JPH10205537A - Rotary hydrostatic bearing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suitably control a change in a bearing gap even if the temperature of a housing is adjusted. SOLUTION: A rotary hydrostatic bearing device provided with a housing 2, a radial hydrostatic bearing 4 fixed to the housing 2 and a rotary shaft 1 supported by the bearing 4, wherein the material of the rotary shaft 1 is smaller in a coefficient of linear thermal expansion than the material of the housing 2, has temperature control means 16, 22 for controlling the temperature of the housing 2 such that the temperature of a contact part of the housing 2 with the other unit is not changed when the rotating shaft is stopped and when it is rotated. The housing 2 and the rotary shaft 1 are constituted by the materials whose coefficients of linear thermal expansion are made different from each other to make the bearing gap 8 nearly constant by a difference in the coefficients of the linear thermal expansion when the rotary shaft is stopped and when it is rotated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrostatic bearing device used for a main shaft of a precision machine tool, and in particular, when mounted on a precision machine tool, does not adversely affect the machine tool body such as thermal deformation. Also, the present invention relates to a rotary type hydrostatic bearing device which can prevent a contact accident between a bearing and a shaft and a deterioration in machining accuracy due to a change in rigidity because a bearing gap does not change between a stop and a rotation.

[0002]

2. Description of the Related Art Conventionally, a hydrostatic bearing having little friction and high rotation accuracy has been used for a main shaft of a machine tool requiring high processing accuracy. In particular, a porous drawing type hydrostatic bearing using a porous material for the bearing has a high bearing rigidity and is used for a main spindle of a machine tool aiming at a surface roughness on the order of nanometers.

However, in order to obtain high rigidity in a porous bearing, the clearance between the bearings must be reduced to several μm. For this reason, the pressurized fluid in the narrow bearing gap is cut off during the rotation of the main shaft, and the amount of heat generated is greater than that of the other throttle type bearings. In order to cool this, a cooling channel is provided in the main shaft housing. However, since there is no cooling means in the rotating shaft, the rotating shaft thermally expands, the bearing gap is reduced, and there is a risk of burning.

[0004] In order to avoid this, Japanese Patent Application No. Hei 3-2767 has been proposed.
No. 73 proposes that the rotating shaft be made of silicon nitride or silicon carbide. That is, a general carbon steel is used as a material of the housing, and a material having a smaller linear thermal expansion coefficient than that of the housing is used for the rotating shaft to suppress the reduction of the bearing gap.

[0005]

However, when heat generated by rotation of the spindle is conducted to other parts of the machine tool so as not to cause thermal deformation, that is, the temperature of the contact part of the spindle housing with other units is controlled by the temperature of the spindle. When the housing is cooled under the condition that the temperature is the same as that at the time of stop, the bearing gap fluctuation is strictly controlled at the submicron level under the condition that the linear thermal expansion coefficient of the rotating shaft material is smaller than that of the housing material as described above. There is a problem that you can not. For example, in the above conventional example, the housing is made of carbon steel and the rotating shaft is made of silicon nitride. In this configuration, the bearing gap changes by about 1 to 2 μm during rotation. In the case of a porous static pressure bearing, a change in the gap of 1 μm reduces the bearing rigidity and heat generation to about 20.
%, And stable surface roughness and shape accuracy cannot be obtained.

An object of the present invention is to provide a rotary type hydrostatic bearing device which can appropriately suppress the fluctuation of the bearing gap even when the temperature of the housing is adjusted. It is to make.

[0007]

According to the present invention, there is provided a housing comprising: a housing; a radial static pressure bearing fixed to the housing; and a rotating shaft supported by the static pressure bearing. In a rotary type hydrostatic bearing device in which the linear thermal expansion coefficient of the material of the rotating shaft is smaller than that of the material of the housing, the temperature of a contact portion of the housing with another device does not change between a stop and a rotation. Temperature control means for controlling the temperature of the housing, and the difference in linear thermal expansion coefficient between the material of the housing and the material of the rotary shaft is such that the bearing gap is substantially constant between when stopped and when rotated. The housing and the rotation shaft are made of a material.

According to this configuration, for example, when a rotary type hydrostatic bearing device is applied to a main shaft of a machine tool, heat generated during rotation of the main shaft is not transmitted to other portions of the machine tool so that the housing is not connected to that portion. By controlling the temperature of the contact part to be constant and appropriately selecting the linear thermal expansion coefficient of the material of the spindle housing and the material of the rotating shaft as described above, fluctuations in the bearing gap are suppressed, and stable machining accuracy is achieved. can get.

[0009]

In a more specific embodiment, for example, the difference in the coefficient of linear thermal expansion is 3.7 to 5.8 × 10 ^-6.
By setting (1 / ° C.), the fluctuation of the bearing gap can be suppressed.

Such a difference in linear thermal expansion coefficient is, for example, as follows.
It can be obtained by using alumina ceramics for the material of the housing and silicon nitride for the material of the rotating shaft.
A porous material can be used as the material of the hydrostatic bearing.

As the temperature control means, for example, a sensor for detecting a temperature of a contact portion of the housing with another device;
A coolant circulation device for cooling the inside of the housing,
A control device for controlling the temperature of the housing by controlling the temperature or flow rate of the coolant based on the output of the sensor can be used. In this case, for example, in a high-rigidity spindle of a porous drawing system to which the bearing device of the present invention is applied, the temperature of the contact portion to another unit is the same as that at the time of rotation even at the time of rotation, so that the contact portion is The temperature or the flow rate of the coolant for housing cooling is controlled while detecting the temperature with the sensor. Further, by using alumina ceramics and silicon nitride as materials for the housing and the rotating shaft as described above, the fluctuation of the bearing gap can be reduced to a submicron level. For this reason, it is possible to suppress a change in the rigidity of the main spindle itself and a change in the amount of generated heat, and also prevent the other parts of the machine tool from being thermally deformed due to the heat generated by the main spindle, so that high machining accuracy can be stably obtained as a whole machine tool. .

[0012]

FIG. 1 shows the configuration of a rotary hydrostatic bearing device used for a main shaft of a machine tool according to an embodiment of the present invention. The bearing device part is shown in cross section. In the figure, reference numeral 1 denotes a disk-shaped rotating shaft having a coefficient of linear thermal expansion of 2.6 × 10
It is made of silicon nitride of about ^-6 (1 / ° C.). Reference numerals 2 and 3 denote a radial housing and a thrust housing made of alumina ceramics having a coefficient of linear thermal expansion of about 7.1 × 10 ⁻⁶ (1 / ° C.). Numerals 4 and 5 denote a radial bearing and a thrust bearing made of a carbon-based self-lubricating porous material, which are fixed to the radial housing 2 and the thrust housing 3, respectively, and make the compressed air supplied from the air supply holes 6 and 7 porous. Air is squeezed out from the bearing surface. As a result, the distance between the rotating shaft 1 and the radial housing 2 and the thrust housing 3 is about 5 μm.
A compressed air layer having high rigidity is formed in the bearing gaps 8 and 9 of the rotary shaft 1 so that the rotating shaft 1 can be levitated and supported without contact.

Reference numeral 10 denotes a ring-shaped carbon steel fixed to the rotating shaft 1, and is attracted in the axial direction by a ring-shaped magnet 11 fixed opposite to the thrust housing 3. The suction force and the repulsive force with the thrust bearing 5 are balanced, so that the thrust gap 9 is kept constant.

Reference numeral 12 denotes a motor coil fixed to the thrust housing 3, and reference numeral 13 denotes a motor magnet integrated with the rotary shaft 1, and both form a direct drive motor. Since this motor has no contact portion, high rotational accuracy and durability can be obtained. Reference numeral 14 denotes an encoder disk of an encoder for detecting the spindle rotation speed (the rotation speed of the rotary shaft 1), and reference numeral 15 denotes a signal detection unit of the encoder. The spindle speed is controlled by a motor driver (not shown) receiving the output of the encoder and controlling the output of the motor. Reference numeral 16 denotes a radial cooling water passage mainly for suppressing heat generation of the radial bearing 4, and reference numeral 17 denotes a thrust cooling water passage mainly for absorbing heat generation of the thrust bearing 5 and heat of the motor coil 12.

When the above-described spindle is mounted on a machine tool, it is fixed to a slider such as 18. In a high-precision machine tool, since the slider 18 also uses a porous static pressure bearing like 19, the bearing gap between the porous static pressure bearing 19 and its guide 20 is as small as about 5 μm.
For this reason, the main shaft generates heat, and even if the heat is slight, the slider 18
In this case, the bearing gap of the slider 18 changes, causing a decrease in rigidity or a decrease in straightness, thereby impairing machining accuracy.

Therefore, the contact portion 2 of the spindle with the slider 18 is provided.
It is necessary to keep the temperature of 1 constant. A thermistor 22 having a resolution of 0.01 ° C. is embedded and adhered to this portion as a temperature detection sensor. 23 is a thermometer connected to the thermistor 22, 24 is a temperature controller for controlling the temperature based on the output of the thermometer 23, 25 is a radial cooling water passage 16
And a constant-temperature water circulating device for circulating constant-temperature water in the thrust cooling water passage 17; flow control valves for cooling the radial housing 2 and the thrust housing 3 for controlling the circulation of the constant-temperature water by a temperature control device 24; It is.

For example, when the diameter of the rotating shaft is φ240 mm, the width of the radial bearing 4 is 45 mm, and the width of the thrust bearing is 35 mm, very large rigidities of 800 N / μm and 1000 N / μm are obtained. . However, when the bearing clearance is set to 5 μm and 3000 rpm
, The calorific value of the four radial bearings is 156.
W, the calorific value of the 5 thrust bearings is 88 W, and the calorific value of the motor coil 12 is 26 W. If not cooled at all, the temperature of the housing rises to about 10 ° C., causing large thermal deformation of the slider 18.

In order to prevent this, the slider 18 of the spindle is used.
Measure the temperature of the contact area with the
When a difference between the initial value and the present value occurs, the temperature control device 24 recognizes the difference and sets the flow control valves 26, 2 so that the difference becomes zero.
7 to increase or decrease the flow rate. Radial housing 2
And the flow rate flowing through the thrust housing 3 is always the ratio of their calorific values, that is, 156: (88 + 26). When cooled at this ratio, the entire housing thermally deforms substantially uniformly. In this case, the set temperature of the constant temperature water circulation device 24 is constant. This is because controlling the flow rate with the flow rate control valves 26 and 27 can perform control with less time delay than controlling the temperature of the cooling water with the constant temperature water circulation device 25. 30
The contact portion 2 of the spindle with the slider 18 at the time of rotation at 00 rpm
Assuming that the initial temperature is 23 ° C. and the temperature of the cooling water is 21.5 ° C., about 3 [l / min] of the radial housing 2 and about 2 0.2 [l / min].

By the cooling method described above, heat generated by the rotation of the spindle does not adversely affect the slider 18. But,
The bearing clearance of the main shaft varies depending on the constituent materials. In FIG.
In the case where the above cooling is performed at 3000 rpm, the radial bearing gap 8 changes depending on the material of the rotating shaft 1. The horizontal axis indicates the axial position of the radial bearing 4 and the vertical axis indicates the amount of change from the set gap of 5 μm. Further, the material of the housings 2 and 3 has high specific rigidity and is relatively workable.
It is an alumina ceramic having a linear thermal expansion coefficient α = 7.1 × 10 ⁻⁶ (1 / ° C.). In FIG. 2, a is a carbon steel having a linear thermal expansion coefficient α = 10.7 × 10 ⁻⁶ (1 / ° C.), b is an alumina ceramic, and c is a linear thermal expansion coefficient α = 4.0 × 10 ^−6.
(1 / ° C.) silicon carbide: d = linear thermal expansion coefficient α = 2.6
It is a calculated value when silicon nitride of × 10 ⁻⁶ (1 / ° C.) is used as the material of the rotating shaft 1.

It is understood that when the coefficient of linear thermal expansion is large, the thermal expansion of the rotating shaft 1 is naturally large, and the bearing gap 8 is reduced. The reason why the linear thermal expansion coefficient and the reduction amount of the bearing gap 8 are not completely proportional is that the influence of the thermal conductivity of the material is small. In a material other than silicon nitride, the gap is reduced by 1 μm or more and the bearing rigidity is increased, but the heat generated by the heat insulation of the air film is increased. Since the bearing gap 8 and the calorific value are in a proportional relationship, if the bearing gap 8 decreases by 1 μm, the calorific value increases by 20%. For this reason, if the contact portion 21 of the spindle housing with the slider is cooled so as to have a constant temperature, the rotating shaft 1 mainly expands, and finally the gap 8 disappears, resulting in seizure.

When the material of the rotary shaft 1 is silicon nitride, there is a slight increase or decrease depending on the bearing position, but there is no change in the bearing gap 8 in total. Therefore, there is no danger of a change in rigidity or seizure, and the heat generated by the spindle does not thermally deform other parts of the machine tool.

The linear thermal expansion coefficients of dense alumina ceramics and silicon nitride that can be used as a structure slightly differ depending on the purity, but α = 6.5 to 7.7 × 10 ⁻⁶ , respectively.
(1 / ° C.), α = 1.9 to 2.8 × 10 ⁻⁶ (1 / ° C.), and if the material is in this range, the fluctuation of the bearing gap can be suppressed to submicron or less under the above cooling conditions. it can. That is, the material of the rotating shaft 1 has a smaller linear thermal expansion coefficient than the material of the housing, and the difference is 3.7 to 5.8 × 10 ^−6.
If the material combination is (1 / ° C.), the above effects can be expected. However, this does not apply when the thermal conductivity coefficient of the material of the rotating shaft 1 is twice or more that of the material of the housing 2. For example, when carbon steel is used as the material of the housing 2, low thermal expansion cast iron having a linear thermal expansion coefficient α of about 7 × 10 ⁻⁶ (1 / ° C.) or alumina ceramics can be used as the material of the rotating shaft 1.

[0023]

As described above, according to the present invention, the temperature of the housing is controlled so that the temperature of the contact portion of the housing with the other device does not change between the time of stop and the time of rotation. The difference between the linear thermal expansion coefficient of the material and the material of the rotating shaft is such that the bearing and the rotating shaft are made of a material that makes the bearing clearance almost constant between when stopped and when rotating. can do. Therefore, it is possible to prevent the heat generation of the main shaft of the machine tool to which the present invention is applied from thermally deforming other parts of the machine tool, and at the same time, to suppress the fluctuation of the bearing clearance of the main shaft. Seizure can be prevented, and stable machining accuracy can be obtained for the entire machine tool.

[Brief description of the drawings]

FIG. 1 is a view showing a configuration of a rotary hydrostatic bearing device used for a main shaft of a machine tool according to an embodiment of the present invention.

FIG. 2 is a graph showing how a bearing gap of a main shaft changes depending on a constituent material of a rotating shaft.

[Explanation of symbols]

1: rotating shaft, 2: radial housing, 3: thrust housing, 4: radial bearing, 5: thrust bearing,
16: radial cooling channel, 17: thrust cooling channel, 2
2: temperature sensor, 23: thermometer, 24: temperature controller,
25: constant temperature water circulation device, 26: radial flow control valve, 27: thrust flow control valve.

Claims (4)

[Claims] 1. A housing comprising: a housing; a radial static pressure bearing fixed to the housing; and a rotating shaft supported by the static pressure bearing, wherein the linear heat of the material of the rotating shaft is more than the material of the housing. In a rotary type hydrostatic bearing device having a small expansion coefficient, a temperature control means for controlling a temperature of the housing so that a temperature of a contact portion of the housing with another device does not change between a stop time and a rotation time, The housing and the rotating shaft are made of a material such that a difference in linear thermal expansion coefficient between the material of the housing and the material of the rotating shaft makes the bearing gap substantially constant between a stop and a rotation. Rotary type hydrostatic bearing device. 2. The difference in linear thermal expansion coefficient between 3.7 and 5.
2. The method according to claim 1, wherein the temperature is 8.times.10.sup.- ⁶ (1 / .degree. C.).
A rotary type hydrostatic bearing device as described in the above. 3. The material according to claim 1, wherein the housing material is alumina ceramics, the material of the rotating shaft is silicon nitride, and the material of the hydrostatic bearing is a porous material. Rotary hydrostatic bearing device. 4. The temperature control means includes: a sensor for detecting a temperature of a portion of the housing in contact with another device; a coolant circulating device for cooling the inside of the housing; and a temperature or flow rate of the coolant. The rotary hydrostatic bearing device according to any one of claims 1 to 3, further comprising a control device that controls the temperature of the housing by controlling the temperature of the housing based on an output of the sensor.