Grease composition and steering gear device

Abstract

To provide a grease composition that exhibits excellent lubricating performance even when used in a location susceptible to poor lubrication.SOLUTION: A grease composition comprises a base oil, a thickening agent, and a resin additive. The base oil contains a trimellitic acid ester and a poly-α-olefin, wherein a ratio of the trimellitic acid ester to the total amount thereof is 10.0 mass% or more and 60.0 mass% or less. The thickening agent comprises lithium 12-hydroxystearate and lithium stearate, wherein a ratio of the lithium 12-hydroxystearate to the total amount thereof is 5.0 mass% or more and 95.0 mass% or less. The resin additive is a cellulose particle, wherein an average diameter of the cellulose particle is 1 μm or more and 20 μm or less. A ratio of the resin additive to the total amount of the base oil and the thickening agent is 1.0 mass% or more and 8.0 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] A rack and pinion included in a 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 applies grease to the meshing portion between the rack teeth and the pinion teeth to suppress wear of the rack teeth and the pinion teeth. This reduces the amount of change in the clearance between the rack teeth and the pinion teeth, and maintains the steering performance of the electric power steering device. Greases for use in steering gear devices are proposed in, for example, Patent Documents 1 to 3. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2001-064665 A [Patent Document 2] JP 2009-203374 A [Patent Document 3] JP 2004-269722 A Summary of the Invention

[0004] A grease composition according to one embodiment of the present disclosure includes a base oil, a thickener, and a resin additive, The base oil comprises a trimellitic acid ester and a poly-α-olefin, the ratio of the trimellitate ester to the total amount of the trimellitate ester and the poly-α-olefin is 10.0% by mass or more and 60.0% by mass or less; The thickener comprises lithium 12-hydroxystearate and lithium stearate, the ratio of the lithium 12-hydroxystearate to the total amount of the lithium 12-hydroxystearate and the lithium stearate is 5.0% by mass or more and 95.0% by mass or less; The resin additive is a cellulose particle, The average diameter of the cellulose particles is 1 μm or more and 20 μm or less, The proportion of the resin additive to the total amount of the base oil and the thickener is 1.0 mass % or more and 8.0 mass % or less.

[0005] A steering gear device according to one aspect of the present disclosure includes: Housing and a rack shaft having rack teeth and capable of reciprocating along an axial direction; a pinion shaft having pinion teeth that mesh with the rack teeth; a rack guide mechanism that biases the rack teeth against the pinion teeth; a grease composition interposed between the rack teeth and the pinion teeth which mesh with each other and between a peripheral surface of the rack shaft and a portion of the rack guide mechanism which is pressed against the rack shaft; Equipped with The above grease composition is the grease composition of the present disclosure. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a configuration diagram illustrating an example of a dual pinion type electric power steering device in which the grease composition of the present disclosure is packed. [Diagram 2] 2 is a cross-sectional view taken along line AA in FIG. 1. [Diagram 3] FIG. 2 is a cross-sectional view of FIG. [Figure 4] FIG. 1 is a configuration diagram that illustrates an example of a column-type electric power steering device in which the grease composition of the present disclosure is packed. [Diagram 5] 5 is a cross-sectional view taken along line AA in FIG. 4. [Figure 6] 1 is a graph showing the evaluation results of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] <Problems to be Solved by the Invention of the Present Disclosure> In a steering gear device having a rack and pinion, wear of the rack teeth and the pinion teeth is suppressed by interposing a grease composition in the meshing portion between the rack teeth and the pinion teeth of the rack and pinion. The steering gear device also includes a rack guide mechanism for biasing the rack teeth against the pinion teeth, and wear of both is suppressed by interposing a grease composition between the rack shaft and a portion of the rack guide mechanism that is pressed against the rack shaft. Therefore, there is a demand for a grease composition that can satisfactorily lubricate not only the meshing portion between the rack teeth and the pinion teeth in the rack and pinion of the steering gear device, but also the sliding portion between the rack shaft and the rack guide mechanism.

[0008] However, it was difficult for the grease compositions proposed in Patent Documents 1 to 3 to fully meet such demands. The meshing portions between the rack teeth and the pinion teeth and the sliding portions between the rack shaft and the rack guide mechanism are lubricated portions that are prone to poor lubrication, and it has been difficult to lubricate such portions satisfactorily. In addition, the portion of the rack guide mechanism that is pressed against the rack shaft is made of a resin such as PTFE, and a grease composition designed for lubricating the meshing portion between the steel rack teeth and the steel pinion teeth may not be able to satisfactorily lubricate the sliding portion between the rack shaft and the rack guide mechanism.

[0009] <Effects of the Invention of the Present Disclosure> The grease composition of the present disclosure is easily adsorbed to the friction surface of a lubricated member, and even when used in a portion that is prone to poor lubrication, it exhibits excellent lubricating performance and can suppress wear of the friction surface of the lubricated member. In addition, it is less likely to damage resin members such as PTFE. Therefore, the grease composition is suitable for use in a steering gear device equipped with a rack and pinion.

[0010] The steering gear device of the present disclosure uses the grease composition of the present disclosure, and therefore can suppress wear of the rack teeth and pinion teeth, the peripheral surface of the rack shaft that slides against the rack guide mechanism, and the portion pressed against the rack shaft of the rack guide mechanism over a long period of time, thereby allowing the steering gear device to maintain steering performance for a long period of time.

[0011] <Summary of the embodiment of the present disclosure> Below, an overview of the embodiments of the present disclosure will be listed and described. (1) The grease composition of the present disclosure comprises a base oil, a thickener, and a resin additive, The base oil comprises a trimellitic acid ester and a poly-α-olefin, the ratio of the trimellitate ester to the total amount of the trimellitate ester and the poly-α-olefin is 10.0% by mass or more and 60.0% by mass or less; The thickener comprises lithium 12-hydroxystearate and lithium stearate, the ratio of the lithium 12-hydroxystearate to the total amount of the lithium 12-hydroxystearate and the lithium stearate is 5.0% by mass or more and 95.0% by mass or less; The resin additive is a cellulose particle, The average diameter of the cellulose particles is 1 μm or more and 20 μm or less, The proportion of the resin additive to the total amount of the base oil and the thickener is 1.0 mass % or more and 8.0 mass % or less.

[0012] The grease composition is easily adsorbed to the friction surface of the lubricated member and is not easily removed from the friction surface because the base oil contains a predetermined amount of a trimellitic ester and a poly-α-olefin, and the thickener contains a predetermined amount of lithium stearate and lithium 12-hydroxystearate. In addition, the grease composition contains a resin additive, and the resin additive is cellulose particles, so that the friction surfaces of the lubricated members can be prevented from contacting each other. In addition, since the cellulose particles are soft and easily deformed, they are easily interposed between the friction surfaces of the lubricated members, and are unlikely to damage resin members such as PTFE. Furthermore, since the cellulose particles have high oil absorption, they are likely to form an oil film when interposed between the friction surfaces. Therefore, the grease composition is suitable for lubricating the friction surfaces of lubricated members that are prone to be in poor lubrication environments, and for suppressing wear of these friction surfaces.

[0013] (2) The grease composition according to (1) above further comprises an extreme pressure additive, The extreme pressure additive comprises molybdenum dialkyldithiocarbamate, A preferred ratio of the molybdenum dialkyldithiocarbamate to the total amount of the base oil and the thickener is 0.5% by mass or more and 5.0% by mass or less. A grease composition containing molybdenum dialkyldithiocarbamate can form a tribo-reactive film on the friction surface, and therefore the grease composition is more likely to suppress wear on the friction surface of the lubricated member.

[0014] (3) The steering gear device of the present disclosure includes a housing and a rack shaft having rack teeth and capable of reciprocating along an axial direction; a pinion shaft having pinion teeth that mesh with the rack teeth; a rack guide mechanism that biases the rack teeth against the pinion teeth; a grease composition interposed between the rack teeth and the pinion teeth which mesh with each other, and between a peripheral surface of the rack shaft and a portion of the rack guide mechanism which is pressed against the rack shaft. The grease composition is the grease composition described above in (1) or (2).

[0015] In the steering gear device, the grease composition interposed between the meshing 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, is made of the grease composition of the present 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 clearance of the lubricated portion due to wear is small, and deterioration of steering performance, etc. is unlikely to occur.

[0016] <Details of the embodiment of the present disclosure> Hereinafter, embodiments of the present disclosure will be described. In addition, in this disclosure, the embodiments of the invention should be considered to be illustrative and not restrictive in all respects. The scope of the invention is defined by the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0017] First, an embodiment of a steering gear device in which the grease composition of the present disclosure is used will be described, and then an embodiment of the grease composition of the present disclosure will be described. The grease composition of the present disclosure is used, for example, in dual pinion type electric power steering devices, column type electric power steering devices, etc.

[0018] (Dual pinion type electric power steering device) FIG. 1 is a schematic diagram showing an example of a dual pinion type electric power steering device 1 including a steering gear device 3. Fig. 2 is a cross-sectional view taken along line AA in Fig. 1, showing a part of the steering gear device 3. In Fig. 2, the lower side of the drawing corresponds to the lower side in the vertical direction when mounted on a vehicle. Fig. 3 is a cross-sectional view taken along line BB in Fig. 1, showing a part of the steering gear device 3. In Fig. 3, the lower side of the drawing corresponds to the lower side in the vertical direction when mounted on a vehicle.

[0019] The dual pinion type electric power steering device 1 includes a steering wheel 10, a steering shaft 2, a first pinion shaft 32, a rack shaft 31, a housing 33, two rack bushes 30, 34, two bearings 35, 36, a first rack guide mechanism 39, and a steering assist device 5. The steering assist device 5 includes a controller 50, a torque sensor 51, an electric motor 52, a reduction gear mechanism 53, a second pinion shaft 54, two bearings 55, 56, a worm housing 57, and a second rack guide mechanism 59. The reduction gear mechanism 53 includes a worm 531 and a worm wheel 532.

[0020] A driver of an automobile equipped with this dual pinion type electric power steering device 1 steers the automobile by rotating a steering wheel 10. The steering shaft 2 includes a column shaft 21, a first universal joint 23, an intermediate shaft 22, and a second universal joint 24. The first universal joint 23 includes a first yoke (not shown), a plurality of first rolling elements (not shown), a first cross (not shown), a plurality of second rolling elements (not shown), and a second yoke (not shown). The second universal joint 24 includes a third yoke (not shown), a plurality of third rolling elements (not shown), a second cross (not shown), a plurality of fourth rolling elements (not shown), and a fourth yoke (not shown).

[0021] The column shaft 21 has one end in the extension direction fixed to the steering wheel 10. The column shaft 21 has the other end in the extension direction fixed to a first yoke of a first universal joint 23. The column shaft 21 is rotatable around a central axis in the extension direction. The first yoke is pivotally 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 pivotally 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 second pair of trunnions intersect at an angle of 90 degrees.

[0022] The second yoke of the first universal joint 23 fixes one end of the intermediate shaft 22 in the extension direction. The third yoke of the second universal joint 24 is fixed to the other end of the intermediate shaft 22 in the extension direction. The third yoke is pivotally 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 pivotally 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 fourth pair of trunnions intersect at an angle of 90 degrees. The fourth yoke of the second universal joint 24 fixes one end of the first pinion shaft 32 in the extension direction. As a result, when the driver turns the steering wheel 10, the column shaft 21 rotates around its central axis in the extension direction, the intermediate shaft 22 also rotates around its central axis in the extension direction, and the first pinion shaft 32 also rotates around its central axis in the extension direction.

[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 bushes 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 a steering gear device 3 as a rack-and-pinion steering device. In Fig. 1, the housing 33 is represented by a virtual line (two-dot chain line), and its interior is shown.

[0024] The first pinion shaft 32 extends from the upper side to the lower side in the vertical direction of the automobile. The first pinion shaft 32 has a serration portion 324, a first shaft portion 322, a first pinion tooth portion 320, and a first boss portion 323 from one end side to the other end along the extension direction. Serrations are formed in the serration portion 324. The fourth yoke of the second universal joint 24 is fixed to the serrations of the serration portion 324. The first shaft portion 322 has a cylindrical shape. The first pinion tooth portion 320 has first pinion teeth 321 formed on the entire circumferential surface. The extension direction of the first pinion teeth 321 has an angle that is not 90 degrees with respect to the extension direction of the central axis of the first pinion shaft 32. The first boss portion 323 has a cylindrical shape.

[0025] The housing 33 has a first opening 332 on the steering wheel 10 side, and the side opposite the first opening 332 is sealed. 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, the inner ring is fixed to the first shaft portion 322, the outer ring is fixed to the housing 33, and the balls roll between the inner ring and the outer ring. The second bearing 36 is a roller bearing. The second bearing 36 includes rollers and an outer ring, the outer ring is fixed to the housing 33, and the rollers roll between the outer peripheral 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, a lid 37 through which the first pinion shaft 32 passes is fixed to a first opening 332 of the housing. A seal is fixed to the lid 37, and the seal is slidable on an 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 part of the first shaft portion 322 of the first pinion shaft 32 from the outside in the radial direction.

[0027] The rack shaft 31 includes, from one end in the extending direction to the other end, 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. The first rack tooth portion 310 has first rack teeth 311 formed on a part of its circumferential direction, and the other part of its circumferential direction is a cylindrical surface 312 whose central axis is the extending direction of the rack shaft 31. The second rack tooth portion 314 has second rack teeth 315 formed on a part of its circumferential direction, and the other part of its circumferential direction is a cylindrical surface 313 whose central axis is the extending direction of the rack shaft 31. The outer circumferential surface of the first cylindrical portion 316, the outer circumferential surface of the second cylindrical portion 317, and the outer circumferential surface of the third cylindrical portion 318 are each a cylindrical surface whose central axis is the extending direction of the rack shaft 31. The extension direction of the first rack teeth 311 forms an angle other than 90 degrees with respect to the extension direction of the rack shaft. The extension direction of the second rack teeth 315 forms an angle other than 90 degrees with respect to the extension direction of the rack shaft 31. When the angle of the first rack teeth 311 with respect to the extension direction of the rack shaft 31 is X, the angle of the second rack teeth 315 with respect to the extension direction of the rack shaft 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 accommodated inside the housing 33 along the extending direction of the housing 33. The first cylindrical portion 316 at one end in the extending direction of the rack shaft 31 protrudes from the second opening 333 at one end in the extending direction of the housing 33. The third cylindrical portion 318 at the other end in the extending direction of the rack shaft 31 protrudes from the third opening 334 at the other end in the extending direction of the housing 33. The housing 33 has a fourth opening 335. The fourth opening 335 is located closer to the other end side in the extending direction of the housing 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 extension direction of the housing 33, in the radial direction with the extension direction of the housing 33 as its central axis, and in a direction perpendicular to the first opening 332. The sixth opening 337 is located at approximately the same position as the fourth opening 335 in the extension direction of the housing 33, in the radial direction with the extension direction of the housing 33 as its central axis, and in a direction perpendicular to the fourth opening 335.

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

[0031] The first rack guide mechanism 39 is fixed to the housing 33. The first rack guide mechanism 39 is fixed to the fifth opening 336. The fifth opening 336 is located on the cylindrical surface 312 side, which is the other part of the first rack tooth portion 310 of the rack shaft 31 in the circumferential direction, of the position where the first pinion shaft 32 meshes with the rack shaft 31 in the extending direction of the housing 33.

[0032] The first rack guide mechanism 39 has a first support yoke 391, a first sheet member 392, a first coil spring 393, and a first plug 394. The first sheet member 392 is sandwiched between a cylindrical surface 312, which is the other circumferential part of the first rack tooth portion 310 of the rack shaft 31, and a cylindrical surface of the first support yoke 391. The first sheet member 392 is fixed to the first support yoke 391. The first sheet member 392 and the cylindrical surface 312, which is the other circumferential part of the first rack tooth portion 310 of the rack shaft 31, are in slidable contact with each other via a grease composition G. The first sheet member 392 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is in contact with the cylindrical surface 312 via the grease composition G. The first plug 394 is fixed to the 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 shortened in length relative to its free length with the first plug 394 fixed in 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 vertically from the upper side to the lower side of the automobile. The second pinion shaft 54 ​​has, from one end to the other end along the extension direction, a fitting portion 544, a second shaft portion 542, a second pinion teeth portion 540, and a second boss portion 543. The fitting portion 544 is cylindrical. The second shaft portion 542 is cylindrical. The second pinion teeth portion 540 has second pinion teeth 541 formed on the entire circumferential surface. The extension direction of the second pinion teeth 541 is at an angle other than 90 degrees with respect to the extension direction of the central axis of the second pinion shaft 54. The second boss portion 543 is cylindrical.

[0034] The worm wheel 532 is fitted into the fitting portion 544. The worm 531 is fixed to an output shaft 521 of the 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 disposed in the internal space of the worm housing 57 via 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 disposed in the internal space of the worm housing 57. The worm wheel 532 is disposed in the internal space of the worm housing 57. The worm housing 57 has an eighth opening 572 at the vertically upper side, 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 from the eighth opening 572. The eighth opening is closed by a lid 58. The worm housing 57 has a ninth opening 573 on the opposite side to the eighth opening 572. A 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. A ninth opening 573 of the worm housing 57 and a fourth opening 335 of the housing 33 communicate with each other to seal the internal space from the external space.

[0037] The third bearing 55 is a ball bearing. 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 roll between the inner ring and the outer ring. Bearing 56 is a roller bearing. Bearing 56 includes rollers and an outer ring, with the outer ring fixed to the housing 33, and the rollers roll between the outer peripheral surface of the second boss portion 543 and the outer ring.

[0038] Second pinion teeth 541 formed on second pinion tooth portion 540 of second pinion shaft 54 ​​and second rack teeth 315 formed on second rack tooth portion 314 of rack shaft 31 are in rolling and slidable contact with each other via grease composition G. Second pinion teeth 541 and second rack teeth 315 mesh with each other via grease composition G. When second pinion shaft 54 ​​rotates relative to housing 33 about its central axis in the extension direction, rack shaft 31 moves linearly relative to housing 33 in the extension direction of housing 33.

[0039] A second rack guide mechanism 59 is fixed to the housing 33. 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, which is the other part of the second rack tooth portion 314 of the rack shaft 31 in the circumferential direction, of the position where the second pinion shaft 54 ​​meshes with the rack shaft 31 in the extending direction of the housing 33.

[0040] The second rack guide mechanism 59 has a second support yoke 591, a second sheet member 592, a second coil spring 593, and a second plug 594. The second sheet member 592 is sandwiched between the cylindrical surface 313, which is the other part in the circumferential direction of the second rack tooth portion 314 of the rack shaft 31, and the cylindrical surface of the second support yoke 591. The second sheet member 592 and the cylindrical surface 313, which is the other part in the circumferential direction of the second rack tooth portion 314 of the rack shaft 31, are in slidable contact with each other via a grease composition G. The second sheet member 592 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is 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 shortened to be shorter than its free length with the second plug 594 fixed in the sixth opening 337. Thus, the second seat member 592 is pressed against the rack shaft 31 with respect to the housing 33.

[0041] The torque sensor 51 detects the steering torque applied to the steering wheel 10 by the driver through the column shaft 21. The reduction gear mechanism 53 is an assembly in which a worm 531 that rotates integrally with the output shaft 521 of the electric motor 52 and a worm wheel 532 that rotates integrally with the second pinion shaft 54 ​​are meshed together. A 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 detected by the torque sensor 51, the vehicle speed, and the like, and transmits the rotational force of the output shaft 521 of the electric motor 52, which has been reduced in speed by the reduction gear mechanism 53, to the second pinion shaft 54. The rotational force of the second pinion shaft 54 ​​is applied to the second rack teeth 315 from the second pinion teeth 541 as a steering assist force.

[0042] The housing 33 is fixed to an automobile (not shown) with the extension direction of the housing 33 coinciding with the vehicle width direction. 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 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 turning the left and right front wheels 14, 14, which are steered wheels.

[0043] A grease composition G is enclosed in the housing 33. The grease composition G is interposed between the rolling sliding surfaces of the first pinion teeth 321 and the first rack teeth 311, which come into contact as the first pinion teeth 321 and the first rack teeth 311 mesh with each other, thereby lubricating the area between the two rolling sliding surfaces. The grease composition G is interposed between the sliding surface of the first sheet member 392 and the sliding surface of the cylindrical surface 312, which is the other part in the circumferential direction of the first rack tooth portion 310 of the rack shaft 31, which come into contact as the first sheet member 392 and the rack shaft 31 are pressed against each other, thereby lubricating the area between the two sliding surfaces. The grease composition G is interposed between the rolling and sliding surfaces of the second pinion teeth 541 and the second rack teeth 315, which come into contact as the second pinion teeth 541 and the second rack teeth 315 mesh with each other, thereby lubricating the space between the two rolling and sliding surfaces. The grease composition G is interposed between the sliding surface of the second sheet member 592 and the sliding surface of the cylindrical surface 313, which is the other circumferential part of the second rack tooth portion 314 of the rack shaft 31, which come into contact as the second sheet member 592 and the rack shaft 31 are pressed against each other, thereby lubricating the space between the two sliding surfaces.

[0044] The steering gear device 3 configured in this manner is filled with the grease composition of the present disclosure as the grease composition G. The grease composition of the present disclosure can satisfactorily 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. This can reduce the amount of wear in these portions.

[0045] (Column type electric power steering device) FIG. 4 is a schematic diagram showing an example of a column type electric power steering device 601 including a steering gear device 603. Fig. 5 is a cross-sectional view taken along line AA in Fig. 4, showing a part of the steering gear device 603. In Fig. 5, the lower side of the drawing corresponds to the lower side in the vertical direction when mounted on a vehicle.

[0046] The column type electric power steering device 601 includes a steering wheel 610, a steering shaft 602, a pinion shaft 632, a rack shaft 631, a housing 633, two rack bushes 630 and 634, two bearings 635 and 636, a rack guide mechanism 639, and a steering assist device 4. A driver who drives an automobile equipped with this column type electric power steering device 601 performs steering operation by rotating the steering wheel 610. The steering shaft 602 includes a column shaft 621, a first universal joint 623, an intermediate shaft 622, and a second universal joint 624. The first universal joint 623 includes 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 includes 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] The column shaft 621 has a steering wheel 610 fixed to one end in the extension direction. The column shaft 621 has a first yoke of a first universal joint 623 fixed to the other end in the extension direction. The column shaft 621 is rotatable around a central axis in the extension direction. The first yoke is pivotally 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 pivotally 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 axis of the first pair of trunnions and the central axis of the second pair of trunnions intersect at an angle of 90 degrees.

[0048] The second yoke of the first universal joint 623 fixes one end of the intermediate shaft 622 in the extension direction. The third yoke of the second universal joint 624 is fixed to the other end of the intermediate shaft 622 in the extension direction. The third yoke is pivotally 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 pivotally 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 fourth pair of trunnions intersect at an angle of 90 degrees. The fourth yoke of the second universal joint 624 fixes one end of the pinion shaft 632 in the extension direction. As a result, when the driver turns the steering wheel 610, the column shaft 621 rotates around its central axis in the extension direction, the intermediate shaft 622 also rotates around its central axis in the extension direction, and the pinion shaft 632 also rotates around its central axis in the extension direction.

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

[0050] The pinion shaft 632 extends vertically from the upper side to the lower side of the automobile. The pinion shaft 632 has, from one end to the other end along the extension direction, a serration portion 724, a shaft portion 722, a pinion teeth portion 720, and a boss portion 723. Serrations are formed in the serration portion 724. The fourth yoke of the second universal joint 624 is fixed to the serrations of the serration portion 724. The shaft portion 722 has a cylindrical shape. The pinion teeth portion 720 has pinion teeth 721 formed on the entire circumferential surface. The extension direction of the pinion teeth 721 has an angle that is not 90 degrees with respect to the extension direction of the central axis of the pinion shaft 632. The boss portion 723 has a cylindrical shape.

[0051] Housing 633 has a first opening 732 on the steering wheel 610 side, and the side opposite to first opening 732 is sealed. Pinion shaft 632 is housed inside housing 633. Pinion shaft 632 is rotatably supported by two bearings 635, 636 relative to 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 shaft portion 722 and the outer ring fixed to housing 633, and the balls roll between the inner ring and the outer ring. Bearing 636 is a roller bearing. Bearing 636 includes rollers and an outer ring, with the outer ring fixed to housing 633, and the rollers roll between the outer peripheral surface of boss portion 723 and the outer ring.

[0052] With the pinion shaft 632 and the two bearings 635, 636 inserted into the housing 633, a lid 637 through which the pinion shaft 632 passes is fixed to a first opening 732 of the housing. A seal is fixed to the lid 637, and the seal is slidable on an 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 part of the shaft portion 722 of the pinion shaft 632 from the outside in the radial direction.

[0053] The rack shaft 631 includes, from one end in the extension direction to the other end, a first cylindrical portion 716, a rack teeth portion 710, and a second cylindrical portion 717. The rack teeth portion 710 has rack teeth 711 formed on one portion in the circumferential direction, and the other portion in the circumferential direction is a cylindrical surface 712 whose central axis is the extension direction of the rack shaft 631. The outer circumferential surface of the first cylindrical portion 716 and the outer circumferential surface of the second cylindrical portion 717 are each cylindrical surfaces whose central axis is the extension direction of the rack shaft 631. The extension direction of the rack teeth 711 is at an angle that is not 90 degrees with respect to the extension direction 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 extending direction and a third opening 734 at the other end. The rack shaft 631 is accommodated inside the housing 633 along the extending direction of the housing 633. One end in the extending direction of the rack shaft 631 protrudes from the second opening 733 at one end in the extending direction of the housing 633. The other end in the extending direction of the rack shaft 631 protrudes from the third opening 734 at the other end in the extending direction of the housing 633.

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

[0057] The housing 633 is fixed to an automobile (not shown) with the extension direction of the housing 633 coinciding with the vehicle width direction. 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 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 turning the left and right front wheels 14, 14, which are steered wheels.

[0058] A rack guide mechanism 639 is fixed to the housing 633. The housing 633 has a fourth opening 736 on the cylindrical surface 712 side, which is the other part of the rack tooth portion 710 of the rack shaft 631 in the circumferential direction, at a position where the pinion shaft 632 meshes with the rack shaft 631 in the extending direction.

[0059] The rack guide mechanism 639 has a support yoke 791, a seat member 792, a coil spring 793, and a plug 794. The seat member 792 is sandwiched between a cylindrical surface 712, which is the other part in the circumferential direction of the rack tooth portion 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 part in the circumferential direction of the rack tooth portion 710 of the rack shaft 631, are in slidable contact with each other via a grease composition G. The sheet member 792 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is 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 contacts one end of the coil spring 793. The support yoke 791 contacts the other end of the coil spring 793. The coil spring 793 is shortened to a length shorter than the free length with the plug 794 fixed in the fourth opening 736. Thus, the seat member 792 is pressed against the rack shaft 631 against the housing 633.

[0060] The steering assist device 4 includes a controller 40, a torque sensor 41 that detects the steering torque applied to the steering wheel 610 by the driver, 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 is meshed with a worm wheel 432 that rotates integrally with the column shaft 621. A 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 detected by the torque sensor 41, the vehicle speed, and the like, and the rotational force of the output shaft 421 of the electric motor 42 that has been reduced in speed by the reduction mechanism 43 is applied to the column shaft 621 as a steering assist force.

[0061] Grease composition G is enclosed in housing 633. Grease composition G is interposed between the rolling sliding surfaces of pinion teeth 721 and rack teeth 711, which come into contact as pinion teeth 721 and rack teeth 711 mesh with each other, thereby lubricating the area between the two rolling sliding surfaces. Grease composition G is interposed between the sliding surface of sheet member 792 and the sliding surface of cylindrical surface 712, which is the other part in the circumferential direction of rack tooth portion 710 of rack shaft 631, which come into contact as sheet member 792 and rack shaft 631 are pressed against each other, thereby lubricating the area between the two sliding surfaces.

[0062] The steering gear device 603 configured in this manner is filled with the grease composition of the present disclosure as the grease composition G. The grease composition of the present disclosure can satisfactorily lubricate the meshing portion between the pinion teeth 721 and the first rack teeth 711, and the sliding portion between the seat member 792 of the rack guide mechanism 639 and the rack shaft 631. This can reduce the amount of wear in these portions.

[0063] The grease composition of the present disclosure can be enclosed and used in the above-mentioned dual pinion type electric power steering device, column type electric power steering device, etc.

[0064] <Grease composition> A grease composition according to an embodiment of the present disclosure includes a base oil, a thickener, and a resin additive.

[0065] (Base oil) The base oil is a mixture containing poly-α-olefins (PAOs) and trimellitic esters.

[0066] Examples of the poly-α-olefin include those obtained by oligomerizing or polymerizing α-olefins such as 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, and 1-tetradecene, and further hydrogenating these. The preferred poly-α-olefins are PAO4 to PAO10, which are oligomerized from 1-decene.

[0067] The preferred base oil kinematic viscosity of the poly-α-olefin at 40° C. is 20 to 60 mm 2 More preferably, the base oil kinematic viscosity (40°C) is 25 to 55 mm 2 / s.

[0068] The preferred trimellitic ester is a trimellitic triester, since it is suitable for improving the heat resistance of the grease composition. The trimellitic acid triester may be, for example, a reaction product of trimellitic acid with a monoalcohol having 6 to 18 carbon atoms. Among these, a preferred trimellitic acid triester is a reaction product of trimellitic acid with a monoalcohol having 8 and / or 10 carbon atoms. Specific examples of the trimellitic triester include tri-2-ethylhexyl trimellitate, tri-normal alkyl (C8, C10) trimellitate, triisodecyl trimellitate, and tri-normal octyl trimellitate. As the trimellitic acid triester, only one type of trimellitic acid triester may be used, or two or more types of trimellitic acid triesters may be used in combination.

[0069] The preferred base oil kinematic viscosity of the trimellitic triester at 40° C. is 37 to 57 mm 2 / s.

[0070] 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. Since the base oil contains 10% by mass or more of trimellitic acid ester, the grease composition is easily adsorbed to the friction surface of the lubricated member. In addition, by making the ratio of the trimellitic acid ester in the base oil of the grease composition 60% by mass or less, the grease composition is prevented from corroding the lubricated member. In general, a base oil with a high ratio of ester oil may corrode peripheral rubber parts.

[0071] (Thickener) The grease composition of the present disclosure contains lithium 12-hydroxystearate and lithium stearate as thickeners. While lithium stearate can be highly effective in reducing friction, a grease composition using it alone as a thickener tends to increase the torque of a steering gear device at low temperatures. Therefore, the above grease composition uses lithium stearate in combination with lithium 12-hydroxystearate to suppress the increase in torque of a steering gear device at low temperatures.

[0072] The proportion of lithium 12-hydroxystearate in the thickener relative 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 ratio of lithium 12-hydroxystearate is less than 5.0% by mass, the combined effect may not be fully exhibited.If the ratio of lithium 12-hydroxystearate is more than 95.0% by mass, the combined effect may not be fully exhibited. The proportion 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] (Resin additives) The grease composition of the present disclosure includes a resin additive. The resin additive is a cellulose particle. The cellulose particle is a solid powder. The cellulose particles are softer than additives made of metal compounds. Therefore, the grease composition containing the cellulose particles is easily interposed between the friction surfaces of the lubricated members. In addition, the grease composition sealed in an electric power steering device or the like is unlikely to damage resin members such as PTFE.

[0074] The shape of the cellulose particles is preferably spherical. In particular, this is because they are easily interposed between the friction surfaces of the lubricated members and are less likely to damage members made of resin such as PTFE. Hereinafter, cellulose particles having a spherical shape are also referred to as spherical cellulose particles. The average diameter of the cellulose particles is 1 μm or more and 20 μm or less. If the average diameter is less than 1 μm, the thickness of the oil film formed is unlikely to be large. On the other hand, if the average diameter exceeds 20 μm, the cellulose particles are unlikely to intervene between the friction surfaces.

[0075] The preferred average diameter of the cellulose particles is 8 μm or more and 16 μm or less. The average diameter (D50) of the cellulose particles is measured by a particle size distribution analyzer using a laser diffraction / scattering method. As the cellulose particles, commercially available products can also be used.

[0076] The content of the resin additive in the grease composition is 1.0% by mass or more and 8.0% by mass or less based on the total amount of the base oil and the thickener. If the proportion of the resin additive is less than 1.0% by mass, the grease composition will not provide a sufficient wear suppression effect, whereas if the proportion of the resin additive exceeds 8.0% by mass, the proportion of oil in the grease composition will decrease, resulting in a deterioration in the lubricity of the grease composition. A preferred ratio of the resin additive is 2.0% by mass or more and 6.0% by mass or less based on the total amount of the base oil and the thickener.

[0077] (Extreme Pressure Additives) The preferred grease composition further includes an extreme pressure additive. The preferred extreme pressure additive includes molybdenum dialkyldithiocarbamate. A grease composition containing molybdenum dialkyldithiocarbamate as an extreme pressure additive together with resin fine particles can further reduce friction on the friction surface of a lubricated member.

[0078] When the grease composition contains molybdenum dialkyldithiocarbamate (hereinafter also referred to as MoDTC), the proportion of MoDTC is preferably 0.5 mass % or more and 5.0 mass % or less based on the total amount of the base oil and thickener. In this case, MoDTC is suitable for forming a tribo-reactive film on the friction surface of the lubricated member, and the tribo-reactive film can reduce the wear of the friction surface.

[0079] When the MoDTC ratio is less than 0.5 mass%, the effect of adding MoDTC is difficult to obtain, whereas when the MoDTC ratio exceeds 5.0 mass%, the total proportion of additives is high, and the grease composition becomes hard, which may make it difficult for the grease composition to penetrate into the friction surface of the lubricated member. The proportion of MoDTC is more preferably 0.6 mass% or more and 3.0 mass% or less based on the total amount of the base oil and the thickener, and even more preferably 1.5 mass% or more and 2.5 mass% or less based on the total amount of the base oil and the thickener. Setting the proportion of MoDTC to 1.5 mass % or more and 2.5 mass % or less is extremely 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 a resin such as a fluororesin.

[0080] An example of the MoDTC is a compound represented by the following formula (1).

[0081] [ka]

[0082] (In the formula, R 1 ~R 4 are each independently a linear or branched alkyl group.

[0083] As the MoDTC, a commercially available product may be used, for example, ADEKA SAKURA-LUBE 200, ADEKA SAKURA-LUBE 165, ADEKA SAKURA-LUBE 525, ADEKA SAKURA-LUBE 600 (all manufactured by ADEKA Corporation).

[0084] The grease composition may contain other additives besides the cellulose particles and MoDTC, as long as the effects of the present invention are not impaired. Examples of the other additives include antioxidants, rust inhibitors, antiwear agents, dyes, color stabilizers, thickeners, structure stabilizers, metal deactivators, and viscosity index improvers. When the grease composition contains other additives, the total content of the other additives in the grease composition is preferably 15 mass % or less based on the total mass of the base oil and the thickener.

[0085] The preferred worked penetration of the grease composition is from No. 00 penetration to No. 2 penetration. By adjusting the worked penetration within this range, when the grease composition is enclosed in a steering gear device, leakage of the grease composition from the steering gear device can be sufficiently suppressed and the grease composition can be made to flow satisfactorily onto the friction surface of the member to be lubricated.

[0086] As described above, the grease composition of the present disclosure can be suitably used in automobile steering gear devices and the like. The above grease composition can also be used as a grease composition to be filled into rolling bearings and the like.

[0087] <Method of producing grease composition> The grease composition can be produced by mixing the respective components. Specifically, for example, the grease composition can be produced in the following manner.

[0088] (1) Lithium stearate and lithium 12-hydroxystearate are added to a poly-α-olefin, and the mixture is heated with stirring (e.g., to 230° C.) until the lithium stearate and lithium 12-hydroxystearate are dissolved in the poly-α-olefin.

[0089] (2) The poly-α-olefin having the lithium stearate and lithium 12-hydroxystearate dissolved therein is then cooled, and when it has been cooled to a predetermined temperature (e.g., 150°C), the trimellitate ester is mixed in and the mixture is further cooled to precipitate the lithium stearate and lithium 12-hydroxystearate, thereby preparing the base grease. After cooling, the base grease may be subjected to a homogenization treatment using a roll or the like, if necessary.

[0090] (3) The cellulose particles and, if necessary, MoDTC and other additives are added to the base grease prepared in step (2) and mixed. The above grease composition can be produced through these steps (1) to (3). EXAMPLES

[0091] Next, the present invention will be described in more detail based on examples, but the present invention is not limited to only these examples.

[0092] In the examples and comparative examples, the following raw materials were used. Base oil: Poly-α-olefin: PAO8 (base oil kinematic viscosity at 40°C is 46mm 2 / s) Trimellitic acid ester: Trimex N-08NB (Kao Corporation, trimellitic acid triester)

[0093] Thickener: Lithium stearate Lithium 12-hydroxystearate

[0094] Resin Additives: Spherical cellulose particles (A): Rengo Co., Ltd., Viscopearl D-5, average diameter (D50) = 10.9 μm Spherical cellulose particles (B): Rengo Co., Ltd., Viscopearl D-10, average diameter (D50) = 14.3 μm

[0095] Extreme Pressure Additives: Molybdenum dialkyldithiocarbamate (MoDTC): Sakuralube 600 (manufactured by ADEKA)

[0096] Example 1 (1) 7.7 parts by mass of lithium stearate and 3.3 parts by mass of lithium 12-hydroxystearate were added to 71.0 parts by mass of poly-α-olefin, and the added poly-α-olefin was heated to 230°C while being stirred, so that the lithium stearate and lithium 12-hydroxystearate were dissolved in the poly-α-olefin. Thereafter, the poly-α-olefin having lithium stearate and lithium 12-hydroxystearate dissolved therein was allowed to cool while being stirred, and when it had cooled to 150°C, 18.0 parts by mass of the trimellitic acid ester was mixed in. Thereafter, the poly-α-olefin having the trimellitic acid ester mixed therein was allowed to continue to cool while being stirred, and was cooled to 60°C. This resulted in the preparation of a base grease in which lithium stearate and lithium 12-hydroxystearate were precipitated in a base oil containing poly-α-olefin and trimellitic acid ester.

[0097] (2) The base grease was then homogenized in a triple roll mill under the following processing conditions: Roll gap: 50μm Pressure between rolls: 1MPa Rotation speed: 200r / min Processing temperature: 25℃ It is.

[0098] (3) 5.0 parts by mass of spherical cellulose particles (A) and 2.0 parts by mass of MoDTC were added to the homogenized base grease, and the added base grease was mixed using a planetary centrifugal mixer at a rotational speed of 2000 rpm for 3 minutes to prepare a grease composition. This grease composition is the grease composition of Example 1.

[0099] Example 2 The grease composition of Example 2 was produced through the same process as in Example 1, except that the spherical cellulose particles (B) were used instead of the spherical cellulose particles (A).

[0100] Example 3 The grease composition of Example 3 was produced through the same process as in Example 2, except that MoDTC was not added.

[0101] Comparative Example 1 The grease composition of Comparative Example 1 was a base grease obtained through steps (1) and (2) of Example 1.

[0102] Comparative Example 2 The grease composition of Comparative Example 2 was produced through the same steps as in Example 1, except that the spherical cellulose particles (A) were not added.

[0103] Friction and wear tests were carried out on the grease compositions produced in the Examples and Comparative Examples, and the wear resistance performance under poor lubrication conditions was evaluated. The results are shown in Table 2 and FIG. (Friction and wear test) The test was conducted based on the standard ASTM D5707. The test equipment used was an SRV2 vibration friction and wear tester (manufactured by OPTIMOL). In this test, a cylindrical roller for bearings (made of SUJ2, Φ15mm×22mm, Ra0.1μm) was used as the upper test piece, and a PTFE sheet (Φ24mm×16mm, Ra0.5μm) was used on the top of the lower test piece. In this test, with the upper test piece pressed against the lower test piece with a load of 100 N, the upper test piece was reciprocated along the axial direction of the cylindrical roller for 60 minutes, and the wear depth of the flat plate was measured. Details of the test conditions are shown in Table 1.

[0104] [Table 1]

[0105] [Table 2]

[0106] As shown in the results in Table 2 and FIG. 6, it has become clear that the grease composition according to the embodiment of the present disclosure can reduce the amount of wear on the friction surface of a lubricated member in a poor lubrication environment. [Explanation of symbols]

[0107] 1. Dual pinion type electric power steering device 2 Steering shaft 3. Steering gear device 33 Housing 31 Rack shaft 310 first rack tooth portion 311 1st rack tooth 312 Cylindrical Surface 313 Cylindrical Surface 314 Second rack tooth part 315 Second rack tooth 32 First pinion shaft 320 First pinion tooth portion 321 1st pinion tooth 392 First sheet member 54 Second pinion shaft 540 Second pinion teeth 541 Second pinion tooth 592 Second sheet member 601 Column type electric power steering device 602 Steering shaft 603 Steering gear device 633 Housing 631 Rack shaft 710 Rack teeth 711 Rack teeth 712 Cylindrical Surface 632 Pinion shaft 720 Pinion teeth 721 Pinion teeth 792 Sheet material Grease composition

Claims

1. The present invention relates to a composition comprising a base oil, a thickener, and a resin additive, The base oil comprises a trimellitic acid ester and a poly-α-olefin, a ratio of the trimellitate ester to the total amount of the trimellitate ester and the poly-α-olefin is 10.0 mass% or more and 60.0 mass% or less, The thickener includes lithium 12-hydroxystearate and lithium stearate, the ratio of the lithium 12-hydroxystearate to the total amount of the lithium 12-hydroxystearate and the lithium stearate is 5.0% by mass or more and 95.0% by mass or less, The resin additive is a cellulose particle, The average diameter of the cellulose particles is 1 μm or more and 20 μm or less, The ratio of the resin additive to the total amount of the base oil and the thickener is 1.0 mass% or more and 8.0 mass% or less. Grease composition.

2. Further, it contains an extreme pressure additive, The extreme pressure additive comprises a molybdenum dialkyldithiocarbamate; 2. The grease composition according to claim 1, wherein a ratio of the molybdenum dialkyldithiocarbamate to a total amount of the base oil and the thickener is 0.5 mass % or more and 5.0 mass % or less.

3. Housing and a rack shaft having rack teeth and capable of reciprocating along an axial direction; a pinion shaft having pinion teeth that mesh with the rack teeth; a rack guide mechanism that biases the rack teeth against the pinion teeth; a grease composition interposed between the rack teeth and the pinion teeth that mesh with each other, and between a peripheral surface of the rack shaft and a 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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