JP2016142378A - Top-type ball screw - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a top-type ball screw which can easily and stably fix a top member without causing a variation, and is reduced in cost by improving assembling workability.SOLUTION: In a top-type ball screw having a top member 5 which is fit to a top window 6 protrusively formed at a flank of a nut 3, and in which a connecting groove 5a with a rolling passage as a circulation route is formed, a pair of recesses 10, 10 which are bulged from an external peripheral face of the top window 6 are formed at an opening part of the top window 6 at an inside diameter side, the top member 5 is formed of a sintered metal which is molded of MIM, the top member 5 linearly extends so as to be inclined from a root part 9a which is curvedly formed at an edge of a bottom face 5b, a pair of tongue pieces 9, 9 are protrusively formed so that their tips protrude rather than an external peripheral face of the flank 12, and the tongue pieces 9, 9 are locked to the recesses 10, 10 in elastically-deformed states.SELECTED DRAWING: Figure 3

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 FIGS. This piece-type ball screw 51 includes a screw shaft 52 having a helical thread groove 52a formed on the outer peripheral surface, and a nut externally fitted to the screw shaft 52 and having a helical screw groove 53a formed on the inner peripheral surface. 53, a large number of balls 54 accommodated in a rolling path formed by the opposing screw grooves 52a and 53a, and a piece member 55 serving as a circulation member for these balls 54.

  A cylindrical window 53 having a substantially circular cross section is formed in the body of the cylindrical nut 53 so as to penetrate the inner and outer peripheral surfaces and cut out a part of the thread groove 53a. A piece member 55 serving as a circulation member for the ball 54 is fitted into the piece window 56. A connecting groove 55a for connecting adjacent one rounds of the screw groove 53a is formed inside the piece member 55.

  As shown in FIGS. 7A and 7B, a pair of arms 55b and 55b project from the opposite ends in the circumferential direction at both ends of the piece member 55 in the axial direction. The arms 55b and 55b are formed in a substantially circular cross section, and are engaged with the thread groove 53a of the nut 53 via a predetermined radial clearance, so that the positioning of the piece member 55 and the piece member 55 are arranged in the radial direction from the piece window 56. It is provided to prevent it from slipping out. Here, the portion of the thread groove 53a where the arms 55b and 55b are engaged becomes a non-ball circulating portion.

  The piece member 55 includes a body portion 55c from which arms 55b and 55b are projected, and a head portion 55d extending from the body portion 55c to the outer diameter side (the radially outer side of the nut 53). The trunk portion 55 c has a substantially circular cross section so as to correspond to the shape (circular hole) of the piece window 56 of the nut 53 and is fitted to the piece window 56.

  The head portion 55d is formed with concave portions 55e whose outer diameter surfaces are recessed relative to other portions on both sides (both sides in the circumferential direction of the nut 53), and rises from the concave portions 55e toward the outer diameter side and has a pair of arcuate circumferential surfaces. The guide walls 55f and 55f are arranged to face each other. The outer surfaces of the guide walls 55f and 55f have an R shape so as to correspond to the shape (circular hole) of the piece window 56. Further, the inner side surfaces of the guide walls 55f, 55f have an R shape that extends in an R shape around the axis of the piece member. The R-shaped portion on the inner side surface has a smaller R shape than the outer side surface, and the wall thickness of the guide walls 55f and 55f is substantially the same.

  When fixing the piece member 55, first, the piece member 55 is fitted into the piece window 56 of the nut 53 from the inner diameter side of the nut 53. Next, the arms 55 b and 55 b are engaged with the thread groove 53 a of the nut 53. In this state, a core (not shown) is put in the nut 53, and the caulking punch 57 is pushed in from the outer diameter side. The caulking punch 57 has a tapered pressurizing surface 57 a at its tip, and the pair of guide walls 55 f and 55 f of the piece member 55 are crimped so that the tip bends obliquely outward, and is fastened to the nut 53. It will be in an engaged state.

  Here, the processed surface 57a of the caulking punch 57 first makes point contact with the end portions 55g and 55g on the inner side surfaces of the guide walls 55f and 55f, as shown in FIG. When the caulking punch 57 is pushed downward in this state, the guide walls 55f and 55f are plastically deformed in all directions as indicated by arrows in FIG. 7B with the end portions 55g and 55g as the base points. . As described above, since the inner side surfaces of the guide walls 55f and 55f are formed in an R shape, the cross-sectional area of the guide walls 55f and 55f to be caulked is reduced, and the caulking can be fixed only by plastically deforming a smaller portion. As a result, the amount of spring back after plastic deformation due to caulking is reduced, and the backlash after caulking of the piece member 55 is reduced.

  In addition, although the processing surface 57a of the caulking punch of this example is tapered, the processing surface can be flat so as to face the guide walls 55f and 55f. Further, the machining surface 57a of the caulking punch can have an R dimension larger than the R dimension of the inner surface of the guide walls 55f and 55f. In any case, if the processed surface 57a of the caulking punch is initially in point contact with the end portions 55g and 55g of the inner surface of the guide walls 55f and 55f, when the caulking punch 57 is pushed downward, The guide walls 55f and 55f are plastically deformed in all directions as indicated by the arrows in FIG. 7B with the end portions 55g and 55g as base points. By doing so, the piece member 55 is stably fixed to the nut 53 (for example, refer to Patent Document 1).

JP 2008-057597 A

  In such a conventional piece-type ball screw 51, arms 55 b and 55 b are projected from both ends of the piece member 55, and these arms 55 b and 55 b are engaged with the thread groove 53 a of the nut 53. The positioning and the feature that the piece member 55 can be prevented from coming out radially outward from the piece window 56 are provided. However, it is necessary to incorporate the piece member 55 from the inwardly inclined side of the nut 53. When the size of the piece member 55 is small or the inner diameter of the nut 53 is small, the operation becomes difficult.

  Further, since the inner side surfaces of the guide walls 55f and 55f of the piece member 55 are formed in an R shape, the cross-sectional area of the guide walls 55f and 55f to be caulked is reduced, and the smaller portions can be caulked and fixed simply by plastic deformation. Therefore, the spring back amount after plastic deformation by caulking is reduced, and the backlash after caulking of the piece member 55 is reduced. However, with such caulking, there are individual differences in performance depending on the operator, and the posture of the piece member 55 with respect to the nut 53 may be deteriorated. Thereby, a level | step difference arises between the piece member 55 and the nut 53, and there exists a possibility of causing a rotation defect and a short life.

  The present invention has been made in view of such circumstances, and has been conceived of a so-called snap-fit piece member in which the piece member can be assembled from the outer diameter side of the nut and the piece member does not move without being caulked and fixed. An object of the present invention is to provide a piece-type ball screw that can easily and stably fix a piece member without variation, and that is improved in assembling workability and reduced in cost.

  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. A nut formed with a screw groove, a plurality of balls accommodated in a rolling path formed by the opposing screw groove, and a piece window formed in a body portion of the nut, and A piece-type ball screw provided with a connecting member having a connecting groove having a circulation path as a circulation path, and a pair of recesses bulged from the outer peripheral surface of the piece window at the opening on the inner diameter side of the piece window And a pair of tongue pieces projecting from the piece member, and these tongue pieces are locked in a state of being elastically deformed in the recess.

  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 formed with the connecting groove having the rolling path as the circulation path, the inner diameter side of the piece window A pair of recesses bulging from the outer peripheral surface of the piece window are formed in the opening of the piece, and a pair of tongue pieces project from the piece member, and the tongue pieces are elastically deformed into the recesses. Since it is locked, it is possible to provide a snap-fit piece member that does not move with respect to the piece window without caulking and fixing the piece member as in the prior art, and stabilizes the posture of the piece member with respect to the nut. Thus, it is possible to prevent a step from being generated between the piece window and the piece member to be easily and stably fixed without variation.

  Preferably, as in the invention described in claim 2, the tongue piece of the piece member extends in a linearly inclined manner from a root portion formed by bending at the edge portion of the bottom surface, and the tip portion thereof is a trunk portion. If it is formed so as to protrude from the outer peripheral surface, the assembling workability can be improved and the cost can be reduced.

  According to a third aspect of the present invention, the tongue piece of the piece member extends in a straight line from a root portion formed by bending at the edge portion of the bottom surface, and the tip portion has an arc shape and the main body. If it is formed so that it protrudes from the outer peripheral surface of the body part, it can improve the workability of assembling and increase the resistance to elastic deformation due to assembling, and insert the piece window. After that, it is possible to improve the catching property to the recess of the piece window, and to fix it easily and more stably without variation.

  According to a fourth aspect of the present invention, a flat surface is formed on the opposed inner peripheral surface of the piece window, and a flat portion corresponding to the flat surface is formed on the piece member. If the flat portion of the piece member is engaged with the flat surface of the piece, the piece member can be easily positioned with respect to the nut so that the connecting groove of the piece member is connected to match the screw groove of the nut. .

  Further, as in the invention described in claim 5, if the piece member is made of sintered metal formed by MIM, even if it has a high workability and a complicated shape, it can be easily and accurately obtained. While being able to shape | mold to a shape and a dimension, it can be made to insert in the piece window of a nut, elastically deforming a tongue piece.

  Further, as in the invention described in claim 6, the piece member may be formed by injection molding synthetic resin.

  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 the thread grooves facing each other, 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 a piece member formed, a pair of recesses bulging from the outer peripheral surface of the piece window are formed in the opening on the inner diameter side of the piece window, and the piece member A pair of tongue pieces projectingly and these tongue pieces are locked in a state of elastic deformation in the recess, so that it is not necessary to caulk and fix the piece member as in the prior art. A snap-fit piece member that does not move can be provided, and the posture of the piece member with respect to the nut is stable. To prevent the level difference is formed between the bridge window Te, the bridge member may variations without easily and stably fixed.

(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. It is a cross-sectional perspective view which shows the nut with which the piece member which concerns on this invention was mounted | worn. (A) is principal part sectional drawing along the III-III line of FIG. 2, (b) is a bottom view of (a). (A) is a perspective view which shows the piece member which concerns on this invention, (b) is the IVb arrow directional view of (a), (c) is sectional drawing along the IVc-IVc line of (b). . 4A is a perspective view showing a modification of the piece member of FIG. 4, FIG. 4B is a view taken along the arrow Vb in FIG. 4A, and FIG. 4C is a cross-sectional view taken along the line Vc-Vc in FIG. It is. (A) is a top view which shows the conventional piece type | formula ball screw, (b) is a longitudinal cross-sectional view same as the above. It is a figure which shows the fixing method of the piece member of FIG. 6, (a) is a side view, (b) is a top view. It is the elements on larger scale which show the detail at the time of fixation of the piece member of FIG.

  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, and a roller formed by the opposing screw groove. A plurality of balls accommodated in the movement path, and a piece member fitted to a piece window formed in the trunk portion of the nut and having a connecting groove having the rolling path as a circulation path. In the piece-type ball screw, a pair of recesses bulging from the outer peripheral surface of the piece window is formed in the opening on the inner diameter side of the piece window, and the piece member is made of sintered metal formed by MIM. A pair of tongue pieces projecting so that the piece member extends in a straight line from the base portion formed by bending at the edge of the bottom surface, and the tip portion protrudes from the outer peripheral surface of the body portion. These tongue pieces are locked in a state of being elastically deformed in the recesses.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
1A is a plan view showing an embodiment of a piece type ball screw according to the present invention, FIG. 1B is a longitudinal sectional view of the same, and FIG. 2 is a state in which the piece member according to the present invention is mounted. FIG. 3 (a) is a cross-sectional view of an essential part taken along line III-III in FIG. 2, (b) is a bottom view of (a), and FIG. 4 (a) is the present invention. The perspective view which shows the piece member which concerns on (b), IVb arrow directional view of (a), (c) is sectional drawing along the IVc-IVc line of (b), FIG.5 (a) is a figure. The perspective view which shows the modification of 2 piece members, (b) is a Vb arrow directional view of (a), (c) is sectional drawing along the Vc-Vc line of (b).

  A piece ball screw 1 shown in FIGS. 1 (a) and 1 (b) has a screw shaft 2 having a spiral thread groove 2a formed on the outer peripheral surface thereof, and is externally fitted to the screw shaft 2 and spiraled on the inner peripheral surface thereof. Nut 3 in which a thread groove 3a is formed, a large number of balls 4 accommodated in a rolling path formed by opposing screw grooves 2a and 3a, and a piece member 5 serving as a circulating member for these balls 4 And.

  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 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 grooves 5a of the piece member 5. After overcoming the thread of the screw shaft 2, the thread returns to the adjacent screw groove 3a and rolls along the screw grooves 2a and 3a again. In addition, although the thing in which the piece window 6 and the piece member 5 were formed in the cross-sectional substantially circular shape was illustrated here, not only circular but an oval shape may be sufficient, for example.

  As shown in FIG. 2, the connecting groove 5 a of the piece member 5 is formed to be curved in an S shape so as to smoothly connect the adjacent screw grooves 3 a and 3 a of the nut 3. Then, the connecting window 5a is connected to the screw groove 3a so that both the frame window edges 7, 7 coincide with the groove edge portion 8 of the adjacent screw groove 3a of the nut 3. 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, a tongue piece 9 to be described later is projected on both sides of the piece member 5 and is locked in a recess 10 formed in the piece window 6 of the nut 3. The tongue piece 9 positions the piece member 5 with respect to the nut 3 and prevents the piece member 5 from coming out of the piece window 6 in the radial direction.

  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.

  The ball 4 is assembled by attaching the piece member 5 to the piece window 6 of the nut 3 from the outer diameter side of the nut 3 and then applying the nut 3 from the shaft end of the screw shaft 2 to attach the ball 4 to both screw grooves 2a and 3a. The nut 3 is rotated while being sequentially inserted therebetween, and the nut 3 is moved to the screw shaft 2. 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. 3A, a pair of recesses 10 bulging from the outer peripheral surface of the piece window 6 are formed in the opening on the inner diameter side of the piece window 6 of the nut 3. Moreover, as shown in (b), the flat surfaces 6a and 6a are formed in the internal peripheral surface which the piece window 6 opposes. On the other hand, the piece member 5 is formed with flat portions 11 and 11 corresponding to the flat surface 6 a of the piece window 6. And the tongue piece 9 of the piece member 5 is latched in the recess 10 of the piece window 6 in an elastically deformed state. In this way, by engaging the flat portion 11 of the piece member 5 with the flat surfaces 6 a and 6 a of the piece window 6, the nut 3 is connected so that the connecting groove 5 a matches the screw groove 3 a of the nut 3. On the other hand, the positioning of the piece member 5 can be facilitated.

  In this embodiment, as shown to Fig.4 (a)-(c), the piece member 5 comprises a column shape, while the flat parts 11 and 11 are formed in the outer peripheral surface which the trunk | drum 12 opposes, and a bottom face ( In the figure, a connecting groove 5a is formed on the upper side 5b, and tongue pieces 9, 9 projecting from the bottom surface 5b on the side of the connecting groove 5a so as to incline toward the upper surface (lower side in the figure). Specifically, the tongue pieces 9, 9 are formed on the opposing outer peripheral surface of the body portion 12, and extend from the root portion 9a formed by bending to the edge of the bottom surface 5b so as to incline linearly, and the tip portion 9b. Is formed so as to protrude from the outer peripheral surface of the body portion 12.

  Then, as shown in FIG. 3 (a), by inserting the piece member 5 into the piece window 6 from the outer diameter side of the nut 3, the tongue piece 9 passes through the piece window 6, and the inside diameter of the piece window 6. The piece member 5 is fixed to the piece window 6 while being elastically deformed into the recess 10 formed in the opening on the side. As a result, it is possible to provide a so-called snap-fit piece member 5 that does not move with respect to the piece window 6 without caulking and fixing the piece member 5 as in the prior art, and the posture of the piece member 5 with respect to the nut 3. To prevent the formation of a step between the frame window 6 and the frame member 5 can be easily and stably fixed without variation, and the assembling workability can be improved and the cost can be reduced. it can.

  Here, the material of the piece member 5 is exemplified by a sintered alloy formed by MIM, but is not limited thereto, and can be inserted into the piece window 6 of the nut 3 while elastically deforming the tongue piece 9. For example, a thermoplastic synthetic resin such as PA (polyamide) 66 filled with 10 to 40 wt% of a fibrous reinforcing material such as GF (glass fiber) may be formed by injection molding. As a result, the cost and weight of the ball screw can be further reduced.

  A piece member 13 shown in FIG. 5 is a modification of the piece member 5 described above. This piece member 13 is basically different from that described above in the configuration of the tongue piece 9, and other detailed explanations will be given by attaching the same reference numerals to the same parts or parts having the same functions. Omitted.

  This piece member 13 is formed from a sintered alloy by MIM, like the piece member 5 described above, and has a columnar shape as shown in FIG. At the same time, a connecting groove 5a is formed in the bottom surface (upper side in the figure) 5b, and the tongue pieces 14 and 14 extend obliquely from the bottom surface 5b on the side of the connecting groove 5a toward the upper surface (lower side in the figure). Is protruding.

  Specifically, the tongue pieces 14 and 14 are formed on the outer peripheral surfaces of the body portion 12 facing each other, and extend inclining linearly from a root portion 9a formed by bending at the edge of the bottom surface 5b, (c) As shown in FIG. 4, the tip end portion 14a is formed in a circular arc shape so as to protrude from the outer peripheral surface of the body portion 12 in a state of being connected without being separated from the main body. As a result, as with the piece member 5 described above, as a snap-fit piece member 13, the workability of assembling can be improved, the resistance to elastic deformation due to assembling is increased, and after inserting the piece member 13, The catching property of the frame window 6 to the recess 10 can be improved, and it can be fixed more easily and more stably without variation.

  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.

DESCRIPTION OF SYMBOLS 1 Piece type ball screw 2 Screw shaft 2a, 3a Screw groove 3 Nut 4 Ball 5, 13 Piece member 5a Connecting groove 5b Bottom face of piece member 6 Piece window 6a Flat face of piece window 7 Piece window edge 8 Groove edge portions 9, 14 Tongue piece 9a Tongue root base part 9b, 14a Tongue piece tip 10 Cone window recess 11 Flat part 12 Piece member trunk 51 Piece ball screw 52 Screw shaft 52a, 53a Screw groove 53 Nut 54 Ball 55 Piece Member 55a Connection groove 55b Arm 55c Body 55f Guide wall 55g End 56 Frame window 57 Caulking punch 57a Processing surface

Claims (6)

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 accommodated in a rolling path formed by the opposing thread grooves;
In a piece type ball screw provided with a piece member fitted to a piece window drilled in the trunk portion of the nut and having a connecting groove having the rolling path as a circulation path,
The opening on the inner diameter side of the piece window is formed with a pair of recesses bulging from the outer peripheral surface of the piece window, and a pair of tongue pieces projecting from the piece member, the tongue pieces are A piece-type ball screw which is locked in a state of being elastically deformed in a recess.   The tongue piece of the piece member extends so as to linearly incline from a root portion formed by bending at the edge of the bottom surface, and the tip portion is formed so as to protrude from the outer peripheral surface of the trunk portion. The piece type ball screw according to 1.   The tongue piece of the piece member extends in a straight line from the root portion formed by bending at the edge of the bottom surface, and the tip portion is arc-shaped and connected to the main body, and is more than the outer peripheral surface of the trunk portion. The piece type ball screw according to claim 1, wherein the piece type ball screw is formed so as to protrude.   A flat surface is formed on the opposed inner peripheral surface of the piece window, a flat portion corresponding to the flat surface is formed on the piece member, and the flat portion of the piece member is engaged with the flat surface of the piece window. The piece type ball screw according to claim 1.   The piece type ball screw according to any one of claims 1 to 4, wherein the piece member is made of a sintered metal formed by MIM.   The piece-type ball screw according to claim 1, wherein the piece member is formed by injection molding a synthetic resin.

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