Nut for ball screw device, method for manufacturing the nut for ball screw device, and ball screw device

Abstract

A nut for a ball screw device is provided that can improve the slidability of the outer peripheral surface and ensure the fatigue strength of the ball screw groove on the nut side. [Solution] The nut 3 has an outer peripheral surface 10 having a sliding surface 21 and an inner peripheral surface 11 having a portion provided with a nut-side ball screw groove 12, and at least the sliding surface 21 of the outer peripheral surface 10 is composed of a compound layer 17, and at least the portion of the inner peripheral surface 11 provided with the nut-side ball screw groove 12 is composed of a diffusion layer 16.

Description

[Technical Field]

[0001] The present disclosure relates to a nut for a ball screw device, a method for manufacturing the nut for a ball screw device, and a ball screw device. [Background technology]

[0002] Ball screw devices use balls to roll between a screw shaft and a nut, which allows for higher efficiency than sliding screw devices, which have direct contact between the screw shaft and the nut. For this reason, ball screw devices are incorporated into various types of machinery, such as electric brake systems and automatic manual transmissions (AMTs) for automobiles, and positioning devices for machine tools, to convert the rotational motion of a drive source such as an electric motor into linear motion.

[0003] The ball screw device has a screw shaft having a shaft-side ball screw groove on its outer peripheral surface, a nut having a nut-side ball screw groove on its inner peripheral surface, and a plurality of balls arranged between the shaft-side ball screw groove and the nut-side ball screw groove.

[0004] Depending on the application, a ball screw device uses one of the screw shaft and the nut as a rotational motion element and the other as a linear motion element. For example, JP-A 2004-502097 discloses that a ball screw device is incorporated into a brake device, and the screw shaft is used as a rotational motion element and the nut is used as a linear motion element. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2004-502097 Summary of the Invention [Problem to be solved by the invention]

[0006] JP-A-2004-502097 discloses a structure in which a nut constituting a ball screw device is used as a piston constituting a brake device, and the nut slides inside a cylinder constituting the brake device.

[0007] When the outer peripheral surface of the nut is used as a sliding surface, as in the ball screw device described in JP 2004-502097 A, the outer peripheral surface of the nut is required to have high slidability. Furthermore, when the ball screw device described in JP 2004-502097 A is used with a portion of the outer peripheral surface of the nut exposed to the cylinder, the outer peripheral surface of the nut is prone to corrosion due to exposure to the outside air. If corrosion occurs, this can cause a decrease in the slidability of the outer peripheral surface of the nut. Therefore, when a nut is used in an application where the outer peripheral surface is exposed to the outside air, corrosion prevention is also required.

[0008] On the other hand, the nut-side ball screw groove provided on the inner peripheral surface of the nut is required to have sufficient fatigue strength because it is subjected to repeated loads from the balls.

[0009] An object of the present disclosure is to provide a nut for a ball screw device that can improve the slidability of the outer peripheral surface and ensure the fatigue strength of the nut-side ball screw groove. [Means for solving the problem]

[0010] A nut for a ball screw device according to one aspect of the present disclosure comprises an outer peripheral surface having a sliding surface and an inner peripheral surface provided with a nut-side ball screw groove, at least the sliding surface of the outer peripheral surface being constituted by a compound layer, and at least the portion of the inner peripheral surface provided with the nut-side ball screw groove being constituted by a diffusion layer.

[0011] In the nut for a ball screw device according to one aspect of the present disclosure, a diffusion layer may further be present radially inward of the compound layer.

[0012] A nut for a ball screw device according to one aspect of the present disclosure can have a cylindrical shape with a bottom.

[0013] A nut for a ball screw device according to one aspect of the present disclosure can have a bottomless cylindrical shape.

[0014] The nut for a ball screw device according to one aspect of the present disclosure may have a circulation groove on the inner circumferential surface.

[0015] In the nut for a ball screw device according to one aspect of the present disclosure, the entire outer circumferential surface can be formed by the compound layer.

[0016] A ball screw device according to one aspect of the present disclosure comprises a screw shaft having a shaft-side ball screw groove on its outer peripheral surface and undergoing rotational motion when in use, a nut having a nut-side ball screw groove on its inner peripheral surface and undergoing linear motion when in use, and a plurality of balls arranged between the shaft-side ball screw groove and the nut-side ball screw groove, wherein the nut is constituted by a nut for a ball screw device according to any one of the aspects of the present disclosure.

[0017] A method for manufacturing a nut for a ball screw device according to one aspect of the present disclosure is a method for manufacturing a nut for a ball screw device according to any one aspect of the present disclosure, a step of forming a diffusion layer on at least a portion of the inner peripheral surface of a first intermediate material having the nut-side ball screw groove formed on its inner peripheral surface and on at least a portion of the outer peripheral surface of the first intermediate material, thereby obtaining a second intermediate material; and forming a compound layer on the radially outer side of the diffusion layer provided on the outer peripheral surface of the second intermediate material, while preventing at least the inner peripheral surface of the second intermediate material from coming into contact with the furnace atmosphere.

[0018] In a method for manufacturing a nut for a ball screw device according to one aspect of the present disclosure, The diffusion layer is formed by subjecting the first intermediate material to a gas nitriding treatment; and The compound layer is formed by subjecting the second intermediate material to gas soft nitriding treatment. [Effects of the Invention]

[0019] According to the nut for a ball screw device of the present disclosure, the slidability of the outer peripheral surface can be improved, and the fatigue strength of the nut-side ball screw groove can be ensured. [Brief explanation of the drawings]

[0020] [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 showing a first example of a nut. [Figure 3] FIG. 3 is an enlarged schematic view of part A in FIG. [Figure 4] 4(A) to 4(D) are schematic cross-sectional views showing the manufacturing process of the nut of the first example in the order of steps. [Figure 5] FIG. 5 is a diagram corresponding to FIG. 1 and illustrating a second example of an embodiment of the present disclosure. [Figure 6] FIG. 6 is a diagram corresponding to FIG. 2 and relating to the second example. [Figure 7] 7(A) to 7(D) are diagrams of the second example, corresponding to FIGS. 4(A) to 4(D). DETAILED DESCRIPTION OF THE INVENTION

[0021] [Example 1] A first example of an embodiment of the present disclosure will be described with reference to FIGS. 1 to 4. FIG.

[0022] [Overall configuration of ball screw device] The ball screw device 1 of this embodiment is incorporated into, for example, an electric brake booster device (EHB) or an electric brake device (EMB), and is used to convert the rotational motion of an electric motor, which is a drive source, into linear motion.

[0023] The ball screw device 1 comprises a screw shaft 2 having a shaft-side ball screw groove 8 on its outer peripheral surface and rotating when in use, a nut 3 having a nut-side ball screw groove 12 on its inner peripheral surface 11 and moving linearly when in use, and a plurality of balls (steel balls) 4 arranged between the shaft-side ball screw groove 8 and the nut-side ball screw groove 12.

[0024] The screw shaft 2 is a rotational motion element that is rotationally driven by an electric motor, which is a drive source, via a reduction mechanism or the like, and that rotates when in use. The screw shaft 2 is inserted into the inside of the nut 3 and is arranged coaxially with the nut 3. The nut 3 is prevented from co-rotating with the screw shaft 2 by a rotation prevention mechanism (not shown), and is a linear motion element that moves linearly when in use. Therefore, the ball screw device 1 of this example is used in a mode in which the screw shaft 2 is rotationally driven and the nut 3 is moved linearly.

[0025] A spiral load path 5 is provided between the outer peripheral surface of the screw shaft 2 and the inner peripheral surface of the nut 3, and is composed of a shaft-side ball screw groove 8 and a nut-side ball screw groove 12. A plurality of balls 4 are arranged in a rollable manner in the load path 5. When the screw shaft 2 and the nut 3 are rotated relative to each other, the balls 4 that reach the end point of the load path 5 are returned to the start point of the load path 5 through a circulation groove (not shown) formed on the outer peripheral surface of the screw shaft 2. The structure of each component of the ball screw device 1 will be described below.

[0026] 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 and the nut 3. Furthermore, one axial side refers to the right side in Figures 1 to 4, and the other axial side refers to the left side in Figures 1 to 4.

[0027] <Screw shaft> The screw shaft 2 of this example is made of metal and has a threaded portion 6 and a fitting shaft portion 7 arranged adjacent to one axial side of the threaded portion 6. The threaded portion 6 and the fitting shaft portion 7 are arranged coaxially and are constructed integrally with each other. The fitting shaft portion 7 has an outer diameter smaller than that of the threaded portion 6.

[0028] The threaded portion 6 has a spiral shaft-side ball screw groove 8 and a circulation groove (not shown) on its outer peripheral surface. The shaft-side ball screw groove 8 is formed by grinding, cutting, or rolling on the outer peripheral surface of the threaded portion 6. In this example, the shaft-side ball screw groove 8 has one thread. The cross-sectional groove shape (groove bottom shape) of the shaft-side ball screw groove 8 is a gothic arch groove or a circular arc groove. The outer peripheral surface of the threaded portion 6 has at least the shaft-side ball screw groove 8, and does not need to have a circulation groove.

[0029] The circulation groove has a roughly S-shape, smoothly connecting adjacent portions of the shaft-side ball screw groove 8 in the axial direction, and connecting the start point and end point of the load path 5. Therefore, the balls 4 that have reached the end point of the load path 5 are returned to the start point of the load path 5 through the circulation groove. The start point and end point of the load path 5 are interchanged depending on the direction of relative displacement (relative rotation direction) between the screw shaft 2 and the nut 3 in the axial direction.

[0030] The circulation groove has a substantially semicircular cross section. The circulation groove has a groove width slightly larger than the diameter of the balls 4 and a groove depth that allows the balls 4 moving in the circulation groove to climb over the threads of the nut-side ball screw groove 12, which will be described later.

[0031] The circulation groove can be formed directly on the outer peripheral surface of the threaded portion 6. Alternatively, it can be formed on the outer peripheral surface of the threaded portion 6 by providing an attachment hole on the outer peripheral surface of the threaded portion 6 and fixing a circulation part with a circulation groove to the inside of the attachment hole. In this example, the circulation groove is formed on the outer peripheral surface of the threaded portion 6 by, for example, cutting.

[0032] To the mating shaft portion 7, a spur gear or a carrier that constitutes a planetary gear that constitutes a reduction mechanism for driving the screw shaft 2 to rotate is fixed so that it cannot rotate relative to the mating shaft portion 7, or the rotating shaft of a motor is directly connected.

[0033] <nut> The nut 3 is made of alloy steel, and is slidably fitted into a cylinder 9 that constitutes an electric brake booster device or an electric brake device, functioning as a piston. In this example, when the ball screw device 1 is in use, the other axial end of the nut 3 is exposed to the outside from the cylinder 9.

[0034] The nut 3 has an outer peripheral surface 10 and an inner peripheral surface 11 .

[0035] The outer peripheral surface 10 of the nut 3 is configured in the shape of a cylindrical surface. The outer peripheral surface 10 of the nut 3 has a sliding surface 21 that slides in the axial direction against the inner peripheral surface of the cylinder 9. A seal ring (not shown) is provided between the portion of the outer peripheral surface 10 of the nut 3 that is apart from the sliding surface 21 and the inner peripheral surface of the cylinder 9. In addition, a boot (not shown) can be provided so as to span between the portion of the outer peripheral surface 10 of the nut 3 that is exposed to the outside from the cylinder 9 and the opening edge of the cylinder 9.

[0036] The inner peripheral surface 11 of the nut 3 faces the outer peripheral surface of the threaded portion 6 constituting the screw shaft 2 over the entire circumference, with a radial gap therebetween. In this example, the inner peripheral surface 11 of the nut 3 has a spiral nut-side ball screw groove 12 and threads provided between the nut-side ball screw grooves 12. It is also possible to provide a circulation groove on the inner peripheral surface 11 of the nut 3 without providing a circulation groove on the outer peripheral surface of the screw shaft 2. In this case, the circulation groove can be formed directly on the inner peripheral surface 11 of the nut 3, or a circulation part with a circulation groove can be fixed to a mounting hole formed in the nut 3 to provide a circulation groove on the inner peripheral surface 11 of the nut 3.

[0037] The nut-side ball screw groove 12 is formed on the inner peripheral surface 11 of the nut 3 by, for example, grinding, cutting, rolling tapping, or cutting tapping. The nut-side ball screw groove 12 has the same lead as the shaft-side ball screw groove 8.

[0038] With the threaded portion 6 of the screw shaft 2 inserted inside the nut 3, the shaft-side ball screw groove 8 and the nut-side ball screw groove 12 are arranged to face each other in the radial direction, thereby forming a load path 5. In this example, the number of threads in the nut-side ball screw groove 12 is one, just like the shaft-side ball screw groove 8. The cross-sectional groove shape of the nut-side ball screw groove 12 is also a gothic arch groove or a circular arc groove, just like the shaft-side ball screw groove 8.

[0039] The shape of the nut 3 is arbitrary as long as it has at least an outer peripheral surface 10 and an inner peripheral surface 11, and it has at least a bottomed cylindrical shape or a bottomless cylindrical shape.

[0040] The nut 3 in this example is a cap nut having a cylindrical shape with a bottom, and is composed of a cylindrical tubular portion 13 and a disk-shaped bottom portion 14 that closes the end opening on the other axial side of the tubular portion 13.

[0041] The nut 3 has a compound layer 17 on at least the sliding surface 21 of its outer circumferential surface 10, and a diffusion layer 16 on at least the portion of its inner circumferential surface 11 where the nut-side ball screw groove 12 is provided.

[0042] More specifically, as shown schematically in Figures 3 and 4(D), the nut 3 is composed of a base material 15, a diffusion layer 16, and a compound layer 17. In Figures 3 and 4, the base material 15 is depicted as a solid pattern, the diffusion layer 16 is depicted as a matte pattern, and the compound layer 17 is depicted as a diagonal checkered pattern. Also, in Figures 3 and 4, the thicknesses of the diffusion layer 16 and the compound layer 17 are exaggerated.

[0043] The compound layer 17 needs to be formed at least on the sliding surface 21 of the outer peripheral surface 10 of the nut 3 where slidability is required. When the entire outer peripheral surface 10 of the nut 3 constitutes the sliding surface 21, the compound layer 17 is formed on the entire outer peripheral surface 10 of the nut 3. When the outer peripheral surface of the nut 3 has a portion that does not function as the sliding surface 21, it is not necessary to form the compound layer 17 on that portion, but even in this case, the compound layer 17 can be formed on the entire outer peripheral surface 10. Note that a diffusion layer 16 is present radially inward (below) of the compound layer 17.

[0044] The diffusion layer 16 must be formed at least in the portion of the inner circumferential surface 11 of the nut 3 that is provided with the nut-side ball screw groove 12 and that requires sufficient fatigue strength. The diffusion layer 16 does not need to be formed in the portion of the inner circumferential surface 11 of the nut 3 that is not provided with the nut-side ball screw groove 12, but the diffusion layer 16 can also be formed on the entire inner circumferential surface 11 of the nut 3, including that portion.

[0045] In this example, the entire outer peripheral surface 10 of the nut 3 is made up of the compound layer 17. That is, the outer peripheral surface 10 of the nut 3 is made up of a cylindrical surface whose outer diameter does not change over the entire axial length, and the entire outer peripheral surface 10 functions as the sliding surface 21, so the entire outer peripheral surface 10 is made up of the compound layer 17.

[0046] In the nut 3 of this example, the side surface on the other axial side of the bottom portion 14 that constitutes the outer bottom surface is also made up of the compound layer 17. However, when implementing the present disclosure, the side surface on the other axial side of the bottom portion 14 that constitutes the outer bottom surface of the nut 3 does not have to be made up of the compound layer 17. In other words, this portion may be made up of the diffusion layer 16 or the surface of the base material 15.

[0047] In this example, the entire inner peripheral surface 11 of the nut 3, including the portion where the nut-side ball screw groove 12 is formed, is formed by the diffusion layer 16.

[0048] In the nut 3 of this example, the side surface on one axial side of the bottom portion 14 that constitutes the inner bottom surface and the end surface on one axial side of the tubular portion 13 are also formed with the diffusion layer 16. However, when implementing the present disclosure, the side surface on one axial side of the bottom portion 14 that constitutes the inner bottom surface of the nut 3 and the end surface on one axial side of the tubular portion 13 do not have to be formed with the diffusion layer 16. In other words, these parts can be formed with the surface of the base material 15.

[0049] The base material 15 corresponds to the so-called core. The base material 15 refers to a region that is not affected by nitrogen that has penetrated by the gas nitriding treatment described later. The portion of the nut 3 excluding the diffusion layer 16 and the compound layer 17 is the base material 15.

[0050] The chemical composition of the base material 15 is not particularly limited. For example, the base material 15 can be SCM420H, which contains, by mass%, 0.17% to 0.23% C, 0.15% to 0.35% Si, 0.55% to 0.95% Mn, 0.03% to 0.03% S, 0.25% to 0.25% Ni, 0.85% to 1.25% Cr, 0.15% to 0.3% Mo, and 0.3% to 0.3% Cu, with the balance being Fe and impurities. In addition to SCM420H, other base materials such as SCr420H can also be used as the base material 15.

[0051] The hardness of the base material 15 is not particularly limited, but it is preferable that the hardness be such that the bending fatigue strength required for the nut 3 to function as a piston is sufficiently ensured.

[0052] The diffusion layer 16 is formed on the surface of the base material 15. The diffusion layer 16 refers to a layer in the surface layer portion of the steel material where nitrogen diffuses (solid solution) and nitride-forming elements such as Cr and Al in the components of the base material 15 form fine alloy nitrides.

[0053] The nitrogen concentration of the diffusion layer 16 is not particularly limited as long as it is higher than the nitrogen concentration of the base material 15 .

[0054] There are no particular limitations on the thickness of the diffusion layer 16. The thickness of the diffusion layer 16 varies depending on the conditions of the gas nitriding treatment, and is, for example, about 0.1 mm or more and 0.3 mm or less.

[0055] The hardness of the diffusion layer 16 is not particularly limited, but for example, the Rockwell hardness of the surface of the diffusion layer 16 is preferably HRC54 or higher, and more preferably HRC58 or higher. The Rockwell hardness of the diffusion layer 16 is higher than that of the base material 15, but lower than that of the compound layer 17.

[0056] The diffusion layer 16 has high hardness and is superior in toughness and impact resistance compared to the compound layer 17. Therefore, the diffusion layer 16 is suitable for forming a portion provided with the nut-side ball screw groove 12 that receives repeated loads from the balls 4.

[0057] The compound layer 17 is formed radially outward (on top) of the diffusion layer 16. The compound layer 17 refers to a layer (white layer) containing compounds of iron and nitrogen (iron-nitrogen compounds). Examples of compounds of iron and nitrogen include ε (epsilon: Fe2-3N) compounds, γ' (gamma prime: Fe4N) compounds, and composite compounds thereof.

[0058] There is no particular limitation on the thickness of the compound layer 17. The thickness of the compound layer 17 is sufficiently smaller than the thickness of the diffusion layer 16. The thickness of the compound layer 17 varies depending on the conditions of the gas soft nitriding treatment, and is, for example, about 5 μm or more and 20 μm or less.

[0059] The hardness of the compound layer 17 is not particularly limited, but for example, the Rockwell hardness of the surface of the compound layer 17 is preferably HRC 58 or more. The compound layer 17 has a higher Rockwell hardness than the diffusion layer 16.

[0060] The compound layer 17 has lower toughness and is weaker against impact loads than the diffusion layer 16, but has higher hardness and superior sliding properties. Therefore, the compound layer 17 is suitable for forming the outer peripheral surface 10 including the sliding surface 21 of the nut 3, which slides against the inner peripheral surface of the cylinder 9.

[0061] In this example, the outer peripheral surface 10 of the nut 3 is made up of a compound layer 17, but in order to further enhance the corrosion resistance of the outer peripheral surface 10, an oxide film (oxide layer) of triiron tetroxide (Fe3O4) of about several microns in thickness can also be formed on the surface of the compound layer 17.

[0062] <Nut manufacturing method> In this example, a method for manufacturing a nut for a ball screw device according to an example of the present disclosure is applied. Hereinafter, a method for manufacturing the nut 3 of this example will be described with reference to Figures 4(A) to 4(D).

[0063] The manufacturing method of the nut 3 in this example includes a shape processing step for manufacturing a first intermediate material 18 having the same dimensions and shape as the finished nut 3, a gas nitriding treatment step, and a gas soft nitriding treatment step. Of these steps, the shape processing step is a pre-process corresponding to a preparation stage, and does not constitute the manufacturing method of the present disclosure.

[0064] 《Shape processing process》 In the shaping process of this example, a steel material (not shown) having the same chemical composition as the base material 15 of the nut 3 is subjected to cutting and / or rolling to obtain a first intermediate material 18 shown in FIG. 4(A). The first intermediate material 18 has the same dimensions and shape as the finished nut 3. The first intermediate material 18 is made entirely of a metal having the same chemical composition as the base material 15, and does not have a diffusion layer 16 or a compound layer 17. Note that heat treatment and grinding can also be additionally performed in the shaping process.

[0065] <Gas nitriding process> In the gas nitriding process, the first intermediate material 18 is subjected to gas nitriding while controlling the nitriding potential. As a result, a second intermediate material 19 is obtained, the inner and outer circumferential surfaces of which are constituted by the diffusion layer 16, as shown in FIG. 4(B). An example of specific conditions for the gas nitriding process is as follows.

[0066] The treatment temperature in the gas nitriding treatment is preferably in the range of 500° C. to 600° C. If the treatment temperature is less than 500° C., it becomes difficult to ensure a sufficient thickness of the diffusion layer 16. On the other hand, if the treatment temperature exceeds 600° C., hard and brittle austenite is formed, which may prevent the diffusion layer 16 from being stably formed.

[0067] The holding time at the above treatment temperature is preferably 15 hours or more. If the treatment time is less than 15 hours, it is not possible to form a diffusion layer 16 having a sufficient nitrogen concentration and thickness, and the diffusion layer 16 may not have enough hardness.

[0068] Ammonia gas (NH3) is used as the atmosphere in the furnace during nitriding. During nitriding, the nitriding potential (K N ) is used as a parameter to control the atmosphere. Specifically, the hydrogen concentration in the furnace is continuously analyzed by an H2 sensor, and the set K N The NH3 flow rate is controlled by a mass flow controller (MFC) so that the value is

[0069] The gas nitriding treatment described above produces a second intermediate material 19 in which the compound layer 17 of the ε phase and the single γ' phase is not formed on the steel surface. The surface of the second intermediate material 19 is composed of a structure consisting only of a diffusion layer 16. In this example, the diffusion layer 16 is formed over the entire outer and inner peripheral surfaces of the second intermediate material 19. Furthermore, the diffusion layer 16 is also formed on the side surface on the other axial side of the bottom portion 14 constituting the outer bottom surface, the side surface on one axial side of the bottom portion 14 constituting the inner bottom surface, and the end surface on one axial side of the tubular portion 13 of the second intermediate material 19.

[0070] <Gas soft nitriding process> In the gas soft-nitriding process, the second intermediate material 19 is subjected to gas soft-nitriding. As a result, in this example, as shown in FIG. 4(D), a compound layer 17 is formed radially outward of the diffusion layer 16 that constitutes the outer peripheral surface 10 of the second intermediate material 19. However, if the diffusion layer 16 is not formed on the outer peripheral surface 10 of the second intermediate material 19, the compound layer 17 can also be formed on the portion of the outer peripheral surface 10 that is constituted by the surface of the base material 15. Note that even in this case, the gas soft-nitriding process forms the diffusion layer 16 radially inward of the compound layer 17. In this way, the nut 3 is manufactured, which has the compound layer 17 on the outer peripheral surface 10 and has the diffusion layer 16 on the inner peripheral surface 11, including the portion provided with the nut-side ball screw groove 12.

[0071] In this example, when the gas soft-nitriding treatment is performed on the second intermediate material 19, it is necessary to prevent at least the inner peripheral surface of the second intermediate material 19 from coming into contact with the furnace atmosphere. That is, the gas soft-nitriding treatment is performed on the second intermediate material 19 in a state where the portion of the diffusion layer 16 constituting the surface of the second intermediate material 19 where the compound layer 17 is not to be formed is prevented from coming into contact with the furnace atmosphere.

[0072] In this example, since the second intermediate material 19 is composed of a cap nut having a cylindrical shape with a bottom, gas soft nitriding treatment is performed so that the inner surface of the second intermediate material 19 and the side surface on one axial side of the bottom portion 14 that forms the inner bottom surface do not come into contact with the atmosphere inside the furnace.

[0073] There is no particular limitation on the method for preventing the inner peripheral surface of the second intermediate material 19 from coming into contact with the furnace atmosphere. In this example, as shown in FIG. 4(C), the end opening on one axial side of the second intermediate material 19 is blocked with a cover member 20 to prevent the furnace atmosphere from entering the interior of the second intermediate material 19. In the illustrated example, the threaded portion formed on the outer peripheral surface of the small-diameter shaft portion of the cover member 20 is threadedly engaged with the nut-side ball screw groove 12, and the large-diameter shaft portion is abutted against the end face on one axial side of the second intermediate material 19, thereby blocking the end opening on one axial side of the second intermediate material 19. This prevents the inner peripheral surface of the second intermediate material 19 and the side surface on one axial side of the bottom portion 14 constituting the inner bottom surface from coming into contact with the furnace atmosphere. Note that, although not shown, the end opening on one axial side of the second intermediate material can also be blocked by forming the inner peripheral surface of the end on one axial side of the second intermediate material 19 into a cylindrical shape and press-fitting the cylindrical outer peripheral surface of a cover member into that portion. An example of specific conditions for the gas soft nitriding treatment is as follows.

[0074] The treatment temperature in the gas soft-nitriding treatment is preferably in the range of 500°C or higher and 600°C or lower. Although it depends on the gas ratio in the furnace and the treatment time, if the treatment temperature is lower than 500°C, it becomes difficult to form a compound layer 17 with a sufficient thickness. Since the treatment temperature in the gas soft-nitriding treatment is relatively low, the occurrence of dimensional changes and distortion can be suppressed.

[0075] The holding time at the above treatment temperature is preferably in the range of 1 hour to 2.5 hours. Although it depends on the gas ratio in the furnace and the treatment temperature, if the treatment time is less than 1 hour, nitrogen and carbon will not penetrate sufficiently, making it difficult to form a compound layer 17 with sufficient hardness.

[0076] The atmosphere in the furnace during the gas soft-nitriding treatment can be, for example, a mixture of ammonia gas (NH3), nitrogen gas (N2), and carbon dioxide gas (CO2). During the gas soft-nitriding treatment, the nitriding potential (K N ) is not used as a parameter to control the atmosphere.

[0077] By the gas nitriding treatment as described above, a nut 3 is manufactured which has a compound layer 17 on the outer peripheral surface 10 and an inner peripheral surface 11 including the portion provided with the nut side ball screw groove 12 which is constituted by a diffusion layer 16.

[0078] In addition, when forming an oxide film of triiron tetroxide on the surface layer of the compound layer 17 to further enhance the corrosion resistance of the outer peripheral surface 10 of the nut 3, after forming the compound layer 17 on the second intermediate material 19, the oxide film can be formed on the surface layer of the compound layer 17 without removing the cover member 20 and with the inner peripheral surface remaining unexposed. Specifically, the oxide film can be formed on the surface layer of the compound layer 17 by, for example, immersing in a low-temperature heated aqueous solution of caustic soda (NaOH), a so-called black oxide treatment.

[0079] In this example, the nut 3 has the entire outer peripheral surface 10 and the other axial side of the bottom portion 14 that constitutes the outer bottom surface made of a compound layer 17, and the entire inner peripheral surface 11, the one axial side of the bottom portion 14 that constitutes the inner bottom surface, and the end face of the tubular portion 13 on one axial side made of a diffusion layer 16.

[0080] In this example, the gas soft-nitriding process is the final process, and no processing is performed on the nut 3 after the gas soft-nitriding process. In other words, the nut 3 after the gas soft-nitriding process becomes the final product. However, after the gas soft-nitriding process, an additional process of grinding the nut-side ball screw groove 12 can also be performed.

[0081] In this example, a method for manufacturing a nut 3 having a compound layer 17 on the outer peripheral surface 10 and an inner peripheral surface 11 including a portion provided with the nut-side ball screw groove 12 constituted by a diffusion layer 16 by performing a gas nitriding process after a shape processing process, and then a gas soft nitriding process has been described. However, the nut 3 in this example is not limited to the above-mentioned method and can also be manufactured by other methods.

[0082] According to the nut 3 of the ball screw device 1 of this embodiment as described above, the slidability of the outer peripheral surface 10 can be improved, and the fatigue strength of the nut-side ball screw groove 12 can be ensured.

[0083] The nut 3 of the ball screw device 1 of this example has a compound layer 17 on its outer peripheral surface 10, which improves the sliding properties of the outer peripheral surface 10 that constitutes the sliding surface 21. Furthermore, in this example, the nut 3 functions as a piston and is used in a manner in which the other axial end is exposed to the outside from the cylinder 9. Therefore, the outer peripheral surface 10 is exposed to the outside air and is used under conditions in which corrosion is likely to occur. However, the compound layer 17 also has excellent corrosion resistance, which can prevent corrosion from occurring on the outer peripheral surface 10 of the nut 3 and prevent a decrease in the sliding properties of the outer peripheral surface 10 due to corrosion. Furthermore, the compound layer 17 also has excellent wear resistance, which can improve the wear resistance of the outer peripheral surface 10. Furthermore, the diffusion layer 16 is present radially inward of the compound layer 17 on the outer peripheral surface 10, which ensures the strength of the sliding surface 21.

[0084] In the nut 3 of this example, at least the portion of the inner peripheral surface 11 where the nut-side ball screw groove 12 is provided is made of a diffusion layer 16 that is high in hardness, excellent in toughness, and resistant to impacts. This allows the fatigue strength of the nut-side ball screw groove 12 to be improved.

[0085] In the manufacturing method of the nut 3 of this example, the first intermediate material 18 is subjected to gas nitriding treatment, so that the diffusion layer 16 having a sufficient thickness can be formed on the outer peripheral surface and inner peripheral surface of the first intermediate material 18. As a result, the fatigue strength of the nut-side ball screw groove 12 can be sufficiently improved, and fatigue fracture at an internal origin can be effectively prevented on both the outer peripheral surface 10 and the inner peripheral surface 11 of the nut 3.

[0086] In the manufacturing method of the nut 3 in this example, the gas nitriding treatment is performed so that at least the inner surface of the second intermediate material 19 does not come into contact with the furnace atmosphere, and therefore, it is possible to prevent the formation of a compound layer 17 on the diffusion layer 16 that constitutes the nut side ball screw groove 12, which requires toughness.

[0087] [Example 2] A second example of the embodiment of the present disclosure will be described with reference to FIGS. 5 to 7(D).

[0088] The nut 3a constituting the ball screw device 1a of this example has a cylindrical shape without a bottom. In other words, the nut 3a is composed only of a cylindrical portion 13 and does not have a bottom portion 14 (see FIG. 2).

[0089] In this example as well, the nut 3a is slidably fitted into the cylinder 9 and functions as a part of the piston.

[0090] The nut 3 a of this example has a compound layer 17 on the outer peripheral surface 10 , and the inner peripheral surface 11 including the portion where the nut-side ball screw groove 12 is provided is formed by a diffusion layer 16 .

[0091] Specifically, the entire outer peripheral surface 10 of the nut 3a in this example is made up of a compound layer 17, and the entire inner peripheral surface 11 and both end faces in the axial direction are made up of a diffusion layer 16.

[0092] The nut 3a of this example can be manufactured by carrying out a shaping process, a gas nitriding process, and a gas soft-nitriding process, similar to the manufacturing method of the first example of the embodiment of the present disclosure. That is, a first intermediate material 18a having the same dimensions and shape as the nut 3a is formed by the shaping process, and then a second intermediate material 19a having a surface structure consisting only of the diffusion layer 16 is obtained by the gas nitriding process.

[0093] In particular, when manufacturing the nut 3a of this example, in order to prevent the inner peripheral surface of the second intermediate material 19a from coming into contact with the furnace atmosphere when performing the gas soft nitriding treatment process, the end openings on both axial sides of the second intermediate material 19a are closed with cover members 20 to prevent the furnace atmosphere from entering the inside of the second intermediate material 19a, as shown in Fig. 7(C). Specifically, in the illustrated example, for each of the two cover members 20, the threaded portion formed on the outer peripheral surface of the small diameter shaft portion is screwed into the nut-side ball screw groove 12, and the large diameter shaft portion is abutted against the axial end face of the second intermediate material 19a, thereby closing the end openings on both axial sides of the second intermediate material 19a.

[0094] In the case of the nut 3a of this example, which has a bottomless cylindrical shape, the outer peripheral surface 10 has a compound layer 17, and the inner peripheral surface 11, including the portion provided with the nut side ball screw groove 12, is composed of a diffusion layer 16, so that the sliding properties of the outer peripheral surface 10 can be improved and the fatigue strength of the nut side ball screw groove 12 can be ensured.

[0095] Other configurations and effects of the second example are the same as those of the first example.

[0096] When implementing a ball screw device nut according to one aspect of the present disclosure, anti-rotation protrusions and key grooves can be provided on the outer peripheral surface. In this case, a compound layer may be formed on the surfaces of the protrusions and key grooves, or only a diffusion layer may be formed by applying masking or the like during gas soft nitriding. However, because the anti-rotation protrusions slide against grooves provided on the housing side, it is more preferable to form a compound layer by performing gas soft nitriding. [Explanation of symbols]

[0097] 1, 1a Ball screw device 2 screw shaft 3, 3a nut 4 balls 5 Load path 6 Threaded section 7 Mating shaft 8 Ball screw groove on shaft side 9 cylinders 10 Outer surface 11 Inner surface 12 Nut side ball screw groove 13 Cylinder part 14 Bottom 15 Base material 16 Diffusion layer 17 Compound layer 18, 18a First intermediate material 19, 19a Second intermediate material 20 Cover member 21 Sliding surface

Claims

1. A nut for a ball screw device made of alloy steel, a bearing having an outer peripheral surface having a sliding surface and an inner peripheral surface having a portion provided with a nut-side ball screw groove, at least the sliding surface of the outer peripheral surface being constituted by a compound layer containing an iron-nitrogen compound, and at least the portion of the inner peripheral surface provided with the nut-side ball screw groove being constituted by a diffusion layer, A diffusion layer is further present radially inward of the compound layer. Nut for ball screw device.

2. 2. The nut for a ball screw device according to claim 1, which has a cylindrical shape with a bottom.

3. 2. The nut for a ball screw device according to claim 1, which has a bottomless cylindrical shape.

4. 2. The nut for a ball screw device according to claim 1, wherein the inner peripheral surface has a circulation groove.

5. 2. The nut for a ball screw device according to claim 1, wherein the entire outer peripheral surface is formed by the compound layer.

6. a screw shaft having a shaft-side ball screw groove on its outer peripheral surface and rotating during use; a nut having a nut-side ball screw groove on its inner peripheral surface and moving linearly when in use; a plurality of balls arranged between the shaft-side ball screw groove and the nut-side ball screw groove; Equipped with A ball screw device, wherein the nut is the nut for a ball screw device according to any one of claims 1 to 5.

7. A method for manufacturing a nut for a ball screw device according to any one of claims 1 to 5, a step of forming the diffusion layer on at least a portion of the inner peripheral surface of a first intermediate material having the nut-side ball screw groove formed on its inner peripheral surface and on at least a portion of the outer peripheral surface of the first intermediate material, thereby obtaining a second intermediate material; forming the compound layer on the radially outer side of the diffusion layer provided on the outer peripheral surface of the second intermediate material, while preventing at least the inner peripheral surface of the second intermediate material from contacting the furnace atmosphere; Equipped with A manufacturing method for a nut for a ball screw device.

8. The diffusion layer is formed by subjecting the first intermediate material to a gas nitriding treatment; and The compound layer is formed by subjecting the second intermediate material to a gas soft nitriding treatment. A method for manufacturing a nut for a ball screw device according to claim 7.

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