JP2005132879A - Urea lubricating grease composition and electric power steering apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a urea lubricating grease composition exhibiting low and stable torque characteristics from low to high temperatures and affording long-lived lubrication without causing oil film exhaustion even at the high temperatures and to provide an electric power steering apparatus using the composition as a lubricant. <P>SOLUTION: The urea lubricating grease composition comprises (a) an alkyldiurea compound having 600-700 average molecular weight in which 25-50 mol% of the whole alkyl groups is an unsaturated component and the total amine value of a primary amine to be a raw material is within the range of 250-350 as a thickener, (b) a base oil consisting essentially of a synthetic hydrocarbon oil having ≤-40°C pour point and (c) a mixture composed of an oil-soluble organomolybdenum complex, an oil-soluble organozinc compound of dithiocarbamic acid, an oil-soluble organozinc compound of dithiophosphoric acid and an inorganic zinc compound. (1) The amount of the component (a) is 5-15 wt.% based on the total amount and (2) the amount of a component which is the synthetic hydrocarbon oil in the component (b) is ≥80 wt.%. (3) The amount of the component (c) is 1-7 wt.% based on the total amount. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention shows a stable low-torque characteristic from low temperature to high temperature, a long-life urea-based lubricating grease composition capable of maintaining a sufficient oil film even at high temperatures, and the use of this urea-based lubricating grease composition as a lubricant. The present invention relates to an electric power steering apparatus.

  In recent years, urea grease has been used in a wide range of fields because it has a higher dropping point than general-purpose lithium soap greases that use lithium soap as a thickener, and has excellent thermal stability, wear resistance, and lubricity. In the automobile industry, the required values of heat resistance, wear resistance, and friction characteristics of various automobile parts such as CVJ (constant velocity joint) are high, and cases where the superior performance of urea grease is applied gradually is increasing. However, the technological progress of automobiles is remarkable, and the required values of individually configured automobile parts are increasing year by year, so that the present situation is not satisfied.

  In particular, the electric power steering device for automobiles has been remarkably technologically innovative, and the device, which was initially used in some solar cars and mini cars, has recently been installed in a very wide range from small passenger cars to medium-sized passenger cars. In addition, the number of units installed is a field that is growing twice a year.

  Here, the electric power steering device of the automobile will be described in detail. Currently, the mainstream of the automobile power steering device is a hydraulic type. However, in the case of the hydraulic type, since hydraulic oil (power steering fluid) is used, environmental problems are directly addressed. In addition, it is necessary to consider, and the hydraulic pump for generating the hydraulic pressure is driven by the power from the engine and is always driven (even if the steering wheel is not operated), resulting in a loss of engine power. Therefore, it is a cause of worsening fuel consumption.

On the other hand, in the case of an electric power steering device, an electric motor is used as a power assist power source, and the electric motor only needs to be driven by the control device when power assist is necessary. Due to the electricity that is generated, there is very little power loss of the engine, the fuel saving effect is great, and the energy consumption is also very low compared to the hydraulic type.
However, the current electric power steering device has a weak output compared to the hydraulic power steering device, so the electric motor performance is improved and the friction of individual components is reduced as much as possible. It is important to reduce the burden on
Therefore, it is essential that the grease used for these parts has a low and stable friction torque characteristic.

  In addition, since automobiles are used everywhere in the world, electric power steering devices can also be used at extremely cold temperatures of around -40 ° C or at high temperatures of 100 ° C or higher (radiation heat from the engine room + radiation heat from the road surface). Designed and manufactured in consideration of the above, low and stable torque characteristics are obtained over a wide temperature range from low to high temperatures. It has been demanded.

Electric power steering devices can be broadly classified into three types: one is a column assist type electric power steering device, the second is a pinion assist type electric power steering device, and the third is a rack assist type electric power steering device. It is.
The reduction gears of the column assist type electric power steering device and the pinion assist type electric power steering device are usually composed of a metal worm gear and a resin worm wheel, and the power of the electric motor is transmitted to the steering via these gears. And power assist. The contact portion of the gear takes the form of sliding friction between the resin and the metal, and the lubricant is applied to this portion.

Patent Literatures 1 to 6 are literatures on prior art column and pinion assist type lubricants for electric power steering devices. That is,
Patent Document 1 includes (a) a thickener, (b) a base oil having a pour point of −40 ° C. or lower, (c) an organic molybdenum compound, (d) melamine cyanurate, (e) polytetrafluoroethylene, And (f) an automotive steering grease composition comprising molybdenum disulfide. The automotive steering grease composition relates to a grease composition in which the above-described grease composition exhibits appropriate lubricity particularly in a gear meshing portion such as a rack / pinion portion or a hypoid gear of a pinion assist type electric power steering. However, the composition of the grease composition is completely different from the grease composition of the present invention.
Patent Document 2 describes an electric power tearing device using a grease with improved lubrication durability at a high temperature while maintaining a low starting rotational torque at a low temperature as a lubricant. It is described that it is an oil, the thickener is selected from lithium complex soaps or urea compounds, and the lubrication improver is selected from solid lubricants and oily substances. The electric power steering device includes an electric motor for generating a steering assist force and a reduction device that reduces the rotation speed by a gear mechanism connected to a rotation shaft of the motor, and at least one of reduction gears of the gear mechanism. One is made of synthetic resin, and the synthetic resin gear is disclosed as an electric power steering device lubricated with grease, but the grease composition has a description of a urea-based thickener in the claim, Although only in this part, the specific composition and effect are not disclosed at all.
Patent Document 3 describes a lubricating grease composition including a base oil and a thickener, in which a fluororesin powder is blended with the base oil, and a speed reducer for an electric power steering apparatus. It is shown that it is used for such as.
Patent Document 4 describes a lubricating grease composition containing a base oil and a thickener, in which Li stearate and Li hydroxystearate are used in combination as a thickener. It is shown to be used for a reduction gear of a power steering apparatus.
The reduction gears of the electric power steering apparatus described in Patent Literature 3 and Patent Literature 4 and the specific lubrication sites thereof are reduction gears using a worm wheel made of synthetic resin such as polyamide resin. It is disclosed that the role of the lubricating grease composition that contributes to reducing friction at the lubrication site in the sliding portion (friction surface) between the worm wheel and the metal worm shaft is important. Is completely different from the grease composition of the present invention.
Patent Document 5 describes a grease composition for resin lubrication in which montan wax is added to grease containing a thickener and a base oil. It is disclosed that the speed reducer of the electric power steering device and the specific lubrication site are a worm wheel gear made of resin (polyamide) and a speed reduction mechanism part of a steel worm gear, but the grease composition is claimed. There is a description of a urea-based thickener in the section, and although only in this part, it matches, but since it is a montan wax as an essential component or a grease composition for resin lubrication, it is completely different from the grease composition of the present invention. Different.
Patent Document 6 describes a lubricating grease composition containing a base oil and a thickener, and a lubricating grease composition containing a polyethylene oxide wax, and is used for a reduction gear of an electric power steering device or the like. It has been shown. The reduction gear of the electric power steering device and the specific lubrication site are the friction surfaces of the reduction gear consisting of a worm wheel made of synthetic resin and a metal worm shaft, and the grease composition is the grease composition of the present invention. It is completely different from things.
The electric power steering apparatus according to the present invention is an apparatus shown in the attached drawing disclosed in Patent Document 7, that is, a rack assist type electric power composed of ball screw structures 16a and 19c and bearings of a first bearing 21 and a second bearing 22. Although it is a steering device, this device assists power in the axial direction with a ball screw connected to a rack shaft. Since this ball screw structure is similar to the ball screw structure mounted on machine tools, etc., lithium grease that is frequently used in these machine tools has been used as the lubricating grease for this part. .
However, as the demand for mounting on automobiles has been increasing, it has been widely used in the process of remarkable progress in other performances such as improvement of assist power and durability and low torque performance of electric power steering devices. However, with lithium-based greases, satisfactory durability cannot be obtained, stable steering characteristics from high temperatures to low temperatures cannot be fully exhibited, and there are also problems with low torque characteristics in lower temperature environments. It was.

JP 2001-64665 A JP 2002-308125 A JP 2002-363589 A JP 2002-363590 A JP 2002-371290 A JP 2003-3185 A JP 2003-158856 A

  The present invention shows a urea-based lubricating grease composition that exhibits low and stable torque characteristics from low temperatures to high temperatures, and that does not cause oil film breakage even at high temperatures and provides long-life lubrication, and the composition as a lubricant. It is an object of the present invention to provide an electric power steering device used.

The present inventors have selected urea grease as a grease that can effectively exhibit heat resistance, wear resistance, and friction characteristics that cannot be obtained with conventional lithium grease, and accurately grasp the environment and required performance of an automobile. In addition, based on the knowledge obtained through estimation of the mechanism of lubrication and the results of numerous experiments and prototype studies, the urea-based lubricating grease composition of the present invention has a low and stable torque from low temperature to high temperature in its application. It has become clear that the grease exhibits properties and does not cause oil film breakage even at high temperatures and can provide a long-life lubrication, thus leading to the present invention. That is,
The first of the present invention is (a) an alkyldiurea compound having an average molecular weight of 600 to 700 as a thickener, and 25 to 50 mol% of the total alkyl groups are unsaturated components, and a primary material used as a raw material A diurea compound in which the total amine number of the amine is in the range of 250 to 350;
(B) a base oil mainly composed of a synthetic hydrocarbon oil having a pour point of −40 ° C. or lower,
(C) As an additive, comprising an oil-soluble organic molybdenum complex, an oil-soluble organic zinc compound of dithiocarbamic acid, an oil-soluble organic zinc compound of dithiophosphoric acid, and an inorganic zinc compound,
(1) The component (a) is 5 to 15% by weight of the total amount,
(2) The component which is an internally synthesized hydrocarbon oil of component (b) is 80% by weight or more,
(3) The component (c) is 1 to 7% by weight of the total amount,
The present invention relates to a urea-based lubricating grease composition.
A second aspect of the present invention relates to an electric power steering apparatus using the urea-based lubricating grease composition according to claim 1 as a lubricant.
A third aspect of the present invention relates to the electric power steering apparatus according to claim 2, wherein the lubricant is used in a bearing apparatus using rolling of a ball.

In the present invention, when the average molecular weight of the thickener (a) is less than 600 and the average molecular weight exceeds 700, optimum electric power intervention cannot be obtained in the electric power steering apparatus, and stable torque characteristics are obtained. Not enough.
In addition, when the unsaturated component is less than 25 mol% of all alkyl groups, an appropriate oil effect cannot be obtained in the electric power steering apparatus, and stable torque characteristics cannot be sufficiently obtained. When it exceeds 50 mol%, it is difficult to ensure sufficient heat resistance in the electric power steering apparatus, and the lifetime is expected to be shortened. Furthermore, if the total amine value of the primary amine is outside of 250 to 350, it becomes difficult to obtain optimum grease intervention, proper oily effect and sufficient heat resistance in the electric power steering apparatus, and stable torque It is expected that the characteristics cannot be sufficiently obtained or the life is shortened.

Examples of the synthetic hydrocarbon oil as component (b) in the present invention include poly α-olefin, bolibene, ethylene and α-olefin co-oligomer, etc., all of which have a pour point of −40 ° C. or less. The optimal effect is demonstrated in the steering device. When lubricating oil having a pour point higher than −40 ° C. is used for grease, the grease itself becomes hard at low temperatures, and a predetermined steering torque characteristic at low temperatures cannot be obtained in this apparatus.
The base oil used in the grease composition uses these synthetic hydrocarbon oils as a main component, and the characteristics of the synthetic hydrocarbon oil are that the pour point is low and the fluidity can be maintained even at low temperatures. Because it has good oxidation stability and excellent viscosity temperature characteristics (high VI), it can maintain a stable oil film with little decrease in viscosity even at high temperatures, and it can be used for materials such as synthetic rubber and synthetic resin. It has excellent features such as no adverse effects such as swelling.
Mineral oil, ester-based synthetic oil, polyglycol-based synthetic oil, silicone-based synthetic oil, or fluorine-based synthetic oil can be used as part of the base oil used in the present grease composition in the steering device. Not suitable for use as a main component. When mineral oil is used as the main component of such base oil, low-temperature properties and heat resistance are insufficient, and ester synthetic oils are concerned with the adverse effects of swelling synthetic rubbers and synthetic resins and reducing hardness. Synthetic oils are not as heat resistant or lubricating as synthetic hydrocarbon oils.
Silicone-based synthetic oils are excellent in heat resistance, but it is difficult to obtain satisfactory lubricity. Fluorine-based synthetic oils are extremely excellent in heat resistance but have poor affinity with thickeners and are extremely expensive. Become.
In addition, the main component in (b) means that the synthetic hydrocarbon oil whose pour point of lubricating oil is -40 degrees C or less contains 80 weight% or more.
In the present invention, if the component (a) is less than 5% by weight of the total amount, the grease becomes too soft and there is a risk of leakage, and if it exceeds 15% by weight, it becomes too hard and the flow resistance increases. Therefore, there is concern about an increase in torque. Moreover, when the component which is an internal synthetic hydrocarbon oil of (b) component is less than 80 weight%, the stable low torque from normal temperature to low temperature cannot be obtained.
Component (c) is 1 to 7% by weight of the total amount, preferably 1.5 to 6% by weight, more preferably 2 to 5% by weight. When the component (c) is less than 1.0% by weight of the total amount, the effect on the catching characteristics of the handle is not recognized, and when it is more than 7.0% by weight, the cost increases and a remarkable effect is obtained. I can't.

  The oil-soluble organic molybdenum complex which is the component (c) of claim 1 of the present invention is an organic molybdenum complex described in Japanese Patent Publication No. 5-66435. A compound that dissolves 5% or more. The dithiocarbamic acid oil-soluble organic zinc compounds include zinc sulfide diethyldithiocarbamate, zinc sulfide dipropyldithiocarbamate, zinc sulfide dibutyldithiocarbamate, zinc sulfide dipentyldithiocarbamate, zinc sulfide dihexyldithiocarbamate, zinc sulfide didecyldithiocarbamate. Zinc sulfide diisobutyldithiocarbamate, zinc sulfide di (2-ethylhexyl) dithiocarbamate, zinc sulfide diamyldithiocarbamate, zinc sulfide dilauryldithiocarbamate, zinc sulfide distearyl dithiocarbamate, zinc sulfide diphenyldithiocarbamate, etc., ditolyldithiocarbamine sulfide Zinc oxide, zinc sulfide dixylyldithiocarbamate, zinc sulfide diethylphenyldithiocarbamate, dipropylphenyldisulfide Zinc ocarbamate, zinc sulfide dibutylphenyl dithiocarbamate, zinc sulfide dipentylphenyldithiocarbamate, zinc sulfide dihexylphenyldithiocarbamate, zinc sulfide diheptylphenyldithiocarbamate, zinc sulfide dioctylphenyldithiocarbamate, zinc sulfide dinonylphenyldithiocarbamate, zinc sulfide Zinc dithiocarbamates such as zinc decylphenyldithiocarbamate, zinc sulfide didodecylphenyldithiocarbamate, zinc sulfide ditetradecylphenyldithiocarbamate, and zinc sulfide dihexadecylphenyldithiocarbamate. In addition, dithiophosphoric acid oil-soluble organic zinc compounds include zinc sulfide diethyldithiophosphate, zinc sulfide dipropyldithiophosphate, zinc sulfide dibutyldithiophosphate, zinc sulfide dipentyldithiophosphate, zinc sulfide dihexyldithiophosphate, sulfide didecyldithiophosphate. Zinc, zinc sulfide diisobutyldithiophosphate, zinc sulfide di (2-ethylhexyl) dithiophosphate, zinc sulfide diamyldithiophosphate, zinc sulfide dilauryldithiophosphate, zinc sulfide distearyl dithiophosphate, zinc sulfide diphenyldithiophosphate, etc. Zinc dithiophosphate, zinc sulfide dixylyl dithiophosphate, zinc sulfide diethylphenyl dithiophosphate, zinc sulfide dipropylphenyl dithiophosphate, zinc sulfide dibutylphenyl dithiophosphate, zinc sulfide dipentylphenyl dithiophosphate, Zinc dihexylphenyl dithiophosphate, zinc sulfide diheptylphenyl dithiophosphate, zinc sulfide dioctylphenyl dithiophosphate, zinc sulfide dinonylphenyl dithiophosphate, zinc sulfide didecylphenyl dithiophosphate, zinc sulfide dododecylphenyl dithiophosphate, sulfide ditetradecyl sulfide Of zinc dithiophosphate, zinc sulfide dihexadecylphenyl dithiophosphate and the like, it refers to a compound that is soluble in synthetic hydrocarbon oil or mineral oil by at least 5% at room temperature. Furthermore, examples of the inorganic zinc compound include zinc carbonate, zinc oxide, zinc pyrophosphate, and zinc phosphate.

  According to the present invention, a urea-based lubricating grease composition that exhibits low and stable torque characteristics from low temperatures to high temperatures, and that does not cause oil film breakage even at high temperatures and can provide long-life lubrication, and the composition as a lubricant. The electric power steering apparatus used can be provided.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention in detail, this invention is not limited at all by these Examples.

Examples 1-4
Base oil and diisocyanate were put into a sealed grease prototype at the blending ratio shown in Table 1, heated to 60 ° C. with stirring, and a raw material in which various amines and base oils were mixed and dissolved was added from a hopper and reacted. In order to complete the reaction, the mixture was heated to 170 ° C. with further stirring and held for 30 minutes, then cooled to 80 ° C., and additives were added at the blending ratios shown in Table 1, and octyldiphenylamine was added as an antioxidant. 0.0% extra was added. After allowing to cool to about 60 ° C., a grease was obtained by treating with three rolls.

Comparative Examples 1-3
Base oil and diisocyanate were put into a sealed grease prototype at the blending ratio shown in Table 2, heated to 60 ° C. with stirring, and a raw material in which various amines and base oils were mixed and dissolved was added from a hopper and reacted. In order to complete the reaction, the mixture was heated to 170 ° C. with further stirring and held for 30 minutes, then cooled to 80 ° C., and additives were added at the blending ratios shown in Table 1, and octyldiphenylamine was added as an antioxidant. 0.0% extra was added. After allowing to cool to about 60 ° C., a grease was obtained by treating with three rolls.
Comparative Example 4
Comparative Example 4 shown in Table 2 is a lithium-based synthetic oil grease that is frequently used in conventional electric power steering devices.

アミンＡは、炭素数８の飽和アルキル基を主体（９０％以上）とする平均分子量１３０の直鎖状一級アミン（工業用カプリルアミン）。
アミンＢは、炭素数１８の飽和アルキル基を主体（９０％以上）とする平均分子量２７０の直鎖状一級アミン（工業用ステアリルアミン）。
アミンＣは、炭素数１８の不飽和アルキル基が約５０％ならびに炭素数１８から炭素数１４の飽和もしくは不飽和アルキル基を含有する平均分子量２５５の直鎖状一級アミン（工業用牛脂アミン）。
アミンＤは、炭素数１８の不飽和アルキル基を主体（７０％以上）とする平均分子量２６０の直鎖状一級アミン（工業用オレイルアミン）。
実施例及び比較例に示す鉱油の４０℃の動粘度は、１０１．５ｍｍ^２／ｓで流動点は−１５℃であり、合成炭化水素油Ａ（ＣＡＳＮｏ．６８０３７−０１−４）の４０℃の動粘度は、１９．０ｍｍ^２／ｓで流動点は−６６．１℃で、また、合成炭化水素油Ｂ（ＣＡＳＮｏ．６８０３７−０１−４）の４０℃の動粘度は、３９６．２ｍｍ^２／ｓで流動点は−３６℃である。
添加剤Ａは、有機モリブテン錯体で特公平５−６６４３５号公報に記載されている油溶解性有機モリブテン化合物、
添加剤Ｂは、油溶解性の１級Ｚｎ−ＤＴＰ（プライマリーＺｎジチオホスフェート）、
添加剤Ｃは、油溶解性のＺｎ−ＤＴＣ（Ｚｎジチオカーバメート）、
添加剤Ｄは、酸化亜鉛を示す。 The diisocyanates shown in Tables 1 and 2 have a molecular weight of 250 and have the following chemical structure.

Amine A is a linear primary amine having an average molecular weight of 130 (industrial caprylamine) mainly composed of a saturated alkyl group having 8 carbon atoms (90% or more).
Amine B is a linear primary amine (industrial stearylamine) having an average molecular weight of 270 mainly composed of a saturated alkyl group having 18 carbon atoms (90% or more).
Amine C is a linear primary amine (industrial beef tallow amine) having an average molecular weight of 255 containing about 50% of an unsaturated alkyl group having 18 carbon atoms and a saturated or unsaturated alkyl group having 18 to 14 carbon atoms.
Amine D is a linear primary amine (industrial oleylamine) having an average molecular weight of 260 mainly composed of an unsaturated alkyl group having 18 carbon atoms (70% or more).
The kinematic viscosity at 40 ° C. of the mineral oil shown in Examples and Comparative Examples is 101.5 mm ² / s and the pour point is −15 ° C., and 40 ° C. of synthetic hydrocarbon oil A (CAS No. 68037-01-4). The kinematic viscosity is 19.0 mm ² / s, the pour point is −66.1 ° C., and the synthetic hydrocarbon oil B (CAS No. 68037-01-4) has a kinematic viscosity at 40 ° C. of 396.2 mm ² / s. In p, the pour point is -36 ° C.
Additive A is an oil-soluble organic molybdenum compound described in JP-B-5-66435 as an organic molybdenum complex,
Additive B is oil soluble primary Zn-DTP (primary Zn dithiophosphate),
Additive C is oil-soluble Zn-DTC (Zn dithiocarbamate),
Additive D represents zinc oxide.

The properties of the examples and comparative examples in each table were performed according to the following test methods.
Consistency: JIS K2220
Dropping point: JIS K2220
Evaporation amount: JIS K2220B method was carried out at a temperature of 99 ° C. for 22 hours.

From these experimental results, the following became clear.
(1) The urea grease of the present invention exhibited stable torque characteristics that are low from low to high.
(2) The urea grease of the present invention can be sufficiently lubricated at the lubrication interface, has little torque variation, and has a very long life at high temperatures.
As an electric power steering device to which the present invention is applied, for example, there is one described in Japanese Patent Application Laid-Open No. 2003-158856. The structure of this thing is as having described in the same gazette, As shown in FIG. 2 of the gazette, the main structure is arrange | positioned so that the rack shaft 16 connected with the wheel and this rack shaft 16 may be surrounded. A ball screw mechanism including a motor 14a, a ball screw groove 16a provided on the outer peripheral surface of the rack shaft 16, and a ball screw nut 19c attached to one end of the motor shaft 19 of the motor 14a is provided. In this configuration, the ball screw mechanisms 16 a and 19 c and the first bearing 21 and the second bearing 22 that support both ends of the motor shaft 19 correspond to “bearing devices using ball rolling”.
Samples using the greases of Examples and Comparative Examples as lubricants were prepared for the bearings of the electric power steering apparatus, and performance comparison of these samples was performed according to the following test method.

1. Steering wheel steering test When the driver operates (steers) the steering wheel, if there is a part where the ball does not roll smoothly in the bearing device, a region where the steering torque changes suddenly (protrudes) appears during steering operation. , You may feel a sense of incongruity as the handle gets caught. Therefore, in this test, the steering operation was performed with the electric power steering device at room temperature, and the steering torque at that time was measured.
Specifically, after steering from the left steering end to the right steering end with respect to the electric power steering apparatus, a steering wheel operation for returning to the left steering end is performed, and the load torque at that time is measured. Then, the average value of the measured torque (average torque value), the amount of protrusion (protruding torque value) when the protruding torque is measured at a predetermined value or more with respect to this average torque value, and the protruding torque value appears. The number of times (protrusion torque appearance frequency) was comparatively evaluated (relative evaluation). In each of the greases of Examples 1 to 4, the appearance frequency of the projecting torque was low as compared with the grease of Comparative Example 4 as a standard, and a stable and smooth steering feeling was obtained.
The greases of Examples 1 to 4 all had a low protruding torque appearance frequency and were stable as compared with Comparative Example 4 which is a lithium synthetic oil grease. Similarly to the examples, even in Comparative Examples 1 to 3 which are urea-based lubricating grease compositions, the additive weight is within the technical range of the present invention (claim 1 “(c) component is 1 to 1 of the total amount). In Comparative Example 3, which is 0.8% by weight that deviates from “7% by weight”), the frequency of appearance of the protruding torque is significantly higher than in any of the Examples, indicating a significant difference between the greases of Examples 1 to 4. Therefore, by using the grease of the embodiment for the bearing device of the electric power steering device, the electric power exhibiting a stable torque characteristic in which the phenomenon that the steering torque suddenly changes during the steering wheel operation ("steering of the steering wheel") hardly occurs. A steering device is obtained.

2. Low temperature to normal temperature performance test When the viscosity of the grease increases in a low temperature range, the preload applied to the bearing device increases, and the steering feeling may be impaired (the steering wheel becomes heavy). Therefore, in this test, the ambient temperature was decreased from 20 ° C. to −40 ° C. in units of 10 ° C., and the preload applied to the ball screw mechanisms 16a and 19c was measured.
Specifically, the preload is measured by the ball screw mechanisms 16a and 19c alone, and the measured value is calculated based on the reduction ratio of the meshing portion of the rack and pinion mechanism (rack shaft 16 and pinion shaft 17). In terms of steering torque (steering torque) as a steering device, the performance in a temperature range from low temperature to normal temperature, particularly at −30 ° C. and 20 ° C., was comparatively evaluated.
As a result, it is clear that the greases of Examples 1 to 4 are reduced in both the low temperature preload (−30 ° C.) and the normal temperature preload (20 ° C.) as compared with the grease of Comparative Example 4 which is a lithium-based synthetic oil grease. Became. Further, even in Comparative Examples 1 to 3 which are urea-based lubricating grease compositions, the pour point of the base oil is within the technical scope of the present invention (Claim 1 “(b) Synthetic carbonization having a pour point of −40 ° C. or lower”. In Comparative Examples 1 and 2, which are -37.6 ° C. and −27.6 ° C. deviating from “base oil mainly composed of hydrogen”, both the low temperature preload and the normal temperature preload are remarkably higher than any of the examples. A significant difference of ~ 4 grease is shown. Therefore, by using the grease of the embodiment for the bearing device of the electric power steering device, an electric power steering device that exhibits a much lower torque characteristic than the conventional one in a low temperature range can be obtained.

3. High temperature performance and durability performance test When the viscosity of the grease decreases in a high temperature range, the oil film to be generated by the grease in the bearing device may be almost lost (oil film breakage). In addition, oil film breakage may occur due to deterioration and consumption of grease due to long-term use. This oil film breakage hinders smooth rolling of the ball in the bearing device and impairs the steering feeling, and when the seizure (carbonization) occurs, in the worst case, the bearing device may be damaged. . Therefore, in this test, a test was performed in which the bearing device was operated a predetermined number of times at a high temperature of about 120 ° C.
Specifically, the first bearing 21 or the second bearing 22 of the electric power steering apparatus is rotated forward and backward independently, and the bearings are damaged or greased at 60,000 rotations and 120,000 rotations. The presence or absence of carbonization was confirmed, and high temperature performance and durability performance were compared and evaluated.
As a result, it was found that high temperature performance and durability performance were improved by using the grease of the present invention (Comparative Example 4 was caused by grease carbonization and bearing breakage during 120,000 rotations). Therefore, by using the grease of the embodiment for the bearing device of the electric power steering device, an electric power steering device that is less likely to cause oil film breakage or seizure in a high temperature range and that can maintain high reliability even after long-term use is obtained. It is done.

Claims (3)

(A) An alkyldiurea compound having an average molecular weight of 600 to 700 as a thickener, wherein 25 to 50 mol% of the total alkyl groups are unsaturated components, and the total amine value of the primary amine as a raw material is 250 A diurea compound in the range of ~ 350,
(B) a base oil mainly composed of a synthetic hydrocarbon oil having a pour point of −40 ° C. or lower,
(C) As an additive, comprising an oil-soluble organic molybdenum complex, an oil-soluble organic zinc compound of dithiocarbamic acid, an oil-soluble organic zinc compound of dithiophosphoric acid, and an inorganic zinc compound,
(1) The component (a) is 5 to 15% by weight of the total amount,
(2) The component which is an internally synthesized hydrocarbon oil of component (b) is 80% by weight or more,
(3) The component (c) is 1 to 7% by weight of the total amount,
A urea-based lubricating grease composition characterized by   An electric power steering apparatus, wherein the urea-based lubricating grease composition according to claim 1 is used as a lubricant. 3. The electric power steering apparatus according to claim 2, wherein the lubricant is used in a bearing device using rolling of a ball.