as a means for eliminating this defect.

Reference numeral 2 is a polygon mirror, 3 is a casing, 4 is a rotating shaft, 5 is a rotor, and 6 is a stator.

Reference numeral 7 denotes a thrust bearing, which has a structure in which a permanent magnet 8 is fixed to the lower end surface of the rotating shaft 4 and another permanent magnet 9 is fixed to the casing 3 so as to face the permanent magnet 8.

The permanent magnets 8 and 9 have the same magnetic poles opposed to each other, and float and support the rotating shaft 4 by magnetic repulsive force.

[Problems to be Solved by the Invention]

Since the weight of the portion supported by the thrust bearing 7 is as heavy as about 400 gr, it is necessary to use a ferrite magnet having a strong magnetic force as the permanent magnets 8 and 9.

Ferrite magnets are cast products and their mechanical strength is not sufficient. On the other hand, the rotation speed of the rotary shaft 4 is tens of thousands rpm, which is an ultrahigh speed. For this reason, a considerable centrifugal force acts on the permanent magnet 8, and the permanent magnet 8 may be destroyed in some cases, resulting in a problem of lack of reliability.

It is an object of the present invention to provide a rotary polygon mirror device capable of improving reliability.

[Means for solving the problem]

The present invention relates to a rotary polygon mirror device comprising a rotating body to which a polygonal mirror is fixed, and a casing that accommodates the rotating body and the polygonal mirror therein, and an annular permanent magnet magnetized in the thickness direction and the circle. A fixed magnetic circuit portion fixed to the casing by an annular yoke that sandwiches the annular permanent magnet and protrudes inward from the annular permanent magnet; a central annular yoke portion; A rotating magnetic circuit portion fixed to the rotating body, which is composed of upper and lower annular yoke portions projecting outward from the central annular yoke portion,
When the rotating body is displaced in the axial direction, the restoring force generated between the fixed magnetic circuit portion and the rotating magnetic circuit portion urges the upper and lower annular yoke portions so as to face the annular yoke. , A magnetic thrust bearing that floats and supports the rotating body in the thrust direction, and the end portion side of the rotating body serves as a piston, which fits into a fixed cylinder with a minute gap, It is configured to have a vibration damping means for generating a vibration damping force by the synergistic effect of the compressibility of the internal air and the viscous resistance effect acting on the air passing through the minute voids.

[Action]

In this magnetic thrust bearing, the permanent magnet is provided only on the fixed side, the permanent magnet is not provided on the rotating side, and the rotating side is composed of only the yoke, and centrifugal force does not act on the permanent magnet. Therefore, there is no fear that the magnetic thrust bearing will be damaged during high-speed rotation, and reliability is improved.

The structure provided with the vibration damping means acts to stabilize the floating of the rotating body.

The configuration in which the end side of the rotating body serves as the piston acts so that the vibration damping means is skillfully incorporated.

〔Example〕

FIG. 2 shows a rotary polygon mirror device 20 according to an embodiment of the present invention.

 Reference numeral 21 is a polygon mirror, and 22 is a casing.

A shaft 23 is vertically fixed to the casing 22, and a sleeve 24 as a rotating body is fitted to the shaft 23. The rotating system is an outer rotor type.

The peripheral surface of the shaft 23 and the inner peripheral surface of the sleeve 24 form a dynamic pressure air journal bearing 25. This bearing 25 has a sleeve 24
It is formed over a wide range in the axial direction by effectively utilizing the structure in which is fitted to the shaft 23. That is, the bearing 25 is formed in a sufficiently wide range on the upper and lower sides except the central portion in the axial direction, and the sleeve 24 rotates stably.

 The polygon mirror 21 is fixed to the upper part of the sleeve 24.

A rotor 26 is fixed to the center of the sleeve 24.

 27 is a stator, which is fixed to the casing 22.

Reference numeral 30 denotes a magnetic thrust bearing which is an essential part of the present invention, and is provided on the lower end side of the sleeve 24.

The magnetic thrust bearing 30 includes a fixed magnetic circuit portion 31 fixed to a casing 22 and
The rotating magnetic circuit unit 32 is fixed to the sleeve 24.

The fixed magnetic circuit unit 31 is composed of an annular ferrite magnet 33 magnetized in the thickness direction (axial direction) and annular yokes 34, 35 that sandwich the annular ferrite magnet 33, and a step inside the casing 22. It is fixed on the portion 22a.

The inner diameters of the yokes 34 and 35 are equal, D ₁ , and smaller than the inner diameter D _{2 of the} magnet 33. As a result, the inner peripheral portion 3 of the yoke 34, 35
4a and 35a project from the magnet 33 toward the inner peripheral side.

The rotating magnetic circuit section 32 has a central annular yoke section 36,
It is composed of the upper and lower annular yoke portions 37, 38, has no magnet, and is fixed to the sleeve 24.

The outer diameters of the yoke portions 37 and 38 are the same, D ₃ , and the yoke portion 3
Larger than 6 outer diameter D ₄ . As a result, the outer peripheral portions 37a, 38a of the yoke portions 37, 38 project outward from the yoke portion 36.

The above-mentioned yokes 34, 35 face the yoke portions 37, 38 via the magnetic gaps 39, 40, respectively.

The magnetic flux 41 from the magnet 33 is the yoke 34 → magnetic gap 39 → yoke part 37 → yoke part 36 → yoke part 38 → magnetic gap 40 → yoke 35.
→ A closed loop returning to the magnet 33 is formed, and the fixed magnetic circuit unit 31 and the rotating magnetic circuit unit 32 form a magnetic circuit.

When the sleeve 24 is displaced in the axial direction (downward) from the position P ₀ shown in FIG. 1, the flow of the magnetic flux 41 becomes as shown in FIG. Regarding the magnetic gaps 39, 40, the magnetic flux 41 runs diagonally, the magnetic resistance in this portion increases, the magnetomotive force consumption in the magnetic gaps 39, 40 increases, and the magnetic energy stored therein increases.

This change in the magnetic energy acts as a reaction force against the displacement, that is, a restoring force for returning the sleeve 24 to the position shown in FIG.

The relationship between the displacement Z and the restoring force F is as shown by the line 42 in FIG.

The displacement Z is positive in the downward direction and negative in the upward direction in FIG.

 The restoring force F is also positive in the downward direction and negative in the upward direction in the figure.

In the vicinity of Z = 0, the above relationship is linear, and when the displacement Z is positive, a negative restoring force F is generated and the sleeve 24 is pulled back to the stable position shown in FIG.

When the displacement Z is negative, a positive restoring force acts and the sleeve 24 is pulled back to the stable position shown in FIG.

As described above, when the sleeve 24 is displaced in any of the upper and lower directions from the stable position shown in FIG. 1, the restoring force F for returning the stable position to the stable position acts on the sleeve 24, and the stable position of the sleeve 24 is shown in FIG. It is surely and quickly returned to.

The characteristics shown in FIG. 4 are characteristics that cannot be obtained with the bearing shown in FIG. 8, and the return of the sleeve 24 to the stable position is performed more reliably and quickly than in the conventional case.

Further, referring again to the magnetic thrust bearing 30, the thicknesses of the yokes 34, 35, the yoke portions 36, 37, 38, and the ferrite magnet 33 are all set to be equal to t (= 2 mm).

As a result, as shown in FIG. 1, the yoke 34 and the yoke portion are
37, and the yoke 35 and the yoke portion 38 are opposed to each other over the entire thickness, and this position is truly the stable position P ₀ .

Further, if the sleeve 24 deviates slightly from the stable position P _{0 in} either the upper or lower direction, the restoring force F is immediately generated as shown in FIG.

Also, in FIG. ₁ , the difference between the dimensions D ₁ and D ₂ is 0.6 mm,
The width g of the magnetic gaps 39, 40 is as narrow as 0.3 mm. Therefore, the amount of increase in the magnetic resistance due to the displacement of the sleeve 24 from the stable position is large, the amount of super-magnetic force consumed in the magnetic gaps 39 and 40 is also increased, and the amount of the magnetic energy stored therein is increased. 4 middle line
The inclination angle α of 42 becomes large, and even if the displacement Z is small, the restoring force F
Is large.

As a result, even if the sleeve 24 is slightly displaced from the stable position P ₀ , the sleeve 24 exerts a sufficiently strong restoring force, and the sleeve 24 is reliably returned to the stable position.

As a result, the sleeve 24 (polyhedral mirror 21) is supported by the dynamic air journal bearing 25 in the radial direction and is supported by the magnetic thrust bearing 30 in the thrust direction, and rotates at a stable position at high speed.

Looking at the thrust direction, the sleeve 24 floats not only during high-speed rotation but also during start-up and stop, and is in non-contact with the casing 27. To start.

Further, a ferrite magnet is not incorporated in the rotating magnetic circuit section 32, and all are yoke sections 36, 37, 38. Therefore, even if a large centrifugal force acts on the rotating magnetic circuit unit 32 due to high-speed rotation, the rotating magnetic circuit unit 32 will not be damaged, and
The magnetic thrust bearing 30 has higher reliability than the conventional one.

Next, a vibration damping device provided in the rotary polygon mirror device 20 described above.
50 will be described.

 FIG. 5 is an enlarged view of the vibration damping device 50 in FIG.

51 is a cylinder, which fixes the lower end of the shaft 23 to the casing
A circular concave portion 53 of the lower plate 53 fixed to the 22 and a peripheral surface of the shaft 23 protruding from the center are formed into an annular concave shape.

Reference numeral 54 denotes a cylindrical piston, which is constituted by the lower end portion of the sleeve 24 and is fitted in the cylinder 51.

A gap 55 having a minute dimension h in the radial direction is provided between the piston portion 54 and the cylinder 51.

The effect of the compressibility of the air present in the cylinder 51 and the air gap
A damping force is generated by a synergistic effect with the viscous resistance effect of the air passing through 55.

As a result, the sleeve 24 (polyhedral mirror 21) is stably maintained in the floating state and rotates at high speed.

Next, a stopper mechanism that limits excessive displacement of the sleeve 25 in the axial direction will be described with reference to FIG.

Reference numerals 60 and 61 are ring-shaped pads to which a solid lubricant is applied.

The pad 60 is supported by a spring 62 and is attached to the bottom of the cylinder 51.

The pad 61 is supported by a spring 63 and attached to the lower surface of the top plate portion of the casing 22.

The pad 60 is at the position where the characteristics of the sleeve 24 are reversed in FIG.
It is arranged to receive the lower end of the sleeve 24 at a position P ₂ in front of P ₁ . Pad 61, the sleeve 24 are arranged so as to receive the upper end of the sleeve 24 at the position P ₄ of the front of Figure 4 in position P _3.

As a result, even when the sleeve 24 is displaced in the axial direction by an impact force during transportation of the laser beam printer, for example, it is received by the pad 60 or 61, and excessive displacement is limited.

As for the magnetic thrust bearing 30, with respect to the downward direction, further deviation is limited in a state before the yoke portion 38 deviates from the position where the yoke portion 38 faces the yoke 35 and the yoke portion 37 faces the yoke 35. . When the impact force disappears, the magnetic thrust bearing 30 returns to the stable state shown in FIG.

In the upward direction, further deviation is limited in a state before the yoke portion 37 is displaced from the position facing the yoke 34 and the yoke portion 38 faces the yoke 34. When the impact force disappears, the magnetic thrust bearing 30 returns to the stable state shown in FIG.

Further, the solid lubricant applied to the pads 60 and 61 prevents damage to the sleeve 24 when the sleeve 24 contacts the pads 60 and 61 during high speed rotation.

FIG. 6 shows a rotary polygon mirror device 70 according to a second embodiment of the present invention.
Indicates.

This device 70 has a structure in which the magnetic thrust bearing 30 is arranged on the upper portion. The other configuration is the same as the configuration shown in FIG. 2, and the substantially corresponding parts are designated by the same reference numerals and the description thereof is omitted.

FIG. 7 shows a main part of a rotary polygon mirror device 80 according to a third embodiment of the present invention. In the figure, those parts corresponding to the parts shown in FIGS. 1 and 2 are designated by the same reference numerals, and a description thereof will be omitted.

The rotating system of this device 80 is an inner rotor type, and the rotating magnetic circuit unit 32 is fixed to a rotating shaft 81 which is a rotating body.

 82 is a dynamic pressure air journal bearing.

Reference numeral 83 denotes a vibration damping device, which includes a cylinder 84 and a piston 85 formed by a lower end portion of the rotary shaft 81.

〔The invention's effect〕

As described above, according to the present invention, since the magnetic thrust bearing has a configuration in which the permanent magnet is not provided on the rotating side,
There is no risk of damage to the magnetic thrust bearing during high-speed rotation,
The reliability can be improved.

Further, by providing the vibration damping means, the floating of the rotating body can be stably maintained.

Further, since the end portion side of the rotating body is used as the piston, the vibration damping means can be made small, and thus the rotary polygon mirror device can be made as small as possible.

[Brief description of drawings]

FIG. 1 is a diagram showing a magnetic thrust bearing of a rotary polygon mirror device of the present invention, FIG. 2 is a diagram showing a rotary polygon mirror device according to a first embodiment of the present invention, and FIG. 3 is a sleeve axially displaced. FIG. 4 is a diagram showing the relationship between the displacement of the magnetic thrust bearing and the restoring force, FIG. 5 is an enlarged view of the vibration damping device in FIG. 2, and FIG. FIG. 7 is a view showing a rotary polygon mirror device according to a second embodiment of the present invention, FIG. 7 is a view showing a main part of a rotary polygon mirror device according to a third embodiment of the present invention, and FIG. 8 is a conventional rotary polygon mirror. FIG. In the figure, 20, 70 and 80 are rotary polygon mirror devices, 21 is a polygon mirror, 22 is a casing, 23 is a shaft, 24 is a sleeve, 25 and 82 are dynamic air journal bearings, 30 is a magnetic thrust bearing, and 31 is fixed. Magnetic circuit part, 32 is a rotating magnetic circuit part, 33 is an annular ferrite magnet, 34,35 are annular yokes, 36,37,38 are yoke parts, 39,40 are magnetic air gaps, 41 is magnetic flux, 50,83 are Vibration suppressors, 51 and 84 are cylinders, 54 and 85 are pistons, 55 is a gap, 60 and 61 are pads, and 81 is a rotating shaft.

Claims (1)

[Claims] 1. A rotary polygon mirror device comprising a rotating body to which a polygonal mirror is fixed and a casing for accommodating the rotating body and the polygonal mirror, wherein an annular permanent magnet magnetized in a thickness direction and A fixed magnetic circuit portion fixed to the casing, which is composed of an annular yoke which sandwiches the annular permanent magnet and protrudes inward from the annular permanent magnet, a central annular yoke portion and upper and lower sides of the annular magnetic yoke portion. And a rotating magnetic circuit portion fixed to the rotating body, which is composed of upper and lower annular yoke portions projecting outward from the central annular yoke portion, and when the rotating body is displaced in the axial direction, By the restoring force generated between the fixed magnetic circuit portion and the rotating magnetic circuit portion, the upper and lower annular yoke portions are urged so as to face the annular yoke, and the rotating body is floated and supported in the thrust direction. Magnetism It has a last bearing, and the end side of the rotating body becomes a piston, which fits into a fixed cylinder with a minute gap and passes through the effect of compressibility of air in the cylinder and the minute gap. A rotary polygon mirror device, characterized in that it has a vibration damping means for generating a vibration damping force by a synergistic effect of a viscous resistance effect acting on the air.