Patent Description:
In these years, electric power steering has been widely applied in need of responsiveness and quietness during steering as improvements in driving comfort of automobiles. The electric power steering is divided into column-assisted, pinion-assisted, and rack-assisted types and the like, and the demand for the rack-assisted type has been expected to expand in the market of electric power steering in the future because the rack-assisted type has high responsiveness and output. Along with this trend, greases used for ball screw parts in rack-assisted electric power steering devices are also demanded to meet various requests such as an increase in service life, a reduction in torque over a wide speed region, and further, reductions in noise and vibration. Particularly, suppression of torque variations has been an important technological challenge for improving the responsiveness and the quietness while the driver is steering.

Conventional techniques of grease compositions that can be applied to ball screw parts in rack-assisted electric power steering devices include Patent Literatures <NUM> and <NUM>.

Patent Literature <NUM> describes a grease composition that use a diurea compound in which an aliphatic moiety has unsaturated components together with a fatty acid metallic salt and an amide compound in a predetermined ratio as a thickener, uses a lubricating base oil containing a synthetic hydrocarbon oil having a pour point of -<NUM> or less as a main component as a base oil, and predetermined amounts of predetermined additives. This grease has been reported as a grease having a long service life that can significantly reduce irregular friction variations, exhibits low and stable torque properties in a wide temperature range, and can maintain a sufficient oil film even at a high temperature.

Patent Literature <NUM> describes a grease composition that uses a mixture of an urea-based compound that has an average molecular weight of <NUM> to <NUM> in which a linear chain hydrocarbon group has an unsaturated component, a fatty acid metallic salt, and an amide compound, as a thickener. This grease has been reported as a grease that can significantly reduce irregular friction variations, and can achieve stable friction properties and lubricity.

An object of the present invention is to provide a grease composition for a ball screw part of rack-assisted electric power steering, that suppresses torque variations to improve responsiveness and quietness during steering using a rack-assisted electric power steering device and achieves a low torque even over a wide speed region range.

The invention is defined in the appended claims,.

The present invention makes it possible to provide a grease composition for a ball screw part of rack-assisted electric power steering, that can achieve both suppression of torque variation and a low torque over a wide speed region.

Thickeners include soap thickeners represented by a Li soap and a Li complex soap, urea-based thickeners represented by diurea, inorganic thickeners represented by organoclay and silica, organic thickeners represented by PTFE, and the like. Among these, diurea represented by the formula (I) is used and is excellent in heat resistance.

R<NUM>-NHCONH-R<NUM>-NHCONH-R<NUM>     (I).

wherein R<NUM> and R<NUM> may be the same or different, and each represent a straight chain alkyl group having <NUM> to <NUM> carbon atoms, an aryl group having <NUM> or <NUM> carbon atoms, or a cyclohexyl group, and preferably a straight chain alkyl group having <NUM> to <NUM> carbon atoms. R<NUM> is a divalent aromatic hydrocarbon group having <NUM> to <NUM> carbon atoms, preferably a group derived from tolylene diisocyanate or diphenylmethane-<NUM>,<NUM>'-diisocyanate, and more preferably a group derived from diphenylmethane-<NUM>,<NUM>'-diisocyanate. As the diurea of the formula (I), it is most preferable to contain a compound in which one of R<NUM> and R<NUM> is a straight chain alkyl group having <NUM> carbon atoms, the other thereof is a straight chain alkyl group having <NUM> carbon atoms, and R<NUM> is a group derived from diphenylmethane-<NUM>,<NUM>'-diisocyanate.

An exemplary Li complex soap is composed of a lithium salt of a hydroxy fatty acid having one or more hydroxyl groups and having <NUM> to <NUM> carbon atoms and a lithium salt of a fatty dicarboxylic acid having <NUM> to <NUM> carbon atoms. The above-described hydroxy fatty acid includes <NUM>-hydroxystearic acid, <NUM>-hydroxylauric acid, <NUM>-hydroxypalmitic acid, e.g., <NUM>-hydroxystearic acid. The above-described fatty dicarboxylic acid includes azelaic acid, sebacic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, undecanedioic acid, and dodecanedioic acid, e.g., azelaic acid. One Li complex soap is composed of the lithium salt of <NUM>-hydroxystearic acid and the lithium salt of azelaic acid.

The thickener of the present invention is a thickener containing a diurea compound of the formula (I) in which one of R<NUM> and R<NUM> is a straight chain alkyl group having <NUM> carbon atoms, the other thereof is a straight chain alkyl group having <NUM> carbon atoms, and R<NUM> is a group derived from diphenylmethane-<NUM>,<NUM>'-diisocyanate.

The content of the thickener in the composition of the present invention is an amount that makes it possible to adjust the worked penetration of the grease composition of the present invention to within a range of <NUM> to <NUM>, and is normally <NUM> to <NUM>% by mass, preferably <NUM> to <NUM>% by mass, and more preferably <NUM> to <NUM>% by mass, based on the total mass of the composition.

Base oils are a mineral oil or a synthetic oil. One base oil may be used alone or two or more base oils may be used in combination, like, as the mineral oil, a paraffin-based mineral oil, a naphthene-based mineral oil, or a mixture of these, and, as the synthetic oil, any of various synthetic oils such as ester-based synthetic oils represented by diesters and polyolesters; synthetic hydrocarbon oils represented by poly-α-olefin (PAO) and polybutene; ether-based synthetic oils represented by alkyl diphenyl ethers and polypropylene glycol; silicone oils, fluorinated oils.

The base oil according to the invention contains poly-α-olefin. In the case where the base oil contains a base oil other than poly-α-olefin, the poly-α-olefin may be contained in an amount of preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, further preferably <NUM>% by mass or more, and most preferably <NUM>% by mass, based on the total mass of the base oil. It is preferable because when the percentage of the poly-α-olefin in the base oil is as described above, an excellent low-temperature performance can be achieved.

The kinematic viscosity of the base oil in the present invention is such that the kinematic viscosity at <NUM> is <NUM> to <NUM><NUM>/s, preferably <NUM> to <NUM><NUM>/s, and further preferably <NUM> to <NUM><NUM>/s, from the viewpoint of the low-temperature performance and the viewpoint of suppressing the torque in high speed region at a low level.

As the base oil of the present invention, PAO having a kinematic viscosity at <NUM> of <NUM> to <NUM><NUM>/s is particularly preferable.

The content of the base oil in the grease composition of the present invention is an amount that is normally used for production of greases, and is, for example, <NUM> to <NUM>% by mass, and is preferably <NUM> to <NUM>% by mass, and more preferably <NUM> to <NUM>% by mass, from the viewpoint of consistency.

The grease composition of the present invention comprises at least one compound selected from the group consisting of a Ca sulfonate, a fatty acid, and a triglyceride , as defined in the appended claims.

Among these, a Ca sulfonate and a fatty acid are preferable, and a Ca sulfonate is more preferable.

The fatty acid includes, and in case of the invention's composition it is, a linear or branched, saturated or unsaturated fatty acid having <NUM> to <NUM>, and preferably <NUM> to <NUM> carbon atoms. A mixture of two or more of these may also be used. A saturated fatty acid is preferable, and a straight chain saturated fatty acid having <NUM> to <NUM> carbon atoms is particularly preferable. The preferable specific examples include stearic acid and palmitic acid.

The triglyceride includes triglycerides in each of which a fatty acid residue constituting the triglyceride has preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM> carbon atoms. A mixture of two or more of these may also be used. Preferable specific examples include castor oil collected from natural oils and/or fats and hydrogenated castor oil.

The Ca sulfonate may be neutral or basic, but is preferably neutral. A mixture of two or more of these may also be used. In the case of a basic Ca sulfonate, the base number (in the case of a mixture, the base number of the mixture) is preferably <NUM> mgKOH/g or less. Preferable specific examples include neutral calcium dinonylnaphthalenesulfonate. Note that in the Specification, the base number is a value measured in accordance with JIS K2501.

As the essential additive of the present invention, neutral calcium dinonylnaphthalenesulfonate is used.

Without being bound to any theory, when an additive predetermined herein is used, it is possible to form a coating film on the surface of a ball screw. It is considered that this coating film thus formed contributes to suppression of torque variation.

The content of the essential additive of the present invention is preferably, and in case of the invention's composition it is, <NUM> to <NUM>% by mass, more preferably <NUM> to <NUM>% by mass, and further preferably <NUM> to <NUM>% by mass, based on the total mass of the composition. When the additive predetermined herein is contained in a content within this range, it is preferable because torque variation can be effectively suppressed.

In the Specification, the term "worked penetration" refers to <NUM>-stroke worked penetration, and can be measured in accordance with JIS K2220 <NUM>. The worked penetration of the present invention is <NUM> to <NUM>, and preferably <NUM> to <NUM>. A worked penetration of <NUM> or more is excellent in terms of torque. A worked penetration of <NUM> or less is excellent in terms of anti-spattering property and anti-rundown property.

The grease composition of the present invention may comprise additives that are normally used for greases as necessary. The content of these additives is normally <NUM> to <NUM>% by mass, and preferably <NUM> to <NUM>% by mass, based on the total amount of the grease composition. Such additives include, for example, an antioxidant, an inorganic passivator, a rust preventive, an oiliness improver, an antiwear agent, an extreme pressure agent, and a solid lubricant. The grease composition of the present invention preferably contains at least one of an antioxidant, a rust preventive, an oiliness improver, and an extreme pressure agent from the viewpoint of oxidation resistance, rust resistance, boundary lubrication, and durability.

The grease composition of the present invention is applied to a ball screw part of rack-assisted electric power steering. It is preferable that all elements constituting the ball screw are steel.

In poly-α-olefin (PAO), <NUM> mol of <NUM>',<NUM>-diphenylmethane diisocyanate was reacted with <NUM> mol of octylamine and <NUM> mol of stearylamine, followed by heating and cooling, and thereafter, additives were blended in a ratio shown in Table <NUM> or Table <NUM>, followed by kneading using a three roll mill to obtain grease compositions of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>. Note that % by mass in Table <NUM> and Table <NUM> was based on the total mass of each grease composition.

To the thread groove portion of a ball screw made of steel, <NUM> of each test grease was applied, and the ball screw was placed in a constant temperature oven with ambient temperature set to <NUM>. The screw shaft was reciprocated <NUM> times at a speed of <NUM>/s and within a stroke range of <NUM>. Thereafter, the screw shaft was reciprocated <NUM> times at a speed of each of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>/s in order from the lowest speed. This was counted as <NUM> cycle, and <NUM> cycles were conducted in total. The force ("operating force") generated by reciprocating the screw shaft of the ball screw at a predetermined speed was sampled at predetermined intervals, and an average value of the operating forces per reciprocation was calculated by dividing the sum of the operating forces per reciprocation by the number of times of sampling the data. The average value in the three times of reciprocation at <NUM>/s and <NUM>/s in the fifth cycle was judged based on the criteria described below to evaluate whether or not a low torque was achieved over a wide speed region.

The average value of the operating forces at <NUM>/s was.

In the above-described evaluation of torques, when the reciprocation was conducted three times at <NUM>/s in the fifth cycle, among variations (peaks) of the operating force generated in the first reciprocation, the first to fifth peak heights in descending order of variation were calculated. In the same manner, the first to fifth peak heights in descending order of variation were calculated for variations generated during the second reciprocation and during the third reciprocation, and an average value of all the <NUM> peaks was obtained, and evaluated based on the criteria described below.

Note that the reason why the above average value was obtained at the lowest speed among the set speeds is because the effect of the steering wheel being caught, which affects the torque variation, is more likely to occur at a low speed than at a high speed. In addition, an automobile is normally test-driven in the site of the factory, or the like, after the automobile is manufactured at the factory and before the automobile is delivered. That is, it is considered that the event where the steering wheel is caught is lessened so that the steering wheel gets into a stable state by the time of delivery. In the torque test as well, since the steering wheel was reached to a stable state by the fifth cycle at the latest, the grease composition was evaluated from variations at the fifth cycle with the assumption of the drive after the delivery.

Claim 1:
A grease composition for a ball screw part of rack-assisted electric power steering, comprising:
a thickener;
a base oil which contains poly-α-olefin; and
based on the total mass of the composition, <NUM> to <NUM>% by mass of at least one compound selected from the group consisting of a Ca sulfonate, a fatty acid, and a triglyceride, wherein
a kinematic viscosity at <NUM> of the base oil is <NUM> to <NUM><NUM>/s,
a worked penetration of the composition measured in accordance with JIS K2220 <NUM> is <NUM> to <NUM>, and
the thickener is represented by formula (I):

        R<NUM>-NHCONH-R<NUM>-NHCONH-R<NUM>     (I)

wherein R<NUM> and R<NUM> may be the same or different, and each represent a straight chain alkyl group having <NUM> to <NUM> carbon atoms, an aryl group having <NUM> or <NUM> carbon atoms, or a cyclohexyl group, R<NUM> is a divalent aromatic hydrocarbon group having <NUM> to <NUM> carbon atoms,
characterized in that
the Ca sulfonate is neutral calcium dinonylnaphthalenesulfonate, the fatty acid is a linear or branched, saturated or unsaturated fatty acid having <NUM> to <NUM> carbon atoms, or the compound is the triglyceride.