WO2024236661A1 - Grease composition and steering gear device - Google Patents

Abstract

A grease composition comprising a base oil, a thickener, and a resin additive, wherein: the base oil comprises a trimellitic acid ester and a poly-α-olefin; the ratio of the trimellitic acid ester to the total amount of the trimellitic acid ester and poly-α-olefin is 10.0 mass% to 60.0 mass%; 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 lithium stearate is 5.0 mass% to 95.0 mass%; the resin additive is crosslinked methacrylic acid ester microparticles and/or crosslinked acrylic acid ester microparticles; and the ratio of the resin additive to the total amount of the base oil and thickener is 1.0 mass% to 8.0 mass%.

Description

Grease composition and steering gear device

The present disclosure relates to a grease composition and a steering gear device.

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.

JP 2001-064665 A JP 2009-203374 A JP 2004-269722 A

A grease composition according to one embodiment of the present disclosure includes a base oil, a thickener, and a resin additive,
The base oil contains 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 mass% or more and 60.0 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 at least one of crosslinked methacrylic acid ester fine particles and crosslinked acrylic acid ester fine particles,
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.

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;
The above grease composition is the grease composition of the present disclosure.

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. 2 is a cross-sectional view taken along line AA of FIG. 1. This is a cross-sectional view taken along line B-B of FIG. 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. This is a cross-sectional view taken along line AA in FIG. 1 is a graph showing the evaluation results of Examples and Comparative Examples.

<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.

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.

<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.

The steering gear device of the present disclosure uses the grease composition of the present disclosure, and therefore can suppress wear over a long period of time on the rack teeth and pinion teeth, the peripheral surface of the rack shaft that comes into sliding contact with the rack guide mechanism, and the portion that is pressed against the rack shaft of the rack guide mechanism. As a result, the steering gear device can maintain steering performance for a long period of time.

<Overview 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 contains 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 mass% or more and 60.0 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 at least one of crosslinked methacrylic acid ester fine particles and crosslinked acrylic acid ester fine particles,
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.

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 acid 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 crosslinked (meth)acrylic acid ester fine particles as a resin additive, which can prevent the friction surfaces of the lubricated members from coming into contact with each other. Furthermore, the crosslinked (meth)acrylic acid ester fine particles have a low relative compressive strength and are easily deformed, so they tend to be interposed between the friction surfaces of the lubricated members and are unlikely to damage resin members such as PTFE.
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.
In this specification, the term "crosslinked (meth)acrylic acid ester fine particles" is a concept that includes both crosslinked methacrylic acid ester fine particles and crosslinked acrylic acid ester fine particles.

(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 mass % or more and 5.0 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.

(3) The steering gear device of the present disclosure includes a housing,
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 the peripheral surface of the rack shaft and a portion of the rack guide mechanism which is pressed against the rack shaft, the grease composition being the grease composition of (1) or (2) above.

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 is suppressed on the rack teeth, pinion teeth, the circumferential surface of the rack shaft that comes into sliding contact with the rack guide mechanism, and the portion of the rack guide mechanism that is pressed against the rack shaft. Therefore, the amount of change in clearance of the lubricated parts due to wear is small, and deterioration of steering performance, etc. is unlikely to occur.

<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.

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.

(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 the steering gear device is 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.

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 mechanism 53, a second pinion shaft 54, two bearings 55, 56, a worm housing 57, and a second rack guide mechanism 59. The reduction mechanism 53 includes a worm 531 and a worm wheel 532.

A driver of an automobile equipped with this dual pinion type electric power steering device 1 steers the vehicle by rotating the 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).

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 its central axis in the extension direction. The first yoke is swingably 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 swingably 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.

The second yoke of the first universal joint 23 fixes one end of the intermediate shaft 22 in its extension direction. The intermediate shaft 22 fixes the third yoke of the second universal joint 24 to the other end in its extension direction. The third yoke is swingably 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 swingably 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 its 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.

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, 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 steering device. In FIG. 1, the housing 33 is represented by a virtual line (two-dot chain line), and its interior is illustrated.

The first pinion shaft 32 extends vertically from the top to the bottom of the vehicle. The first pinion shaft 32 has, from one end to the other along the extension direction, a serration portion 324, a first shaft portion 322, a first pinion tooth portion 320, and a first boss portion 323. 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 is cylindrical. 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 is at an angle other than 90 degrees with respect to the extension direction of the central axis of the first pinion shaft 32. The first boss portion 323 is cylindrical.

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.

When the first pinion shaft 32, the first bearing 35, and the second bearing 36 are inserted into the housing 33, a lid 37 through which the first pinion shaft 32 passes is fixed to the first opening 332 of the housing. A seal is fixed to the lid 37, and the seal is capable of sliding on the outer peripheral 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.

The rack shaft 31 has, from one end in the extension 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 one part of the circumferential direction, and the other part of the circumferential direction is a cylindrical surface 312 whose central axis is the extension direction of the rack shaft 31. The second rack tooth portion 314 has second rack teeth 315 formed on one part of the circumferential direction, and the other part of the circumferential direction is a cylindrical surface 313 whose central axis is the extension direction of the rack shaft 31. The outer peripheral surface of the first cylindrical portion 316, the outer peripheral surface of the second cylindrical portion 317, and the outer peripheral surface of the third cylindrical portion 318 are each cylindrical surfaces whose central axis is the extension direction of the rack shaft 31. The extension direction of the first rack teeth 311 is at 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 is at an angle other than 90 degrees with respect to the extension direction of the rack shaft 31. If 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.

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 extension direction and a third opening 334 at the other end. The rack shaft 31 is stored inside the housing 33 along the extension direction of the housing 33. The first cylindrical portion 316 at one end in the extension direction of the rack shaft 31 protrudes from the second opening 333 at one end in the extension direction of the housing 33. The third cylindrical portion 318 at the other end in the extension direction of the rack shaft 31 protrudes from the third opening 334 at the other end in the extension direction of the housing 33. The housing 33 has a fourth opening 335. The fourth opening 335 is located closer to the other end in the extension 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 in the extension direction of the housing 33 as the first opening 332, 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 in the extension direction of the housing 33 as the fourth opening 335, 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.

The first rack bush 30 is fixed to one end of the housing 33 in the extension direction. The first rack bush 30 is fixed to the housing 33 adjacent to the second opening 333. The first rack bush 30 can slide on the outer peripheral 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 extension direction. The second rack bush 34 is fixed to the housing 33 adjacent to the third opening 334. The second rack bush 34 can slide on the outer peripheral surface of the third cylindrical portion 318 of the rack shaft 31.

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 mesh with each other via the grease composition G. When the first pinion shaft 32 rotates relative to the housing 33 around the central axis in its extension direction, the rack shaft 31 moves linearly relative to the housing 33 in the extension direction of the housing 33.

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 circumferential part of the first rack tooth portion 310 of the rack shaft 31, at the position where the first pinion shaft 32 meshes with the rack shaft 31 in the extension direction of the housing 33.

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 the 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 sliding 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 shorter than its free length with the first plug 394 fixed in the fifth opening 336. Therefore, the first seat member 392 is pressed against the rack shaft 31 against the housing 33.

The second pinion shaft 54 extends vertically from the top to the bottom of the vehicle. The second pinion shaft 54 has, from one end to the other along the extension direction, an engagement portion 544, a second shaft portion 542, a second pinion tooth portion 540, and a second boss portion 543. The engagement portion 544 is cylindrical. The second shaft portion 542 is cylindrical. The second pinion tooth 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 that is not 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.

The worm wheel 532 is fitted into the fitting portion 544. The worm 531 is fixed to the output shaft 521 of the electric motor 52. The electric motor 52 is fixed to the 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 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.

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 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 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 tooth portion 540, and the second boss portion 543 protrude from the ninth opening 573 of the worm housing 57.

The worm housing 57 is fixed to the housing 33. The ninth opening 573 of the worm housing 57 communicates with the fourth opening 335 of the housing 33, sealing the internal space from the external space.

The third bearing 55 is a ball bearing. The bearing 55 includes an inner ring, an outer ring, and balls. The inner ring is fixed to the second shaft portion 542 and the outer ring is fixed to the worm housing 57. The balls roll between the inner ring and the outer ring. The bearing 56 is a roller bearing. The bearing 56 includes rollers and an outer ring. The outer ring is fixed to the housing 33. The rollers roll between the outer peripheral surface of the second boss portion 543 and the outer ring.

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 slidable contact via the grease composition G. The second pinion teeth 541 and the second rack teeth 315 mesh with each other via the grease composition G. When the second pinion shaft 54 rotates relative to the housing 33 about the central axis in its extension direction, the rack shaft 31 moves linearly relative to the housing 33 in the extension direction of the housing 33.

The second rack guide mechanism 59 is fixed to the housing 33. The second rack guide mechanism 59 is fixed to the sixth opening 337. The sixth opening 337 is located on the cylindrical surface 313 side, which is the other circumferential part of the second rack tooth portion 314 of the rack shaft 31, at the position where the second pinion shaft 54 meshes with the rack shaft 31 in the extension direction of the housing 33.

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 circumferential part 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 is fixed to the second support yoke 591. The second sheet member 592 and the cylindrical surface 313, which is the other circumferential part of the second rack tooth portion 314 of the rack shaft 31, are in sliding contact with each other via the 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 shorter than its free length with the second plug 594 fixed to the sixth opening 337. Therefore, the second sheet member 592 is pressed against the rack shaft 31 against the housing 33.

The torque sensor 51 detects the steering torque applied to the steering wheel 10 by the driver through the column shaft 21. The reduction mechanism 53 is an assembly in which a worm 531 that rotates integrally with the output shaft 521 of the electric motor 52 is meshed with a worm wheel 532 that rotates integrally with the second pinion shaft 54. 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 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 in speed by the reduction 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.

The housing 33 is fixed to an automobile (not shown) with the extension direction of the housing 33 aligned with the vehicle width direction. Ball joint sockets 11, 11 are fixed to one end and the other end of the rack shaft 31, 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 the steered wheels.

Grease composition G is sealed in housing 33. Grease composition G is interposed between the rolling sliding surfaces of first pinion teeth 321 and first rack teeth 311, which come into contact as first pinion teeth 321 and first rack teeth 311 mesh with each other, thereby lubricating the area between the two rolling sliding surfaces. Grease composition G is interposed between the sliding surface of first sheet member 392 and the sliding surface of cylindrical surface 312, which is the other circumferential part of first rack tooth portion 310 of rack shaft 31, which come into contact as first sheet member 392 and 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 sliding surface of the second pinion teeth 541 and the rolling sliding surface of 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 area between the two rolling 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 area between the two sliding surfaces.

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 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 portion between the first seat member 392 of the first rack guide mechanism 39 and the rack shaft 31, and the sliding 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.

(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.

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, 634, two bearings 635, 636, a rack guide mechanism 639, and a steering assist device 4. A driver who drives a vehicle equipped with this column type electric power steering device 601 steers the vehicle 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).

The column shaft 621 has one end in the extension direction fixed to the steering wheel 610. The column shaft 621 has the other end in the extension direction fixed to a first yoke of a first universal joint 623. The column shaft 621 is rotatable around a central axis in the extension direction. The first yoke is swingably 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 swingably 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.

The second yoke of the first universal joint 623 fixes one end of the intermediate shaft 622 in its extension direction. The intermediate shaft 622 fixes a third yoke of the second universal joint 624 to the other end in its extension direction. The third yoke is swingably 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 swingably 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 its 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.

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

The pinion shaft 632 extends vertically from the top to the bottom of the vehicle. The pinion shaft 632 has, from one end to the other along the extension direction, a serration portion 724, a shaft portion 722, a pinion tooth 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 is cylindrical. The pinion tooth portion 720 has pinion teeth 721 formed on the entire circumferential surface. The extension direction of the pinion teeth 721 is at an angle other than 90 degrees with respect to the extension direction of the central axis of the pinion shaft 632. The boss portion 723 is cylindrical.

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

When the pinion shaft 632 and the two bearings 635, 636 are inserted into the housing 633, a lid 637 through which the pinion shaft 632 passes is fixed to the first opening 732 of the housing. A seal is fixed to the lid 637, and the seal can slide 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.

The rack shaft 631 has, from one end in the extension direction to the other end, a first cylindrical portion 716, a rack tooth portion 710, and a second cylindrical portion 717. The rack tooth 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 peripheral surface of the first cylindrical portion 716 and the outer peripheral 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 to the extension direction of the rack shaft 631.

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 extension direction and a third opening 734 at the other end. The rack shaft 631 is stored inside the housing 633 along the extension direction of the housing 633. One end of the rack shaft 631 in the extension direction protrudes from the second opening 733 at one end of the housing 633 in the extension direction. The other end of the rack shaft 631 in the extension direction protrudes from the third opening 734 at the other end of the housing 633 in the extension direction.

The first rack bush 630 is fixed to one end of the housing 633 in the extension direction. The first rack bush 630 is fixed to the housing 633 adjacent to the second opening 733. The first rack bush 630 can slide on the outer peripheral 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 extension direction. The second rack bush 634 is fixed to the housing 633 adjacent to the third opening 734. The second rack bush 634 can slide on the outer peripheral surface of the second cylindrical portion 717 of the rack shaft 631.

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

The housing 633 is fixed to an automobile (not shown) with the extension direction of the housing 633 aligned with the vehicle width direction. Ball joint sockets 11, 11 are fixed to one end and the other end of the rack shaft 631, 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 the steered wheels.

The 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 circumferential part of the rack tooth portion 710 of the rack shaft 631, at the position where the pinion shaft 632 meshes with the rack shaft 631 in the extension direction.

The rack guide mechanism 639 has a support yoke 791, a sheet member 792, a coil spring 793, and a plug 794. The sheet member 792 is sandwiched between a cylindrical surface 712, which is the other circumferential part of the rack tooth portion 710 of the rack shaft 631, and the cylindrical surface of the support yoke 791. The sheet member 792 is fixed to the support yoke 791. The sheet member 792 and the cylindrical surface 712, which is the other circumferential part of the rack tooth portion 710 of the rack shaft 631, are in sliding 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 shorter than its 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.

The steering assist device 4 has 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 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, vehicle speed, etc., 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.

Grease composition G is enclosed within housing 633. Grease composition G is present 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 present between the sliding surface of sheet member 792 and the sliding surface of cylindrical surface 712, which is the other circumferential part 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.

The steering gear device 603 configured in this manner is filled with the grease composition of the present disclosure as 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 portion between the sheet member 792 of the rack guide mechanism 639 and the rack shaft 631. This can reduce the amount of wear in these portions.

The grease composition disclosed herein can be enclosed and used in the above-mentioned dual pinion type electric power steering device, column type electric power steering device, etc.

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

(Base oil)
The base oil is a mixture containing poly-α-olefin (PAO) and trimellitic ester.

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.

The kinematic viscosity of the base oil of the poly-α-olefin at 40° C. is preferably 20 to 60 mm ² /s, and more preferably 25 to 55 mm ² /s.

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, tri-isodecyl 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.

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

The proportion 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.
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.

(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 and lithium 12-hydroxystearate in combination to suppress the increase in torque of a steering gear device at low temperatures.

The ratio of lithium 12-hydroxystearate in the thickener 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.
When the ratio of lithium 12-hydroxystearate is less than 5.0% by mass, the combined effect may not be fully exhibited, and when 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.

(Resin Additives)
The grease composition of the present disclosure includes a resin additive.
The resin additive is at least one of crosslinked methacrylic acid ester fine particles and crosslinked acrylic acid ester fine particles.
The resin additive is softer than additives made of metal compounds. Also, the relative compressive strength is small. Therefore, the grease composition containing the resin additive is likely to be interposed between the friction surfaces of the lubricated members. Also, the grease composition sealed in an electric power steering device or the like is unlikely to damage resin members such as PTFE.

Examples of the crosslinked (meth)acrylic acid ester fine particles include those made of an alkyl methacrylate-ethylene glycol dimethacrylate copolymer, an alkyl acrylate-ethylene glycol dimethacrylate copolymer, and the like.
Specific examples of the alkyl methacrylate-ethylene glycol dimethacrylate copolymer include n-butyl methacrylate-ethylene glycol dimethacrylate copolymer (CAS number: 26794-61-6) and methyl methacrylate-ethylene glycol dimethacrylate copolymer (CAS number: 25777-71-3).

As the crosslinked (meth)acrylic acid ester fine particles, commercially available products can also be used.
Commercially available products of the crosslinked (meth)acrylic acid ester fine particles include Techpolymer (registered trademark) BMX series, ARX series, MBX series, and AFX series (all manufactured by Sekisui Plastics Co., Ltd.).

The crosslinked (meth)acrylic acid ester fine particles preferably have a particle size (D50) of 1 μm or more and 40 μm or less, and more preferably have a particle size (D50) of 5 μm or more and 20 μm or less.
The particle size (D50) of the crosslinked (meth)acrylic acid ester fine particles is measured by the electrical sensing zone method (also called the Coulter principle).

The ratio of the resin additive in the grease composition is 1.0 mass % or more and 8.0 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 mass %, the grease composition will not provide a sufficient wear suppression effect, whereas if the proportion of the resin additive exceeds 8.0 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 3.0% by mass or more and 6.0% by mass or less with respect to the total amount of the base oil and the thickener.

(Extreme Pressure Additives)
The preferred grease composition further comprises an extreme pressure additive. The preferred extreme pressure additive comprises 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.

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

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.

An example of the MoDTC is a compound represented by the following formula (1):

(In the formula, R ¹ to R ⁴ are each independently a linear or branched alkyl group.)

As the MoDTC, commercially available products can also be used. Examples of the 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 Corporation).

The grease composition may contain other additives other than the crosslinked (meth)acrylic acid ester fine 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.

The preferred worked penetration of the above 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.

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.

<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.

(1) Lithium stearate and lithium 12-hydroxystearate are added to 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.

(2) Thereafter, the poly-α-olefin having the lithium stearate and lithium 12-hydroxystearate dissolved therein is cooled, and when it has been cooled to a predetermined temperature (e.g., 150° C.), the trimellitic ester is mixed therein, 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.

(3) The crosslinked (meth)acrylic acid fine 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 steps (1) to (3).

Next, the disclosed invention will be explained in more detail based on examples, but the disclosed invention is not limited to only the examples.

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.: 46 mm ² /s)
Trimellitic acid ester: Trimex N-08NB (Kao Corporation, trimellitic acid triester)

Thickener:
Lithium stearate Lithium 12-hydroxystearate

Resin additives:
(A) Cross-linked butyl methacrylate microparticles (manufactured by Sekisui Chemical Co., Ltd., Cross-linked Techpolymer BM30X-8, n-butyl methacrylate-ethylene glycol dimethacrylate copolymer), particle size (D50): 8.01 μm
(B) Crosslinked acrylic ester fine particles (manufactured by Sekisui Chemical Co., Ltd., Techpolymer ARX-806, acrylic acid alkyl ester-ethylene glycol dimethacrylate copolymer), particle size (D50): 9.02 μm
(C) Cross-linked methyl methacrylate: Sekisui Chemical Co., Ltd., Techpolymer MBX-8, methyl methacrylate-ethylene glycol dimethacrylate copolymer), particle size (D50): 7.94 μm

Extreme Pressure Additives:
Molybdenum dialkyldithiocarbamate (MoDTC): Sakuralube 600 (manufactured by ADEKA Corporation)

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. with stirring, so that the lithium stearate and lithium 12-hydroxystearate were dissolved in the poly-α-olefin.
Thereafter, the dissolved poly-α-olefin was allowed to cool while being stirred, and 18.0 parts by mass of the trimellitic acid ester was mixed into the dissolved poly-α-olefin when it had cooled to 150° C. Thereafter, the mixed poly-α-olefin 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.

(2) Next, the base grease was homogenized in a triple roll mill under the following processing conditions:
Roll gap: 50 μm
Pressure between rolls: 1 MPa
Rotation speed: 200 r/min
Processing temperature: 25°C
It is.

(3) 5.0 parts by mass of resin additive (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 centrifugal mixer at a rotation speed of 2000 rpm for a time of 3 minutes to complete the grease composition. This grease composition is the grease composition of Example 1.

Example 2
The grease composition of Example 2 was produced through the same process as in Example 1, except that the resin additive (B) was used instead of the resin additive (A).

Example 3
The grease composition of Example 3 was produced through the same process as in Example 1, except that the resin additive (C) was used instead of the resin additive (A).

Example 4
The grease composition of Example 4 was produced through the same process as in Example 1, except that MoDTC was not added.

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

(Comparative Example 2)
The grease composition of Comparative Example 2 was produced through the same process as in Example 1, except that the resin additive (A) was not added.

The grease compositions prepared in the Examples and Comparative Examples were subjected to friction and wear tests to evaluate the anti-wear performance under poor lubrication conditions. The results are shown in Table 2 and FIG.
(Friction and wear test)
The test was carried out based on the standard ASTM D5707. The test device used was an SRV2 vibration friction and wear tester (manufactured by OPTIMOL).
In this test, a cylindrical roller for bearings (made of SUJ2, Φ15 mm×22 mm, Ra 0.1 μm) was used as the upper test piece, and a PTFE sheet (Φ24 mm×16 mm, Ra 0.5 μm) was used on the upper part 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 generated in the flat plate was measured.
Details of the test conditions are shown in Table 1.

As shown in Table 2 and Figure 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 the lubricated member in a poor lubrication environment.

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: first rack tooth, 312: cylindrical surface, 313: cylindrical surface, 314: second rack tooth portion, 315: second rack tooth,
32: first pinion shaft, 320: first pinion tooth portion, 321: first pinion teeth, 392: first seat member 54: second pinion shaft, 540: second pinion tooth portion, 541: second pinion teeth, 592: second seat member 601: column type electric power steering device, 602: steering shaft, 603: steering gear device, 633: housing 631: rack shaft, 710: rack tooth portion, 711: rack teeth, 712: cylindrical surface 632: pinion shaft, 720: pinion tooth portion, 721: pinion teeth, 792: seat member G Grease composition

Claims (3)

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 at least one of crosslinked methacrylic acid ester fine particles and crosslinked acrylic acid ester fine particles,
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. 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. 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, wherein the grease composition is the grease composition according to claim 1 or 2.

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