Ball screw device

Abstract

Provided is a ball screw device capable of holding a sufficient amount of lubricant in a circulation groove. This ball screw device comprises: a screw shaft having a shaft-side spiral groove in the outer peripheral surface thereof; a nut 3 having, in the inner peripheral surface thereof, a nut-side spiral groove 7 and a substantially S-shaped circulation groove 9 connecting an end point and a start point of a spiral load path 8 composed of the shaft-side spiral groove 5 and the nut-side spiral groove 7; and a plurality of balls 4 arranged on the inner side of the load path 8 and the circulation groove 9. The circulation groove 9 has, in the surface thereof, a recessed lubricant holding part 11 for holding the lubricant.

Description

Ball screw device

[0001] The present disclosure relates to a ball screw device, which is a mechanical element for converting linear motion into rotational motion or rotational motion into linear motion.

[0002] A ball screw device rolls balls between a screw shaft and a nut, which provides higher efficiency than a sliding screw device in which the screw shaft and the nut are in direct contact. For this reason, ball screw devices are incorporated into various mechanical devices, such as electric brake devices and automatic manual transmissions (AMTs) for automobiles and positioning devices for machine tools, in order to convert the rotational motion of a drive source such as an electric motor into linear motion.

[0003] The ball screw device includes a screw shaft having a shaft-side spiral groove on its outer circumferential surface, a nut having a nut-side spiral groove on its inner circumferential surface, and a plurality of balls arranged to roll freely between the shaft-side spiral groove and the nut-side spiral groove. A lubricant such as grease is filled between the outer circumferential surface of the screw shaft and the inner circumferential surface of the nut to enable the balls to roll smoothly.

[0004] The shaft-side spiral groove and the nut-side spiral groove are arranged radially opposite each other to form a spiral load path. The start and end points of the load path are connected by a circulation means. The circulation means returns balls that have reached the end point of the load path to the start point of the load path, allowing the balls to circulate infinitely. The start and end points of the load path are interchanged depending on the direction of relative displacement (relative rotation direction) between the screw shaft and the nut in the axial direction.

[0005] Depending on the application, the ball screw device uses one of the screw shaft and the nut as a rotational motion element, and the other of the screw shaft and the nut as a linear motion element.

[0006] In ball screw devices, it has been common practice to use circulation parts such as a bearing, a tube, an end deflector, etc. as circulation means. JP 2014-062570 A discloses a structure of a ball screw device in which circulation parts are omitted and a substantially S-shaped circulation groove is formed directly on the inner peripheral surface of the nut as circulation means in order to reduce the size and cost of the ball screw device.

[0007] JP 2014-062570 A

[0008] As shown in Figure 12, the circulation groove 101 formed on the inner peripheral surface of the nut 100 has a groove width larger than the diameter of the balls 102. Furthermore, the balls 102 inside the circulation groove 101 are pushed and moved by the balls 102 positioned behind them in the direction of movement of the balls 102. For this reason, the balls 102 inside the circulation groove 101 are arranged alternately in the groove width direction (the left-right direction in Figure 12), and move while rubbing against the surface of the circulation groove 101.

[0009] This makes it easier for the balls 102 to scrape out the lubricant in the circulation groove 101 into the load path 103. Furthermore, the remaining state of the lubricant may vary depending on the position of the circulation groove 101. As a result, metal contact may occur between adjacent balls 102, between the balls 102 and the circulation groove 101, or both, which may cause damage such as indentations on the surface of the balls 102, the surface of the circulation groove 101, or both.

[0010] An object of the present disclosure is to provide a ball screw device that can retain a sufficient amount of lubricant in the circulation groove.

[0011] A ball screw device according to one aspect of the present disclosure includes a screw shaft, a nut, and a plurality of balls.

[0012] The screw shaft has a shaft-side spiral groove on the outer circumferential surface.

[0013] The nut has, on its inner surface, a nut-side spiral groove and a roughly S-shaped circulation groove that connects the end point and start point of a spiral load path formed by the shaft-side spiral groove and the nut-side spiral groove.

[0014] The plurality of balls are disposed inside the load path and the circulation groove.

[0015] The circulation groove has a recessed lubricant holding portion on its surface for holding a lubricant. Although grease is preferred as the lubricant, lubricants include not only grease but also lubricating oil (oil).

[0016] In the ball screw device according to one aspect of the present disclosure, the surface roughness of the circulation groove is greater than the surface roughness of the nut-side spiral groove, and the surface roughness of the circulation groove refers to the surface roughness of the entire surface of the circulation groove, including the portion of the surface of the circulation groove where the lubricant retaining portion is formed.

[0017] In a ball screw device according to one aspect of the present disclosure, the circulation groove has a surface roughness of 1.6 μm or more in terms of arithmetic mean roughness Ra.

[0018] In a ball screw device according to one aspect of the present disclosure, the surface of the nut-side spiral groove is a ground surface, and the surface of the circulation groove is a non-ground surface.

[0019] In a ball screw device according to one aspect of the present disclosure, the surface of the circulation groove is a blasted surface, that is, a surface processed by shot blasting.

[0020] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portions are arranged irregularly.

[0021] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portions are arranged regularly.

[0022] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portion is a hole-shaped recess or a groove-shaped recessed groove.

[0023] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portion is provided on the entire surface of the circulation groove.

[0024] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portion is provided partially on the surface of the circulation groove.

[0025] In the ball screw device according to one aspect of the present disclosure, the lubricant retaining portion is provided on both sides of the circulation groove in the longitudinal direction.

[0026] In a ball screw device according to one aspect of the present disclosure, the lubricant retaining portion is provided in a scooping portion provided in a part of both side portions in the length direction of the circulation groove.

[0027] According to the ball screw device of one aspect of the present disclosure, a sufficient amount of lubricant can be held in the circulation groove.

[0028] FIG. 1 is a cross-sectional view of a ball screw device according to a first embodiment of the present disclosure. FIG. 2 is a cross-sectional view of a nut constituting the ball screw device according to the first embodiment. FIG. 3 is a developed view of the inner peripheral surface of the nut constituting the ball screw device according to the first embodiment. FIG. 4 is a partially enlarged view of FIG. 2. FIG. 5 is a view corresponding to FIG. 4 of a ball screw device according to a second embodiment of the present disclosure. FIG. 6 is a cross-sectional view taken along line A-A in FIG. 5. FIG. 7 is a view corresponding to FIG. 4 of a ball screw device according to a third embodiment of the present disclosure. FIG. 8 is a view corresponding to FIG. 4 of a ball screw device according to a fourth embodiment of the present disclosure. FIG. 9 is a view corresponding to FIG. 4 of a ball screw device according to a fifth embodiment of the present disclosure. FIG. 10 is a view corresponding to FIG. 4 of a ball screw device according to a sixth embodiment of the present disclosure. FIG. 11 is a view corresponding to FIG. 4 of a ball screw device according to a seventh embodiment of the present disclosure. FIG. 12 is a view shown for explaining problems with ball screw devices having a conventional structure.

[0029] First Example A first example of an embodiment of the present disclosure will be described with reference to FIGS. 1 to 4. FIG.

[0030] The ball screw device 1 of this embodiment can be widely used for converting linear motion into rotational motion or rotational motion into linear motion. In particular, the ball screw device 1 of this embodiment can be incorporated into mechanical devices such as electric brake devices (EMB), electric brake booster devices (EHB), and speed ratio change devices, and used for converting rotational motion into linear motion.

[0031] The ball screw device 1 includes a screw shaft 2 , a nut 3 , and a plurality of balls 4 .

[0032] In the following description, unless otherwise specified, the axial direction, radial direction, and circumferential direction refer to the axial direction, radial direction, and circumferential direction with respect to the screw shaft 2. The axial direction, radial direction, and circumferential direction of the screw shaft 2 coincide with the axial direction, radial direction, and circumferential direction of the nut 3.

[0033] The screw shaft 2 is made of a metal such as an iron-based alloy and has a generally cylindrical shape. The screw shaft 2 is inserted into the nut 3 and is disposed coaxially with the nut 3.

[0034] The screw shaft 2 has a shaft-side spiral groove 5 on its outer peripheral surface. The cross-sectional groove shape (groove bottom shape) of the shaft-side spiral groove 5 is not limited to, but can be a Gothic arch groove, a circular arc groove, or the like. The shaft-side spiral groove 5 is formed on the outer peripheral surface of the screw shaft 2 by, for example, cutting or rolling, and grinding. The screw shaft 2 has a land portion 6 between adjacent shaft-side spiral grooves 5. The shaft-side spiral groove 5 can have one or any number of grooves greater than two, but in this example, it has one groove.

[0035] The nut 3 is made of a metal such as an iron-based alloy and has a generally cylindrical shape. The nut 3 has a nut-side spiral groove 7 on its inner circumferential surface. The nut 3 has multiple nut-side spiral grooves 7 on its inner circumferential surface. In this example, the nut 3 has four nut-side spiral grooves 7 on its inner circumferential surface. The nut-side spiral groove 7 has the same lead as the shaft-side spiral groove 5. The nut-side spiral groove 7 has the same number of threads as the shaft-side spiral groove 5, which is one in this example. The cross-sectional groove shape of the nut-side spiral groove 7 is not limited to this, but can be a gothic arch groove, a circular arc groove, or the like. The nut-side spiral groove 7 is formed on the inner circumferential surface of the nut 3 by, for example, cutting, such as cutting tapping, or rolling, such as rolling tapping, and grinding.

[0036] The shaft-side spiral groove 5 and the nut-side spiral groove 7 are arranged radially opposite to each other and form a spiral load path 8. Multiple load paths 8 can be provided. In this example, four load paths 8 are formed between the shaft-side spiral groove 5 and the nut-side spiral groove 7. Each load path 8 makes approximately one revolution around the outer periphery of the shaft-side spiral groove 5. Note that even when the shaft-side spiral groove 5 and the nut-side spiral groove 7 have two grooves, each load path 8 also makes approximately one revolution around the outer periphery of the shaft-side spiral groove 5.

[0037] The nut 3 has a substantially S-shaped circulation groove 9 on its inner peripheral surface that connects the end point and start point of the load path 8. The circulation groove 9 has a substantially S-shape when viewed from the radially inside of the nut 3. One end of the circulation groove 9 in the length direction is connected to the end point of the load path 8, and the other end of the circulation groove 9 in the length direction is connected to the start point of the load path 8. The middle portion of the circulation groove 9 in the length direction is arranged so as to cross the land portion 6 of the screw shaft 2.

[0038] The circulation groove 9 has a substantially semicircular cross section. The circulation groove 9 has a groove width slightly larger than the diameter of the ball 4. The circulation groove 9 is deeper than the nut-side spiral groove 7, and has a groove depth that allows the ball 4 moving in the circulation groove 9 to climb over the land portion 6.

[0039] The circulation groove 9 is formed directly on the inner peripheral surface of the nut 3 by, for example, cutting or forging. The circulation groove 9 is machined on the inner peripheral surface of the nut 3 separately from or simultaneously with the nut-side spiral groove 7.

[0040] The nut 3 has a plurality of circulation grooves 9 on its inner circumferential surface. The number of circulation grooves 9 that the nut 3 has on its inner circumferential surface depends on the number of load paths 8, but is not particularly limited. Furthermore, the arrangement positions of the circulation grooves 9 are not particularly limited. It is preferable that the plurality of circulation grooves 9 are arranged at equal intervals in the circumferential direction. In this example, the nut 3 has four circulation grooves 9 on its inner circumferential surface, the same number as the nut-side spiral grooves 7. The four circulation grooves 9 are evenly spaced at 90 degrees.

[0041] The balls 4 are steel balls having a predetermined diameter, and are arranged so as to roll freely between the shaft-side spiral groove 5 and the nut-side spiral groove 7, i.e., in the load path 8, and are also arranged inside the circulation groove 9, circulating between the load path 8 and the inside of the circulation groove 9. The balls 4 arranged in the load path 8 roll while receiving a compressive load, whereas the balls 4 inside the circulation groove 9 move by being pushed by the balls 4 behind them in the direction of movement of the balls 4, without receiving a compressive load.

[0042] As the screw shaft 2 or the nut 3 rotates, the balls 4 that have reached the end point of the loaded path 8 are returned to the start point of the loaded path 8 through the circulation groove 9. As a result, the balls 4 circulate endlessly within the loaded path 8. The start point and end point of the loaded path 8 are interchanged depending on the relative rotation direction of the screw shaft 2 and the nut 3.

[0043] In the ball screw device 1, grease (not shown) is filled as a lubricant in the annular space 10 between the outer circumferential surface of the screw shaft 2 and the outer circumferential surface of the nut 3 to enable smooth rolling of the balls 4. The ball screw device 1 may also be provided with a sealing member at the opening of the annular space 10 to prevent leakage of grease from the annular space 10. Note that although grease is preferably used as the lubricant, lubricating oil (oil) may also be filled instead of grease.

[0044] The lubricant filled in the annular space 10 adheres to the surfaces of the balls 4, the shaft-side spiral groove 5, the nut-side spiral groove 7, and the circulation groove 9. The lubricant adheres to the balls 4 and flows.

[0045] In order to prevent the lubricant in the circulation groove 9 from running out, the circulation groove 9 has a recessed lubricant retaining portion 11 on its surface that retains the lubricant. The lubricant retaining portion 11 has a shape recessed from the surface of the circulation groove 9.

[0046] The lubricant retaining portion 11 is not provided on the surface of the nut-side spiral groove 7 .

[0047] The lubricant holding portion 11 can have any shape as long as it can hold the lubricant. For example, the lubricant holding portion 11 can be formed of, but is not limited to, a minute recess (dimple), a hole-shaped recess, a groove-shaped recess, or the like.

[0048] Depending on the shape of the lubricant holding portion 11, there may be any number of lubricant holding portions 11 equal to or greater than one. When the lubricant holding portion 11 is configured by a plurality of lubricant holding portions 11, the respective lubricant holding portions 11 may be arranged irregularly or regularly.

[0049] Furthermore, the lubricant retaining portion 11 can be provided on the entire surface of the circulation groove 9 .

[0050] Alternatively, the lubricant holding portion 11 may be provided partially on the surface of the circulation groove 9. In this case, the lubricant holding portion 11 may be provided on both side portions in the length direction of the circulation groove 9. Preferably, the lubricant holding portion 11 may be provided on scooping portions provided on parts of both side portions in the length direction of the circulation groove 9.

[0051] In this example, the lubricant holding portion 11 is made up of a plurality (countless) of lubricant holding portions 11. The lubricant holding portions 11 are provided over the entire surface of the circulation groove 9 and are arranged irregularly (randomly).

[0052] The lubricant holding portion 11 is composed of minute recesses (cavities). The lubricant holding portion 11 is preferably formed by shot blasting (dimple processing) the surface of the circulation groove 9. In this case, the lubricant holding portion 11 is a minute recess having a specific or unspecific (irregular) shape that is formed on the surface of the circulation groove 9 by the impact of steel balls, which serve as shot material. The lubricant holding portion 11 includes not only a single independent recess, but also a recess formed by a series of multiple recesses. Note that in FIG. 4, the openings of the lubricant holding portion 11 are schematically shown as circles all having the same diameter.

[0053] Due to the formation of the lubricant holding portion 11, the surface roughness of the circulation groove 9 is greater than the surface roughness of the nut-side spiral groove 7. In this example, the surface roughness of the circulation groove 9 is 1.6 μm or more in arithmetic mean roughness Ra, and the surface roughness of the nut-side spiral groove 7 is less than 1.6 μm in arithmetic mean roughness Ra. The surface roughness of the circulation groove 9 refers to the surface roughness of the entire surface of the circulation groove 9, including the portion of the surface of the circulation groove 9 where the lubricant holding portion 11 is formed.

[0054] If the surface roughness of the circulation groove 9 is less than 1.6 μm in arithmetic mean roughness Ra, the lubricant holding portion 11 is not formed sufficiently, making it difficult to hold a sufficient amount of lubricant (preferably grease) in the circulation groove 9.

[0055] The nut-side spiral groove 7 has a ground surface, whereas the circulation groove 9 has an unground surface. The circulation groove 9 has a roughened surface. Examples of roughening methods include shot blasting, mechanical processing such as knurling, transfer processing, electric discharge processing, electron beam processing, and laser processing, and chemical processing such as etching. In this example, the surface of the circulation groove 9 has been shot blasted.

[0056] The surface of the circulation groove 9 has properties according to the type of roughening process. The cross-sectional shape of the circulation groove 9 on whose surface the lubricant holding portion 11 is formed can be, for example, a substantially triangular wave shape, a substantially sawtooth shape, etc. When the roughening process is shot blasting, the surface of the circulation groove 9 is an uneven blasted surface and has a non-periodic roughness curve.

[0057] [Manufacturing Method of Nut] An example of a manufacturing method of the nut 3 constituting the ball screw device 1 of this embodiment will be described.

[0058] <Circulation Groove Forming Process> First, a metallic material having a substantially cylindrical shape is prepared. Then, the inner peripheral surface of the material is forged or machined to form the circulation groove 9, thereby obtaining a first intermediate material. The lubricant retaining portion 11 is not formed on the surface of the circulation groove 9 formed on the inner peripheral surface of the first intermediate material.

[0059] <Nut-side spiral groove forming process> The inner peripheral surface of the first intermediate material is subjected to cutting processing such as cutting tapping to form the nut-side spiral groove 7, thereby obtaining a second intermediate material. The inner peripheral surface of the second intermediate material is provided with the nut-side spiral groove 7 and the circulation groove 9. The circulation groove forming process and the nut-side spiral groove forming process can also be performed simultaneously.

[0060] <Heat Treatment Step> The second intermediate material is subjected to heat treatment such as carburizing and quenching or induction hardening, and tempering to obtain a third intermediate material.

[0061] <Surface Roughening (Shot Blasting) Process> The third intermediate material, more specifically, at least the inner peripheral surface of the third intermediate material, is roughened to obtain a fourth intermediate material. In this example, shot blasting (dimple processing) is performed by impacting at least the inner peripheral surface of the third intermediate material with steel balls as projectiles. This removes oxide scale from at least the inner peripheral surface of the third intermediate material and forms recessed lubricant retaining portions 11 on at least the inner peripheral surface of the third intermediate material. Therefore, at the stage where the surface roughening process, in this example, shot blasting, is completed, lubricant retaining portions 11 (in this example, minute recesses) are formed not only on the surface of the circulation groove 9 but also on the surface of the nut-side spiral groove 7.

[0062] <Grinding Process> Then, the nut-side spiral groove 7 provided on the inner peripheral surface of the fourth intermediate material is ground to obtain the finished nut 3. In this process, the circulation groove 9 is not ground. The irregularities (lubricant retaining portion 11) formed on the surface of the nut-side spiral groove 7 are removed by the grinding process.

[0063] When manufacturing the nut 3, other steps may be inserted between the steps described above.

[0064] According to the ball screw device 1 of this embodiment, a sufficient amount of grease as a lubricant can be held in the circulation groove 9 .

[0065] That is, because the ball screw device 1 of this example has a concave lubricant retaining portion 11 on the surface of the circulation groove 9, even when the balls 4 rub against the surface of the circulation groove 9 as they move, the lubricant is retained in the lubricant retaining portion 11, reducing the amount of lubricant scraped out from the circulation groove 9 to the load path 8. Therefore, a sufficient amount of lubricant can be retained in the circulation groove 9, preventing the lubricant in the circulation groove 9 from running out. As a result, lubricant can be supplied to the balls 4 moving inside the circulation groove 9, preventing metallic contact between adjacent balls 4 and between the balls 4 and the circulation groove 9. Therefore, damage such as indentations can be suppressed from occurring on the balls 4 and the circulation groove 9.

[0066] Since the surface roughness of the circulation groove 9 is greater than the surface roughness of the nut-side spiral groove 7, the flow speed of the lubricant flowing inside the circulation groove 9 is slower than the flow speed of the lubricant flowing inside the nut-side spiral groove 7. For this reason, the lubricant tends to accumulate in the circulation groove 9, and the lubricant is therefore more likely to be retained in the circulation groove 9.

[0067] In this example, the lubricant retaining portion 11 is provided on the entire surface of the circulation groove 9, so that the lubricant can be retained regardless of the position of the circulation groove 9. Therefore, it is possible to prevent the amount of lubricant retained in the circulation groove 9 from varying depending on the position of the circulation groove 9.

[0068] In this example, the lubricant retaining portions 11 are minute recesses formed by shot blasting, and their shapes tend to be complex, so they have excellent lubricant retaining capacity, allowing a sufficient amount of lubricant to be retained in the circulation groove 9.

[0069] By changing the particle size, hardness, amount, and speed of the shot material used in the shot blasting process, the depth, size, and number of the lubricant holding portion 11 can be easily changed. Therefore, the amount of lubricant held in the circulation groove 9 can be easily adjusted. In addition, the processing cost of the lubricant holding portion 11 can be reduced.

[0070] Second Example A second example of the embodiment of the present disclosure will be described with reference to FIGS. 5 and 6. FIG.

[0071] In this example, the lubricant retaining portion 11a is not a minute recess formed by shot blasting as in the first example, but is formed on the surface of the circulation groove 9a by forging using a punch.

[0072] In this example, the lubricant holding portion 11a is a hole-shaped recess.

[0073] The multiple lubricant holding portions 11a formed on the surface of the circulation groove 9a are independent of each other and all have the same shape. In this example, as shown in FIG. 6, the lubricant holding portions 11a are configured in a hemispherical or spherical crown shape. Therefore, the lubricant holding portions 11a have circular (including elliptical) opening shapes. The depth and diameter of the openings of the lubricant holding portions 11a are larger than those of the lubricant holding portions 11 formed by shot blasting in the first example.

[0074] The shape of the lubricant retaining portion 11a formed by forging using a punch is not limited to a hemispherical shape (spherical crown shape), but can also be configured as, for example, a conical shape, a cylindrical shape, a pyramidal shape, a prismatic shape, or the like.

[0075] In this example, similar to the first example, the lubricant retaining portions 11a are provided on the entire surface of the circulation groove 9a and are arranged irregularly.

[0076] The density, shape, depth, diameter of the opening, etc. of the lubricant retaining portion 11a can be varied depending on the position of the circulation groove 9a.

[0077] In this example, the surface roughness of the circulation groove 9a is greater than the surface roughness of the nut-side spiral groove 7 due to the formation of the lubricant retaining portion 11a. The surface roughness of the circulation groove 9a is 1.6 μm or more in arithmetic mean roughness Ra.

[0078] An example of a method for manufacturing the nut 3 constituting the ball screw device 1 of this embodiment will be described.

[0079] First, a metallic blank having a generally cylindrical shape is prepared. The inner peripheral surface of the blank is forged using a punch to form a circulation groove 9a, thereby obtaining a first intermediate blank. The punch used in this example has a tip surface with the same number of protrusions as the lubricant holding portions 11a to be formed in the circulation groove 9a. Each protrusion has an outer surface shape that matches the inner surface shape of the lubricant holding portion 11a. By pressing the tip of the punch having such a configuration against the inner peripheral surface of the blank, the shape of the tip of the punch is transferred to the inner peripheral surface of the blank, forming a circulation groove 9a that is generally S-shaped overall and has multiple recessed lubricant holding portions 11a on its surface.

[0080] After the circulation groove forming step, the nut side spiral groove forming step and the heat treatment step are carried out in sequence, as in the manufacturing method of the nut 3 of the first example, to obtain a third intermediate material.

[0081] In this example, after the heat treatment step, the nut-side spiral groove 7 provided on the inner peripheral surface of the third intermediate material is ground without being subjected to a shot blasting step, to obtain the finished nut 3. Note that the oxide scale formed on the nut-side spiral groove 7 and the circulation groove 9a by the heat treatment can be removed by a processing method other than shot blasting.

[0082] However, a shot blasting process can also be carried out before the grinding process. In this case, two types of lubricant holding portions 11, 11a are formed on the surface of the circulation groove 9a: a lubricant holding portion 11a formed by transferring a protrusion provided on the tip surface of the punch, and a lubricant holding portion 11 formed by impact of a projectile on the surface of the circulation groove. In addition, in the grinding process, irregularities formed on the surface of the nut-side spiral groove 7 in the shot blasting process are removed.

[0083] In this example, the lubricant holding portion 11a is formed by forging using a punch, which prevents variations in the shape, depth, and size of the lubricant holding portion 11a formed on the surface of the circulation groove 9a. This prevents variations in the amount of lubricant (preferably grease) that can be held in the circulation groove 9a from varying between nuts 3. Furthermore, since the lubricant holding portion 11a can be machined simultaneously with the circulation groove 9a, the number of steps required to machine the nut 3 can be reduced.

[0084] The other configurations and effects of the second example are the same as those of the first example.

[0085] Third Example A third example of the embodiment of the present disclosure will be described with reference to FIG.

[0086] This example is a modification of the second example, and differs from the second example only in the arrangement of the lubricant holding portions 11b.

[0087] In this example, a plurality of hole-shaped lubricant holding portions 11b are regularly arranged over the entire surface of the circulation groove 9b. The number of lubricant holding portions 11b formed on the surface of the circulation groove 9b is not particularly limited and can be appropriately set depending on the depth of the lubricant holding portions 11b, the diameter of the openings, and the like. Furthermore, the arrangement of the lubricant holding portions 11b can be, for example, unequally spaced in the width direction, length direction, or both directions of the circulation groove 9b while maintaining a regular arrangement. Specifically, a lubricant holding portion group 11b formed by arranging a plurality of lubricant holding portions 11b at equal intervals in each of the width direction and length direction of the circulation groove 9b can be arranged in the width direction or length direction of the circulation groove 9b at a pitch different from the pitch of the lubricant holding portions constituting the lubricant holding portion group (the pitch in the width direction or length direction of the circulation groove).

[0088] It is preferable that the lubricant holding portions 11b are arranged at equal intervals in the width direction and length direction of the circulation groove 9b. In this example, the lubricant holding portions 11b are arranged at three equally spaced locations in the width direction of the circulation groove 9b and at 12 equally spaced locations in the length direction.

[0089] In this example, the lubricant holding portions 11b are regularly arranged on the surface of the circulation groove 9b, thereby effectively preventing the amount of grease held in the circulation groove 9b from varying depending on the position of the circulation groove 9b.

[0090] Other configurations and effects of the third example are the same as those of the first and second examples.

[0091] Fourth Example A fourth example of the embodiment of the present disclosure will be described with reference to FIG.

[0092] The lubricant retaining portion 11c is formed on the surface of the circulation groove 9c by forging using a punch, but is not a hole-shaped recess but a groove-shaped recessed groove.

[0093] The lubricant holding portion 9c may be configured with a groove having any shape, such as a straight line, a curved line, or a grid pattern. The cross-sectional groove shape of the lubricant holding portion 9c may be a substantially C-shape, a substantially V-shape, a substantially U-shape, or the like. The cross-sectional groove shape of the lubricant holding portion 9c may also vary along the length of the lubricant holding portion 9c.

[0094] The lubricant holding portion 11c in this example is a linear recessed groove. The lubricant holding portion 11c extends linearly in the axial direction of the nut 3. In other words, the lubricant holding portion 11c extends linearly in the width direction of the circulation groove 9c.

[0095] The multiple lubricant holding portions 11c formed on the surface of the circulation groove 9c all have the same shape. The cross-sectional groove shape of the lubricant holding portion 11c is approximately C-shaped. Furthermore, the cross-sectional groove shape of the lubricant holding portion 11c does not change over the entire length of the lubricant holding portion 11c. Therefore, the depth and opening width of the lubricant holding portion 11c are constant over the entire length of the lubricant holding portion 11c.

[0096] The lubricant holding portions 11c are regularly arranged over the entire surface of the circulation groove 9c. The lubricant holding portions 11c are spaced apart in the groove width direction of the lubricant holding portions 11c and are arranged parallel to one another. The lubricant holding portions 11c are preferably arranged at equal intervals in the length direction of the circulation groove 9c. The number of lubricant holding portions 11c is not particularly limited and can be set appropriately depending on the depth, opening width, etc. of the lubricant holding portions 11c. In this example, 13 lubricant holding portions 11c are arranged at equal intervals in the length direction of the circulation groove 9c.

[0097] In this example, the circulation groove forming step is performed using a punch having, on its tip surface, the same number of ridges as the number of lubricant holding portions 11c to be formed in the circulation groove 9c. Each ridge has an outer surface shape that matches the inner surface shape of the lubricant holding portion 11c. By pressing the tip of the punch having such a configuration against the inner circumferential surface of the material, the shape of the tip of the punch is transferred to the inner circumferential surface of the material, forming a circulation groove 9c that is generally S-shaped overall and has multiple lubricant holding portions 11c in the shape of recesses on its surface.

[0098] In this example, the center line O of the circulation groove 9c is approximately aligned with the direction of movement of the ball 4 inside the circulation groove 9c. ９ The angle of intersection between the direction of rotation of the lubricant retaining portion 11c and the lubricant retaining portion 11c is approximately 90 degrees for the lubricant retaining portions 11c formed at both ends of the circulation groove 9c in the longitudinal direction, and is approximately 60 degrees or more for the lubricant retaining portion 11c formed in the middle of the circulation groove 9c in the longitudinal direction. Therefore, each lubricant retaining portion 11c can sufficiently retain grease. Therefore, it is possible to prevent the amount of grease retained in the circulation groove 9c from varying depending on the position of the circulation groove 9c.

[0099] Other configurations and effects of the fourth example are the same as those of the first to third examples.

[0100] Fifth Example A fifth example of the embodiment of the present disclosure will be described with reference to FIG.

[0101] This example is a modification of the fourth example, and differs from the fourth example only in the extending direction of the lubricant retaining portion 11d.

[0102] In this example, the lubricant holding portion 11d extends linearly in the circumferential direction. ７ It extends linearly in a direction substantially parallel to the

[0103] The lubricant holding portions 11d are regularly arranged over the entire surface of the circulation groove 9d. The lubricant holding portions 11d are spaced apart in the groove width direction of the lubricant holding portions 11d and are arranged parallel to one another. The lubricant holding portions 11d are preferably arranged at equal intervals in the axial direction of the nut 3. The number of lubricant holding portions 11d is not particularly limited and can be set appropriately depending on the depth, opening width, etc. of the lubricant holding portions 11d. In this example, six lubricant holding portions 11d are arranged at equal intervals in the axial direction of the nut 3.

[0104] In this example, since the overall length of each lubricant holding portion 11d is long, the amount of lubricant (preferably grease) that can be held by the lubricant holding portion 11d is large, and thus a sufficient amount of lubricant can be held in the circulation groove 9d.

[0105] Other configurations and effects of the fifth example are the same as those of the first to fourth examples.

[0106] Sixth Example A sixth example of the embodiment of the present disclosure will be described with reference to FIG.

[0107] This example is a modified example of the fourth or fifth example, and only the extending direction of the lubricant holding portion 11e differs from the extending direction of the lubricant holding portion 11c of the fourth example and the lubricant holding portion 11d of the fifth example.

[0108] The lubricant retaining portion 11e is aligned with the center line O of the nut-side spiral groove 7. ７ The center line O of the lubricant retaining portion 11c extends linearly and is inclined with respect to the center line O of the lubricant retaining portion 11c. ７ In this example, the inclination angle of the lubricant holding portion 11c with respect to the center line O ７ The angle of inclination with respect to the center line O of the nut-side spiral groove 7 is approximately 45 degrees. ７ extends linearly when the inner circumferential surface of the nut 3 is expanded (see FIG. 3).

[0109] The lubricant holding portions 11e are regularly arranged over the entire surface of the circulation groove 9e. The lubricant holding portions 11e are spaced apart in the groove width direction of the lubricant holding portions 11e and are arranged parallel to one another. The number of lubricant holding portions 11e is not particularly limited and can be set appropriately depending on the depth of the lubricant holding portions, the size of the opening width, etc. In this example, 12 lubricant holding portions 11e are arranged at equal intervals on the surface of the circulation groove 9e.

[0110] In this example, the center line O of the circulation groove 9e is approximately aligned with the direction of movement of the ball 4 inside the circulation groove 9e. ９ The angle of intersection between the direction of the circulating groove 9e and the lubricant holding portion 11e approaches 90 degrees as it approaches the longitudinal center of the circulating groove 9e. This allows a large amount of lubricant (preferably grease) to be held in the longitudinal center of the circulating groove 9e. This reduces the amount of lubricant that flows from the circulating groove 9e to the load path 8.

[0111] Other configurations and effects of the sixth example are the same as those of the first to fifth examples.

[0112] Seventh Example A seventh example of the embodiment of the present disclosure will be described with reference to FIG.

[0113] This example is a modification of the first example, and differs from the first example only in the range in which the lubricant retaining portion 11 is formed.

[0114] In this example, the lubricant holding portion 11 is provided partially on the surface of the circulation groove 9 f. That is, the lubricant holding portion 11 is not provided on the entire surface of the circulation groove 9 f, and there are areas on the surface of the circulation groove 9 f where the lubricant holding portion 11 is provided and areas where the lubricant holding portion 11 is not provided.

[0115] The area provided with the lubricant holding portion 11 and the area not provided with the lubricant holding portion 11 can be disposed at any position in the longitudinal direction of the circulation groove 9f.

[0116] In this example, when the circulation groove 9f is divided into three parts along its length, the lubricant holding portion 11 is provided on both sides of the circulation groove 9f along its length, but is not provided in the middle part along the length of the circulation groove 9f.

[0117] In this example, the lubricant retaining portion 11 is not provided on the entire surface of both sides of the circulation groove 9f in the length direction, but is provided partially on the surface of both sides of the circulation groove 9f in the length direction. Specifically, the lubricant retaining portion 11 is provided only on the surface of the ball scooping portion 12 provided on part of both sides of the circulation groove 9f in the length direction and in its vicinity. The ball scooping portion 12 is a portion that scoops up the balls 4 from the load path 8 into the circulation groove 9f. The ball scooping portion 12 is provided on the surface of the circulation groove 9f within the area that is parallel to the center line O of the circulation groove 9f. ９ The curved portion Oc is located at the boundary between the straight portion Os and the curved portion Oc, and is located farther from the center of curvature of the curved portion Oc.

[0118] Alternatively, the lubricant retaining portion 11 may be provided on the entire surface of both sides of the circulation groove 9f in the longitudinal direction.

[0119] To manufacture the nut 3 of this example, the portion of the surface of the circulation groove 9 f that is not the ball scoop portion 12 and its vicinity is masked and then subjected to a shot blasting process, which allows the lubricant retaining portion 11 to be formed only on the portion of the surface of the circulation groove 9 f where the ball scoop portion 12 is provided and its vicinity.

[0120] In this example, a lubricant holding portion 11 is provided at and near the ball scooping portion 12, which is the portion of the surface of the circulation groove 9f that rubs particularly strongly against the ball 4, thereby effectively preventing metal contact between the ball scooping portion 12 and the ball 4.

[0121] In this example, the surface roughness of the circulation groove 9f is 1.6 μm or more in terms of arithmetic mean roughness Ra.

[0122] Other configurations and effects of Example 7 are the same as those of Example 1. In Examples 2 to 6, similar to Example 7, the formation range of the lubricant retaining portions 11a to 11e can be a part of the surface of the circulation grooves 9a to 9e, preferably both sides in the longitudinal direction of the circulation grooves 9a to 9e.

[0123] The first to seventh examples of the embodiment of the present disclosure can be implemented in any suitable combination as long as no contradiction occurs.

[0124] REFERENCE SIGNS LIST 1 ball screw device 2 screw shaft 3 nut 4 ball 5 shaft-side spiral groove 6 land portion 7 nut-side spiral groove 8 load path 9, 9a to 9f circulation groove 10 annular space 11, 11a to 11e lubricant holding portion 12 ball scoop portion 100 nut 101 circulation groove 102 ball 103 load path

Claims

1. A ball screw device comprising: a screw shaft having a shaft-side spiral groove on its outer peripheral surface; a nut having a nut-side spiral groove and a generally S-shaped circulation groove on its inner peripheral surface that connects the end point and start point of a spiral load path formed by the shaft-side spiral groove and the nut-side spiral groove; and a plurality of balls arranged inside the load path and the circulation groove, wherein the circulation groove has a concave lubricant retaining portion on its surface that retains a lubricant.

2. A ball screw device as set forth in claim 1, wherein the surface roughness of the circulation groove is greater than the surface roughness of the nut-side spiral groove.

3. A ball screw device according to claim 1 or 2, wherein the surface roughness of the circulation groove is 1.6 μm or more in terms of arithmetic mean roughness Ra.

4. A ball screw device according to any one of claims 1 to 3, wherein the surface of the nut-side spiral groove is a ground surface, and the surface of the circulation groove is a non-ground surface.

5. A ball screw device according to any one of claims 1 to 4, wherein the surface of the circulation groove is a blasted surface.

6. A ball screw device according to any one of claims 1 to 5, wherein the lubricant retaining portions are arranged irregularly.

7. A ball screw device according to any one of claims 1 to 5, wherein the lubricant retaining portions are regularly arranged.

8. A ball screw device according to any one of claims 1 to 4, wherein the lubricant retaining portion is a hole-shaped recess or a groove-shaped recess.

9. A ball screw device according to any one of claims 1 to 8, wherein the lubricant retaining portion is provided on the entire surface of the circulation groove.

10. A ball screw device according to any one of claims 1 to 8, wherein the lubricant retaining portion is provided partially on the surface of the circulation groove.

11. A ball screw device according to claim 10, wherein the lubricant retaining portions are provided on both sides of the circulation groove in the longitudinal direction.

12. A ball screw device according to claim 11, wherein the lubricant retaining portion is provided in a scooping portion provided in a portion of both sides of the circulation groove in the longitudinal direction.

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