JP2010078060A - Roller type ball screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly reliable roller type ball screw capable of smoothly circulating a ball, by restraining the occurrence of looseness of a piece member, by preventing abrasion of a caulking member in the piece member. <P>SOLUTION: The piece member 5 has a body part 10 molded by MIM from a sintered alloy and having a cross section formed in a substantially circular shape in response to a piece window 6, and a head part 11 extending outward in the radial direction of a nut 3 from this body part 10. A pair of arms 9 are projected on both sides of the body part 10, and the piece member 5 is positioned by engaging these arms 9 with a screw groove 3a of the nut 3. The head part 11 is formed with a recessed part 12 of recessing an outer diameter surface, and eight guide walls 13 rising from this recessed part 12 and having a circular arc-shaped peripheral surface, are formed in equal arrangement in the peripheral direction. The piece member 5 is fixed to the nut 3 by a caulking part 14 substantially orthogonal to a lead wire L of the screw groove 3a and formed by plastically deforming opposed respective two guide walls 13 among the eight guide walls 13 to the edge part side of the piece window 6. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a ball screw used for various machine tools such as an electric discharge machine and a tapping center, or an electric power steering or an actuator of an automobile, and more particularly to a piece type ball screw using a piece member that is a ball circulation part. .

  The ball screw is composed of a screw shaft having a helical thread groove formed on the outer peripheral surface, a nut externally fitted to the screw shaft, and a spiral screw groove formed on the inner peripheral surface, and both opposing screw grooves. It is equipped with a large number of balls accommodated in the formed rolling path and a circulation mechanism having the rolling path as a circuit path, and is used as a motion conversion mechanism that linearly moves the screw shaft by rotating the nut, for example. ing.

  Generally, there are various types of ball screws having different ball circulation mechanisms, and one of them is called a piece type. This piece-type ball screw has a threaded groove connection path, and a circulation piece member with a rolling path as a circulation path is attached to the nut. The structure is relatively simple and compact. There are advantages you can do.

  A typical example of such a piece-type ball screw is shown in FIG. This piece type ball screw is provided with a piece member fitting opening (piece window) 51 having an oval shape in the nut body 50a of the rotary nut 50 so as to penetrate the inner and outer peripheral surfaces. 52 is fitted from the inner diameter side.

  The piece member 52 is formed with a connecting groove 52a for connecting adjacent circumferential portions of the inner screw groove 53, and engages with the inner screw groove 53 of the rotary nut 50 so that the piece member 52 is pivoted with respect to the nut body 50a. A pair of arms 54 positioned in the direction are integrally provided. The pair of arms 54 are provided at both ends of the piece member 52 in the axial direction so as to protrude opposite to each other in the circumferential direction, and are formed in a semicircular cross-sectional shape that fits into the inner screw groove 53. Here, the portion of the inner screw groove 53 with which the arm 54 is engaged becomes a non-ball circulating portion.

  Both side edges in the circumferential direction of the rotary nut 50 in the piece member 52 are recessed portions 55 whose outer diameter surfaces are recessed from other portions, and rise from the recessed portions 55 to the outer diameter side and face the circumferential direction of the piece member 52. A pair of guide walls 56 are provided along the side surface. The opening 51 for fitting the piece member of the nut main body 50a is provided with an engaging step portion 57 at the opening edge of a pair of opposed inner side surfaces, and the width of the opening side portion is slightly wider than the engaging step portion 57. ing.

  The piece member 52 is fitted into the piece member fitting opening 51 of the nut main body 50a from the inner diameter side, the pair of arms 54 is engaged with the inner screw groove 53, and the guide wall 56 is plastically deformed to deform the nut main body 50a. Fixed to. The fixing by the plastic deformation is performed by caulking and fixing the guide wall 56 to a pair of opposed inner side surfaces of the piece member fitting opening 51. Specifically, the piece wall 52 is fixed with certainty by crimping the guide wall 56 to the engagement step portion 57 to engage with each other. The piece member 52 with the arm 54 is used to prevent the piece member 52 from being detached and positioned by the arm 54, to obtain high-precision and high-strength performance, and to easily and reliably fix the piece member 52. (For example, refer to Patent Document 1).

  However, in such a conventional piece type ball screw, the arm 54 of the piece member 52 is prevented from being detached and positioned, and when processed accurately, the arm 54 engages with the inner thread groove 53 of the nut 50. However, when the curvature of the arm 54 changes due to the processing error of the piece member 52, a gap is formed between the arm 54 and the inner groove 53 as shown in FIG. At the same time, the tip of the arm 54 may come into contact with the edge to increase the surface pressure and cause wear. Due to such wear of the arm 54, there is a possibility that looseness occurs between the crimping portion 60 of the piece member 52 and the piece member fitting opening 51, resulting in poor circulation of the ball 59.

  As a solution to these problems, a piece member 61 as shown in FIG. 7 is known. The arm 62 of the piece member 61 is formed in an arc shape along the screw groove 53 of the nut 50, and the curvature radius R ′ of the arc along the width center of the arm 62 is the curvature radius of the groove center of the screw groove 53. A position where the arm 62 and the inner surface of the screw groove 53 are in contact with each other with no load is defined as the tip of the arm 62.

In this type of structure for preventing the piece member 62 from falling off, the arm 62 receives a pulling load applied to the connecting groove 61 a of the piece member 61. The unloading load in this case is due to centrifugal force due to the revolution of the ball 59 or due to radial component force when the ball 64 rides on the inner surface of the thread groove 53 of the nut 50 from the connecting groove 61a of the piece member 61. Can be mentioned. When such a falling load is received, the arm 62 is deformed to cause poor circulation of the ball 59 or to reduce the reliability of the piece member 61 from dropping from the nut 50. When a load is applied to the piece member 61 from the loaded state, the arm 62 is elastically deformed so that the entire arm 62 comes into contact with the inner surface of the thread groove 53 of the nut 50, and the contact area between the two increases. Become. Therefore, excessive plastic deformation of the arm 62 is prevented, and the rigidity against the pull-out force of the piece member 61 can be improved, and poor circulation of the ball 59 due to deformation of the arm 62 can be suppressed (see, for example, Patent Document 2). ).
JP 2001-289301 A JP 2006-189124 A

  However, such a conventional fixing method of the piece member 52 is, for example, as shown in FIG. 6, by an arm 54 formed integrally with the piece member 52 and a caulking portion 60 protruding outside the piece member 52. The nut 50 is fixed to the nut 50 in a state of being sandwiched between the inner diameter side and the outer diameter side. In this case, for the convenience of assembly, there is a mounting clearance between the piece member fitting opening 51 and the piece member 52 on the nut 50 side. For this reason, even if the piece member 52 is fixed to the nut 50 by caulking (plastic deformation) at the time of assembly, the piece member 52 is continuously connected to the piece member 52 by the unloading load repeatedly applied by the balls 64 passing through the piece member 52. Vibration occurs. Therefore, even if the wear on the arm 54 side is prevented, if the strength of the crimping portion 60 is not sufficient, the wear of the crimping portion 60 is developed by continuous vibration, and the piece member 52 is rattled. .

  The present invention has been made in view of such circumstances, and prevents wear of the caulking portion in the piece member, suppresses the occurrence of rattling of the piece member, and provides high reliability in which smooth ball circulation is obtained. The purpose is to provide a piece-type ball screw.

  In order to achieve such an object, the invention according to claim 1 of the present invention includes a screw shaft having a spiral thread groove formed on the outer peripheral surface thereof, and is externally fitted to the screw shaft and spirally formed on the inner peripheral surface. And a plurality of balls accommodated in a rolling path formed by opposing screw grooves, and a piece window drilled in a body portion of the nut, and the rolling A piece-type ball screw including a piece member having a connection groove having a circulation path as a circulation path, the piece member being inserted into the piece window, and a radial direction of the nut from the case portion And a pair of arms projecting from both sides of the body portion, and the piece members are positioned by engaging the arms with screw grooves of the nut, and the head portion In addition, a recessed portion whose outer diameter surface is recessed more than other portions is formed, rising from this recessed portion, At least four guide walls having a peripheral surface are formed at equal intervals in the circumferential direction, and among the plurality of guide walls, each of the opposing guide walls that are substantially orthogonal to and opposed to the lead wire of the thread groove in the nut is the piece window. The piece member is fixed to the nut by a caulking portion formed by plastic deformation on the edge side of the nut.

  Thus, in the piece-type ball screw having the piece member that is fitted to the piece window formed in the trunk portion of the nut and has the connection groove having the rolling path as the circulation path, the piece member is It has a body part that is inserted into the window and a head part that extends from the body part to the radially outward side of the nut, and a pair of arms projecting from both sides of the body part, and these arms are engaged with the thread grooves of the nut. In addition to positioning the piece member, the head is formed with a recessed portion whose outer diameter surface is recessed from the other portion, and at least four guide walls having an arcuate circumferential surface rising from the recessed portion. A caulking formed in the circumferential direction at equal intervals, and formed by plastically deforming the opposing guide walls on the edge side of the piece window, substantially perpendicular to the lead wire of the thread groove of the nut among these guide walls Since the piece member is fixed to the nut by the It is possible to easily optimize the position of the door wall, improve workability, reduce the generation of stress that is loaded on the crimping portion when the ball passes and tilts the piece member, It is possible to provide a highly reliable piece-type ball screw that prevents wear of the caulking portion, suppresses the occurrence of rattling of the piece member, and provides a smooth circulation of the ball.

  Preferably, if a slit extending in the radial direction is formed in advance in the guide wall as in the invention described in claim 2, the plastic deformation can be made smooth and the caulking load can be reduced. It is possible to improve the performance.

  According to a third aspect of the present invention, eight guide walls are formed, and two opposing guide walls of the eight guide walls are plastically deformed so that the caulking portion is formed. If formed, the position of the guide wall can be further optimized without reducing the strength of the crimped portion.

  Further, as in the invention according to claim 4, the inclination angle of the edge portion of the piece window is set in a range of 20 to 45 °, and the guide wall is in plastic contact with the edge portion and is plastically deformed. If the caulking portion is formed, it is possible to prevent the caulking portion from being cracked during plastic deformation, and the spring back of the caulking portion causes play in the piece member, and the positioning accuracy and fixing force Therefore, the reliability of the caulking portion can be improved by securing a desired fixing force.

  Moreover, if the piece window is formed in a substantially circular cross section and the piece member is formed in a circular shape corresponding to the piece window as in the invention described in claim 5, the man-hour for processing the piece window is reduced. The cost can be reduced and the cost can be reduced, and the piece member can be easily and accurately positioned in the axial direction with respect to the nut, and the reliability can be improved.

  Further, as in the invention described in claim 6, if the piece member is formed from a sintered alloy by MIM, even if it has a high workability and a complicated shape, the desired shape / Can be molded to dimensions.

  In addition, as in the invention described in claim 7, if the piece member includes a metal powder made of SUS630 and the surface hardness is set in a range of 20 to 33 HRC by solution heat treatment, toughness and high hardness are achieved. Can be secured.

  Further, as in the invention according to claim 8, the piece member includes a metal powder made of a carburized material, and is hardened by carburizing and quenching so that the surface hardness is in a range of 30 to 40 HRC, and the high-frequency temper is used. If the guide wall is tempered and the surface hardness is set in the range of 15 to 30 HRC, the toughness of the guide wall serving as the crimping portion is increased and cracking occurs when the piece member is crimped and fixed to the nut. And the like can be prevented.

  The piece ball screw according to the present invention includes a screw shaft having a helical thread groove formed on the outer peripheral surface, a nut externally fitted to the screw shaft, and a spiral screw groove formed on the inner peripheral surface, A plurality of balls accommodated in a rolling path formed by opposing screw grooves, and a connecting groove that is fitted to a piece window drilled in a body portion of the nut and has the rolling path as a circulation path. In the piece-type ball screw provided with the formed piece member, the piece member includes a body portion that is inserted into the piece window, and a head portion that extends from the body portion to the radially outward side of the nut. A pair of arms projecting on both sides of the body, and the piece members are positioned by engaging the arms with the thread grooves of the nut, and the outer surface of the head is more external than the other parts. Is formed with a dent, and rises from the dent and has at least four arcuate peripheral surfaces. Guide walls are formed in a circumferentially equidistant manner, and among these guide walls, the guide walls that are substantially orthogonal to the lead wires of the screw grooves in the nut and that are opposed to each other are plastically deformed to the edge side of the piece window. Since the piece member is fixed to the nut by the caulking portion formed in this manner, the position of the guide wall that becomes the caulking portion can be easily optimized, and workability can be improved. It is possible to reduce the occurrence of stress that is applied to the caulking part when the ball passes and tilts the piece member, prevents wear of the caulking part, suppresses the occurrence of rattling of the piece member, and smoothly circulates the ball. It is possible to provide an obtained highly reliable piece type ball screw.

  A screw shaft having a spiral thread groove formed on the outer peripheral surface, a nut externally fitted to the screw shaft and having a spiral thread groove formed on the inner peripheral surface, and a roller formed by both opposing screw grooves. A piece comprising a plurality of balls accommodated in a movement path, and a piece member that is fitted to a piece window formed in the trunk portion of the nut and has a connecting groove having the rolling path as a circulation path. In the type ball screw, the piece member is formed from a sintered alloy by MIM, and a body portion having a substantially circular cross section corresponding to the piece window, and a radially outer side of the nut from the body portion And a pair of arms projecting on both sides of the body, and the pieces are positioned by engaging the arms with the threaded grooves of the nut, A dent with an outer surface that is recessed from the part is formed, rising from this dent, and arcuate Eight guide walls having a surface are formed in a circumferentially equidistant manner, and among these eight guide walls, two guide walls that are substantially orthogonal to the lead wire of the thread groove in the nut and that face each other The piece member is fixed to the nut by a caulking portion formed by plastic deformation on the edge side of the window.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
1A and 1B show an embodiment of a piece-type ball screw according to the present invention, in which FIG. 1A is a plan view, FIG. 2B is a longitudinal sectional view, FIG. 2A is a sectional perspective view of a nut, and FIG. FIG. 3 is a perspective view showing a piece member, and FIG. 4 is a plan view showing a fixed state of the piece member according to the present invention.

  This piece-type ball screw 1 includes a screw shaft 2 having a helical thread groove 2a formed on the outer peripheral surface, and a nut externally fitted to the screw shaft 2 and having a helical screw groove 3a formed on the inner peripheral surface. 3, a large number of balls 4 accommodated in a rolling path formed by both opposing screw grooves 2 a and 3 a, and a piece member 5 serving as a circulation member for these balls 4.

  The cross-sectional shape of each thread groove 2a, 3a may be a circular arc shape or a Gothic arc shape, but here, it has a Gothic arc shape that allows a large contact angle with the ball 4 and a small axial clearance. Is formed. Thereby, the rigidity with respect to an axial load becomes high and generation | occurrence | production of a vibration can be suppressed.

  The cylindrical nut 3 has a generally circular piece window 6 having a substantially circular cross section that penetrates the inner and outer peripheral surfaces and cuts out a part of the screw groove 3a. A circular piece member 5 is fitted. A connecting groove 5a is formed on the inner side of the piece member 5 so as to connect adjacent ones of the screw grooves 3a, and the ball 4 rolls between the connecting groove 5a and the substantially one part of the screw groove 3a. Constitutes the road. A large number of balls 4 interposed between the inner and outer screw grooves 2a and 3a in the rolling path roll along the screw grooves 2a and 3a, and are guided by the connecting groove 5a of the piece member 5 to be screw shafts. 2 over the thread 2 and return to the adjacent thread groove 3a, and roll along the thread grooves 2a and 3a again. Here, although the cross section of the piece member 5 is exemplified from the viewpoint of workability of the piece window 6, the shape is not limited to this, and may be, for example, an oval shape.

  As shown in FIG. 2A, the connecting groove 5a of the piece member 5 is formed to be curved in an S shape so as to smoothly connect the adjacent screw grooves 3a, 3a of the nut 3. Therefore, as shown in FIG. 2B, the connecting window 5a is connected to the screw groove 3a so that both the frame window edges 7 and 7 are aligned with the groove edge 8 of the adjacent screw groove 3a of the nut 3. ing. The depth of the connecting groove 5a is set to a depth at which the ball 4 can exceed the thread of the screw groove 2a in the screw shaft 2 within the connecting groove 5a. Further, an arm 9 having a round bar shape with a substantially circular cross section is provided on both sides of the piece member 5 and is engaged with the thread groove 3a of the nut 3 via a predetermined radial clearance. The arm 9 positions the piece member 5 in the axial direction with respect to the nut 3, and prevents the piece member 5 from coming out of the piece window 6 radially outward.

  In this embodiment, the piece member 5 is made of a sintered alloy prepared by adjusting a metal powder into a plastic shape and molding it with an injection molding machine. In this injection molding, first, metal powder and a binder made of plastic and wax are kneaded by a kneader, and the kneaded product is granulated into pellets. The granulated pellets are molded by so-called MIM (Metal Injection Molding), which is supplied to a hopper of an injection molding machine and pushed into a mold in a heated and melted state. A sintered alloy formed by such an MIM can be easily and accurately formed into a desired shape / dimension even if it has a high workability and a complicated shape.

  As the metal powder, a material that can be subsequently carburized and hardened, for example, C (carbon) is 0.13 wt%, Ni (nickel) is 0.21 wt%, Cr (chromium) is 1.1 wt%, Cu (copper) Exemplifies SCM415 which is 0.04 wt%, Mn (manganese) is 0.76 wt%, Mo (molybdenum) is 0.19 wt%, Si (silicon) is 0.20 wt%, and the rest is Fe (iron). Can do. The piece member 5 is performed by adjusting the carburizing quenching and tempering temperatures. In addition to this, the material of the piece member 5 contains 3.0 to 10.0 wt% of Ni, and is excellent in workability and corrosion resistance (FEN8 of Japan Powder Metallurgy Industry Standard), or C is 0.1. It may be a precipitation hardening stainless steel SUS630 made of 07 wt%, Cr 17 wt%, Ni 4 wt%, Cu 4 wt%, and the rest made of Fe or the like. This SUS630 can appropriately increase the surface hardness in the range of 20 to 33 HRC by solution heat treatment, and can ensure toughness and high hardness.

  When the piece member 5 is formed of a carburized material such as SCM415, the piece member 5 is hardened by carburizing and quenching and tempering temperature adjustment, or by carburizing and quenching so that the surface hardness is in the range of 30 to 40 HRC. At the same time, the caulking portion 14 shown in FIG. 2B, that is, the guide wall 13 shown in FIG. 3 is tempered using the high-frequency temper device, and the hardness is set to be in the range of 15 to 30 HRC. . As a result, the toughness of the guide wall 13 that becomes the crimping portion 14 is increased, and it is possible to prevent cracks and the like from occurring when the piece member 5 is crimped and fixed to the nut 3.

  The ball 4 is assembled by attaching the piece member 5 to the piece window 6 of the nut 3 from the inner diameter side of the nut 3 and then applying the nut 3 from the shaft end of the screw shaft 2 so that the ball 4 is placed between the screw grooves 2a and 3a. The nut 3 is rotated while being sequentially inserted into the screw shaft 1, and the nut 3 is moved to the screw shaft 1. In addition, after the piece member 5 is mounted on the piece window 6 of the nut 3, the ball 4 may be similarly inserted using a temporary shaft.

  Here, as shown in FIG. 3, the piece member 5 includes a trunk portion 10 on which arms 9, 9 project, and a head extending from the trunk portion 10 to the outer diameter side (the radially outer side of the nut 3). Part 11. The trunk portion 10 is formed in a circular cross section so as to correspond to the shape (circular hole) of the piece window 6 of the nut 3 and is fitted to the piece window 6. Further, the head portion 11 is formed with a recessed portion 12 whose outer diameter surface is recessed compared to other portions, and the eight guide walls 13 having an arcuate peripheral surface rise from the recessed portion 12 toward the outer diameter side in the circumferential direction. It is formed equally. Thereby, optimization of the position of the guide wall 13 used as the crimping part 14 mentioned later can be aimed at easily, and workability | operativity can be improved.

  The piece member 5 is fitted and inserted into the piece window 6, and after engaging the arms 9 and 9 with the thread groove 3 a of the nut 3, the guide wall 13 is attached to the piece window 6 using a crimping jig (not shown). The piece member 5 is fixed to the nut 3 by a caulking portion 14 formed by plastic deformation until it is in close contact with the edge 6a side (see FIG. 2B). Further, the edge portion 6a of the piece window 6 is formed in a tapered shape having a predetermined inclination angle θ, and is set in a range of 20 to 45 °. If the amount of crimping further increases, cracking occurs in the crimped portion 14 and the reliability is impaired. If the inclination angle θ is less than 20 °, the piece member 5 is loose due to the springback of the crimped portion 14. This causes a problem in positioning accuracy and fixing force, which is not preferable.

  In the present embodiment, as shown in FIG. 4, eight guide walls 13 are formed on the head portion 11 of the piece member 5 at equal intervals in the circumferential direction, and among these eight guide walls 13, the nut 3 A caulking portion 14 is formed by plastically deforming two guide walls 13 and 13 that are substantially orthogonal to the lead wire L of the screw groove 3a and that are opposed to each other in the direction indicated by the arrows in the figure. As a result, it is possible to reduce the occurrence of stress that is applied to the caulking portion 14 when the ball passes and tilts the piece member 5, prevents wear of the caulking portion 14, and suppresses the occurrence of rattling of the piece member 5. Therefore, it is possible to provide a highly reliable piece-type ball screw that can obtain a smooth circulation of the ball. The caulking portion 14 is not limited to the two guide walls 13 and 13 facing each other, but may be formed by plastic deformation of all eight, but this increases caulking work and increases manufacturing costs. At the same time, there is a possibility that rattling may occur in the caulking portion 14 that has already been plastically deformed in the caulking step, which is not preferable.

  Here, the number of guide walls 13 to be the caulking portions 14 is eight, but is not limited to this, and at least four or more are preferable. That is, with the three, the guide wall 13 is not disposed at the opposite fixed portion, and when the number is nine or more, the rigidity of the guide wall 13 itself may be reduced depending on the size of the ball screw, and the strength may be reduced. Because there is. Although not shown, by forming slits extending in the radial direction in advance in the guide wall 13 serving as the caulking portion 14, plastic deformation can be made smooth and the caulking load can be reduced. It is possible to improve the performance.

  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 piece type ball screw according to the present invention can be applied to a piece type ball screw used for an electric actuator of an automobile or the like.

(A) is a top view which shows one Embodiment of the piece-type ball screw which concerns on this invention. (B) is a longitudinal cross-sectional view same as the above. (A) is a cross-sectional perspective view which shows the nut which concerns on this invention. (B) is a cross-sectional view same as the above. It is a perspective view which shows the piece member which concerns on this invention. It is a top view which shows the fixed state of the piece member which concerns on this invention. (A) is a longitudinal cross-sectional view which shows the conventional piece type | formula ball screw. (B) is a cross-sectional view of the same as above. It is principal part sectional drawing which shows the fixed state of the conventional piece member. It is principal part sectional drawing which shows the attachment state to the nut of the conventional piece member.

Explanation of symbols

1 ······ Poma type ball screw 2 ·············· Screw shafts 2a, 3a Nut 4 ... Ball 5 ... Piece member 5a ... Connection groove 6 ... Piece Window 6a ... Edge 7 ... Frame window edge 8 ... Groove edge 9 ... ··· Arm 9a ··································································· Portion 13 ························································································· ········· Opening 52, 61 for piece member fitting ··· Piece member 52a, 61a ··· Connection groove 53 ..... Internal thread grooves 54, 62 ..... Arm 55 ....... Recessed portion 56 ....... Guide wall 57 ... ··········· Step 58 ··········································································· ····· Lead wire R of screw groove ···················································· Radius θ ・ ・ ・ ・ ・ Inclined angle of edge

Claims (8)

A screw shaft having a helical thread groove formed on the outer peripheral surface;
A nut externally fitted to the screw shaft and having a spiral thread groove formed on the inner peripheral surface;
A plurality of balls housed in a rolling path formed by opposing screw grooves;
In a piece-type ball screw provided with a piece member that is fitted to a piece window drilled in the trunk portion of the nut and has a connection groove having the rolling path as a circulation path,
The piece member includes a trunk portion to be inserted into the piece window, and a head portion extending from the trunk portion to the radially outward side of the nut, and a pair of arms projecting from both sides of the trunk portion, The arm member is engaged with the thread groove of the nut to position the piece member, and the head is formed with a recessed portion whose outer diameter surface is recessed from the other portion, rising from the recessed portion, At least four guide walls having an arcuate circumferential surface are formed at equal intervals in the circumferential direction, and among the plurality of guide walls, each of the opposing guide walls that are substantially orthogonal to the lead wire of the thread groove in the nut is opposed to the piece. A piece-type ball screw, wherein the piece member is fixed to the nut by a crimping portion formed by plastic deformation on the edge side of the window.   The piece type ball screw according to claim 1, wherein a slit extending in a radial direction is formed in the guide wall in advance.   The piece according to claim 1 or 2, wherein eight guide walls are formed, and two of the eight guide walls are plastically deformed to form the crimped portion. Ball screw.   The angle of inclination of the edge of the piece window is set in a range of 20 to 45 °, and the caulking portion is formed by plastic deformation in a state where the guide wall is in close contact with the edge. 3. The piece type ball screw according to any one of 3 above.   The piece type ball screw according to any one of claims 1 to 4, wherein the piece window is formed in a substantially circular cross section, and the piece member is formed in a circular shape corresponding to the piece window.   The piece-type ball screw according to any one of claims 1 to 5, wherein the piece member is formed from a sintered alloy by MIM.   The piece type ball screw according to any one of claims 1 to 5, wherein the piece member includes a metal powder made of SUS630, and the surface hardness is set in a range of 20 to 33HRC by a solution heat treatment.   The piece member includes a metal powder made of a carburized material, and is hardened by carburizing and quenching so that the surface hardness is in a range of 30 to 40 HRC, and the guide wall is tempered by a high-frequency temper, and the surface hardness is 15 The piece type ball screw according to claim 1, which is set in a range of ˜30 HRC.

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