Rolling device

Abstract

Provided is a rolling device in which heat generation during rolling is suppressed and peeling resistance is improved. A rolling bearing 1 is a rolling device including: an inner ring 2 having a raceway surface 2a in contact with a ball 4; and a grease composition 7 for lubricating a contact part between the ball 4 and the inner ring 2. The inner ring 2 has a nitrided layer formed on the raceway surface 2a. The average nitrogen concentration at the surface is 0.10 mass% or more. The hardness at the surface is 800 Hv to 1,000 Hv. The residual austenite amount at the surface is 20 vol.% or less. The grease composition 7 contains base oil and a thickener. The kinematic viscosity of the base oil at 40°C is 12 mm2 / s to 26 mm2 / s. The maximum value of the storage modulus of the grease composition 7 is within a range of 50,000 Pa or less under viscoelasticity measurement conditions in which the temperature of the grease composition 7 is 25°C, the amount of strain is 1×10-5 to 5×10-3, and the frequency is 1 Hz.

Description

Rolling mechanism

[0001] The present invention relates to a rolling device that contains a grease composition, etc.

[0002] For example, the spindle of a machine tool is preferably designed to rotate at high speed to increase machining efficiency, and various lubrication technologies are applied to its bearings. Suitable lubrication methods for high-speed spindles include, for example, air-oil lubrication and oil mist lubrication. However, these lubrication methods require ancillary equipment such as compressed air and oil supply devices, which contribute to increased initial and running costs of machine tools. In contrast, grease lubrication is a preferable lubrication method because it requires less maintenance.

[0003] Rolling bearings for machine tool spindles, which are sealed with grease, require high-speed durability. At high rotational speeds, the amount of heat generated by the flow of grease inside the bearing increases, which can cause peeling and other issues. In such cases, there are concerns about a decrease in machining accuracy and a shortened bearing lifespan.

[0004] In recent years, from the perspective of reducing environmental impact, the electrification of automobiles has been progressing, mainly with regard to electric vehicles (BEVs: Battery Electric Vehicles), plug-in hybrid electric vehicles (PHEVs: Plug-in Hybrid Electric Vehicles), and hybrid electric vehicles (HEVs: Hybrid Electric Vehicles). For example, electric axles (e-axles) that replace engines, electric brakes, electric VTC (Variable Valve Control), and electric compressors are being applied.

[0005] In electric vehicles like those described above, miniaturization and weight reduction of units, as well as higher rotational speeds and higher output of motors, are desired in order to extend the driving range with less power (to improve energy efficiency). Accordingly, it is desirable that rolling devices such as rolling bearings also meet these requirements. For example, in rolling bearings, the amount of heat generated increases as the motor rotates at higher speeds and becomes smaller. Also, in rolling bearings, it is highly likely that the viscosity of lubricating oil will decrease and the amount of grease will decrease in order to reduce torque. If this happens, the amount of heat generated in the rolling bearings will increase even further. Thus, in rolling bearings used in electric axles, electric brakes, electric VTCs, and electric compressors of electric vehicles, the lubrication conditions become more severe, and the concern about peeling increases.

[0006] In such rolling bearings, conventionally, for example, with regard to grease compositions, grease compositions have been proposed that prevent overheating and grease leakage in bearings by specifying the kinematic viscosity of the base oil at 40°C and the number of carbon atoms in the lithium soap thickener (see Patent Document 1). On the other hand, regarding the steel material of the bearing, for example, Patent Document 2 describes a rolling device comprising a first rolling component and a second rolling component that contacts the first rolling component, in order to improve peeling resistance, the hardness of the surface of the rolling component of the second rolling component is increased so that its arithmetic mean roughness is greater than that of the surface of the rolling component of the first rolling component.

[0007] Japanese Patent Publication No. 2006-199771, Japanese Patent Publication No. 6833330

[0008] However, there was room for further investigation in terms of approaches from both the grease composition and the steel material of the bearing.

[0009] This invention has been made in view of these circumstances, and aims to provide a rolling device that suppresses heat generation during rolling and has improved peeling resistance.

[0010] The rolling device of the present invention comprises a steel member having a surface that contacts a mating member, and a grease composition for lubricating the contact portion between the mating member and the steel member, wherein the steel member has a nitrogen-infiltrated layer formed on its surface, the average nitrogen concentration on the surface is 0.10% by mass or more, the hardness on the surface is 800 Hv or more and 1000 Hv or less, the amount of residual austenite on the surface is 20% by volume or less, and the grease composition comprises a base oil and a thickener, and the kinematic viscosity of the base oil at 40°C is 12 mm² ２ / s or more 26mm ２ The value is less than or equal to / s, and the temperature of the grease composition is 25°C, and the strain is 1 × 10⁻⁶. －５ ~5 x 10 －３ The grease composition is characterized in that the maximum storage modulus of the grease under viscoelastic measurement conditions at a frequency of 1 Hz is within the range of 50,000 Pa or less.

[0011] The above-mentioned steel member is characterized by containing 0.95% by mass or more and 1.10% by mass of carbon, less than 0.30% by mass of silicon, less than 0.50% by mass of manganese, less than 0.0080% by mass of sulfur, and 1.4% by mass or more and 1.6% by mass of chromium, with the remainder being iron and unavoidable impurities.

[0012] The above-mentioned nitrided layer is characterized in that, in a cross-sectional view perpendicular to the surface, the area ratio of undissolved carbides with a particle size of 5.0 μm or less is 17% or less. Furthermore, the average carbon concentration on the surface is characterized in that it is 0.60% by mass or more and 1.5% by mass or less.

[0013] In a cross-sectional view perpendicular to the surface of the above-mentioned nitriding layer, the area ratio of chromium nitrides with a particle size of 1.0 μm or less is 1.0% to 10%. Furthermore, in the nitriding layer, the area ratio of chromium nitrides with a particle size of 1.0 μm or less is 100 μm. ２ It is characterized by having five or more of them.

[0014] The thickener is a diurea compound, barium soap, or lithium complex soap, and is characterized in that the content of the thickener relative to the total amount of the base oil and the thickener is 10% to 30% by mass.

[0015] The above grease composition is further characterized by containing an antioxidant and a rust inhibitor.

[0016] The rolling device described above is a rolling bearing comprising an inner ring and an outer ring which are raceway members, a plurality of rolling elements interposed between the inner ring and the outer ring, and a cage which holds the rolling elements, characterized in that at least one of the raceway members and the rolling elements is made of steel, and the grease composition is sealed in the bearing space between the inner ring and the outer ring.

[0017] The rolling device of the present invention comprises a steel member having a surface that contacts a mating member, and a grease composition that lubricates these contact areas. By combining a steel member whose surface hardness and metallic structure are specified, with a grease composition whose base oil viscosity and storage modulus are specified, heat generation during rolling can be suppressed, and peeling resistance can be improved.

[0018] This is a cross-sectional view of a rolling bearing as an example of the rolling device of the present invention. This is an enlarged cross-sectional view of the inner ring surface. This is a diagram showing a test using a rheometer. This is a diagram showing a spindle using a rolling bearing as the rolling device of the present invention.

[0019] A rolling bearing, as an example of a rolling device of the present invention, will be described with reference to Figure 1. Figure 1 is an axial cross-sectional view of an angular contact ball bearing, which is an example of a rolling bearing. As shown in Figure 1, the angular contact ball bearing 1 has an inner ring 2 and an outer ring 3 which are raceway members, a plurality of balls 4 as rolling elements, and a cage 5. In the angular contact ball bearing 1, the bearing space in which the balls 4 are held by the cage 5 between the inner ring 2 and the outer ring 3 is sealed by a seal member 6 fixed to a locking groove provided on the inner circumferential surface of the outer ring 3. The seal member 6 is a contact seal in which the inner diameter side seal lip portion contacts the seal groove. At least a grease composition 7 is sealed around the balls 4. The line connecting the contact points of the balls 4 and the inner ring 2 and outer ring 3 has a contact angle α with respect to the radial direction, and can withstand radial loads and unidirectional axial loads. In the angular contact ball bearing 1 of Figure 1, the grease composition is sealed in the bearing space formed by the inner ring 2, the outer ring 3 and the balls 4.

[0020] Furthermore, a circumferential groove-shaped grease pocket is formed on the inner surface of the outer ring 3, providing an even greater protection against leakage of the grease composition.

[0021] The sealing member 6 may be made of metal or a rubber molded body alone, or it may be a composite of a rubber molded body and a metal plate, plastic plate, or ceramic plate. Due to its durability and ease of adhesion, a composite of a rubber molded body and a metal plate is preferred, as shown in Figure 1. Furthermore, as will be described later, it may be a non-contact seal.

[0022] (Regarding steel members) In the rolling mechanism of the present invention, the steel member having a surface that contacts the mating member is made of a predetermined steel member. In the angular contact ball bearing 1 of Figure 1, for example, the inner ring 2 is made of a predetermined steel member, and the explanation follows.

[0023] The inner ring 2 has a width surface that constitutes one end face in the axial direction, the other width surface which is the opposite end face, an inner circumferential surface, and an outer circumferential surface, which serve as the rolling surface or contact surface with the rolling member. The inner circumferential surface and the outer circumferential surface extend along the circumferential direction. The inner ring 2 is fitted onto the shaft (not shown) at its inner circumferential surface. The outer circumferential surface has a raceway surface 2a, which is the portion of the outer circumferential surface that contacts the ball 4.

[0024] The inner ring 2 is made of steel that has been hardened and tempered. The steel constituting the inner ring 2 contains martensite and retained austenite. The surface of the inner ring 2 (one width surface, the other width surface, the inner circumferential surface, and the outer circumferential surface) has been subjected to nitriding treatment, and a nitrided layer 2a has been formed on the surface of the inner ring 2 (see Figure 2).

[0025] The steel constituting the inner ring 2 may be bearing steel. Bearing steel refers to high-carbon chromium steel with a carbon concentration of 0.90% by mass or more and 1.1% by mass or less, and a chromium concentration of 0.3% by mass or more and 2.1% by mass or less. Specific examples of bearing steel include SUJ1, SUJ2, SUJ3, SUJ4, SUJ5, etc., as specified in ISO 683-17 standards; 50100, 51100, 52100, etc., as specified in ASTM standards; 100Cr6, as specified in ISO standards; and GCr15, as specified in GB standards.

[0026] The carbon concentration in the steel constituting the inner ring 2 may be, for example, 0.95% by mass or more and 1.10% by mass or less. Further, the steel constituting the inner ring 2 may contain 1.40% by mass or more and 1.60% by mass or less of chromium, less than 0.30% by mass of silicon, less than 0.50% by mass of manganese, and less than 0.0080% by mass of sulfur. For example, when the silicon concentration in the steel is less than 0.30% by mass and the manganese concentration is less than 0.50% by mass, silicon-manganese-based nitrides are less likely to precipitate, and chromium-based nitrides described later are likely to precipitate finely. More specifically, the steel constituting the inner ring 2 contains 0.95% by mass or more and 1.10% by mass or less of carbon, less than 0.30% by mass of silicon, less than 0.50% by mass of manganese, less than 0.0080% by mass of sulfur, and 1.4% by mass or more and 1.6% by mass or less of chromium, and the balance may consist of iron and unavoidable impurities. Note that the steel constituting the inner ring 2 may not contain silicon, manganese, and sulfur.

[0027] Fig. 2 shows an enlarged cross-sectional view of the raceway surface of the inner ring 2. As shown in Fig. 2, a nitrided layer 2a is formed on the surface of the inner ring 2. In the nitrided layer 2a, nitrogen is dissolved in the steel. The average nitrogen concentration on the surface of the inner ring 2 is 0.10% by mass or more. The average nitrogen concentration is, for example, 0.60% by mass or less. Further, the average carbon concentration on the surface of the inner ring 2 is, for example, 0.60% by mass or more and 1.5% by mass or less. The average nitrogen concentration and the average carbon concentration on the surface of the inner ring 2 are measured using EPMA (Electron Probe Micro Analyzer). In this measurement, a calibration curve is created using a standard sample with clear nitrogen and carbon concentrations.

[0028] The hardness on the surface of the inner ring 2 is 800 Hv or more and 1000 Hv or less, and may be 800 Hv or more and 900 Hv or less. The hardness on the surface of the inner ring 2 is measured by the Vickers hardness test method defined in ISO 6507. The load when measuring the hardness on the surface of the inner ring 2 is 300 g. The hardness on the surface of the inner ring 2 is measured at three or more locations, and the average value of the measured values is adopted. In addition, when it is difficult to measure the hardness on the surface of the inner ring 2, the hardness at a position where the depth from the surface of the inner ring 2 is 50 μm is regarded as the hardness on the surface of the inner ring 2.

[0029] The amount of retained austenite on the surface of the inner ring 2 is 20% by volume or less. The amount of retained austenite is, for example, 10% by volume or more. The amount of retained austenite on the surface of the inner ring 2 is measured by the X-ray diffraction method. This measurement is performed using a Cr tube type X-ray diffractometer. In the Cr tube type X-ray diffractometer, the wavelength of the Cr-Kα line is 2.29093×10 －１０ m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm×2 mm. When obtaining a sample for measurement from the inner ring 2, it is preferably electrolytically polished so that the retained austenite does not undergo processing-induced transformation.

[0030] Undissolved carbides are contained in the nitrided layer 2a. This undissolved carbide is chromium carbide (Fe,Cr) ３ C that remained undissolved in the matrix phase of the steel constituting the inner ring 2. In the nitrided layer 2a, the area ratio of undissolved carbides with a particle size of 5.0 μm or less is, for example, 20% or less in a cross-sectional view perpendicular to the surface of the inner ring 2, and preferably 17% or less. The area ratio of the undissolved carbides is, for example, 8% or more.

[0031] The area ratio of undissolved carbides with a particle size of 5.0 μm or less is measured by image analysis of a microscopic observation image. Specifically, in a cross section perpendicular to the above surface, in a field of view including a depth of 50 μm from the surface, at least 100 μm ２ or more in area, preferably 500 μm ２The area is measured by performing image analysis on a microscope image of the area described above. The cross section perpendicular to the surface is the cut surface, and after polishing, an etching solution is used to expose undissolved carbides on the surface. Next, the cross section is photographed using a high-magnification optical microscope or laser microscope capable of discriminating 5.0 μm of undissolved carbides (hereinafter, the image obtained by the microscope is referred to as the "microscope image"). By performing image processing on the obtained microscope image, the area ratio of undissolved carbides in the microscope image is calculated. If the area does not reach the above level in one field of view, including the area from the surface to a depth of 50 μm, multiple field-of-view microscope images are acquired.

[0032] Furthermore, in the nitrided layer 2a, the maximum particle size of undissolved carbides is, for example, 2 μm or less. This maximum particle size can also be calculated by performing image processing on the obtained microscope image, similar to the area ratio described above. The maximum particle size of undissolved carbides refers to the diameter of the undissolved carbide with the largest diameter among the undissolved carbides observed in the microscope image. Specifically, in a cross section perpendicular to the surface, undissolved carbides and chromium nitrides are identified by image processing (in detail, by binarizing the image by converting it to grayscale, outlining the edges of the grain and the matrix phase based on the difference in contrast, and then filling in the gaps), the area of ​​each grain is determined, and the maximum particle size is determined by converting the area of ​​the grain with the largest area to an equivalent diameter of a circle.

[0033] Chromium nitrides may be present in the nitriding layer 2a. Chromium nitrides are chromium nitride or nitrides in which a portion of the chromium site of chromium nitride is replaced by elements other than chromium. In the nitriding layer 2a, the area ratio of chromium nitrides with a particle size of 1.0 μm or less may be 1.0% to 10% in a cross-sectional view perpendicular to the surface of the inner ring 2. In the nitriding layer 2a, the area ratio of chromium nitrides with a particle size of 1.0 μm or less may be 1.5% to 2.5% in a cross-sectional view perpendicular to the surface of the inner ring 2. Furthermore, in the nitriding layer 2a, the number of chromium nitrides with a particle size of 1.0 μm or less may be 100 μm ２ There may be five or more per unit area. In the nitrided layer 2a, the number of chromium nitrides with a particle size of 1.0 μm or less is 100 μm.２ There may be eight or more per unit area. Note that 100 μm in the nitrided layer 2a ２ The number of chromium nitride particles with a particle size of 1.0 μm or less present in the area is, for example, 20 or less.

[0034] The particle size, area ratio, and number of chromium nitrides are measured by image analysis of microscopic observations. Specifically, in a cross section perpendicular to the surface, a field of view including a depth of 50 μm from the surface is measured to at least 100 μm. ２ The area is measured by performing image analysis on a microscope image covering the area described above. The cross-section perpendicular to the surface is the cut surface, and after polishing, an etching solution is used to expose undissolved carbides on the surface. Next, the cross-section is photographed using a high-magnification optical microscope or laser microscope capable of discriminating chromium nitrides with a particle size of 1.0 μm or less. By performing image processing on the obtained microscope image, the area ratio of undissolved carbides in the microscope image is calculated. If the area does not reach the above level in a single field of view including the surface to a depth of 50 μm, multiple field-of-view microscope images are acquired.

[0035] (Method for manufacturing steel components) The method for manufacturing the inner ring 2 includes a preparation step S1, a nitrogen treatment step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6.

[0036] In preparation step S1, the workpiece to be processed is prepared. Following preparation step S1, nitriding treatment step S2 is performed. In nitriding treatment step S2, nitriding treatment is performed on the workpiece to be processed. The nitriding treatment of the workpiece to be processed is performed by heating and holding the workpiece in an atmospheric gas containing a nitrogen source. The heating temperature and nitrogen concentration in the atmospheric gas in nitriding treatment step S2 are set so that a compound layer is not formed on the surface of the workpiece to be processed. As a result of nitriding treatment step S2, nitrogen penetrates from the surface of the workpiece to the interior of the workpiece, and nitrogen is dissolved in the matrix phase of the steel that makes up the workpiece. Nitriding treatment step S2 is performed so that nitrogen reaches the position that will become the surface of the inner ring 2 after post-treatment step S6 (the position that will become the nitriding layer 2a). Note that in nitriding treatment step S2, nitriding and carburizing treatment may be performed instead of nitriding treatment. That is, the atmospheric gas may contain a carbon source. Following nitriding treatment step S2, quenching step S3 is performed.

[0037] In the quenching process S3, the workpiece to be processed is quenched. The quenching of the workpiece is performed on the steel A that makes up the workpiece. １ The component to be heated is heated to a temperature above its transformation point and held thereafter, and then the component to be processed is M Ｓ This is carried out by cooling to a temperature below the transformation point. The quenching process S3 generates martensite and retained austenite in the steel constituting the workpiece. The quenching process S3 may be repeated on the workpiece. Repeated quenching process S3 refines the crystal grains and improves the effect of the cooling process S4. The half-width or dislocation density of martensite and retained austenite is adjusted by the cooling rate in the quenching process S3. After the quenching process S3, the cooling process S4 is performed.

[0038] In the cooling process S4, the workpiece is subjected to sub-zero treatment or cryo-treatment (ultra-sub-zero treatment). If sub-zero treatment is performed, the workpiece is cooled to a temperature below -100°C or below room temperature. If cryo-treatment is performed, the workpiece is cooled to a temperature below -100°C. In the cooling process S4, a portion of the retained austenite in the steel constituting the workpiece is transformed into martensite. Before the cooling process S4, the workpiece may be tempered at a low temperature or cleaned to prevent cracking. The half-width or dislocation density of martensite and retained austenite is adjusted by the time between the quenching process S3 and the cooling process S4, and by the cooling rate in the cooling process S4. After the cooling process S4, the tempering process S5 is performed.

[0039] In the tempering process S5, tempering is performed on the workpiece. Tempering of the workpiece involves the A of the steel that makes up the workpiece. １ This is done by heating the workpiece to a temperature below the transformation point. This heating temperature is, for example, 180°C or lower. When tempering is performed, the dislocation density of martensite and retained austenite in the steel that makes up the workpiece decreases, resulting in a decrease in hardness. However, if the cooling process S4 is performed, the dislocation density of martensite and retained austenite does not decrease as easily after tempering, and the hardness and yield strength do not decrease as easily after tempering. After the tempering process S5, the post-treatment process S6 is performed.

[0040] In the post-processing step S6, machining such as grinding and polishing is performed on the surface of the workpiece. As a result, the inner ring 2 is formed.

[0041] In the nitrided layer 2a, nitrogen is dissolved in the matrix phase of the steel constituting the inner ring 2, and thus the hardness of the surface of the inner ring 2 increases due to solid solution strengthening. Furthermore, as described above, since the inner ring 2 undergoes a cooling process S4 followed by a tempering process S5, the decrease in hardness on the surface of the inner ring 2 due to tempering is suppressed. As a result, a hardness of 800 Hv or more is obtained on the surface of the inner ring 2. In addition, the decrease in undissolved carbides and the increase in fine chromium nitrides also contribute to the improvement in hardness of the inner ring 2.

[0042] Furthermore, austenite has a lower yield strength compared to martensite, making it more susceptible to plastic deformation, which causes peeling. However, the amount of retained austenite on the surface of the inner ring 2 is suppressed to 20% or less. In addition, since the inner ring 2 undergoes a cooling process S4 followed by a tempering process S5, the decrease in dislocation density in the retained austenite and, consequently, the decrease in yield strength associated with tempering are suppressed. In this way, plastic deformation is suppressed on the surface of the inner ring 2 due to the reduction in the amount of retained austenite.

[0043] Furthermore, if the undissolved carbides present in the nitriding layer 2a become coarser, stress concentration is more likely to occur around the undissolved carbides when the ball 4 contacts the raceway surface of the inner ring 2, which can cause peeling. The higher the carbon concentration in the steel, the greater the amount of undissolved carbides in the steel, and the more likely the undissolved carbides are to become coarser. For example, if the average carbon concentration on the surface is 1.5 mass% or less, the increase in undissolved carbides present in the nitriding layer 2a is suppressed, and the coarsening of the undissolved carbides is suppressed. Also, in the inner ring 2, since the nitrogen concentration in the steel is 0.1 mass% or more, the chromium in the undissolved carbides combines with nitrogen to precipitate chromium nitrides, reducing the amount of undissolved carbides and suppressing the increase and coarsening of the undissolved carbides present in the nitriding layer 2a.

[0044] In Figure 1 above, the inner ring 2 is described as a predetermined steel member, but the outer ring 3 and ball 4 may also be made of the same steel member. In that case, the outer ring 3 and ball 4 may be made of steel with the same composition as the inner ring 2, and the same nitrogen-infiltrating layer as described above may be formed on the surface of the outer ring 3 and the surface of the ball 4.

[0045] (Regarding the grease composition) In a rolling mechanism comprising a steel member having a surface that contacts a mating member and a grease composition that lubricates these contact areas, for example, during high-speed rotation, there is a growing concern that peeling may occur as a result of heat generation due to grease flow within the rolling mechanism. The grease composition used in the present invention uses a base oil with relatively low viscosity and keeps the storage modulus of the grease composition within a predetermined range to suppress bearing heat generation and, consequently, improve peeling resistance.

[0046] The grease composition used in the present invention comprises a base oil and a thickener, with various additives added as needed. The grease composition is suitable for temperatures of 25°C and strain levels of 1 × 10⁻⁶. －５ ~5 x 10 －３ The maximum storage modulus at a frequency of 1 Hz is within the range of 50,000 Pa or less.

[0047] Dynamic viscoelasticity is a method of evaluating viscoelasticity by applying vibrational (periodic) strain or stress to a linear viscoelastic material and measuring the resulting stress or strain. The viscoelasticity obtained in this way is called dynamic viscoelasticity, and the resulting modulus of elasticity is expressed in the form of a complex number. In this invention, the grease composition is used as the linear viscoelastic material.

[0048] If the modulus of elasticity in dynamic viscoelasticity is given by the complex modulus G* = G' + iG'', then the real part G' of the complex modulus G* corresponds to the storage modulus, and the imaginary part G'' corresponds to the loss modulus. The storage modulus G' represents the elastic component of dynamic viscoelasticity. Specifically, it is the ratio of the elastic stress in phase with the strain generated when an external force is applied to the grease composition, and represents the energy that can be elastically stored from the external force received by the grease composition. On the other hand, the loss modulus G'' represents the viscous component of dynamic viscoelasticity. Specifically, it is the ratio of the strain in phase with a different phase from the strain generated when an external force is applied to the grease composition, and represents the energy that is dissipated as heat from the external force received by the grease composition.

[0049] Thus, the storage modulus can be considered an indicator of the dimensional stability of grease. Furthermore, even with the same grease, the storage modulus changes significantly with strain. In the case of grease, the storage modulus is high under low strain conditions, and under high strain conditions, viscosity takes over and the storage modulus decreases. This low strain (specifically, strain amount 1 × 10⁻⁶) －５ ~5 x 10 －３ By setting the maximum storage modulus in the bearing to 50,000 Pa or less, within an appropriate range, the grease composition exhibits appropriate dimensional stability even during high-speed rotation, thereby suppressing bearing heat generation.

[0050] The maximum value of the storage modulus is preferably 6000 Pa or more. For example, during high-speed rotation, the grease composition is easily separated by centrifugal force, and the thickener, which is agitated and sheared within the rolling gear, is destroyed, causing the grease composition to soften and making it easier for the grease composition to leak out of the rolling gear. By setting the storage modulus to 6000 Pa or more, leakage to the outside of the rolling gear can be suppressed. The maximum value of the storage modulus is more preferably 8000 Pa to 40000 Pa, and may also be 8000 Pa to 20000 Pa.

[0051] In this invention, the storage modulus is defined as a temperature of 25°C and a strain of 1 × 10⁻⁶. －５ ~5 x 10 －３ The measurement is performed using a rheometer at a frequency of 1 Hz. Preferably, a rheometer with a parallel plate type cell, as shown in Figure 3, is used. Specific measurement conditions are shown in the examples.

[0052] The base oil in the grease composition used in the present invention has a kinematic viscosity of 12 mm² at 40°C (or the kinematic viscosity of the mixed oil in the case of a mixed oil). ２ / s or more 26mm ２ The kinematic viscosity is less than or equal to / s. Keeping it within this range makes it easier to suppress bearing heat generation and also makes it easier to suppress leakage outside the bearing even at high rotational speeds. In addition, it makes it easier to prevent grease evaporation degradation at high temperatures. The above kinematic viscosity is 15 mm². ２ / s ~ 22mm ２ / s is preferable.

[0053] As a base oil, any oil commonly used in rolling bearings can be used without particular restrictions. Examples include mineral oils such as paraffinic mineral oil and naphthenic mineral oil, synthetic hydrocarbon oils such as poly-α-olefin (PAO) oil and alkylbenzene oil, ester oils, ether oils, silicone oils, and fluorinated oils. These base oils may be used individually or in combination of two or more types.

[0054] Among the base oils mentioned above, it is preferable to use ester oil or a base oil mixed with ester oil, due to its compatibility with other extreme pressure additives and rust inhibitors that may be contained in the grease.

[0055] The thickener used in the grease composition of the present invention is not particularly limited, and general thickeners commonly used in the field of grease can be used. For example, soap-based thickeners such as metal soaps and complex metal soaps, and non-soap-based thickeners such as bentonite, silica gel, urea compounds, and urea-urethane compounds can be used. Examples of metal soaps include sodium soap, calcium soap, barium soap, aluminum soap, and lithium soap, while examples of urea compounds and urea-urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, and diurethane compounds.

[0056] Diurea compounds are obtained by reacting a diisocyanate component with a monoamine component. Examples of diisocyanate components include phenylenediisocyanate and diphenylmethane diisocyanate (MDI). Examples of monoamine components include alicyclic monoamines such as cyclohexylamine, aromatic monoamines such as p-toluidine, and aliphatic monoamines such as octylamine.

[0057] Among the thickeners mentioned above, it is preferable to use aliphatic diurea compounds alone or in mixtures with others, as this makes it easier to set the storage modulus within the desired range. Examples of the latter include diurea compounds (also referred to as fatty / alicyclic diureas) that use aliphatic monoamines and alicyclic monoamines as monoamine components, and diurea compounds (also referred to as fatty / aromatic diureas) that use aliphatic monoamines and aromatic monoamines.

[0058] The above-mentioned thickener is contained in an amount of, for example, 5% to 30% by mass relative to the total amount of the base oil and the thickener, and is preferably contained in an amount of 10% to 30% by mass, as this makes it easier to set the storage modulus within a desired range. For example, in the case of diurea-based thickeners, it is more preferable to contain 10% to 20% by mass, and may also be 10% to 16% by mass.

[0059] Furthermore, the grease composition used in the present invention may contain other additives, to the extent that they do not impair the objectives of the present invention. Examples include antioxidants such as amine-based, phenol-based, and sulfur-based compounds, rust inhibitors such as sulfonates, and oily agents such as esters and alcohols.

[0060] The amount of antioxidant added is preferably 0.1% by mass or more and less than 3% by mass relative to the total amount of the base grease (100% by mass) consisting of the base oil and thickener. Within this range, oxidative degradation of the grease can be suppressed, and a decrease in lubrication performance can be prevented. Examples of antioxidants include amine-based antioxidants such as phenyl-1-naphthylamine, phenyl-2-naphthylamine, diphenyl-p-phenylenediamine, dipyridylamine, phenothiazine, N-methylphenothiazine, N-ethylphenothiazine, 3,7-dioctylphenothiazine, p,p'-dioctyldiphenylamine, and N,N'-diisopropyl-p-phenylenediamine, as well as phenol-based antioxidants such as 2,2'-methylenebis(6-tert-butyl-4-methylphenol). These antioxidants may be used individually or in combination of two or more. It is preferable to use amine-based antioxidants as the antioxidant.

[0061] The type of rust inhibitor is not particularly limited, and can be used as follows: ester-based rust inhibitors; sulfonate-based rust inhibitors; linear fatty acids such as lauric acid and stearic acid, or carboxylic acid-based rust inhibitors such as succinic acid and alkyl succinic acid; carboxylic acid salt-based rust inhibitors such as fatty acids and metal salts (cobalt, manganese, zinc) of naphthenic acid; or amine-based rust inhibitors such as alkoxyphenylamine. Among these rust inhibitors, it is preferable to use at least one selected from ester-based rust inhibitors and sulfonate-based rust inhibitors. As ester-based rust inhibitors, partial esters of polyhydric alcohols such as sorbitan, sorbitol, pentaerythritol, sucrose, and glycerin with carboxylic acids such as oleic acid and lauric acid, or succinic acid half-esters such as alkyl succinic acid half-esters and alkenyl succinic acid half-esters can be used. These ester-based rust inhibitors may be used alone or in combination of two or more.

[0062] The amount of rust inhibitor added is preferably 0.1% by mass or more and less than 3% by mass relative to the total amount of the base grease (100% by mass) consisting of the base oil and thickener.

[0063] In the present invention, the grease composition preferably contains an antioxidant and a rust inhibitor as additives. By adding these additives, the storage modulus can be easily adjusted to a desired range, and as shown in the examples described later, the storage modulus can be reduced by adding, for example, a sulfonate-based rust inhibitor.

[0064] The consistency of the above grease composition (ISO 2137) is preferably in the range of 200 to 350. If the consistency is less than 200, oil separation is poor and lubrication may be inadequate. On the other hand, if the consistency exceeds 350, the grease becomes soft and easily leaks out of the bearing, which is undesirable. The above consistency is more preferably in the range of 250 to 340.

[0065] When the rolling device of the present invention is a rolling bearing, the amount of grease to be sealed is preferably 10% to 40% (volume ratio) of the static space volume in the bearing's internal space. If it is less than 10 volume%, the amount of grease necessary for lubrication will be insufficient and it will easily be depleted, and if it exceeds 40 volume%, it will be more prone to heat generation due to increased torque caused by agitation. Here, the static space volume is the volume of space in the space between the inner ring, outer ring, and seal member that the rolling elements and cage do not pass through when the bearing rotates. The amount of grease to be sealed may be 10% to 30% of the static space volume in the bearing's internal space, or it may be 10% to 20%.

[0066] In the angular contact ball bearing 1 shown in Figure 1, a contact seal is used as the sealing member, but a non-contact seal with a gap between the seal lip portion and the raceway ring portion opposite it may also be used. For example, a seal groove may be provided on the raceway ring portion opposite the seal lip portion of the sealing member (for example, the outer circumferential surface of the inner ring), and a simple labyrinth may be formed between the seal lip portion and the seal groove.

[0067] In addition to the angular contact ball bearings shown in Figure 1, other types of rolling bearings that can be used include deep groove ball bearings, cylindrical roller bearings, tapered roller bearings, self-aligning roller bearings, needle roller bearings, thrust cylindrical roller bearings, thrust tapered roller bearings, thrust needle roller bearings, and thrust self-aligning roller bearings.

[0068] Figure 4 shows an example of a spindle device for a machine tool spindle using the angular contact ball bearings shown in Figure 1. As shown in Figure 4, the spindle device 11 has two rows of angular contact ball bearings 1 on the front side and a single row of cylindrical roller bearings 15 on the rear side, and is a high-speed specification driven by a built-in motor 12 equipped with a stator 13 and rotor 14 located in the center. The rotating shaft driven by the built-in motor 12 is supported by the angular contact ball bearings 1 and the cylindrical roller bearings 15. The cylindrical roller bearing 15 consists of an inner ring 16, an outer ring 17, cylindrical rollers 18, and a cage 19. For bearing preloading, for example, a fixed-position preloading method is employed. For example, in the fixed-position preloading method, there is a concern about heat generation, but by combining predetermined steel members as raceway members and rolling elements with a grease composition having physical properties within a specific range of base oil viscosity and storage modulus of grease, it is possible to operate at a low temperature rise, and consequently improve peeling resistance.

[0069] The rolling device of the present invention may include any device comprising two members that are in contact with each other via a grease composition, and is not limited to rolling bearings. Other examples include constant velocity joints, linear guide devices, and ball screws.

[0070] The present invention will be specifically described by the following test examples, but the invention is not limited in any way by these examples.

[0071] [Test Example A] First, steel members were examined. The test pieces for Test Examples A1 to A8 were cylindrical in shape with an outer diameter of 40 mm and an inner diameter of 28 mm. SUJ2 was used for Test Examples A1 to A6. For Test Examples A7 to A8, a steel was used that had the same composition as SUJ2 except for the carbon content, with a carbon concentration that was 0.2 mass% higher. In Test Examples A1 to A8, grinding and superfinishing were performed in the post-processing step S6 to achieve an arithmetic mean roughness (Ra) of 0.02 μm on the outer surface. Details of Test Examples A1 to A8 are shown in Table 1.

[0072] In test examples A1 to A3, the heat treatment consisted of a nitrogen immersion process S2, a quenching process S3, a cooling process S4 (cryotreatment), and a tempering process S5. In test examples A1 to A3, the average nitrogen concentration, average carbon concentration, presence of undissolved carbides, and presence of chromium nitrides on the surface of the test specimens were adjusted by changing the heating temperature and holding time in the nitrogen immersion process S2. In addition, the amount of retained austenite on the surface of the test specimens was adjusted by changing the holding time and cooling temperature in the cooling process S4.

[0073] In Test Example A4, the heat treatment consisted of a nitrogen immersion process S2 and a tempering process S5. In Test Example A5, the heat treatment consisted of a nitrogen immersion process S2, a quenching process S3, and a tempering process S5. In Test Example A6, the heat treatment consisted of a standard quenching and tempering process S5. In Test Examples A7 and A8, the heat treatment consisted of a nitrogen immersion process S2, a quenching process S3, and tempering at 200°C.

[0074] Next, peeling tests were performed on test examples A1 to A8. For the peeling tests, a cylindrical mating material with an outer diameter of 40 mm and an inner diameter of 28 mm was used. The sub-radius of curvature on the outer surface of the mating material was set to 60 mm. The mating material was made of SUJ2 and underwent quenching and tempering as heat treatment. The hardness on the outer surface of the mating material was 760 Hv, and the surface roughness (Ra) on the outer surface of the mating material was set to 0.75 μm. During the peeling tests, the mating material was rotated at a rotational speed of 2000 rpm. During the peeling tests, test examples A1 to A8 were driven by contact between their outer surfaces and the outer surface of the mating material. In this peeling test, additive-free turbine oil (ISO VG46) was used for lubrication between test examples A1 to A8 and the mating material, and the oil film parameter was set to 0.6. A load of 2254 N was applied between test examples A1 to A8 and the mating material. This corresponds to a maximum contact pressure of 2.3 GPa. In the peeling test, the test time was 240 minutes, and the test temperature was room temperature.

[0075] After the peeling test, the surfaces of test examples A1 to A8 were observed using an optical microscope. In addition, optical microscope images were taken of the areas where peeling occurred, and the peeling area ratio was calculated by image analysis. More specifically, firstly, optical microscope images were taken in the three fields of view with the largest peeling areas. Secondly, in each image, the area ratio of microcracks present in the peeling area (the ratio of the crack area to the field of view area) was calculated using binarization processing of image analysis software. Thirdly, the peeling area ratio was obtained by averaging the above area ratios for the three fields of view. The results are shown in Table 1.

[0076]

[0077] As shown in Table 1, test examples A1 to A3, in which the hardness of the surface of the test specimen was 800 Hv or higher, the average nitrogen concentration was 0.10 mass% or higher, and the amount of retained austenite was 20 volume% or less, had a peeling area ratio of less than 2%. In contrast, test examples A4 to A8, which did not meet all of these requirements, had a peeling area ratio of 2% or higher.

[0078]

[0079] Table 2 shows other parameters for Test Examples A1 to A3. As shown in Table 2, in Test Examples A1 to A3, the average carbon concentration on the surface was between 0.60% by mass and 1.5% by mass, and the area ratio of undissolved carbides with a particle size of 5.0 μm or less in the nitrided layer was 17% or less. In Test Examples A7 and A8, the area ratios of undissolved carbides with a particle size of 5.0 μm or less in the nitrided layer were 18% and 26%, respectively.

[0080] Furthermore, as shown in Table 2, in Test Examples A1 and A2, the area ratio of chromium nitrides with a particle size of 1.0 μm or less in the nitridation layer was 1.0% or more. Also, the area ratio of chromium nitrides with a particle size of 1.0 μm or less in the nitridation layer was 100 μm. ２ There were five or more of these present. These requirements are more favorable in terms of improved peel resistance, as seen in comparison with test example A3.

[0081] [Test Example B] Next, grease compositions were examined. Eleven test greases were prepared as shown in Tables 3 and 4. Tables 3 and 4 show the kinematic viscosity at 40°C, the type of thickener, and the amount of thickener relative to the base grease for each test grease. Note that each grease composition in Test Examples B1 to B5 contains antioxidants and rust inhibitors as additives.

[0082] <Measurement of Storage Modulus> The storage modulus was measured using a viscoelasticity measuring device (HAAKE MARS). As shown in Figure 3, each test grease G was sandwiched between two parallel disc plates 8 with a diameter of φ25 mm to a thickness of 1 mm. Periodic strain was applied to each test grease by rotating the upper plate 9 due to vibration, and the shear stress as the response was measured. The measurement conditions are as follows: Frequency: 1 Hz Strain amount of grease: 1 × 10⁻⁶ －５ ~5 x 10 －３ Grease temperature: 25°C

[0083] From the following formula, frequency 1 Hz, strain 1 × 10⁻⁶ －５ ~5 x 10 －３ The maximum storage modulus G' at a temperature of 25°C was determined. The results are shown in Tables 3 and 4. G' = (σ ０ / γ ０ ) cosδ Here, σ ０ γ represents the stress at t=0, ０ δ represents the strain at t=0, and δ represents the phase difference (the difference in response time when strain is applied).

[0084] <Bearing Test> A test bearing was prepared by sealing test grease into a 20mm inner diameter deep groove ball bearing 6204LLB (non-contact seal). The test conditions were an ambient temperature of 25°C and a rotational speed of 10,000 mins. －１The axial load Fa and radial load Fr were set to 67 N, and the amount of grease filled was set to 30% of the stationary space volume inside the bearing. The bearing was then rotated for 24 hours. This test observed the heat generated during churning, a condition in which the bearing torque fluctuates due to grease movement caused by agitation. The evaluation items for this test were the temperature measured during bearing operation and the presence or absence of leakage from the bearing. For temperature, "A" was assigned if the outer ring temperature remained below 60°C, and "B" if it exceeded 60°C. For leakage from the bearing, the appearance was observed after operation. "A" was assigned if no test grease leaked from the seal groove, and "B" if leakage occurred. The results are shown in Tables 3 and 4.

[0085] <High-Temperature Storage Test> The weight loss of a predetermined amount (approximately 1 g) of grease composition was measured when it was left at a high temperature of 120°C for 100 hours. A weight loss of less than 20% by mass was classified as "A," and a weight loss of 20% by mass or more was classified as "B." The results are shown in Tables 3 and 4.

[0086]

[0087]

[0088] As shown in Table 3, the kinematic viscosity of the base oil at 40°C is 12 mm². ２ / s or more 26mm ２ In Test Examples B1 to B5, which used grease compositions with a storage modulus of elasticity of 50,000 Pa or less and a maximum storage modulus of elasticity of 50,000 Pa or less measured above, the bearing temperature remained below 60°C during operation, there was no leakage outside the bearing, and the weight loss due to high-temperature storage was less than 20% by mass.

[0089] On the other hand, in test examples B6 and B10-B11, where the kinematic viscosity of the base oil at 40°C was the same as in the above test examples, the temperature rise during operation was greater, and the bearing temperature exceeded 60°C. It is thought that the higher storage modulus made it easier to maintain the shape of the grease composition, which increased the resistance when the balls overcame the grease composition on the raceway surface, resulting in higher torque and increased heat generation.

[0090] Note that the difference between test example B5 and test example B11 is the presence or absence of a sulfonate-based rust inhibitor as an additive; test example B5 includes the additive. In test example B5, the inclusion of the sulfonate-based rust inhibitor reduces the storage modulus compared to test example B11, resulting in improved heating characteristics. Furthermore, the difference between test example B2 and test example B10 is the amount of thickener; in test example B2, the amount of thickener affects the storage modulus, bringing it within an appropriate range, resulting in improved heating characteristics.

[0091] Furthermore, the kinematic viscosity of the base oils in test examples B8 to B9 at 40°C was 5 mm². ２ In the grease composition with a storage modulus of 20 / s, a weight loss of 20% or more by mass occurred when left at high temperatures. On the other hand, even if the storage modulus satisfies the above range, if the kinematic viscosity of the base oil at 40°C is relatively high (for example, test example B7), heat generation is large, and as a result, the surface pressure is considered to be large.

[0092] Thus, the grease compositions of Test Examples B1 to B5 can suppress heat generation during rotation compared to the grease compositions of Test Examples B6 to B11, and can also suppress the increase in surface pressure and base oil reduction associated with increased heat generation, which in turn leads to improved peeling resistance. Furthermore, grease leakage can be effectively prevented.

[0093] Based on the results of the above-mentioned Test Examples A and B, it is possible to suppress heat generation during the rotation of the rolling mechanism by combining the specified steel components and grease composition, and to suppress peeling even when the lubricating oil is depleted and lubrication is poor.

[0094] The rolling device of the present invention suppresses heat generation during rolling and improves peeling resistance, so it can be widely used in rolling devices where peeling is a concern, such as rolling bearings used in electric axles, electric brakes, electric VTCs, and electric compressors of electric vehicles, and rolling bearings used in the spindles of machine tools.

[0095] 1. Angular contact ball bearing 2. Inner ring 3. Outer ring 4. Balls (rolling elements) 5. Cage 6. Seal member 7. Grease composition 8. Rheometer 9. Upper plate 10. Lower plate 11. Spindle device 12. Built-in motor 13. Stator 14. Rotor 15. Cylindrical roller bearing 16. Inner ring 17. Outer ring 18. Cylindrical roller 19. Cage

Claims

1. A rolling device comprising a steel member having a surface that contacts a mating member, and a grease composition for lubricating the contact portion between the mating member and the steel member, wherein the steel member has a nitrogen-infiltrated layer formed on its surface, the average nitrogen concentration on the surface is 0.10% by mass or more, the hardness on the surface is 800 Hv or more and 1000 Hv or less, and the amount of residual austenite on the surface is 20% by volume or less, and the grease composition comprises a base oil and a thickener, and the kinematic viscosity of the base oil at 40°C is 12 mm² ２ / s or more 26mm ２ The value is less than or equal to / s, and the temperature of the grease composition is 25°C, and the strain amount is 1 × 10⁻⁶. －５ ~5 x 10 －３ A rolling device characterized in that the maximum value of the storage modulus of the grease composition under viscoelastic measurement conditions at a frequency of 1 Hz is within the range of 50,000 Pa or less.

2. The rolling device according to claim 1, characterized in that the steel member contains 0.95% by mass or more and 1.10% by mass of carbon, less than 0.30% by mass of silicon, less than 0.50% by mass of manganese, less than 0.0080% by mass of sulfur, and 1.4% by mass or more and 1.6% by mass of chromium, with the remainder being iron and unavoidable impurities.

3. The rolling device according to claim 1, characterized in that, in a cross-sectional view perpendicular to the surface in the nitrided layer, the area ratio of undissolved carbides with a particle size of 5.0 μm or less is 17% or less.

4. The rolling device according to claim 1, characterized in that, in a cross-sectional view perpendicular to the surface in the nitrided layer, the area ratio of chromium nitrides with a particle size of 1.0 μm or less is 1.0% or more and 10% or less.

5. The rolling device according to claim 1, characterized in that the thickener is a diurea compound, barium soap, or lithium complex soap, and the content of the thickener relative to the total amount of the base oil and the thickener is 10% to 30% by mass.

6. The rolling device according to claim 1, characterized in that the grease composition further comprises an antioxidant and a rust inhibitor.

7. The rolling device is a rolling bearing comprising an inner ring and an outer ring which are raceway members, a plurality of rolling elements interposed between the inner ring and the outer ring, and a cage which holds the rolling elements, wherein at least one of the raceway members and the rolling elements is the steel member, and the grease composition is sealed in the bearing space between the inner ring and the outer ring, as described in claim 1.

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