JP7350608B2 - Grease compositions and rolling ball bearings - Google Patents

Description

The present invention relates to a grease composition and a rolling ball bearing containing the grease composition.

A lubricating grease composition is sealed inside the rolling ball bearing for the purpose of reducing rolling friction and sliding friction. Rolling ball bearings filled with grease compositions have a long life, do not require an external lubrication unit, and are inexpensive, so they are often used in general-purpose applications such as automobiles and industrial equipment.

Bearing torque (also referred to as rotational torque) in rolling ball bearings is an important characteristic for products, and lower torque is required from the viewpoint of energy and resource conservation. Grease behavior such as channeling and churning is involved in the rotational torque of rolling ball bearings. In the case of channeling, the grease is swept away during rotation, reducing the amount of grease adhering to the rolling element surfaces and raceway surfaces, which tends to result in low torque. On the other hand, in the case of churning, the grease that has been scraped away by rotation returns to the raceway surface, so that the amount of grease adhering to the rolling element surface and raceway surface is always large, which tends to result in high torque. Therefore, it is desired to develop a grease that exhibits channeling behavior.

For example, Patent Document 1 discloses that the grease composition contains a base oil and a thickener, the thickener is lithium 12-hydroxystearate, and the mass ratio of the thickener to the total mass of the grease composition is 15% or less. A grease composition having a yield stress of 2 kPa or more is disclosed. In Patent Document 1, the channeling property is improved by increasing the yield stress, and the torque is reduced.

JP2013-23644A

Incidentally, in recent years, the usage conditions of rolling ball bearings have become more severe, and there is a demand for rolling ball bearings that have low torque and have excellent bearing life even under high load conditions and high temperature conditions. Although the rotational torque is evaluated in Patent Document 1, the bearing life is not considered, and there is room for improvement in terms of long-term use.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a grease composition that has low torque and excellent bearing life, and a rolling ball bearing containing the same.

The grease composition of the present invention is a grease composition containing a base oil and a thickener, and the grease composition has a shear stress of 0.2 Pa or less at a shear rate of 20,000 s ^-1 at 25°C, and , characterized in that the yield stress at 25°C measured by dynamic viscoelasticity measurement using a rheometer is 1600 Pa or more.

The thickener is a diurea compound obtained by reacting a diisocyanate component and a monoamine component, and is characterized in that the monoamine component is an aliphatic monoamine and an alicyclic monoamine.

It is characterized in that the alicyclic monoamine includes dicyclohexylamine.

The base oil is characterized in that it contains a synthetic hydrocarbon oil as a main component and has a kinematic viscosity of 30 mm ² /s to 80 mm ² /s at 40°C.

The rolling ball bearing of the present invention includes an inner ring and an outer ring, balls interposed between the inner ring and the outer ring, a resin cage that holds the balls, and a grease composition sealed around the balls. A rolling ball bearing characterized in that the above-mentioned grease composition is the grease composition of the present invention.

The above rolling ball bearing is characterized in that it is used in a rotational speed range of 2000 min ^-1 or less.

The grease composition of the present invention contains a base oil and a thickener, has a shear stress of 0.2 Pa or less at a shear rate of 20,000 s ^-1 at 25°C, and a yield stress of 1,600 Pa or more. The grease has a low viscosity in the shear speed range, and the channeling properties of the grease are improved, so rotational torque can be reduced and the bearing life is also excellent.

The rolling ball bearing of the present invention includes an inner ring and an outer ring, balls interposed between the inner ring and outer ring, a resin retainer that holds the balls, and a grease composition of the invention sealed around the balls. Since the grease is used at a rotational speed of 2000 min ^-1 or less, the viscosity of the grease is particularly low in the rotational speed range used, contributing to a reduction in rotational torque. Furthermore, excellent bearing life can be obtained even under high load conditions and high temperature conditions.

1 is a sectional view showing an example of a rolling ball bearing of the present invention. FIG. 2 is a partial perspective view of the cage in FIG. 1. FIG. It is a figure which shows the state of the grease in a cage|retainer pocket. FIG. 1 is a schematic diagram showing an example of a rheometer.

In grease lubrication of rolling ball bearings, it is important to reduce the shear resistance of the grease interposed between the balls and cage pocket surfaces in order to reduce torque. In order to reduce this shear resistance, the inventors of the present invention have conducted extensive studies focusing on grease viscosity (viscosity) and channeling properties, and as a result, have determined that the shear stress and yield stress at a predetermined shear rate are within a predetermined range. It was discovered that it exhibits low torque and long life. The present invention is based on such knowledge.

An example of the rolling ball bearing of the present invention will be explained based on FIGS. 1 and 2. FIG. 1 is a partial sectional view of a deep groove ball bearing incorporating a resin crown-shaped cage as a rolling ball bearing of the present invention, and FIG. 2 is a partial perspective view of this crown-shaped cage. As shown in FIG. 1, in the deep groove ball bearing 1, an inner ring 2 having a raceway surface 2a on the outer peripheral surface and an outer ring 3 having a raceway surface 3a on the inner peripheral surface are arranged concentrically. A plurality of balls 4 are interposed between the raceway surface 2a of the inner ring and the raceway surface 3a of the outer ring. The plurality of balls 4 are held by a crown-shaped holder 5. Further, the deep groove ball bearing 1 includes an annular seal member 6 provided at openings at both axial ends of the inner and outer rings, and a bearing inner space composed of an inner ring 2, an outer ring 3, a cage 5, and a seal member 6. It is lubricated by a grease composition 7 encapsulated in. This grease composition 7 corresponds to the grease composition of the present invention.

As shown in FIG. 2, the crown-shaped retainer 5 has a pair of opposing retaining claws 8 formed at a constant pitch in the circumferential direction on the upper surface of the annular main body 5a, and the opposing retaining claws 8 are moved close to each other. In addition, a pocket 9 for holding a ball, which is a rolling element, is formed between the holding claws 8. A flat portion 10 serving as a reference surface for rising the holding claws 8 is formed between the back surfaces of the holding claws 8 that are adjacent to each other and are formed on the edges of the adjacent pockets 9 . Inside the bearing, in this pocket 9, when grease enters the pocket gap between the retainer 5 and the balls (churning), it becomes susceptible to the shear resistance of the grease.

The grease composition of the present invention contains a base oil and a thickener, and various additives are added as necessary. In particular, the grease composition has a shear stress of 0.2 Pa or less at a shear rate of 20,000 s ^-1 at 25°C, and a yield stress at 25°C measured by dynamic viscoelasticity measurement using a rheometer. It is characterized by a pressure of 1600 Pa or more.

The yield stress of the grease composition is measured by a dynamic viscoelastic measurement method based on JIS K 7244 using a rheometer. Specifically, by changing the rocking angle with a rheometer under predetermined conditions, the storage elastic modulus G' representing the elastic component of the grease and the loss elastic modulus G'' representing the viscous component are actually measured, and the ratio (tan δ The shear stress value at which G″/G′) becomes 1 is defined as the yield stress. Note that the storage elastic modulus G' corresponds to the energy that can be stored elastically within the external force that the grease composition receives, and the loss elastic modulus G'' corresponds to the energy that can be stored elastically within the external force that the grease composition receives. This corresponds to the energy dissipated as .

The conditions for dynamic viscoelasticity measurement are preferably a frequency of 1 Hz and a temperature of 25°C. Further, as the rheometer, it is preferable to use a rheometer having a parallel plate type cell. This rheometer is suitable for measuring the yield stress of a grease composition because it is capable of applying a constant stress.

Here, FIG. 3 shows a photograph of the state of grease adhesion inside the bearing, taken with an X-ray CT scanner using a model bearing. In FIG. 3, the inner and outer rings, balls, retainer, and seals are made of resin so that X-rays can pass through them. Furthermore, 5% by mass of tungsten was added to the grease as a tracer to facilitate contrast between the grease and the members. This bearing was operated while measuring torque, and a churning product (torque 13 Nmm) that stopped initially (5 hours) and a channeling product (torque 5 Nmm) that stopped for a long time (23 hours) were observed. As shown in FIG. 3, it can be seen that there is a large difference in the amount of grease in the pocket gap between the retainer and balls during channeling and churning. That is, while no grease is present in the pocket gap during channeling, the presence of grease during churning provides shear resistance.

Since the grease composition of the present invention has a yield stress of 1600 Pa or more at 25°C, the grease composition is scraped through during rotation, and the grease composition that was once repelled from the raceway surface is positioned and difficult to be introduced into the raceway surface. Become. For example, in a ball bearing in which the inner ring rotates, the grease composition moves from the raceway surface to the inner diameter surface of the outer ring due to centrifugal force, and is deposited there as a lump. As a result, a channeling state occurs in which the amount of grease adhering to the ball surface and raceway surface decreases, and the rotational torque decreases. Note that the bearing is lubricated by the deposited grease composition or its separated oil flowing back to the raceway surface.

In the present invention, the yield stress of the grease composition is preferably 1700 Pa or more. The higher the yield stress, the easier it is to maintain a stable channeling state by preventing the grease composition from moving to the raceway surface due to driving forces such as vibration and temperature rise. Further, it is possible to suppress the bearing life from being shortened due to heat generated due to an increase in rotational torque. On the other hand, the upper limit of the yield stress is, for example, 5000 Pa, preferably 3000 Pa. This is because if the yield stress becomes high, it becomes difficult to supply lubricating components, and the bearing life may be shortened. Preferably, the yield stress of the grease composition is 1700 Pa to 2500 Pa.

Subsequently, the shear stress of the grease composition can be calculated using a rheometer. It is preferable to use a rheometer having a cone-plate type cell. An overview of such a rheometer is shown in FIG. As shown in FIG. 4, the rheometer 11 is composed of a cone plate type cell 12 and a horizontal disk plate 13, and the cell 12 and plate 13 are arranged so that they touch at one point (with a slight gap). and a sample of grease 14 is placed between them. In this rheometer, the shear rate applied to the grease 14 is the same at any position, independent of the distance from the cell center. The conditions for rheology measurements are: (1) rotational speed dependence at constant temperature and constant rotation, (2) vibration frequency dependence at constant temperature and constant shear strain, and (3) dynamic viscoelastic shear at constant frequency. There is stress dependence, etc. For example, under the condition (1), a value obtained when the shear stress becomes constant after rotation at a constant temperature and in a constant direction for a predetermined period of time is used.

Furthermore, the shear stress at a shear rate outside the measurement range of the rheometer can be calculated (predicted) using Herschel-Bulkley's equation, which is a general flow equation for non-Newtonian fluids. The Herschel-Berkeley equation is expressed by the following equation.

The shear speed is the shear speed applied to the grease present in the pocket gap between the balls and the retainer, and can be calculated from the set bearing rotation speed. For example, assuming that the balls are located at the center of the cage pocket, when the inner ring of the ball bearing (6204) is rotated at 1800 min ^-1 to 10000 min ^-1 , the shear rate of the grease in the pocket is 24000 s ^-1 to 130000 s ^-1 becomes. Further, the yield stress and each constant can be specified based on evaluation of the rheological properties of grease using a rheometer. The above formula calculates the viscosity of the grease composition at a predetermined shear rate. Further, the viscosity at a shear rate outside the measurement range can be extrapolated from the calculated viscosity. By multiplying the obtained viscosity by the shear rate, the shear stress at the shear rate can be calculated.

In the above grease composition, the shear stress at a shear rate of 20,000 s ^-1 at 25° C. is preferably 0.1 Pa or less, and more preferably 0.08 Pa or less. The lower limit of the shear stress is, for example, 0.05 Pa.

The grease composition of the present invention is not particularly limited as long as it contains a base oil and a thickener, and the above-mentioned yield stress and shear stress are within predetermined ranges. As the base oil, those commonly used in the field of grease can be used. As the base oil, for example, highly refined oil, mineral oil, ester oil, ether oil, synthetic hydrocarbon oil (PAO oil), silicone oil, fluorine oil, and mixed oils thereof can be used. Note that, as shown in the examples below, containing ester oil in the base oil may lead to a decrease in life time and an increase in bearing torque, so it is preferable to use a base oil excluding ester oil. Ester oil is a compound that has an ester group in its molecule and is liquid at room temperature. Examples include polyol ester oil, phosphate ester oil, polymer ester oil, aromatic ester oil, carbonate ester oil, and diester oil. It will be done.

Among the above base oils, base oils containing PAO oil as a main component are preferred. In this case, the content of the PAO oil is 50% by mass or more, preferably 80% by mass or more, based on the entire base oil (mixed oil). In particular, it is preferable to use a base oil consisting only of PAO oil (100% PAO oil).

The kinematic viscosity of the base oil (in the case of a mixed oil, the kinematic viscosity of the mixed oil) is, for example, 30 mm ² /s to 100 mm ² /s, and 30 mm ² /s to 80 mm ² /s at 40°C. is preferred. The lower the kinematic viscosity of the base oil, the more suitable it is from the viewpoint of reducing torque, but this may shorten the life of the bearing. Therefore, by setting the speed to 30 mm ² /s to 80 mm ² /s, it is easier to achieve both low torque and long life. More preferably, the kinematic viscosity at 40° C. is 30 mm ² /s to 50 mm ² /s.

The thickener for the grease composition of the present invention is not particularly limited, and any commonly used thickener in the field of grease can be used. For example, soap-based thickeners such as metal soaps and composite 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 soap, calcium soap, aluminum soap, lithium soap, etc., and examples of urea compounds and urea-urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, and diurethane compounds. Among these, diurea compounds are preferred because of their excellent high-temperature durability.

A diurea compound is obtained by reacting a diisocyanate component and a monoamine component. Among the diurea compounds, aliphatic/alicyclic diurea compounds are particularly preferred. The aliphatic/alicyclic diurea compound is obtained using an aliphatic monoamine and an alicyclic monoamine as monoamine components. Here, the blending ratio (for example, mol %) of aliphatic monoamine and alicyclic monoamine is not particularly limited, but it is preferable that the alicyclic monoamine is larger than the aliphatic monoamine. Specifically, it is preferable that the alicyclic monoamine be 60 mol % or more based on the total monoamine.

Examples of the diisocyanate component constituting the diurea compound include phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate (MDI), octadecane diisocyanate, decane diisocyanate, hexane diisocyanate, and the like. Examples of aliphatic monoamines include hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine, stearylamine, and oleylamine. Examples of the alicyclic monoamine include cyclohexylamine and dicyclohexylamine, and it is particularly preferable to include dicyclohexylamine.

Furthermore, as the diurea compound, an alicyclic diurea compound using an alicyclic monoamine, an aliphatic diurea compound using an aliphatic monoamine, and an aromatic diurea compound using an aromatic monoamine (p-toluidine, etc.) can also be used. .

Base grease is obtained by blending a thickener with base oil. Base grease using a diurea compound as a thickener is produced by reacting a diisocyanate component and a monoamine component in a base oil. The proportion of the thickener in the entire grease composition is, for example, 5% to 40% by mass, preferably 10% to 30% by mass, and more preferably 10% to 20% by mass. If the content of the thickener is less than 5% by mass, the thickening effect will be reduced and it will be difficult to form a grease. Moreover, if it exceeds 40% by mass, the resulting base grease will become too hard, making it difficult to obtain the desired effect.

Furthermore, known additives can be added to the grease composition as needed. Examples of additives include extreme pressure agents such as organic zinc compounds and organic molybdenum compounds, antioxidants such as amine-based, phenol-based, and sulfur-based compounds, anti-wear agents such as sulfur-based and phosphorus-based compounds, and polyhydric alcohols. Examples include rust preventive agents such as esters, friction reducers such as molybdenum disulfide and graphite, and oil-based agents such as esters and alcohols.

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

The above-mentioned grease composition is encapsulated in the rolling ball bearing of the present invention. Since this grease composition has a low viscosity at a shear rate of 20,000 s ^-1 , it is suitable for bearings used in a relatively low rotational speed range as rolling ball bearings. The operating speed range of this rolling ball bearing is preferably 2000 min ^-1 or less, more preferably 1600 min ^-1 or less.

In addition, as shown in the examples below, the rolling ball bearing of the present invention exhibits low torque under high load conditions and low rotation conditions, and also has excellent high temperature durability. Suitable for rolling ball bearings used in conditions. For example, it can be used in bearings for low-speed motors used in high-temperature environments, segment roll bearings in continuous casting equipment, and the like.

Regarding the rolling ball bearing of the present invention, although a deep groove ball bearing is shown in FIG. 1 above, the form of the rolling ball bearing is not limited to this. For example, it may be applied to angular contact ball bearings and automobile hub bearings that use balls as rolling elements. The rolling ball bearing of the present invention has a very wide range of industrial applications and can be used in various types of equipment.

For Example 1 and Comparative Examples 1 to 5, grease compositions were obtained by mixing the base oil and the thickener in the formulation composition (% by mass) shown in Table 1. Shear stress and yield stress were calculated using each of the obtained grease compositions.

(1) Shear stress Using a cone-plate type rheometer (diameter 20 mm, cone angle 1°), the shear rate was increased from 1 s ^-1 to 8000 s ^-1 at a temperature of 25 °C and a frequency of 1 Hz for each grease composition. I let it happen. The shear stress that resulted in a steady flow at each shear rate was determined, and the grease viscosity at each shear rate was calculated using the Herschel-Berkeley equation described above. Using the obtained viscosity of each grease, the viscosity of the grease at a shear rate of 20,000 s ⁻¹ was extrapolated and calculated. The calculated grease viscosity was multiplied by the shear rate of 20,000 s ^-1 to calculate the shear stress at the shear rate of 20,000 s ^-1 . The results are shown in Table 1.

(2) Yield Stress Using a parallel plate type (gap 1 mm) rheometer having an upper plate and a lower plate, dynamic viscoelasticity measurements were performed on each grease composition according to the following conditions. Specifically, a grease composition is sandwiched between an upper plate and a lower plate, periodic shear stress due to vibration is applied to the grease composition, and the storage modulus G' and loss modulus G'' are determined from the response. The shear stress value at the point where the obtained storage modulus G' and loss modulus G'' overlapped was defined as the yield stress. The results are shown in Table 1.
Shear stress: Increases from 10Pa to 3000Pa Measurement frequency: 1Hz
Measurement temperature: 25℃
Each plate: 25mm in diameter

(3) High-temperature grease life test The grease composition obtained above was sealed in a deep groove ball bearing (TS3-6204ZZC3 manufactured by NTN Corporation) to prepare bearings for high-temperature grease life test. Each of the obtained bearings was rotated at a rotational speed of 10,000 min ^-1 under conditions of a temperature of 150° C., an axial load of 67 N, and a radial load of 67 N, and the time until seizure occurred was measured. Grease life time of 3000 hours or more was considered acceptable. The results are shown in Table 1.

(4) Bearing Torque Test The grease composition obtained above was encapsulated in a deep groove ball bearing (6204T2LLBC3 manufactured by NTN Corporation) having a resin cage to prepare bearings for a bearing torque test. Each of the obtained bearings was rotated at a rotational speed of 1600 min ⁻¹ , a normal temperature, an axial load of 8 kgf, and a radial load of 0 kgf. In the test, it was rotated for 30 minutes, and the average value for 20 to 30 minutes was taken as the torque value (mNm). A torque value of 30 or less was considered acceptable. The results are shown in Table 1.

As shown in Table 1, the grease composition (Example 1) which has a shear stress of 0.2 Pa or less at a shear rate of 20,000 s ^-1 at 25°C and a yield stress of 1,600 Pa or more has low torque and long service life. and passed both tests. On the other hand, in Comparative Examples 1 and 2, each physical property value (shear stress and yield stress) was not significantly different from Example 1, but the high temperature grease life test showed a short life. Furthermore, in Comparative Examples 4 and 5, although the kinematic viscosity of the base oil was comparable to that of Example 1, the yield stress of the grease composition was low, and the torque test showed relatively high torque.

From the above, the grease composition according to the present invention has a low viscosity and a high yield stress value in a predetermined shear rate range, so the grease shear resistance between the balls and the cage pocket surface is reduced, and the torque is reduced. This not only makes it possible to realize longer lifespans, but also contributes to longer lifespans.

Since the grease composition of the present invention has low torque and excellent service life, it has an extremely wide range of industrial applications and can be used in various types of equipment. It is particularly suitable for rolling ball bearings used under high load and high temperature conditions.

1 Deep groove ball bearing 2 Inner ring 3 Outer ring 4 Balls 5 Cage 6 Seal member 7 Grease composition 8 Holding claw 9 Pocket 10 Flat part 11 Rheometer 12 Cone plate type cell 13 Horizontal disk plate 14 Grease

Claims (4)

A grease composition comprising a base oil and a thickener,
The grease composition has a shear stress of 0.2 Pa or less at a shear rate of 20,000 s ^-1 at 25°C, and a yield stress of 1,600 Pa or more at 25°C as measured by dynamic viscoelasticity measurement using a rheometer. and
The base oil contains a synthetic hydrocarbon oil as a main component, and has a kinematic viscosity at 40° C. of 30 mm ² /s to 80 mm ² /s,
The thickener is a diurea compound obtained by reacting a diisocyanate component and a monoamine component, and the monoamine component is an aliphatic monoamine and an alicyclic monoamine. A grease composition characterized in that the agent is contained in an amount of 10% by mass to 30% by mass . The grease composition according to claim 1 , wherein the alicyclic monoamine includes dicyclohexylamine. A rolling ball bearing comprising an inner ring and an outer ring, balls interposed between the inner ring and the outer ring, a resin cage holding the balls, and a grease composition sealed around the balls,
A rolling ball bearing characterized in that the grease composition is the grease composition according to claim 1 or 2 . The rolling ball bearing according to claim 3 , wherein the rolling ball bearing is used in a rotational speed range of 2000 min ^-1 or less.

Images