Grease composition, ball screw, and ball screw device

Abstract

Provided is a grease composition including a base oil, urea, and an additive. The base oil includes at least a polyglycol oil. The additive includes molybdenum dithiocarbamate, zinc dithiophosphate, calcium sulfonate, MCA, and an antioxidant. The proportion of urea to the entire grease composition is 6.0-11.0 mass%. As pertains to the proportion of the additive to the entire grease composition, the proportion of molybdenum dithiocarbamate is 3.0-7.0 mass%, the proportion of zinc dithiophosphate is 0.7-5.0 mass%, the proportion of calcium sulfonate is 0.7-5.0 mass%, the proportion of MCA is 3.0-7.0 mass%, and the proportion of the antioxidant is 0.3-3.0 mass%.

Description

Grease composition, ball screw, and ball screw device

[0001] The present disclosure relates to a grease composition, a ball screw, and a ball screw device.

[0002] An electric brake for an automobile is equipped with a ball screw device. To improve the service life of the ball screw included in the ball screw device, it is effective to suppress wear between the balls and the screw shaft of the ball screw. A grease composition is used in the ball screw to suppress wear between the screw shaft and the balls. Meanwhile, the ball screw device also includes, as other components of the ball screw, a moved member, a case, and a seal. The moved member is fixed to one of the nut and the screw shaft of the ball screw. The case rotatably supports the other of the nut and the screw shaft of the ball screw. The seal is fixed to the case and slides axially on either the moved member, the nut, or the screw shaft, or is fixed to either the moved member, the nut, or the screw shaft and slides axially on the case. The seal defines a space between the case and either the moved member, the nut, or the screw shaft, and an external space. The seal is made of EPDM (ethylene propylene diene rubber).

[0003] EPDM may swell due to a grease composition. Therefore, a grease composition that does not cause EPDM to swell is selected as the grease composition used in a ball screw. For example, a grease composition using a polyglycol-based base oil has been proposed as a grease composition that does not cause EPDM to swell (see, for example, Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2008-298107

[0005] While a ball screw using the grease composition described in Patent Document 1 is expected to suppress swelling of EPDM members, the grease composition described in Patent Document 1 is not effective enough in suppressing wear between the screw shaft and the balls. Therefore, a new grease composition is needed to further improve the life of the ball screw.

[0006] The grease composition of the present disclosure is a grease composition comprising a base oil, urea, and an additive, wherein the base oil comprises at least a polyglycol oil, and the additive comprises molybdenum dithiocarbamate, zinc dithiophosphate, calcium sulfonate, melamine cyanuric acid (MCA), and an antioxidant, and the proportion of urea relative to the entire grease composition is 6.0 to 11.0 mass%, the proportion of molybdenum dithiocarbamate relative to the entire grease composition is 3.0 to 7.0 mass%, the proportion of zinc dithiophosphate relative to the entire grease composition is 0.7 to 5.0 mass%, the proportion of calcium sulfonate relative to the entire grease composition is 0.7 to 5.0 mass%, the proportion of MCA relative to the entire grease composition is 3.0 to 7.0 mass%, and the proportion of the antioxidant relative to the entire grease composition is 0.3 to 3.0 mass%.

[0007] The ball screw of the present disclosure includes a screw shaft, a nut, and a plurality of balls, and the grease composition of the present disclosure is disposed at the locations of rolling and sliding contact between the balls and the screw shaft and at the locations of rolling and sliding contact between the balls and the nut.

[0008] The ball screw device of the present disclosure comprises the ball screw of the present disclosure, a moved member, a case, and a seal, wherein the moved member is fixed to one of the nut and screw shaft of the ball screw, the case rotatably supports the other of the nut and screw shaft of the ball screw, the seal is fixed to the case and slides axially on either the moved member, the nut, or the screw shaft, or is fixed to either the moved member, the nut, or the screw shaft and slides axially on the case, and the seal defines a space between the case and either the moved member, the nut, or the screw shaft, and an outside space, and the seal is made of EPDM.

[0009] The grease composition of the present disclosure can effectively suppress wear of lubricated members without causing swelling of EPDM members. The ball screw and ball screw device of the present disclosure have a long life because the screw shaft, nut, and balls are less likely to wear.

[0010] 1 is a perspective view of a ball screw; FIG. 2 is a partial cross-sectional view of a ball screw device including the ball screw shown in FIG. 1, a case, and a part of a seal; FIG. 3 is a perspective view of a circulation member; FIG. 4 is an exploded perspective view of the circulation member shown in FIG.

[0011] <Outline of Embodiments of the Invention of the Present Disclosure> Below, outlines of embodiments of the invention of the present disclosure will be listed and described. (1) The grease composition of the present disclosure is a grease composition comprising a base oil, urea, and an additive, wherein the base oil comprises at least a polyglycol oil, and the additive comprises molybdenum dithiocarbamate, zinc dithiophosphate, calcium sulfonate, MCA, and an antioxidant, wherein the proportion of urea relative to the entire grease composition is 6.0 to 11.0 mass%, the proportion of molybdenum dithiocarbamate relative to the entire grease composition is 3.0 to 7.0 mass%, the proportion of zinc dithiophosphate relative to the entire grease composition is 0.7 to 5.0 mass%, the proportion of calcium sulfonate relative to the entire grease composition is 0.7 to 5.0 mass%, the proportion of MCA relative to the entire grease composition is 3.0 to 7.0 mass%, and the proportion of the antioxidant relative to the entire grease composition is 0.3 to 3.0 mass%.

[0012] The grease composition contains polyglycol oil as a base oil, which makes it less likely to swell EPDM.Furthermore, the grease composition contains a predetermined amount of specific additives, which provides a good wear-inhibiting effect on lubricated members.

[0013] (2) In the grease composition (1) above, the proportion of molybdenum dithiocarbamate relative to the entire grease composition is preferably 4.0 to 6.0 mass%. (3) In either the grease composition (1) or (2) above, the proportion of zinc dithiophosphate relative to the entire grease composition is preferably 2.0 to 4.0 mass%. (4) In any of the grease compositions (1) to (3) above, the proportion of calcium sulfonate relative to the entire grease composition is preferably 1.0 to 2.0 mass%. (5) In any of the grease compositions (1) to (4) above, the proportion of MCA relative to the entire grease composition is preferably 4.0 to 6.0 mass%. (6) In any of the grease compositions (1) to (5) above, the proportion of antioxidant relative to the entire grease composition is preferably 0.5 to 2.0 mass%.

[0014] (7) A ball screw according to the present disclosure includes a screw shaft, a nut, and a plurality of balls, and any one of the grease compositions (1) to (6) is disposed at the locations of rolling and sliding contact between the balls and the screw shaft and at the locations of rolling and sliding contact between the balls and the nut.

[0015] The ball screw is a ball screw in which the screw shaft, nut, and balls are resistant to wear. Furthermore, even if the ball screw includes an EPDM member, the EPDM member is resistant to swelling.

[0016] (8) A ball screw according to (7), comprising a moved member, a case, and a seal, wherein the moved member is fixed to one of the nut and screw shaft of the ball screw, and the case rotatably supports the other of the nut and screw shaft of the ball screw, and the seal is fixed to the case and slides axially on either the moved member, the nut, or the screw shaft, or is fixed to either the moved member, the nut, or the screw shaft and slides axially on the case, and the seal defines a space between the case and either the moved member, the nut, or the screw shaft, and an outside space, and the seal is made of EPDM.

[0017] The ball screw device is a ball screw device in which the screw shaft, the nut, and the balls are resistant to wear. Furthermore, even if the ball screw device includes members made of EPDM, the members made of EPDM are resistant to swelling.

[0018] <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.

[0019] The grease composition of the present disclosure is suitable for use in a ball screw. In this specification, first, an embodiment of a ball screw device and a ball screw of the present disclosure will be described. Then, an embodiment of a grease composition of the present disclosure will be described.

[0020] [Overall Structure of Ball Screw] Fig. 1 is a perspective view of an example of a ball screw. Fig. 2 is a partial cross-sectional view of a ball screw device including the ball screw shown in Fig. 1, a case, and a portion of a seal. The ball screw device 1 of this embodiment includes a ball screw 10, a moved member 7, a case 8, and a seal 9. The ball screw 10 of this embodiment includes a screw shaft 11, a cylindrical nut 12, a plurality of balls 13, and a grease composition G. The nut 12 includes a nut body 15 and two circulation members 16, 17 attached to both axial sides of the nut body 15. The central axis C of the screw shaft 11 and the central axis of the nut 12 coincide. This central axis C is also the central axis of the ball screw 10.

[0021] The moved member 7 is fixed to a nut 12 of a ball screw 10. A case 8 rotatably supports a screw shaft 11 of the ball screw 10. A seal 9 is fixed to the case 8 and slides axially on the moved member 7. The seal 9 defines a space between the case 8 and the moved member 7 and an outside space. The seal 9 is made of EPDM (ethylene propylene diene rubber).

[0022] The screw shaft 11 is connected to a driving device such as a motor (not shown). The driving device drives the screw shaft 11 to rotate around its own central axis C. When the screw shaft 11 rotates toward a first side in the circumferential direction (forward rotation), the nut 12 moves toward a first side in the axial direction, and when the screw shaft 11 rotates toward a second side in the circumferential direction (reverse rotation), the nut 12 moves toward a second side in the axial direction.

[0023] In the present disclosure, the direction along the central axis C is defined as the "axial direction." This axial direction also includes a direction parallel to the central axis C. A direction perpendicular to the central axis C is defined as the "radial direction," and a direction along a circle centered on the central axis C is defined as the "circumferential direction."

[0024] The ball screw 10 of this embodiment is applied to, for example, an automobile brake device, particularly an electric booster. For example, an output shaft of the drive device (motor output shaft) (not shown) is connected to the second axial side of the screw shaft 11, which is the left side in FIG. 2 . A member connected to a piston of the brake device is provided on the first axial side, which is the right side of the nut 12 in FIG. 2 . In the ball screw 10, the member connected to the piston of the brake device is a moved member 7. The moved member 7 of this embodiment has a cylindrical portion with the same outer diameter as the nut 12 on its second axial side. When the screw shaft 11 rotates, the nut 12 moves to the first axial side, thereby pushing the moved member 7 toward the first axial side. As a result, the ball screw 10 of this embodiment generates a braking force in the automobile brake device.

[0025] The screw shaft 11 is a cylindrical member and has one first helical groove 21 on its outer periphery. As shown in FIG. 2 and as described above, the nut 12 has a nut body 15 and two circulation members 16, 17. The nut body 15 is cylindrical and has a second helical groove 22 on its inner periphery. The second helical groove 22 has the same pitch and number of threads as the first helical groove 21. The first helical groove 21 and the second helical groove 22 can face each other. A helical passage formed between the first helical groove 21 and the second helical groove 22 is a rolling path 23. Note that the first helical groove 21 and the second helical groove 22 may have multiple threads.

[0026] A plurality of balls 13 are arranged in the rolling path 23. As the screw shaft 11 rotates, the balls 13 move while rolling along the rolling path 23. A load acts between the first spiral groove 21 and the ball 13 and between the second spiral groove 22 and the ball 13. The surfaces of the first spiral groove 21 and the second spiral groove 22 are hardened to bear the load. To harden the surfaces of the first spiral groove 21 and the second spiral groove 22, the nut body 15 and the screw shaft 11 are subjected to heat treatment. Specifically, the nut body 15 and the screw shaft 11 are subjected to quenching (carburizing and quenching) and tempering.

[0027] The nut body 15 has a cylindrical central cylindrical portion 30, a cylindrical first end cylindrical portion 31, and a cylindrical second end cylindrical portion 32. The central cylindrical portion 30, the first end cylindrical portion 31, and the second end cylindrical portion 32 are provided in a single steel cylindrical block. The central portion of the single cylindrical block is the central cylindrical portion 30, and the portions on either side of it are the first end cylindrical portion 31 and the second end cylindrical portion 32. The outer diameters of the first end cylindrical portion 31 and the second end cylindrical portion 32 are the same as the outer diameter of the central cylindrical portion 30, while the inner diameters of the first end cylindrical portion 31 and the second end cylindrical portion 32 are larger than the inner diameter of the central cylindrical portion 30.

[0028] The second spiral groove 22 is provided on the inner peripheral surface of the central cylindrical portion 30. Furthermore, a through hole 33 that is long in the axial direction is provided in the central cylindrical portion 30. The ball 13 passes through the through hole 33 that is long in the axial direction. The central cylindrical portion 30 has a first end face 34 facing a first axial side and a second end face 35 facing a second axial side. The first end face 34 and the second end face 35 are each annular surfaces facing the axial direction and are surfaces along an imaginary plane perpendicular to the central axis C. The first axial side of the through hole 33 opens to the first end face 34, and the second axial side of the through hole 33 opens to the second end face 35.

[0029] The first cylindrical end portion 31 surrounds the first end face 34 from the radially outer side. The first cylindrical end portion 31 is provided on a first axial side of the radially outer portion of the central cylindrical portion 30. The second cylindrical end portion 32 surrounds the second end face 35 from the radially outer side. The second cylindrical end portion 32 is provided on a second axial side of the radially outer portion of the central cylindrical portion 30. With this configuration, the nut body 15 has a stepped hole shape on both axial sides. The inner circumferential surface 31a of the first cylindrical end portion 31 is a cylindrical surface and is provided with a circumferential groove 41 along part of its axial direction. The inner circumferential surface 32a of the second cylindrical end portion 32 is a cylindrical surface and is provided with a circumferential groove 42 along part of its axial direction. The inner circumferential surfaces 31a, 32a are each cylindrical surfaces centered on the central axis C.

[0030] As described above, when the threaded shaft 11 rotates forward and the nut 12 moves toward the first axial side, the nut 12 pushes the moved member 7 toward the first axial side. That is, the end face 39 on the first axial side of the first tubular end portion 31 comes into contact with the moved member 7 from the axial direction and further pushes the moved member 7 toward the first axial side. This generates a braking force in the brake device. In this embodiment, the end face 39 on one axial side of the first tubular end portion 31 is a load transmission surface that transmits the thrust of the nut 12 toward the first axial side to the moved member 7.

[0031] When the screw shaft 11 reverses and the nut 12 moves to the second axial side from a state in which braking force is being generated in the brake device, the thrust of the nut 12 against the moved member 7 is released. This releases the braking by the brake device. The moved member 7 may be configured to follow the nut 12 to both the first axial side and the second axial side. In this embodiment, the end face 39 on the first axial side of the first tubular end portion 31 is a load transmission surface that contacts the moved member 7 from the axial direction. On the other hand, although not shown, in a modified example, the end portion on the first axial side of the first tubular end portion 31 is structured to fit closely with the end portion of the moved member 7. With this structure, the nut 12 and the moved member 7 move together.

[0032] Fig. 3 is a perspective view of the circulation member 16. Fig. 4 is an exploded perspective view of the circulation member 16 shown in Fig. 3. The circulation member 16 of this embodiment is made of polyamide 66 containing 30 mass % glass fiber, and is composed of two molded parts 16a and 16b that can be separated in the axial direction. The circulation member 16 of this embodiment is an annular member.

[0033] A first passage 36 is formed in a first circulation member 16 disposed on a first axial side of the nut 12 (see FIG. 2 ). The first passage 36 is configured by a groove and a hole formed between a side surface 45 on the other axial side of the first circulation member 16 and an inner circumferential surface 46. A second passage 37 is formed in a second circulation member 17 disposed on a second axial side. The second passage 37 is configured by a groove and a hole formed between a side surface 47 on one axial side of the second circulation member 17 and an inner circumferential surface 48. The second circulation member 17 has the same configuration as the first circulation member 16, and is attached to the nut body 15 with its axial orientation opposite.

[0034] The circulation path 38 of the nut 12 is formed by the first passage 36, the through hole 33 of the central cylindrical portion 30, and the second passage 37. The balls 13 are also arranged in the circulation path 38.

[0035] As the screw shaft 11 rotates, the balls 13 in the rolling path 23 roll along the rolling path 23 (first spiral groove 21 and second spiral groove 22) and apply an axial force to the nut 12, causing the nut 12 to move in the axial direction. The balls 13 pass through the circulation path 38 from the end of the rolling path 23 on the first side in the axial direction, and can return to the end of the rolling path 23 on the second side in the axial direction depending on the movement stroke of the nut 12. In other words, the balls 13 can circulate through the rolling path 23 and the circulation path 38.

[0036] As described above, the first circulation member 16 has the first passage 36 that connects the through hole 33 and the rolling path 23 (the second spiral groove 22 that constitutes the rolling path 23) on the first axial side of the nut 12. The first passage 36 has a function of transferring the balls 13 that have passed through the rolling path 23 to the through hole 33 when the screw shaft 11 rotates, and conversely, a function of returning the balls 13 that have passed through the through hole 33 to the rolling path 23 when the screw shaft 11 rotates.

[0037] The second circulation member 17 has a second passage 37 that connects the through hole 33 and the rolling path 23 (the second spiral groove 22 that constitutes the rolling path 23) on the second axial side of the nut 12. The second passage 37 has a function of returning the balls 13 that have passed through the through hole 33 to the rolling path 23 when the screw shaft 11 rotates, and conversely, a function of transferring the balls 13 that have passed through the rolling path 23 to the through hole 33 when the screw shaft 11 rotates.

[0038] 2 , the first circulation member 16 is disposed radially inside the first cylindrical end portion 31 and in contact with the first end face 34. The ball screw 10 further includes a first retaining ring 18 that fits into a first circumferential groove 41 formed on the inner circumference of the first cylindrical end portion 31. The retaining ring 18 and the first end face 34 sandwich the first circulation member 16 in the axial direction, thereby positioning and fixing the first circulation member 16 relative to the nut body 15.

[0039] The second circulation member 17 is disposed radially inside the second cylindrical end portion 32 and in contact with the second end face 35. The ball screw 10 further includes a second retaining ring 19 that fits into a second circumferential groove 42 formed on the inner circumference of the second cylindrical end portion 32. The retaining ring 19 and the second end face 35 sandwich the second circulation member 17 in the axial direction, thereby positioning and fixing the second circulation member 17 relative to the nut body 15.

[0040] The first retaining ring 18 for fixing the first circulation member 16 and the second retaining ring 19 for fixing the second circulation member 17 are C-shaped retaining rings of the same configuration. As described above, the first circulation member 16 and the second circulation member 17 are fixed to the nut body 15 by the first retaining ring 18 and the second retaining ring 19, respectively.

[0041] [Regarding Circulation Members 16, 17] The first circulation member 16 will be described with reference to Figures 3 and 4. As described above, the circulation member 16 is made of polyamide 66 containing 30% by mass of glass fiber and is composed of two resin molded parts 16a, 16b that can be separated in the axial direction. By fitting the second resin molded part 16b into a recessed portion 51 formed on the side of the first resin molded part 16a, the first resin molded part 16a and the second resin molded part 16b are combined to form a single circulation member 16. The central axis C1 of the first resin molded part 16a and the central axis C2 of the second resin molded part 16b coincide with each other. Each of the two resin molded parts 16a, 16b is manufactured by injection molding.

[0042] Each surface of the resin molded products 16a, 16b is configured in a shape that allows them to be demolded by dividing the molding die in the axial direction. The first resin molded product 16a has a recessed portion 51 for receiving the second resin molded product 16b, as well as a groove 52 for forming the first passage 36. All of the surfaces forming the recessed portion 51 and the groove 52 are parallel to the central axis C1 of the resin molded product 16a or are visible when the resin molded product 16a is viewed in the axial direction. The second resin molded product 16b has grooves 53a, 53b formed therein to form the first passage 36. All of the surfaces forming the grooves 53a, 53b are parallel to the central axis C2 of the resin molded product 16b or are visible when the resin molded product 16a is viewed in the axial direction (the opposite direction from the first resin molded product 16a). The resin molded products 16a, 16b are not forcibly removed from the molding die when the molding die is divided in the axial direction.

[0043] The first resin molded product 16a and the second resin molded product 16b are positioned relative to each other in the circumferential direction, so that the convex portions 54 fit into the cutout portions 55. The first resin molded product 16a of this embodiment has the convex portions 54. The second resin molded product 16b of this embodiment has the cutout portions 55 into which the convex portions 54 fit. By fitting the convex portions 54 into the cutout portions 55, the second resin molded product 16b is positioned relative to the first resin molded product 16a in the circumferential direction.

[0044] Furthermore, the protrusion 54 of the first resin molded product 16a fits into a hole (not shown) provided in the first end face 34 of the central cylindrical portion 30 of the nut body 15 (see FIG. 2). The circulation member 16 is positioned in the circumferential direction and attached to the nut body 15. As a result, the through hole 33 of the central cylindrical portion 30 and the first passage 36 of the circulation member 16 communicate with each other.

[0045] The second circulation member 17 on the other axial side is attached to the nut body 15 in the same manner as the first circulation member 16 .

[0046] [Regarding Grease Composition G] As described above, the ball screw 10 includes the screw shaft 11 having the first spiral groove 21 formed on its outer periphery, the nut 12 having the nut body 15 having the second spiral groove 22 formed on its inner periphery and having the through hole 33 extending in the axial direction, and a plurality of balls 13 arranged in the rolling path 23 formed between the first spiral groove 21 and the second spiral groove 22. The ball screw device 1 also includes the ball screw 10, the moved member 7, the case 8, and the seal 9.

[0047] In addition to the nut body 15, the nut 12 has, on a first axial side, a first circulation member 16 and a first retaining ring 18 which are separate from the nut body 15. The first circulation member 16 has a first passage 36 which connects the through hole 33 and the rolling path 23. The nut 12 further has, on a second axial side, a second circulation member 17 and a second retaining ring 19 which are separate from the nut body 15. The second circulation member 17 has a second passage 37 which connects the through hole 33 and the rolling path 23.

[0048] In the ball screw 10 configured in this manner, the grease composition G is disposed in the first spiral groove 21 of the screw shaft 11, the second spiral groove 22 of the nut body 15, the balls 13, and the through hole 33 of the nut body 15. Therefore, in the ball screw 10, the rolling and sliding contact between the balls 13 and the surface of the rolling path 23, and the rolling and sliding contact of the balls 13 with the wall surface of the through hole 33 are lubricated by the grease composition G. As a result, in the ball screw 10, wear of the balls 13, the first spiral groove 21 of the screw shaft 11, the second spiral groove 22 of the nut body 15, and the through hole 33 of the nut body 15 is suppressed.

[0049] Furthermore, the grease composition G is allowed to move into the space between the case 8 and the moved member 7. The seal 9 of the ball screw device 1 prevents the grease composition G from leaking into the external space from the space between the case 8 and the moved member 7. The grease composition G adheres to the seal 9 made of EPDM.

[0050] As already explained, EPDM is a material that has poor grease resistance and is prone to swelling due to a grease composition. However, the ball screw device 1 of this embodiment uses the grease composition G according to an embodiment of the present disclosure, which makes it difficult for the EPDM seal 9 to swell. Swelling of the seal 9 prevents deterioration of the sealing performance of the seal 9, which prevents leakage of the grease composition G and entry of foreign matter from the external space. Therefore, the ball screw device 1 of this embodiment suppresses wear of the screw shaft 11, the nut 12, and the balls 13, and also suppresses swelling of the seal 9. As a result, the ball screw 10 and the ball screw device 1 of this embodiment have a long life.

[0051] As described above, the grease composition according to the embodiment of the present disclosure can be suitably used in ball screws and ball screw devices having components made of EPDM. Next, the grease composition according to the embodiment of the present disclosure will be described.

[0052] <Grease Composition> A grease composition according to an embodiment of the present disclosure includes a base oil, urea, and an additive. The base oil includes at least a polyglycol oil. The base oil may be composed solely of a polyglycol oil, or may be a mixed oil containing other base oils. When the base oil is a mixed oil, the content of the polyglycol oil relative to the total base oil is preferably 50.0 mass% or more.

[0053] Examples of the polyglycol oil include polyoxyethylene glycol, polyoxypropylene glycol, polyoxyethylene polyoxypropylene glycol, polyoxyethylene monobutyl ether, polyoxypropylene monobutyl ether, polyoxyethylene polyoxypropylene monobutyl ether, and derivatives thereof. When the base oil is a mixed oil of a polyglycol oil and another base oil, the other base oil is preferably an ester oil.

[0054] When the base oil is a mixed oil of polyglycol oil and ester oil, the proportion of polyglycol oil relative to the total base oil is preferably 50.0 mass% or more. A grease composition containing a base oil with a polyglycol oil proportion of less than 50.0 mass% is likely to swell EPDM components. The base oil may have a proportion of polyglycol oil of 100.0 mass% relative to the mass of the base oil. In other words, as described above, the base oil may be composed solely of polyglycol oil.

[0055] Examples of the ester oil include a reaction product of trimethylolpropane and a fatty acid, a reaction product of pentaerythritol and a fatty acid, a reaction product of dipentaerythritol and a fatty acid, and mixtures thereof.

[0056] The preferred kinematic viscosity of the base oil at 40°C is 40 to 80 mm ２ / s. The kinematic viscosity of the base oil at 40 ° C. is 40 mm ２ A grease composition having a kinematic viscosity of less than 80 mm / s at 40°C has poor heat resistance. ２ When a grease composition exceeding this value is used in a ball screw, it is expected that the torque of the ball screw will increase.

[0057] The grease composition contains urea, which functions as a thickener. The reason why a grease composition containing urea is preferred is that it exhibits excellent wear resistance when used in combination with the additives described below.

[0058] The urea may be any known urea. Specifically, examples of the urea include diurea, triurea, tetraurea, and polyurea (excluding diurea, triurea, and tetraurea). These may be used alone or in combination of two or more.

[0059] The preferred urea is diurea. The diurea is a reaction product of an amine compound and a diisocyanate compound. Examples of the amine compound include alkylamine, alkylphenylamine, and cyclohexylamine. Examples of the diisocyanate compound include aliphatic diisocyanates, alicyclic diisocyanates, and aromatic diisocyanates.

[0060] The preferred diurea is represented by the following structural formula (1): １ -NHCONH-R ２ -NHCONH-R ３ ...(1) (In formula (1), R １ and R ３ are each independently -C ｎ H ２ｎ＋１ (n is an integer of 6 to 10), or an alkyl group represented by R ４ -C ６ H １０ - (R ４ is hydrogen, a 2-methyl group, a 3-methyl group, or a 4-methyl group), and R ２ is -(CH ２ ) ６ -, -C ６ H ３ (CH ３ ) - or -C ６ H ４ -CH ２ -C ６ H ４ - is.)

[0061] In the above formula (1), R ２ Ga-C ６ H ３ (CH ３In the case of R ), the phenylene group of the preferred diurea is bonded at the 2,4 or 2,6 positions with the methyl group at the 1 position. ２ Ga-C ６ H ４ -CH ２ -C ６ H ４ In the case of R 1 -, both phenylene groups of the preferred diurea are bonded at the para position. １ and R ３ is C ６ H １１ (cyclohexyl group). ２ is -C ６ H ４ -CH ２ -C ６ H ４ - is.

[0062] To obtain the diurea, the amine compound and the diisocyanate compound can be reacted under various conditions. The preferred reaction is in a base oil, since this produces a diurea compound with high uniform dispersibility as a thickener. The reaction between the amine compound and the diisocyanate compound may be carried out by adding a base oil in which the diisocyanate compound has been dissolved to the base oil in which the amine compound has been dissolved, or by adding a base oil in which the amine compound has been dissolved to the base oil in which the diisocyanate compound has been dissolved.

[0063] The temperature and time for the reaction of the amine compound with the diisocyanate compound are not particularly limited, and conditions similar to those usually employed in this type of reaction may be employed. The preferred reaction temperature is 50 to 100°C in view of the solubility and volatility of the amine compound and the diisocyanate compound.

[0064] The reaction time is preferably 0.5 to 2.0 hours from the viewpoint of completing the reaction with the amine compound and the diisocyanate compound and shortening the production time of the grease composition.

[0065] In the grease composition, the content of the urea is 8 to 20% by mass relative to the total amount of the base oil and the urea. If the urea content is less than 8% by mass, the urea as a thickener will not be able to sufficiently retain the base oil, and when the grease composition is filled into a ball screw and used, there is a risk of the base oil scattering or leakage. On the other hand, if the urea content exceeds 20% by mass, the grease composition will become hard, and when the grease composition is filled into a ball screw and used, there is a risk of the torque of the ball screw increasing or the fluidity of the grease composition decreasing, thereby shortening the life of the ball screw. The preferred content of the urea is 8 to 15% by mass relative to the total amount of the base oil and the urea.

[0066] The grease composition contains a predetermined amount of a plurality of additives (the following additives (a) to (e)). In the following description, the content is a mass ratio relative to the total amount of the grease composition.

[0067] (a) Molybdenum dithiocarbamate (MoDTC) is contained in a proportion of 3.0 to 7.0 mass%. MoDTC forms a film on the surface of a lubricated member (e.g., the surface of the spiral groove of a screw shaft, the surface of the spiral groove of a nut, the surface of a through hole of a nut, etc. in a ball screw). A surface on which a sufficient film is formed can suppress wear of the lubricated member. On the other hand, a grease composition with a MoDTC content of less than 3.0 mass% cannot form a good film on the surface of a lubricated member. A surface on which a sufficient film is not formed cannot sufficiently suppress wear of the lubricated member. Furthermore, the effect of a grease composition with a MoDTC content of more than 7.0 mass% is hardly improved compared to the effect of a grease composition with a MoDTC content of 7.0 mass%. A preferred MoDTC proportion is 4.0 to 6.0 mass%.

[0068] As the molybdenum dithiocarbamate (MoDTC), a conventionally known MoDTC can be used. The MoDTC is a compound represented by the following formula (1):

[0069]

[0070] (In the formula, R １ ~R ４are each independently a linear or branched alkyl group.) In the embodiment of the present disclosure, the MoDTC may be one type of MoDTC, or two or more types of MoDTCs having different structures.

[0071] (b) Zinc dithiophosphate (ZnDTP) is contained in a proportion of 0.7 to 5.0 mass%. Zinc dithiophosphate (ZnDTP) forms a film on the surface of a lubricated member. A surface on which a sufficient film is formed can suppress wear of the lubricated member. On the other hand, a grease composition with a ZnDTP content of less than 0.7 mass% cannot form a good film on the surface of a lubricated member. A surface on which a sufficient film is not formed cannot sufficiently suppress wear of the lubricated member. Furthermore, the effect of a grease composition with a ZnDTP content of more than 5.0 mass% is hardly improved compared to the effect of a grease composition with a ZnDTP content of 5.0 mass%. A preferred proportion of ZnDTP is 2.0 to 4.0 mass%.

[0072] As the ZnDTP, a conventionally known ZnDTP can be used. The ZnDTP is, for example, a compound represented by the following formula (2):

[0073]

[0074] (In formula (2), R ５ ~R ８ are each independently a primary alkyl group, a secondary alkyl group, or an aryl group.) In the embodiment of the present disclosure, the ZnDTP may be one type of ZnDTP, or two or more types of ZnDTP having different structures.

[0075] (c) The calcium sulfonate is contained in a proportion of 0.7 to 5.0 mass%. In this case, calcium sulfonate can suppress the formation of rust in the ball screw and reduce friction between the screw shaft 11 and the balls 13, as well as friction between the nut 12 and the balls 13. On the other hand, a grease composition containing less than 0.7 mass% calcium sulfonate cannot sufficiently suppress the formation of rust or increase in friction. Furthermore, the effect of a grease composition containing more than 5.0 mass% calcium sulfonate is hardly improved compared to the effect of a grease composition containing 5.0 mass% calcium sulfonate. Furthermore, if the calcium sulfonate content exceeds 5.0 mass%, the amount of lubricating component becomes relatively small. The preferred calcium sulfonate proportion is 1.0 to 2.0 mass%.

[0076] (d) MCA (Melamine Cyanuric Acid) is contained in a proportion of 3.0 to 7.0 mass%. In this case, MCA is present between lubricated members and can suppress wear of the lubricated members. On the other hand, a grease composition with an MCA content of less than 3.0 mass% cannot sufficiently suppress wear of the lubricated members. Furthermore, the effect of a grease composition with an MCA content of more than 7.0 mass% is hardly improved compared to the effect of a grease composition with an MCA content of 7.0 mass%. The preferred MCA proportion is 4.0 to 6.0 mass%. MCA is an organic adduct of melamine and isocyanuric acid in a ratio of approximately 1:1.

[0077] (e) The antioxidant is contained in a proportion of 0.3 to 3.0 mass%. A grease composition containing this proportion of antioxidant can effectively prevent deterioration of the grease composition. On the other hand, a grease composition containing less than 0.3 mass% of antioxidant cannot sufficiently suppress deterioration of the grease composition. Furthermore, the effect of a grease composition containing more than 3.0 mass% of antioxidant is hardly improved compared to the effect of a grease composition containing 3.0 mass% of antioxidant. The preferred proportion of antioxidant is 0.5 to 2.0 mass%.

[0078] The antioxidant is not particularly limited. Any known antioxidant can be used. Specific examples of the antioxidant include amine-based antioxidants and phenol-based antioxidants.

[0079] The grease composition may further contain additives other than the above (a) to (e), such as dyes, color stabilizers, thickeners, structural stabilizers, metal deactivators, and viscosity index improvers.

[0080] The preferred worked penetration (60W) of the grease composition measured according to a method in accordance with JIS K 2220.7 is 310 to 340. This is because when used in a ball screw, the grease composition having a worked penetration (60W) in this range is likely to be present at the rolling and sliding contact points between the screw shaft and balls and the rolling and sliding contact points between the nut and balls, and is also unlikely to leak from the ball screw device.

[0081] <Method for producing grease composition> The grease composition can be produced, for example, by first producing a base grease through a step of producing urea in a base oil, and then adding additives to the obtained base grease and stirring to mix each component into the base grease.

[0082] As described above, the grease composition of the present disclosure can be suitably used for lubricating ball screws, and in particular, the grease composition of the present disclosure can be suitably used for lubricating ball screw devices in automotive brake systems.

[0083] Next, the present invention will be described in more detail based on examples, but the present invention is not limited to the examples.

[0084] (Raw materials for the grease composition) Base oil: Polyglycol oil (polyoxypropylene monobutyl ether, kinematic viscosity of the base oil (40°C) is 70 mm ２ / s) Thickeners: Lithium complex soap (reaction product of lithium hydroxide with 12-hydroxystearic acid and azelaic acid), Diurea (reaction product of cyclohexylamine and 4,4'-diphenylmethane diisocyanate (MDI)), Additives: (a) Molybdenum dithiocarbamate (molybdenum di-n-butyldithiocarbamate), (b) Zinc dithiophosphate (zinc dialkyldithiophosphate (C1-12)), (c) Calcium sulfonate (calcium salt of alkylbenzenesulfonic acid), (d) MCA (melamine cyanurate), (e) Antioxidant (IRGANOX L57 (1.0 mass%), IGRANOX L135 (0.2 mass%))

[0085] Example 1

[0086] 1) Base oil (polyglycol oil), cyclohexylamine, and 4,4'-diphenylmethane diisocyanate (MDI) were weighed. The base oil was heated to 55°C. 2) Half of the base oil (55°C) and MDI were placed in stainless steel container A. The base oil and MDI were stirred, and the MDI was dissolved in the base oil. 3) The remaining half of the base oil (55°C) and cyclohexylamine were placed in another stainless steel container B. The base oil and cyclohexylamine were stirred, and the cyclohexylamine was dissolved in the base oil.

[0087] 4) The amine solution in stainless steel container B was gradually poured into stainless steel container A containing the isocyanate solution. The temperature inside stainless steel container A was then raised to 130°C. 5) The mixture in stainless steel container A was stirred while continuing to be heated, and the temperature was maintained at 130°C for 30 minutes. 6) The heating of stainless steel container A was then stopped, and the mixture was allowed to cool naturally to below 80°C, yielding a base grease.

[0088] 7) While the obtained base grease was being stirred, the above-mentioned additives were added to the base grease in the order of (a) to (e). After all the additives had been added to the base grease, the base grease to which all the additives had been added was stirred for 60 minutes. 8) As a finishing treatment, a three-roll mill treatment was carried out, and the grease composition was completed.

[0089] (Comparative Example 1) 1) First, 12-hydroxystearic acid was dissolved in a base oil (polyglycol oil). Next, an aqueous solution of lithium hydroxide was added to the base oil. After that, the base oil was heated at 170°C for 10 minutes while being stirred, and the lithium hydroxide and 12-hydroxystearic acid reacted with each other.

[0090] 2) Next, azelaic acid was dissolved in a base oil containing a reaction product of 12-hydroxystearic acid and lithium hydroxide. Next, an aqueous solution of lithium hydroxide was added to the base oil. The base oil was then heated to 180°C for 10 minutes while stirring, and the lithium hydroxide and azelaic acid reacted. After completion of the reaction, the base oil was cooled to 80°C at a cooling rate of 1.5°C / min, and a base grease was obtained. In this process, the amounts of lithium hydroxide, 12-hydroxystearic acid, and azelaic acid were adjusted so that the amount of lithium complex soap produced would be 6.5% by mass based on the total grease composition.

[0091] 4) While the obtained base grease was being stirred, the above-mentioned additives were added to the base grease in the order of (a) to (e). After all the additives had been added to the base grease, the base grease to which all the additives had been added was stirred for 60 minutes. Thereafter, the base grease to which all the additives had been added was allowed to cool to room temperature.

[0092] 5) As a finishing treatment, a three-roll mill treatment was carried out, and the grease composition was completed.

[0093] The physical properties of the grease compositions prepared in the Examples and Comparative Examples were measured. Furthermore, the wear resistance of the above grease compositions was evaluated when the grease compositions prepared in the Examples and Comparative Examples were used. The results are shown in Table 2.

[0094] (Measurement of Worked Penetration) The worked penetration (60W) of the grease compositions produced in the Examples and Comparative Examples was measured by a method in accordance with JIS K 2220.7.

[0095] (Wear Amount) A ball-on-disk friction and wear test was conducted on the grease compositions produced in the Examples and Comparative Examples using a friction and wear tester (SRV reciprocating friction and wear tester), and the wear amount (wear cross-sectional area) was evaluated. The test conditions are shown in Table 1. The results are shown in Table 2. The wear cross-sectional area is the cross-sectional area of ​​a circle cut on a plane including the center of the steel ball before the test minus the cross-sectional area of ​​the circle cut on a plane including the center of the steel ball and the center of the worn surface of the steel ball after the test. In this test method, the grease compositions produced in the Examples and Comparative Examples were applied to a disk made of SCM415, and a steel ball (diameter 10 mm) made of SUJ2 with a load (100 N) applied was brought into contact with the grease composition. In this state, the disk was oscillated at a speed of 33 Hz and an amplitude of 1.5 mm for 20 minutes. The wear cross-sectional area (μm ２ The temperature of the disc during the test was 70°C. This evaluation was carried out twice. The results are shown in Table 2 as the average of the two results.

[0096]

[0097]

[0098] As shown in Table 2, it has become clear that the grease composition according to the embodiment of the present disclosure can exhibit good lubricating performance capable of suppressing wear of lubricated members.

[0099] 1: Ball screw device 7: Moved member 8: Case 9: Seal 10: Ball screw 11: Screw shaft 12: Nut 13: Ball 15: Nut body 16, 17: Circulation member 16a, 16b: Resin molded product 18, 19: Retaining ring 21: First spiral groove 22: Second spiral groove 23: Rolling path 30: Central cylindrical portion 31: First end cylindrical portion 31a: Inner peripheral surface 32: Second end cylindrical portion 32a: Inner peripheral surface 33: Through hole 34: First end face 35: Second end face 36: First passage 37: Second passage 39: End face 41, 42: Circumferential groove G Grease composition

Claims

1. A grease composition comprising a base oil, urea, and an additive, wherein the base oil comprises at least a polyglycol oil, and the additive comprises molybdenum dithiocarbamate, zinc dithiophosphate, calcium sulfonate, melamine cyanuric acid (MCA), and an antioxidant, wherein a proportion of urea in the entire grease composition is 6.0 to 11.0 mass%, a proportion of molybdenum dithiocarbamate in the entire grease composition is 3.0 to 7.0 mass%, a proportion of zinc dithiophosphate in the entire grease composition is 0.7 to 5.0 mass%, a proportion of calcium sulfonate in the entire grease composition is 0.7 to 5.0 mass%, and a proportion of MCA in the entire grease composition is 3.0 to 7.0 mass%, A grease composition, wherein the proportion of the antioxidant in the entire grease composition is 0.3 to 3.0 mass %.

2. The grease composition according to claim 1, wherein the proportion of molybdenum dithiocarbamate in the entire grease composition is 4.0 to 6.0 mass %.

3. The grease composition according to claim 1, wherein the ratio of zinc dithiophosphate to the entire grease composition is 2.0 to 4.0 mass %.

4. The grease composition according to claim 1, wherein the ratio of calcium sulfonate to the entire grease composition is 1.0 to 2.0 mass %.

5. The grease composition according to claim 1, wherein the ratio of the MCA to the entire grease composition is 4.0 to 6.0 mass %.

6. The grease composition according to claim 1, wherein the ratio of the antioxidant to the entire grease composition is 0.5 to 2.0 mass %.

7. A ball screw comprising a screw shaft, a nut, and a plurality of balls, wherein the grease composition according to any one of claims 1 to 6 is disposed at the points of rolling and sliding contact between the balls and the screw shaft, and at the points of rolling and sliding contact between the balls and the nut.

8. A ball screw device comprising the ball screw according to claim 7, a moved member, a case, and a seal, wherein the moved member is fixed to one of the nut and screw shaft of the ball screw, and the case rotatably supports the other of the nut and screw shaft of the ball screw, and the seal is fixed to the case and slides axially on either the moved member, the nut, or the screw shaft, or is fixed to either the moved member, the nut, or the screw shaft and slides axially on the case, and the seal defines a space between the case and either the moved member, the nut, or the screw shaft, and an outside space, and the seal is made of EPDM.

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