Grease composition and steering gear device

Abstract

This grease composition comprises a base oil, a thickener, and an additive, wherein: the thickener is lithium soap; the additive includes a molybdenum dialkyl dithiocarbamate and octadecanamide; the ratio of the molybdenum dialkyl dithiocarbamate to the total mass of the grease composition is 0.1%-4.0% by mass; and the ratio of octadecanamide to the total mass of the grease composition is 0.5%-10.0% by mass.

Description

Grease composition and steering gear device

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

[0002] A rack and pinion included in a steering gear device used in an electric power steering device includes a rack shaft with rack teeth and a pinion shaft with pinion teeth. This steering gear device suppresses wear of the rack teeth and pinion teeth by interposing grease between the meshing portions of the rack teeth and the pinion teeth. The grease lubricates the gap 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.

[0003] JP 2001-064665 A JP 2009-203374 A JP 2004-269722 A

[0004] A grease composition according to one embodiment of the present disclosure is a grease composition comprising a base oil, a thickener, and an additive, wherein the thickener is lithium soap, the additive comprises molybdenum dialkyldithiocarbamate and octadecane amide, the proportion of the molybdenum dialkyldithiocarbamate relative to the total mass of the grease composition is 0.1 mass% or more and 4.0 mass% or less, and the proportion of the octadecane amide relative to the total mass of the grease composition is 0.5 mass% or more and 10.0 mass% or less.

[0005] A steering gear device according to one aspect of the present disclosure comprises: 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 urges the rack teeth against the pinion teeth; and a grease composition interposed between the meshing rack teeth and the pinion teeth 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 of the present disclosure.

[0006] 1 is a structural diagram schematically showing an example of a dual-pinion type electric power steering device in which the grease composition of the present disclosure is packed. FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. FIG. 3 is a cross-sectional view taken along line B-B in FIG. 1. FIG. 4 is a structural diagram schematically showing an example of a column type electric power steering device in which the grease composition of the present disclosure is packed. FIG. 5 is a cross-sectional view taken along line A-A in FIG. 6. FIG. 7 is a graph showing evaluation results of examples and comparative examples.

[0007] <Problem to be Solved by the Invention of the Present Disclosure> A steering gear device having a rack and pinion suppresses wear of the rack teeth and pinion teeth 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 the grease composition is also interposed between the rack shaft and a portion of the rack guide mechanism that is pressed against the rack shaft, thereby suppressing wear of both. Therefore, there is a need for a grease composition that can effectively lubricate not only the meshing portion between the rack teeth and the pinion teeth of the rack and pinion of the steering gear device, but also the sliding portion between the rack shaft and the rack guide mechanism.

[0008] However, the grease compositions proposed in Patent Documents 1 to 3 had difficulty fully meeting these requirements. The meshing portions between the rack teeth and pinion teeth and the sliding portions between the rack shaft and rack guide mechanism are lubricated portions that are prone to poor lubrication, so it was not easy to adequately lubricate these portions. Furthermore, because the portion of the rack guide mechanism that is pressed against the rack shaft is made of a resin such as PTFE, a grease composition designed to lubricate the meshing portions between the steel rack teeth and steel pinion teeth sometimes fails to adequately lubricate the sliding portions between the steel rack shaft and the resin rack guide mechanism.

[0009] Effect of the Invention of the Present Disclosure The grease composition of the present disclosure can lubricate even friction surfaces that tend to be poorly lubricated, and is suitable for suppressing wear on such friction surfaces. Furthermore, the grease composition of the present disclosure is suitable for simultaneously suppressing wear on the friction surfaces of lubricated members made of steel and those made of resin.

[0010] The steering gear device of the present disclosure uses the grease composition of the present disclosure, which makes it possible to suppress wear over a long period of time on the rack teeth and 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, thereby enabling the steering gear device to maintain steering performance over a long period of time.

[0011] <Outline of Embodiments of the Invention of the Present Disclosure> Below, an outline of embodiments of the invention of the present disclosure will be listed and described. (1) A grease composition of the present disclosure is a grease composition including a base oil, a thickener, and an additive, wherein the thickener is lithium soap, the additive includes molybdenum dialkyldithiocarbamate and octadecaneamide, the proportion of the molybdenum dialkyldithiocarbamate relative to the total mass of the grease composition is 0.1% by mass or more and 4.0% by mass or less, and the proportion of the octadecaneamide relative to the total mass of the grease composition is 0.5% by mass or more and 10.0% by mass or less. Grease composition.

[0012] The grease composition contains molybdenum dialkyldithiocarbamate and the octadecane amide as additives. Therefore, the grease composition is likely to form a triboreactive film on friction surfaces made of steel. The grease composition can also form a lubricating film that prevents direct contact between the friction surfaces of lubricated members. Therefore, the grease composition can lubricate even friction surfaces that tend to be poorly lubricated, and is suitable for suppressing wear on such friction surfaces. The grease composition is also suitable for simultaneously suppressing wear on the friction surfaces of lubricated members made of steel and those made of resin.

[0013] (2) In the grease composition of (1) above, the thickener is preferably lithium 12-hydroxystearate. In this case, the grease composition can be produced at low cost while maintaining good sliding properties.

[0014] (3) In the grease composition of (1) or (2), the base oil preferably contains a poly-α-olefin and a mineral oil. In this case, the grease composition can be improved in heat resistance while keeping costs down.

[0015] (4) A 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 urges the rack teeth against the pinion teeth; and a grease composition interposed between the meshing rack teeth and the pinion teeth and between the circumferential 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 described in any one of (1) to (3) above.

[0016] The grease composition interposed between the meshing rack teeth and pinion teeth of the steering gear device, and between the circumferential surface of the rack shaft and the portion of the rack guide mechanism that is pressed against the rack shaft, is made of the grease composition of the present disclosure. In this case, wear is suppressed on the rack teeth, pinion teeth, the circumferential surface of the rack shaft that is in sliding contact with the rack guide mechanism, and the portion of the rack guide mechanism that is pressed against the rack shaft. As a result, the amount of change in clearance of the lubricated parts that changes due to wear is small, and deterioration of the steering performance of the steering gear device is less likely to occur.

[0017] <Details of the Embodiments of the Invention of the Present Disclosure> Hereinafter, embodiments of the present disclosure will be described. Note that in this disclosure, the embodiments of the invention are to be considered as illustrative in all respects and not restrictive. The scope of the rights of the present invention is defined by the claims, and it is intended to include all modifications within the meaning and scope equivalent to the claims.

[0018] First, an embodiment of a steering gear device in which the grease composition of the present disclosure is used will be described, followed by an embodiment of the grease composition of the present disclosure, which is used in, for example, a dual-pinion type electric power steering device, a column type electric power steering device, etc.

[0019] (Dual pinion type electric power steering device) Fig. 1 is a configuration diagram that schematically shows an example of a dual pinion type electric power steering device 1 that includes a steering gear device 3. Fig. 2 is a cross-sectional view taken along line A-A in Fig. 1 that shows a part of the steering gear device 3. In Fig. 2, the bottom of the drawing corresponds to the bottom side in the vertical direction when the device is mounted on a vehicle. Fig. 3 is a cross-sectional view taken along line B-B in Fig. 1 that shows a part of the steering gear device 3. In Fig. 3, the bottom of the drawing corresponds to the bottom side in the vertical direction when the device is mounted on a vehicle.

[0020] 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 speed reduction mechanism 53, a second pinion shaft 54, two bearings 55, 56, a worm housing 57, and a second rack guide mechanism 59. The speed reduction mechanism 53 includes a worm 531 and a worm wheel 532.

[0021] 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).

[0022] The column shaft 21 has one end in its extension direction fixed to the steering wheel 10. The other end in its extension direction is fixed to a first yoke of a first universal joint 23. The column shaft 21 is rotatable around its central axis in its extension direction. The first yoke is pivotally fitted to a first pair of trunnions on the same central axis of the first cross shaft via a plurality of first rolling elements. The second yoke is pivotally fitted to a second pair of trunnions on the same central axis of the first cross shaft via a plurality of second rolling elements. The central axes of the first pair of trunnions and the second pair of trunnions intersect at a 90-degree angle.

[0023] The second yoke of the first universal joint 23 fixes one end of the intermediate shaft 22 in the extension direction. The other end of the intermediate shaft 22 fixes the third yoke of the second universal joint 24. The third yoke is pivotally fitted to a third pair of trunnions on the same central axis of the second cross shaft via a plurality of third rolling elements. The fourth yoke is pivotally fitted to a fourth pair of trunnions on the same central axis of the second cross shaft via a plurality of fourth rolling elements. The central axes of the third pair of trunnions and the fourth pair of trunnions intersect at a 90-degree angle. 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 rotates 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.

[0024] Of the dual-pinion type electric power steering device 1, the first pinion shaft 32, the rack shaft 31, the housing 33, the two rack bushes 30 and 34, the first bearing 35, the second bearing 36, the first rack guide mechanism 39, the electric motor 52, the speed reduction mechanism 53, the second pinion shaft 54, the third bearing 55, the fourth bearing 56, the worm housing 57, and the second rack guide mechanism 59 constitute a steering gear device 3 as a rack-and-pinion steering device. In Figure 1, the housing 33 is represented by a virtual line (two-dot chain line), and its interior is also shown.

[0025] The first pinion shaft 32 extends vertically from the upper side to the lower side of the vehicle. The first pinion shaft 32 has, from one end to the other along its extension, 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. A 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. First pinion teeth 321 are formed on the entire circumferential surface of the first pinion tooth portion 320. 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.

[0026] The housing 33 has a first opening 332 on the steering wheel 10 side, and is sealed on the side opposite the first opening 332. The first pinion shaft 32 is housed inside the housing 33. The first pinion shaft 32 is rotatably supported relative to the housing 33 by two bearings 35, 36. The first bearing 35 is a ball bearing. The first bearing 35 includes an inner ring, an outer ring, and balls, with the inner ring fixed to the first shaft portion 322 and the outer ring fixed to the housing 33, and the balls 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, with the outer ring fixed to the housing 33, and the rollers roll between the outer peripheral surface of the first boss portion 323 and the outer ring.

[0027] With the first pinion shaft 32, the first bearing 35, and the second bearing 36 inserted into the housing 33, a lid 37, through which the first pinion shaft 32 passes, is fixed to the first opening 332 of the housing. A seal is fixed to the lid 37, and the seal is slidable on the outer peripheral surface 322b of the first shaft portion 322 of the first pinion shaft 32. A cover member 38 is also 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.

[0028] 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 first rack teeth 311 formed on one portion in the circumferential direction, and the other portion in 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 portion in the circumferential direction, and the other portion in the circumferential direction is a cylindrical surface 313 whose central axis is the extension direction of the rack shaft 31. The outer peripheral surfaces of the first cylindrical portion 316, the second cylindrical portion 317, and the third cylindrical portion 318 are each cylindrical surfaces whose central axis is the extension direction of the rack shaft 31. The extending direction of the first rack teeth 311 forms an angle that is not 90 degrees with respect to the extending direction of the rack shaft 31. The extending direction of the second rack teeth 315 forms an angle that is not 90 degrees with respect to the extending direction of the rack shaft 31. If the angle of the first rack teeth 311 with respect to the extending direction of the rack shaft 31 is X, the angle of the second rack teeth 315 with respect to the extending direction of the rack shaft 31 is π-X.

[0029] 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 accommodated 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.

[0030] 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 is slidable 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 is slidable on the outer peripheral surface of the third cylindrical portion 318 of the rack shaft 31.

[0031] 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 slidable 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 the extension direction thereof, the rack shaft 31 moves linearly relative to the housing 33 in the extension direction of the housing 33.

[0032] 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 portion of the first rack tooth portion 310 of the rack shaft 31, at a position where the first pinion shaft 32 meshes with the rack shaft 31 in the extending direction of the housing 33.

[0033] The first rack guide mechanism 39 includes a first support yoke 391, a first seat member 392, a first coil spring 393, and a first plug 394. The first seat member 392 is sandwiched between a cylindrical surface 312, which is the other circumferential portion of the first rack tooth portion 310 of the rack shaft 31, and the cylindrical surface of the first support yoke 391. The first seat member 392 is fixed to the first support yoke 391. The first seat member 392 and the cylindrical surface 312, which is the other circumferential portion of the first rack tooth portion 310 of the rack shaft 31, are in slidable contact with each other via a grease composition G. The first seat member 392 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is in contact with the cylindrical surface 312 via the grease composition G. The first plug 394 is fixed to the fifth opening 336 of the housing 33. The first plug 394 contacts one end of the first coil spring 393. The first support yoke 391 contacts the other end of the first coil spring 393. The first coil spring 393 is shortened to be 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 in the housing 33.

[0034] The second pinion shaft 54 ​​extends vertically from the upper side to the lower side of the vehicle. The second pinion shaft 54 ​​has, from one end to the other along its extension, a fitting portion 544, a second shaft portion 542, a second pinion teeth portion 540, and a second boss portion 543. The fitting portion 544 is cylindrical. The second shaft portion 542 is also cylindrical. The second pinion teeth portion 540 has second pinion teeth 541 formed on the entire circumferential surface. The extension direction of the second pinion teeth 541 is at an angle 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.

[0035] 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 a worm housing 57. The worm housing 57 has a seventh opening 571. The output shaft 521 of the electric motor 52 is disposed in the internal space of the worm housing 57 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.

[0036] The worm 531 is disposed in the internal space of the worm housing 57. The worm wheel 532 is disposed in the internal space of the worm housing 57. The worm housing 57 has an eighth opening 572 at the vertically upper side, and the assembly of the second pinion shaft 54 ​​and the worm wheel 532 is inserted into the internal space of the worm housing 57 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 teeth portion 540, and the second boss portion 543 protrude from the ninth opening 573 of the worm housing 57.

[0037] The worm housing 57 is fixed to the housing 33. A 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.

[0038] 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, and the rollers roll between the outer peripheral surface of the second boss portion 543 and the outer ring.

[0039] Second pinion teeth 541 formed on second pinion tooth portion 540 of second pinion shaft 54 ​​and second rack teeth 315 formed on second rack tooth portion 314 of rack shaft 31 are in rolling and slidable contact via grease composition G. Second pinion teeth 541 and second rack teeth 315 mesh with each other via grease composition G. When second pinion shaft 54 ​​rotates relative to housing 33 about the central axis in the extension direction thereof, rack shaft 31 moves linearly relative to housing 33 in the extension direction of housing 33.

[0040] A 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 portion of the second rack tooth portion 314 of the rack shaft 31, at a position where the second pinion shaft 54 ​​meshes with the rack shaft 31 in the extending direction of the housing 33.

[0041] The second rack guide mechanism 59 includes a second support yoke 591, a second seat member 592, a second coil spring 593, and a second plug 594. The second seat member 592 is sandwiched between the cylindrical surface 313, which is the other circumferential portion of the second rack tooth portion 314 of the rack shaft 31, and the cylindrical surface of the second support yoke 591. The second seat member 592 is fixed to the second support yoke 591. The second seat member 592 and the cylindrical surface 313, which is the other circumferential portion of the second rack tooth portion 314 of the rack shaft 31, are in slidable contact with each other via a grease composition G. The second seat member 592 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is in contact with the cylindrical surface 313 via the grease composition G. The second plug 594 is fixed to the sixth opening 337 of the housing 33. The second plug 594 contacts one end of the second coil spring 593. The second support yoke 591 contacts the other end of the second coil spring 593. The second coil spring 593 is shortened to be shorter than its free length with the second plug 594 fixed in the sixth opening 337. Therefore, the second seat member 592 is pressed against the rack shaft 31 in the housing 33.

[0042] The torque sensor 51 detects the steering torque applied by the driver to the steering wheel 10 via the column shaft 21. The reduction gear mechanism 53 is an assembly in which a worm 531, which rotates integrally with the output shaft 521 of the electric motor 52, is meshed with a worm wheel 532, which 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 detected by the torque sensor 51, the vehicle speed, etc., and transmits the rotational force of the output shaft 521 of the electric motor 52, which has been reduced in speed by the reduction gear mechanism 53, to the second pinion shaft 54. The rotational force of the second pinion shaft 54 ​​is applied to the second rack teeth 315 via the second pinion teeth 541 as a steering assist force.

[0043] 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 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 left and right front wheels 14, 14, which are steered wheels, are steered by the rack shaft 31 moving linearly in the extension direction of the housing 33.

[0044] Grease composition G is sealed within housing 33. Grease composition G is present between the rolling and sliding surfaces of first pinion teeth 321 and first rack teeth 311, which come into contact when first pinion teeth 321 and first rack teeth 311 mesh with each other, thereby lubricating the gap between the two rolling and sliding surfaces. Grease composition G is present between the sliding surface of first seat member 392 and the sliding surface of cylindrical surface 312, which is the other circumferential portion of first rack tooth portion 310 of rack shaft 31, which come into contact when first seat member 392 and rack shaft 31 are pressed against each other, thereby lubricating the gap between the two sliding surfaces. Grease composition G is interposed between the rolling and sliding surfaces of second pinion teeth 541 and second rack teeth 315, which come into contact when second pinion teeth 541 and second rack teeth 315 mesh with each other, thereby lubricating the gap between the two rolling and sliding surfaces. Grease composition G is interposed between the sliding surface of second sheet member 592 and the sliding surface of cylindrical surface 313, which is the other circumferential part of second rack tooth portion 314 of rack shaft 31, which come into contact when second sheet member 592 and rack shaft 31 are pressed against each other, thereby lubricating the gap between the two sliding surfaces.

[0045] The steering gear device 3 configured as described above 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 reduces the amount of wear in these portions.

[0046] (Column-type electric power steering device) Fig. 4 is a configuration diagram that schematically shows an example of a column-type electric power steering device 601 that includes a steering gear device 603. Fig. 5 is a cross-sectional view taken along line A-A in Fig. 4 that shows a part of the steering gear device 603. In Fig. 5, the bottom of the drawing corresponds to the bottom in the vertical direction when the device is mounted on a vehicle.

[0047] 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 bushings 630 and 634, two bearings 635 and 636, a rack guide mechanism 639, and a steering assist device 4. A driver of a vehicle equipped with this column-type electric power steering device 601 performs steering operation by rotating the steering wheel 610. The steering shaft 602 includes a column shaft 621, a first universal joint 623, an intermediate shaft 622, and a second universal joint 624. The first universal joint 623 includes a first yoke (not shown), a plurality of first rolling elements (not shown), a first cross shaft (not shown), a plurality of second rolling elements (not shown), and a second yoke (not shown). The second universal joint 624 includes a third yoke (not shown), a plurality of third rolling elements (not shown), a second cross shaft (not shown), a plurality of fourth rolling elements (not shown), and a fourth yoke (not shown).

[0048] The column shaft 621 has a steering wheel 610 fixed to one end thereof in the extension direction. A first yoke of a first universal joint 623 is fixed to the other end thereof in the extension direction. The column shaft 621 is rotatable about its central axis in the extension direction. The first yoke is pivotally fitted to a first pair of trunnions on the same central axis of the first cross shaft via a plurality of first rolling elements. The second yoke is pivotally fitted to a second pair of trunnions on the same central axis of the first cross shaft via a plurality of second rolling elements. The central axes of the first pair of trunnions and the second pair of trunnions intersect at a 90-degree angle.

[0049] The second yoke of the first universal joint 623 fixes one end of the intermediate shaft 622 in the extension direction. The other end of the intermediate shaft 622 fixes the third yoke of the second universal joint 624. The third yoke is pivotally fitted to a third pair of trunnions on the same central axis of the second cross shaft via a plurality of third rolling elements. The fourth yoke is pivotally fitted to a fourth pair of trunnions on the same central axis of the second cross shaft via a plurality of fourth rolling elements. The central axes of the third pair of trunnions and the fourth pair of trunnions intersect at a 90-degree angle. 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 rotates 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.

[0050] Of the column-type electric power steering device 601, a pinion shaft 632, a rack shaft 631, a housing 633, two rack bushes 630 and 634, two bearings 635 and 636, and a rack guide mechanism 639 constitute a steering gear device 603 as a rack-and-pinion steering device. In Figure 4, the housing 633 is represented by a virtual line (two-dot chain line), and its interior is also shown.

[0051] The pinion shaft 632 extends vertically from the upper side to the lower side of the vehicle. The pinion shaft 632 has, from one end to the other along its extension, 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. A 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 over the entire circumferential surface. The extension direction of the pinion teeth 721 is at an angle that is not 90 degrees with respect to the extension direction of the central axis of the pinion shaft 632. The boss portion 723 is cylindrical.

[0052] Housing 633 has a first opening 732 on the steering wheel 610 side, and the side opposite first opening 732 is sealed. Pinion shaft 632 is housed inside housing 633. Pinion shaft 632 is rotatably supported relative to housing 633 by two bearings 635, 636. Bearing 635 is a ball bearing. Bearing 635 includes an inner ring, an outer ring, and balls, with the inner ring fixed to shaft portion 722 and the outer ring fixed to housing 633, and the balls roll between the inner ring and the outer ring. Bearing 636 is a roller bearing. Bearing 636 includes rollers and an outer ring, with the outer ring fixed to housing 633, and the rollers roll between the outer peripheral surface of boss portion 723 and the outer ring.

[0053] With the pinion shaft 632 and the two bearings 635, 636 inserted into the housing 633, a lid 637, through which the pinion shaft 632 passes, is fixed to a first opening 732 of the housing. A seal is fixed to the lid 637, and the seal is slidable on an outer peripheral surface 722b of the shaft portion 722 of the pinion shaft 632. A cover member 638 is also 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.

[0054] The rack shaft 631 includes, 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. Rack teeth 711 are formed on one portion of the rack tooth portion 710 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 with respect to the extension direction of the rack shaft 631.

[0055] 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 housed inside the housing 633 along the extension direction of the housing 633. One end in the extension direction of the rack shaft 631 protrudes from the second opening 733 at one end in the extension direction of the housing 633. The other end in the extension direction of the rack shaft 631 protrudes from the third opening 734 at the other end in the extension direction of the housing 633.

[0056] 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 is slidable 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 is slidable on the outer peripheral surface of the second cylindrical portion 717 of the rack shaft 631.

[0057] Pinion teeth 721 formed on pinion tooth portion 720 of pinion shaft 632 and rack teeth 711 formed on rack tooth portion 710 of rack shaft 631 are in rolling and slidable contact via grease composition G. Pinion teeth 721 and rack teeth 711 mesh with each other via grease composition G. When pinion shaft 632 rotates relative to housing 633 about the central axis in the extension direction thereof, rack shaft 631 moves linearly relative to housing 633 in the extension direction of housing 633.

[0058] 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 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 left and right front wheels 14, 14, which are steered wheels, are steered by the rack shaft 631 moving linearly in the extension direction of the housing 633.

[0059] A rack guide mechanism 639 is fixed to the housing 633. The housing 633 has a fourth opening 736 on the cylindrical surface 712 side, which is the other part of the rack tooth portion 710 of the rack shaft 631 in the circumferential direction, at a position where the pinion shaft 632 meshes with the rack shaft 631 in the extension direction.

[0060] The rack guide mechanism 639 includes a support yoke 791, a seat member 792, a coil spring 793, and a plug 794. The seat member 792 is sandwiched between a cylindrical surface 712, which is the other circumferential portion of the rack tooth portion 710 of the rack shaft 631, and the cylindrical surface of the support yoke 791. The seat member 792 and the cylindrical surface 712, which is the other circumferential portion of the rack tooth portion 710 of the rack shaft 631, are in slidable contact with each other via a grease composition G. The seat member 792 includes a metal layer, such as bronze, and a resin layer, such as PTFE, and the resin layer is in contact with the cylindrical surface 712 via the grease composition G. The plug 794 is fixed to the fourth opening 736 of the housing 633. The plug 794 contacts one end of the coil spring 793. The support yoke 791 contacts the other end of the coil spring 793. The coil spring 793 is shortened to a length less than the free length while fixing the plug 794 in the fourth opening 736. Thus, the seat member 792 is pressed against the rack shaft 631 against the housing 633.

[0061] 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 speed 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 speed 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 speed reduction mechanism 43 is applied to the column shaft 621 as steering assist force.

[0062] Grease composition G is sealed 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 portion 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.

[0063] The steering gear device 603 configured in this manner is filled with the grease composition of the present disclosure as the grease composition G. The grease composition of the present disclosure can effectively lubricate the meshing portion between the pinion teeth 721 and the first rack teeth 711, and the sliding portion between the seat member 792 of the rack guide mechanism 639 and the rack shaft 631. This reduces the amount of wear in these portions.

[0064] The grease composition of the present disclosure can be enclosed and used in the above-mentioned dual pinion type electric power steering device, column type electric power steering device, etc.

[0065] <Grease Composition> A grease composition according to an embodiment of the present disclosure includes a base oil, a thickener, and an additive.

[0066] (Base Oil) Known base oils can be used as the base oil. The base oil is preferably non-polar. The base oil preferably contains poly-α-olefin (PAO) and mineral oil. In this case, the grease composition can be produced at low cost and has good heat resistance.

[0067] Examples of the poly-α-olefins include oligomers or polymers of α-olefins such as 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, and 1-tetradecene, and further hydrogenated versions of these. Preferred poly-α-olefins are PAO4 to PAO10, which are oligomers of 1-decene. Two or more of the poly-α-olefins can also be used in combination.

[0068] The preferred kinematic viscosity of the base oil of the poly-α-olefin at 100°C is 3 to 11 mm ２ More preferably, the kinematic viscosity (100°C) of the base oil is 5 to 9 mm ２ / s.

[0069] As the mineral oil, known mineral oils can be used. Examples of the mineral oil include paraffinic mineral oils and naphthenic mineral oils. Two or more types of the mineral oils can be mixed and used. The preferred base oil kinematic viscosity of the mineral oil at 100°C is 5 to 15 mm ２ / s.

[0070] When the base oil is composed of a poly-α-olefin and a mineral oil, the preferred ratio of the poly-α-olefin to the total amount of the poly-α-olefin and the mineral oil is 30 to 50 mass %. If the poly-α-olefin content in the base oil is low, the heat resistance and low-temperature operability will be insufficient. If the poly-α-olefin content in the base oil is high, the grease composition will be expensive.

[0071] The preferred kinematic viscosity of the base oil containing poly-α-olefin and mineral oil at 100°C is 6 to 10 mm ２ / s.

[0072] (Thickener) In the grease composition of the present disclosure, the thickener is a lithium soap. Examples of the lithium soap include lithium 12-hydroxystearate and lithium stearate. Two or more types of the lithium soaps can also be mixed and used.

[0073] A preferred lithium soap is 12-hydroxystearate. A grease composition in which the thickener is lithium 12-hydroxystearate is likely to ensure good sliding properties.

[0074] In the grease composition, the proportion of the thickener relative to the total mass of the grease composition is preferably 5% by mass or more and 10% by mass or less, in which case the grease composition has good leakage resistance when filled in a steering gear device and is more likely to flow onto the friction surfaces of lubricated members.

[0075] (Additives) The grease composition of the present disclosure contains molybdenum dialkyldithiocarbamate and octadecane amide as additives, and therefore the grease composition can reduce friction on the friction surfaces of lubricated members.

[0076] In the grease composition, the proportion of the molybdenum dialkyldithiocarbamate (hereinafter also referred to as MoDTC) relative to the total mass of the grease composition is 0.1 mass % or more and 4.0 mass % or less. In this case, the grease composition is suitable for forming a tribo-reactive film on the friction surfaces of pinion teeth and rack teeth, which are made of steel. Therefore, the grease composition is suitable for reducing wear on the friction surfaces, which are prone to poor lubrication.

[0077] If the proportion of MoDTC is less than 0.1% by mass, the grease composition, when used in a steering gear device, cannot suppress wear caused by the meshing of steel pinion teeth and steel rack teeth. On the other hand, if the proportion of MoDTC exceeds 4.0% by mass, the adhesion of MoDTC to the steel rack shaft in the grease composition interferes with the adhesion of octadecaneamide to the rack shaft. As a result, the grease composition becomes less able to suppress wear between the steel rack shaft and the PTFE layer in sliding contact with the circumferential surface of the rack shaft using octadecaneamide. The preferred proportion of MoDTC is 0.5% by mass or more and 3.0% by mass or less. The more preferred proportion of MoDTC is 1.0% by mass or more and 3.0% by mass or less.

[0078] The MoDTC may be, for example, a compound represented by the following formula (1):

[0079]

[0080] (In the formula, R １ ~R ４ are each independently a linear or branched alkyl group.

[0081] Commercially available MoDTCs may also be used, such as ADEKA SAKURA-LUBE 600 (manufactured by ADEKA Corporation) and MOLYVAN A (manufactured by Vanderbilt Corporation).

[0082] In the grease composition, the proportion of octadecaneamide relative to the total mass of the grease composition is 0.5 mass % or more and 10.0 mass % or less. In this case, the grease composition is suitable for forming a stable lubricating film that suppresses direct contact between the friction surfaces of lubricated members. Therefore, the grease composition can reduce wear not only on steel friction surfaces but also on resin friction surfaces such as PTFE.

[0083] If the proportion of the octadecane amide is less than 0.5% by mass, when the grease composition is used in a steering gear device, the performance of suppressing wear between the steel rack shaft and the PTFE layer that is in sliding contact with the circumferential surface of the rack shaft is poor. On the other hand, even if the proportion of the octadecane amide in the grease composition exceeds 10.0% by mass, the effect of suppressing wear of the PTFE layer is hardly improved, and other performances required of the grease composition may be impaired. The preferred proportion of the octadecane amide is 1.0% by mass or more and 8.0% by mass or less.

[0084] The grease composition may contain additives other than MoDTC and octadecaneamide, as long as the effects of the present invention are not impaired. Examples of such additives include oiliness agents, antioxidants, rust inhibitors, corrosion inhibitors, anti-wear agents, dyes, color stabilizers, thickeners, structural 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 thickener.

[0085] The preferred worked penetration of the grease composition is No. 00 to No. 2. By adjusting the worked penetration within this range, the grease has sufficient leakage resistance when filled into a steering gear device and can flow smoothly onto the friction surfaces of the members to be lubricated.

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

[0087] <Method for producing grease composition> The grease composition can be produced, for example, by the following procedure. The method for producing the grease composition will be described using as an example a grease composition in which the base oil is a mixed base oil of poly-α-olefin and mineral oil and the thickener is lithium 12-hydroxystearate.

[0088] (1) 12-hydroxystearic acid is dissolved in mineral oil (e.g., 80°C). Separately, lithium hydroxide is dissolved in pure water (e.g., 80°C).

[0089] (2) Mineral oil in which 12-hydroxystearic acid has been dissolved is mixed with an aqueous solution of lithium hydroxide, and the mixture is stirred at 100-150°C for 30-40 minutes to react the 12-hydroxystearic acid with the lithium hydroxide. The mixture is then heated to 200-250°C, the water evaporates, and a mixture of the reactant and mineral oil is obtained. Next, poly-α-olefin is added to a vessel containing the mixture of the reactant (lithium 12-hydroxystearate) and mineral oil, and the mixture of the reactant, mineral oil, and poly-α-olefin is cooled to 100°C or below while stirring to form a base grease.

[0090] (3) MoDTC, octadecaneamide, and other additives, if necessary, are added to and mixed with the base grease prepared in step (2). The base grease with the additives mixed therein is then subjected to a homogenization treatment using a roll or the like, if necessary. In step (3), one or both of a mineral oil and a poly-α-olefin may be added to the base grease with the additives mixed therein, if necessary. The grease composition described above is produced through steps (1) to (3).

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

[0092] In the examples and comparative examples, the following raw materials were used: Base oil: Poly-α-olefin: PAO6 (base oil kinematic viscosity at 100°C: 6 mm ２ / s) Mineral oil: Diana Fresia N-150 (base oil kinematic viscosity at 100 ° C. is 10 mm ２ / s) and Samic 679 (base oil kinematic viscosity at 100 ° C. is 11 mm ２ / s) mixed oil

[0093] Thickener: Lithium 12-hydroxystearate

[0094] Additives: Molybdenum dialkyldithiocarbamate (MoDTC): MOLYVAN A manufactured by Vanderbilt; Octadecanamide: Lion Specialty Chemicals Co., Ltd. (melting point: 98-108°C, purity: 100% by mass); Oiliness agent: LUBRIZOL 5283C manufactured by Lubrizol; Others: rust inhibitor, antioxidant, corrosion inhibitor

[0095] (Example 1) (1) In a container A, 500.0 g of 12-hydroxystearic acid was completely dissolved in 1,254.6 g of mineral oil at 80° C. (2) In a container B, 69.8 g of lithium hydroxide was completely dissolved in 210 g of purified water at 80° C.

[0096] (3) The aqueous solution in container B was transferred to container A, and the solution in container A was mixed with the solution in container B. The 12-hydroxystearic acid and lithium hydroxide were reacted for 30 minutes with thorough stirring at 100°C, and the mixture was then heated to 230°C. Next, 675.6 g of poly-α-olefin (PAO6) was added to container A, and the mixture was cooled to below 100°C with stirring to form a base grease. The total amount of base grease produced was 2,500 g, and the mass ratio of base oil to thickener was 80:20. The mass ratio of mineral oil to PAO6 in the produced base oil was 65:35, and the kinematic viscosity of this base oil at 100°C was 8.5 mm ２ / s.

[0097] (4) To this base grease, additives were added in the formulation shown in Table 1, and then base oil (mineral oil to PAO6 in a mass ratio of 65:35) was added to obtain the formulation shown in Table 3. The base grease to which the additives and base oil had been added was homogenized in a three-roll mill to complete the grease composition of Example 1. The homogenization conditions were: roll gap: 50 μm; treatment temperature: 25° C.

[0098] (Example 2) A grease composition of Example 2 was completed in the same manner as in Example 1, except that the base oil added in step (4) was adjusted so that the base oil content was 86.7 mass %, and the amount of octadecanamide was 3.0 mass %.

[0099] (Example 3) A grease composition of Example 3 was completed in the same manner as in Example 1, except that the base oil added in step (4) was adjusted so that the base oil content was 84.7 mass %, and the amount of octadecanamide was 5.0 mass %.

[0100] Example 4 A grease composition of Example 4 was completed in the same manner as in Example 1, except that the base oil added in step (4) was adjusted so that the base oil content was 82.7 mass % and the amount of octadecanamide was 7.0 mass %.

[0101] (Comparative Example 1) The grease composition of Comparative Example 1 was completed in the same manner as in Example 1, except that the base oil added in step (4) was adjusted so that the base oil content was 89.0 mass % and the thickener content was 7.4 mass %, and octadecanamide was not blended.

[0102] (Comparative Example 2) A grease composition of Comparative Example 2 was completed in the same manner as in Example 2, except that the base oil added in step (4) was adjusted so that the base oil content was 88.7 mass % and MoDTC was not blended.

[0103] (Comparative Example 3) A grease composition of Comparative Example 3 was prepared in the same manner as in Example 2, except that the base oil added in step (4) was adjusted so that the base oil content was 83.7 mass % and the amount of MoDTC was 5.0 mass %.

[0104] (Comparative Example 4) The grease composition of Comparative Example 4 was completed in the same manner as Comparative Example 1, except that the base oil added in step (4) was adjusted so that the base oil content was 86.0 mass % and the amount of oiliness agent was 3.0 mass %.

[0105] The worked penetration of the grease compositions produced in the Examples and Comparative Examples was measured. The results are shown in Table 3. Friction and wear tests were conducted on the grease compositions produced in the Examples and Comparative Examples to evaluate their wear resistance in poor lubrication environments. Two types of friction and wear tests were conducted using test specimens made of different materials. The results are shown in Table 3 and Figure 6.

[0106] (Measurement of Worked Penetration) The worked penetration (60W) was measured by a method in accordance with "7. Penetration test method" of JIS K 2220 (2013).

[0107] (Friction and Wear Test 1) The test was conducted based on the ASTM D5707 standard. An SRV2 vibration friction and wear tester (manufactured by OPTIMOL) was used as the test equipment. 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 test piece with a flat plate with a PTFE sheet (Φ24 mm × 16 mm, Ra 0.5 μm) on top was used as the lower test piece. In this test, the upper test piece was pressed against the lower test piece with a load of 100 N, and the upper test piece was reciprocated along the axial direction of the cylindrical roller for 60 minutes, and the wear depth of the flat plate was measured. Details of the test conditions are shown in Table 1.

[0108]

[0109] (Friction and Wear Test 2) Grease compositions were evaluated in the same manner as in Friction and Wear Test 1, except for the following changes: The lower test piece was changed to a flat plate made of SUJ2 (Φ24 mm × 7.8 mm, Ra 0.2 μm). The load pressing the upper test piece against the lower test piece was changed to 250 N. The test time was changed to 10 minutes. Details of the test conditions are shown in Table 2. Friction and Wear Test 2 was conducted on the grease compositions prepared in Example 2 and Comparative Examples 1 to 3.

[0110]

[0111]

[0112] As shown in the results in Table 3, it has become clear that the grease composition according to the embodiment of the present disclosure can reduce the amount of wear on the friction surface of a lubricated member in a poor lubrication environment.

[0113] 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 tooth 792: Seat member G Grease composition

Claims

1. A grease composition comprising a base oil, a thickener, and an additive, wherein the thickener is lithium soap, and the additive comprises molybdenum dialkyldithiocarbamate and octadecane amide, the proportion of the molybdenum dialkyldithiocarbamate relative to the total mass of the grease composition is 0.1 mass% or more and 4.0 mass% or less, and the proportion of the octadecane amide relative to the total mass of the grease composition is 0.5 mass% or more and 10.0 mass% or less.

2. The grease composition according to claim 1, wherein the thickener is lithium 12-hydroxystearate.

3. The grease composition of claim 1, wherein the base oil comprises a poly-α-olefin and a mineral oil.

4. A steering gear device comprising: 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 urges the rack teeth against the pinion teeth; and a grease composition interposed between the meshing rack teeth and pinion teeth, and between the circumferential 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 a grease composition according to any one of claims 1 to 3.

Images