JPH03213718A - Static-pressure air bearing - Google Patents

Abstract

PURPOSE:To heighten a spring constant so as to hold sufficient stability by setting the depth of a pocket to be thin to the same degree as a shaft gap, and forming an autogeneous throttle inside the pocket. CONSTITUTION:A complex throttle system static-pressure air bearing is provided with a shaft 11, a bearing body 12 and a nozzle 13. The depth of a pocket 14 continued from the nozzle 13 is set to be thin enough for its outogeneous throttling action to function. The ratio of pocket depth H to the size Cr of a shaft gap G is set to be within the range of H/Cr=0.5-1.5. In this case, the maximum spring constant value and a high damping coefficient can be obtained. The spring constant can be thus heightened to maintain sufficient stability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a hydrostatic air bearing used in, for example, ultra-precision processing machines and precision measuring instruments.

[Prior Art] Conventionally, this type of static air bearing has a throttle type.
The main known methods are the self-throttle drawing method and the pocketed orifice drawing method.

FIGS. 8 and 9 show models of the self-throttle and pocketed orifice throttling systems, respectively, and FIGS. 10 and 11 show the positions where the air flow is throttled, respectively.

In the drawings, ■ is the shaft, 2 is the bearing body, and G is the shaft gap therebetween. As shown in Fig. 8, in the self-throttling method, air at a supply pressure Ps is injected directly into the shaft gap G from the nozzle 3, and the position A where this air flow is throttled is located at the shaft gap G, as shown in Fig. 10. This is near the exit of nozzle 3 of G. on the other hand,
The pocketed orifice aperture method is shown in Figure 9.
Following the orifice 4 is a pocket 5 with considerable depth.
As shown in FIG. 11, the position B where the airflow is restricted remains within the nozzle 4.

[Problems to be Solved by the Invention] Generally, in this type of static air bearing, secondary pressure is generated downstream of the throttle relative to the supply pressure Ps due to the presence of a throttle, and this pressure causes fluctuations in the shaft clearance G. - In boat fishing, a spring constant called stiffness occurs in the bearing.

In the case of self-control aperture, it is common to make the nozzle diameter small, the shaft gap G small, and the aperture area small in order to increase the spring constant.
If this happens, the bearing loses its original function, so the shaft clearance G must be reduced (
There is a limit to what can be done, and the disadvantage is that the spring constant cannot be made very high.

On the other hand, the pocketed orifice aperture has the advantage that the spring constant can be set higher than the self-adjustment aperture due to the existence of the pocket 5, but in the relationship between the shaft clearance G and the spring constant, the spring constant is the maximum. Near the size Cr of the shaft gap G, the damping coefficient decreases extremely, and in some cases becomes a negative value, so that the bearing body tends to be in an unstable state such as self-excited vibration.

Figures 12 and 13 show how the pressure distribution in the shaft clearance G changes with respect to shaft clearance fluctuations in the self-control aperture and the pocketed orifice aperture, respectively, where Pa is the atmospheric pressure and Ps
indicates the supply pressure, Po indicates the pressure inside the pocket 5, and the hatched pressure change is the spring constant. Since the pocket 5 exists in the pocketed orifice aperture, the amount of change in pressure is large with respect to fluctuations in the shaft clearance G, and a higher spring constant can be set than in the case of a self-control aperture. However, in the relationship between the shaft clearance G and the spring constant, the shaft clearance G where the spring constant is maximum
In the vicinity of the magnitude Cr, the damping coefficient decreases extremely or becomes negative, and the bearing is in an unstable state where self-excited vibration is likely to occur. Therefore, in order to ensure a positive damping coefficient and stabilize the shaft, the shaft clearance must be set at the expense of the spring constant, which does not necessarily make full use of the advantages of the pocketed orifice restrictor. The problem is that there is no.

The purpose of the present invention is to improve the above-mentioned problems by making the depth of the pocket at the nozzle tip shallow enough to form a state of confinement within the pocket, thereby increasing the spring constant and stability. Our goal is to provide good hydrostatic air bearings.

[Means for Solving the Problems] In order to achieve the object of the present invention, in a static pressure air bearing according to the present invention, in a static pressure air bearing having a constriction type having an end pocket at the nozzle tip, the pocket The relationship between the depth H and the size Cr of the shaft clearance is expressed as: H/Cr=0.
5 to 1.5.

[Function J] In the hydrostatic air bearing having the above configuration, by making the pocket depth as shallow as the shaft gap, a self-limiting aperture is also formed within the pocket, and the effective cross-sectional area of the aperture is equal to the effective cross-sectional area of the nozzle. Since it is smaller than the cross-sectional area, even if the nozzle diameter is the same, the maximum spring constant value can be obtained with a narrower shaft clearance than with a pocketed orifice throttle. Further, since the pocket depth is shallow, a decrease in the damping coefficient is prevented.

[Example] The present invention will be described in detail based on the example illustrated in FIGS. 1 to 7.

FIG. 1 shows a compound throttle type static pressure air bearing according to the present invention, where 11 is a shaft, 12 is a bearing body, and 13 is a nozzle.
The depth of the pocket 14 following this nozzle 13 is shallow enough to allow the self-diaphragm action to function.Here, the ratio of the pocket depth H to the size Cr of the shaft clearance G is expressed as H/Cr= It has been confirmed through experiments that a maximum spring constant value and a high damping coefficient can be obtained when the range is from 0.5 to 1.5.

Figure 2 (al (bl) is a dimensional diagram of the specific bearing dimensions and nozzle position (arrow N) of the radial bearing for the data shown in Figures 3 to 6 as an example, and Figure 7 is These are dimensional drawings of the nozzle's self-control, pocketed orifice, and compound apertures. Also, the data shown here is for the case where the supply pressure is 5 kgf/cm2G.

In FIG. 3, the horizontal axis is H/Cr, the vertical axis is the spring constant, that is, the maximum spring constant value, for the optimal bearing clearance, and the vertical axis is data showing the relationship between the spring constant value and the optimal bearing clearance. Similarly, FIG. 4 shows data in which the horizontal axis is H/Cr and the vertical axis is the dimensionless damping coefficient with respect to the optimum clearance of the bearing.

From these two data, it is clear that the maximum spring constant is obtained when H/Cr is within the range of 0.5 to 1.5, and the value of the damping coefficient is high at the maximum spring constant value.

As shown in FIG. 1, when the depth of the pocket 14 is made shallow to approximately the same extent as the size Cr of the shaft gap G, the relationship between the nozzle diameter d and H and Cr is πd''/4>π(H+C
r) When the following holds true, the position C where the airflow is restricted is pocket 1
4 and the shaft gap G. The aperture type having this function will be referred to as a compound aperture.

Figure 5 shows the relationship between the shaft clearance G size Cr and the spring constant in the compound throttle type static pressure air bearing according to the present invention, when the supply pressure is 5 Kgf/cm'' and a pocketed orifice aperture, F is a composite aperture according to the present invention,
J stands for the conventional self-discipline aperture, and P stands for the pocketed orifice aperture.

With compound orifice F, the effective cross-sectional area of the aperture is smaller, so even if the nozzle diameter is the same, the maximum value of the spring constant can be obtained with a narrower axial clearance than with the conventional pocketed orifice aperture P.
Moreover, the maximum spring constant value is larger than that of the pocketed orifice diaphragm P. Furthermore, since the maximum spring constant value can be obtained in a narrow gap, the amount of air used can be significantly reduced compared to the conventional pocketed orifice throttle P.

Figure 6 shows the relationship between the shaft clearance G size Cr and the dimensionless damping coefficient in comparison with the conventional method.
represents the unstable region of the pocketed orifice diaphragm P. As can be seen from FIG. 6, the composite aperture F of the present invention
Since the depth of the pocket 14 is shallow, it is possible to prevent the damping coefficient from decreasing, and even at the shaft clearance G size Cr that indicates the maximum spring constant value, the damping coefficient does not decrease as much as the pocketed orifice orifice P. , the bearing body can hold stabilization.

The compound orifice F can be applied to a static air table that has the same function as a static air bearing, and can also be applied to optimal control of active static air bearings that control the nozzle pocket depth. be able to.

[Effects of the Invention] As explained above, the hydrostatic air bearing according to the present invention has a self-control function also in the pocket, thereby obtaining a larger spring constant with a narrower shaft clearance than an orifice orifice with a pocket. This makes it possible to miniaturize the shaft clearance and reduce the amount of air consumed. Furthermore, since the size of the shaft clearance with the maximum spring constant deviates significantly from the unstable region, it is possible to maintain sufficient stability even if the shaft clearance varies greatly.

[Brief explanation of drawings]

Drawings 1 to 7 show examples of the hydrostatic air bearing according to the present invention, FIG. 1 is a principle diagram of a compound aperture, and FIG.
(bl is the dimensional drawing of the radial bearing used in the experiment,
Figure 4 is a characteristic diagram of the change in the maximum spring constant, Figure 4 is a characteristic diagram of the change in dimensionless damping coefficient at the maximum spring constant value, Figure 5 is a characteristic diagram of the relationship between the spring constant and the shaft clearance, Fig. 6 is a characteristic diagram of the relationship between the dimensionless damping function and the shaft clearance, Fig. 7 is a dimensional diagram of the nozzle, Figs. Figure 9 is a configuration diagram of the orifice aperture with a pocket, Figure 10 is an explanatory diagram of the orifice aperture with a pocket, Figure 11 is an explanatory diagram of the orifice aperture with a pocket, and Figure 12 is the pressure of the orifice aperture. Graph diagram of distribution state,
FIG. 13 is a graph of the pressure distribution state of the pocketed orifice throttle. Reference numeral 11 is a shaft, 12 is a bearing body, 13 is an orifice, I
4 is a pocket, and G is a shaft gap.

Claims (1)

[Claims] 1. In a static air bearing with a throttle type having an end pocket at the nozzle tip, the relationship between the depth H of the pocket and the size Cr of the shaft clearance is H/Cr = 0.5 to 1.5. A hydrostatic air bearing characterized by: