Grease composition and steering gear device

Abstract

To provide a grease composition that exhibits excellent lubrication performance even when used in a part that is likely to be in a poor lubrication state.SOLUTION: A grease composition comprises a base oil, a thickener and an additive, wherein the base oil contains a trimellitic acid ester and a poly-α-olefin, the thickener contains 12-hydroxystearic acid lithium and lithium stearate, the additive contains molybdenum dialkyldithiocarbamate and an urea-based additive, and the base oil, the thickener, and the additive are respectively included at predetermined ratios to a total amount of the base oil, the thickener, and the additive.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a grease composition and a steering gear device. [Background technology]

[0002] The rack and pinion in the steering gear system used in electric power steering systems comprises a rack shaft with rack teeth and a pinion shaft with pinion teeth. This steering gear system uses grease at the meshing portion between the rack teeth and pinion teeth to suppress wear on the rack teeth and pinion teeth. By suppressing this wear, this steering gear system reduces the amount of change in the clearance between the rack teeth and pinion teeth, thereby maintaining the steering performance of the electric power steering system. Examples of greases used in steering gear systems include those proposed in Patent Documents 1 to 3. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2001-064665 [Patent Document 2] Japanese Patent Publication No. 2009-203374 [Patent Document 3] Japanese Patent Publication No. 2004-269722 [Overview of the project] [Problems that the invention aims to solve]

[0004] A steering gear system having a rack and pinion incorporates a grease composition between the meshing portion of the rack teeth and pinion teeth in the rack and pinion, thereby suppressing wear on the rack teeth and pinion teeth. Furthermore, the steering gear system includes a rack guide mechanism for biasing the rack teeth onto the pinion teeth, and this grease composition is interposed between the rack shaft and the portion of the rack guide mechanism that presses against the rack shaft, thereby suppressing wear on both. Therefore, the grease composition used in the steering gear device described above is required to have the ability to properly lubricate not only the meshing portion between the rack teeth and pinion teeth in the rack and pinion, but also the sliding portion between the rack shaft and the rack guide mechanism.

[0005] However, the grease compositions proposed in Patent Documents 1 to 3 were unable to adequately meet these requirements. The meshing areas between the rack teeth and pinion teeth, and the sliding contact areas between the rack shaft and the rack guide mechanism, are prone to poor lubrication, and it has not been easy to properly lubricate these areas. Furthermore, the part of the rack guide mechanism that is pressed against the rack shaft is made of a resin such as PTFE, and the grease composition designed for lubricating the meshing portion between the steel rack teeth and the steel pinion teeth cannot adequately lubricate the sliding contact portion between the rack shaft and the rack guide mechanism. [Means for solving the problem]

[0006] Under these circumstances, the inventors have discovered a grease composition that can easily penetrate into lubricated parts that tend to be poorly lubricated and can effectively lubricate the friction surfaces of lubricated components, thereby completing the present invention.

[0007] (1) The grease composition of the present invention comprises a base oil, a thickener, and an additive. The above base oil contains trimellitic acid ester and poly-α-olefin, The trimellitic acid ester is present in an amount of 10.0% by mass or more and 60.0% by mass or less relative to the total amount of the trimellitic acid ester and the poly-α-olefin. The above thickener comprises lithium 12-hydroxystearate and lithium stearate. The lithium 12-hydroxystearate mentioned above is present in an amount of 5.0% by mass or more and 95.0% by mass or less relative to the total amount of lithium 12-hydroxystearate and lithium stearate mentioned above. The above additive includes molybdenum dialkyldithiocarbamate and a urea-based additive. The molybdenum dialkyldithiocarbamate mentioned above is present in proportion to the total amount of the trimellitic acid ester, poly-α-olefin, lithium 12-hydroxystearate, lithium stearate, molybdenum dialkyldithiocarbamate, and urea-based additives, which is 1.5% by mass or more and 8.0% by mass or less. The above urea-based additive has an average diameter of 0.2 μm or more for particles with a diameter of 0.2 μm or more, and a minimum diameter of 1.0 μm. The above urea-based additive is present in proportion to the total amount of the above trimellitic acid ester, the above poly-α-olefin, the above lithium 12-hydroxystearate, the above lithium stearate, the above molybdenum dialkyldithiocarbamate, and the above urea-based additive, which is 1.2% by mass or more and 3.4% by mass or less.

[0008] The above grease composition is suitable for lubricating the friction surfaces of lubricated components that tend to be in poorly lubricated environments, and for suppressing wear on these friction surfaces. The above grease composition contains a predetermined amount of trimellitic acid ester and poly-α-olefin as a base oil, and a predetermined amount of lithium stearate and lithium 12-hydroxystearate as a thickener, so it is easily adsorbed to the friction surface of the lubricated member and is difficult to detach from the friction surface. Furthermore, since the above grease composition contains a predetermined amount of molybdenum dialkyldithiocarbamate, it readily forms a tribo-reaction film on the friction surface. Furthermore, because the above grease composition contains a predetermined amount of urea-based additives (additives containing urea compounds) of a specific size, it can interpose between the friction surfaces of the lubricated components, thereby suppressing contact between the friction surfaces. In addition, urea-based additives are less likely to damage the friction surfaces.

[0009] (2) In the grease composition of (1) above, the urea-based additive is preferably a mixture of a urea compound and a styrene-based polymer. In this case, it is more suitable for suppressing contact between the friction surfaces of the lubricated components without damaging the friction surfaces. This is thought to be because the urea-based additive can easily interpose between the friction surfaces of the lubricated components while maintaining an appropriate aggregated state.

[0010] (3) The steering gear device of the present invention is Housing and A rack shaft having rack teeth and capable of reciprocating along the axial direction, A pinion shaft having pinion teeth that mesh with the rack teeth mentioned above, A rack guide mechanism that biases the rack teeth to the pinion teeth, The present invention comprises a grease composition interposed between the rack teeth and pinion teeth that mesh with each other, and between the circumferential surface of the rack shaft and the portion of the rack guide mechanism that is pressed against the rack shaft. The above grease composition is the grease composition of (1) or (2) above.

[0011] In the above steering gear device, the grease composition interposed between the rack teeth and pinion teeth that mesh with each other, and between the circumferential surface of the rack shaft and the portion of the rack guide mechanism that presses against the rack shaft, is made of the grease composition of the present invention. In this case, wear of the rack teeth, pinion teeth, the circumferential surface of the rack shaft that slides against the rack guide mechanism, and the portion of the rack guide mechanism that presses against the rack shaft is suppressed. Therefore, the amount of change in the clearance of the lubricated parts due to wear is small, and a decrease in steering performance is less likely to occur. [Effects of the Invention]

[0012] The grease composition of the present invention exhibits excellent lubricating performance even when used in sites prone to poor lubrication conditions, and can suppress wear of the friction surface of a lubricated member. Therefore, the grease composition of the present invention is suitable as a grease composition for use in a steering gear device equipped with a rack and pinion.

[0013] Since the grease composition of the present invention is used in the steering gear device of the present invention, wear of rack teeth and pinion teeth, the peripheral surface slidingly contacting the rack guide mechanism of the rack shaft, and the portion pressed against the rack shaft in the rack guide mechanism can be suppressed over a long period of time. Therefore, the above steering gear device can maintain steering performance for a long period of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] [Figure 1] It is a configuration diagram schematically showing an example of a dual-pinion type electric power steering device in which the grease composition of the present invention is enclosed. [Figure 2] It is an A-A cross-sectional view of Fig. 1. [Figure 3] It is a B-B cross-sectional view of Fig. 1. [Figure 4] It is a configuration diagram schematically showing an example of a column-type electric power steering device in which the grease composition of the present invention is enclosed. [Figure 5] It is an A-A cross-sectional view of Fig. 4. [Figure 6] It is a process diagram for explaining an example of a method for producing a urea-based additive. [Figure 7] It is a process diagram for explaining another example of a method for producing a urea-based additive. [Figure 8] It is a process diagram for explaining another example of a method for producing a urea-based additive. [Figure 9] It is a graph showing evaluation results of Examples and Comparative Examples. MODE FOR CARRYING OUT THE INVENTION

[0015] Embodiments of the present invention will be described below. In this invention, the embodiments of the invention should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is defined by the claims and includes all modifications in the sense and scope equivalent to the claims.

[0016] First, an embodiment of a steering gear device in which the grease composition of the present invention is used will be described, and then an embodiment of the grease composition of the present invention will be described. <Steering Gear System> The grease composition of the present invention is used, for example, in dual-pinion type electric power steering systems, column type electric power steering systems, and the like.

[0017] (Dual-pinion type electric power steering system) Figure 1 is a schematic diagram showing an example of a dual-pinion type electric power steering system 1, including a steering gear unit 3. Figure 2 is a cross-sectional view AA of Figure 1, showing a part of the steering gear device 3. In Figure 2, the lower part of the drawing corresponds to the vertical downward side when mounted in the vehicle. Figure 3 is a cross-sectional view of BB in Figure 1, showing a part of the steering gear device 3. In Figure 3, the lower part of the drawing corresponds to the vertical lower side when mounted in the vehicle.

[0018] The dual-pinion type electric power steering system 1 comprises a steering wheel 10, a steering shaft 2, a first pinion shaft 32, a rack shaft 31, a housing 33, two rack bushings 30 and 34, two bearings 35 and 36, a first rack guide mechanism 39, and a steering assist device 5. The steering assist device 5 comprises a controller 50, a torque sensor 51, an electric motor 52, a reduction mechanism 53, a second pinion shaft 54, two bearings 55 and 56, a worm housing 57, and a second rack guide mechanism 59. The reduction mechanism 53 comprises a worm 531 and a worm wheel 532.

[0019] A driver operating a vehicle equipped with this dual-pinion type electric power steering system 1 steers by rotating the steering wheel 10. The steering shaft 2 comprises a column shaft 21, a first universal joint 23, an intermediate shaft 22, and a second universal joint 24. The first universal joint 23 comprises a first yoke (not shown), a plurality of first rolling elements (not shown), a first cross shaft (not shown), a plurality of second rolling elements (not shown), and a second yoke (not shown). The second universal joint 24 comprises a third yoke (not shown), a plurality of third rolling elements (not shown), a second cross shaft (not shown), a plurality of fourth rolling elements (not shown), and a fourth yoke (not shown).

[0020] The column shaft 21 has a steering wheel 10 fixed to one end in the extending direction. The column shaft 21 has a first yoke of a first universal joint 23 fixed to the other end in the extending direction. The column shaft 21 is rotatable about a central axis in the extending direction. The first yoke is pivotably fitted to a first pair of trunnions on the same central axis of the first cross shaft via a plurality of first rolling elements. The second yoke is pivotably fitted to a second pair of trunnions on the same central axis of the first cross shaft via a plurality of second rolling elements. The central axes of the first pair of trunnions and the central axes of the second pair of trunnions intersect at a 90-degree angle.

[0021] The second yoke of the first universal joint 23 fixes one end of the intermediate shaft 22 in the extending direction. The intermediate shaft 22 fixes the third yoke of the second universal joint 24 to the other end in the extending direction. The third yoke is pivotably fitted to a third pair of trunnions on the same central axis of the second cross shaft via a plurality of third rolling elements. The fourth yoke is pivotably fitted to a fourth pair of trunnions on the same central axis of the second cross shaft via a plurality of fourth rolling elements. The central axes of the third pair of trunnions and the central axes of the fourth pair of trunnions intersect at a 90-degree angle. The fourth yoke of the second universal joint 24 fixes one end of the first pinion shaft 32 in the extending direction. As a result, when the driver rotates the steering wheel 10, the column shaft 21 rotates around its central axis in the direction of extension, the intermediate shaft 22 also rotates around its central axis in the direction of extension, and the first pinion shaft 32 also rotates around its central axis in the direction of extension.

[0022] In the dual-pinion type electric power steering device 1, the first pinion shaft 32, the rack shaft 31, the housing 33, the two rack bushings 30 and 34, the first bearing 35, the second bearing 36, the first rack guide mechanism 39, the electric motor 52, the reduction mechanism 53, the second pinion shaft 54, the third bearing 55, the fourth bearing 56, the worm housing 57, and the second rack guide mechanism 59 constitute the steering gear device 3 as a rack and pinion type steering device. Figure 1 shows the housing 33 represented by a dashed line (two-dot chain line) and illustrates its interior.

[0023] The first pinion shaft 32 extends vertically from the top to the bottom of the automobile. The first pinion shaft 32 has a serrated portion 324, a first shaft portion 322, a first pinion tooth portion 320, and a first boss portion 323 along its extending direction from one end to the other. The serrations are formed on the serrated portion 324. The serrations on the serrated portion 324 are used to fix the fourth yoke of the second universal joint 24. The first shaft portion 322 is cylindrical in shape. The first pinion tooth portion 320 has first pinion teeth 321 formed on its entire circumferential surface. The extending direction of the first pinion teeth 321 is at an angle that is not 90 degrees with respect to the extending direction of the central axis of the first pinion shaft 32. The first boss portion 323 is cylindrical in shape.

[0024] The housing 33 has a first opening 332 on the steering wheel 10 side, and is sealed on the opposite side of the first opening 332. The first pinion shaft 32 is housed inside the housing 33. The first pinion shaft 32 is rotatably supported by two bearings 35, 36 relative to the housing 33. The first bearing 35 is a ball bearing. The first bearing 35 includes an inner ring, an outer ring, and balls, with the inner ring fixed to the first shaft portion 322 and the outer ring fixed to the housing 33, and the balls rolling between the inner and outer rings. The second bearing 36 is a roller bearing. The second bearing 36 includes rollers and an outer ring, with the outer ring fixed to the housing 33, and the rollers rolling between the outer circumferential surface of the first boss portion 323 and the outer ring.

[0025] With the first pinion shaft 32, the first bearing 35, and the second bearing 36 inserted into the housing 33, the first opening 332 of the housing is fixed with a cover 37 through which the first pinion shaft 32 passes. A seal is fixed to the cover 37 and is slidable on the outer circumferential surface 322b of the first shaft portion 322 of the first pinion shaft 32. Furthermore, a cover member 38 is fixed to the housing 33. The cover member 38 covers a portion of the first shaft portion 322 of the first pinion shaft 32 from the radial outside.

[0026] The rack shaft 31 comprises a first cylindrical portion 316, a first rack tooth portion 310, a second cylindrical portion 317, a second rack tooth portion 314, and a third cylindrical portion 318, extending from one end to the other in the direction of extension. The first rack tooth portion 310 has first rack teeth 311 on a part of its circumferential direction, and the other part of the first rack tooth portion 310 has a cylindrical surface 312 with the direction of extension of the rack shaft 31 as its central axis. The second rack tooth portion 314 has second rack teeth 315 on a part of its circumferential direction, and the other part of the first rack tooth portion 310 has a cylindrical surface 313 with the direction of extension of the rack shaft 31 as its central axis. The outer circumferential surfaces of the first cylindrical portion 316, the second cylindrical portion 317, and the third cylindrical portion 318 are each cylindrical surfaces with the direction of extension of the rack shaft 31 as their central axis. The extending direction of the first rack tooth 311 is at an angle that is not 90 degrees with respect to the extending direction of the rack axis. The extending direction of the second rack tooth 315 is at an angle that is not 90 degrees with respect to the extending direction of the rack axis 31. If the angle of the first rack tooth 311 with respect to the extending direction of the rack axis 31 is X, then the angle of the second rack tooth 315 with respect to the extending direction of the rack axis 31 is π-X.

[0027] The housing 33 extends in a direction different from the first opening 332 on the steering wheel 10 side and has a second opening 333 at one end in the extending direction and a third opening 334 at the other end. The rack shaft 31 is housed inside the housing 33 along the extending direction of the housing 33. A first cylindrical portion 316 at one end of the rack shaft 31 in the extending direction protrudes from the second opening 333 at one end of the housing 33 in the extending direction. A third cylindrical portion 318 at the other end of the rack shaft 31 in the extending direction protrudes from the third opening 334 at the other end of the housing 33 in the extending direction. The housing 33 has a fourth opening 335. The fourth opening 335 is located on the other end of the housing in the extending direction than the first opening 332. The housing 33 further has a fifth opening 336 and a sixth opening 337. The fifth opening 336 is located at approximately the same position as the first opening 332 in the extending direction of the housing 33, in the radial direction with the extending direction of the housing 33 as its central axis, and perpendicular to the first opening 332. The sixth opening 337 is located at approximately the same position as the fourth opening 335 in the extending direction of the housing 33, in the radial direction with the extending direction of the housing 33 as its central axis, and perpendicular to the fourth opening 335.

[0028] A first rack bush 30 is fixed to one end of the housing 33 in the extending direction. The first rack bush 30 is fixed to the housing 33 adjacent to the second opening 333. The first rack bush 30 is slidable on the outer circumferential surface of the first cylindrical portion 316 of the rack shaft 31. A second rack bush 34 is fixed to the other end of the housing 33 in the extending direction. The second rack bush 34 is fixed to the housing 33 adjacent to the third opening 334. The second rack bush 34 is slidable on the outer circumferential surface of the third cylindrical portion 318 of the rack shaft 31.

[0029] The first pinion teeth 321 formed on the first pinion tooth portion 320 of the first pinion shaft 32 and the first rack teeth 311 formed on the first rack tooth portion 310 of the rack shaft 31 are in rolling and sliding contact via the grease composition G. The first pinion teeth 321 and the first rack teeth 311 are meshed via the grease composition G. When the first pinion shaft 32 rotates relative to the housing 33 about its central axis in its extending direction, the rack shaft 31 moves linearly relative to the housing 33 in the extending direction of the housing 33.

[0030] A first rack guide mechanism 39 is fixed to the housing 33. The first rack guide mechanism 39 is fixed to a fifth opening 336. The fifth opening 336 is located on the cylindrical surface 312 side, which is the other circumferential part of the first rack teeth 310 of the rack shaft 31, at the position where the first pinion shaft 32 engages with the rack shaft 31 in the extending direction of the housing 33.

[0031] The first rack guide mechanism 39 includes a first support yoke 391, a first seat member 392, a first coil spring 393, and a first plug 394. The first seat member 392 is sandwiched between a cylindrical surface 312, which is the other circumferential part of the first rack teeth 310 of the rack shaft 31, and the cylindrical surface of the first support yoke 391. The first seat member 392 is fixed to the first support yoke 391. The first seat member 392 and the cylindrical surface 312, which is the other circumferential part of the first rack teeth 310 of the rack shaft 31, are in sliding contact via a grease composition G. The first seat member 392 includes a metal layer, such as bronze, and a resin layer, such as PTFE, with the resin layer in contact with the cylindrical surface 312 via the grease composition G. The first plug 394 is fixed to a fifth opening 336 of the housing 33. The first plug 394 contacts one end of the first coil spring 393. The first support yoke 391 contacts the other end of the first coil spring 393. The first coil spring 393 is shorter than its free length when the first plug 394 is fixed to the fifth opening 336. Thus, the first seat member 392 is pressed against the rack shaft 31 relative to the housing 33.

[0032] The second pinion shaft 54 ​​extends from the upper to the lower side in the vertical direction of the automobile. The second pinion shaft 54 ​​has a fitting portion 544, a second shaft portion 542, a second pinion tooth portion 540, and a second boss portion 543 along its extending direction from one end to the other. The fitting portion 544 is cylindrical in shape. The second shaft portion 542 is cylindrical in shape. The second pinion tooth portion 540 has second pinion teeth 541 formed on its entire circumferential surface. The extending direction of the second pinion teeth 541 is at an angle that is not 90 degrees with respect to the extending direction of the central axis of the second pinion shaft 54. The second boss portion 543 is cylindrical in shape.

[0033] A worm wheel 532 is fitted into a mating portion 544. A worm 531 is fixed to the output shaft 521 of an electric motor 52. The electric motor 52 is fixed to a worm housing 57. The worm housing 57 has a seventh opening 571. The output shaft 521 of the electric motor 52 is positioned in the internal space of the worm housing 57 through the seventh opening 571. The electric motor 52 is fixed to the worm housing 57 so as to close the seventh opening 571 of the worm housing 57.

[0034] The worm 531 is located in the internal space of the worm housing 57. The worm wheel 532 is located in the internal space of the worm housing 57. The worm housing 57 has an eighth opening 572 located vertically upward, and the assembly of the second pinion shaft 54 ​​and the worm wheel 532 is inserted into the internal space of the worm housing 57 through the eighth opening 572. The eighth opening is closed by a cover 58. The worm housing 57 has a ninth opening 573 on the opposite side of the eighth opening 572. Part of the second shaft portion 542 of the second pinion shaft 54, the second pinion teeth portion 540, and the second boss portion 543 protrude from the ninth opening 573 of the worm housing 57.

[0035] The worm housing 57 is fixed to the housing 33. The ninth opening 573 of the worm housing 57 and the fourth opening 335 of the housing 33 are in communication, sealing the internal space from the external space.

[0036] The third bearing 55 is a ball bearing. The bearing 55 includes an inner ring, an outer ring, and balls, with the inner ring fixed to the second shaft portion 542 and the outer ring fixed to the worm housing 57, and the balls rolling between the inner and outer rings. The bearing 56 is a roller bearing. The bearing 56 includes rollers and an outer ring, with the outer ring fixed to the housing 33, and the rollers rolling between the outer circumferential surface of the second boss portion 543 and the outer ring.

[0037] The second pinion teeth 541 formed on the second pinion tooth portion 540 of the second pinion shaft 54 ​​and the second rack teeth 315 formed on the second rack tooth portion 314 of the rack shaft 31 are in rolling and sliding contact via the grease composition G. The second pinion teeth 541 and the second rack teeth 315 are meshed via the grease composition G. When the second pinion shaft 54 ​​rotates relative to the housing 33 about its central axis in its extending direction, the rack shaft 31 moves linearly relative to the housing 33 in the extending direction of the housing 33.

[0038] The housing 33 has a second rack guide mechanism 59 fixed to it. The second rack guide mechanism 59 is fixed to a sixth opening 337. The sixth opening 337 is located on the cylindrical surface 313 side of the second rack teeth 314 of the rack shaft 31, at the position where the second pinion shaft 54 ​​engages with the rack shaft 31 in the extending direction of the housing 33.

[0039] The second rack guide mechanism 59 includes a second support yoke 591, a second seat member 592, a second coil spring 593, and a second plug 594. The second seat member 592 is sandwiched between the cylindrical surface 313, which is the other circumferential part of the second rack teeth 314 of the rack shaft 31, and the cylindrical surface of the second support yoke 591. The second seat member 592 is fixed to the second support yoke 591. The second seat member 592 and the cylindrical surface 313, which is the other circumferential part of the second rack teeth 314 of the rack shaft 31, are in sliding contact via a grease composition G. The second seat member 592 includes a metal layer, such as bronze, and a resin layer, such as PTFE, with the resin layer in contact with the cylindrical surface 313 via the grease composition G. The second plug 594 is fixed to the sixth opening 337 of the housing 33. The second plug 594 contacts one end of the second coil spring 593. The second support yoke 591 contacts the other end of the second coil spring 593. The second coil spring 593 is shorter than its free length when the second plug 594 is fixed to the sixth opening 337. Thus, the second seat member 592 is pressed against the rack shaft 31 relative to the housing 33.

[0040] The torque sensor 51 detects the steering torque applied by the driver to the steering wheel 10 via the column shaft 21. The reduction mechanism 53 is an assembly in which a worm 531, which rotates integrally with the output shaft 521 of the electric motor 52, and a worm wheel 532, which rotates integrally with the second pinion shaft 54, are meshed together. Motor current is supplied from the controller 50 to the electric motor 52. The controller 50 controls the electric motor 52 based on the steering torque and vehicle speed detected by the torque sensor 51, and transmits the rotational force of the output shaft 521 of the electric motor 52, which has been reduced by the reduction mechanism 53, to the second pinion shaft 54. The rotational force of the second pinion shaft 54 ​​is applied as a steering assist force from the second pinion teeth 541 to the second rack teeth 315.

[0041] The housing 33 is fixed to an automobile (not shown) with its extending direction aligned with the width direction of the vehicle. Ball joint sockets 11, 11 are fixed to one end and the other end of the rack shaft 31, respectively, and tie rods 12, 12 connected to these ball joint sockets 11, 11 are connected via knuckle arms 13, 13 to the raceways of rolling bearings that rotatably support a pair of left and right front wheels 14, 14. The rack shaft 31 moves linearly in the extending direction of the housing 33, thereby steering the left and right front wheels 14, 14, which are the steering wheels.

[0042] Grease composition G is sealed inside the housing 33. Grease composition G lubricates the space between the rolling and sliding surfaces of the first pinion teeth 321 and the first rack teeth 311, which come into contact when the first pinion teeth 321 and the first rack teeth 311 mesh with each other. Grease composition G lubricates the space between the sliding surface of the first seat member 392 and the sliding surface of the cylindrical surface 312, which is the other circumferential part of the first rack teeth portion 310 of the rack shaft 31, which come into contact when the first seat member 392 and the rack shaft 31 are pressed against each other. Grease composition G lubricates the space between the rolling and sliding surfaces of the second pinion teeth 541 and the second rack teeth 315, which come into contact when the second pinion teeth 541 and the second rack teeth 315 mesh with each other. Grease composition G lubricates the space between the sliding surface of the second seat member 592 and the sliding surface of the cylindrical surface 313, which is the other circumferential part of the second rack teeth portion 314 of the rack shaft 31, which come into contact when the second seat member 592 and the rack shaft 31 are pressed against each other.

[0043] The grease composition of the present invention is sealed as grease composition G in the steering gear device 3 configured in this manner. The grease composition of the present invention can effectively lubricate the meshing portion between the first pinion teeth 321 and the first rack teeth 311, the meshing portion between the second pinion teeth 541 and the second rack teeth 315, the sliding contact portion between the first seat member 392 of the first rack guide mechanism 39 and the rack shaft 31, and the sliding contact portion between the second seat member 592 of the second rack guide mechanism 59 and the rack shaft 31. Therefore, the grease composition of the present invention can reduce the amount of wear on these parts.

[0044] (Column-type electric power steering system) Figure 4 is a schematic diagram showing an example of a column-type electric power steering system 601, including a steering gear unit 603. Figure 5 is a cross-sectional view AA of Figure 4, showing a part of the steering gear device 603. In Figure 5, the lower part of the drawing corresponds to the vertical lower side when mounted on the vehicle.

[0045] The column-type electric power steering system 601 comprises a steering wheel 610, a steering shaft 602, a pinion shaft 632, a rack shaft 631, a housing 633, two rack bushings 630 and 634, two bearings 635 and 636, a rack guide mechanism 639, and a steering assist device 4. A driver operating a vehicle equipped with this column-type electric power steering system 601 steers by rotating the steering wheel 610. The steering shaft 602 comprises a column shaft 621, a first universal joint 623, an intermediate shaft 622, and a second universal joint 624. The first universal joint 623 comprises a first yoke (not shown), a plurality of first rolling elements (not shown), a first cross shaft (not shown), a plurality of second rolling elements (not shown), and a second yoke (not shown). The second universal joint 624 comprises a third yoke (not shown), a plurality of third rolling elements (not shown), a second cross shaft (not shown), a plurality of fourth rolling elements (not shown), and a fourth yoke (not shown).

[0046] A steering wheel 610 is fixed to one end of the column shaft 621 in the extending direction. The first yoke of a first universal joint 623 is fixed to the other end of the column shaft 621 in the extending direction. The column shaft 621 is rotatable about its central axis in the extending direction. The first yoke is pivotably fitted to a first pair of trunnions, which lie on the same central axis of the first cross shaft, via a plurality of first rolling elements. The second yoke is pivotably fitted to a second pair of trunnions, which lie on the same central axis of the first cross shaft, via a plurality of second rolling elements. The central axes of the first pair of trunnions and the central axes of the second pair of trunnions intersect at a 90-degree angle.

[0047] The second yoke of the first universal joint 623 fixes one end of the intermediate shaft 622 in the extending direction. The intermediate shaft 622 fixes the third yoke of the second universal joint 624 to the other end in the extending direction. The third yoke is pivotably fitted via a plurality of third rolling elements to a third pair of trunnions that lie on the same central axis of the second cross shaft. The fourth yoke is pivotably fitted via a plurality of fourth rolling elements to a fourth pair of trunnions that lie on the same central axis of the second cross shaft. The central axes of the third pair of trunnions and the central axes of the fourth pair of trunnions intersect at a 90-degree angle. The fourth yoke of the second universal joint 624 fixes one end of the pinion shaft 632 in the extending direction. As a result, when the driver rotates the steering wheel 610, the column shaft 621 rotates around its central axis in the direction of extension, the intermediate shaft 622 also rotates around its central axis in the direction of extension, and the pinion shaft 632 also rotates around its central axis in the direction of extension.

[0048] Of the column-type electric power steering device 601, the pinion shaft 632, the rack shaft 631, the housing 633, the two rack bushings 630 and 634, the two bearings 635 and 636, and the rack guide mechanism 639 constitute the steering gear device 603 as a rack and pinion type steering device. Figure 4 shows the housing 633 represented by a dashed line (two-dot line) and illustrates its interior.

[0049] The pinion shaft 632 extends vertically from the top to the bottom of the automobile. Along its extending direction, the pinion shaft 632 has a serrated portion 724, a shaft portion 722, a pinion tooth portion 720, and a boss portion 723, extending from one end to the other. The serrations are formed on the serrated portion 724. The serrations on the serrated portion 724 are used to secure the fourth yoke of the second universal joint 624. The shaft portion 722 is cylindrical in shape. Pinion teeth 721 are formed on the entire circumferential surface of the pinion tooth portion 720. The extending direction of the pinion teeth 721 is at an angle that is not 90 degrees with respect to the extending direction of the central axis of the pinion shaft 632. The boss portion 723 is cylindrical in shape.

[0050] The housing 633 has a first opening 732 on the steering wheel 610 side, and is sealed on the opposite side of the first opening 732. The pinion shaft 632 is housed inside the housing 633. The pinion shaft 632 is rotatably supported by two bearings 635 and 636 relative to the housing 633. Bearing 635 is a ball bearing. Bearing 635 includes an inner ring, an outer ring, and balls, with the inner ring fixed to the shaft portion 722 and the outer ring fixed to the housing 633, and the balls rolling between the inner and outer rings. Bearing 636 is a roller bearing. Bearing 636 includes rollers and an outer ring, with the outer ring fixed to the housing 633, and the rollers rolling between the outer circumferential surface of the boss portion 723 and the outer ring.

[0051] With the pinion shaft 632 and two bearings 635 and 636 inserted into the housing 633, the first opening 732 of the housing is fixed with a cover 637 through which the pinion shaft 632 passes. A seal is fixed to the cover 637 and is slidable on the outer circumferential surface 722b of the shaft portion 722 of the pinion shaft 632. Furthermore, a cover member 638 is fixed to the housing 633. The cover member 638 covers a portion of the shaft portion 722 of the pinion shaft 632 from the radial outside.

[0052] The rack shaft 631 comprises a first cylindrical portion 716, a rack tooth portion 710, and a second cylindrical portion 717, extending from one end to the other in the direction of extension. The rack tooth portion 710 has rack teeth 711 on a part of its circumferential direction, and the rack tooth portion 710 has a cylindrical surface 712 on the other part of its circumferential direction with the direction of extension of the rack shaft 631 as its central axis. The outer circumferential surfaces of the first cylindrical portion 716 and the outer circumferential surfaces of the second cylindrical portion 717 are cylindrical surfaces with the direction of extension of the rack shaft 631 as their central axis. The direction of extension of the rack teeth 711 is at an angle that is not 90 degrees with respect to the direction of extension of the rack shaft 631.

[0053] The housing 633 extends in a direction different from the first opening 732 on the steering wheel 610 side and has a second opening 733 at one end in the direction of extension and a third opening 734 at the other end. The rack shaft 631 is housed inside the housing 633 along the direction of extension of the housing 633. One end of the rack shaft 631 in the direction of extension protrudes from the second opening 733 at the one end of the housing 633 in the direction of extension. The other end of the rack shaft 631 in the direction of extension protrudes from the third opening 734 at the other end of the housing 633 in the direction of extension.

[0054] A first rack bush 630 is fixed to one end of the housing 633 in the extending direction. The first rack bush 630 is fixed to the housing 633 adjacent to the second opening 733. The first rack bush 630 is slidable on the outer circumferential surface of the first cylindrical portion 716 of the rack shaft 631. A second rack bush 634 is fixed to the other end of the housing 633 in the extending direction. The second rack bush 634 is fixed to the housing 633 adjacent to the third opening 734. The second rack bush 634 is slidable on the outer circumferential surface of the second cylindrical portion 717 of the rack shaft 631.

[0055] The pinion teeth 721 formed on the pinion teeth portion 720 of the pinion shaft 632 and the rack teeth 711 formed on the rack teeth portion 710 of the rack shaft 631 are in rolling and sliding contact via the grease composition G. The pinion teeth 721 and the rack teeth 711 are meshed via the grease composition G. When the pinion shaft 632 rotates relative to the housing 633 about its central axis in the direction of its extension, the rack shaft 631 moves linearly relative to the housing 633 in the direction of its extension.

[0056] The housing 633 is fixed to an automobile (not shown) with its extension direction aligned with the width direction of the vehicle. Ball joint sockets 11, 11 are fixed to one end and the other end of the rack shaft 631, respectively, and tie rods 12, 12 connected to these ball joint sockets 11, 11 are connected via knuckle arms 13, 13 to the raceways of rolling bearings that rotatably support a pair of left and right front wheels 14, 14. The rack shaft 631 moves linearly in the extension direction of the housing 633, thereby steering the left and right front wheels 14, 14, which are the steering wheels.

[0057] The housing 633 has a rack guide mechanism 639 fixed to it. The housing 633 has a fourth opening 736 on the cylindrical surface 712 side, which is the other side in the circumferential direction of the rack teeth 710 of the rack shaft 631, at the position where the pinion shaft 632 engages with the rack shaft 631 in the extending direction.

[0058] The rack guide mechanism 639 includes a support yoke 791, a seat member 792, a coil spring 793, and a plug 794. The seat member 792 is sandwiched between the cylindrical surface 712, which is the other circumferential part of the rack teeth 710 of the rack shaft 631, and the cylindrical surface of the support yoke 791. The seat member 792 is fixed to the support yoke 791. The seat member 792 and the cylindrical surface 712, which is the other circumferential part of the rack teeth 710 of the rack shaft 631, are in sliding contact via a grease composition G. The seat member 792 includes a metal layer, such as bronze, and a resin layer, such as PTFE, with the resin layer in contact with the cylindrical surface 712 via the grease composition G. The plug 794 is fixed to the fourth opening 736 of the housing 633. The plug 794 is in contact with one end of the coil spring 793. The support yoke 791 is in contact with the other end of the coil spring 793. With the plug 794 fixed to the fourth opening 736, the coil spring 793 is shorter than its free length. Thus, the seat member 792 is pressed against the rack shaft 631 relative to the housing 633.

[0059] The steering assist device 4 includes a controller 40, a torque sensor 41 that detects the steering torque applied by the driver to the steering wheel 610, an electric motor 42, and a reduction mechanism 43 that reduces the rotational force of the output shaft 421 of the electric motor 42 and transmits it to the column shaft 621. The reduction mechanism 43 is an assembly in which a worm 431 that rotates integrally with the output shaft 421 of the electric motor 42 and a worm wheel 432 that rotates integrally with the column shaft 621 are meshed together. Motor current is supplied from the controller 40 to the electric motor 42. The controller 40 controls the electric motor 42 based on the steering torque and vehicle speed detected by the torque sensor 41, and the rotational force of the output shaft 421 of the electric motor 42, which has been reduced by the reduction mechanism 43, is applied to the column shaft 621 as a steering assist force.

[0060] Grease composition G is sealed inside the housing 633. Grease composition G lubricates the space between the rolling and sliding surfaces of the pinion teeth 721 and the rack teeth 711, which come into contact when the pinion teeth 721 and rack teeth 711 mesh with each other. Grease composition G lubricates the space between the sliding surface of the seat member 792 and the sliding surface of the cylindrical surface 712, which is the other circumferential part of the rack teeth portion 710 of the rack shaft 631, which come into contact when the seat member 792 and the rack shaft 631 are pressed against each other.

[0061] The grease composition of the present invention is sealed as grease composition G in the steering gear device 603 configured in this manner. The grease composition of the present invention can effectively lubricate the meshing portion between the pinion teeth 721 and the first rack teeth 711, and the sliding contact portion between the seat member 792 of the rack guide mechanism 639 and the rack shaft 631. Therefore, the grease composition of the present invention can reduce the amount of wear on these parts.

[0062] The grease composition of the present invention can be used by sealing it in the dual-pinion type electric power steering device, column type electric power steering device, etc.

[0063] <Grease composition> A grease composition according to an embodiment of the present invention comprises a base oil, a thickener, and an additive.

[0064] (Base oil) The above base oil consists of a mixture containing poly-α-olefin (PAO) and trimellitic acid ester.

[0065] Examples of the poly-α-olefins mentioned above include α-olefins such as 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, and 1-tetradecene, which are oligomerized or polymerized, and further, those which are hydrogenated. As the above poly-α-olefin, PAO4 to PAO10, which are oligomerized 1-decene, are preferred.

[0066] The preferred kinematic viscosity of the base oil of the above poly-α-olefin at 40°C is 20-60 mm². 2 The kinematic viscosity of the base oil is / s. A more preferable kinematic viscosity of the base oil (at 40°C) is 25-55 mm². 2 It is / s.

[0067] As the trimellitic acid ester mentioned above, trimellitic acid triester is preferred. The reason why trimellitic acid triester is preferred is that it is suitable for improving the heat resistance of the grease composition. Examples of trimellitic acid triesters include reaction products of trimellitic acid with a monoalcohol having 6 to 18 carbon atoms. Among these, reaction products of trimellitic acid with a monoalcohol having 8 and / or 10 carbon atoms are preferred. Specific examples of the trimellitic acid triesters mentioned above include tri-2-ethylhexyl trimellitic acid, tri-normal alkyl (C8, C10) trimellitic acid, triisodecyl trimellitic acid, and tri-normal octyl trimellitic acid. The trimellitic acid triesters mentioned above may be used individually or in combination of two or more types.

[0068] The preferred kinematic viscosity of the base oil of the above trimellitic acid triester at 40°C is 37-57 mm². 2 It is / s.

[0069] The ratio of the above trimellitic acid ester to the total amount of the above trimellitic acid ester and the above poly-α-olefin is 10.0% by mass or more and 60.0% by mass or less. Because the base oil contains 10% by mass or more of trimellitic acid ester, the grease composition readily adheres to the friction surface of the lubricated member. Furthermore, by keeping the proportion of trimellitic acid ester in the base oil of the grease composition at 60% by mass or less, the grease composition avoids corroding the lubricated member. Generally, if the proportion of ester oil is high, the grease composition may corrode surrounding rubber parts.

[0070] (Thickener) The grease composition of the present invention contains lithium 12-hydroxystearate and lithium stearate as thickeners. While lithium stearate can be highly effective in reducing friction, its use alone tends to increase torque at low temperatures. The above grease composition uses both lithium stearate and lithium 12-hydroxystearate, thus suppressing the increase in torque at low temperatures.

[0071] In the above thickener, the ratio of lithium 12-hydroxystearate to the total amount of lithium 12-hydroxystearate and lithium stearate is 5.0% by mass or more and 95.0% by mass or less. If the above proportion of lithium 12-hydroxystearate is less than 5.0% by mass, the above combined effects may not be fully realized. Also, if the above proportion of lithium 12-hydroxystearate exceeds 95.0% by mass, the above combined effects may not be fully realized. The preferred ratio of lithium 12-hydroxystearate to the total amount of lithium 12-hydroxystearate and lithium stearate is 20.0% by mass or more and 75.0% by mass or less, and the more preferred ratio is 25.0% by mass or more and 60.0% by mass or less.

[0072] (Additives) The grease composition of the present invention contains molybdenum dialkyldithiocarbamate and a urea-based additive as additives.

[0073] [Molybdenum dialkyldithiocarbamate] The above-mentioned molybdenum dialkyldithiocarbamate can act as an extreme pressure additive. By using a grease composition containing the above-mentioned molybdenum dialkyldithiocarbamate, wear on the friction surface of the lubricated component can be reduced.

[0074] In the above grease composition, the ratio of molybdenum dialkyldithiocarbamate (hereinafter also referred to as MoDTC) to the total amount of trimellitic acid ester, poly-α-olefin, lithium 12-hydroxystearate, lithium stearate, molybdenum dialkyldithiocarbamate, and urea-based additive (hereinafter also referred to as the MoDTC ratio) is 1.5% by mass or more and 8.0% by mass or less.

[0075] In this case, the grease composition is suitable for forming a tribo-reaction film on the friction surface of the lubricated member, thereby reducing wear on the friction surface. In particular, the grease composition is suitable for simultaneously suppressing wear on the friction surface of a lubricated member made of steel, and suppressing wear on the friction surface of a lubricated member made of resin such as fluororesin. If the above MoDTC ratio is less than 1.5% by mass, the grease composition will not achieve the effect of adding MoDTC. On the other hand, if the above MoDTC ratio exceeds 8.0% by mass, the grease composition will harden, and it will be difficult for the grease composition to penetrate between the friction surfaces of the lubricated members.

[0076] Examples of the above-mentioned MoDTC include compounds represented by the following formula (1).

[0077] [ka]

[0078] (In the formula, R 1 ~R 4 Each of these is independently a linear or branched alkyl group.

[0079] Commercially available products can be used as the above-mentioned ModDTC. Examples of such commercially available products include ADEKA Sakura Lube 200, ADEKA Sakura Lube 165, ADEKA Sakura Lube 525, and ADEKA Sakura Lube 600 (all manufactured by ADEKA).

[0080] [Urea-based additives] The above-mentioned urea-based additives are additives that contain urea compounds. The above urea-based additives have an average diameter of 0.2 μm or larger particles that is between 0.2 μm and 1.0 μm. Because it contains urea-based additives of this size, the grease composition can easily penetrate the lubricated parts (for example, the meshing portion between the pinion teeth and rack teeth in the steering gear device described above, or the sliding contact portion between the seat member and the rack shaft of the rack guide mechanism). Therefore, by using the grease composition containing the above urea-based additives, the friction surfaces of the lubricated parts are less likely to come into contact with each other, and wear of the friction surfaces is suppressed. On the other hand, if the average value of the above diameter exceeds 1.0 μm, the grease composition becomes less likely to penetrate between the friction surfaces of the lubricated components.

[0081] The ratio of the above urea-based additive to the total amount of the above trimellitic acid ester, the above poly-α-olefin, the above lithium 12-hydroxystearate, the above lithium stearate, the above molybdenum dialkyldithiocarbamate, and the above urea-based additive (hereinafter also referred to as the urea-based additive ratio) is 1.2% by mass or more and 3.4% by mass or less. By using a grease composition in which the above urea-based additive ratio is within the above range, the grease composition can significantly reduce wear on the friction surface of the lubricated component.

[0082] As the above-mentioned urea-based additive, a mixture of a urea compound and a styrene-based polymer is preferred, in which the urea compound and the styrene-based polymer are intertwined. The urea-based additive, in which a urea compound and a styrenic polymer exist in an intertwined state, can be produced by the method described below.

[0083] Examples of the urea compound include urea compounds such as diurea, triurea, tetraurea, polyurea (excluding diurea, triurea, and tetraurea), urea-urethane compounds, and mixtures of these.

[0084] For the reason that the above grease composition has favorable heat resistance, the preferred urea compound is diurea represented by the following structural formula (1). R 1 -NHCONH-R 2 -NHCONH-R 3 ···(1) (In formula (1), R 1 and R 3 each independently represent an amino residue, and R 2 represents a diisocyanate residue.) The diurea represented by the above structural formula (1) is a reaction product of an amine compound and a diisocyanate compound.

[0085] The above amine compound may be any one known as an amine compound for synthesizing diurea known as a thickener. Examples of the above amine compound include alkylamines, alkylphenylamines, cyclohexylamine, and the like. Preferred amine compounds are alkylamines from the viewpoints that the grease composition has favorable low-torque properties and that the grease composition has favorable heat resistance.

[0086] The above diisocyanate compound may be any one known as a diisocyanate compound for synthesizing diurea known as a thickener. Examples of the above-mentioned diisocyanate compounds include 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI), a mixture of 2,4-TDI and 2,6-TDI, and 4,4′-diphenylmethane diisocyanate (MDI).

[0087] The above-mentioned styrene-based polymer is a polymer that contains styrene or a derivative thereof as a monomer component. The styrene-based polymer described above may be a homopolymer of styrene or its derivatives, or it may be a copolymer of a first monomer component selected from styrene and its derivatives with another monomer component. The other monomer component may be styrene or its derivatives, as long as it is different from the first monomer component. Examples of the above copolymers include random copolymers, alternating copolymers, block copolymers, and graft copolymers.

[0088] Examples of the above-mentioned styrene homopolymers include atactic polystyrene, isotactic polystyrene, poly-p-methylstyrene, poly-p-ethylstyrene, poly-p-isopropylstyrene, and poly-α-methylstyrene.

[0089] Examples of the copolymers mentioned above include copolymers of a first monomer component selected from styrene and its derivatives with styrene or its derivatives other than the first monomer component. Examples of the copolymers mentioned above include copolymers of the first monomer component and an alkadiene. Examples of alkadienes include butadiene, isoprene, pentadiene, and hexadiene.

[0090] Styrene-isoprene copolymer is preferred as the copolymer. In the above-mentioned styrene-isoprene copolymer, the ratio (molar ratio) of styrene to isoprene should be styrene:isoprene = 1:9 to 9:1. The above copolymer is not limited to copolymers of two monomer components, but may also be copolymers of three or more monomer components.

[0091] The preferred number-average molecular weight of the above styrene-based polymer is between 10,000 and 500,000, and a more preferred number-average molecular weight is between 20,000 and 200,000. The above number-average quantities are measured using gel permeation chromatography.

[0092] Commercially available styrene-based polymers can be used as described above. Examples of commercially available products include Lubrizol® 7306 (manufactured by Lubrizol Nippon Co., Ltd.), 7308, 7460, Infineum® SV140 (manufactured by Infineum Inc.), 150, 160, Septon® 1001 (manufactured by Kuraray Inc.), 1020, and others.

[0093] The preferred content of the styrene polymer is 2% by mass or more and 30% by mass or less relative to the total amount of the urea compound and the styrene polymer. In this case, it is easy to adjust the average diameter of the urea additive to within the above range. The preferred content of the styrene polymer is 2% by mass or more and 20% by mass or less, relative to the total amount of the urea compound and the styrene polymer, and more preferably 2% by mass or more and 9% by mass or less.

[0094] As stated above, the size of the urea-based additives is such that the average diameter of particles with a diameter of 0.2 μm or more is between 0.2 μm and 1.0 μm. The average diameter of the urea-based additives is calculated by assuming the shape of the particles is a perfect sphere and determining the average diameter from the volume of the particles. The average diameter of the above urea-based additives is measured using a confocal laser microscope with a laser beam of 488 nm wavelength as the excitation light. Observation of urea-based additives using a confocal laser microscope is not possible due to the resolution limitations of the microscope, which prevents the observation of particles smaller than 0.2 μm in diameter. Therefore, the above grease composition specifies the average diameter of urea-based additive particles with a diameter of 0.2 μm or larger as the average value of the urea-based additive particles. Furthermore, since observation of urea-based additives using a confocal laser microscope is not possible for particles smaller than 0.2 μm in diameter, the lower limit of the above average diameter is 0.2 μm. However, this does not mean that the above grease composition does not contain urea-based additives with a diameter of less than 0.2 μm.

[0095] When a grease composition containing the above-mentioned urea-based additive is irradiated with laser light of a wavelength of 488 nm, the urea compound contained in the urea-based additive emits fluorescence, and the urea compound is observed as a fluorescence image. Furthermore, in the observation of the urea-based additive in the above-mentioned grease composition, the fluorescence image of the urea compound observed with a confocal laser fluorescence microscope is considered to be the fluorescence image of the particles of the urea-based additive. Furthermore, the size of the urea-based additive can be determined by measuring the volume of the urea-based additive particles in the observed fluorescence images, assuming the shape of the urea-based additive particles is a perfect sphere, calculating the diameter of the urea-based additive particles from the measured volume, and then calculating the average value. The average value of the above diameter can be calculated using commercially available analysis software.

[0096] The above grease composition may contain additives other than molybdenum dialkyldithiocarbamate and urea-based additives (hereinafter referred to as "other additives" in this specification), as long as the effects of the present invention are not impaired. Other additives mentioned above include, for example, antioxidants, rust inhibitors, wear inhibitors, dyes, color stabilizers, thickeners, structural stabilizers, metal deactivators, and viscosity index improvers. If the above grease composition contains other additives, the total mass of the other additives in the grease composition is preferably 15% by mass or less relative to the total mass of the base oil and the thickener.

[0097] The consistency of the above grease composition is preferably between 00 and 2. By adjusting the consistency of the grease composition to this range, sufficient leak resistance can be ensured when it is sealed in a steering gear device, and it can also flow smoothly onto the friction surface of the lubricated component.

[0098] As described above, the grease composition of the present invention can be suitably used in automobile steering gear systems and the like. The above grease composition can also be used as a grease composition for sealing in rolling bearings and the like.

[0099] <Method for producing grease composition> The above grease composition is manufactured by mixing each of its constituent components. Specifically, the above grease composition can be manufactured, for example, by the following procedure.

[0100] (1) Add lithium stearate and lithium 12-hydroxystearate to poly-α-olefin and heat while stirring (for example, 230°C) to dissolve the lithium stearate and lithium 12-hydroxystearate in the poly-α-olefin.

[0101] (2) Subsequently, the poly-α-olefin in which lithium stearate and lithium 12-hydroxystearate are dissolved is cooled, and when it has cooled to a predetermined temperature (for example, 150°C), trimellitic acid ester is mixed in, and cooling is continued to precipitate lithium stearate and lithium 12-hydroxystearate to prepare the base grease. After cooling, homogenization treatment using rolls or the like may be performed as needed.

[0102] (3) Add MoDTC, a urea-based additive, and any other additives as needed to the base grease prepared in step (2) and mix. The above-mentioned grease composition can be manufactured by going through these steps (1) to (3).

[0103] Furthermore, the above-mentioned urea-based additive can be manufactured by the following method. The urea-based additive manufactured by the following method is a urea-based additive in which the urea compound and the styrene-based polymer are intertwined. The above-mentioned urea-based additive is produced by mixing an amine compound and an isocyanate compound in a predetermined molar ratio in the presence of a styrene-based polymer, and reacting the amine compound with the isocyanate compound. Here, the method for producing the above-mentioned urea-based additive will be explained using the case where a diisocyanate compound is used as the isocyanate compound and diurea is synthesized as the urea compound as an example.

[0104] (Method of manufacturing urea-based additives) The above urea-based additives are manufactured, for example, by one of the following manufacturing methods A to C.

[0105] [Manufacturing method A] Figure 6 is a process diagram illustrating an example of a manufacturing method (manufacturing method A) for urea-based additives. (1) Prepare predetermined amounts of the amine compound, the diisocyanate compound, the styrene polymer, solvent A, and solvent B. Specific examples of amine compounds, diisocyanate compounds, and styrene polymers are as described above.

[0106] Preferred solvents A and B each have a lower boiling point than the prepared styrene-based polymer and dissolve the prepared styrene-based polymer. Specific examples of solvents A and B include, for example, toluene, hexane, ethyl acetate, tetrahydrofuran, p-xylene, m-xylene, o-xylene, and methyl acetate. It is preferable to avoid using substances that react with isocyanate groups, such as substances having amine groups or hydroxyl groups, or substances that react with amine groups, as solvents A and B. Preferred solvents A and B have lower viscosity than the prepared styrene-based polymer. In this invention, the viscosity of the solvent and the styrene-based polymer is measured using a Cannon-Fenske viscometer according to the method specified in JIS Z8803:2011.

[0107] Solvent A and solvent B may be the same or different. It is preferable that solvent A and solvent B are the same. In a subsequent step, when mixing mixture A containing solvent A and mixture B containing solvent B, the fact that solvents A and B are identical ensures that mixture A and mixture B mix reliably, which is suitable for promoting the reaction between the amine compound and the diisocyanate compound. Furthermore, when removing solvents A and B in a subsequent step, the fact that solvents A and B are identical makes it easier to select the removal method and conditions.

[0108] (2) Next, a portion of the styrene polymer and an amine compound are added to solvent A to obtain a mixture A (S111). At this time, the timing of adding the styrene polymer and amine compound to solvent A is not particularly limited. (a) A solution may be prepared by dissolving a styrene polymer in solvent A, and then an amine compound may be dissolved or dispersed in the resulting solution to make a mixture A. (b) A mixture may be prepared by dissolving or dispersing an amine compound in solvent A, and then a styrene polymer may be dissolved in the resulting mixture to obtain mixture A. (c) The amine compound and the styrene polymer may be added to solvent A simultaneously, and then all components may be mixed to obtain mixture A. In this case, the amount of the amine compound is, for example, 5% by mass or more and 60% by mass or less, relative to 100% by mass of solvent A. Furthermore, the amount of styrene polymer is, for example, 0.3% by mass or more and 30% by mass or less relative to 100% by mass of solvent A.

[0109] (3) Separately from step (2) above, the remaining styrene polymer and diisocyanate compound are added to solvent B to obtain mixed solution B (S112). At this time, the timing of adding the styrene polymer and diisocyanate compound to solvent B is not particularly limited. (a) A solution may be prepared by dissolving a styrene polymer in solvent B, and then a diisocyanate compound may be dissolved or dispersed in the resulting solution to obtain a mixed solution B. (b) A mixture may be prepared by dissolving or dispersing a diisocyanate compound in solvent B, and then a styrene polymer may be dissolved in the resulting mixture to obtain mixture B. (c) The diisocyanate compound and the styrene polymer may be added to solvent B simultaneously, and then all components may be mixed to obtain mixture B. In this case, the amount of the diisocyanate compound is, for example, 5% by mass or more and 60% by mass or less relative to 100% by mass of solvent B. Furthermore, the amount of styrene-based polymer is, for example, 0.3% by mass or less (30% by mass) relative to 100% by mass of solvent B.

[0110] (4) Next, mixture A and mixture B are mixed to react the amine compound with the diisocyanate compound to synthesize diurea (S113). Mixing of mixture A and mixture B can be done by stirring mixture A while adding mixture B dropwise, or by stirring mixture B while adding mixture A dropwise. The mixing of mixture A and mixture B may be carried out at room temperature or under heating. When mixing liquid mixture A and liquid mixture B under heating, a preferred heating temperature is, for example, 40°C to 110°C.

[0111] Mixture A and Mixture B may be mixed in such a ratio of 2 to 2.2 moles of amine compound to 1 mole of diisocyanate compound. The reaction time between the amine compound and the diisocyanate compound is not particularly limited; it should be sufficient for the reaction to proceed fully. Specifically, the reaction time is, for example, between 0.2 hours and 5 hours.

[0112] In steps (2) to (4) above, the mixing of the amine compound, diisocyanate compound, and styrene polymer into their respective solvents, as well as the mixing of mixture A and mixture B, is carried out using, for example, a mechanical stirrer or a magnetic stirrer. The preferred method for mixing mixture A and mixture B is to use a mechanical stirrer, as this makes it easier to uniformly mix each component.

[0113] By going through steps (1) to (4) as described above, a mixture containing diurea, a styrene-based polymer, solvent A, and solvent B can be obtained.

[0114] (5) Remove solvent A and solvent B from the mixture obtained in step (4) above (S114). The method for removing solvents A and B is not particularly limited. For example, solvents A and B can be vaporized at room temperature, or by heating, reducing pressure, stirring, etc., as appropriate if necessary. The specific method can be appropriately selected depending on the types of solvents A and B, and the following methods are examples. For example, one method involves leaving the above mixture at room temperature and atmospheric pressure to vaporize solvent A and solvent B. Another method involves heating the mixture under atmospheric pressure at a temperature lower than the boiling points of solvents A and B to vaporize them. In this case, the heating conditions are, for example, heating under atmospheric pressure in a constant temperature bath at 40°C for 5 to 10 hours. These methods can be combined.

[0115] (6) Next, the mixture remaining after removing solvent A and solvent B is washed (S115). This washing process removes any unreacted amine compounds or diisocyanate compounds that remain in the mixture. Here are some specific examples of cleaning methods, such as the following: First, the mixture after removing solvents A and B is mixed with water, filtered through a membrane filter, and the residue is collected. Then, the residue is heated at a temperature lower than the boiling point of water and lower than the boiling point of the styrene polymer to vaporize the water adhering to the residue, and the water is removed from the residue. At this time, the heating conditions are, for example, heating in a high-temperature bath at 80°C under atmospheric pressure for 5 to 10 hours.

[0116] (7) The washed mixture is recovered to obtain a urea-based additive containing diurea and a styrene-based polymer (S116). The resulting urea-based additive is usually in powder form. This urea-based additive may be subjected to pulverization as needed. Pulverization can improve the fineness and uniformity of the urea-based additive. Furthermore, pulverization can also be used to adjust the particle size of the urea-based additive. When performing the above-mentioned grinding process, it is preferable to use a small grinder (for example, Lab Millser, manufactured by Osaka Chemical Co., Ltd.) because it can be carried out with simple equipment and at low cost.

[0117] The above-mentioned urea-based additive can be manufactured through this process.

[0118] [Manufacturing method B] Figure 7 is a process diagram illustrating another example (manufacturing method B) of a method for producing urea-based additives. This manufacturing method B includes the steps (S121 to S126) shown in the figure. This manufacturing method B is the same as the manufacturing method for urea-based additives described above, except that the mixed solution A' (S121) obtained by adding an amine compound to solvent A is replaced with mixed solution A (S111 of manufacturing method A) obtained by adding a portion of the styrene polymer and the amine compound to solvent A. In this manufacturing method B, the styrene-based polymer is not added to mixture A', but only to mixture B.

[0119] Mixture A' is obtained by adding an amine compound to solvent A (S121). In this case, the amount of the amine compound is, for example, 5% by mass or more and 60% by mass or less, relative to 100% by mass of solvent A, as in S111 of production method A. The mixing of the amine compound into solvent A can be carried out in the same manner as in S111 of production method A, for example, using a mechanical stirrer or a magnetic stirrer. The mixing of the preferred amine compound into solvent A is carried out using a mechanical stirrer.

[0120] [Manufacturing method C] Figure 8 is a process diagram illustrating another example (manufacturing method C) of a method for producing urea-based additives. This manufacturing method C includes the steps shown in the figure (S131 to S136). This manufacturing method C is the same as the manufacturing method for urea-based additives described above, except that the mixed solution B' (S132) obtained by adding a diisocyanate compound to solvent B is replaced with mixed solution B (S112 in manufacturing method A) obtained by adding a portion of a styrene polymer and a diisocyanate compound to solvent B. In this manufacturing method C, the styrene-based polymer is not added to mixture B', but only to mixture A.

[0121] Mixture B' is obtained by adding a diisocyanate compound to solvent B (S132). At this time, the amount of the diisocyanate compound is, for example, 5% by mass or more and 60% by mass or less, relative to 100% by mass of solvent B, as in S112 of production method A. The mixing of the diisocyanate compound into solvent B can be carried out in the same manner as in S112 of production method A, for example, using a mechanical stirrer or a magnetic stirrer. The preferred mixing of the diisocyanate compound into solvent B is carried out using a mechanical stirrer.

[0122] [Examples of manufacturing methods A-C] The steps for removing solvent A and solvent B (S114, S124, S134) and washing the mixture (S115, S125, S135) may be performed in reverse order. In this case, for example, the following methods can be employed. The above mixture, in which diurea is dispersed in solvents A and B, is placed in a separatory funnel, and water is then added to the separatory funnel to transfer the unreacted amine compound and unreacted diisocyanate compound to the aqueous phase. Next, the water containing the unreacted amine compound and diisocyanate compound is removed from the separatory funnel. Subsequently, solvents A and B are removed from the mixture washed using the separatory funnel by the method of removing solvents A and B (S114, S124, S134).

[0123] The steps of washing the mixture (S115, S125, S135) are not mandatory and may be omitted. [Examples]

[0124] Next, the present invention will be described in more detail based on embodiments of the present invention, but the present invention is not limited to these embodiments.

[0125] The following raw materials were used in the examples / comparative examples. Base oil: Poly-α-olefin:PAO8 (base oil kinematic viscosity at 40°C is 46 mmHg) 2 / s) Trimellitate ester: Trimex N-08NB (manufactured by Kao Corporation, trimellitate tryster)

[0126] Thickener: Lithium stearate 12-Lithium hydroxystearate

[0127] Additives: Molybdenum dialkyldithiocarbamate (MoDTC): Sakura Lube 600 (manufactured by ADEKA) Urea-based additives: Manufactured by the following method. The raw materials used were octylamine, MDI (4,4'-diphenylmethane diisocyanate), and a styrene polymer (styrene-isoprene copolymer: Lubrizol 7306, manufactured by Lubrizol Nippon Co., Ltd.), and the solvent used was toluene.

[0128] (Manufacturing of urea-based additives) (1) A styrene-isoprene copolymer was dissolved in toluene. Furthermore, a predetermined amount of octylamine was mixed into the resulting solution to obtain mixture A. (2) Separately from the above step (1), a predetermined amount of MDI was mixed with a solution of styrene-isoprene copolymer dissolved in toluene to obtain mixed solution B. Here, the amount of styrene-isoprene copolymer added to obtain mixture A and the amount of styrene-isoprene copolymer added to obtain mixture B were the same.

[0129] In steps (1) and (2), the mixing ratio of octylamine to MDI (octylamine:MDI) was 2:1 in molar ratio, and the amount of diurea produced was 40% by mass relative to 100% by mass of toluene. Furthermore, the amount of styrene-isoprene copolymer added was set to be 7.00% by mass of the total amount of diurea and styrene-isoprene copolymer, as described later, in the mixture of diurea and styrene-isoprene copolymer.

[0130] Mixture A was prepared by adding styrene-isoprene copolymer and octylamine while stirring toluene with a mechanical stirrer. Furthermore, the mixture B was prepared by adding styrene-isoprene copolymer and MDI while stirring toluene with a mechanical stirrer.

[0131] (3) While stirring mixture A with a mechanical stirrer, mixture B was added dropwise to mixture A and the two were mixed. After the dropwise addition of mixture B was complete, octylamine and MDI were reacted at room temperature while stirring was continued for 0.5 hours to produce diurea.

[0132] (4) Subsequently, the mixture containing diurea, styrene-isoprene copolymer, and toluene was left at room temperature for 24 hours to evaporate and remove the toluene, thereby completing the mixture of diurea and styrene-isoprene copolymer (urea-based additive).

[0133] (Comparative Example 1: Homogenized base grease) (1) 7.7 parts by mass of lithium stearate and 3.3 parts by mass of lithium 12-hydroxystearate were added to 71.2 parts by mass of poly-α-olefin, and the mixture was heated to 230°C while stirring to dissolve the lithium stearate and lithium 12-hydroxystearate in the poly-α-olefin. Subsequently, the mixture was allowed to cool while stirring, and 17.8 parts by mass of trimellitic acid ester were mixed in when it had cooled to 150°C. Then, the cooling process was continued while stirring until it cooled to 60°C. This resulted in the preparation of a base grease containing precipitated lithium stearate and lithium 12-hydroxystearate.

[0134] The base oil contained in this base grease has a mass ratio of 4:1 between poly-α-olefin and trimellitic acid ester. The mass ratio of lithium stearate to lithium 12-hydroxystearate in the thickener contained in this base grease is 7:3. The mass ratio of base oil to thickener in this base grease is 89:11.

[0135] (2) Next, homogenization was performed using a three-roll mill. The processing conditions at this time were: Roll gap: 50 μm Roller pressure: 1 MPa Rotation speed: 200 r / min Processing temperature: 25℃ That's what I decided.

[0136] Through this process, a homogenized base grease was obtained. The obtained base grease was used as the grease composition for Comparative Example 1.

[0137] (Example 1) In Comparative Example 1, 96.2 parts by mass of the homogenized base grease was mixed with 1.9 parts by mass of MoDTC and 1.9 parts by mass of a urea-based additive. The mixture was then mixed using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0138] The average particle size of the urea-based additives contained in the obtained grease composition was measured using a confocal laser microscope (Leica Microsystems, TCS SP08). The configuration of the confocal laser microscope is shown in Table 1. This device has built-in software (Leica Application Suite X (LAS X) Version 4.4.0) that can calculate the average particle size of the observed material. This software calculates the volume of the urea-based additive for each particle from the acquired 3D image, calculates the diameter of each particle assuming that the shape of each particle is a perfect sphere, and calculates the average value of the diameters of each particle as the measured value (average particle size). The average diameter of the obtained urea-based additives was 0.6 μm.

[0139] [Table 1]

[0140] (Comparative Example 2) In Comparative Example 1, 98.0 parts by mass of the homogenized base grease was mixed with 2.0 parts by mass of MoDTC. The mixture was then mixed using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0141] (Comparative Example 3) In Comparative Example 1, 97.1 parts by mass of the homogenized base grease was mixed with 1.9 parts by mass of MoDTC and 1.0 part by mass of a urea-based additive. The mixture was then mixed using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0142] (Comparative Example 4) In Comparative Example 1, 93.4 parts by mass of the homogenized base grease was mixed with 1.9 parts by mass of MoDTC and 4.7 parts by mass of a urea-based additive. The mixture was then mixed using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0143] (Comparative Example 5) In Comparative Example 1, 89.3 parts by mass of the homogenized base grease was mixed with 1.8 parts by mass of MoDTC and 8.9 parts by mass of a urea-based additive. The mixture was then mixed using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0144] The grease compositions prepared in the examples and comparative examples were subjected to friction and wear tests to evaluate their wear resistance under poor lubrication conditions. The results are shown in Table 3 and Figure 9. Figure 9 shows the results for Example 1 and Comparative Examples 2-5.

[0145] (Friction and wear test) The tests were conducted in accordance with the ASTM D5707 standard. The test equipment used was an SRV2 vibration friction and wear tester (manufactured by OPTIMOL). In this test, the upper test specimen was a cylindrical bearing roller made of SUJ2 (Φ15mm × 22mm, Ra0.1μm), and the upper part of the lower test specimen was a flat PTFE sheet (Φ24mm × 16mm, Ra0.5μm). This test involves pressing the upper test specimen against the lower test specimen with a load of 100N, and then reciprocating the cylindrical roller (the upper test specimen) along the axial direction of the cylindrical roller against the flat PTFE sheet (the upper part of the lower test specimen) for 60 minutes, measuring the depth of the wear marks formed on the flat sheet. Details of the test conditions are shown in Table 2.

[0146] [Table 2]

[0147] [Table 3]

[0148] As shown in Table 3 and Figure 9, the grease composition according to the embodiment of the present invention can reduce the amount of wear on the friction surface of the lubricated member in a poor lubrication environment. In particular, as is clear from the graph shown in Figure 9, by setting the urea-based additive ratio to 1.2% by mass or more and 3.4% by mass or less, the amount of wear in friction and wear tests can be significantly reduced. [Explanation of Symbols]

[0149] 1. Dual-pinion type electric power steering system 2 Steering shaft 3. Steering gear system 33 Housing 31 Rack axis 310 First rack tooth 311 First rack tooth 312 Cylindrical surface 313 Cylindrical surface 314 Second rack tooth 315 Second rack tooth 32 First pinion shaft 320 First pinion tooth 321 First pinion tooth 392 First sheet member 54 Second pinion shaft 540 Second pinion tooth 541 Second pinion tooth 592 Second sheet member 601 Column-type electric power steering system 602 Steering shaft 603 Steering gear system 633 Housing 631 Rack axis 710 Rack teeth 711 Rack teeth 712 Cylindrical surface 632 Pinion shaft 720 Pinion teeth 721 Pinion teeth 792 Sheet material G Grease Composition

Claims

1. It contains a base oil, a thickener, and an additive. The base oil comprises trimellitic acid ester and poly-α-olefin, The trimellitic acid ester is present in an amount of 10.0% by mass or more and 60.0% by mass or less relative to the total amount of the trimellitic acid ester and the poly-α-olefin. The thickener comprises lithium 12-hydroxystearate and lithium stearate. The lithium 12-hydroxystearate is present in proportion to the total amount of lithium 12-hydroxystearate and lithium stearate, which is 5.0% by mass or more and 95.0% by mass or less. The aforementioned additive comprises molybdenum dialkyldithiocarbamate and a urea-based additive. The proportion of the molybdenum dialkyldithiocarbamate to the total amount of the trimellitic acid ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, the molybdenum dialkyldithiocarbamate, and the urea-based additive is 1.5% by mass or more and 8.0% by mass or less. The urea-based additive has an average diameter of 0.2 μm or more for particles with a diameter of 0.2 μm or more, and a minimum diameter of 1.0 μm. The urea-based additive is present in proportion to the total amount of trimellitic acid ester, poly-α-olefin, lithium 12-hydroxystearate, lithium stearate, molybdenum dialkyldithiocarbamate, and the urea-based additive, which is 1.2% by mass or more and 3.4% by mass or less. Grease composition.

2. The grease composition according to claim 1, wherein the urea-based additive is a mixture of a urea compound and a styrene-based polymer.

3. Housing and A rack shaft having rack teeth and capable of reciprocating along the axial direction, A pinion shaft having pinion teeth that mesh with the rack teeth, A rack guide mechanism that biases the rack teeth to the pinion teeth, The present invention comprises a grease composition interposed between the interlocking rack teeth and pinion teeth, and between the circumferential surface of the rack shaft and the portion of the rack guide mechanism pressed against the rack shaft. A steering gear device wherein the grease composition is the grease composition according to claim 1 or 2.

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