JP2001065572A - Ceramic combination rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ceramic combination rolling bearing to satisfy characteristics demanded for a bearing for a high speed spindle. SOLUTION: In a ceramic combination rolling bearing consisting of a rolling body, an inner ring, and an outer ring, all of which are formed of a ceramic material, a ceramic material consisting of sialon or a silicon nitride, being comparatively low in density and high in fracture toughness and thermal shock properties is formed in the rolling body. A ceramic material consisting of zirconia or alumina, having a thermal expansion factor comparatively similar to that of a steel and being low in thermal conductance and high in a fracture strength and a Young's modulus is used in the inner ring, and a ceramic material, consisting of silicon carbide, aluminum nitride, or alumina, being comparatively high in thermal conductance and low in a thermal expansion factor is used in an outer ring.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic combination rolling bearing using a ceramic material for a rolling element, an inner ring and an outer ring.

[0002]

2. Description of the Related Art Conventionally, a rolling bearing using silicon nitride (Si ₃ N ₄ ) for a rolling element and bearing steel for an inner ring and an outer ring is generally known as a ceramic combination rolling bearing. In addition, an all-ceramic bearing using silicon nitride also exists on the inner ring and the outer ring.

[0003] These rolling bearings have excellent characteristics due to silicon nitride, and the density of silicon nitride is 3.3 (g / c).
m ³ ), which is relatively small, which helps to reduce the contact stress of the outer ring due to the centrifugal force of the rolling elements.

[0004]

However, when the above-mentioned ceramic combination rolling bearing is applied to a high-speed spindle, there are the following problems, and improvement thereof has been demanded. That is, in the case of a rolling bearing using silicon nitride for the rolling element and bearing steel for the inner and outer rings, there is a possibility that seizure may occur due to heat generation, and the rotational speed N of the bearing is limited. In terms of a general DmN value (= pitch circle diameter × rotation speed) corresponding to the rotation speed N, at present, the DmN value has reached its limit.

[0005] As a countermeasure against seizure due to such heat generation, oil jet lubrication has been used to increase the DmN value. However, when oil jet lubrication is used, friction increases due to an increase in the stirring resistance of the rolling bearing lubricating oil, which increases the load on the drive motor. Will be.

On the other hand, an all-ceramic bearing in which silicon nitride is also used for the inner ring and the outer ring is advantageous for increasing the temperature of the bearing. Since the amount of thermal expansion due to the temperature rise is small, the interference may be excessive and the inner ring may be damaged. In addition, when the outer ring is made of silicon nitride, the heat conductivity is small, so that the temperature generated by the heat generated in the rolling bearing increases, which is disadvantageous for releasing the heat.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a ceramic combination rolling bearing which can satisfy the characteristics required for a bearing for a high-speed spindle.

[0008]

According to a first aspect of the present invention, there is provided a ceramic combination rolling bearing using a ceramic material for a rolling element, an inner ring and an outer ring. For the rolling element, a ceramic material made of sialon or silicon nitride having relatively low density, high fracture toughness, and high thermal shock resistance is used. A ceramic material made of zirconia or alumina having a low conductivity and a high breaking strength and a high Young's modulus is used. The outer ring is made of silicon carbide, aluminum nitride or alumina having a relatively high thermal conductivity and a low thermal expansion coefficient. Characterized by using a ceramic material composed of

As described above, it is preferable to use sialon or silicon nitride for the rolling elements, zirconia or alumina for the inner ring, and silicon carbide, aluminum nitride or alumina for the outer ring. preferable.

Further, a composite material of silicon carbide whiskers having each ceramic material as a base may be used. further,
A composite material as it is, such as a composite material composed of silicon carbide and silicon nitride, may be used.

[0011] In the rolling bearing in which the rolling element, the inner ring and the outer ring are formed using such a ceramic material, the rolling materials are different from each other, so that the rolling friction is small and the temperature rise itself can be suppressed. Furthermore, even when used for a high-speed spindle, the inner ring does not reach breaking strength due to the difference between the thermal expansion of the shaft and the inner ring and the expansion due to centrifugal force.

Further, since the material is made in consideration of the flow of heat with respect to heat generation, a rise in temperature can be suppressed. For example, when silicon carbide is used as the material of the outer ring, the thermal conductivity is 1.6 to 4 times that of the bearing steel, and when aluminum nitride is used, it is 6 times that of the bearing steel. Therefore, it is possible to sufficiently rotate at high speed even with oil mist and oil air.

Furthermore, it is desirable to use an angular contact ball bearing, which is a rolling bearing using the above-mentioned material, by incorporating it into a spindle of a constant pressure preload system, and to suppress a change in bearing clearance (in the case of an angular contact ball bearing, a contact angle). Can be.

Further, since the flow of heat from the inner ring to the rotating shaft is reduced, the elongation of the shaft can be reduced, and in the machine tool, the change in the position of the tool tip is reduced, thereby realizing high-precision machining. it can. In particular, when applied to a spindle of a built-in motor drive system, an inlet through-hole and an outlet through-hole of a system separate from lubrication of a bearing are provided in a housing near both ends of a rotor, and a room temperature or lower (for example, It is possible to suppress heat generation by flowing air cooled to (−30 ° C.). As described above, the heat generated by the rotor of the built-in motor, which is another heat source other than the bearing, is cooled by the air at room temperature or lower, whereby the temperature increase of the rotating shaft can be suppressed. In this case, the cooling air is preferably introduced from the tool mounting side, and changes in the tool position are suppressed due to elongation due to the temperature rise of the rotating shaft on the tool side.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the ceramic combination rolling bearing of the present invention will be described. The ceramic combination rolling bearing of the present embodiment is applied to an angular contact ball bearing.

[Spindle Structure] FIG. 1 is a sectional view showing the structure of a spindle of a built-in motor drive system using an angular ball bearing. In the spindle of the built-in motor drive system, the rotating shaft 18 is supported by a pair of angular ball bearings 13 and 15 provided inside both ends of the housing 11. Spacers 21 and 23 are provided at intermediate positions between the pair of angular ball bearings 13 and 15.

A rotor 24 is attached to the rotation shaft 18 located at the center of the inside of the housing. A stator 26 facing the rotor 24 is provided inside the housing 11. Further, a groove 11a for cooling water is formed on an inner wall of the housing 11 facing the stator 26. A balance ring 27 is provided on the rotation shaft 18 before and after the rotor 24.

Further, each part of the rotating shaft 18 protruding from both ends of the housing 11 is provided with inner ring tightening nuts 31, 33.
Is provided, and the amount of tightening of the inner ring can be adjusted. Further, outer ring tightening nuts 35 and 36 are provided on the portion of the rotating shaft 18 outside the inner ring tightening nuts 31 and 33, respectively.
Is provided, and the amount of tightening of the outer ring can be adjusted.

A preload sleeve 39 is inserted into the end of the right housing 11 through a guide ball 38. The preload sleeve 39 is urged rightward in FIG. The stop 43 prevents rotation. Tip 3 of preload sleeve 39
9a is bent so as to lock the outer ring 15a of the angular ball bearing 15, and the outer rings 15a of the pair of angular ball bearings 15 sandwiched between the preload sleeve 39 and the outer ring tightening nut 36 are fixed to the outer ring tightening nut. Tightening is adjusted by 36.

[Lubricating and Cooling Structure] FIG. 2 is a sectional view showing a lubricating and cooling structure in a spindle of a built-in motor drive system. Entrance through-holes 51 and 52 are formed in a radial portion of the housing 11 located at an intermediate position between the pair of angular ball bearings 13 and 15.
Oil mist or oil air and cooling air (for example, minus 30 ° C.) are supplied from the outside of the housing 11 through the through holes 51 and 52 to the pair of angular ball bearings 1.
3 and 15, that is, the outer ring 13a, 15a, the inner ring 13
b, 15b and the rolling elements 13c, 15c, respectively. An outlet through-hole 54 is formed in a radial portion on the substantially central left side of the housing 11. Similarly,
An outlet through-hole 55 is formed in a radial portion on the substantially central right side of the housing 11 and the preload sleeve 39. Oil mist (oil air) and cooling air flowing into the housing 11 through the inlet through holes 51 and 52 absorb heat while passing through the inside of the angular ball bearings 13 and 15, respectively, and the outlet through holes 54 and 55, respectively. Out or from the axial direction to the outside (see arrow a in the figure). Here, the arrow d in the figure represents the flow of heat.

An inlet through-hole 61 and an outlet through-hole 62 communicating with the groove 11a are formed in a radial portion of the inner wall of the housing 11 facing the stator 26, and the cooling air flowing through the inlet through-hole 61 is formed. Water (arrow b
After the heat is absorbed by the stator 26, flows out of the outlet through-hole 62.

Further, a cooling air inlet through hole 65 and an outlet through hole 66 are provided in a radial portion of the housing 11 located outside the cooling water inlet through hole 61 and the outlet through hole 62. Formed through hole 6 for the entrance
The cooling air that has flowed in through hole 5 absorbs heat while passing between stator 26 and rotor 24 and then flows out of housing 11 through outlet through hole 66 (arrow c).
reference).

As described above, by providing a cooling system for cooling the angular ball bearings 13, 15 and the rotor 24 as heat generating portions and a lubrication system for lubricating the angular ball bearings 13, 15 separately, a cooling effect is obtained. Can be increased.

[Materials] The angular ball bearings 13 and 15 used for the spindle having the above structure are composed of ceramic combination rolling bearings. The materials of the rolling elements, the inner ring and the outer ring will be examined below. FIG. 3 is a table showing characteristics of various ceramic materials.

Rolling element When an angular contact ball bearing is used for a high-speed spindle,
Since an increase in the contact force between the rolling element and the outer ring due to centrifugal force poses a problem, it is desirable that the material of the rolling element has a low density. FIG. 3 shows that the density of silicon carbide, sialon, and silicon nitride is relatively small.

In a machining center (M / C) on which a built-in motor drive type spindle is mounted, the rotation speed rises (rotation speed: 0 → maximum rotation speed) and falls (rotation speed: maximum rotation speed → 0). Since it is important to reduce the time of the impact, and more and more impact movement is required, it is desired that the fracture toughness is large. FIG. 3 shows that sialon and silicon nitride have relatively high fracture toughness. It is also desired that the thermal shock resistance is large, and similarly, it can be seen that sialon and silicon nitride have relatively large thermal shock resistance.

Furthermore, it is desired that the change in clearance (change in the contact angle in the case of the angular ball bearing of the present embodiment) with respect to the temperature rise is small, that is, the difference in the coefficient of thermal expansion between the inner ring and the outer ring is small. This will be described in detail later. Further, in order to release heat to the outside of the housing, it is desired that the heat conductivity is large. FIG. 3 shows that silicon carbide has the largest thermal conductivity (thermal conductivity).

Inner Ring In order to prevent stress destruction due to the difference in the amount of thermal expansion between the rotating shaft and the inner ring due to temperature rise, it is desirable that the material of the inner ring has a small difference in thermal expansion coefficient from the rotating shaft. In FIG. 3, it can be seen that the thermal expansion coefficients of zirconia and alumina are large, and the difference from the thermal expansion coefficient of the rotating shaft (bearing steel) is relatively small.

It is desirable that the breaking strength and the Young's modulus are large, and zirconia is the largest. Further, it is desirable that the fracture toughness is also large, and it can be seen that zirconia, sialon, and silicon nitride have relatively large fracture toughness.

On the other hand, it is desirable that the change in interference due to the difference in the amount of expansion between the rotating shaft and the inner ring due to centrifugal force be small. This will be described in detail later. Further, it is desirable that the density and the Young's modulus are large. In FIG. 3, silicon carbide, alumina, silicon nitride, and sialon are appropriate.

Furthermore, in order to efficiently release the heat generated by the bearing, it is desirable that the thermal conductivity is large, and silicon carbide and silicon nitride are suitable. However, when it is assumed that heat is released to the outer ring side, it is preferable that the thermal conductivity is rather small. In this case, zirconia and sialon are appropriate. In any case, the thermal conductivity is determined by the combination of the rolling element and the outer ring.

Outer ring In order to release heat generated by the bearing to the housing as much as possible, silicon carbide, alumina and silicon nitride having high thermal conductivity are desired. In particular, when zirconia is used for the inner ring, this characteristic of thermal conductivity is important.

Further, it is desirable that the change in the clearance with respect to the temperature rise be small, and that the coefficient of thermal expansion be close to those of the inner ring and the rolling element. However, the priority of the characteristic for this request is relatively low. In addition, zirconia, sialon, and silicon nitride having high fracture toughness are desired.

As described above, the priority order of the characteristics with respect to the requirements of the angular contact ball bearing is higher in the order of the rolling element, the inner ring, and the outer ring, and the ceramic materials satisfying the characteristics with respect to these requirements are arranged as follows. is there. FIG. 4 is a table showing necessary characteristics of the rolling elements, the inner ring and the outer ring, and corresponding ceramic materials.

As a result of the above examination, the most suitable ceramic material as a combination of the rolling element, the inner ring and the outer ring is a rolling element: sialon, an inner ring: zirconia, and an outer ring: silicon carbide.

Next, a technical study will be made when a rolling bearing made of a ceramic material selected as shown in FIG. 4 is used for a spindle. That is, thermal expansion of the rotating shaft and the inner ring due to temperature rise and expansion due to centrifugal force are analyzed and examined. The bearing clearance (contact angle in the case of an angular contact ball bearing) is also examined. Here, an angular contact ball bearing having a representative name of 85BNC is used. This bearing has dimensions of outer diameter Do = 130φ, inner diameter Di = 85φ, and pitch circle diameter Dm (PCD) = 106.357φ. The rotation speed N is 25,000 rpm (DmN
Value ≒ 2.6 million).

[Study 1 (thermal expansion of shaft and inner ring)]
First, the increase in the interference due to the thermal expansion between the rotating shaft and the inner ring will be examined. Assuming that the temperature rise is ΔT ° C. and the thermal expansion coefficients of the rotating shaft and the inner ring are αs and αi, respectively, the increase δt of the interference is expressed by the following equation (1).

[0038]

Δt = (αs−αi) Di · ΔT Here, the thermal expansion coefficient αs of the rotating shaft made of steel is 12.
5 × 10 ⁻⁶ / ° C. The thermal expansion coefficient αi of the inner ring made of zirconia is 10.5 × 10 ⁻⁶ / ° C. The thermal expansion coefficient αi of the inner ring made of alumina is 8.1 × 10
⁻⁶ / ° C.

Therefore, in the case of zirconia, the increase δt of the interference due to the temperature rise is 2.1 × 10 ⁻⁴ ΔT (m
m), and 3.7 × 10 ⁻⁴ ΔT (m
m).

Next, the circumferential stress σtθ (maximum stress σtθma) of the inner ring caused by the increase in the interference δt.
x) is calculated. That is, when the internal pressure Pa acts in the radial direction of the inner ring, the stress σtθmax in the circumferential direction of the inner ring is expressed by Expression (2) assuming that the ratio k (= b / a) between the inner diameter a and the outer diameter b of the inner ring. Is done.

[0041]

Σtθmax = Pa · (k ² +1) / (k ² −1) Further, the internal pressure Pa is obtained by Expression (3).

[0042]

(Equation 3)

Here, the modulus of longitudinal elasticity of steel Eb = 2.08 ×
10 ⁴ kg / mm ² , the modulus of longitudinal elasticity of zirconia Ea = 2.
1 × 10 ⁴ kg / mm ² , alumina longitudinal modulus Ea =
3.90 × 10 ⁴ kg / mm ² , and Poisson's ratio νa, νb = 0.3.

According to equations (2) and (3), the stress of the inner ring is
5.20 × 10 for zirconia ^-2ΔT (kg / mm
^Two), 18.9 × 10 for alumina^-2ΔT (kg /
mm^Two).

At a bearing temperature increase ΔT = 100 ° C., 5.2 kg for zirconia strength of 100 kg / mm ² .
/ Mm ^2, to strength 60 kg / mm ² of alumina, because it is 18.9 kg / mm ^2, both the material can be sufficiently withstand use at 120 ° C. ([Delta] T = equivalent to 100 ° C.).

The expansion amount δt2 of the outer diameter of the inner ring is the sum of the amount of thermal expansion of the inner ring and the amount of expansion due to interference, and in the case of zirconia and alumina, respectively, 1.19 × ΔT
(Μmφ), and 1.11 × ΔT (μmφ).

FIG. 5 is a graph showing changes in the thermal expansion amount δt2 of the outer diameter of the inner ring and the maximum value σtθmax of the stress in the radial direction of the inner ring with respect to the temperature rise ΔT in zirconia and alumina.

[Study 2 (Expansion of Shaft and Inner Ring by Centrifugal Force)] Next, the increase in interference of the inner ring due to centrifugal force δc1, the stress σθmax in the circumferential direction of the inner ring, and the expansion amount δc2 of the outer diameter of the inner ring Are the equations (4), (5),
Required by (6).

[0049]

(Equation 4)

[0050]

(Equation 5)

[0051]

(Equation 6)

Here, ρa: mass of inner ring material (6.12 × 10 ⁻¹⁰ kg · sec ² / mm ^{2 for} zirconia, 3.98 × 10 ⁻¹⁰ kg · sec ² / mm ^{2 for} alumina)
² ), ρb: mass of shaft material (7.96 × 10 ⁻¹⁰ kg · s)
ec ² / mm ² ), angular velocity ω: 2.62 × 10 ³ rad /
sec at 25,000 rpm, modulus of longitudinal elasticity Ea of inner ring material: 2.08 × 10 ⁴ kg / mm ² , modulus of longitudinal elasticity Eb of shaft material: 2.10 × 10 ⁴ kg / mm ² , inner diameter of inner ring a: 42 0.5 mm, outer diameter b of the inner ring: 49.5 mm, and Poisson's ratio νa = νb = 0.3.

As a result, the increase δc1 in the interference of the inner ring due to the centrifugal force is −36.5 μmφ and −8.2 μmφ in the case of zirconia and alumina, respectively.

Further, in the case of zirconia and alumina, the circumferential stress σcθmax generated in the inner diameter direction of the inner ring is as follows.
They are 18.6 kg / mm ² and 12.2 kg / mm ² , respectively.

The expansion amount δc2 of the outer diameter of the inner ring is 42.4 μmφ for zirconia and alumina, respectively.
It becomes 14.9 μmφ.

[Study 3 (Summary of Interlock and Stress between Shaft and Inner Ring)] The above calculated values are summarized below when the inner ring is made of zirconia and alumina. (A) In case of expansion due to temperature rise ΔT Maximum stress σtθmax in the circumferential direction of the inner diameter of the inner ring = 5.2
0 × 10 ⁻² × ΔT (kg / mm ² ), 18.9 × 10 ⁻²
× ΔT (kg / mm ² ) Increase in interference δt = 0.17 × ΔT (μm), 0.0
37 × ΔT (μm) The amount of expansion of the outer diameter of the inner ring δt2 = 1.19 × ΔT (μm),
1.11 × ΔT (μm) (b) In the case of expansion due to centrifugal force, the maximum stress σcθmax in the circumferential direction of the inner diameter of the inner ring = 18.
6 (kg / mm ² ), 12.2 (kg / mm ² ) Increase in interference δc1 = −36.5 μmφ, −8.2 μ
mφ Outer diameter expansion amount of inner ring δc2 = 42.4 μmφ, 14.9 μm
mφ In the actual spindle, it is necessary to have an interference at the maximum rotation speed and the maximum bearing temperature. At this time, the interference increases with an increase in temperature, and decreases with an increase in rotation. In the machining center, the operation between the maximum rotation speed and the rotation stop is always repeated. FIG. 6 is a graph showing the change over time in the bearing temperature and interference when the operation is repeated between the maximum rotation speed and the rotation stop.

The interference is ensured at point A (near the temperature rise and rotation speed maximum) in the figure, and the interference is maximized at the point B (near the temperature rise maximum and rotation speed 0) in the figure. This means that the interference at point A (normal temperature) is a value that gives the decrease in interference due to centrifugal force, and the interference at point B is an increase in interference due to temperature rise and an increase in interference due to centrifugal force. The value is obtained by adding minutes. The same applies to a change in the inner diameter stress of the inner ring and a change in the outer diameter dimension of the inner ring.

Assuming that the temperature rise ΔT = 50 ° C., the increase in the interference at the points A and B, the maximum stress, and the expansion of the outer diameter are as follows. However, the minimum clearance at point A is set to zero.

(A) Point A: Increase in interference δt = 36.5 μmφ (zirconia),
8.2 μmφ (alumina) Maximum stress σθmax = 11.2 kg / mm ² (zirconia), 4.2 kg / mm ² (alumina) Outer diameter expansion of inner ring δt2 = 33.1 μmφ (zirconia), 6.9 μmφ (alumina) (B) Point B: Increase in interference δt = 45.0 μmφ (zirconia), 2
6.7 μmφ (alumina) Maximum stress σθmax = 25.9 kg / mm ² (zirconia), 24.8 kg / mm ² (alumina) Outer diameter expansion of inner ring δt2 = 102 μmφ (zirconia), 74.1 μmφ (alumina) In addition, the maximum stress at the points A and B is much lower than the fracture strength (100 kg / mm ² ) in the case of zirconia, and lower than the fracture strength (60 kg / mm ² ) in the case of alumina. When either zirconia or alumina is used, neither is destroyed. When alumina is used, the breaking strength is about 1
/ 2 or less. Also, the temperature rise ΔT = 1
Even in the case of 00 ° C. (corresponding to an operating temperature of 120 ° C.), in the case of zirconia, the maximum stress is 28.5 (kg / mm ² ).
In the case of alumina, the maximum stress is 34.1 (k
g / mm ² ), so it can withstand sufficiently.

The change in the outer diameter of the inner ring was 68.1 μmφ in the case of zirconia and 66.2 in the case of alumina.
μmφ, and both have large amounts of change. This is because, in zirconia, the expansion due to centrifugal force is large and the expansion due to temperature rise is small, while in alumina, the expansion due to centrifugal force is small and the expansion due to temperature rise is almost the same value. .

[Study 4 (Change in Bearing Clearance (Contact Angle))] The change in bearing clearance (contact angle) will be examined. (A) The amount of expansion of the inner groove (outer diameter) due to the centrifugal force is determined by the rotational speed N = 25,000 rpm according to the aforementioned equation (6).
42.4 μm in the case of zirconia as calculated by
φ, in the case of alumina, 14.9 μmφ. (B) The change in the clearance due to the temperature rise is related to the change in the outer ring groove diameter and the expansion (× 2) of the rolling element. Here, the rolling element diameter is 9.525, and the outer ring groove diameter is Doi = 115.89.
When the material of the outer ring is silicon carbide, the coefficient of thermal expansion α is 4.2 × 10 ⁻⁶ / ° C., and when the material of the rolling elements is sialon, the coefficient of thermal expansion α is 1. 6 × 10
⁻⁶ / ° C.

Therefore, when the outer ring is made of silicon carbide,
When the inner ring is made of zirconia (coefficient of thermal expansion α = 1.19 × 10 ⁻³ ), the amount of change δ1 in the clearance due to temperature rise is δ1 =
4.2 × 10 ^-6 × 115.896 × ΔT-1.6 × 10
^-6 * 9.525 * 2 * [Delta] T-1.19 * 10 < ^-3 > * [Delta] T =
−0.733 × 10 ⁻³ × ΔT, and when the inner ring is alumina, δ1 = −0.653 × 10 ⁻³ × ΔT.

Similarly, when the outer race is made of alumina, the variation δ2 in the clearance due to the temperature rise is δ2 = −0.293 × 10 ⁻³ × ΔT when the inner race is made of zirconia, and the inner race is made of alumina. Where δ2 = −0.213 × 10
⁻³ × ΔT.

Therefore, when ΔT = 50 ° C. and the inner ring is made of zirconia, the amount of change in the clearance δ1, δ2 due to the temperature rise
Is as follows. In addition, () shows the case where the inner ring is made of alumina.

Δ1 = −36.7 μmφ (−32.7 μmφ) δ2 = −14.7 μmφ (−10.7 μmφ) Here, the geometrical clearance of the bearing δo = 23 to 33 μmφ,
The contact angle α = 15 °. Even if the material of the rolling element is silicon nitride, the change amounts δ1 and δ2 of the clearance can be regarded as the same value.

From the above calculation results, the most suitable combination of ceramic materials is as follows.

Roller: Sialon or silicon nitride Inner ring: Alumina Outer ring: Alumina At this time, as described above, the change in the clearance due to the centrifugal force is 14.9 μmφ (N = 25000 rpm), and the change in the clearance due to the temperature rise of the bearing is as follows. , As described above, -1
0.7 μmφ (ΔT = 50 ° C.). When the inner and outer rings are made of alumina, the coefficient of thermal expansion, Young's modulus, density,
The intensities are each a balanced intermediate value.

However, the change in the clearance is 14.9 + 1.
Since 0.7 = 25.6 μmφ, which is almost equivalent to the geometric clearance, it is necessary to determine the initial clearance in consideration of temperature rise and centrifugal force.

[Study 5 (Design of Appropriate Clearance)] FIG. 7 is a diagram showing a design of an appropriate clearance. The appropriate clearance δo is represented by Expression (7).

[0070]

Δo = 2 × (Δo + Δi) = 2 × (ro + ri−d) (1-cosα) Here, using ro = ri = 4.97 and d = 9.525, the equation (7) becomes: δo = 0.830 × (1-cos
α). The appropriate clearance δo for the contact angle α is as follows. That is, when the contact angle α is 10 °, 15 °,
Clearance δ at 20 ゜, 25 ゜, 30 ゜, 35δ
o are 12.6, 28.3, 50.1, and 77., respectively.
8, 111, and 150.

FIG. 8 is a graph showing the relationship between the clearance δo and the contact angle α. When the minimum clearance (maximum temperature rise, maximum rotation speed) is the design clearance 28.3 μm and the contact angle α = 15 °, the maximum clearance 53.9 (= 28.3 + 25.6) μ
When m (temperature rise, rotation speed 0), the contact angle α = 21 °. The contact angle α = 21 ° to 15 ° is a general value for an angular ball bearing, and there is no particular problem.

In the figure, temperature change ΔT = 50 °, N = 250
The change point A 'of the clearance from the contact angle α = 15 ° when receiving the centrifugal force of 00 rpm is as follows: inner ring: zirconia, outer ring:
Silicon carbide, rolling element: Sialon. The change point A is the case where the inner ring is alumina, the outer ring is alumina, and the rolling element is sialon. Contact angle α = 15 ° ~ 2
It changes within the range up to 9.4 °. However, such a change in the contact angle can be used by using a constant pressure preload method. Therefore, all combinations of materials shown in FIG. 4 can be used.

[Study 6 (Contact Stress between Outer Ring Groove and Rolling Element Due to Centrifugal Force)] The contact stress between the outer ring groove and the rolling element due to centrifugal force will be examined. FIG. 9 is a view showing the relationship between the contact stress between the outer ring groove and the rolling element due to centrifugal force. Assuming that the centrifugal force acting on the rolling elements is Fe, the centrifugal force Fe is expressed by the following equation (8).

[0074]

Fe = m · Dm / 2 · (2π · Nm) ² where m is the mass of the rolling element, Dm is the pitch circle diameter of the bearing,
ρ is the density of the rolling elements. If the material of the rolling elements is sialon (silicon nitride), the density ρ is 3.2 (3.3). Also, from the bearing dimensions, the bearing pitch diameter Dm = 1.
06.357φ, the diameter d of the rolling element d = 9.525, the contact angle α = 15 ° to 20 °, and the rotation speed N = 25000 rpm. Therefore, the centrifugal force Fe = 11.2 kg.

Also, the load component Q at the bearing contact point due to centrifugal force
Is: Q = Fe / cos α = 11.9 kg (α = 20 °)
It is. The maximum stress σmax of the contact ellipse at the contact point due to the load component Q is obtained by Expression (9). Where 2
a is the major axis of the contact ellipse, and 2b is the minor axis of the contact ellipse.

[0076]

Σmax = 3Q / 2πab Further, from the outer ring groove diameter 115.896 μmφ, the outer ring groove radius 4.97R, and the rolling element diameter d = 9.525, Expression (9) becomes Expression (10).

[0077]

(Equation 10)

When the material of the rolling element is silicon nitride and the material of the outer ring is alumina, the longitudinal elastic modulus E _I and Poisson's ratio 1 / m _I of silicon nitride are 3.00 × 10 ⁴ (kg / m 2).
m ² ) and 03, and the longitudinal elastic modulus E _II and the Poisson's ratio 1 / m _II of alumina are 3.90 × 10 ⁴ (kg / m ² ), respectively.
m ² ) and 0.3, the maximum stress σmax is 137
kg / mm ² . The material of the rolling element is silicon nitride,
When the material of the outer ring is silicon carbide, similarly, the longitudinal elastic modulus E _II and the Poisson's ratio 1 / m _II of the hydrocarbon are 3.00 × 10 ⁴ (kg / mm ² ) and 0.3, respectively. ,
The maximum stress σmax is 107 kg / mm ² .

As described above, the maximum stress σmax is 107〜
It is in the range of 137 kg / mm ² , and the value in this range is a proven value for bearing steel, and no particular problem occurs.

[Summary] As described above, in this embodiment,
For the rolling bearing composed of a combination of ceramic materials shown in FIG. 4, technical calculations were conducted assuming an angular ball bearing with a shaft diameter of 85φ and a DmN value of 2.6 million. It can be used with a DmN value of 3,000,000 or more.

Therefore, the ceramic combination rolling bearing using the ceramic material shown in FIG. 4 can satisfy the characteristics required as a bearing for a high-speed spindle.

[0082]

According to the present invention, since the rolling element, the inner ring and the outer ring are formed using the above-described ceramic material, the rolling materials are different from each other, so that the rolling friction is small and the temperature rise itself can be suppressed. Furthermore, even when used for a high-speed spindle, the inner ring does not reach breaking strength due to the difference between the thermal expansion of the shaft and the inner ring and the expansion due to centrifugal force.

Further, since the material is made in consideration of the heat flow with respect to heat generation, the temperature rise itself can be suppressed. For example, when silicon carbide is used for the material of the outer ring,
Thermal conductivity is 1.6 to 4 times that of bearing steel, and 6 times that of bearing steel when aluminum nitride is used. Therefore, it is possible to sufficiently rotate at high speed even with oil mist and oil air.

Further, since the flow of heat from the inner ring to the rotating shaft is reduced, the elongation of the shaft can be reduced.
High-precision processing can be realized. In particular, when applied to a spindle of a built-in motor drive system, an inlet through-hole and an outlet through-hole of a different system from that for lubrication of the bearing are provided in a housing near both ends of the rotor, and a room temperature or lower (for example, , Minus 30 ° C.), it is possible to suppress heat generation by flowing air cooled to −30 ° C.). As described above, the heat generated by the rotor of the built-in motor, which is another heat source other than the bearing, is cooled by the air at room temperature or lower, whereby the temperature increase of the rotating shaft can be suppressed. In this case, the cooling air is preferably introduced from the tool mounting side,
Due to the elongation of the rotary shaft on the tool side due to the temperature rise, the change in the tool position is suppressed.

As described above, the ceramic combination rolling bearing of the present invention can satisfy the characteristics required for a bearing for a high-speed spindle.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the structure of a spindle of a built-in motor drive system using an angular ball bearing.

FIG. 2 is a sectional view showing a lubrication and cooling structure in a spindle of a built-in motor drive system.

FIG. 3 is a table showing characteristics of various ceramic materials.

FIG. 4 is a table showing required characteristics of rolling elements, inner races and outer races, and corresponding ceramic materials.

FIG. 5 shows the thermal expansion amount δt2 of the outer diameter of the inner ring in zirconia and alumina, and the maximum value σtθma of the radial stress of the inner ring.
5 is a graph showing a change in x with respect to a temperature rise ΔT.

FIG. 6 is a graph showing the change over time in the bearing temperature and interference when the operation is repeated between the maximum rotation speed and the rotation stop.

FIG. 7 is a diagram showing a design of an appropriate clearance.

FIG. 8 is a graph showing a relationship between a clearance δo and a contact angle α.

FIG. 9 is a diagram showing the relationship between the contact stress between the outer ring groove and the rolling element due to centrifugal force.

[Explanation of symbols]

 11 Housing 13, 15 Angular contact ball bearing 13a, 15a Outer ring 13b, 15b Inner ring 13c, 13c Rolling element 18 Rotating shaft 24 Rotor 26 Stator 51, 52, 61, 65 Inlet through hole 54, 55, 62, 66 Outlet through hole

Claims (1)

[Claims] 1. A ceramic combination rolling bearing using a ceramic material for a rolling element, an inner ring and an outer ring, wherein the rolling element has a relatively low density, a high fracture toughness, and a high thermal shock resistance. A ceramic material made of zirconia or alumina having a thermal expansion coefficient relatively close to that of steel, a small thermal conductivity, and a high fracture strength and a large Young's modulus for the inner ring; A ceramic combination rolling bearing using a ceramic material made of silicon carbide, aluminum nitride, or alumina having relatively high thermal conductivity and low thermal expansion coefficient.