Grease composition and grease-sealed bearing

Abstract

Provided are: a grease composition capable of preventing grease leakage and suppressing heat generation inside a bearing even under a high-speed rotation condition; and a grease-sealed bearing in which the grease composition is enclosed. A grease composition 7 is enclosed in a rolling bearing 1 and comprises a base oil and a thickener, wherein: the base oil has a kinematic viscosity at 40°C of less than 40 mm2 / s; the maximum storage elastic modulus at a temperature of 25°C and a frequency of 1 Hz is 10000-50000 Pa; the thickener is a urea compound or barium soap; and the content of the thickener with respect to the total amount of the base oil and the thickener is 5-30% by mass.

Description

Grease composition and grease-filled bearing

[0001] The present invention relates to a grease composition and a grease-filled bearing filled with the grease composition, and in particular to a grease composition used in a bearing that supports a rotating shaft such as a main shaft (spindle) that rotates at high speed in a machine tool.

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

[0003] Grease-filled rolling bearings for machine tool spindles are required to have high-speed durability. High-speed rotation can lead to grease leakage outside the bearing and heat generated by grease flow inside the bearing, shortening the bearing's lifespan.

[0004] For example, Patent Document 1 proposes a grease composition that prevents grease leakage by specifying the kinematic viscosity of the base oil at 40°C and the carbon number of the lithium soap thickener. Also, Patent Document 2 proposes that in a bearing device lubricated with a grease composition, compressed air that is at a lower temperature than the bearing is sent from the outside to suppress temperature increases in the bearing and thereby extend its lifespan.

[0005] JP 2006-199771 A JP 2018-169040 A

[0006] However, in the grease composition of Patent Document 1, the kinematic viscosity of the base oil and the carbon number of the lithium soap are specified, but even if the grease composition has the same composition, depending on the proportion of thickener in the total amount of grease, grease leakage and heat generation may increase, potentially shortening the life of the bearing. Furthermore, in Patent Document 2, heat generation inside the bearing is suppressed, but compressed air passes through the bearing, potentially shortening its life due to grease leakage. Furthermore, a device and structure for feeding compressed air are required, which may increase manufacturing costs.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a grease composition that can prevent grease leakage and suppress heat generation inside a bearing even under high-speed rotation conditions, and a grease-filled bearing in which the grease composition is filled.

[0008] The grease composition of the present invention is a grease composition that is filled into a rolling bearing and contains a base oil and a thickener, wherein the kinematic viscosity of the base oil at 40°C is 40 mm ２ / s, and the maximum storage modulus at a temperature of 25° C. and a frequency of 1 Hz is 10,000 Pa to 50,000 Pa.

[0009] The kinematic viscosity of the base oil at 40°C is 15 mm ２ / s or more 30mm ２ / s or less.

[0010] The thickener is a urea compound or barium soap, and the content of the thickener relative to the total amount of the base oil and the thickener is 10% by mass to 30% by mass.

[0011] The kinematic viscosity of the base oil at 40°C is 15 mm ２ / s or more 30mm ２ / s, the thickener is a diurea compound obtained by reacting a diisocyanate component with a monoamine component, the monoamine component contains an aromatic monoamine, and the content of the thickener relative to the total amount of the base oil and the thickener is 10% by mass to 20% by mass.

[0012] The grease-sealed bearing of the present invention is a rolling bearing comprising an inner ring and an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, a cage that holds the rolling elements, a grease composition that is sealed in the bearing space between the inner ring and the outer ring, and a seal member that seals the bearing space, wherein the grease composition is the grease composition of the present invention.

[0013] The rolling elements are characterized in that they are made of ceramic.

[0014] The rolling bearing has a dm-n value (the pitch circle diameter dm of the rolling element [unit: mm] and the number of revolutions of the bearing ring n [unit: min －１ ]) 100 x 10 ４ The bearing is characterized in that it is a bearing for a machine tool main spindle used as described above.

[0015] The sealing member has one end fixed to one of the inner and outer rings and a seal lip portion at the other end facing the other ring without contacting it, and is characterized in that the amount of grease filled in the rolling bearing is 10% to 30% of the static space volume within the bearing.

[0016] The grease composition of the present invention is filled into a rolling bearing, contains a base oil and a thickener, and the kinematic viscosity of the base oil at 40°C is 40 mm ２ / s, and the maximum storage modulus at a temperature of 25°C and a frequency of 1 Hz is 10,000 Pa to 50,000 Pa. Therefore, even under high-speed rotation conditions, leakage to the outside of the bearing is prevented and heat generation inside the bearing can be suppressed.

[0017] The grease-sealed bearing of the present invention comprises an inner ring, an outer ring, a plurality of rolling elements, a cage, the grease composition of the present invention, and a seal member. The bearing further uses ceramic rolling elements and has a dm·n value of 100×10 ４ The bearing can be suitably used as a bearing for the main spindle of a machine tool as described above.

[0018] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

[0019] The present inventors have conducted extensive research into grease compositions used to lubricate rolling bearings, particularly those used under high-speed rotation conditions, and have found that the storage modulus of a grease composition is related to grease leakage and heat generation inside the bearing, and that by adjusting the storage modulus and the kinematic viscosity of the base oil within a predetermined range, grease leakage can be prevented and heat generation in the bearing can be suppressed, even under high-speed rotation conditions. The present invention is based on this finding.

[0020] An example of a rolling bearing of the present invention will be described with reference to FIG. 1 . FIG. 1 is an axial cross-sectional view of an angular contact ball bearing. As shown in FIG. 1 , the angular contact ball bearing 1 is an angular contact ball bearing in which the bearing space between an inner ring 2 and an outer ring 3, in which balls 4 are held in a cage 5, 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 seal lip portion on the inner diameter side contacts the seal groove. A grease composition 7 is sealed around at least the balls 4. A straight line connecting the contact points between the balls 4 and the inner and outer rings 2 and 3 has a contact angle α with respect to the radial direction, allowing the bearing to withstand a radial load and an axial load in one direction. In the present invention, the grease composition of the present invention is sealed in the bearing space formed by the inner ring 2, outer ring 3, and balls 4.

[0021] A circumferential groove-like grease pocket is formed on the inner peripheral surface of the outer ring 3, which serves to further prevent leakage of the grease composition.

[0022] In angular contact ball bearing 1, the iron-based metallic material constituting the bearing members of inner ring 2, outer ring 3, and balls 4 is any material commonly used as a bearing material, such as high carbon chromium bearing steel (SUJ1, SUJ2, SUJ3, SUJ4, SUJ5, etc.; JIS G 4805), carburized steel (SCr420, SCM420, etc.; JIS G 4053), stainless steel (SUS440C, etc.; JIS G 4303), high-speed steel (M50, etc.), cold-rolled steel, etc.

[0023] Furthermore, the balls 4 can be made of not only iron-based metal materials but also lightweight ceramic materials. The use of ceramic materials is preferable because, for example, they can suppress the increase in preload due to centrifugal force during high-speed rotation. Examples of ceramic materials include silicon nitride, silicon carbide, aluminum oxide (alumina), zirconium oxide (zirconia), sialon, and glass. Among these, silicon nitride, which has excellent heat resistance, is more preferable.

[0024] The sealing member 6 may be a metal or rubber molded body alone, or may be a composite of a rubber molded body and a metal plate, a plastic plate, or a ceramic plate. From the viewpoint of durability and ease of attachment, a composite of a rubber molded body and a metal plate, as shown in Figure 1, is preferred. Also, a non-contact seal may be used, as described below.

[0025] The angular contact ball bearing 1 is suitable for use under high speed rotation conditions. For example, under high speed conditions, the dm·n value is 80×10 ４ or more, preferably 100×10 ４ The upper limit of the dm·n value is not particularly limited, but is, for example, 250×10 ４ is.

[0026] In rolling bearings, when the bearing rotates at high speeds, centrifugal force tends to cause the grease composition to separate from the oil, and the thickener is destroyed by stirring and shearing within the bearing, softening the grease composition and making it more likely to leak out of the bearing. Furthermore, when the bearing rotates at high speeds, heat generated by the grease flowing inside the bearing tends to shorten the life of the bearing. In response to this, the present invention uses a base oil with a relatively low viscosity and limits the storage modulus of the grease composition to a predetermined range, thereby preventing grease leakage and suppressing heat generation in the bearing.

[0027] The grease composition of the present invention contains a base oil and a thickener, and optionally contains various additives. The grease composition has a maximum storage modulus in the range of 10,000 Pa to 50,000 Pa at a temperature of 25°C and a frequency of 1 Hz.

[0028] Dynamic viscoelasticity is a method of evaluating viscoelasticity by applying oscillatory (periodic) strain or stress to a linear viscoelastic body and measuring the corresponding stress or strain. The viscoelasticity obtained in this manner is called dynamic viscoelasticity, and the obtained elastic modulus of dynamic viscoelasticity is expressed in the form of a complex number. In the present invention, the grease composition is considered to be a linear viscoelastic body.

[0029] If the elastic modulus in dynamic viscoelasticity is defined as 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 refers to the ratio of elastic stress that is in phase with the strain that occurs when an external force is applied to the grease composition, and corresponds to the energy that can be elastically stored out of 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 refers to the ratio of the strain that occurs out of phase with the strain that occurs when an external force is applied to the grease composition, and corresponds to the energy that is dissipated as heat out of the external force received by the grease composition.

[0030] Thus, the storage modulus can be said to be an index showing the shape stability of a grease. Furthermore, even for the same grease, the storage modulus changes significantly depending on the strain. In the case of grease, the storage modulus is high under low strain conditions, and under high strain conditions, viscosity dominates and the storage modulus becomes small. This low strain (specifically, a strain of 1×10 －５ ~5 x 10 －３ By setting the maximum storage modulus at 10,000 Pa to 50,000 Pa, the grease composition exhibits adequate shape stability even during high-speed rotation, preventing grease leakage and suppressing heat generation in the bearing. For example, if the storage modulus exceeds 50,000 Pa, the fluidity decreases and heat generation becomes more likely. The storage modulus may also be 10,000 Pa to 30,000 Pa, or 10,000 Pa to 20,000 Pa.

[0031] In the present invention, the storage modulus is measured using a rheometer at a temperature of 25° C. and a frequency of 1 Hz. As the rheometer, it is preferable to use a rheometer having a parallel plate type cell as shown in Fig. 2. Specific measurement conditions are shown in the Examples.

[0032] The base oil used in the grease composition of the present invention has a kinematic viscosity at 40°C (in the case of a mixed oil, the kinematic viscosity of the mixed oil) of 40 mm ２ From the viewpoint of suppressing heat generation in the bearing, the kinematic viscosity is less than 35 mm ２ / s is preferably less than 30 mm ２ On the other hand, the kinematic viscosity is preferably less than 10 mm / s. ２ / s or more is preferable, and 15 mm ２ / s or more is more preferable. ２ If the viscosity is less than 16 mm / s, the base oil tends to evaporate. ２ / s ~ 22 mm ２ / s.

[0033] The base oil can be any oil normally used in rolling bearings without any particular limitations. 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 oil, ether oil, silicone oil, fluorine oil, etc. These base oils may be used alone or in combination of two or more.

[0034] Among the above base oils, it is preferable to use an ester oil or a base oil containing an ester oil in view of compatibility with other extreme pressure additives and rust inhibitors contained in the grease.

[0035] The thickener used in the grease composition of the present invention is not particularly limited, and any of the conventional thickeners typically used in the field of greases can be used. For example, soap-based thickeners such as metal soaps and complex metal soaps, and non-soap-based thickeners such as bentone, silica gel, urea compounds, and urea-urethane compounds can be used. Examples of metal soaps include sodium soaps, calcium soaps, barium soaps, aluminum soaps, and lithium soaps. Examples of urea compounds and urea-urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, and diurethane compounds.

[0036] The diurea compound is obtained by reacting a diisocyanate component with a monoamine component. Examples of the diisocyanate component include phenylene diisocyanate and diphenylmethane diisocyanate (MDI). Examples of the monoamine component include alicyclic monoamines such as cyclohexylamine, aromatic monoamines such as p-toluidine, and aliphatic monoamines such as octylamine.

[0037] Among the thickeners described above, aromatic diurea alone or a mixture of aromatic diurea and other compounds is preferred because it is easy to adjust the storage modulus within the desired range. For example, the aromatic diurea compound can be obtained by reacting a diisocyanate component with an aromatic monoamine.

[0038] The thickener is preferably contained in an amount of 5 to 30% by mass based on the total amount of the base oil and the thickener, and more preferably in an amount of 10 to 30% by mass because this makes it easier to adjust the storage modulus within the desired range. In the case of a diurea-based thickener, the thickener may be contained in an amount of 10 to 20% by mass.

[0039] The grease composition of the present invention may further contain other additives within the scope of the present invention, such as antioxidants such as amine-based, phenol-based, and sulfur-based compounds, rust inhibitors such as sulfonates, and oiliness agents such as esters and alcohols.

[0040] The worked penetration (JIS K 2220) of the grease composition is preferably in the range of 200 to 350. If the penetration is less than 200, oil separation is small and lubrication may be poor. On the other hand, if the penetration exceeds 350, the grease becomes soft and tends to leak out of the bearing, which is undesirable.

[0041] In the grease-sealed bearing of the present invention, the amount of grease sealed in is preferably 10% to 40% (volume ratio) of the static space volume in the bearing interior. If it is less than 10% by volume, the amount of grease required for lubrication will be insufficient and the grease will be prone to depletion, while if it exceeds 40% by volume, the increased torque caused by stirring will make it more likely to generate heat. Here, the static space volume refers to the volume of 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 sealed in may be 10% to 30% or may be 10% to 20% of the static space volume in the bearing interior.

[0042] Although the angular contact ball bearing 1 shown in Figure 1 uses a contact seal as the seal member, a non-contact seal may also be used, in which a gap is provided between the seal lip and the opposing raceway ring portion. For example, a seal groove may be provided in the raceway ring portion (e.g., the outer peripheral surface of the inner ring) opposing the seal lip of the seal member, and a simple labyrinth may be formed by the seal lip and seal groove. The grease composition of the present invention has a storage modulus within a specified range, and can prevent grease leakage even in a non-contact seal.

[0043] As the rolling bearing of the present invention, in addition to the angular contact ball bearing shown in FIG. 1, 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, thrust self-aligning roller bearings, etc. can also be used.

[0044] Figure 3 shows an example of a spindle device for a machine tool main spindle that uses the angular contact ball bearing of Figure 1. As shown in Figure 3, 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 of high-speed specification driven by a built-in motor 12 equipped with a stator 13 and a rotor 14 located in the center. The rotating shaft driven by the built-in motor 12 is supported by the angular contact ball bearing 1 and the cylindrical roller bearing 15. The cylindrical roller bearing 15 is composed of an inner ring 16, an outer ring 17, cylindrical rollers 18, and a cage 19.

[0045] The present invention will be specifically explained by way of examples and comparative examples, but is not limited to these examples in any way.

[0046] Examples 1 to 3, Comparative Examples 1 to 7 Ten types of test greases were prepared as shown in Tables 1 and 2. Tables 1 and 2 show the kinematic viscosity at 40°C and the type of thickener for each test grease.

[0047] <Measurement of storage modulus> The storage modulus was measured using a viscoelasticity measuring device (HAAKE MARS). As shown in Figure 2, each test grease G was sandwiched between parallel disk plates 8 with a diameter of 25 mm to a thickness of 1 mm. For each test grease, a periodic strain was applied by vibration by rotating the upper plate 9, and the shear stress as a response was measured. The measurement conditions are as follows: Frequency: 1 Hz, Strain: 1 x 10 －５ ~5 x 10 －３ Temperature: 25℃

[0048] From the formula below, the frequency is 1 Hz and the distortion is 1 x 10 －５ ~5 x 10 －３ The maximum storage modulus G' at 25°C was determined. The results are shown in Tables 1 and 2. G' = (σ ０ / γ ０ ) cos δ where σ ０ denotes the stress at t = 0, and γ ０ indicates the strain at t=0, and δ indicates the phase difference (the shift in response time when strain is applied).

[0049] <Bearing test> The test grease was filled into an angular contact ball bearing 5S-2LA-BNS014CLLB (non-contact seal) with an inner diameter of 70 mm to prepare a test bearing. The test conditions were an ambient temperature of 25°C and a rotational speed of 21,500 min －１ , Maximum surface pressure at stable temperature during operation is 1.8 to 2.0 GPa, dm・n value is approximately 200 × 10 ４ The evaluation was carried out with the amount of grease filled being 15% of the static space volume inside the bearing. The load was a fixed position preload method commonly used in machine tools. Ceramic balls were used as the rolling elements.

[0050] The bearing life was evaluated as the time until the test bearing became too hot to rotate. Note that the bearing life in Tables 1 and 2 was calculated based on the life of Comparative Example 1. The results are shown in Tables 1 and 2.

[0051] Leakage outside the bearing was evaluated by visually inspecting each test bearing after operation. If the result was "yes," it was confirmed that the test grease had leaked from the seal groove. The results are shown in Tables 1 and 2.

[0052]

[0053]

[0054] As shown in Table 1, the kinematic viscosity of the base oil at 40°C is 40 mm ２ The test bearings of Examples 1 to 3, which used grease compositions having a storage modulus of less than 10,000 Pa to 50,000 Pa measured above, exhibited a bearing life that was 3.0 times or more longer than that of Comparative Example 1.

[0055] In contrast, Comparative Examples 1 to 3 and 6 to 7, which had base oil viscosities equivalent to those of Examples 1 to 3, had short lifespans. In Comparative Examples 1 and 2, the bearings generated more heat during operation than in Examples 1 to 3. This is thought to be because the heat generated increased the surface pressure during stable operation, leading to early failure to rotate. The storage modulus of the grease compositions of Comparative Examples 1 and 2 exceeded 50,000 Pa.

[0056] On the other hand, the elastic storage modulus of Comparative Examples 6 and 7 was lower than that of Examples 1 to 3, being 5000 Pa or less. In these cases, leakage to the outside of the bearing was confirmed, and the leakage of the test grease resulted in a short life. Since the amount of grease enclosed was only 15% of the static space volume inside the bearing, the impact of leakage is thought to be particularly large.

[0057] From the above results, it is preferable that the maximum storage modulus is 10,000 Pa to 50,000 Pa from the viewpoint of preventing grease leakage outside the bearing under high speed rotation conditions and suppressing heat generation in the bearing.

[0058] In addition, even if the storage modulus satisfies this range, if the kinematic viscosity of the base oil at 40°C is 40 mm ２ / s or more, the grease compositions had a short life (Comparative Examples 4 and 5). These grease compositions have a high viscosity, so they tend to starve, and compared to low viscosity products, they generate more heat and the surface pressure increases, which is thought to have led to early failure of rotation. ２ / s or less is preferred.

[0059] The grease composition of the present invention is suitable for use as a grease for rolling bearings used under high-speed rotation conditions, specifically, as a grease for rolling bearings in spindle devices for machine tool main shafts, because it prevents leakage of grease outside the bearing, allows operation with low temperature rise, and has a long life, even under high-speed rotation conditions.

[0060] REFERENCE SIGNS LIST 1 angular contact ball bearing 2 inner ring 3 outer ring 4 balls (rolling elements) 5 cage 6 sealing 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 rollers 19 cage

Claims

1. A grease composition that is enclosed in a rolling bearing and contains a base oil and a thickener, wherein the kinematic viscosity of the base oil at 40°C is 40mm ２ / s, and the grease composition has a maximum storage modulus of 10,000 Pa to 50,000 Pa at a temperature of 25°C and a frequency of 1 Hz.

2. The kinematic viscosity of the base oil at 40°C is 15 mm ２ / s or more 30mm ２ 2. The grease composition according to claim 1, wherein the viscosity of the grease composition is less than 1 / s.

3. A grease composition according to claim 1, characterized in that the thickener is a urea compound or barium soap, and the content of the thickener relative to the total amount of the base oil and the thickener is 5% by mass to 30% by mass.

4. The kinematic viscosity of the base oil at 40°C is 15 mm ２ / s or more 30mm ２ / s, the thickener is a diurea compound obtained by reacting a diisocyanate component with a monoamine component, the monoamine component contains an aromatic monoamine, and a content of the thickener relative to the total amount of the base oil and the thickener is 10% by mass to 20% by mass.

5. A rolling bearing comprising an inner ring and an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, a cage that holds the rolling elements, a grease composition that is sealed in the bearing space between the inner ring and the outer ring, and a seal member that seals the bearing space, wherein the grease composition is the grease composition defined in claim 1.

6. A grease-filled bearing according to claim 5, wherein said rolling elements are made of ceramics.

7. The rolling bearing has a dm-n value of 100 x 10 ４ 6. A grease-sealed bearing according to claim 5, which is a bearing for a main spindle of a machine tool used as described above.

8. A grease-sealed bearing according to claim 5, characterized in that the sealing member has one end fixed to one of the inner and outer rings and the seal lip portion at the other end facing the other ring without contacting it, and the amount of grease sealed in the rolling bearing is 10% to 30% of the static space volume within the bearing.

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