JP2013099840A - Threaded shaft, method for manufacturing the same, and ball screw unit - Google Patents

Abstract

A hollow screw shaft having a small wall thickness with excellent accuracy and strength, and a method for manufacturing the hollow screw shaft. In addition, a small and lightweight ball screw unit is provided.
The outer peripheral surface of a steel rod-shaped material 20 is rolled to form a helical thread groove 3a, and then the outer peripheral surface is subjected to heat treatment. Then, the rod-shaped material 20 subjected to the heat treatment was subjected to a boring process to form a tubular shape, and the hollow screw shaft 3 was manufactured. The hardness of the thread groove surface of the screw shaft 3 is more than Hv600, the hardness of the inner peripheral surface is more than Hv180 and less than Hv400, and the wall thickness is 0.65 mm or more and 50% or less of the inner radius. The ball screw 1 including the screw shaft 3, the rolling bearing 30, the fixed shaft 32, and the inclusion of the motor 38 are combined to form a ball screw unit. These inclusions are included in the hollow portion of the screw shaft 3.
[Selection] Figure 5

Description

  The present invention relates to a screw shaft of a ball screw and a manufacturing method thereof. The present invention also relates to a ball screw unit including a ball screw.

  Conventionally, a ball screw unit in which a member such as a motor and a gear is combined with a ball screw is known (see, for example, Patent Document 1). Such a ball screw unit is required to be reduced in size and weight. However, since members such as a motor and gears are provided on the outer peripheral portion of the ball screw nut, there is a limit to downsizing. Therefore, a technique for reducing the weight by using a screw shaft of a ball screw as a hollow shaft and reducing the size by housing members such as a motor and gears in the hollow portion has been proposed.

Japanese Patent No. 4,697,785

However, since a hollow screw shaft is manufactured by rolling or heat-treating the tubular material, deformation may occur due to the rolling or heat treatment, which may reduce the accuracy of the thread groove shape and the roundness of the screw shaft. It was. Further, when heat treatment is performed on a tubular material having a small thickness, there is a possibility that the toughness is lowered and the tube material is likely to be damaged. If the wall thickness of the tubular material is increased, deformation due to rolling or heat treatment is reduced, and the above-described decrease in accuracy and toughness is suppressed. However, there is a possibility that weight reduction may be insufficient.
Accordingly, it is an object of the present invention to solve the above-described problems of the prior art and to provide a hollow screw shaft having a small thickness and having excellent accuracy and strength, and a method for manufacturing the same. Another object is to provide a small and lightweight ball screw unit.

  In order to solve the above problems, an aspect of the present invention has the following configuration. That is, the manufacturing method of the screw shaft according to one aspect of the present invention is a method of manufacturing a hollow screw shaft used for a ball screw, wherein the outer peripheral surface of the rod-shaped material is rolled to form a helical screw groove. A thread groove forming step for forming the heat treatment, a heat treatment step for heat-treating the outer peripheral surface of the rod-shaped material on which the screw groove is formed, and a boring step for performing a boring process on the heat-treated rod-shaped material to form a tubular shape It is characterized by providing.

Such a method for manufacturing a screw shaft may include an end portion removing step for removing an end portion of the rod-shaped material between the heat treatment step and the boring step. Moreover, you may provide the thread groove removal process which removes the edge part of a thread groove between the heat treatment process and the boring process, or after the boring process.
The screw shaft according to another aspect of the present invention is a hollow screw shaft used for a ball screw and having a helical screw groove on the outer peripheral surface, and the hardness of the screw groove surface exceeds Hv600, The hardness of the inner peripheral surface is more than Hv180 and less than Hv400, and the wall thickness is 0.65 mm or more and 50% or less of the inner radius.

Furthermore, a ball screw unit according to another aspect of the present invention includes a hollow screw shaft having a helical screw groove on an outer peripheral surface, and a nut having a screw groove facing the screw groove of the screw shaft on an inner peripheral surface. A ball screw including a plurality of balls movably loaded in a spiral ball rolling path formed by the both screw grooves, and an inclusion disposed in a hollow portion of the screw shaft. The hardness of the screw groove surface of the screw shaft is more than Hv600, the hardness of the inner peripheral surface is more than Hv180 and less than Hv400, and the wall thickness is 0.65 mm or more and 50% or less of the inner radius. And
In such a ball screw unit, the inclusion may be at least one of a motor, a sensor, an encoder, a damping material, a bearing, a bush, and a shaft.

The screw shaft manufacturing method of the present invention manufactures a hollow screw shaft by boring a rod-shaped material that has been subjected to rolling and heat treatment, so that a hollow screw with a small wall thickness having excellent accuracy and strength. A shaft can be manufactured.
Moreover, since the screw shaft of the present invention has excellent strength, it is difficult to cause damage despite being a hollow screw shaft having a small wall thickness.
Furthermore, the ball screw unit of the present invention is lightweight because the screw shaft is hollow. Moreover, since the inclusions provided in the ball screw unit are arranged in the hollow portion of the screw shaft, the size is small.

It is process drawing explaining one Embodiment of the manufacturing method of the screw shaft which concerns on this invention. It is sectional drawing of a ball screw. It is principal part sectional drawing of the nut used for the ball screw of FIG. It is a side view explaining the structure of one Embodiment of the ball screw unit which concerns on this invention. It is AA sectional drawing of the ball screw unit of FIG. It is a side view explaining the structure of another embodiment of the ball screw unit which concerns on this invention. It is BB sectional drawing of the ball screw unit of FIG. FIG. 7 is a perspective view of the ball screw unit of FIG. 6 with a support shaft removed.

DESCRIPTION OF EMBODIMENTS Embodiments of a screw shaft, a manufacturing method thereof, and a ball screw unit according to the present invention will be described in detail with reference to the drawings.
An example of a method for producing a hollow screw shaft used for a ball screw will be described with reference to FIG. In addition, about the screw axis | shaft shown in FIG. 1 and its raw material, the lower half has a side surface, and the upper half has a cross section (a cross section cut by a plane along the axial direction).

  First, the outer peripheral surface of the steel rod-shaped material 20 was rolled to form a helical thread groove 3a (thread groove forming step). Since the thread groove 3a is processed by rolling, mass production of the screw shaft 3 can be performed efficiently. And the heat treatment was performed to the rod-shaped raw material 20 in which this thread groove 3a was formed (heat treatment process). Thereby, the surface (screw groove surface) of the screw groove 3a is hardened. As the heat treatment, a transformation-type heat treatment such as carburizing, carbonitriding, or induction hardening can be used. If carburizing treatment, carbonitriding treatment, or induction hardening is employed as the heat treatment, only the surface layer portion 20a (particularly, the portion where the thread groove 3a is formed) on the outer peripheral surface of the rod-shaped material 20 is cured, and the central portion is not. It can be cured.

In addition, it is more preferable to employ induction hardening as the heat treatment. If induction hardening is used, grain boundary oxidation due to heat treatment can be suppressed. Further, if induction hardening is used, the heat treatment process can be inlined, so that mass production of the screw shaft 3 can be performed efficiently.
Next, the rod-shaped material 20 subjected to the heat treatment was subjected to a boring process, and through holes along the axial direction were provided between both end surfaces to form a tubular shape (boring step). As described above, if the center portion of the rod-shaped material 20 is not cured, the boring process is easy. In this way, a hollow and lightweight screw shaft 3 was obtained.

  When a hollow screw shaft is manufactured by rolling or heat-treating a tubular material as in the past, deformation may occur due to the rolling or heat treatment, which may reduce the accuracy of the thread groove shape and the roundness of the screw shaft. However, if the hollow screw shaft 3 is manufactured by performing the boring process after rolling or heat-treating the rod-shaped material 20 as in this embodiment, deformation due to rolling or heat-treatment is suppressed (for example, The deformation due to the heat treatment is about the same as that of a solid screw shaft), and a highly accurate hollow screw shaft 3 with excellent accuracy of the thread groove shape and roundness of the screw shaft 3 can be obtained.

  Further, since only the surface layer portion 20a on the outer peripheral surface of the rod-shaped material 20 can be cured by heat treatment, the surface layer portion on the outer peripheral surface of the hollow screw shaft 3 can be appropriately adjusted by adjusting the diameter of the through hole formed by boring. Only 20a is cured, and the other portion 20b can be uncured. As a result, the hollow screw shaft 3 has excellent toughness and strength due to the uncured portion 20b, so that even if the wall thickness is small, breakage hardly occurs.

Further, since the hardness of the outer peripheral surface is harder than the hardness of the inner peripheral surface of the hollow screw shaft 3, the damping coefficient of the inner peripheral surface is larger than that of the outer peripheral surface. The acoustic characteristics are superior to those in the case where is more than the hardness of the outer peripheral surface.
The hardness of the thread groove surface (outer peripheral surface) of the hollow screw shaft 3 needs to exceed Hv600. Thereby, in addition to the said effect resulting from hardness, the hardness of the thread groove surface which is a track surface of a rolling element can fully be ensured.

  Further, the hardness of the inner peripheral surface of the hollow screw shaft 3 needs to be less than Hv400. Thereby, in addition to the said effect resulting from hardness, the toughness of the screw shaft 3 whole can be maintained. Furthermore, compared with the case where the inner peripheral surface of the screw shaft 3 is hard, a member (for example, a detent) that slides with the screw shaft 3 is less likely to be worn. In order to further suppress wear of the member that slides with the screw shaft 3, it is preferable that the inner peripheral surface of the screw shaft 3 and the member that slides with the screw shaft 3 have the same hardness.

Furthermore, the hardness of the inner peripheral surface of the hollow screw shaft 3 needs to exceed Hv180. Thereby, in addition to the said effect resulting from hardness, the intensity | strength of an internal peripheral surface can be maintained.
Furthermore, the thickness of the hollow screw shaft 3 needs to be 0.65 mm or more and 50% or less of the inner radius (that is, 25% or less of the inner diameter). Thereby, it is possible to achieve both excellent strength and sufficient weight reduction. Here, the thickness of the hollow screw shaft 3 means the radially innermost portion (that is, the groove bottom portion of the screw groove 3a) and the inner peripheral surface of the outer peripheral surface on which the screw groove 3a is formed. It means radial distance.

  When the heat treatment is carburizing or carbonitriding, the material of the rod-shaped material 20 is chromium steel or chromium molybdenum steel (for example, SCM420, SCM415) having a carbon content of 0.10 to 0.25% by mass. When the heat treatment is induction hardening, carbon steel having a carbon content of 0.4 to 0.6% by mass (for example, S53C, SAE4150) is preferable. By setting it as such a material, the said effect resulting from hardness can be acquired reliably.

  The screw shaft 3 and the manufacturing method thereof according to this embodiment show an example of the present invention, and the present invention is not limited to the screw shaft 3 and the manufacturing method thereof according to this embodiment. For example, although the manufacturing method of the screw shaft 3 of this embodiment was a method provided with a thread groove formation process, a heat treatment process, and a boring process, it is good also as a method further provided with processes other than these. An example of this will be described below.

  In the present embodiment, the thread groove 3a is formed on the outer peripheral surface of the rod-shaped material 20 by rolling, but there is an approach or separation operation of two roll dies at the rolling start portion and the rolling end portion of the rod-shaped material 20. Depending on the roll die shape, the twist angle of the roll die, etc., there is a possibility that an incomplete thread portion may occur. Since the incomplete thread portion has a larger diameter than the effective thread portion, even if rolling is performed on the end portion of the rod-shaped material 20, the screw shaft 3 and the nut 5 are screwed by the incomplete thread portion formed at the end portion. There is a risk that the screw shaft 3 may not be smoothly screwed into the nut screw in which the ball 9 is inserted into the screw groove 3a. As described above, the assembly of the screw shaft 3 and the nut 5 is deteriorated due to the incomplete thread portion, which causes a decrease in the productivity of the ball screw 1.

Further, the outer peripheral surface in which the thread groove 3a is formed is hardened by heat treatment. However, when induction hardening is adopted as the heat treatment, the end portion of the rod-shaped material 20 cannot be sufficiently heat treated and is sufficiently hardened. There is a risk that an incompletely heat-treated portion not formed is formed.
Therefore, it is preferable to perform an end portion removing step for removing the end portion of the rod-shaped material 20 between the heat treatment step and the boring step. As a result, the incomplete thread portion and the incomplete heat treatment portion formed in the rod-shaped material 20 are removed, so that the above-described problem does not occur. In addition, the method of removing the edge part of the rod-shaped raw material 20 is not specifically limited, However, Cutting, grinding, etc. are mention | raise | lifted. When carburizing or carbonitriding is adopted as the heat treatment, an incomplete heat treatment portion is not formed, but the end surface of the hardened rod-shaped material 20 is removed and the non-hardened surface is exposed, so that the boring process is performed. Easy to apply.

Further, since residual stress is generated at the end portion of the screw groove 3a (end portion in the continuous direction of the spirally continuous screw groove 3a) by the heat treatment, the end portion of the screw groove 3a may expand. As a result, since the compressive stress is applied to the central portion of the screw groove 3a (the central portion in the continuous direction of the screw groove 3a), the accuracy of the screw groove 3a may be reduced.
Therefore, it is preferable to perform a thread groove removing step of removing the end of the thread groove 3a between the heat treatment step and the boring step or after the boring step. As a result, the compressive stress applied to the screw groove 3a is released, so that the accuracy of the screw groove 3a is increased. The method for removing the end portion of the screw groove 3a is not particularly limited, and examples thereof include cutting and grinding. For example, if the outer peripheral surfaces of both ends of the rod-shaped material 20 or the screw shaft 3 are scraped off by cutting, the screw grooves 3 a formed on the outer peripheral surfaces of both ends of the rod-shaped material 20 or the screw shaft 3 can be removed.

The ball screw 1 shown in FIG. 2 was manufactured by combining the hollow screw shaft 3 manufactured as described above, the nut 5 and the ball 9 manufactured by a conventional method. Since the ball screw 1 includes a hollow screw shaft 3, the ball screw 1 is lightweight.
As a ball circulation type using a ball circulation path, a tube type, a top type, etc. are common, but a part of the inner peripheral surface of the nut is recessed to form a groove, and this groove is circulated through the ball. You may employ | adopt the ball circulation form used as a path. In the case of tube type and top type, separate members (return tube, top) that make up the ball circulation path are attached to the nut, but in the circulation type with the concave groove as the ball circulation path, the ball is integrated with the nut. Since the circulation path is formed, it is not necessary to attach another member to the nut.

  Therefore, the effort and cost of attaching another member to the nut can be reduced. Further, it is not necessary to provide a sleeve on the outer peripheral portion of the frame so as to prevent the frame from falling off the nut. In addition, since there is no top hole for attaching a top to the nut, even if another member such as a bearing is press-fitted to the outer peripheral surface of the nut and fitted, it is difficult for the nut to be deformed and the reduction in roundness can be suppressed. it can.

  Here, the structure and operation of the ball circulation type ball screw 1 having the concave groove 13 as the ball circulation path 11 will be described with reference to FIGS. As shown in FIG. 2, the ball screw 1 includes a hollow screw shaft 3 having a helical screw groove 3a on the outer peripheral surface, and a helical screw groove 5a facing the screw groove 3a of the screw shaft 3 on the inner peripheral surface. And a plurality of balls 9 movably loaded in a spiral ball rolling path 7 formed by both screw grooves 3a, 5a, and the ball 9 from the end point of the ball rolling path 7 to the starting point. And a ball circulation path 11 for return circulation.

  The ball circulation path 11 is integrally formed on the inner peripheral surface of the nut 5. More specifically, the concave groove 13 formed by recessing a part of the cylindrical inner peripheral surface of the nut 5 by, for example, forging or cutting is used as the ball circulation path 11. Therefore, unlike the case of a ball circulation type such as a tube type or a piece type, another member constituting the ball circulation path 11 is not attached. Since the separate member is not used, there is no possibility that a step having an edge portion, which occurs at the boundary portion between the two members when the separate member is used, is generated.

  In addition, the ball circulation path 11 (concave groove 13) is linear at both ends that are connected to the ball rolling path 7 (screw groove 5a), and an intermediate portion located between the both ends is curved. It has become a shape. Both ends of the intermediate portion and the both end portions are smoothly connected, and the overall shape of the ball circulation path 11 (concave groove 13) when viewed from the center of the nut 5 is substantially S-shaped. ing. However, the overall shape of the ball circulation path 11 is not limited to a substantially S shape.

  Since such a ball circulation path 11 is provided, the ball 9 that travels around the screw shaft 3 while moving in the ball rolling path 7 and reaches the end point of the ball rolling path 7 is one of the ball circulation paths 11. , And is scooped up into the ball circulation path 11 (intermediate part) from the vicinity of the boundary between the end part and the intermediate part and sinks into the nut 5 (radially outward). Then, it passes over the land portion of the screw shaft 3 (the thread of the screw groove 3a) through the intermediate portion of the ball circulation path 11 and reaches the other end of the ball circulation path 11, from which the starting point of the ball rolling path 7 is reached. Returned to

  When such a ball screw 1 is rotated relative to a nut 5 and a screw shaft 3 that are screwed to the screw shaft 3 via a ball 9, the screw shaft 3 and the nut 5 are moved via the rolling of the ball 9. And move relative to each other in the axial direction. An endless ball path is formed by the ball rolling path 7 and the ball circulation path 11, and the ball 9 circulates infinitely in the ball path. Therefore, the screw shaft 3 and the nut 5 are continued. Relative movement.

  The cross-sectional shape of the ball circulation path 11 (the cross-sectional shape when cut along a plane perpendicular to the longitudinal direction of the ball circulation path 11) may be an arc shape (single arc shape) or a gothic arc shape. Further, the cross-sectional shape of the screw grooves 3a and 5a (the cross-sectional shape when cut along a plane perpendicular to the longitudinal direction of the screw grooves 3a and 5a) may be arcuate (single arc) or gothic arc-shaped. . Further, the ball circulation path 11 and the ball rolling path 7 are smoothly connected. That is, the ball circulation path 11 and the ball rolling path 7 are arranged so that the locus of contact between the ball 9 and the inner surface of the concave groove 13 and the locus of contact between the ball 9 and the inner surface of the screw groove 5a are smoothly continuous. Is connected. As a result, the ball 9 circulates smoothly in the ball passage.

  A ball screw unit combining the ball screw 1 of the present embodiment and various members (corresponding to the inclusion, which is a constituent element of the present invention, and may also be referred to as “inclusion” hereinafter). It can be. In the ball screw unit, the member is included in the hollow portion of the screw shaft 3 of the ball screw 1. Although the kind of said member is not specifically limited, Apparatuses, such as a motor, a sensor, an encoder, machine elements, such as a bearing, a bush, and a shaft, and a damping material (vibration absorption material) are mention | raise | lifted. These members may be used alone or in combination of two or more.

Such a ball screw unit is lightweight because the screw shaft 3 is hollow. Further, since the member provided in the ball screw unit is arranged not in the outer peripheral portion of the nut 5 but in the hollow portion of the screw shaft 3, the size is small. Furthermore, since the ball screw unit is light and small and has a small radial dimension, it is hardly affected by centrifugal force or the like and has a small inertia (inertia). As a result, the ball screw unit of the present embodiment has high-precision positioning performance and high responsiveness.
Applications of the ball screw 1 and the ball screw unit of the present embodiment are not particularly limited, but can be suitably used for automobile parts (particularly, brake actuators for automobiles), positioning devices, and the like.

Hereinafter, the ball screw unit of the present embodiment will be described more specifically.
[First embodiment]
4 is a side view of the ball screw unit of the first embodiment, and FIG. 5 is a cross-sectional view taken along the line AA of the ball screw unit of FIG.
A fixed shaft 32 is coaxially inserted into the hollow portion of the screw shaft 3 of the ball screw 1 described above. Further, rolling bearings 30 and 30 are arranged at both axial ends in the hollow portion of the screw shaft 3, respectively, and the outer diameter surface of the rolling bearing 30 is fitted to the inner peripheral surface of the screw shaft 3. Is fitted to the outer peripheral surface of the fixed shaft 32. Thereby, the screw shaft 3 is supported so as to be rotatable with respect to the fixed shaft 32.

  Further, a rotor 34 made of, for example, a magnet is fixed to the inner peripheral surface of the screw shaft 3 by conventional fixing means such as press-fitting and bonding. In addition, a stator 36 is fixed to the outer peripheral surface of the fixed shaft 32 so as to face the rotor 34, and a motor 38 is configured by the rotor 34 and the stator 36. When the motor 38 is driven, the screw shaft 3 rotates and the nut 5 moves linearly in the axial direction. In this ball screw unit, the rolling bearing 30, the fixed shaft 32, and the motor 38 are inclusions.

  The ball screw unit of the first embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small). Furthermore, since the rolling bearing conventionally fitted to the outer peripheral surface of the screw shaft end for supporting the screw shaft is included in the hollow portion of the screw shaft 3, the axial dimension is also reduced. . Further, since the screw shaft 3 can be directly rotated by the motor 38 without using a gear or the like, the ball screw unit of the first embodiment has no torque transmission loss. Also, regeneration is possible during reverse rotation. Furthermore, the rotation angle can be detected.

[Second Embodiment]
In a ball screw unit including the ball screw 1, an internal gear (not shown) is formed on the inner peripheral surface of the screw shaft 3 along the circumferential direction. A motor (not shown) is contained in the hollow portion of the screw shaft 3, and an external gear that meshes with the internal gear is attached to the rotation shaft of the motor by conventional fixing means such as press-fitting and irregular fitting. It has been. When this motor is driven, the screw shaft 3 rotates and the nut 5 moves linearly in the axial direction. In this ball screw unit, the motor and the both gears are inclusions.
The ball screw unit of the second embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small).

[Third embodiment]
In a ball screw unit including a ball screw 1 of a type in which a screw shaft 3 rotates and a nut 5 moves linearly, an encoder (not shown) is attached to the inner peripheral surface of the screw shaft 3 and a sensor for detecting a signal from the encoder (Not shown) was enclosed in the hollow portion of the screw shaft 3. In this ball screw unit, the encoder and the sensor are inclusions.
The ball screw unit of the third embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small). Further, the rotation of the screw shaft 3 can be detected by the sensor, and the rotation speed, the rotation direction, etc. can be measured. If the control unit for controlling the rotation of the screw shaft 3 is also included in the hollow portion of the screw shaft 3, the ball screw unit can be further downsized.

[Fourth embodiment]
In a ball screw unit including a ball screw 1 of a type in which a nut 5 rotates and a screw shaft 3 moves linearly, a displacement sensor (not shown) that detects linear movement of the screw shaft 3 is included in a hollow portion of the screw shaft 3. . In this ball screw unit, the displacement sensor is an inclusion.
The ball screw unit of the fourth embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small). Further, the displacement of the screw shaft 3 can be detected by a displacement sensor, and the moving speed, moving direction, and the like can be measured.

[Fifth embodiment]
In a ball screw unit including a ball screw 1 of the type in which the screw shaft 3 rotates and the nut 5 moves linearly, a balancer (not shown) that maintains the dynamic balance of the screw shaft 3 is attached to the inner peripheral surface of the screw shaft 3. In this ball screw unit, the balancer is an inclusion.
The ball screw unit of the fifth embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small). Further, since the balance of the dynamics of the screw shaft 3 can be kept good by the balancer, the sound and vibration generated when the ball screw 1 is driven are suppressed. Further, as a result of the reduction of vibration, the life of the ball screw 1 is extended.

[Sixth embodiment]
In the ball screw unit including the ball screw 1, a damping material (not shown) is included in the hollow portion of the screw shaft 3. The damping material may be filled in the entire hollow portion of the screw shaft 3 or may be disposed in part. In this ball screw unit, the damping material is an inclusion.
The ball screw unit of the sixth embodiment is lightweight because the hollow screw shaft 3 is used. Moreover, since the inclusion is enclosed in the hollow part of the screw shaft 3, it is small (especially the dimension of radial direction is small). Furthermore, since the eigenvalue of the screw shaft 3 changes depending on the damping material, it is possible to optimize the acoustic characteristics of the ball screw 1 (the sound generated when the ball screw 1 is driven can be reduced). Further, since the vibration generated when the ball screw 1 is driven by the damping material is reduced, the life of the ball screw 1 is extended.

[Seventh embodiment]
FIG. 6 is a side view of the ball screw unit of the seventh embodiment, and FIG. 7 is a cross-sectional view taken along the line BB of the ball screw unit of FIG.
A support shaft 40 is coaxially inserted into the hollow portion of the screw shaft 3 of the ball screw 1 described above. The support shaft 40 may be a solid shaft as shown in FIG. 7, but may be a hollow shaft. In addition, annular bushes 42 and 42 are disposed at both axial ends in the hollow portion of the screw shaft 3, respectively, and are fitted into annular grooves 44 and 44 formed on the inner peripheral surface of the screw shaft 3. .

  Since the inner peripheral surface of the bush 42 is in sliding contact with the outer peripheral surface of the support shaft 40, the screw shaft 3 is supported by the support shaft 40 via the bush 42 so as to be linearly movable. Therefore, when the nut 5 is rotated, the screw shaft 3 moves relative to the support shaft 40 in the axial direction. In this ball screw unit, the support shaft 40 and the bush 42 are inclusions.

The ball screw unit of the seventh embodiment is lightweight because the hollow screw shaft 3 is used. Further, since the inclusion is contained in the hollow portion of the screw shaft 3, the screw shaft 3 is small (particularly in the radial direction).
If the support shaft 40 is made of a material having excellent slidability, or the outer peripheral surface of the support shaft 40 is coated with a film having excellent slidability, the bush 42 can be omitted, and the screw shaft 3 and the outer peripheral surface of the support shaft 40 can be configured to be in sliding contact with each other. By doing so, the inclination of the screw shaft 3 due to an uneven load or the like is suppressed, so that the occurrence of radial galling is suppressed and the life of the ball screw 1 is extended. Moreover, forward / reverse operation efficiency is improved by suppressing the occurrence of twisting.

  As shown in FIGS. 7 and 8, a linear groove 40 a extending in the axial direction across both ends of the support shaft 40 is provided on the outer peripheral surface of the support shaft 40, and a radial direction is provided on the inner peripheral surface of the screw shaft 3. You may provide the convex part 46 which protrudes toward inward and the front-end | tip part is distribute | arranged in the groove | channel 40a. In the example of FIGS. 7 and 8, the convex portions 46 are respectively provided at both axial ends of the inner peripheral surface of the screw shaft 3. When the convex portion 46 is provided, the bush 42 is a member having a substantially annular C-shaped cross section having a slit extending in the axial direction (cross section cut by a plane orthogonal to the axial direction). The bush 42 is attached so that the convex portion 46 is disposed in the slit.

  With such a configuration, since the convex portion 46 is disposed in the groove 40 a and in the slit of the bush 42, the screw shaft 3 and the bush 42 are prevented from rotating relative to the support shaft 40. When the screw shaft 3 is manufactured, the convex portion 46 may be integrally formed on the inner peripheral surface of the screw shaft 3 (see FIG. 7), or the convex portion 46 separate from the screw shaft 3 may be formed as a screw shaft. You may fix to the inner peripheral surface of 3 by press-fitting etc.

DESCRIPTION OF SYMBOLS 1 Ball screw 3 Screw shaft 3a Thread groove 5 Nut 5a Thread groove 7 Ball rolling path 9 Ball 11 Ball circulation path 13 Concave groove 20 Rod material 30 Rolling bearing 32 Fixed shaft 34 Rotor 36 Stator 38 Motor 40 Support shaft 42 Bush

Claims (6)

  A method of manufacturing a hollow screw shaft used for a ball screw, a thread groove forming step of forming a spiral thread groove by rolling the outer peripheral surface of a rod-shaped material, and a rod shape in which the thread groove is formed A method of manufacturing a screw shaft, comprising: a heat treatment step of performing a heat treatment on an outer peripheral surface of the material; and a boring step of forming a tube by subjecting the heat-treated rod-shaped material to a boring process.   The method for manufacturing a screw shaft according to claim 1, further comprising an end portion removing step for removing an end portion of the rod-shaped material between the heat treatment step and the boring step.   The screw shaft manufacturing method according to claim 1, further comprising a thread groove removing step of removing an end of the thread groove between the heat treatment step and the boring step or after the boring step.   A hollow screw shaft used for a ball screw and having a helical thread groove on an outer peripheral surface, the hardness of the thread groove surface is more than Hv600, and the hardness of the inner peripheral surface is more than Hv180 and less than Hv400, A screw shaft characterized by having a wall thickness of 0.65 mm or more and 50% or less of the inner radius. A hollow screw shaft having a spiral screw groove on the outer peripheral surface, a nut having a screw groove on the inner peripheral surface facing the screw groove of the screw shaft, and a spiral ball rolling path formed by the both screw grooves A ball screw including a plurality of balls loaded in a freely rolling manner, and an inclusion disposed in a hollow portion of the screw shaft,
The screw groove surface of the screw shaft has a hardness exceeding Hv600, the hardness of the inner peripheral surface is more than Hv180 and less than Hv400, and the wall thickness is 0.65 mm or more and 50% or less of the inner radius. Ball screw unit to be used.   The ball screw unit according to claim 5, wherein the inclusion is at least one of a motor, a sensor, an encoder, a damping material, a bearing, a bush, and a shaft.

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