JP2006329265A - Rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing hard to heat and superior in impression resistance even when used with high speed rotation. <P>SOLUTION: An angular ball bearing comprises an inner ring 1 having a raceway surface 1a on the outer peripheral face, an outer ring 2 having a raceway surface 2a on the inner peripheral face, and a plurality of rolling elements 3 rollingly arranged between both raceway surfaces 1a, 2a. It is used for rotatably supporting the spindle of a machine tool. The inner ring 1 and the outer ring 2 are formed of bearing steel and their raceway surfaces 1a, 2a each have a hardened layer formed with a surface hardness of Hv900-Hv1500 by surface hardening treatment. The rolling elements are formed of ceramic. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a rolling bearing for a machine tool spindle.

The important performances required for rolling bearings for machine tool spindles include low heat build-up and pressure scar resistance. For example, Patent Document 1 discloses a technique for improving the pressure scar resistance of rolling bearings for machine tool spindles. There is a thing of description.
In a machine tool, in order to suppress thermal displacement, a decrease in service life in the case of grease lubrication, and a decrease in allowable rotational speed, a rolling bearing having low heat generation, that is, a rolling bearing with a small temperature rise during rotation is desired. Therefore, rolling elements made of ceramic (for example, silicon nitride) are often used.
JP 2005-76679 A Japanese Utility Model Publication No. 5-86026 Japanese Patent No. 3241491 Japanese Patent Laid-Open No. 2001-20037

However, due to the recent increase in the speed of machine tools, low exothermic properties have become insufficient only by making the rolling elements ceramic. In addition, since ceramic has a high Young's modulus and a high contact pressure at the rolling contact portion, it also has a problem that pressure scar resistance is insufficient. Therefore, there are many cases in which the rolling bearing for the machine tool spindle is damaged when the spindle of the machine tool collides with a workpiece or the like due to an NC program error or the like.
Accordingly, an object of the present invention is to solve the above-described problems of the prior art, and to provide a rolling bearing that hardly generates heat even when used at a high speed rotation and has an excellent pressure resistance.

  In order to solve the above problems, the present invention has the following configuration. That is, the rolling bearing of claim 1 according to the present invention includes an inner ring, an outer ring, and a plurality of rolling elements that are arranged to freely roll between the inner ring and the outer ring, and rotates the spindle of the machine tool. In the freely supported rolling bearing, at least one of the inner ring and the outer ring includes a hardened layer formed by a surface hardening process, the surface hardness is set to Hv900 or more and Hv1500 or less, and the rolling element is made of ceramic. It is characterized by being.

Heat generation of a rolling bearing is considered to be caused by elastic hysteresis loss of the material, rolling friction due to an oil film, friction due to sliding, resistance of a cage, and the like. Since the rolling bearing for the machine tool spindle rotates at a high speed such that the dmN value exceeds 1 million, a ceramic ball bearing is adopted and is lubricated by a small amount of lubricating oil or grease such as oil-air lubrication. Under such conditions, the rolling friction moment and rolling contact slip (spin sliding, gyro sliding, etc.) due to the oil film can be kept very small, and as a result, the proportion of heat generation caused by elastic hysteresis loss Becomes larger. The rolling friction moment Mr due to elastic hysteresis is generally expressed by the following equation.
Mr = 3/8 · α · b · Q
Here, α is an elastic hysteresis constant, b is a contact ellipse radius (radius in the rolling direction), and Q is a rolling element load.

The present inventors pay attention to the fact that hard steel having a surface hardness of Hv900 or higher has a small elastic hysteresis constant. If the rolling bearing has such an inner ring and outer ring made of such steel, it is used at high speed rotation. It was also found that heat generation is less likely to occur and is suitable for a machine tool spindle.
On the other hand, since the Vickers hardness and yield stress of steel are in a substantially proportional relationship, the pressure scar resistance can be improved by increasing the surface hardness. Since the ceramic rolling element has a high Young's modulus, compared with a rolling bearing using a steel rolling element, the surface pressure increases even with the same load, but by making the surface hardness Hv900 or more, Better indentation resistance can be ensured than when steel rolling elements are used. In addition, it is practically difficult to make the surface hardness of steel exceed Hv1500.

  The rolling bearing of the present invention hardly generates heat even when used at high speed rotation, and has excellent pressure resistance.

Embodiments of a rolling bearing according to the present invention will be described in detail with reference to the drawings.
FIG. 1 is a partial longitudinal sectional view showing a structure of an angular ball bearing which is an embodiment of a rolling bearing according to the present invention. The angular ball bearing of FIG. 1 includes a plurality of inner ring 1 having a raceway surface 1a on an outer peripheral surface, an outer ring 2 having a raceway surface 2a on an inner peripheral surface, and a plurality of rolls disposed between both raceway surfaces 1a and 2a. Rolling elements (balls) 3.

The inner ring 1 and the outer ring 2 are made of bearing steel, and at least the raceway surfaces 1a and 2a are formed with a hardened layer (not shown) by surface hardening treatment, and the surface hardness is set to Hv900 or more and Hv1500 or less. . Moreover, the rolling element 3 is comprised with the ceramic.
As bearing steel used for the inner ring 1 and the outer ring 2, for example, carbon is 0.2 mass% or more and 2.3 mass% or less, chromium is 2 mass% or more and 7 mass% or less, silicon is 1 mass% or less, 1% by mass or less of manganese, 10% by mass or less of molybdenum, 1.97% by mass to 22% by mass of tungsten, 0.5% by mass to 6% by mass of vanadium, 18% by mass or less of cobalt, and the balance Is iron and inevitable impurities, carbon is 0.2 mass% or more and 0.7 mass% or less, chromium is 0.5 mass% or more and 3.0 mass% or less, silicon is 0.4 mass% or more 2 0.0% by mass or less, molybdenum containing 0.5% by mass or more and 3.0% by mass or less, oxygen containing 10ppm or less, and the balance being iron and inevitable impurities.

  Examples of the surface hardening treatment include carbonitriding, nitriding, and shot peening. Specific examples of the nitriding treatment include salt bath nitriding treatment, gas nitriding treatment, ion nitriding treatment, and the like. Specific examples of the shot peening process include a WPC (Wonder Process Craft) process (a shot peening process provided by Fuji Seisakusho Co., Ltd. and Fuji Machine Sales Co., Ltd.).

The depth of the hardened layer formed by carbonitriding or nitriding is preferably 0.1 to 0.3 mm because it is preferably cured to the maximum shear stress depth of contact stress.
By forming a nitride layer having a high nitrogen concentration on the surface by carbonitriding or nitriding, the friction coefficient can be reduced. Further, the apparent elastic limit can be increased by applying compressive residual stress by shot peening. Carbonitriding, nitriding, and shot peening may be used in combination.

Such an angular ball bearing is suitable as a rolling bearing that rotatably supports the main shaft of the machine tool. A built-in motor drive type spindle, which is a machine tool incorporating the angular ball bearing of this embodiment, will be described below.
In the spindle of FIG. 2, a rotating shaft 18 is pivotally supported by a pair of angular ball bearings 13, 13, 15, 15 provided inside the both ends of the housing 11. Spacers 21 and 23 are provided at intermediate positions of the pair of angular ball bearings 13, 13, 15, and 15.

A rotor 24 is attached to the rotary shaft 18 located at the inner center of the housing, and a stator 26 facing the rotor 24 is provided inside the housing 11. Further, a cooling water groove 11 a is formed on the inner wall of the housing 11 facing the stator 26. Further, a balance ring 27 is provided on the rotary shaft 18 before and after the rotor 24.
Further, inner ring tightening nuts 31 and 33 are attached to respective portions of the rotating shaft 18 protruding from both ends of the housing 11.

  A preload sleeve 39 is inserted into the end of the housing 11 on the right side in FIG. 2 via a guide ball 38, and the preload sleeve 39 is urged to the right in the figure by a preload spring 42, and a detent 43 Is prevented from rotating by. The tip portion 39a of the preload sleeve 39 is bent so as to lock the outer ring 15a of the angular ball bearing 15, and a pair of angular ball bearings 15 and 15 sandwiched between the preload sleeve 39 and the outer ring tightening nut 36. The outer rings 15 a and 15 a are fixed to the preload sleeve 39 by an outer ring tightening nut 36.

Since the angular ball bearings 13 and 15 used for such a spindle rotate at a high speed, the inner ring and the outer ring are made of steel and the rolling elements are made of ceramic as described above.
When angular contact ball bearings are used for high-speed spindles, the increase in contact force between the rolling element and the outer ring due to centrifugal force becomes a problem. Therefore, the rolling element is made of silicon carbide, sialon, silicon nitride, Ceramics such as zirconia are preferred.

  In a machining center (M / C) equipped with a built-in motor drive type spindle, the rotation speed rises (that is, the time from the rotation speed reaching 0 to the highest point) and falls (that is, the rotation speed is the highest). It is important to shorten the time from the point to 0), and more and more shocking motion is required. Therefore, it is desired that the rolling element material has a high fracture toughness, but the fracture toughness of sialon and silicon nitride is relatively high. Furthermore, the material of the rolling element is desired to have a high thermal shock resistance, but the thermal shock resistance of sialon and silicon nitride is relatively high.

  Furthermore, since it is desired that the rolling bearing incorporated in the spindle has a small clearance change (change in the contact angle in the case of the angular ball bearing of this embodiment) with respect to the temperature rise, the thermal expansion coefficients of the inner ring and the outer ring are desired. The difference is preferably small. In order to release heat to the outside of the spindle housing, it is desirable that the outer ring has a high thermal conductivity, but silicon carbide has a high thermal conductivity (thermal conductivity).

〔Example〕
Hereinafter, the present invention will be described more specifically with reference to examples. A test bearing (referred to as Example 1) having the same configuration as the angular ball bearing of FIG. 1 (inner and outer rings are made of steel and the rolling elements are made of ceramic) was prepared. The angular ball bearing has an inner diameter of 70 mm, an outer diameter of 100 mm, a width of 20 mm, a ball PCD of 90 mm, a contact angle of 18 °, a ball diameter of 8.73 mm, and a number of balls of 25. Moreover, the hardened layer by the surface hardening process is formed in the track surface of the inner ring and the outer ring, and the surface hardness is set to Hv1000. Further, this angular ball bearing is lubricated with grease.

Such an angular ball bearing was loaded with an axial load of 1000 N and rotated at a rotational speed of 5000 to 20000 min ⁻¹ , and the dynamic torque at that time was measured. Further, a bearing of Comparative Example 1 having the same configuration as that of Example 1 except that the surface hardness of the hardened layer is Hv750 is prepared, and the dynamic torque is measured in the same manner as described above. Compared with. The results are shown in the graph of FIG. Since the numerical value obtained by multiplying the dynamic torque by the rotational speed indicates the heat generation amount, it can be determined that the heat generation amount is small when the dynamic torque is low.

From the graph of FIG. 3, it can be seen that the heat generation amount of the angular ball bearing of Example 1 is about 25% less than that of the angular ball bearing of Comparative Example 1. In particular, it can be seen that the effect of reducing heat generation is high in a high-speed region where the rotation speed exceeds 10,000 min ⁻¹ (dmN value is 900,000). If the calorific value is small, the deterioration of the grease is suppressed and the angular ball bearing has a long life.

  Next, the pressure resistance of the angular ball bearing was evaluated. The evaluated angular ball bearing has the same configuration as that of Example 1 except that the angular ball bearings of Example 1 and Comparative Example 1 described above and the surface hardness of the hardened layer formed on the raceway surface are set to Hv900. Are angular ball bearings (Example 2) and angular ball bearings (Comparative Example 2) having the same configuration as Comparative Example 1 except that the rolling elements are made of SUJ2.

Vickers hardness and yield stress of steel are approximately proportional. Generally, when the maximum surface pressure (kgf / mm ² ) of the rolling contact portion is ½ of Vickers hardness Hv, indentation harmful to machine tools occurs. To do. When the allowable load of the angular ball bearing was calculated based on this, the results shown in Table 1 were obtained. In addition, the Young's modulus of SUJ2 which comprises the steel and rolling element which comprise the ring of each angular ball bearing was 210 GPa, and the Young's modulus of the ceramic which comprises a rolling element was calculated to 320 GPa.

From the results shown in Table 1, when the surface hardness of the hardened layer is Hv900 or higher, even when a ceramic rolling element is used, the pressure resistance is higher than that of a conventional angular contact ball bearing using a SUJ2 rolling element. It turns out that it is excellent.
In the present embodiment, an angular ball bearing has been described as an example of a rolling bearing, but the present invention is also suitable for a cylindrical roller bearing capable of rotating at high speed. The present invention can also be applied to various types of various rolling bearings. For example, radial type rolling bearings such as deep groove ball bearings, self-aligning ball bearings, tapered roller bearings, needle roller bearings, and self-aligning roller bearings, and thrust type rolling bearings such as thrust ball bearings and thrust roller bearings. is there.

It is a fragmentary longitudinal cross-section which shows the structure of the angular ball bearing which is one Embodiment of the rolling bearing which concerns on this invention. It is sectional drawing which shows the structure of the spindle incorporating the angular ball bearing. It is a graph which shows the correlation with the rotational speed of an angular contact ball bearing, and dynamic torque.

Explanation of symbols

1 inner ring 1a raceway surface 2 outer ring 2a raceway surface 3 rolling element

Claims (1)

  A rolling bearing that includes an inner ring, an outer ring, and a plurality of rolling elements that are rotatably arranged between the inner ring and the outer ring, and that rotatably supports a main shaft of a machine tool. At least one is provided with the hardened layer formed by the surface hardening process, the surface hardness is Hv900 or more and Hv1500 or less, The said rolling element is comprised by the ceramic, The rolling bearing characterized by the above-mentioned.

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