JP2024070569A - Grease composition and steering gear device - Google Patents

Abstract

[Problem] To provide a grease composition that exhibits excellent lubricating performance even when used in areas prone to poor lubrication. [Solution] A grease composition containing a base oil, a thickener, and an additive, wherein the base oil contains a trimellitic acid ester and a poly-α-olefin, the thickener contains lithium 12-hydroxystearate and lithium stearate, and the additive contains molybdenum dialkyldithiocarbamate and a urea-based additive, and the base oil, thickener, and additive are each contained in a predetermined ratio relative to the total amount of the base oil, thickener, and additive. [Selected Figure] Figure 1

Description

The present invention 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. By suppressing wear, this steering gear device reduces the amount of change in the clearance between the rack teeth and the pinion teeth, thereby maintaining the steering performance of the electric power steering device.
Greases for use in steering gear devices include those proposed in, for example, Patent Documents 1 to 3.

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

A steering gear device having a rack and pinion has a grease composition interposed between the meshing portion of the rack and pinion between the rack teeth and the pinion teeth, and this grease composition suppresses wear of the rack teeth and the pinion teeth. The steering gear device also has a rack guide mechanism for biasing the rack teeth against the pinion teeth, and this grease composition is interposed between the rack shaft and a portion of the rack guide mechanism that is pressed against the rack shaft, suppressing wear of both.
Therefore, the grease composition used in the above-mentioned steering gear device is required to have the ability to satisfactorily lubricate not only the meshing portion between the rack teeth and pinion teeth in the rack and pinion, but also the sliding portion between the rack shaft and the rack guide mechanism.

However, the grease compositions proposed in Patent Documents 1 to 3 do not 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 becoming poorly lubricated, and it has not been easy 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 to lubricate the meshing portion between the steel rack teeth and the steel pinion teeth is unable to sufficiently lubricate the sliding portion between the rack shaft and the rack guide mechanism.

Under these circumstances, the inventors discovered a grease composition that can easily penetrate into lubricated areas that are prone to poor lubrication and can effectively lubricate the friction surfaces of lubricated members, thus completing the present invention.

(1) The grease composition of the present invention comprises a base oil, a thickener, and an additive,
The base oil contains a trimellitic acid ester and a poly-α-olefin,
the trimellitate ester accounts for 10.0% by mass or more and 60.0% by mass or less based on the total amount of the trimellitate ester and the poly-α-olefin;
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 additive includes molybdenum dialkyldithiocarbamate and a urea-based additive,
the proportion of the molybdenum dialkyldithiocarbamate to the total amount of the trimellitic acid ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, the molybdenum dialkyldithiocarbamate, and the urea-based additive is 1.5% by mass or more and 8.0% by mass or less;
The urea-based additive has an average diameter of particles having a diameter of 0.2 μm or more of 0.2 μm or more and 1.0 μm or less,
The proportion of the urea-based additive to the total amount of the trimellitic acid ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, the molybdenum dialkyldithiocarbamate, and the urea-based additive is 1.2 mass % or more and 3.4 mass % or less.

The above 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.
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, since the grease composition contains a predetermined amount of molybdenum dialkyldithiocarbamate, it is easy to form a tribo-reactive film on the friction surface.
Furthermore, since the grease composition contains a specific amount of a urea-based additive (an additive containing a urea compound) of a specific size, it is interposed between the friction surfaces of the lubricated members, and can suppress contact between the friction surfaces. In addition, the urea-based additive is unlikely to damage the friction surfaces.

(2) In the grease composition of (1) above, the urea-based additive is preferably a mixture of a urea compound and a styrene-based polymer.
In this case, it is more suitable for suppressing contact between the friction surfaces of the lubricated members without damaging the friction surfaces, because the urea-based additive is likely to be present between the friction surfaces of the lubricated members while maintaining an appropriate state of aggregation.

(3) The steering gear device of the present invention comprises:
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 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 invention. In this case, wear is suppressed on 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. 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.

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

The steering gear device of the present invention uses the grease composition of the present invention, which 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. Therefore, the steering gear device can maintain steering performance for a long period of time.

FIG. 1 is a schematic diagram showing an example of a dual-pinion type electric power steering device in which the grease composition of the present invention 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 schematic diagram showing an example of a column-type electric power steering device in which the grease composition of the present invention is packed. This is a cross-sectional view taken along line AA in FIG. FIG. 2 is a process diagram illustrating an example of a method for producing a urea-based additive. FIG. 2 is a process diagram for explaining another example of the method for producing a urea-based additive. FIG. 2 is a process diagram for explaining another example of the method for producing a urea-based additive. 1 is a graph showing the evaluation results of Examples and Comparative Examples.

Hereinafter, an embodiment of the present invention will be described.
In addition, in the present invention, the embodiments of the invention should be considered to be illustrative and not restrictive in all respects. The scope of the rights of the present invention is defined by the claims, and includes 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 invention is used will be described, and then an embodiment of the grease composition of the present invention will be described.
<Steering gear device>
The grease composition of the present invention 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 the steering gear device is 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 a vehicle 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 the first yoke of the first universal joint 23. The column shaft 21 is rotatable around the 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 the extension direction. The intermediate shaft 22 fixes the third yoke of the second universal joint 24 to the other end in the 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 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.

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 shown 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 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.

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 can slide on the outer peripheral surface 322b of the first shaft portion 322 of the first pinion shaft 32. Furthermore, a cover member 38 is fixed to the housing 33. The cover member 38 covers a part of the first shaft portion 322 of the first pinion shaft 32 from the radial outside.

The rack shaft 31 includes, 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 a first rack tooth 311 in a part of the circumferential direction, and the first rack tooth portion 310 has a cylindrical surface 312 with the extension direction of the rack shaft 31 as its central axis in the other part of the circumferential direction. The second rack tooth portion 314 has a second rack tooth 315 in a part of the circumferential direction, and the first rack tooth portion 310 has a cylindrical surface 313 with the extension direction of the rack shaft 31 as its central axis in the other part of the circumferential direction. 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 a cylindrical surface with the extension direction of the rack shaft 31 as its central axis. 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 of 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 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 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 a cylindrical surface of 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 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.

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 sliding 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 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 in 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 and a worm wheel 532 that rotates integrally with the second pinion shaft 54 are meshed together. A motor current is supplied from the controller 50 to the electric motor 52. The controller 50 controls the electric motor 52 based on the steering torque and vehicle speed detected by the torque sensor 51, and transmits the rotational force of the output shaft 521 of the electric motor 52, which has been reduced in speed by the reduction mechanism 53, to the second pinion shaft 54. The rotational force of the second pinion shaft 54 is applied from the second pinion teeth 541 to the second rack teeth 315 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, 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 the steered wheels.

Grease composition G is enclosed in the housing 33. The grease composition G is interposed between the rolling sliding surface of the first pinion tooth 321 and the rolling sliding surface of the first rack tooth 311, which come into contact when the first pinion tooth 321 and the first rack tooth 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 when 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 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 grease composition of the present invention is enclosed as grease composition G in the steering gear device 3 configured in this manner. The grease composition of the present invention can effectively lubricate the meshing portion between the first pinion teeth 321 and the first rack teeth 311, the meshing portion between the second pinion teeth 541 and the second rack teeth 315, the sliding 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. Therefore, the grease composition of the present invention 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 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 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 the extension direction. The intermediate shaft 622 fixes the third yoke of the second universal joint 624 to the other end in the 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 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.

Of 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 steering device. Figure 4 shows the housing 633 with a virtual line (two-dot chain line) and illustrates its interior.

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 housed 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, the inner ring is fixed to the shaft portion 722, the outer ring is fixed to the housing 633, and the balls roll between the inner ring and the outer ring. The bearing 636 is a roller bearing. The bearing 636 includes rollers and an outer ring, the outer ring is fixed to the housing 633, and the rollers roll between the outer peripheral surface of the boss portion 723 and the outer ring.

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 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. Furthermore, a cover member 638 is 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.

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 in one part of the circumferential direction, and the rack tooth portion 710 has a cylindrical surface 712 with the extension direction of the rack shaft 631 as its central axis in the other part of the circumferential direction. 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 with the extension direction of the rack shaft 631 as its central axis. 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 sliding contact with each other via the grease composition G. The pinion teeth 721 and the rack teeth 711 mesh with each other via the grease composition G. When the pinion shaft 632 rotates relative to the housing 633 about the 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, 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 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 part of the rack tooth portion 710 of the rack shaft 631 in the circumferential direction, 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 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. Therefore, 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 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 and a worm wheel 432 that rotates integrally with the column shaft 621 are meshed. A motor current is supplied from the controller 40 to the electric motor 42. The controller 40 controls the electric motor 42 based on the steering torque detected by the torque sensor 41, the vehicle speed, etc., and the rotational force of the output shaft 421 of the electric motor 42 that has been reduced by the reduction mechanism 43 is applied to the column shaft 621 as a steering assist force.

Grease composition G is enclosed in housing 633. Grease composition G is interposed between the rolling sliding surface of pinion tooth 721 and the rolling sliding surface of rack tooth 711, which come into contact as pinion tooth 721 and rack tooth 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 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 grease composition of the present invention is enclosed as grease composition G in the steering gear device 603 configured in this manner. The grease composition of the present invention can effectively lubricate the meshing portion between the pinion teeth 721 and the first rack teeth 711, and the sliding portion between the sheet member 792 of the rack guide mechanism 639 and the rack shaft 631. Therefore, the grease composition of the present invention can reduce the amount of wear in these portions.

The grease composition of the present invention 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>
The grease composition according to an embodiment of the present invention contains a base oil, a thickener, and an 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 those obtained by hydrogenating these.
As the poly-α-olefin, PAO4 to PAO10, which are oligomerized 1-decene, are preferred.

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.

As the trimellitic acid ester, a trimellitic acid triester is preferred because 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 reaction product of trimellitic acid with a monoalcohol having 8 and/or 10 carbon atoms is preferred.
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.
The trimellitic triesters may be used alone or in combination of two or more kinds.

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, since the ratio of the trimellitic acid ester in the base oil of the grease composition is 60% by mass or less, the grease composition is prevented from corroding the lubricated member. In general, if the ratio of the ester oil is large, the grease composition may corrode the surrounding rubber parts.

(Thickener)
The grease composition of the present invention contains lithium 12-hydroxystearate and lithium stearate as thickeners.
Although lithium stearate can be highly effective in reducing friction, when used alone it tends to increase torque at low temperatures.The above grease composition uses lithium stearate and lithium 12-hydroxystearate in combination, so that the increase in torque at low temperatures is suppressed.

In the thickener, the proportion of lithium 12-hydroxystearate to the total amount of lithium 12-hydroxystearate and lithium stearate is 5.0% by mass or more and 95.0% by mass or less.
If the proportion of lithium 12-hydroxystearate is less than 5.0% by mass, the combined effect may not be fully exhibited, and if the proportion 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.

(Additive)
The grease composition of the present invention contains, as additives, molybdenum dialkyldithiocarbamate and a urea-based additive.

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

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

In this case, the grease composition is suitable for forming a tribo-reactive film on the friction surface of the lubricated member, thereby reducing wear of the friction surface. In particular, the grease composition is suitable for suppressing wear on the friction surface of the lubricated member made of steel and on the friction surface of the lubricated member made of resin such as fluororesin.
If the MoDTC ratio is less than 1.5 mass %, the effect of adding MoDTC to the grease composition cannot be obtained, whereas if the MoDTC ratio exceeds 8.0 mass %, the grease composition becomes hard and the grease composition becomes difficult to penetrate between the friction surfaces of the lubricated members.

The above-mentioned MoDTC can be, for example, 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 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).

[Urea-based additives]
The urea-based additive is an additive containing a urea compound.
The urea-based additive has particles having a diameter of 0.2 μm or more, and the average diameter of the particles is 0.2 μm or more and 1.0 μm or less.
Since the grease composition contains a urea-based additive having such dimensions, it is easy for the grease composition to penetrate into the lubricated parts (for example, the meshing part between the pinion teeth and the rack teeth in the steering gear device described above, and the sliding part between the seat member and the rack shaft of the rack guide mechanism, etc.) Therefore, by using a grease composition containing the urea-based additive, the friction surfaces of the lubricated parts are less likely to come into contact with each other, and wear of the friction surfaces is suppressed.
On the other hand, if the average diameter exceeds 1.0 μm, the grease composition will have difficulty penetrating between the friction surfaces of the members to be lubricated.

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

As the urea-based additive, a mixture of a urea compound and a styrene-based polymer, in which the urea compound and the styrene-based polymer are present in an entangled state, is preferred.
The urea-based additive in which the urea compound and the styrene-based polymer are present in an entangled state can be produced by the method described below.

Examples of the urea compounds include urea compounds such as diurea, triurea, tetraurea, polyurea (excluding diurea, triurea, and tetraurea), urea-urethane compounds, and mixtures thereof.

The preferred urea compound is a diurea represented by the following structural formula (1), because the grease composition has good heat resistance.
R ¹ -NHCONH-R ² -NHCONH-R ³ ... (1)
(In formula (1), ^R1 and ^R3 each independently represent an amino residue, and ^R2 represents a diisocyanate residue.)
The diurea represented by the above structural formula (1) is a reaction product of an amine compound and a diisocyanate compound.

The amine compound may be any known amine compound for synthesizing diurea, which is known as a thickening agent.
Examples of the amine compound include alkylamines, alkylphenylamines, and cyclohexylamines.
A preferred amine compound is an alkylamine, since the grease composition has good low torque properties and good heat resistance.

The diisocyanate compound may be any known diisocyanate compound for synthesizing diurea, which is known as a thickening agent.
Examples of the diisocyanate compound include 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI), a mixture of 2,4-TDI and 2,6-TDI, and 4,4'-diphenylmethane diisocyanate (MDI).

The styrene-based polymer is a polymer containing styrene or a derivative thereof as a monomer component.
The styrene-based polymer may be a homopolymer of styrene or a derivative thereof, or a copolymer of a first monomer component selected from styrene and its derivatives with another monomer component, which may be styrene or a derivative thereof, provided that the other monomer component is different from the first monomer component.
Examples of the copolymer include random copolymers, alternating copolymers, block copolymers, and graft copolymers.

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

The copolymer may be, for example, a copolymer of a first monomer component selected from styrene and its derivatives, and styrene or a derivative thereof other than the first monomer component.
The copolymer may be, for example, a copolymer of the first monomer component with an alkadiene, such as butadiene, isoprene, pentadiene, or hexadiene.

As the copolymer, a styrene-isoprene copolymer is preferred.
In the styrene-isoprene copolymer, the ratio (molar ratio) of styrene to isoprene may be styrene:isoprene=1:9 to 9:1.
The copolymer is not limited to a copolymer of two types of monomer components, but may be a copolymer of three or more types of monomer components.

The number average molecular weight of the styrene-based polymer is preferably from 10,000 to 500,000, and more preferably from 20,000 to 200,000.
The number average molecular weight is determined using gel permeation chromatography.

As the styrene-based polymer, a commercially available product can be used.
Specific examples of commercially available products include Lubrizol (registered trademark) 7306 (manufactured by Lubrizol Japan Corporation), Lubrizol 7308, Lubrizol 7460, Infineum (registered trademark) SV140 (manufactured by Infineum Corporation), Infineum SV150, Infineum SV160, Septon (registered trademark) 1001 (manufactured by Kuraray Co., Ltd.), and Septon 1020.

The content of the styrene-based polymer is preferably 2% by mass or more and 30% by mass or less based on the total amount of the urea compound and the styrene-based polymer, in which case it is easy to adjust the average diameter of the urea-based additive within the above range.
The content of the styrene-based polymer is preferably 2% by mass or more and 20% by mass or less, and more preferably 2% by mass or more and 9% by mass or less, based on the total amount of the urea compound and the styrene-based polymer.

As described above, the size of the urea-based additive is such that the average diameter of the particles having a diameter of 0.2 μm or more is 0.2 μm or more and 1.0 μm or less. The average diameter of the urea-based additive is the average diameter calculated from the volume of the particles, assuming that the shape of the urea-based additive particles is a perfect sphere.
The average diameter of the urea-based additive is measured using a confocal laser microscope with a laser beam having a wavelength of 488 nm as excitation light.
When observing urea-based additives with a confocal laser microscope, due to the resolution of the confocal laser microscope, particles with a diameter of less than 0.2 μm cannot be observed. Therefore, the grease composition specifies the average diameter of urea-based additive particles with a diameter of 0.2 μm or more as the average diameter of the urea-based additives. In addition, when observing urea-based additives with a confocal laser microscope, since particles with a diameter of less than 0.2 μm cannot be observed, the lower limit of the average diameter is 0.2 μm.
However, this does not mean that the grease composition does not contain urea-based additives with a diameter of less than 0.2 μm.

When a grease composition containing the urea-based additive is irradiated with laser light having a wavelength of 488 nm, the urea compound contained in the urea-based additive emits fluorescence, and the urea compound can be observed as a fluorescent image.
In observing the urea-based additive of the grease composition, a fluorescent image of the urea compound observed with a confocal laser fluorescence microscope is regarded as a fluorescent image of particles of the urea-based additive.
In addition, the size of the urea-based additive can be determined by measuring the volume of the urea-based additive particles in the observed fluorescent image, regarding the shape of the urea-based additive particles as a perfect sphere, calculating the diameter of the urea-based additive particles from the measured volume, and calculating the average value thereof.
The average diameter value may be calculated using commercially available analysis software.

The grease composition may contain additives other than the molybdenum dialkyldithiocarbamate and the urea-based additive (hereinafter, also referred to as “other additives” in this specification) as long as the effects of the present invention are not impaired.
Examples of the other additives include antioxidants, rust inhibitors, anti-wear agents, dyes, color stabilizers, thickeners, structure stabilizers, metal deactivators, and viscosity index improvers.
When the grease composition contains other additives, the total content by mass 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 worked penetration of the above grease composition is preferably from No. 00 penetration to No. 2 penetration.
By adjusting the worked penetration of the grease composition within this range, sufficient leakage resistance can be ensured when the grease composition is filled into a steering gear device, and the grease composition can be made to flow satisfactorily onto the friction surfaces of members to be lubricated.

As described above, the grease composition of the present invention 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 in rolling bearings and the like.

<Method of producing grease composition>
The grease composition is produced by mixing the respective components. Specifically, the grease composition can be produced, for example, by the following procedure.

(1) Add lithium stearate and lithium 12-hydroxystearate to poly-α-olefin, and heat with stirring (e.g., 230°C) to dissolve the lithium stearate and lithium 12-hydroxystearate 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), a trimellitic ester is mixed therein, and cooling is continued to cause the lithium stearate and lithium 12-hydroxystearate to precipitate, thereby preparing a base grease.
After cooling, a homogenization treatment using a roll or the like may be carried out as necessary.

(3) MoDTC, the urea-based additive, and other additives to be contained as required are added to the base grease prepared in step (2) and mixed.
The above grease composition can be produced through steps (1) to (3).

The urea-based additive can be produced by the following method. The urea-based additive produced by the following method is a urea-based additive in which a urea compound and a styrene-based polymer are entangled with each other.
The urea-based additive is produced by mixing an amine compound and an isocyanate compound in a predetermined molar ratio in the presence of a styrene-based polymer, and reacting the amine compound with the isocyanate compound. Here, the production method of the urea-based additive will be described using an example in which a diisocyanate compound is used as the isocyanate compound and diurea is synthesized as the urea compound.

(Production method of urea-based additives)
The urea-based additive is produced, for example, by any one of the following production methods A to C.

[Production method A]
FIG. 6 is a process diagram for explaining an example of a method for producing a urea-based additive (production method A).
(1) Predetermined amounts of an amine compound, a diisocyanate compound, a styrene-based polymer, a solvent A, and a solvent B are prepared.
Specific examples of the amine compound, the diisocyanate compound, and the styrene-based polymer are as described above.

Each of the preferable solvents A and B has a boiling point lower than that of the prepared styrene-based polymer and dissolves the prepared styrene-based polymer.
Specific examples of the solvent A and the solvent B include toluene, hexane, ethyl acetate, tetrahydrofuran, p-xylene, m-xylene, o-xylene, methyl acetate, etc. It is preferable to avoid using, as the solvent A and the solvent B, substances that react with substances having an isocyanate group, such as substances having an amine group and substances having a hydroxyl group, or substances that react with substances having an amine group.
The preferred solvents A and B have a lower viscosity than the provided styrene-based polymer.
In the present invention, the viscosity of the solvent and the styrene-based polymer is measured using a Cannon-Fenske viscometer according to the method of JIS Z8803:2011.

The solvent A and the solvent B may be the same or different. It is preferable that the solvent A and the solvent B are the same.
It is suitable for progressing the reaction between the amine compound and the diisocyanate compound that the solvent A and the solvent B are the same so that the mixed liquid A and the mixed liquid B are reliably mixed when the mixed liquid A containing the solvent A and the mixed liquid B containing the solvent B are mixed in a later step. In addition, when the solvent A and the solvent B are removed in the later step, the fact that the solvent A and the solvent B are the same makes it easy to select the removal method and removal conditions.

(2) Next, a part of the styrene-based polymer and an amine compound are added to the solvent A to obtain a mixed solution A (S111).
In this case, the timing of adding the styrene-based polymer and the amine compound to the solvent A is not particularly limited,
(a) A styrene-based polymer may be dissolved in a solvent A to prepare a solution, and then an amine compound may be dissolved or dispersed in the obtained solution to prepare a mixed solution A.
(b) An amine compound may be dissolved or dispersed in a solvent A to prepare a mixed liquid, and then a styrene-based polymer may be dissolved in the resulting mixed liquid to prepare mixed liquid A.
(c) The amine compound and the styrene-based polymer may be added simultaneously to the solvent A, and then all the components may be mixed to prepare the mixed solution A.
In this case, the amount of the amine compound is, for example, 5% by mass or more and 60% by mass or less with respect to 100% by mass of the solvent A.
The amount of the styrene-based polymer is, for example, 0.3% by mass or more and 30% by mass or less relative to 100% by mass of the solvent A.

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

(4) Next, the mixed liquid A and the mixed liquid B are mixed, and the amine compound and the diisocyanate compound are reacted to synthesize diurea (S113).
The mixed liquid A and the mixed liquid B may be mixed by dropping mixed liquid B into mixed liquid A while stirring the mixed liquid A, or by dropping mixed liquid A into mixed liquid B while stirring the mixed liquid B, and then mixing the two.
Mixture of mixed solution A and mixed solution B may be carried out at room temperature or under heating.
When the mixed liquid A and the mixed liquid B are mixed under heating, the heating temperature is preferably, for example, 40° C. or higher and 110° C. or lower.

The mixed solution A and the mixed solution B may be mixed so that the ratio of the diisocyanate compound is 1 mol per 2 to 2.2 mol of the amine compound.
The time for reacting the amine compound with the diisocyanate compound is not particularly limited as long as the reaction proceeds sufficiently. Specifically, the reaction time is, for example, 0.2 hours or more and 5 hours or less.

In the above steps (2) to (4), the mixing of the amine compound, the diisocyanate compound, and the styrene-based polymer into their respective solvents, and the mixing of mixed liquid A and mixed liquid B are carried out using, for example, a mechanical stirrer or a magnetic stirrer. The preferred method for mixing mixed liquid A and mixed liquid B is to use a mechanical stirrer, as this makes it easier to mix the components uniformly.

By going through steps (1) to (4) like this, a mixture containing diurea, a styrene-based polymer, solvent A, and solvent B can be obtained.

(5) Solvent A and solvent B are removed from the mixture obtained in the above step (4) (S114).
The method for removing solvent A and solvent B is not particularly limited, and examples thereof include vaporizing solvent A and solvent B at room temperature or, if necessary, while appropriately performing heating, decompression, stirring, etc. Specific methods may be appropriately selected depending on the types of solvent A and solvent B, and the following methods can be exemplified.
For example, the mixture may be left at room temperature under atmospheric pressure to vaporize solvent A and solvent B.
Another example is a method in which the mixture is heated under atmospheric pressure at a temperature lower than the boiling points of solvent A and solvent B to vaporize solvent A and solvent B. In this case, the heating conditions are, for example, heating under atmospheric pressure in a thermostatic bath at 40° C. for 5 hours to 10 hours.
These methods may be combined.

(6) Next, the mixture remaining after removing solvent A and solvent B is washed (S115).
By carrying out this washing step, it is possible to remove unreacted amine compounds and diisocyanate compounds remaining in the mixture.
Here, specific examples of the cleaning method include the following methods.
First, the mixture obtained after removing solvent A and solvent B is mixed with water and filtered through a membrane filter to recover the residue. The residue is then heated at a temperature lower than the boiling point of water and lower than the boiling point of the styrene-based polymer to vaporize the water adhering to the residue and remove the water from the residue. The heating conditions are, for example, heating in a high-temperature bath at 80° C. under atmospheric pressure for 5 hours to 10 hours.

(7) The washed mixture is recovered to obtain a urea-based additive containing diurea and a styrene-based polymer (S116).
The obtained urea-based additive is usually in a powder form. This urea-based additive may be subjected to a pulverization treatment as necessary. By subjecting the urea-based additive to a pulverization treatment, it is possible to make the urea-based additive finer and more uniform. In addition, the particle size of the urea-based additive can be adjusted by the pulverization treatment.
When the above-mentioned pulverization treatment is carried out, it is preferable to carry out the pulverization treatment using a small pulverizer (for example, Labo Millser manufactured by Osaka Chemical Co., Ltd.) because it can be carried out using a simple device at low cost.

By going through these steps, the above-mentioned urea-based additive can be produced.

[Manufacturing method B]
FIG. 7 is a process diagram for explaining another example (production method B) of the method for producing a urea-based additive.
This manufacturing method B includes the steps (S121 to S126) as shown in the figure.
This production method B is the same as the production method of the urea-based additive carried out in the same manner as the above-mentioned production method A, except that the mixed liquid A' (S121) obtained by adding the amine compound to the solvent A is replaced with the mixed liquid A (S111 of the production method A) obtained by adding a part of the styrene-based polymer and the amine compound to the solvent A.
In this production method B, the styrene-based polymer is not blended into the mixed liquid A', but is blended only into the mixed liquid B.

The mixed liquid A' is obtained by adding an amine compound to the solvent A (S121).
In this case, the amount of the amine compound is, for example, 5% by mass or more and 60% by mass or less with respect to 100% by mass of the solvent A, similarly to S111 of the production method A.
The amine compound may be mixed into the solvent A using, for example, a mechanical stirrer or a magnetic stirrer, as in S111 of the production method A. The amine compound is preferably mixed into the solvent A using a mechanical stirrer.

[Production Method C]
FIG. 8 is a process diagram for explaining another example (production method C) of the method for producing a urea-based additive.
This manufacturing method C includes the steps (S131 to S136) as shown in the figure.
This production method C is the same as the production method for a urea-based additive carried out in the same manner as the above-mentioned production method A, except that the mixed liquid B' (S132) obtained by adding a diisocyanate compound to the solvent B is replaced with the mixed liquid B (S112 of the production method A) obtained by adding a part of the styrene-based polymer and the diisocyanate compound to the solvent B.
In this production method C, the styrene polymer is blended only in the mixed liquid A, not in the mixed liquid B'.

The mixed liquid B' is obtained by adding a diisocyanate compound to the solvent B (S132).
In this case, the amount of the diisocyanate compound is, for example, 5% by mass or more and 60% by mass or less with respect to 100% by mass of the solvent B, similarly to S112 of the production method A.
The diisocyanate compound may be mixed into the solvent B using, for example, a mechanical stirrer or a magnetic stirrer, as in S112 of the production method A. The diisocyanate compound is preferably mixed into the solvent B using a mechanical stirrer.

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

The steps of washing the mixture (S115, S125, S135) are not essential steps and may be omitted.

Next, the present invention will be described in more detail based on examples of the present invention, but the present 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

Additive:
Molybdenum dialkyldithiocarbamate (MoDTC): Sakuralube 600 (manufactured by ADEKA Corporation)
Urea-based additive: Produced by the following method. The raw materials used were octylamine, MDI (4,4'-diphenylmethane diisocyanate), and a styrene-based polymer (styrene-isoprene copolymer: Lubrizol 7306, manufactured by Lubrizol Japan Co., Ltd.), and the solvent used was toluene.

(Production of urea-based additives)
(1) A styrene-isoprene copolymer was dissolved in toluene. A predetermined amount of octylamine was then mixed into the resulting solution to obtain a mixed solution A.
(2) Separately from the above step (1), a mixture B was obtained by mixing a predetermined amount of MDI with a solution of styrene-isoprene copolymer dissolved in toluene.
Here, the amount of styrene-isoprene copolymer added to obtain mixed liquid A was the same as the amount of styrene-isoprene copolymer added to obtain mixed liquid B.

In the steps (1) and (2), the compounding ratio of the octylamine to the MDI (octylamine:MDI) was 2:1 in terms of molar ratio, and the amount of diurea produced was 40% by mass relative to 100% by mass of toluene.
In addition, the amount of styrene-isoprene copolymer added was set so that the amount of styrene-isoprene copolymer contained in the mixture of diurea and styrene-isoprene copolymer, which will be described later, was 7.00 mass % relative to the total amount of diurea and styrene-isoprene copolymer.

Mixture A was prepared by adding the styrene-isoprene copolymer and octylamine to toluene while stirring it with a mechanical stirrer.
Mixture B was prepared by adding the styrene-isoprene copolymer and MDI to toluene while stirring the toluene with a mechanical stirrer.

(3) While mixing mixed solution A with a mechanical stirrer, mixed solution B was added dropwise to mixed solution A to mix the two.
After the dropwise addition of the mixed solution B was completed, stirring was continued for 0.5 hours while reacting the octylamine with the MDI at room temperature to produce diurea.

(4) The mixture containing diurea, styrene-isoprene copolymer, and toluene was then left at room temperature for 24 hours to evaporate and remove the toluene, completing a mixture of diurea and styrene-isoprene copolymer (urea-based additive).

(Comparative Example 1: Homogenized Base Grease)
(1) 7.7 parts by mass of lithium stearate and 3.3 parts by mass of lithium 12-hydroxystearate were added to 71.2 parts by mass of poly-α-olefin, and the mixture was heated to 230° C. with stirring to dissolve the lithium stearate and lithium 12-hydroxystearate in the poly-α-olefin.
Thereafter, the mixture was allowed to cool while stirring, and 17.8 parts by mass of trimellitic acid ester was mixed in when the mixture had cooled to 150°C. Thereafter, the mixture was allowed to continue to cool while stirring, 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.

The base oil contained in this base grease contains poly-α-olefin and trimellitic acid ester in a mass ratio of 4:1.
In the thickener contained in this base grease, the mass ratio of lithium stearate to lithium 12-hydroxystearate was 7:3.
The mass ratio of the base oil to the thickener contained in this base grease was 89:11.

(2) Next, homogenization was carried out using a triple roll mill. The processing conditions were as follows:
Roll gap: 50 μm
Pressure between rolls: 1 MPa
Rotation speed: 200 r/min
Processing temperature: 25°C
It was decided.

Through these steps, a homogenized base grease was obtained. The resulting base grease was used as the grease composition of Comparative Example 1.

Example 1
1.9 parts by mass of MoDTC and 1.9 parts by mass of a urea-based additive were added to 96.2 parts by mass of the homogenized base grease obtained in Comparative Example 1. Thereafter, the mixture was mixed using a planetary centrifugal mixer at a rotation speed of 2000 rpm for 3 minutes to prepare a grease composition.

The average particle size of the urea-based additive contained in the obtained grease composition was measured using a confocal laser microscope (TCS SP08, manufactured by Leica Microsystems). The configuration of the confocal laser microscope is shown in Table 1.
This device is equipped with software (Leica Application Suite X (LAS X) Version 4.4.0) capable of calculating the average particle diameter of the observed object. This software calculates the volume of the urea-based additive for each particle from the acquired three-dimensional image, calculates the diameter of each particle assuming that the shape of each particle is a perfect sphere, and calculates the average value of the diameters of each particle as the measured value (average particle diameter).
The average diameter of the resulting urea-based additive was 0.6 μm.

(Comparative Example 2)
2.0 parts by mass of MoDTC was added to 98.0 parts by mass of the homogenized base grease obtained in Comparative Example 1. The mixture was then mixed using a planetary centrifugal mixer at a rotation speed of 2000 rpm for 3 minutes to prepare a grease composition.

(Comparative Example 3)
1.9 parts by mass of MoDTC and 1.0 part by mass of a urea-based additive were added to 97.1 parts by mass of the homogenized base grease obtained in Comparative Example 1. The mixture was then mixed using a planetary centrifugal mixer at a rotation speed of 2000 rpm for 3 minutes to prepare a grease composition.

(Comparative Example 4)
1.9 parts by mass of MoDTC and 4.7 parts by mass of a urea-based additive were added to 93.4 parts by mass of the homogenized base grease obtained in Comparative Example 1. Thereafter, the mixture was mixed using a planetary centrifugal mixer at a rotation speed of 2000 rpm for 3 minutes to prepare a grease composition.

(Comparative Example 5)
1.8 parts by mass of MoDTC and 8.9 parts by mass of a urea-based additive were added to 89.3 parts by mass of the homogenized base grease obtained in Comparative Example 1. Thereafter, the mixture was mixed using a planetary centrifugal mixer at a rotation speed of 2000 rpm for 3 minutes to prepare a grease composition.

The grease compositions produced in the Examples and Comparative Examples were subjected to friction and wear tests to evaluate the wear resistance under poor lubrication conditions. The results are shown in Table 3 and FIG.
The results of Example 1 and Comparative Examples 2 to 5 are shown in FIG.

(Friction and wear test)
The test was carried out based on the standard ASTM D5707. The test device was an SRV2 vibration friction and wear tester (manufactured by OPTIMOL).
In this test, the upper test piece was a cylindrical roller for bearings made of SUJ2 (Φ15 mm×22 mm, Ra 0.1 μm), and the upper part of the lower test piece was a flat PTFE sheet (Φ24 mm×16 mm, Ra 0.5 μm).
In this test, with the upper test piece pressed against the lower test piece with a load of 100 N, the upper test piece (a cylindrical roller) is made to move back and forth along the axial direction of the cylindrical roller against the flat PTFE sheet that is the upper part of the lower test piece for 60 minutes, and the depth of the wear marks formed on the flat plate is measured.
Details of the test conditions are shown in Table 2.

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

REFERENCE SIGNS LIST 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 teeth 312 Cylindrical surface 313 Cylindrical surface 314 Second rack tooth portion 315 Second rack teeth 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 oil composition includes a base oil, a thickener, and an 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 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 additive includes molybdenum dialkyldithiocarbamate and a urea-based additive,
the molybdenum dialkyldithiocarbamate accounts for 1.5% by mass or more and 8.0% by mass or less of the total amount of the trimellitic acid ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, the molybdenum dialkyldithiocarbamate, and the urea-based additive;
The urea-based additive has an average diameter of particles having a diameter of 0.2 μm or more of 0.2 μm or more and 1.0 μm or less,
the urea-based additive is contained in an amount of 1.2% by mass or more and 3.4% by mass or less relative to the total amount of the trimellitic acid ester, the poly-α-olefin, the lithium 12-hydroxystearate, the lithium stearate, the molybdenum dialkyldithiocarbamate, and the urea-based additive;
Grease composition. The grease composition according to claim 1, wherein the urea-based additive is a mixture of a urea compound and a styrene-based polymer. 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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