Grease composition and steering gear device

Abstract

Provided is a grease composition comprising a base oil, a thickener, and an extreme pressure additive, wherein the base oil includes trimellitate ester and poly-α-olefin, the content of the trimellitate ester is 10.0-60.0 mass% with respect to the total amount of the trimellitate ester and the poly-α-olefin, the thickener includes lithium 12-hydroxystearate and lithium stearate, the content of the lithium 12-hydroxystearate is 5.0-95.0 mass% with respect to the total amount of the lithium 12-hydroxystearate and the lithium stearate, the extreme pressure additive includes molybdenum dialkyldithiocarbamate, and the content of the molybdenum dialkyldithiocarbamate is 0.6-16.0 mass% with respect to the total amount of the trimellitate ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, and molybdenum dialkyldithiocarbamate.

Description

Technical Field

[0001] The present disclosure relates to a grease composition and a steering gear device. This application claims priority based on International Application No. PCT / JP2022 / 003775 filed on February 1, 2022, and incorporates all the descriptions described in the above international application.

Background Art

[0002] The rack and pinion included in the steering gear device used in an electric power steering device includes a rack shaft having rack teeth and a pinion shaft having pinion teeth. This steering gear device suppresses wear of the rack teeth and pinion teeth by interposing grease at the meshing portion between the rack teeth and the pinion teeth. Thereby, the amount of change in the clearance between the rack teeth and the pinion teeth is reduced, and the steering performance of the electric power steering device is maintained. Examples of the grease used for the steering gear device include those proposed in Patent Documents 1 to 3.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0004] The grease composition according to one aspect of the present disclosure includes a base oil, a thickener, and an extreme pressure additive. The base oil includes a trimellitic acid ester and a 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 extreme pressure additive contains molybdenum dialkyldithiocarbamate, The proportion of the above-mentioned molybdenum dialkyldithiocarbamate to the total amount of the above-mentioned trimellitic acid ester, the above-mentioned poly-α-olefin, the above-mentioned lithium 12-hydroxystearate, the above-mentioned lithium stearate, and the above-mentioned molybdenum dialkyldithiocarbamate is 0.6% by mass or more and 16.0% by mass or less.

[0005] A steering gear device according to one aspect of this disclosure 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, 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, Equipped with The above-mentioned grease composition is the grease composition of this disclosure. [Brief explanation of the drawing]

[0006] [Figure 1] This diagram schematically shows an example of a dual-pinion type electric power steering device containing the grease composition of the present disclosure. [Figure 2] This is a cross-sectional view AA in Figure 1. [Figure 3]This is a cross-sectional view of BB in Figure 1. [Figure 4] This diagram schematically shows an example of a column-type electric power steering device in which the grease composition of this disclosure is enclosed. [Figure 5] This is a cross-sectional view AA in Figure 4. [Figure 6] This graph shows the evaluation results for the examples and comparative examples. [Modes for carrying out the invention]

[0007] <Problems that the present invention aims to solve> In a steering gear system with a rack and pinion, a grease composition is interposed at the meshing portion between the rack teeth and pinion teeth to suppress wear on the rack teeth and pinion teeth. Furthermore, the steering gear system is equipped with a rack guide mechanism for biasing the rack teeth onto the pinion teeth, and a grease composition is also interposed between the rack shaft and the portion of the rack guide mechanism that is pressed against the rack shaft to suppress wear on both. Therefore, the steering gear device described above requires a grease composition that can effectively lubricate not only the meshing portion between the rack teeth and pinion teeth in the rack and pinion system, but also the sliding portion between the rack shaft and the rack guide mechanism.

[0008] However, the grease compositions proposed in Patent Documents 1 to 3 have not been able 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 grease compositions designed for lubricating the meshing portion between the steel rack teeth and the steel pinion teeth sometimes fail to adequately lubricate the sliding contact portion between the rack shaft and the rack guide mechanism.

[0009] <Advantages of the Invention of the Present Disclosure> The grease composition of the present disclosure is likely to adsorb on the friction surface of the lubricated member and is also likely to stay on the friction surface by generating a triboreaction film. Therefore, even when used in a site prone to a lean lubrication state, it exhibits excellent lubricating performance and can suppress wear of the friction surface of the lubricated member. Therefore, it is suitable as a grease composition used for a steering gear device provided with a rack and pinion.

[0010] Since the grease composition of the present disclosure is used in the steering gear device of the present disclosure, it is possible to suppress wear of the rack teeth, pinion teeth, the peripheral surface in sliding contact with the rack guide mechanism of the rack shaft, and the portion pressed against the rack shaft of the rack guide mechanism over a long period of time. Therefore, in the above steering gear device, the steering performance can be maintained for a long time.

[0011] <Outline of Embodiments of the Invention of the Present Disclosure> Hereinafter, the outline of the embodiments of the invention of the present disclosure will be listed and described. (1) The grease composition of the present disclosure contains a base oil, a thickener, and an extreme pressure additive. The base oil contains a trimellitic acid ester and a poly-α-olefin. The proportion of the trimellitic acid ester to the total amount of the trimellitic acid ester and the poly-α-olefin is 10.0% by mass or more and 60.0% by mass or less. The thickener contains lithium 12-hydroxystearate and lithium stearate. The proportion 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. The extreme pressure additive contains molybdenum dialkyldithiocarbamate. The proportion of the above-mentioned molybdenum dialkyldithiocarbamate to the total amount of the above-mentioned trimellitic acid ester, the above-mentioned poly-α-olefin, the above-mentioned lithium 12-hydroxystearate, the above-mentioned lithium stearate, and the above-mentioned molybdenum dialkyldithiocarbamate is 0.6% by mass or more and 16.0% by mass or less.

[0012] 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 as an extreme pressure additive, it readily forms a tribo-reaction film on the friction surface. Therefore, the above-mentioned 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.

[0013] (2) In the grease composition of (1) above, it is preferable that the proportion of molybdenum dialkyldithiocarbamate to the total amount of trimellitic acid ester, poly-α-olefin, lithium 12-hydroxystearate, lithium stearate, and molybdenum dialkyldithiocarbamate is 1.3% by mass or more and 8.3% by mass or less. In this case, the grease composition is more suitable for achieving both wear suppression on the friction surface of a lubricated member made of steel and wear suppression on the friction surface of a lubricated member made of resin.

[0014] (3) The steering gear device of the present disclosure comprises a 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-mentioned grease composition is the grease composition of this disclosure.

[0015] In the above-described 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 is pressed against the rack shaft, is made of the grease composition of this disclosure. 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 is pressed 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.

[0016] <Details of Embodiments of the Invention Disclosed> The embodiments of this disclosure will be described below. In this disclosure, the embodiments of the invention should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims and is intended to include all modifications in the sense and scope of equivalence to the claims.

[0017] First, a steering gear device in which the grease composition of this disclosure is used will be described, and then embodiments of the grease composition of this disclosure will be described. The grease composition of this disclosure is used, for example, in a dual-pinion type electric power steering system, a column type electric power steering system, and the like.

[0018] (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.

[0019] 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.

[0020] 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).

[0021] 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.

[0022] 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.

[0023] 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. In Figure 1, the housing 33 is represented by a dashed line (two-dotted line), and its interior is illustrated.

[0024] 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. 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.

[0025] 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.

[0026] 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. A cover member 38 is further 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.

[0027] 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 formed on a part of its circumferential direction, and the other part is 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 formed on a part of its circumferential direction, and the other part is 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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 to the electric motor 52 from the controller 50. 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.

[0042] The housing 33 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 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 extension direction of the housing 33, thereby steering the left and right front wheels 14, 14, which are the steering wheels.

[0043] A grease composition G is sealed inside the housing 33. The 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. The grease composition G also 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.

[0044] The steering gear device 3 configured in this manner is sealed with the grease composition G of the present disclosure. The grease composition of the present disclosure 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 amount of wear on these parts can be reduced.

[0045] (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 downward side when mounted in the vehicle.

[0046] 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).

[0047] 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.

[0048] 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.

[0049] Of the column-type electric power steering device 601, the pinion shaft 632, rack shaft 631, housing 633, two rack bushings 630 and 634, two bearings 635 and 636, and rack guide mechanism 639 constitute the steering gear device 603 as a rack and pinion type steering device. In Figure 4, the housing 633 is represented by a dashed line (two-dot line), and its interior is illustrated.

[0050] The pinion shaft 632 extends from the top to the bottom in the vertical direction of the automobile. The pinion shaft 632 has a serrated portion 724, a shaft portion 722, a pinion tooth portion 720, and a boss portion 723 along its extending direction from one end to the other. Serrations are formed on the serrated portion 724. The serrations on the serrated portion 724 are used to fix 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.

[0051] 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.

[0052] 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. A cover member 638 is further 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.

[0053] 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 formed on a part of its circumferential direction, while the other part in the circumferential direction is a cylindrical surface 712 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 second cylindrical portion 717 are both 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 to the electric motor 42 from the controller 40. 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.

[0061] A grease composition G is sealed inside the housing 633. The 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. The 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.

[0062] The steering gear device 603 configured in this manner is sealed with the grease composition of the present disclosure as the grease composition G. The grease composition of the present disclosure 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 amount of wear on these parts can be reduced.

[0063] The grease composition of this disclosure can be used by sealing it in the dual-pinion type electric power steering device, column type electric power steering device, etc. as described above.

[0064] <Grease composition> The grease composition according to the embodiments of this disclosure comprises a base oil, a thickener, and an extreme pressure additive.

[0065] (Base oil) The above base oil consists of a mixture containing poly-α-olefin (PAO) and trimellitic acid ester.

[0066] 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 poly-α-olefins mentioned above, PAO4 to PAO10, which are oligomerized 1-decene, are preferred.

[0067] The above poly-α-olefin has a base oil kinematic viscosity of 20-60 mmHg at 40°C. 2 / s is preferred. The kinematic viscosity of the base oil (at 40°C) is 25-55 mm². 2 / s is preferable.

[0068] As the trimellitic acid ester mentioned above, trimellitic acid triester is preferred because 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.

[0069] The above trimellitic acid triester has a base oil kinematic viscosity of 37-57 mm at 40°C. 2 It is preferable that it be / s.

[0070] 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. 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 limiting the proportion of trimellitic acid ester in the base oil to 60% by mass or less, the grease composition avoids corrosion of the lubricated member. Generally, a high proportion of ester oil can corrode surrounding rubber parts.

[0071] (Thickener) The grease composition of this disclosure comprises 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. Therefore, the above grease composition uses lithium stearate and lithium 12-hydroxystearate in combination to suppress the increase in torque at low temperatures.

[0072] The above thickener has a ratio of lithium 12-hydroxystearate to the total amount of lithium 12-hydroxystearate and lithium stearate of 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. Furthermore, if the above proportion of lithium 12-hydroxystearate exceeds 95.0% by mass, the above combined effects may not be fully realized. The ratio of lithium 12-hydroxystearate to the total amount of lithium 12-hydroxystearate and lithium stearate is preferably 20.0% by mass or more and 75.0% by mass or less, and more preferably 25.0% by mass or more and 60.0% by mass or less.

[0073] (Extreme pressure additive) The grease composition of this disclosure contains molybdenum dialkyldithiocarbamate as an extreme pressure additive. Therefore, by using the above-mentioned grease composition, the friction of the friction surface of the lubricated member 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 and poly-α-olefin as base oils, lithium 12-hydroxystearate and lithium stearate as thickeners, and molybdenum dialkyldithiocarbamate (hereinafter also referred to as the MoDTC ratio) is 0.6% by mass or more and 16.0% by mass or less. In this case, it is suitable for forming a tribo-reaction film on the friction surface of the lubricated member, thereby reducing wear on the friction surface.

[0075] If the above MoDTC ratio is less than 0.6% by mass, the effect of adding MoDTC cannot be obtained. On the other hand, if the above MoDTC ratio exceeds 16% by mass, the grease composition becomes hard, and it becomes difficult for the grease composition to penetrate the friction surface of the lubricated member. The above MoDTC ratio is preferably 1.0% by mass or more and 10.0% by mass or less, and more preferably 1.3% by mass or more and 8.3% by mass or less. Setting the above-mentioned MoDTC ratio to 1.3% by mass or more and 8.3% by mass or less is extremely suitable for simultaneously suppressing wear on the friction surface of lubricated members made of steel, and suppressing wear on the friction surface of lubricated members made of resins such as fluororesins.

[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 also 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] The above-mentioned grease composition may contain other additives besides extreme pressure additives, to the extent that they do not impair the effects of the invention disclosed herein. Examples of such other additives include antioxidants, rust inhibitors, wear inhibitors, dyes, color stabilizers, thickeners, structural stabilizers, metal deactivators, viscosity index improvers, and the like. If the above grease composition contains other additives, it is preferable that the total content of the other additives in the grease composition be 15% by mass or less relative to the total mass of the base oil and the thickener.

[0081] The consistency of the above grease composition is preferably between 00 and 2. By adjusting the mixing consistency to this range, sufficient leak resistance can be ensured when the mixture is sealed in the steering gear device, and it can be properly supplied to the friction surface of the lubricated component.

[0082] As described above, the grease composition of this disclosure 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.

[0083] <Method for producing grease composition> The above grease composition can be manufactured by mixing each of its constituent components. Specifically, it can be manufactured, for example, by the following procedure.

[0084] (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.

[0085] (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.

[0086] (3) Add MoDTC 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). [Examples]

[0087] Next, the invention of this disclosure will be described in more detail based on examples, but the invention of this disclosure is not limited to the examples.

[0088] 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) Diester: Sansozyme DOS (manufactured by Shin-Nippon Rika Co., Ltd.) Phthalate esters: Sanso-sizer DIDP (manufactured by Shin-Nippon Rika Co., Ltd.)

[0089] Thickener: Lithium stearate 12-Lithium hydroxystearate

[0090] Additives: Molybdenum dialkyldithiocarbamate (MoDTC): Sakura Lube 600 (manufactured by ADEKA)

[0091] (Example 1) (1) 69.6 parts by mass of poly-α-olefin was mixed with 6.0 parts by mass each of lithium stearate and lithium 12-hydroxystearate, 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 17.4 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 process prepared a base grease from which lithium stearate and lithium 12-hydroxystearate precipitated. The base oil contained in this base grease has a mass ratio of poly-α-olefin to trimellitic acid ester of 4:1.

[0092] (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.

[0093] (3) 1.0 parts by mass of MoDTC was added to the homogenized base grease, and the mixture was prepared using a rotary-revolving mixer at a rotation speed of 2000 rpm for 3 minutes to complete the grease composition.

[0094] (Example 2) A grease composition was prepared in the same manner as in Example 1, except that the amount of poly-α-olefin was 68.8 parts by mass, the amount of trimellitic acid ester was 17.2 parts by mass, and the amount of MoDTC was 2.0 parts by mass.

[0095] (Example 3) A grease composition was prepared in the same manner as in Example 1, except that the amount of poly-α-olefin was 66.7 parts by mass, the amount of trimellitic acid ester was 16.7 parts by mass, the amounts of lithium stearate and lithium 12-hydroxystearate were 5.8 parts by mass each, and the amount of MoDTC was 5.0 parts by mass.

[0096] (Example 4) A grease composition was prepared in the same manner as in Example 1, except that the amount of poly-α-olefin was 63.2 parts by mass, the amount of trimellitic acid ester was 15.8 parts by mass, the amounts of lithium stearate and lithium 12-hydroxystearate were 5.5 parts by mass each, and the amount of MoDTC was 10.0 parts by mass.

[0097] (Example 5) A grease composition was prepared in the same manner as in Example 1, except that the amount of poly-α-olefin was 59.7 parts by mass, the amount of trimellitic acid ester was 14.9 parts by mass, the amounts of lithium stearate and lithium 12-hydroxystearate were 5.2 parts by mass each, and the amount of MoDTC was 15.0 parts by mass.

[0098] (Example 6) (1) 41.6 parts by mass of poly-α-olefin were mixed with 7.4 parts by mass each of lithium stearate and lithium 12-hydroxystearate, 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 41.6 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 process prepared a base grease from which lithium stearate and lithium 12-hydroxystearate precipitated. The base oil contained in this base grease has a mass ratio of poly-α-olefin to trimellitic acid ester of 1:1.

[0099] (2) After homogenization treatment was performed under the same conditions as in Example 1, 2.0 parts by mass of MoDTC were added to the base grease, and then a mixing treatment was performed under the same conditions as in Example 1 to complete the grease composition.

[0100] (Comparative Example 1) (1) 5.5 parts by mass each of lithium stearate and lithium 12-hydroxystearate were added to 89.0 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 to 60°C while stirring to precipitate lithium stearate and lithium 12-hydroxystearate. Furthermore, a homogenization treatment was performed under the same conditions as in Example 1 to complete the grease composition.

[0101] (Comparative Example 2) (1) 70.2 parts by mass of poly-α-olefin was mixed with 6.1 parts by mass each of lithium stearate and lithium 12-hydroxystearate, 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 17.6 parts by mass of trimellitic acid ester were mixed in when it had cooled to 150°C. Then, the mixture was allowed to cool to 60°C while stirring to precipitate lithium stearate and lithium 12-hydroxystearate. Furthermore, a homogenization treatment was performed under the same conditions as in Example 1 to complete the grease composition.

[0102] (Comparative Example 3) (1) 5.4 parts by mass each of lithium stearate and lithium 12-hydroxystearate were added to 87.2 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 to 60°C while stirring to prepare a base grease in which lithium stearate and lithium 12-hydroxystearate precipitated.

[0103] (2) After homogenization treatment was performed under the same conditions as in Example 1, 2.0 parts by mass of MoDTC were added to the base grease, and then a mixing treatment was performed under the same conditions as in Example 1 to complete the grease composition.

[0104] (Comparative Example 4) A grease composition was prepared in the same manner as in Example 1, except that the amount of poly-α-olefin was 69.8 parts by mass, the amount of trimellitic acid ester was 17.5 parts by mass, the amounts of lithium stearate and lithium 12-hydroxystearate were 6.1 parts by mass each, and the amount of MoDTC was 0.5 parts by mass.

[0105] (Comparative Example 5) A grease composition was completed in the same manner as in Example 6, except that a diester was used instead of trimellitic acid ester.

[0106] (Comparative Example 6) A grease composition was completed in the same manner as in Example 6, except that a phthalate ester was used instead of trimellitic acid ester.

[0107] 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. Two types of friction and wear tests were conducted using test specimens made of different materials. The results are shown in Table 3 and Figure 6. (Friction and wear test 1) 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, a cylindrical bearing roller made of SUJ2 (Φ15mm × 22mm, Ra0.1μm) was used as the upper test specimen, and a flat plate made of SUJ2 (Φ24mm × 7.8mm, Ra0.2μm) was used as the lower test specimen. In this test, the upper specimen was pressed against the lower specimen with a load of 250 N, and the upper specimen was moved back and forth along the axial direction of the cylindrical roller for 10 minutes. The width of the wear marks generated on the flat plate was then measured. Details of the test conditions are shown in Table 1.

[0108] [Table 1]

[0109] (Friction and wear test 2) The grease composition was evaluated in the same manner as in friction and wear test 1, except for the following changes. • The upper part of the lower test specimen was changed to a PTFE sheet (Φ24mm × 16mm, Ra0.5μm). The load applied to press the upper specimen against the lower specimen was changed to 100N. • The exam duration was changed to 60 minutes. Details of the test conditions are shown in Table 2.

[0110] [Table 2]

[0111] [Table 3]

[0112] As shown in Table 3, the grease composition according to the embodiment of this disclosure was found to be able to reduce the amount of wear on the friction surface of the lubricated member in a poor lubrication environment. Furthermore, as shown in the graph in Figure 6, it became clear that by setting the MoDTC ratio between 1.3% by mass and 8.3% by mass, it is possible to achieve a wear amount of 0.2 mm or less in friction wear test 1, while also achieving a wear amount of 0.5 mm or less in friction wear test 2. [Explanation of symbols]

[0113] 1: Dual-pinion type electric power steering system, 2: Steering shaft, 3: Steering gear system, 33: Housing 31: Rack shaft, 310: First rack tooth section, 311: First rack tooth, 312: Cylindrical surface, 313: Cylindrical surface, 314: Second rack tooth section, 315: Second rack tooth, 32: First pinion shaft, 320: First pinion teeth, 321: First pinion teeth, 392: First seat member 54: Second pinion shaft, 540: Second pinion teeth, 541: Second pinion teeth, 592: Second seat member 601: Column-type electric power steering system, 602: Steering shaft, 603: Steering gear system, 633: Housing 631: Rack shaft, 710: Rack teeth, 711: Rack teeth, 712: Cylindrical surface 632: Pinion shaft, 720: Pinion teeth, 721: Pinion teeth, 792: Seat member G Grease Composition

Claims

1. It contains a base oil, a thickener, and an extreme pressure 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 extreme pressure additive comprises molybdenum dialkyldithiocarbamate, The molybdenum dialkyldithiocarbamate is present in proportion to the total amount of trimellitic acid ester, poly-α-olefin, lithium 12-hydroxystearate, lithium stearate, and molybdenum dialkyldithiocarbamate, which is 0.6% by mass or more and 16.0% by mass or less. The blending consistency ranges from No. 00 to No.

2. Grease composition.

2. The grease composition according to claim 1, wherein 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, and the molybdenum dialkyldithiocarbamate is 1.3% by mass or more and 8.3% by mass or less.

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 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. A steering gear device wherein the grease composition is the grease composition according to claim 1 or 2.

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