JP2005090570A - Ball screw nut and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a durable low-cost ball screw nut having a relatively accurate ball running groove, and to provide a method of easily manufacturing the same. <P>SOLUTION: The manufacturing method of the ball screw nut formed with the ball running groove, wherein balls run, in the inner diameter surface is provided with a process for opening the predetermined inner diameter surface at the center of a raw material by drilling, a process for finishing the outer diameter surface and the inner diameter surface of the raw material with a turning tool, a process for inserting a tapping tool into the inner diameter surface to cut the ball running groove, a heating process for carburizing and quenching the ball running groove, and a process for shot peening with steel beads after the heating process. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a ball screw nut having a spiral ball rolling groove on which a large number of balls roll and a method for manufacturing the same.

  The ball screw is composed of a ball screw shaft having a spiral ball rolling groove formed on the outer periphery, a ball screw nut having a spiral ball rolling groove formed in the cylindrical surface, and both opposing ball rolling grooves. It is a mechanical element that is composed of a large number of balls that are rotatably accommodated in a configured ball rolling path, and that converts the rotation of the ball screw shaft or the ball screw nut into a translational motion in the axial direction.

  Conventionally, in this ball screw nut, a ball rolling groove is generally formed on the inner periphery by cutting and grinding. That is, a drill hole is used to make a pilot hole, and a spiral ball rolling groove is cut on the peripheral surface of the pilot hole with a cutting tool. Next, after heat treatment such as carburizing and quenching, the outer diameter portion is ground with a cylindrical grinder or the like, and finally the surface of the ball rolling groove cut with a grindstone is ground.

  However, in the conventional ball screw nut in which a ball rolling groove is formed by cutting and grinding, when grinding a ball screw nut having a small inner diameter, the grindstone cannot be inserted into the pilot hole, There were cases where processing was not possible. Also, even if the lead screw is not a ball screw nut with a small inner diameter, the lead angle is large, so that the amount of the grindstone inserted into the lower hole is limited, and grinding may not be possible. Naturally, the grinding process is disadvantageous in terms of cost because it takes time and man-hours for preparation such as centering of the nut.

  A ball screw nut as shown in FIG. 9 is known as a solution to these problems. The ball screw nut 50 has a substantially cylindrical shape, and has a flange 50a at one end thereof for coupling to a conveying machine or the like. A ball rolling groove 50c on which a ball (not shown) rolls is formed on the inner peripheral surface of the ball screw nut 50, and a flat surface portion 50b is formed on the outer peripheral surface. A member such as a return pipe that connects one end and the other end of the ball rolling groove 50c is coupled to the flat surface portion 50b. The ball rolling groove 50c is formed in a Gothic arch shape by combining two arcs having a radius of curvature slightly larger than the radius of the ball by rolling.

The ball screw nut 50 is manufactured by a process as shown in FIG. First, the outer periphery of the cylindrical material 51 is cut using a cutting tool 52, a flange 51a is formed on the material 51 (S1), and a pilot hole 51b is formed on the material 51 using a drill 53 (S2). Next, using the boring tool 54, the lower hole 51b is spread to a normal size (S3), and then the raw material 51 is rotated at a low speed of 100 to 200 rpm, and the rolling tap 55 is inserted into the lower hole 51b. The ball rolling groove 51c is rolled on the inner peripheral surface of 51 (S4). In this way, the surface of the ball rolling groove 51c is burnished. Next, the end mill 56 is used to groove the return portion 51 d for circulating the ball through the material 51. Then, the outer diameter of the material 51 is cut using the cutting tool 52 to finish the outer diameter (S5). Finally, the rolling tap 55 is again passed through the ball rolling groove 51c of the material 51, and the ball rolling groove 51c is deburred.

Thus, since the ball rolling groove 51c is formed on the inner peripheral surface of the material 51 in the rolling process (S4), a grinding process for grinding the ball rolling groove using a conventional grindstone is not necessary, Even with a small-diameter ball screw nut, the ball rolling groove can be easily processed. For this reason, since the processing cost of the ball rolling groove 51c can be reduced and the number of processing steps can be reduced, the lead time can be shortened.
JP 2000-88072 A

  Recently, not only in the field of machine tools and semiconductor manufacturing equipment that require precise positioning, smooth operation, and quietness, but also to generate thrust for transfer machines and press machines that do not require much positioning accuracy. There is also a tendency to control noise and vibration, etc., with higher speeds in applications. In such a conventional ball screw nut 50, unlike the rolling process of the screw shaft, the cold plastic working for the ball rolling groove 50c on the inner diameter surface has a processing problem. That is, since the plastic flow in the axial direction of the hollow nut body can hardly be expected, a large stress is generated in the rolling tap 55 inserted into the inner diameter, and the rolling tap 55 is often damaged.

In addition, as a measure against this breakage, when trying to set a small plastic deformation allowance, if the accuracy of the rough screw is not sufficient, not only the surface accuracy of the ball rolling groove can be expected, but also a stable quality due to residual machining. Problems remain in terms of maintenance. Furthermore, when carburizing and tempering is performed with SCM steel (JIS) or the like, a grain boundary oxidation layer is generated on the surface layer, the surface of the ball rolling groove 50c becomes brittle, and wear due to insufficient surface hardness. There was a fear of progress. In addition, impurities that have entered the processed surface (mussel surface or chatter surface) before the heat treatment have deteriorated the hardenability and further increased the grain boundary oxide layer.

An object of the present invention is to solve such conventional problems, and to provide a ball screw nut having a ball rolling groove that is excellent in durability, low in cost, and relatively high in accuracy, and a method for manufacturing the same.

  In order to achieve such an object, the invention according to claim 1 of the present invention is a ball screw nut in which a ball rolling groove on which a ball rolls is formed on an inner surface, and shot peening treatment is performed on the ball rolling groove. The structure in which the hardened layer by was formed was adopted.

  In this way, a hardened layer formed by shot peening treatment is formed in the ball rolling groove, so that the surface roughness can be improved and the compressive residual stress of the surface can be increased, and the durability is excellent and the cost is relatively low. A ball screw nut having a highly accurate ball rolling groove can be provided.

  In the invention according to claim 2, since the surface roughness of the ball rolling groove is regulated to Ra 1.0 μm or less, it is possible to suppress the generation of noise, vibration and the like even at high speed rotation.

  Moreover, since the surface hardness of the said ball rolling groove is set to the range of Hv700-900, invention of Claim 3 has sufficient abrasion resistance and durability.

  Moreover, since the compression residual stress is formed in the range of -500--1500MPa on the surface of the said ball rolling groove, invention of Claim 4 can improve durability further.

  According to a fifth aspect of the present invention, there is provided a method for manufacturing a ball screw nut in which a ball rolling groove on which a ball rolls is formed on an inner diameter surface, before or after heat treatment, or before and after heat treatment. Since the shot peening process is performed, a ball screw having a specification having a ball rolling groove with relatively high accuracy at low cost and having durability suitable for the use conditions can be provided as appropriate.

  The invention described in claim 6 includes a step of drilling a predetermined inner diameter surface at the center of the material by drilling, and a step of inserting a tapping tool into the inner diameter surface and cutting the ball rolling groove. As a result, the ball rolling groove can be cut efficiently with a single pass, and high-precision grooving can be performed at low cost.

  In addition, since the invention according to claim 7 is subjected to shot peening treatment with silicon carbide beads having a particle diameter of 40 to 60 μm before the heat treatment, the surface roughness of the ball rolling groove and the mushy or chattering. Etc., and the surface hardness and the compressive residual stress of the surface can be increased.

  In addition, since the invention according to claim 8 is subjected to shot peening treatment with steel beads having a particle size of 40 to 60 μm after the heat treatment, a ball screw nut having sufficient wear resistance and durability is provided. Can be provided.

The ball screw nut according to the present invention is a ball screw nut in which a ball rolling groove on which a ball rolls is formed on an inner diameter surface, and since a hardened layer is formed by shot peening treatment on the ball rolling groove, It is possible to provide a ball screw nut having a ball rolling groove that can improve the roughness and increase the compressive residual stress on the surface, has excellent durability, and is relatively low in cost and relatively high in accuracy.

  The ball screw nut manufacturing method according to the present invention is a ball screw nut manufacturing method in which a ball rolling groove on which a ball rolls is formed on an inner diameter surface, and shot peening before or after heat treatment or before and after heat treatment. Since the treatment is performed, it is possible to appropriately provide a ball screw having a specification that has a ball rolling groove with relatively high accuracy at low cost and has durability that matches the use conditions.

  In a ball screw nut manufacturing method in which a ball rolling groove for rolling a ball is formed on an inner diameter surface, a step of drilling a predetermined inner diameter surface at the center of the material by drilling, and an outer diameter surface and an inner diameter surface of the material A step of finishing with a cutting tool, a step of inserting a tapping tool into the inner surface and cutting the ball rolling groove, a heat treatment step of carburizing and quenching the ball rolling groove, and a shot peening treatment after the heat treatment Process.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a longitudinal sectional view showing an embodiment of a ball screw nut according to the present invention.

  The ball screw nut 1 is made of case-hardened steel such as SCM430, and is externally fitted to the ball screw shaft 3, corresponding to the spiral ball rolling groove 4 formed on the outer diameter surface 3a of the ball screw shaft 3. A spiral ball rolling groove 2 is formed on the inner diameter surface 1a by cutting with a tapping tool 9 described later. A ball rolling path is constituted by these ball rolling grooves 2 and 4, and a large number of balls 7 are infinitely circulated by a piece member 6 in which a connecting groove 5 that connects the ball rolling grooves 2 of the ball screw nut 1 is formed. Can do. The ball circulation method is not limited to the piece type, and may be a return tube type or an end cap type.

  In FIG. 2, the manufacturing process of the ball screw nut 1 in one Embodiment of this invention is shown. First, a predetermined inner diameter surface is formed with a drill or the like at the center of a cylindrical material (P1). The outer diameter surface and inner diameter surface of the material are finished with a cutting tool (P2). Next, as shown in FIG. 3, the ball screw nut 1 and the tapping tool 9 are mounted on the chuck 8 a and the tail stock (not shown) of the turning center 8, respectively, and then the inner surface 1 a of the ball screw nut 1 is tapped. The tool 9 is inserted and cutting is performed while NC controlling the respective phases (P3). As a result, the ball rolling groove 2 can be efficiently cut by one pass, and high-precision grooving can be performed at low cost.

As shown in FIG. 4, the tapping tool 9 is formed with a cylindrical portion 9a at the tip so that centering is possible using the inner diameter surface 1a of the ball screw nut 1 as a guide. A cutting edge portion 9b having a gradually larger diameter is formed rearward from the cylindrical portion 9a, and a rectangular chuck portion 9d to be attached to the tailstock is formed at the rear end portion via the shank portion 9c. . As the cutting teeth 9b of the tapping tool 9 rotate, the ball rolling groove 2 is formed on the inner diameter surface 1a of the ball screw nut 1 by cutting. In addition, the outer peripheral shape of the cutting edge portion 9b of the tapping tool 9 can be efficiently cut in one pass by forming the ball rolling groove 2 into, for example, a Gothic arch shape in advance. High groove processing is possible.

After cutting with such a tapping tool 9, a hardened layer is formed on the surface of the ball screw nut 1 in a range of 54 to 64 HRC by carburizing and tempering (P4). The surface hardening treatment by heat treatment is not limited to this, and tempering treatment or induction hardening may be used, or high-carbon chromium steel such as SUJ2 may be used as the material.

Then, after the heat treatment, the ball rolling groove 2 is subjected to shot peening treatment (P5). In the present embodiment, steel beads having high hardness with specific gravity are used, the particle size is 40 to 60 μm, the shot pressure is 0.4 MPa, and the treatment time is 60 sec. I went there. Thereby, the scale by heat processing can be deleted, surface hardness can be improved, and surface hardness and surface compressive residual stress can be increased. In addition to the steel beads, the treatment may be performed with ceramic beads or glass beads.

The surface roughness after shot is improved to Ra 0.89 μm on average compared to Ra 1.05 μm before processing, and generation of noise, vibration, etc. can be suppressed even at high speed rotation. Further, as shown in FIG. 5, the surface hardness of the untreated product has a decrease in hardness in the range of 25 μm from the surface, and a grain boundary oxide layer is observed on the surface. On the other hand, it can be seen that the treated product is improved by increasing the surface hardness. The surface hardness was measured from the surface to a depth of 0.3 mm in the vicinity of the ball 7 in the ball rolling groove 2 contacting.

Next, the life test result of the ball screw incorporating the ball screw nut 1 of the present embodiment is shown in FIG. Here, the specifications of the specimen are: shaft diameter: φ14, lead: 4 mm, ball diameter: φ2.381, circulation number: 1 turn, 4 rows, circulation method is piece type. Moreover, the test conditions are a load application method; constant pressure preload by a spring preload double nut, lubrication; oil VG68, rotation speed: 2000 rpm, stroke: 60 mm, load load: 2400 N.

  The notation in FIG. 6 is a graph showing the ratio between the total number of revolutions until a thrust load is applied to the ball screw nut and peeling occurs and the calculated total number of revolutions (L10 life). As can be seen from this graph, the life ratio is 2 to 3 times that of the unprocessed shot peened product (Comparative Examples 1 to 3), and the shot peened product has sufficient durability. I know that.

The surface hardness is preferably set in the range of Hv 700 to 900 by shot peening. If it is less than Hv700, the improvement of the life cannot be expected, and if it exceeds Hv900, there is a concern that the toughness is lowered. Furthermore, the surface compressive residual stress is preferably in the range of -500 to -1500 MPa. If it is less than −500 MPa, a sufficient improvement in the life cannot be obtained, and if it exceeds −1500 MPa, an increase in stress proportional to the treatment time cannot be expected, and the treatment time becomes longer and the cost increases.

  In FIG. 7, the manufacturing process of the ball screw nut 1 in other embodiment of this invention is shown. The present embodiment is the same as the above-described embodiment from the drilling (P1) of the material to the cutting (P3) of the ball rolling groove 2 by the tapping tool 9, but after the cutting of the ball rolling groove 2, That is, the shot peening process is performed on the ball rolling groove 2 before the heat treatment (P5). In the present embodiment, silicon carbide beads are used, the particle size is 40 to 60 μm, the shot pressure is 0.4 MPa, and the treatment time is 20 sec. I went there. This is because the workpiece is a raw material, and with steel beads, the plastic deformation of the workpiece may increase and the groove shape may collapse, and plastic deformation is unlikely to occur, and the structure of the extreme surface (2 to 3 μm from the surface) This is because silicon carbide beads are suitable for miniaturization and removal of surface impurities caused by shots. Thereby, a whistle, chatter, etc. are improved with the surface roughness of the ball rolling groove 2, and surface hardness and surface compressive residual stress can be raised.

  Next, the life test result of the ball screw incorporating the ball screw nut 1 of this embodiment is shown in FIG. Here, the specifications of the specimen and the test conditions are the same as in the above-described embodiment. As can be seen from this graph, the life ratio is 1.5 to 2 times that of the unprocessed shot peened product (Comparative Examples 1 to 3), and the shot peened product has sufficient durability. You can see that

  In this embodiment, as a matter of course, since the shot peening process is performed after the cutting process and before the heat treatment, the grain boundary oxide layer cannot be removed by the heat treatment, but the surface roughness and the surface compressive residual stress are formed together with the formation of the surface hardness. It is considered that the improvement in the contact state between the ball rolling groove 2 and the ball 7 by smoothing the projection-like cutting surface rather than the numerical improvement contributes to the improvement of the life. . Of course, ball screws with severe use conditions can be shot peened before and after heat treatment to improve wear resistance and durability.

  The embodiment of the present invention has been described above, but the present invention is not limited to such an embodiment, and is merely an example, and various modifications can be made without departing from the scope of the present invention. Of course, the scope of the present invention is indicated by the description of the scope of claims, and further, the equivalent meanings described in the scope of claims and all modifications within the scope of the scope of the present invention are included. Including.

  The ball screw nut according to the present invention can be applied to an automatic manual transmission, an electric brake, an electric power steering, an engine valve operating control actuator used for a vehicle such as an automobile, and also for width control of an electric shock absorber or a CVT pulley. It can also be applied to other actuators.

It is a longitudinal section showing one embodiment of a ball screw nut concerning the present invention. It is explanatory drawing which shows one manufacturing process of the ball screw nut which concerns on this invention. It is the schematic which shows the turning center for enforcing the manufacturing method of the ball screw nut which concerns on this invention. It is a top view which shows the tapping tool used for the process of the ball screw nut which concerns on this invention. It is the data which showed the surface hardness of the ball rolling groove in the ball screw nut concerning the present invention. It is a graph which shows the lifetime test result of the ball screw incorporating the ball screw nut which concerns on this invention. It is explanatory drawing which shows the other manufacturing process of the ball screw nut which concerns on this invention. It is a graph which shows the lifetime test result of the ball screw incorporating the ball screw nut which concerns on this invention. It is a longitudinal cross-sectional view which shows the conventional ball screw nut. It is process drawing which shows the manufacturing method of the conventional ball screw nut.

Explanation of symbols

1 ··············· Ball screw nut 1a ······· Inner diameter surface 2, 4 ······················· Ball rolling groove 3 ······· Ball screw shaft 3a ··· Outer diameter surface 5 ······· Connection groove 6 ······· Member 7 ... Ball 8 ... Turning Center 8a ... Chuck 9 ...・ ・ Tapping tool 9a ・ ・ ・ ・ ・ ・ Cylindrical part 9b ・ ・ ・ ・ ・ ・ Cutting tooth part 9c ・ ・ ・ ・ ・ ・ Shank part 9d ... Chuck part 50 ... Ball screw nuts 50a, 51a ... Flange 50b ... Plane parts 50c, 51c ... Ball rolling groove 51 51b ·························· 51d ·············································································· ···················································································· End mill

Claims (8)

  A ball screw nut in which a ball rolling groove on which a ball rolls is formed on an inner diameter surface, and a hardened layer formed by shot peening is formed on the ball rolling groove.   The ball screw nut according to claim 1, wherein the surface roughness of the ball rolling groove is regulated to Ra 1.0 μm or less.   The ball screw nut according to claim 1 or 2, wherein the surface hardness of the ball rolling groove is set in a range of Hv 700 to 900.   The ball screw nut according to any one of claims 1 to 3, wherein a compressive residual stress is formed in a range of -500 to -1500 MPa on a surface of the ball rolling groove.   In a ball screw nut manufacturing method in which a ball rolling groove on which a ball rolls is formed on an inner diameter surface, a shot screw peening treatment is performed before or after heat treatment or before and after heat treatment. Manufacturing method.   The ball screw nut according to claim 5, comprising a step of drilling a predetermined inner diameter surface in the center of the material, and a step of cutting a ball rolling groove by inserting a tapping tool into the inner diameter surface. Production method.   The method for producing a ball screw nut according to claim 5 or 6, wherein a shot peening treatment using silicon carbide beads having a particle diameter of 40 to 60 µm is performed before the heat treatment.   The method of manufacturing a ball screw nut according to any one of claims 5 to 7, wherein a shot peening process using steel beads having a particle size of 40 to 60 µm is performed after the heat treatment.

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