JPH02102915A - Dynamic pressure fluid bearing device - Google Patents

Abstract

PURPOSE:To obtain a dynamic pressure fluid bearing device of high performance by tightly closing up a gas having smaller density than air in a housing. CONSTITUTION:Pipes 24, 25 provided with valves 22, 23 are respectively arranged on the upper and lower part of a side wall in the housing 21 of a dynamic pressure gas radial bearing device 19. Further, an O-ring 26 is fitted on the inside of the lower edge connecting part to form a pressure tight structure. For example in this case, helium is closed up in this housing 21 at three atmosphere. By closing up a gas having smaller density than air in the housing 21, Reynolds number indicating liability of generation of turbulent flow becomes small, the turbulent flow is hard to generate between a stationary shaft 5 and a rotating member 7, and generation of vibration in the rotating member 7 can be prevented. Hereby, performance of the dynamic pressure fluid bearing device 19 is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a hydrodynamic bearing device used in rotating members of various precision instruments.

2. Description of the Related Art Currently, one type of bearing device for precision equipment is a hydrodynamic bearing device. This hydrodynamic bearing device is, for example, a groove for generating dynamic pressure formed in a shaft or shaft hole with a minute gap, and the dynamic pressure generated when this member rotates to connect the shaft and shaft hole. It is used to support rotating members in the absence of mechanical contact. There are various types of such hydrodynamic bearing devices, such as hydrodynamic gas radial bearings that use gas to hold rotating members in the radial direction.
Used in optical deflectors of laser printers, etc.

Therefore, one conventional example of an optical deflector using such a hydrodynamic bearing device will be explained based on FIG. 2. Here, in the hydrodynamic gas radial bearing device 2, which is a hydrodynamic bearing device used in this optical deflector 1, the radial bearing portion 41s42 is vertically moved by the herringbone groove 3, which is an unevenness for the hydrodynamic gas radial bearing. The rotary member 7 includes a fixed shaft 5 provided in the rotary member 7 and a shaft hole 6 into which the fixed shaft 5 is loosely fitted. The fixed shaft 5 is erected downward from the inner side of the upper surface of the housing 8, which has a sealed structure. A magnet 10 with a through hole 9 formed in the center is disposed at the bottom of the shaft hole 6 facing the lower end of the fixed shaft 5, and a magnet 11 disposed at the bottom of the housing 8. The rotating member 7 is supported against gravity by the repulsive force of the rotating member 7. Below the outer periphery of the rotating member 7, several anisotropically magnetized cylindrical magnet members 12 are attached. Further, an annular stator 14 to which a plurality of coils 13 are attached is disposed on the bottom surface of the housing 8 so as to surround these magnet members 12, forming a polygon motor 15. On the other hand, above the outer periphery of the rotating member 7, a polygon mirror 17 is attached as a member of the optical deflector 1 and faces a transparent window 16 provided in the housing 8. Note that air 18 from which dust and the like have been removed is sealed in the housing 8 of the optical deflector 1 at 1 atmosphere, which corresponds to atmospheric pressure.

In such a configuration, the operation of the optical deflector 1 will be explained first. In this optical deflector 1, a polygon motor 1
When the rotating member 7 is rotated by the force of the rotation of the rotating member 5, the hydrodynamic gas radial bearing device 2 is operated, and the polygon mirror 17 attached to the rotating member 7 rotates without having any mechanical contact portion. Therefore, a laser beam (not shown) emitted from a laser emitting unit (not shown) through the window 16 is reflected by the polygon mirror 17 rotating at a constant speed, becomes scanning light, and passes through the window 16 again. The light is then irradiated onto a photosensitive drum (not shown).

Next, the dynamic pressure gas radial bearing device 2 of this optical deflector 1 is
The operation will be explained. In this hydrodynamic gas radial bearing device 2, the coil 13 to which a perturbation current is sequentially applied sequentially attracts each magnetic pole portion of the magnetic screw member 12, so that the polygon motor 15 is operated and the rotating member 7 is rotated. do. Then, due to the friction between the shaft hole 6 of the rotating member 7 and the air 18, the air 18 existing in the gap between the fixed shaft 5 and the shaft hole 6 is removed.
It also rotates. This rotating air 18 flows along the shape of the herringbone groove 3 and is pumped to the center of each of the two radial bearings 41*42. At this time, the pumped air 18 presses and holds the rotating member 7 horizontally in the entire circumferential direction due to its dynamic pressure. In other words, this rotating member 7 is held by the repulsive force of the magnet 10.11 in the thrust direction and by the dynamic pressure of the two radial bearings 41t42 in the radial direction.
It rotates without any mechanical contact.

In this dynamic pressure gas radial bearing device 2, since the through hole 9 is formed in the magnet 10 at the bottom of the rotating member 7, the lower radial bearing portion 42 can also be effectively pumped.

Furthermore, in this hydrodynamic gas radial bearing device 2, when one device is stopped, the one-rotation member 7 is separated from the magnet 11 by the repulsive force of the magnet 10 at the bottom, making it easy to start up.

Problems to be Solved by the Invention The hydrodynamic gas radial bearing device 2 as described above is effective because it can pivot and rotate the polygon mirror 17 etc. without having any mechanical contact parts. . Here, the hydrodynamic gas radial bearing device 2 is mainly used for precision equipment,
It is required that the rotating member 7 rotate stably without vibration. Generally, hydrodynamic bearing devices are designed so that fluid flows as a laminar flow. However, in actual products, turbulence tends to occur and the rotating member 7 tends to vibrate. Currently, the ease with which turbulence occurs in a Newtonian fluid such as air 18 is determined by the Reynolds number Re.
It is calculated based on the value of This Reynolds number Re is
The flow velocity of the fluid is U, the length of the main part is Q, the density is ρ, and the viscosity is μ
Then, it is expressed as Re=J]LLμ, and the smaller the value of this Reynolds number Re,
Turbulence is said to be less likely to occur. In other words, it can be seen that the smaller the value of the Reynolds number Re, the less vibration occurs in the rotating member 7, and the higher the performance of the hydrodynamic gas radial bearing device 2 is. Here, the flow velocity U corresponds to the rotational speed of the rotating member 7, and this U and the main length Ω are values determined by the equipment conditions, and as mentioned above, the viscosity μ differs between various gases. is minute. Therefore, the density ρ varies depending on the type of gas sealed in the housing 8, but in the conventional dynamic pressure gas radial bearing device 2, the air 18 is used. Although air 18 is a relatively light gas,
Recent hydrodynamic bearing devices are required to have extremely high performance, and the hydrodynamic gas radial bearing device 2 is designed to further reduce the Reynolds number Re, prevent the occurrence of turbulence, and prevent the rotating member 7 from vibrating. It is requested.

In addition, in the optical deflector 1 as described above, the polygon mirror 1
7 is located inside the housing 8, lubricating oil or the like cannot be used as the fluid for this dynamic pressure gas radial bearing device 2. Therefore, air 18 is used as this fluid, but unlike lubricating oil or the like, air 18 has a low viscosity and a small damping force, so it has a weak force to suppress vibrations generated in the rotating member 7 and the like. Furthermore, since gas is a compressible fluid,
Unlike incompressible fluids such as lubricating oil, dynamic pressure gases that generate self-excited vibrations and easily transmit vibrations to the rotating member 7 and polygon mirror 17 currently use compressible fluids such as air 18. The influence of fluid compressibility on the performance of the radial bearing device 2 is calculated by the value of the compressibility constant A. This compressibility constant A is expressed as L ah2 where μ is the viscosity of the gas, U is the relative sliding speed, B is the bearing length in the sliding direction, Pa is the ambient pressure, and h is the bearing clearance. It is said that the smaller the value of this compressibility constant A, the higher the performance of the dynamic pressure gas radial bearing device 1!2. Here, the difference in gas viscosity μ among various gases is minute. Further, the bearing clearance h is determined by manufacturing accuracy, and the relative sliding speed U and bearing length B are determined by the conditions of the equipment. The ambient pressure Pa corresponds to the atmospheric pressure of the air 18 sealed in the housing 8, but in the conventional dynamic pressure gas radial bearing device 2, since Pa = 1 (atn+), the value of is extremely large, indicating that the performance of the hydrodynamic gas radial bearing device 2 is low.

In other words, in the conventional hydrodynamic gas radial bearing device 2, turbulence tends to occur in the hydrodynamic fluid, which tends to cause vibrations in the rotating member 7, and furthermore, the performance as a bearing is adversely affected by the compressibility of the gas. The rotation of the polygon mirror 17 is unstable, making it difficult to obtain a high-performance optical deflector l.

Means for Solving the Problem A hydrodynamic bearing is provided on at least one of the outer circumferential surface of a fixed shaft installed upright in a housing with a sealed structure and the inner circumferential surface of a shaft hole formed in a rotating member into which the fixed shaft is loosely inserted. In a hydrodynamic bearing device with unevenness formed.

A gas with a lower density than air is sealed inside the housing.

By sealing a gas with a lower density than air in the action housing, the Reynolds number, which indicates the likelihood of turbulent flow, is reduced, and turbulent flow is less likely to occur between the fixed shaft and the rotating member.
<9 Since it is possible to prevent vibrations from occurring in rotating members,
The performance of the hydrodynamic bearing device is high, and it is also possible to pressurize and seal gas with a lower density than air in the housing to a level higher than atmospheric pressure, so the dynamic pressure fluid bearing device is inversely proportional to the air pressure. It is possible to reduce the compressibility constant indicating the degree of influence on performance, and it is also possible to obtain a hydrodynamic bearing device with even higher performance even though gas is used as a compressible fluid.

Example An embodiment of the present invention will be described based on FIG.

First, the dynamic pressure gas radial bearing device 19, which is this dynamic pressure fluid bearing device, is also used in the optical deflector 20.

In the housing 21 of this hydrodynamic gas radial bearing device 19, a pipe 24 provided with valves 22 and 23 is provided.
．． 25 are disposed above and below the side walls, respectively, and an O-ring 26 is mounted inside the lower edge joint to provide a pressure-resistant structure. In this housing 21, for example, helium 27 is sealed at, for example, 3 atmospheres. Further, the other configuration is similar to that of the optical deflector 1 described above.

In such a configuration, the optical deflector 20 and the hydrodynamic gas radial bearing device 19 function in the same manner as the optical deflector 1 and the hydrodynamic gas radial bearing device 2 described above.

Therefore, a method for sealing helium 27 at 3 atm inside the housing 21 of this dynamic pressure gas radial bearing device 19 will be explained. First, this optical deflector 20 is assembled in air like a normal device. Therefore, after this, the valves 22 and 23 are opened, a helium 27 cylinder (not shown) is connected to the upper pipe 24, and helium 27 is injected into the housing 21. Then, since the helium 27 is lighter than the air 18, it accumulates in the housing 21 from above, and the air 18 is discharged from the pipe 25.

Then, when the inside of the housing 21 is filled with helium 27, the lower valve 23 is closed. Then, in this state, helium 27 is injected until the injection pressure reaches 3 atmospheres, and the valve 22 is also closed. By doing this,
It can be seen that helium 27 at 3 atmospheres can be easily sealed in the housing 21. In addition, in order to increase the pressure of helium 27 in this way, the housing 21 needs to have a pressure-resistant structure, but as mentioned above, the housing of the hydrodynamic bearing device is designed to protect the device from dust, etc. Since most of the housings are originally of a sealed structure, it can be seen that even if modification is necessary, a pressure-resistant structure housing 21 can be easily obtained by adding an O-ring 26.

Next, the characteristics of this dynamic pressure gas radial bearing device 19 will be explained. This dynamic pressure gas radial bearing device 19 uses helium 27, which is a compressible fluid, pressurized to 3 atmospheres as the dynamic pressure fluid. Therefore, considering the Reynolds number Re, which indicates the ease with which turbulence occurs, from the above formula, the density of helium-27 at 3 atmospheres is given by Re-111 μ.

ρ1=o, 1aX1o-3X3 =0,54X10-'(g/an') On the other hand, the density of air 18 at 1 atm.

ρ2 = 1.29 x 100-3 (/am'), and the density of helium 27 is less than the density of air 18. In other words, it can be seen that the Reynolds number Re is extremely small. Moreover, the viscosity of helium 27 is p, = 19,6X10-' (Pa-8), while the viscosity of air 1
What is the viscosity of 8?

Since μ, = 18,2X10-G (Pa-8), it can be seen that this Reynolds number Re becomes even smaller. It goes without saying that the flow velocity U and main length Q are values determined by design conditions and the like.

Next, regarding the compressibility constant that indicates the degree of influence on the performance of the hydrodynamic gas radial bearing device 19, considering from the above formula, we can see that for A=-e, the helium 27 is sealed at 3 atm. Therefore, the ambient pressure Pa corresponding to the atmospheric pressure of the gas sealed inside the housing 21 is P a = 3 (atm). Furthermore, as described above, the difference in viscosity μ is minute. In other words, it can be seen that in this hydrodynamic gas radial bearing device 19, the value of is approximately 10% lower than that of the conventional hydrodynamic gas radial bearing 2. The length B and the bearing clearance h are also values determined by design conditions and the like.

As mentioned above, in this hydrodynamic gas radial bearing device 2,
It can be seen that both the Reynolds number Re and the compressibility constant become extremely small.

In addition, in the dynamic pressure gas radial bearing device 19 of this embodiment,
Although the gas sealed in the housing 21 is helium 27, the present invention is not limited thereto; for example,
It is also possible to use hydrogen, neon, etc.

Effects of the Invention In the present invention, as mentioned above, by sealing a gas with a lower density than air in the housing, the Reynolds number, which indicates the likelihood of turbulent flow, is reduced, and the gap between the fixed shaft and the rotating member is reduced. The performance of the hydrodynamic bearing device is high because turbulence is less likely to occur and vibrations are prevented from occurring in rotating members.Moreover, the housing is pressurized to a level higher than atmospheric pressure, which is less dense than air. It can also be sealed, so
The compressibility constant, which indicates the degree of influence on the performance of a hydrodynamic bearing device and is inversely proportional to the gas pressure, can be reduced, so even though gas is used as a compressible fluid,
It is also possible to obtain a hydrodynamic bearing device with even higher performance, and since the fluid used is gas, it can be easily used in equipment where contamination by lubricating oil is a concern, such as polygon mirrors in laser printers. It has the following effects:

[Brief explanation of the drawing]

FIG. 1 is a vertical side view of one embodiment of the present invention, and FIG. 2 is a vertical side view of a conventional example. 3... Unevenness, 5... Fixed shaft, 6... Shaft hole, 7...
・Rotating member 519...Dynamic pressure fluid bearing S position, 21...
Housing, 27... gas

Claims (1)

[Scope of Claims] 1. Dynamic pressure fluid is applied to at least one of the outer circumferential surface of a fixed shaft installed upright in a housing with a sealed structure and the inner circumferential surface of a shaft hole formed in a rotating member into which the fixed shaft is loosely inserted. What is claimed is: 1. A dynamic pressure fluid bearing device in which unevenness is formed for a bearing, characterized in that a gas having a lower density than air is sealed in the housing. 2. Claim 1, characterized in that the housing is sealed with a gas having a lower density than air, pressurized to a level higher than atmospheric pressure.
The hydrodynamic bearing device described.