JP2002364648A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a less expensive rolling bearing than the one that uses ceramics for a rolling element, which also has higher seizure resistance by attaining lower heat generation than the one that uses a bearing steel for the rolling element. SOLUTION: In the rolling bearing used for a main spindle for a machine tool, at least either of an inner ring 21 or an outer ring 22 contains 0.2-1.2 wt.% C, 0.7-1.5 wt.% Si, 0.5-1.5 wt.% Mo, 0.5-2.0 wt.% Cr, residual Fe, and steel including inevitable impurity element, and hardening and tempering professes are performed after a carbonitriding process. Thus, the rolling bearing 20 is provided with 0.8-1.3% in surface carbon concentration and 0.2-0.8% in surface nitrogen concentration, having a nitriding layer 23a with hardness of Hv 1200-1500 on a surface of the rolling element 23 using martensitic stainless steel as a base material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing used for a main shaft of a machine tool.

[0002]

2. Description of the Related Art In recent years, various types of machine tools have been used mainly for machining centers to improve machining efficiency and productivity.
Higher speeds such as rotation of the spindle and feed of peripheral devices have been progressing, but users' demands for higher speeds of machine tools have been increasing. It is also required that the spindle of the machine tool be rotated at a higher speed, and the Dmn value of the grease lubricated bearing (Dm; pitch circle diameter of the rolling element (mm),
(n; number of rotations (min ^-1 )) exceeding 1 × 10 ⁶ has recently become rare.

[0003] In order to cope with this, the problems of rolling bearings include, for example, measures against a decrease in service life, measures against an increase in heat generation, measures against seizure under high-speed rotation, measures against an increase in vibration, and measures against an increase in noise. There are many countermeasures. In order to overcome these problems, at present, the mainstream rolling bearings for high-speed spindles employ ceramic rolling elements.

[0004]

A rolling bearing in which the rolling element is made of ceramics is superior to a rolling bearing in which the rolling element is made of bearing steel in seizure resistance and high heat generation under high-speed rotation. However, there is a problem that the cost increases. The present invention has been made in view of the above circumstances, and has as its object to improve the seizure performance at a lower heat generation than rolling bearings in which the rolling elements are made of ceramics and are less expensive than rolling bearings made of ceramics. It is to provide a rolling bearing.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a rolling bearing used for a main shaft of a machine tool, wherein an outer ring and an inner ring each having a bearing steel as a base material and a rolling bearing having a martensitic stainless steel as a base material. A nitrided layer on at least the surface of the rolling element of the moving body, wherein the nitrided layer has an Hv of 1200 to 15
This is achieved by a rolling bearing characterized by having a hardness of 00. Another object of the present invention is to provide a rolling bearing used for a main shaft of a machine tool, wherein at least one of an inner ring and an outer ring has a C content of 0.2 to 1.2% and a Si content of 0.2 to 1.2%.
From 0.7 to 1.5%, Mo from 0.5 to 1.5%, Cr from 0.5 to 2.0%, balance containing Fe and unavoidable impurity elements, and carbonitriding After the quenching and tempering treatment, the surface carbon concentration is reduced to 0.8
And the surface nitrogen concentration is 0.2 to 0.8%.
%, The surface of a rolling element made of martensitic stainless steel as a base material has a nitrided layer, and the nitrided layer is Hv120.
This is achieved by a rolling bearing characterized by having a hardness of from 0 to 1500.

A description will be given taking a ball bearing as an example. Generally, it is clear that the smaller the product of the contact pressure at the contact surfaces of the inner and outer rings with the balls and the sliding speed between the inner and outer rings and the balls, the smaller the amount of heat generated by the rotation of the ball bearings. Has become. In particular, bearings for machine tool spindles are generally used at a high speed under a small amount of lubrication, and oil film rupture easily occurs due to a rise in temperature. Furthermore, at the time of high-speed rotation, a large load is applied between the ball and the outer ring due to the centrifugal force acting on the ball, so that the ball and the outer ring are close to pure rolling contact, and the slip between the ball and the inner ring is increased. The amount of heat generated by the bearing under high-speed rotation largely depends on the friction generated by the slip between the ball and the inner ring. This heat generation may lead to wear of the raceways of the inner ring and the outer ring and seizure of the bearing. The longitudinal modulus of the nitrided layer of the present invention is 2
Since the longitudinal elastic modulus of high carbon chromium bearing steel (SUJ2) is about 208 GPa at about 40 GPa, the contact ellipse of the contact portion between the ball and the groove can be reduced, and the slip can be suppressed. As a result, low heat generation can be achieved,
Seizure and the like can be significantly prevented. In the present invention,
Since the contact state between the inner ring and the outer ring and the rolling element is a state of contact between different materials, seizure resistance is improved as compared with the case where a rolling element made of bearing steel is used. In addition, the base material SU
Martensitic stainless steel such as S440C is manufactured by SU
Since the linear expansion coefficient is smaller than that of bearing steel such as J2, even if the bearing temperature rises and a temperature difference occurs between the inner and outer rings, the preload changes in this rolling bearing compared to a bearing with rolling elements made of bearing steel. And the contact surface pressure between the ball and the groove can be reduced. As bearing steel, high carbon chromium bearing steel (S
In addition to UJ2), carburized steel, heat-resistant steel, stainless steel, alloy tool steel, chromium steel, chromium molybdenum steel, and the like can be used.

As a method for forming a nitrided layer (a layer having an appropriate nitrogen concentration and hardness), a liquid nitriding treatment using a salt bath or the like, a gas nitriding treatment, or an ion nitriding treatment can be mentioned. Of these, the ion nitriding treatment requires a relatively high treatment temperature, and it is difficult to obtain a sufficient hardness of the base even in consideration of the heat resistance of the base material, or it is possible to form a uniform nitrided layer on the surface of the spherical body. Since it may be difficult, it is preferable to use salt bath nitriding or gas nitriding. The preferred nitriding temperature is 480 ° C. or lower. In addition, in particular, when the processing temperature is high, a fragile compound layer (pseudo-ceramic layer composed of a single phase of ξ phase or ε phase) of about several microns may be formed on the outermost surface of the nitride layer. Therefore, the nitriding temperature is more preferably set to 460 ° C. or lower. Further, when the processing temperature is lower, the nitrided layer becomes denser and a rough porous layer is not generated.

In the present invention, the nitride layer has a ξ phase (Fe
₂ N), epsilon phase _{(Fe 2 ~ 3 N),} γ ' phase (Fe ₄ N), Cr
One or more nitrides of N, Cr ₂ N, etc. are precipitated in a large amount on the martensite ground, and have extremely high hardness and high toughness. Can be extremely suppressed. Further, since the base metal is made of martensitic stainless steel and bearing steel, it is possible to secure a sufficient hardness of the underlayer of about HRC 57 to 59 even under the above nitriding conditions. In addition, it is preferable that the rolling element on which the nitrided layer is formed is subsequently subjected to finish processing to reduce the surface roughness to 0.1 μmRa or less. This can reduce the aggressiveness of the surface of the mating member with which the nitride layer surface contacts.

Hereinafter, a method for manufacturing a rolling element will be described. First, using a wire drawn cold, a raw ball is manufactured by cold working or cutting by a header and flashing, etc., and the raw ball is quenched, tempered, and cured by using sub-zero processing in some cases. Let it. Thereafter, grinding is performed to a target size, that is, a size obtained by adding a set allowance to the finished product size, and a semi-processed ball is manufactured. The set allowance specifically means a necessary allowance for performing finishing processing to a target accuracy, but also includes an amount of expansion and contraction due to nitriding.

In the case where the elementary sphere before the nitriding treatment is in a quenched state, the sphericity or the difference between the diameters is very large, and generally a setting allowance of about several tens μm to 100 μm is required. is there. Therefore, if a nitrided layer is formed on the as-quenched elementary sphere, the nitrided layer will be unevenly etched, and the thickness of the nitrided layer will be uneven, and at the same time, the balance of the internal stress caused by the nitriding treatment will be lost. In some cases, it may take a long time for finishing to obtain the required accuracy, or the target accuracy may not be achieved. Further, the durability of the rolling elements may be affected in some cases. These problems are particularly remarkable in the as-quenched elemental ball, but if the accuracy of the semi-finished ball is insufficient, the required accuracy cannot be satisfied or the finishing process requires a long time. In some cases, the sphericity of the semi-processed sphere needs to be 3.0 μm or less, preferably 1.0 μm or less.

If the semi-finished ball is not tempered after quenching, the internal residual stress accumulated during quenching may adversely affect the quality of the finished product. It is good to perform tempering. Furthermore, in order to prevent the occurrence of surface scratches during handling or from the aspect of strength, heat treatment is performed on the production of semi-finished balls, followed by mechanical hardening such as barrel or ball peening.
The hardness may be further increased.

As a result, the obtained rolling element has a very hard and excellent toughness nitrided layer of Hv1200 to 1500 formed on the surface thereof to have a very uniform thickness, and further supports the nitrided layer. Since the base is configured to satisfy HRC 57 or more, durability can be maintained even when the bearing operates under a high load condition.

If the thickness of the nitrided layer is too large, not only does the processing cost increase, but also the expansion and contraction and deformation after nitriding become remarkable, the finishing cost increases, and the physical properties of the steel also increase. 3% or more and 2% Da (2% of diameter)
The following is preferred. Further, for the above-mentioned reason, the uniformity of the thickness of the nitride layer is preferably within 5 μm.

Although the rolling elements of ball bearings have been described above, the same applies to the rolling elements (rollers, etc.) of other rolling bearings.

In Japanese Patent Application Laid-Open No. 2000-45049, by using a carbonitrided steel material having the following composition as a bearing material, plastic deformation is suppressed by the action of strengthening the matrix strength at high temperatures, and seizure resistance is reduced. A bearing with significantly improved is presented. That is, at least one of the inner ring and the outer ring contains 0.2 to 1.2% by weight of C, 0.7 to 1.5% by weight of Si, and 0.5 to 1.5% by weight of Mo as alloy components.
It is formed of a steel material containing 1.5% by weight and Cr at a ratio of 0.5 to 2.0% by weight, and is subjected to carbonitriding and then quenching and tempering to reduce the surface carbon concentration to 0%. .8
And the surface nitrogen concentration is 0.2 to
0.8% by weight. Local adhesion is suppressed by the action of strengthening the matrix strength at high temperature,
Low friction characteristics under high surface pressure. By using a martensitic stainless steel as a base material and having a nitrided layer on the surface and using the nitrided layer in combination with a rolling element having a hardness of Hv1200 to 1500, an outer ring and an inner ring having a bearing steel as a base material can be formed. It generates even less heat and has improved wear resistance and seizure resistance as compared to those having the same.

The effective range of each component element will be described. (1) Si: 0.7 to 1.5% by weight Si is an element having an effect on temper softening resistance, and is a retained austenite which is effective in improving high-temperature strength and preventing indentation origin type peeling in a high-temperature environment. Has the effect of delaying the decomposition of If the Si content is less than 0.7% by weight, the high-temperature strength becomes insufficient and the indentation origin type peeling occurs. Therefore, the lower limit is set to 0.7% by weight. On the other hand, if the Si content exceeds 1.5% by weight, the mechanical strength is reduced and carburization is impaired. Therefore, the upper limit was set to 1.5% by weight.

(2) Mo: 0.5 to 1.5% by weight Mo is an element having an effect on temper softening resistance like Si, and has an effect of improving high-temperature strength. Also, Mo
Acts as a carbide-forming element to form subtle carbides on the carbonitrided surface. If the Mo content is less than 0.5% by weight, the high-temperature strength becomes insufficient and the amount of carbide deposited on the surface becomes insufficient. Therefore, the lower limit is set to 0.5% by weight. On the other hand, if the Mo content exceeds 1.5% by weight, giant carbides are formed at the stage of the raw material, causing the carbides to fall off and reducing the rolling fatigue life of the bearing. And

(3) Cr: 0.5 to 2.0 wt% Cr is an additive element having the same effect as Mo.
If the Cr content is less than 0.5% by weight, the high-temperature strength becomes insufficient and the amount of carbide deposited on the surface becomes insufficient. Therefore, the lower limit was set to 0.5% by weight. On the other hand, C
If the r content exceeds 2.0% by weight, a giant carbide is formed at the stage of the raw material, causing the carbides to fall off and shortening the rolling fatigue life of the bearing.
And

(4) C: 0.2 to 1.2% by weight As described above, if the amount of retained austenite is too large, the retained austenite is decomposed and the shape changes over time, and the dimensional stability of the bearing is impaired. It is. On the other hand, the presence of retained austenite on the raceway surface is effective in preventing indentation origin type peeling. Therefore, it is preferable to control the amount of retained austenite occupying in the entire bearing after the presence of retained austenite on the surface, and it is necessary to suppress the amount of retained austenite in the bearing core. From such a viewpoint, it is preferable that the amount of the average retained austenite in the steel including the surface and the core is 5% by volume or less, and for that purpose, the carbon concentration on which the retained austenite depends is reduced to 1.2% by weight or less. Therefore, the upper limit was set to 1.2% by weight. On the other hand, if the carbon concentration is less than 0.2% by weight, it takes a long time to obtain a desired carburizing depth by the carbonitriding treatment, which leads to an increase in overall cost. 0.2% by weight.

(5) Surface C concentration: 0.8 to 1.3% by weight When carbon is added to the surface by carbonitriding, the martensite structure serving as a matrix can be solid-solution strengthened, and indentations are formed in the very surface layer. It is possible to form a large amount of retained austenite which is effective for prevention of the starting type exfoliation. If the surface carbon concentration is less than 0.8% by weight, the surface hardness becomes insufficient and rolling fatigue life and wear resistance are reduced. Therefore, the lower limit is set to 0.8% by weight. On the other hand, when the surface carbon concentration exceeds 1.3% by weight, giant carbides are precipitated during carbonitriding treatment, and the rolling fatigue life is reduced.
The upper limit was set to 1.3% by weight.

(6) Surface N concentration: 0.2 to 0.8% by weight When nitrogen is added to the surface by carbonitriding, the tempering resistance is improved, the high-temperature strength is increased, and the wear resistance is improved. A large amount of retained austenite that is effective in preventing indentation origin type peeling can be present in the surface layer. If the surface nitrogen concentration is less than 0.2% by weight, the high temperature strength is reduced and the wear resistance is reduced.
And On the other hand, if the surface nitrogen concentration exceeds 0.8% by weight, it becomes difficult to finish the grinding during the production of the bearing, and the productivity of the bearing decreases due to difficult grinding.
8% by weight.

(7) Other component elements It is preferable to add a small amount of Ti as another component element. When Ti is added, fine titanium carbide (Ti
This is because C) and carbonitride (Ti (C + N)) precipitate and disperse in the matrix and improve wear resistance and seizure resistance. In this case, the Ti content is desirably 0.1 to 0.3% by weight. Ti content is 0.1% by weight
If it is less than 0.1%, the effect of carbide precipitation cannot be obtained, so the lower limit is set to 0.1% by weight. On the other hand, if the Ti content exceeds 0.3% by weight, a large precipitate is likely to be formed, which may become a defect and the rolling fatigue life may be reduced, so the upper limit is set to 0.3% by weight. %.
Incidentally, when the size of the titanium precipitate (TiC, Ti (C + N)) is 0.1 μm or less, it contributes to improvement of wear resistance and seizure resistance.

It is desirable that unavoidable impurity elements such as S, P, H, and O are not contained as much as possible. In particular, when the content of oxygen (O) exceeds 12 ppm, oxide-based inclusions are easily formed, which may cause defects and reduce the rolling fatigue life. Therefore, the oxygen content should be less than 12 ppm. desirable.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. An angular contact ball bearing 10 according to a first embodiment of the present invention shown in FIG. 1 includes an inner ring 11, an outer ring 12, and an inner and outer ring 1.
A plurality of balls 13 arranged between the first and second balls 12 and a holder 14 for holding the balls 13 at equal intervals in the circumferential direction are provided. This embodiment is a rolling bearing for supporting a main shaft of a machine tool. In the present embodiment, the inner and outer rings 11, 12 are formed of high carbon chromium bearing steel. The ball 13 is made of martensitic stainless steel as a base material, and has a nitrided layer 13a having a hardness of Hv1200 to 1500 on its surface.

An angular ball bearing 20 according to a second embodiment of the present invention shown in FIG. 2 includes an inner ring 21, an outer ring 22, and inner and outer rings 21 and 21.
A plurality of balls 23 arranged between the two and a holder 24 for holding the balls 23 at equal intervals in the circumferential direction are provided. In this embodiment,
This is a rolling bearing for supporting the main shaft of a machine tool. In the present embodiment, the inner and outer rings 21 and 22 have a weight ratio C of 0.2 to 0.2.
1.2%, Si 0.7-1.5%, Mo 0.5-
It is made of steel containing 1.5%, 0.5 to 2.0% of Cr, balance Fe and unavoidable impurity elements, and is subjected to carbonitriding and then quenching and tempering to reduce the surface carbon concentration to 0%. 0.8 to 1.3%, and the surface nitrogen concentration is 0.2 to 0.8%. The ball 23 is formed of martensitic stainless steel, and a nitride layer 23 a of Hv1200 to 1500 is formed on the surface of the ball 23.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following four types of angular contact ball bearings (Examples 1 and 2 and Comparative Examples 1 and 2) in the form shown in FIG.
2) was prepared. (Example 1) Inner and outer rings are made of high carbon chromium bearing steel (SUJ2)
The rolling element (ball) is made of martensitic stainless steel (SUS440C) and has a nitrided layer on the surface. (Embodiment 2) For the inner and outer rings, C is added to the material as an alloying element.
It contains 94% by weight, 1.02% by weight of Si, 0.88% by weight of Mo, and 1.52% by weight of Cr, and is further carbonitrided to give a surface carbon concentration of 1.15% by weight. Rolling element (ball) with nitrogen concentration of 0.23% by weight
Is a martensitic stainless steel (SUS440C) with a nitrided layer on the surface. (Comparative Example 1) The inner and outer races and rolling elements (balls) are made of high carbon chromium bearing steel (SUJ2). (Comparative Example 2) Inner and outer rings are high carbon chromium bearing steel (SUJ2)
The rolling elements (balls) are made of silicon nitride ceramics.

The angular ball bearings of Examples 1 and 2 and Comparative Examples 1 and 2 all have an inner diameter of 65 mm, an outer diameter of 100 mm,
Grease lubrication (Isoflex NBU15) with width 18mm, ball diameter 7.144mm, number of balls 28, contact angle 18 °
It is. Each of Examples 1 and 2 and Comparative Examples 1 and 2 was assembled into a vertical testing machine 50 as shown in FIG. 3 in a two-row back-to-back combination (DB combination fixed position preload), and the axial spring constant at the time of assembly was 100 N / μm. And incorporated. Example 1
2 or angular contact ball bearings 52 of Comparative Examples 1-2
Supports the rotating shaft 51 of the vertical testing machine 50. An inner ring spacer 53 and an outer ring spacer 54 are arranged between the pair of angular ball bearings 52. The rotating shaft 51 was rotated by belt driving, and the outer ring temperature rise and the high speed limit were observed for each of the examples and comparative examples.

FIG. 4 shows the test results. In FIG. 4, the horizontal axis represents the rotation speed (min ^-1 ), and the vertical axis represents the outer ring temperature rise (° C.). In the case of Comparative Example 1, the seizure occurred at 16000 min ⁻¹ and the raceway surface of the inner ring was worn.
Endured up to 00 min ^-1 . In Example 1, the temperature rise at each rotation speed was also lower than Comparative Example 1. Comparative Example 2 is currently most frequently used for high-speed spindles, and has the best results in both seizure resistance and low heat generation. on the other hand,
In Example 2, a high-speed limit comparable to that of Comparative Example 2 could be obtained, and the temperature rise was not different from Comparative Example 2 up to 19000 min ⁻¹ .

The present invention is not limited to the above-described embodiments and examples, but can be appropriately modified and improved. For example, the present invention can be applied to a double row rolling bearing.

[0030]

As described above, according to the present invention,
A low cost, low temperature rise at high speed rotation, excellent seizure resistance and excellent wear resistance can be provided, and a highly reliable machine tool spindle can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention.

FIG. 2 is a sectional view showing a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a state in which an example of the present invention and a comparative example are incorporated in a vertical testing machine.

FIG. 4 is a graph illustrating test results of the examples of the present invention and comparative examples.

[Explanation of symbols]

 10, 20 Angular contact ball bearing (rolling bearing) 11, 21 Inner ring 12, 22 Outer ring 13, 23 Ball (rolling element) 14, 24 Cage 13a, 23a Nitride layer

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 3J101 AA01 AA32 AA42 AA54 AA62 BA10 BA70 DA02 DA03 EA03 EA06 FA33 FA44 GA31

Claims (2)

[Claims] In a rolling bearing used for a main shaft of a machine tool, at least a surface of a rolling element among an outer ring and an inner ring formed of bearing steel as a base material and a rolling element formed of martensitic stainless steel as a base material is formed. And the nitrided layer is H
A rolling bearing having a hardness of v1200 to 1500. 2. In a rolling bearing used for a main shaft of a machine tool, at least one of an inner ring and an outer ring has a weight ratio of C of 0.2 to 1.2%, Si of 0.7 to 1.5%, and Mo.
0.5-1.5%, Cr 0.5-2.0%, balance F
e and a quenching and tempering treatment after a carbonitriding treatment to make the surface carbon concentration 0.8 to 1.3% and the surface nitrogen concentration to 0.1%. A rolling bearing characterized by having a nitrided layer on the surface of a rolling element made of martensitic stainless steel as a base material, the nitrided layer having a hardness of Hv 1200 to 1500.