JPH07139547A - Gas bearing - Google Patents

Abstract

PURPOSE:To provide a gas bearing which can simply restrain temperature rise even without using a complicated fluid cooling mechanism. CONSTITUTION:Material of emissivity more than 0.4 is used for both, the bearing surface 1a of the movable member and the bearing surface 2a of the supporting member of a gas bearing. Therefore, remarkably much heat than that in a customary case can be taken away from the movable member 1, and excellent baring performance can be insured without forcedly cooling the movable member by water or liquid.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to gas bearings, and more particularly to
The present invention can cope with thermal expansion, which is a problem when used in the field of precision machining, etc., without using a cooling fluid for the movable member.

[0002]

2. Description of the Related Art Generally, a gas bearing has a very small coefficient of friction and generates less heat than a rolling bearing or a hydrostatic bearing.
Further, since it can be rotated with ultra-high precision, it is widely used in machine tools for ultra-precision machining. Figure 4
Is an example of a conventional gas bearing used in a gas bearing spindle that is a main shaft bearing of a machine tool, and a bearing surface (hereinafter, also referred to as a movable bearing surface) 1a of a shaft (rotor) 1 that is a movable member is It is supported by a bearing surface (hereinafter, also referred to as a support-side bearing surface) 2a of a bearing 2 which is a support member so as to be non-contact rotatable.

Both the shaft 1 and the bearing 2 are made of stainless steel, and the bearing surfaces 1a and 2a are ground. Compressed gas 5 supplied from an unillustrated air supply source is sent from the air supply hole 3 to the bearing gap 4, and the shaft 1 is levitationally supported by the pressure of the compressed gas 5. Due to the non-contact structure, less heat is generated in the bearing portion. However, although the amount of heat generated is small, a temperature rise of about 20 to 40 ° C. is inevitable in the case of high speed rotation. Therefore, the bearing 2 is cooled by a cooling mechanism (not shown). When the bearing 2 is cooled, a temperature difference is generated between the bearing 2 and the shaft 1, and heat corresponding to this temperature difference is transferred from the shaft 1 to the bearing 2 to cool the shaft 1. However, axis 1,
Since both the bearing 2 is close to the mirror surface and the heat radiation and the heat absorption are extremely small, the amount of heat transfer between the shaft 1 and the bearing 2 is small, and the heat of the shaft 1 moves to the bearing 2 and the shaft 1 is cooled to a degree. Very small.

FIG. 5 shows another example of a conventional gas bearing. The shaft 1 is made of stainless steel, and the shaft 1 is coated with a coating 6 of molybdenum disulfide (MoS ₂ ). The purpose of applying the coating 6 is to cause seizure when the bearing surfaces 1a and 2a of the shaft 1 and the bearing 2 come into contact with each other when the supply of the compressed gas 5 from the air supply hole 3 to the bearing gap 4 is stopped due to a trouble. In order to prevent accidents, a one-sided coating on the side of the shaft 1 can suffice for this purpose.

By the way, since the coating film of molybdenum disulfide happens to have a large thermal emissivity,
The shaft 1 of FIG. 5 radiates a large amount of heat unlike the shaft 1 of FIG. However, most of the emitted heat is reflected by the bearing surface 2a of the bearing 2 having a small heat absorption rate and returns to the shaft 1. In addition, the large heat emissivity of the shaft 1 causes a great deal of damage and absorbs the heat released by itself (the larger the heat emissivity is, the larger the heat emissivity is. As a result, although the coating 6 increases the thermal emissivity of the shaft 1 to 10 times that in the case without the coating, the amount of heat transfer, that is, the amount of heat transfer from the shaft 1 to the bearing 2 is only about twice. Absent.

As described above, in the case of the conventional gas bearing,
In both cases, the amount of heat transfer from the shaft 1 is small. In other words, the cooling effect of the shaft is small. Therefore, as a result of the temperature of the shaft 1 rising by about 20 to 40 ° C., the shaft 1 thermally expands to change the bearing clearance, resulting in deterioration of bearing performance and deterioration of machining accuracy of the machine tool. In order to avoid this deterioration of accuracy, it is necessary to equilibrate the machine, but for that purpose, for example, the warm-up time (warm-up) time of the machine tool must be lengthened, which results in a loss of time.

Therefore, conventionally, not only the bearing of the gas bearing but also the shaft has been required to be forcibly cooled by a water cooling or oil cooling system.

[0008]

However, in order to supply the cooling fluid such as water or oil to the shaft rotating at a high speed while preventing the leakage, a special technique is required for the seal and the cost is very high. There was a problem. Therefore, the present invention has been made in view of such conventional problems, and an object thereof is to provide a gas bearing that can easily suppress a temperature rise of a movable member.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a gas bearing in which a movable bearing surface provided on a movable member faces a supporting bearing surface provided on a supporting member, and the movable bearing surface and the supporting bearing surface are Both are characterized by a thermal emissivity of 0.4 or more.

[0010]

In the gas bearing, heat is exchanged between the two members at a portion where the bearing surface of the movable member and the bearing surface of the supporting member face each other with a gas film of several μm therebetween. The transfer of this heat is mainly due to heat conduction and heat radiation. However, since the movable member and the supporting member are in non-contact with each other to transfer heat through gas, the specific gravity of heat conduction is relatively small. Therefore, in the present invention, the heat radiation is positively utilized to cool efficiently.

The opposing bearing surfaces of both members constituting the gas bearing must be finished to the highest degree of dimensional accuracy and surface roughness, corresponding to the extremely small bearing clearance of several μm. There is. Usually, both members are made of stainless steel, and the bearing surface thereof is a ground finish surface or a lap finish surface. Such a surface is almost a mirror surface, and therefore the thermal emissivity ε is very small at 0.03 to 0.1. This can be said to be the worst condition from the standpoint of the technique of the present invention of cooling by radiative heat transfer. The inventors of the present invention have conducted extensive studies on overcoming this adverse condition, and as a result, if the thermal emissivity ε of both the movable side bearing surface and the supporting side bearing surface is a high value of 0.4 or more, the thermal radiation The present invention has been completed by finding that the cooling capacity is greatly promoted as compared with the case where the stainless steel material is used as it is, and a sufficient effect is obtained in practical use.

(1) With reference to the drawings, first, the heat balance by radiative heat transfer of the present invention will be examined in detail. FIG. 1 shows a mode of heat transfer due to radiation between two adjacent surfaces. A surface 1a is a bearing surface of a shaft (high temperature side) as a movable member of a gas bearing, and a surface 2a is a bearing as a supporting member. This is the bearing surface (on the low temperature side).

The surface 1a has a temperature T ₁ and a thermal emissivity ε ₁
Then, the heat quantity q ₁ expressed by the equation (1) is released.
Almost all of this heat goes to the surface 2a because the surfaces 1a and 2a are close to each other. Part of the heat reaching the surface 2a is absorbed by the surface 2a (q ₁₂ ). However, most of the rest is reflected by the surface 2a and returns to the surface 1a (q ₁₁ ). A part of the returned heat q ₁₁ is also absorbed by the surface 1a (q ₁₁₁ ), but the rest is reflected by the surface 1 a toward the surface 2 a (q ₁₁₂ ). Such reflection and absorption are repeated all the time.

On the other hand, the surface 2a also radiates a heat quantity q ₂ represented by the equation (2) when the temperature T ₂ and the thermal emissivity ε ₂ are given. Similar to q ₁ , this heat is also repeatedly reflected and absorbed between the surface 1a and the surface 2a. In this way, the surfaces 1a and 2a each radiate heat at the same time. Also,
The heat radiated from one surface has a part to be absorbed by the opposite surface but also a part to be reflected, so that the heat transfer is complicated. When arranged, formulas (3) and (4) are obtained.

[0015] ^{_{q 1 = σT 1 4 Aε 1}} (1) q 2 = σT 2 4 Aε 2 (2) Q 12 = σ (T 1 4 -T 2 4) AF g12 (3) F g12 = 1 / {( 1 / ε ₁ ) + (1 / ε ₂ ) −1} (4) q ₁ : the amount of heat emitted from the surface 1 a q ₂ : the amount of heat emitted from the surface 2 a σ: the Stefan-Boltzmann constant T ₁ : the amount of the surface 1 a Absolute temperature T ₂ : Absolute temperature of surface 2a A: Area ε ₁ : Thermal emissivity of surface 1a ε ₂ : Thermal emissivity of surface 2a Q ₁₂ : Heat quantity transferred from surface 1a to surface 2a F _g12 : Gray surface 1a, Form factor between 2a (when the above formula is between two adjacent surfaces) Thermal emissivity ε: The larger this value is, the better the radiation of heat, the better the absorption of heat, and the less reflection of heat.

Gray surface: A surface with a thermal emissivity ε of 0 <ε <1, that is, a surface of an actual object (a white surface is an imaginary surface with ε = 0, and a black surface is an imaginary surface with ε = 1) Equation (3), As can be seen from (4), if T ₁ , T ₂ , and A are the same, Q _{12 depends on} the size of F _g12 . That is,
If the area and temperature of the surface 1 and the surface 2 are the same, the amount of radiant heat transfer between the two surfaces is proportional to the form factor F _g12 between the gray surfaces.

Now, the type of material of each surface 1a, 2a in the gas bearing is changed, and the combination is made with either a stainless steel polished surface or a graphite polished surface having a one-digit difference in thermal emissivity ε. Table 1 shows the results of calculation of the _view factor F _g12 between gray planes from (4).

[0018]

[Table 1]

As is clear from the table, when only the thermal emissivity ε of one of the shaft and the bearing is increased, the effect of increasing the form factor F _g12 between the gray surfaces is about double. is there. On the other hand, by increasing the thermal emissivity ε of both the shaft and the bearing, the effect of increasing the view factor of 10 times or more between the gray surfaces was obtained. From the above calculation, it has been clarified that the heat transfer amount can be dramatically increased by using a material having a high thermal emissivity for both the shaft and the bearing of the gas bearing.

(2) Next, the temperature of the shaft in that case will be examined. The heat that enters the shaft of the gas bearing can be either heat generated by the motor or heat generated by shearing the air in the bearing gap. When the shaft rotates at high speed, heat generation due to air shearing becomes dominant, so here only heat generation due to air shearing is considered. On the other hand, the heat emanating from the shaft may be due to heat conduction or heat radiation. In this case as well, heat radiation due to heat conduction is smaller than heat radiation due to heat radiation by at least two orders of magnitude. .

When a gas bearing spindle of a machine tool is actually operated, the temperature of air in the bearing clearance is substantially constant at T _A because the gas bearing spindle of the machine tool is operated at constant rotation. At this time, the amount of heat Q _IN entering the shaft is the temperature T _{A of the} air film and the temperature T _A of the shaft.
Proportional to the difference of _{1, Q IN = C (T} A -T 1) (5) C: a coefficient.

The heat gradually warms the shaft. As the shaft warms, the temperature difference between the two becomes smaller and Q _IN gradually decreases. On the other hand, when the shaft warms up, a temperature difference is generated between the shaft and the bearing, and the amount of heat commensurate with the temperature difference is radiated from the shaft. The heat emanating from this axis is not constant and increases as the axis warms. Thus, when the temperature of the shaft rises and the amount of heat entering the shaft and the amount of heat exiting the shaft become equal, the heat balance of the shaft is in equilibrium, and the temperature of the shaft becomes constant at that temperature.

Here, the amount of heat entering the shaft is Q _IN only, and the amount of heat exiting the shaft is only heat Q ₁₂ due to radiation. Since Q ₁₂ = Q _IN is constant in the equilibrium state, the above equations (3), (4), and (5) are combined to obtain the temperature T ₁ of the bearing surface of the shaft from the equation (6). To be σ (T ₁ ⁴ −T ₂ ⁴ ) AF _g12 = C (T _A −T ₁ ) (6) Now, when the thermal emissivity ε ₁ = ε ₂ = 0.05 (the same value as that of the conventional polished stainless steel surface) , The shaft temperature is 326K (5
For gas bearing spindles that are in equilibrium at 3 ° C,
Table 2 shows an example of trial calculation of the shaft temperature when the thermal emissivity ε ₁ and ε ₂ are changed.

[0024]

[Table 2]

A graph of Table 2 is shown in FIG. As is clear from FIG. 2, the thermal emissivities ε ₁ and ε ₂ are 0.
Until 4 epsilon _1, but decreases significantly the temperature of the shaft with increasing epsilon ₂ values, epsilon _1, the degree of temperature drop of the shaft when the value of epsilon ₂ is 0.4 or more is slowed, ε _1, ε ₂ The temperature T ₁ of the shaft does not drop much even if the value of is increased. In other words, the thermal emissivity ε ₁ , ε _{2 of} the conventional polished stainless steel surface is 0.
In the case of 05, the shaft temperature T ₁ changes from 20 ° C to 53 ° C, and the temperature increase ΔT is 33 ° C, while ε ₁ , ε ₂
Is 0.4, ΔT is 9 ° C, ε ₁ , ε ₂ is 0.7, and ΔT is 4 ° C.
It has become. So if ε ₁ and ε ₂ are 0.4,
Temperature rise is about 1/4 or less of the conventional case of ε = 0.05,
If ε ₁ and ε ₂ have a value of 0.7, it can be reduced to ⅛ or less. On the other hand, even if ε ₁ and ε ₂ are increased to 0.8 and 0.9, the effect does not increase so much.

As a reference, the thermal emissivity ε ₁ of various materials,
Table 3 shows the value of ε ₂ .

[0027]

[Table 3]

[0028]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 shows an embodiment of the gas bearing of the present invention. In the gas bearing of this embodiment, a shaft 13 as a movable member is coated with a coating 13 of molybdenum disulfide. However, the coating 13 of molybdenum disulfide enhances the thermal emissivities ε ₁ and ε ₂ of the bearing surface 1a of the shaft, unlike the conventional purpose of preventing seizure between the shaft and the bearing during air supply trouble. Is intended. Further, the bearing 2 which is a supporting member is made of stainless steel, and the bearing 2 is also coated with molybdenum disulfide coating 15. The bearing 2 fixed as a support member is forcibly cooled by a cooling mechanism (not shown).

A compressed gas 5 is sent from the air supply hole 3 formed in the bearing 2 to the bearing clearance 4, and the shaft 1 is levitationally supported by the pressure of the non-contact structure. In this case, the bearing surface 1a of the shaft 1 radiates a large amount of heat because the thermal emissivity ε ₁ , ε _{2 of the} molybdenum disulfide coating 13 is large. Most of the radiated heat is absorbed by the coating 13 of molybdenum disulfide having a high heat absorption coefficient on the bearing surface 2 a of the bearing 2. Since the bearing surface 2a has a large heat absorption coefficient and a small reflectance coefficient, the amount of heat returning to the shaft 1 is very small. Further, the amount of radiant heat transmitted from the bearing 2 to the shaft 1 is small because the bearing 2 is cooled. as a result,
A large amount of heat from the shaft 1 is transferred to the bearing 2 (more than ten times that of the conventional type),
The shaft 1 is efficiently cooled.

The present invention can be applied to a gas bearing for fixing a shaft and rotating a member fitted to the shaft, and is not limited to the static pressure gas bearing as described in the embodiment, but a static pressure air slide ( Static pressure direct acting gas bearings) and rotary and direct acting dynamic pressure gas bearings. Further, since the present invention has a large cooling effect, its constituent members may be forcibly cooled as in the above-described embodiment, but may not be forcibly cooled.

Further, the coating is not limited to molybdenum disulfide, and may be appropriately selected and used from other materials having thermal emissivity ε ₁ and ε ₂ of 0.4 or more, such as graphite and black ceramics. You can Further, instead of coating, both the shaft and bearing members themselves may be formed of a material having a thermal emissivity ε ₁ , ε ₂ of 0.4 or more.

Further, one of the shaft and the bearing has a thermal emissivity ε ₁ ,
epsilon ₂ is coated with 0.4 or more materials, other thermal emissivity epsilon ₁ the member itself, epsilon ₂ may be formed by 0.4 or more materials. Also, for materials with thermal emissivity ε ₁ and ε ₂ of 0.4 or more,
Different shaft side and bearing side may be used.

As a material having a thermal emissivity of 0.4 or more,
Other examples include alumina-based ceramics and magnesia-based ceramics.

[0034]

As described above, according to the present invention, a material having a thermal emissivity of 0.4 or more is used for both the movable side bearing surface and the supporting side bearing surface of the gas bearing. Because the heat of can be removed from the movable member,
It is not necessary to force the movable member to be water-cooled or liquid-cooled, and it is possible to provide a simple and low-cost gas bearing.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a mode of heat transfer by radiation between a shaft and a bearing.

FIG. 2 is a graph showing the relationship between the thermal emissivity of the shaft and the bearing and the temperature of the shaft.

FIG. 3 is a sectional view of an embodiment of the present invention.

FIG. 4 is a cross-sectional view showing an example of a conventional gas bearing.

FIG. 5 is a cross-sectional view showing another example of a conventional gas bearing.

[Explanation of symbols]

 1 movable member (shaft) 1a movable bearing surface 2 support member (bearing) 2a supporting bearing surface

Claims (1)

[Claims] 1. A gas bearing in which a movable bearing surface provided on a movable member faces a supporting bearing surface provided on a supporting member, and both the movable bearing surface and the supporting bearing surface have a thermal emissivity of 0.
A gas bearing characterized by being 4 or more.