Method for manufacturing a ball screw device and a nut for a ball screw device

Abstract

The present invention provides a ball screw device capable of holding a sufficient amount of lubricant in a circulation groove. The ball screw device comprises a screw shaft having a shaft-side helical groove on its outer circumference, a nut 3 having a nut-side helical groove 7 and a substantially S-shaped circulation groove 9 on its inner circumference that connects the end and start points of a helical load path 8 formed by the shaft-side helical groove 5 and the nut-side helical groove 7, and a plurality of balls 4 arranged inside the load path 8 and the circulation groove 9. The circulation groove 9 has a concave lubricant-holding portion 11 on its surface for holding lubricant.

Description

Technical Field

[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.

Background Art

[0002] Since a ball screw device causes balls to roll between a screw shaft and a nut, higher efficiency can be obtained compared to 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 for automobiles, automated manual transmissions (AMTs), 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] A ball screw device includes a screw shaft having an axial spiral groove on its outer peripheral surface, a nut having a nut-side spiral groove on its inner peripheral surface, and a plurality of balls arranged to be rollable between the axial spiral groove and the nut-side spiral groove. A lubricant such as grease is filled between the outer peripheral surface of the screw shaft and the inner peripheral surface of the nut to enable smooth rolling of the balls.

[0004] The axial spiral groove and the nut-side spiral groove are arranged to face each other in the radial direction and constitute a spiral load path. The start point and the end point of the load path are connected by a circulation means. The circulation means returns the balls that have reached the end point of the load path to the start point of the load path and causes the balls to circulate infinitely. The start point and the end point of the load path are interchanged according to the direction of relative displacement (relative rotation direction) in the axial direction of the screw shaft and the nut.

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

[0006] In ball screw devices, it has been common practice to use circulating components such as spools, tubes, and end deflectors as means of circulation. Japanese Patent Publication No. 2014-062570 discloses a ball screw device structure in which, in order to miniaturize and reduce the cost of the ball screw device, circulating components are omitted and a substantially S-shaped circulation groove is directly formed on the inner surface of the nut as the means of circulation. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2014-062570 [Overview of the project] [Problems that the invention aims to solve]

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

[0009] As a result, the lubricant in the circulation groove 101 is more easily scraped out into the load passage 103 by the ball 102. Also, the remaining amount of lubricant may vary depending on the location of the circulation groove 101. Consequently, metal-to-metal contact may occur between adjacent balls 102, between balls 102 and the circulation groove 101, or both, potentially causing damage such as indentations on the surface of the balls 102, the surface of the circulation groove 101, or both.

[0010] The present disclosure aims to provide a ball screw device that can retain a sufficient amount of lubricant in the circulation groove. [Means for solving the problem]

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

[0012] The screw shaft has a helical groove on its outer surface.

[0013] The nut has a nut-side helical groove and a substantially S-shaped circulation groove on its inner circumferential surface that connects the end and start points of a helical load path formed by the shaft-side helical groove and the nut-side helical groove.

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

[0015] The circulation groove has a concave lubricant-holding portion on its surface for holding a lubricant. Grease is preferred as the lubricant, but the lubricant may include not only grease but also lubricating oil.

[0016] In a 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 helical groove. The surface roughness of the circulation groove refers to the surface roughness of the entire surface of the circulation groove, including the portion on which the lubricant-holding portion is formed.

[0017] In a ball screw device according to one aspect of the present disclosure, the surface roughness of the circulation groove is 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 helical 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 a 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 holding portions are regularly arranged.

[0022] In the ball screw device according to one aspect of the present disclosure, the lubricant holding portion is a concave portion having a hole shape or a concave groove having a groove shape.

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

[0025] In the ball screw device according to one aspect of the present disclosure, the lubricant holding portion is provided on both side portions with respect to the length direction of the circulation groove.

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

Advantages of the Invention

[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.

Brief Description of the Drawings

[0028] [Figure 1] FIG. 1 is a cross-sectional view of a ball screw device according to a first example of an embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view of a nut constituting the ball screw device of the first example. [Figure 3] FIG. 3 is a developed view of the inner peripheral surface of the nut constituting the ball screw device of the first example. [Figure 4] FIG. 4 is a partially enlarged view of FIG. 2. [Figure 5] FIG. 5 is a view corresponding to FIG. 4 for a ball screw device according to a second example of an embodiment of the present disclosure. [Figure 6]Figure 6 is a cross-sectional view taken along line AA in Figure 5. [Figure 7] Figure 7 is a diagram corresponding to Figure 4, relating to a third example of a ball screw device according to the embodiments of this disclosure. [Figure 8] Figure 8 is a diagram corresponding to Figure 4, relating to a ball screw device of a fourth example of the embodiments of this disclosure. [Figure 9] Figure 9 is a diagram corresponding to Figure 4, relating to a fifth example of a ball screw device according to the embodiments of this disclosure. [Figure 10] Figure 10 is a diagram corresponding to Figure 4, relating to a sixth example of a ball screw device according to the embodiments of this disclosure. [Figure 11] Figure 11 is a diagram corresponding to Figure 4, relating to the seventh example of a ball screw device according to the embodiments of this disclosure. [Figure 12] Figure 12 is a diagram illustrating the problems with conventional ball screw devices. [Modes for carrying out the invention]

[0029] [Example 1] A first example of the embodiment of this disclosure will be described with reference to Figures 1 to 4.

[0030] The ball screw device 1 in this example is widely applicable to applications that convert linear motion to rotational motion or rotational motion to linear motion. In particular, the ball screw device 1 in this example can be incorporated into mechanical devices such as electric brake devices (EMB), electric brake booster devices (EHB), and gear ratio switching devices, and used for applications that convert rotational motion to linear motion.

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

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

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

[0034] The screw shaft 2 has a helical groove 5 on its outer circumferential surface. The groove shape (groove bottom shape) of the cross-section of the helical groove 5 is not limited to this, but can be a Gothic arch groove, a circular arc groove, etc. The helical groove 5 is formed on the outer circumferential surface of the screw shaft 2 by, for example, cutting, rolling, and grinding. The screw shaft 2 has land portions 6 between adjacent helical grooves 5. The helical groove 5 can have one or any number of grooves, but in this example there is one groove.

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

[0036] The shaft-side helical groove 5 and the nut-side helical groove 7 are arranged radially opposite each other, forming a helical load path 8. Multiple load paths 8 can be provided. In this example, four load paths 8 are formed between the shaft-side helical groove 5 and the nut-side helical groove 7. Each load path 8 circles the outer circumference of the shaft-side helical groove 5 approximately once. Even if there are two shaft-side helical grooves 5 and two nut-side helical grooves 7, each load path 8 circles the outer circumference of the shaft-side helical groove 5 approximately once.

[0037] The nut 3 has a roughly S-shaped circulation groove 9 on its inner circumferential surface, connecting the end and start points of the load path 8. The circulation groove 9 has a roughly S-shape when viewed from the radially inward side of the nut 3. One end of the circulation groove 9 in the longitudinal direction is connected to the end point of the load path 8, and the other end in the longitudinal direction is connected to the start point of the load path 8. The longitudinal middle portion of the circulation groove 9 is positioned to cross the land portion 6 of the screw shaft 2.

[0038] The circulation groove 9 has a roughly semicircular cross-sectional shape. 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 helical groove 7, and has a groove depth that allows the ball 4 moving through the circulation groove 9 to overcome the land portion 6.

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

[0040] The nut 3 has multiple circulation grooves 9 on its inner circumferential surface. The number of circulation grooves 9 on the inner circumferential surface of the nut 3 corresponds to the number of load passages 8, but is not particularly limited. The arrangement position of the circulation grooves 9 is also not particularly limited. It is preferable that the multiple 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 helical grooves 7. The four circulation grooves 9 are arranged at 90-degree intervals.

[0041] The balls 4 are steel balls having a predetermined diameter, and are rotatably positioned between the shaft-side helical groove 5 and the nut-side helical groove 7, i.e., in the load path 8, and are also positioned inside the circulation groove 9, circulating between the load path 8 and the inside of the circulation groove 9. The balls 4 positioned in the load path 8 roll under compressive load, while the balls 4 inside the circulation groove 9 move without compressive load, being pushed by the balls 4 behind them in the direction of their movement.

[0042] As the screw shaft 2 or nut 3 rotates, the ball 4 that has reached the end of the load path 8 is returned to the starting point of the load path 8 through the circulation groove 9. This causes the ball 4 to circulate indefinitely within the load path 8. The starting and ending points of the load path 8 are reversed depending on the relative rotation direction of the screw shaft 2 and 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 surface of the screw shaft 2 and the outer surface of the nut 3 to enable the smooth rolling of the balls 4. The ball screw device 1 may also be equipped with a sealing member at the opening of the annular space 10 to prevent leakage of grease from the annular space 10. While it is preferable to use grease as a lubricant, lubricating oil can be used as a substitute.

[0044] The lubricant filling the annular space 10 adheres to the surfaces of the ball 4, the shaft-side helical groove 5, the nut-side helical groove 7, and the circulation groove 9, respectively. The lubricant adheres to the ball 4 and flows.

[0045] To prevent the lubricant in the circulation groove 9 from being depleted, the circulation groove 9 has a concave lubricant retaining portion 11 on its surface to hold the lubricant. The lubricant retaining portion 11 is 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 helical 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 composed of, but is not limited to, minute recesses (indentations), hole-shaped recesses, groove-shaped concaves, etc.

[0048] The lubricant holding portion 11 can be one or any number depending on its shape. When the lubricant holding portion 11 is composed of multiple lubricant holding portions 11, each lubricant holding portion 11 can be arranged irregularly or regularly.

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

[0050] Alternatively, the lubricant retaining portion 11 may be partially provided on the surface of the circulation groove 9. In this case, the lubricant retaining portion 11 may be provided on both sides of the circulation groove 9 in the longitudinal direction. Preferably, the lubricant retaining portion 11 may be provided on scooping portions provided on part of both sides of the circulation groove 9 in the longitudinal direction.

[0051] In this example, the lubricant holding section 11 is composed of multiple (innumerable) lubricant holding sections 11. The lubricant holding sections 11 are provided across the entire surface of the circulation groove 9 and are arranged irregularly (randomly).

[0052] The lubricant holding portion 11 is composed of minute recesses (indentations). Preferably, the lubricant holding portion 11 is formed by shot blasting (dimple processing) on ​​the surface of the circulation groove 9. In this case, the lubricant holding portion 11 is a minute recess having a specific or unspecified (irregular) shape, formed on the surface of the circulation groove 9 by the impact of steel balls, which are the projectile material. The lubricant holding portion 11 includes not only a single independent recess but also a recess formed by a series of recesses. Note that in Figure 4, the opening of the lubricant holding portion 11 is schematically shown as a circle with all of the same diameter.

[0053] The surface roughness of the circulation groove 9 is greater than that of the nut-side helical groove 7 due to the formation of the lubricant retaining portion 11. In this example, the surface roughness of the circulation groove 9 is 1.6 μm or more in terms of arithmetic mean roughness Ra, while the surface roughness of the nut-side helical groove 7 is less than 1.6 μm in terms of arithmetic mean roughness Ra. Note that 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 retaining portion 11 is formed.

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

[0055] The nut-side helical groove 7 is a ground surface that has been ground, while the circulation groove 9 is an unground surface that has not been ground. The surface of the circulation groove 9 is roughened. For roughening, mechanical processing methods such as shot blasting, knurling, transfer processing, electrical discharge machining, electron beam processing, and laser processing, as well as chemical processing methods such as etching, can be used. In this example, the surface of the circulation groove 9 is shot blasted.

[0056] The surface of the circulation groove 9 has properties corresponding to the type of surface roughening. The cross-sectional shape of the circulation groove 9 formed on the surface of the lubricant holding part 11 can be configured, for example, as a roughly triangular wave shape, a roughly sawtooth shape, etc. When the surface roughening is shot blasting, the surface of the circulation groove 9 is an uneven blast surface and has a non-periodic roughness curve.

[0057] [Method of manufacturing nuts] An example of a manufacturing method for the nut 3 that constitutes the ball screw device 1 in this example will be described.

[0058] <Circulation groove formation process> First, a metal material in a roughly cylindrical shape is prepared. Then, the inner circumferential surface of this material is forged or machined to form circulation grooves 9, thereby obtaining a first intermediate material. No lubricant retaining portion 11 is formed on the surface of the circulation grooves 9 formed on the inner circumferential surface of the first intermediate material.

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

[0060] <Heat treatment process> The second intermediate material is subjected to heat treatment processes such as carburizing, high-frequency induction hardening, and tempering to obtain a third intermediate material.

[0061] <Surface roughening (shot blasting) process> A fourth intermediate material is obtained by roughening the surface of the third intermediate material, or more specifically, the inner circumferential surface of the third intermediate material. In this example, a steel ball, which is the projectile material, is impacted onto at least the inner circumferential surface of the third intermediate material by shot blasting (dimple processing). This removes oxide scale from at least the inner circumferential surface of the third intermediate material and forms a concave lubricant retaining portion 11 on at least the inner circumferential surface of the third intermediate material. Therefore, at the stage when the roughening process, in this example, the shot blasting process, is completed, lubricant retaining portions 11 (miniature recesses in this example) are formed not only on the surface of the circulation groove 9 but also on the surface of the nut-side helical groove 7.

[0062] <Grinding process> Subsequently, the nut-side helical groove 7 provided on the inner circumferential 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 holding portion 11) formed on the surface of the nut-side helical groove 7 are removed by grinding.

[0063] Furthermore, when manufacturing nut 3, other processes can be inserted between the processes described above.

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

[0065] In other words, since the ball screw device 1 in this example has a concave lubricant retaining portion 11 on the surface of the circulation groove 9, even when the balls 4 move while rubbing against the surface of the circulation groove 9, the lubricant is retained in the lubricant retaining portion 11, and the amount of lubricant scraped out from the circulation groove 9 to the load path 8 is reduced. Therefore, a sufficient amount of lubricant can be retained in the circulation groove 9, and the depletion of lubricant in the circulation groove 9 can be prevented. As a result, lubricant can be supplied to the balls 4 moving inside the circulation groove 9, so metal-to-metal contact between adjacent balls 4 can be prevented, as well as metal-to-metal contact between the balls 4 and the circulation groove 9. Therefore, damage such as indentations on the balls 4 and the circulation groove 9 can be suppressed.

[0066] Because the surface roughness of the circulation groove 9 is greater than that of the nut-side helical groove 7, the flow velocity of the lubricant flowing inside the circulation groove 9 is slower than that of the lubricant flowing inside the nut-side helical groove 7. As a result, the lubricant tends to accumulate in the circulation groove 9, making it easier for the lubricant to be retained in the circulation groove 9.

[0067] In this example, since the lubricant retaining portion 11 is provided across the entire surface of the circulation groove 9, the lubricant can be retained regardless of the position of the circulation groove 9. Therefore, it is possible to suppress variations in the amount of lubricant retained in the circulation groove 9 depending on its position.

[0068] In this example, the lubricant holding portion 11 is a minute recess formed by shot blasting, and because its shape tends to be complex, it has excellent lubricant holding ability. Therefore, a sufficient amount of lubricant can be held in the circulation groove 9.

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

[0070] [Example 2] A second example of the embodiment of this disclosure will be described with reference to Figures 5 and 6.

[0071] In this example, the lubricant holding portion 11a is formed on the surface of the circulation groove 9a by forging using a punch, rather than being a minute recess formed by shot blasting as in the first example.

[0072] In this example, the lubricant holding portion 11a is a recessed portion with a hole shape.

[0073] The multiple lubricant-holding portions 11a formed on the surface of the circulation groove 9a are all independent and have the same shape. In this example, as shown in Figure 6, the lubricant-holding portions 11a are configured in a hemispherical or spherical shape. Therefore, the lubricant-holding portions 11a have a circular (including elliptical) opening shape. The depth and diameter of the opening of the lubricant-holding portions 11a are greater than the depth and diameter of the opening of the lubricant-holding portion 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 (spherical crown) shape, but can also be configured in the shape of a cone, cylinder, pyramidal pyramidal or prismatic prism, for example.

[0075] In this example, as in the first example, the lubricant retaining portion 11a is provided across the entire surface of the circulation groove 9a and is arranged irregularly.

[0076] Depending on the position of the circulation groove 9a, the density, shape, depth, and diameter of the opening of the lubricant holding portion 11a can also be varied.

[0077] In this example as well, the surface roughness of the circulation groove 9a is greater than that of the nut-side helical 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 terms of arithmetic mean roughness Ra.

[0078] This section describes an example of a manufacturing method for the nut 3 that constitutes the ball screw device 1 in this example.

[0079] First, a metal material with a roughly cylindrical shape is prepared. Then, a forging process is performed on the inner circumferential surface of the material using a punch to form a circulation groove 9a, thereby obtaining a first intermediate material. The punch used in this example has the same number of protrusions on its tip surface as the number of lubricant holding portions 11a 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 with this 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 9a that is roughly S-shaped overall and has a plurality of concave lubricant holding portions 11a on its surface.

[0080] After the circulation groove formation process, the nut-side helical groove formation process and heat treatment process are carried out sequentially, similar to the manufacturing method of nut 3 in the first example, to obtain a third intermediate material.

[0081] In this example, after the heat treatment process, the nut-side helical groove 7 on the inner circumferential surface of the third intermediate material is ground without performing a shot blasting process to obtain the finished nut 3. Note that the oxide scale formed in the nut-side helical groove 7 and circulation groove 9a by the heat treatment can be removed by processing methods other than shot blasting.

[0082] However, a shot blasting process can also be performed before the grinding process. In this case, two types of lubricant retaining parts 11 and 11a are formed on the surface of the circulation groove 9a: a lubricant retaining part 11a formed by the transfer of a projection provided on the tip surface of the punch, and a lubricant retaining part 11 formed by the collision of the projectile material with the surface of the circulation groove. Furthermore, in the grinding process, the irregularities formed on the surface of the nut-side helical groove 7 in the shot blasting process are removed.

[0083] In this example, since the lubricant retaining portion 11a is formed by forging using a punch, variations in the shape, depth, and size of the lubricant retaining portion 11a formed on the surface of the circulation groove 9a can be prevented. Therefore, variations in the amount of lubricant (preferably grease) that can be held in the circulation groove 9a can be suppressed from one nut 3 to the next. In addition, since the lubricant retaining portion 11a can be machined at the same time as the circulation groove 9a, the number of machining steps for the nut 3 can be reduced.

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

[0085] [Example 3] A third example of the embodiment of this disclosure will be described with reference to Figure 7.

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

[0087] In this example, multiple lubricant-holding portions 11b with a hole shape are regularly arranged across 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 according to the depth of the lubricant-holding portions 11b, the diameter of the openings, etc. Furthermore, the arrangement of the lubricant-holding portions 11b can also be varied; for example, the lubricant-holding portions 11b can be arranged at unequal intervals in the width direction, length direction, or both directions of the circulation groove 9b while maintaining a regular arrangement. Specifically, a group of lubricant-holding portions 11b, formed by arranging multiple lubricant-holding portions 11b at equal intervals in 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 group (the pitch in the width direction or length direction of the circulation groove).

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

[0089] In this example, since the lubricant holding portions 11b are regularly arranged on the surface of the circulation groove 9b, it is possible to effectively suppress variations in the amount of grease held in the circulation groove 9b depending on the position of the circulation groove 9b.

[0090] The other components and effects of the third example are the same as those of the first and second examples.

[0091] [Example 4] A fourth example of the embodiments of this disclosure will be described with reference to Figure 8.

[0092] The lubricant retaining portion 11c is formed on the surface of the circulation groove 9c by forging using a punch, but it is a groove-shaped concave groove rather than a hole-shaped recess.

[0093] The lubricant retaining portion 9c can be composed of grooves of any shape, such as straight, curved, or grid-like. Furthermore, the groove shape of the cross-section of the lubricant retaining portion 9c can be approximately C-shaped, approximately V-shaped, approximately U-shaped, etc. The groove shape of the cross-section of the lubricant retaining portion 9c can also be varied in the longitudinal direction of the lubricant retaining portion 9c.

[0094] In this example, the lubricant retaining portion 11c is a linear groove. Furthermore, the lubricant retaining portion 11c extends linearly in the axial direction of the nut 3. In other words, the lubricant retaining 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 groove shape in the cross-section of the lubricant-holding portion 11c is approximately C-shaped. Furthermore, the groove shape in the cross-section of the lubricant-holding portion 11c does not change along the entire length of the lubricant-holding portion 11c. Therefore, the depth and opening width of the lubricant-holding portion 11 are constant along the entire length of the lubricant-holding portion 11c.

[0096] The lubricant retaining portions 11c are arranged regularly across the entire surface of the circulation groove 9c. Multiple lubricant retaining portions 11c are spaced apart in the groove width direction and arranged parallel to each other. Preferably, the lubricant retaining portions 11c are arranged at equal intervals in the length direction of the circulation groove 9c. The number of lubricant retaining portions 11c is not particularly limited and can be set as appropriate depending on the depth of the lubricant retaining portions 11c, the size of the opening width, etc. In this example, 13 lubricant retaining portions 11c are arranged at equal intervals in the length direction of the circulation groove 9c.

[0097] In this example, the circulation groove formation process is performed using a punch having the same number of protrusions on its tip surface as the number of lubricant-holding portions 11c formed in the circulation groove 9c. Each protrusion has an outer surface shape that matches the inner surface shape of the lubricant-holding portion 11c. By pressing the tip of the punch with this configuration against the inner surface of the material, the shape of the punch tip is transferred to the inner surface of the material, forming a circulation groove 9c that is generally S-shaped and has a plurality of recessed lubricant-holding portions 11c on its surface.

[0098] In this example, the angle of intersection between the direction of the center line O9 of the circulation groove 9c, which is roughly coincided with the direction of movement of the ball 4 inside the circulation groove 9c, and the lubricant retaining portion 11c is approximately 90 degrees at the lubricant retaining portions 11c formed at both ends in the longitudinal direction of the circulation groove 9c, and approximately 60 degrees or more at the lubricant retaining portion 11c formed in the middle of the longitudinal direction of the circulation groove 9c. Therefore, each lubricant retaining portion 11c can sufficiently hold grease. Consequently, it is possible to suppress variations in the amount of grease held in the circulation groove 9c depending on its position.

[0099] The other components and effects of the fourth example are the same as those of the first to third examples.

[0100] [Example 5] A fifth example of the embodiment of this disclosure will be described with reference to Figure 9.

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

[0102] In this example, the lubricant retaining portion 11d extends linearly in the circumferential direction. Specifically, the lubricant retaining portion 11d extends linearly in a direction approximately parallel to the center line O7 of the nut-side helical groove 7.

[0103] The lubricant retaining portions 11d are arranged regularly across the entire surface of the circulation groove 9d. Multiple lubricant retaining portions 11d are spaced apart in the groove width direction and arranged parallel to each other. Preferably, the lubricant retaining portions 11d are arranged at equal intervals in the axial direction of the nut 3. The number of lubricant retaining portions 11d is not particularly limited and can be set as appropriate depending on the depth of the lubricant retaining portions 11d, the size of the opening width, etc. In this example, six lubricant retaining 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 each lubricant-holding portion 11d can hold increases. As a result, a sufficient amount of lubricant can be held in the circulation groove 9d.

[0105] The other components and effects of Example 5 are the same as those of Examples 1 through 4.

[0106] [Example 6] A sixth example of the embodiments of this disclosure will be described with reference to Figure 10.

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

[0108] The lubricant retaining portion 11e extends linearly, inclined with respect to the center line O7 of the nut-side helical groove 7. The inclination angle of the lubricant retaining portion 11c with respect to the center line O7 is arbitrary. In this example, the inclination angle of the lubricant retaining portion 11c with respect to the center line O7 is approximately 45 degrees. Note that the center line O7 of the nut-side helical groove 7 extends linearly when the inner circumferential surface of the nut 3 is unfolded (see Figure 3).

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

[0110] In this example, the angle at which the center line O9 of the circulation groove 9e, which is approximately in line with the direction of movement of the ball 4 inside the circulation groove 9e, intersects with the lubricant holding portion 11e approaches 90 degrees as it approaches the longitudinal middle portion of the circulation groove 9e. Therefore, a large amount of lubricant (preferably grease) can be held in the longitudinal middle portion of the circulation groove 9e. Consequently, the amount of lubricant flowing from the circulation groove 9e to the load passage 8 can be reduced.

[0111] The other components and effects of Example 6 are the same as those of Examples 1 through 5.

[0112] [Case 7] A seventh example of the embodiments of this disclosure will be described with reference to Figure 11.

[0113] This example is a modified version of the first example, and only the formation range of the lubricant holding portion 11 differs from that of the first example.

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

[0115] The area with the lubricant holding portion 11 and the area without the lubricant holding portion 11 can be positioned at any position along the length of the circulation groove 9f.

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

[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 longitudinal direction, but rather partially on the surface of both sides of the circulation groove 9f in the longitudinal direction. Specifically, the lubricant retaining portion 11 is provided only on the surface of the ball scooping portion 12, which is provided on a part of both sides of the circulation groove 9f in the longitudinal direction, and in its vicinity. The ball scooping portion 12 is the part that scoops up the balls 4 from the load path 8 into the circulation groove 9f. The ball scooping portion 12 is located on the surface of the circulation groove 9f at the boundary between the straight portion Os and the curved portion Oc of the center line O9 of the circulation groove 9f, and on the side farther from the center of curvature of the curved portion Oc.

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

[0119] To manufacture the nut 3 in this example, the portion of the surface of the circulation groove 9f that is away from the ball scoop portion 12 and its vicinity is masked, and a shot blasting process is carried out. As a result, the lubricant retaining portion 11 can be formed only on the surface of the circulation groove 9f in the portion where the ball scoop portion 12 is provided and its vicinity.

[0120] In this example, the lubricant holding portion 11 is provided on the ball scooping portion 12 and its vicinity, which are the parts of the surface of the circulation groove 9f that rub particularly strongly against the ball 4. This effectively prevents metal-to-metal contact between the ball scooping portion 12 and the ball 4.

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

[0122] The other configurations and effects of Example 7 are the same as those of Example 1. Furthermore, for Examples 2 to 6, similar to Example 7, the formation range of the lubricant holding portions 11a to 11e can be limited to a portion of the surface of the circulation grooves 9a to 9e, preferably both sides of the circulation grooves 9a to 9e in the longitudinal direction.

[0123] The first to seventh embodiments of this disclosure can be combined as appropriate, as long as they do not create a contradiction. [Explanation of Symbols]

[0124] 1. Ball screw device 2 Screw shaft 3 nuts 4 balls 5 Shaft side spiral groove 6 Land Section 7. Spiral groove on the nut side 8 Load path 9, 9a~9f circulation groove 10 Ring space 11, 11a~11e Lubricant holding section 12 Ball scooping section 100 nuts 101 Circulation groove 102 Balls 103 Load path

Claims

1. A screw shaft having a helical groove on the outer surface, A nut having a nut-side helical groove on its inner circumferential surface, and a substantially S-shaped circulation groove connecting the end and start points of a helical load path formed by the shaft-side helical groove and the nut-side helical groove, A plurality of balls arranged inside the load passage and the circulation groove, Equipped with, The circulation groove has a concave lubricant-holding portion on its surface for holding lubricant, The surface roughness of the circulation groove is greater than the surface roughness of the nut-side helical groove. Ball screw device.

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

3. The ball screw device according to claim 1, wherein the surface of the nut-side helical groove is a ground surface, and the surface of the circulation groove is a non-ground surface.

4. The ball screw device according to claim 1, wherein the surface of the circulation groove is a blasted surface.

5. The ball screw device according to claim 1, wherein the lubricant holding parts are arranged irregularly.

6. The ball screw device according to claim 1, wherein the lubricant holding parts are arranged in a regular pattern.

7. The circulation groove has a plurality of lubricant holding parts, Each of the multiple lubricant-holding portions is a concave groove having a linear groove shape. The ball screw device according to claim 6.

8. The ball screw device according to claim 1, wherein the lubricant holding portion is a recess in the shape of a hole.

9. The ball screw device according to claim 1, wherein the lubricant holding portion is a groove-shaped concave groove.

10. The ball screw device according to claim 1, wherein the lubricant holding portion is provided on the entire surface of the circulation groove.

11. The ball screw device according to claim 1, wherein the lubricant holding portion is partially provided on the surface of the circulation groove.

12. The ball screw device according to claim 11, wherein the lubricant holding portion is provided on both sides with respect to the length direction of the circulation groove.

13. The ball screw device according to claim 12, wherein the lubricant holding portion is provided in scooping portions provided on a portion of both sides in the longitudinal direction of the circulation groove.

14. A method for manufacturing a ball screw device nut, comprising a substantially S-shaped circulation groove having a nut-side helical groove and a concave lubricant-holding portion on its inner circumferential surface for holding lubricant, The process includes a step of forming the circulation groove by forging the inner circumferential surface of a material having a substantially cylindrical shape using a punch, the punch having a projection on its tip surface that has an outer surface shape matching the inner surface shape of the lubricant holding part. A method for manufacturing nuts for ball screw devices.

Images