Screw shaft guide unit and ball screw actuator

Abstract

The present invention reduces energy loss accompanying movement of a screw shaft in an actuator for converting rotary motion of a motor or the like to linear motion. Two guide members (62) are disposed facing each other with a screw shaft (30) sandwiched therebetween. The urging of an O-ring (63) causes guide surfaces (622) of the guide members (62) to be pressed against a cylindrical sliding surface (34) on the outer circumference of the screw shaft. The guide surfaces (622) support the screw shaft (30) so as to enable rotation about an axis O and movement in a Z direction. A rotation-preventing member (64) has a flat rotation-preventing surface (644) and is fixed at a position where a clearance is provided between the rotation-preventing surface (644) and a flat contact surface (33) on the outer circumference of the screw shaft. When the rotation angle of the screw shaft (30) around the axis O reaches a prescribed angle, the contact surface (33) and the rotation-preventing surface (644) interfere with each other, and further rotation of the screw shaft (30) is prevented. When the rotation angle of the screw shaft (30) around the axis O is less than the prescribed angle, a non-contact state between the contact surface (33) and the rotation-preventing surface (644) is maintained.

Description

Screw shaft guide unit and ball screw actuator

[0001] The present invention relates to a structure of a low-energy-loss screw shaft guide unit that supports a screw shaft movably in its axial direction while preventing rotation of the screw shaft about its axis in an actuator such as a ball screw actuator that converts the rotational motion of a motor or the like into linear motion.

[0002] Steer-by-wire systems and the like use an electric actuator that converts the rotational motion of a motor into linear motion using a ball screw. For example, in a steer-by-wire steering device described in Patent Document 1, the steering actuator has a nut-rotating ball screw mechanism that converts the rotational motion of an electric motor into linear motion of a steering shaft connected to the steering wheels via a tie rod and a knuckle arm, and the electric motor is driven in accordance with a detected steering angle to turn the steering wheels.

[0003] In this steering actuator, the steering shaft (screw shaft of the ball screw mechanism) is slidably supported by the inner surface of a cylindrical bush fitted into a circumferential groove provided on the inner periphery of the steering housing so that the steering shaft (screw shaft of the ball screw mechanism) moves axially in response to the rotation of a nut (nut of the ball screw mechanism) fixed to the steering housing, and rotation around the axis relative to the steering housing is regulated by the structure described below.

[0004] The steering housing is provided with a cylindrical rotation-restricting-member accommodating portion with a bottom facing the outer peripheral surface of the steering shaft, between the bushing and the nut. A cylindrical rotation-restricting member is fitted inside this rotation-restricting-member accommodating portion so that it can move toward the outer peripheral surface of the steering shaft. Furthermore, a compressed spring is disposed between the bottom of the rotation-restricting-member accommodating portion and the rotation-restricting member, biasing the rotation-restricting member toward the steering shaft, so that the end face of the rotation-restricting member is pressed against the outer peripheral surface of the steering shaft.

[0005] A V-groove, into which the outer peripheral surface of the steering shaft fits, is formed along the axial direction of the steering shaft in the end face of the rotation restricting member (the surface that is pressed against the outer peripheral surface of the steering shaft). Meanwhile, two inclined surfaces (two inclined surfaces facing the inner wall surfaces on both sides of the V-groove) are provided along the axial direction of the steering shaft on the outer peripheral surface of the steering shaft that are fitted into the V-groove in the end face of the rotation restricting member. The spacing between the inclined surfaces gradually narrows as the V-groove approaches the bottom of the V-groove. The bias of the compressed spring brings the inner wall surfaces on both sides of the V-groove on the end face of the rotation restricting member into slidable surface contact with the two inclined surfaces on the outer periphery of the steering shaft. As the steering shaft moves with rotation of the nut, the rotation restricting portion guides the steering shaft in the axial direction while restricting its rotation around the axis relative to the steering housing.

[0006] Patent No. 7208381

[0007] In the above-described conventional steering actuator, the rotation restricting member is constantly pressed against the steering shaft by the compressed spring, and large friction acts between the contact surfaces of the rotation restricting member and the steering shaft (between the inner wall surface of the V-groove in the rotation restricting member and the opposing inclined surface on the outer periphery of the steering shaft), which may increase energy loss due to movement of the screw shaft.

[0008] The present invention has been made in consideration of the above circumstances, and one of its objects is to reduce energy loss associated with movement of a screw shaft in an actuator that converts rotational movement of a nut by a motor or the like into linear movement of a screw shaft.

[0009] In order to solve the above problems, in the present invention, the screw shaft, which moves in the axial direction due to rotation of the nut, is supported by the guide surface of a resin guide member so that it can rotate around the axis and move in the axial direction, and a rotation prevention surface is arranged with a clearance between it and the outer periphery of the screw shaft, and rotation of the screw shaft around the axis of more than a rotation angle corresponding to the thickness of the clearance is prevented by interference between the rotation prevention surface and the outer periphery of the screw shaft.

[0010] For example, the present invention provides a screw shaft guide unit for supporting a screw shaft in an actuator that converts rotational motion of a nut into linear motion of the screw shaft, comprising: a guide housing into which the screw shaft, which has on its outer periphery a sliding surface shaped like a cylindrical surface and an abutment surface having a surface shape different from that of the sliding surface, is inserted toward the axis of the screw shaft; two resin guide members, which have guide surfaces for contacting the sliding surface of the screw shaft and are arranged opposite each other with the axis center sandwiched between them within the guide housing so that the guide surfaces support the screw shaft rotatably around the axis and movable toward the axis; and rotation preventing means, which is arranged opposite the abutment surfaces with a clearance between them and the abutment surfaces of the screw shaft, and which has a rotation preventing surface that comes into contact with the abutment surface of the screw shaft when the screw shaft rotates around the axis by a predetermined rotation angle or more determined by the thickness of the clearance, and which prevents rotation of the screw shaft by the predetermined rotation angle or more by contact between the rotation preventing surface and the abutment surface of the screw shaft. The present invention also provides a ball screw actuator comprising: the above-mentioned screw shaft guide unit; and a ball screw having the nut and the screw shaft, and converting rotational motion of the nut into linear motion of the screw shaft, wherein the screw shaft has the sliding surface and the abutment surface on its outer periphery, and is supported by the guide surfaces of the two guide members so as to be rotatable around the axis and movable in the direction of the axis, with the clearance provided between the abutment surface and the rotation-preventing surface.

[0011] According to the present invention, a clearance is provided between the outer periphery of the screw shaft and the rotation preventing surface, which prevents rotation of the screw shaft about its axis by interference with the outer periphery of the screw shaft, so that a non-contact state between the outer periphery of the screw shaft and the rotation preventing surface is maintained when the rotation of the screw shaft about its axis remains below a predetermined angle, thereby reducing energy loss associated with the reciprocating movement of the screw shaft.

[0012] Furthermore, since the guide member holds the screw shaft rotatably around its axis and does not have the function of preventing rotation of the screw shaft around its axis, the load in the rotational direction received from the screw shaft is relatively small. Therefore, the guide member does not need to be made of a hard, high-friction material that is more durable than necessary, and can be made of a synthetic resin with a low coefficient of friction with the screw shaft. This further reduces energy loss associated with the reciprocating movement of the screw shaft.

[0013] FIG. 1 is a cross-sectional view showing a portion of the internal structure of a ball screw actuator according to an embodiment of the present invention. FIG. 2(A) is a view showing the internal structure of a ball screw actuator according to an embodiment of the present invention at the position where a ball screw guide unit is arranged, and FIG. 2(B) is a cross-sectional view taken along line A-A in FIG. 2(A). FIG. 3(A) is an external view of the guide housing, and FIGS. 3(B) to 3(D) are front, right side, and bottom views of the guide housing 61. FIGS. 2(E) to 2(F) are cross-sectional views taken along line B-B in FIG. 3(B) and line D-D in FIG. 3(C). FIGS. 4(A) to 4(D) are front, right side, back, and bottom views of a rotation-preventing member 64, and FIG. 4(E) is a cross-sectional view taken along line E-E in FIG. 4(A). Figures 5(A) to (D) are front, bottom, back, and right side views of the guide member 62, Figure 5(E) is a cross-sectional view taken along line F-F in Figure 5(A), and Figure 5(F) is a cross-sectional view taken along line G-G in Figure 5(A). Figure 6 is a diagram showing changes in the positional relationship between the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64 in response to rotation of the screw shaft 30. Figure 7 is a diagram for explaining another method of connecting the guide housing 61 and the rotation preventing member 64. Figure 8 is a cross-sectional view showing a modified example of the ball screw guide unit.

[0014] An embodiment of the present invention will now be described with reference to the accompanying drawings. In this embodiment, a ball screw actuator applicable as a steering actuator for a steer-by-wire system will be given as an example of an actuator having a nut-rotating ball screw mechanism.

[0015] Fig. 1 is a cross-sectional view showing a part of the internal structure of a ball screw actuator according to this embodiment. Fig. 2(A) is a view showing the internal structure of the ball screw actuator according to this embodiment at the position where a ball screw guide unit is arranged, and Fig. 2(B) is a cross-sectional view taken along line A-A in Fig. 2(A). Note that Figs. 1 and 2(A) show only a part of a long screw shaft 30.

[0016] As shown in the figure, the ball screw actuator according to this embodiment has a nut-rotating ball screw mechanism 10 that is composed of a nut 20 rotatably supported on a housing 50 via a bearing 40, a cylindrical screw shaft 30 inserted into the nut 20, and balls (not shown) that circulate in rolling grooves provided on the outer periphery 31 of the screw shaft 30 and the inner periphery of the nut 20, and that rotates the nut 20 in both directions around its axis O by a motor or the like, thereby reciprocating the screw shaft 30 in the direction of its axis O. Furthermore, this ball screw actuator has one or more ball screw guide units 60 attached to the inner periphery of the housing 50 so as to be interposed between the inner periphery of the housing 50 and the outer periphery 31 of the screw shaft 30.

[0017] For the convenience of the following explanation, three mutually orthogonal directions (X direction, Y direction, Z direction) are defined in the arrangement space of the ball screw actuator, including the Z direction along the axis O of the screw shaft 30 (the direction in which the screw shaft 30 moves back and forth), and these directions XYZ are appropriately shown in each figure.

[0018] As shown in the figure, the screw shaft 30 has a cylindrical shape, and on its outer periphery 31, a spiral groove is formed around the axis O as a ball rolling groove 32. In addition, in a section S2 other than the section S1 where the rolling groove 32 is formed (hereinafter referred to as the rolling groove forming section), two flat surfaces 33 having a length in the Z direction corresponding to the maximum stroke of the screw shaft 30 are formed so as to face each other across the axis O. In this embodiment, as these two flat surfaces (hereinafter referred to as abutment surfaces) 33, two flat surfaces parallel to the XZ plane are formed at positions equidistant d in opposite directions from the axis O. Therefore, two cylindrical surface regions 34 of a predetermined width are left on the outer periphery of the screw shaft 30 between these two abutment surfaces 33, facing each other across the axis O. The guide surfaces 622 of the two guide members 62 (described later) provided in the ball screw guide unit 60 are in slidable contact with these cylindrical surface areas (hereinafter referred to as sliding surfaces) 34, and support the screw shaft 30 so that it can rotate around the axis O and move in the Z direction.

[0019] On the other hand, the ball screw guide unit 60 has a cylindrical guide housing 61 that is fitted and fixed to the inner periphery of the housing 50 of the ball screw actuator, two resin guide members 62 that are arranged opposite each other within the guide housing 61 and support the screw shaft 30 rotatably about the axis O and movably in the Z direction, and an O-ring 63 that is arranged between the guide housing 61 and at least one of the two guide members 62 (in this embodiment, between the guide housing 61 and each guide member 62) and urges the guide member 62 in the X direction toward the axis O of the screw shaft 30. Furthermore, in order to restrict rotation of the screw shaft 30 about the axis O that accompanies rotation of the nut 20, the ball screw guide unit 60 is provided with two rotation prevention members 64 that are arranged opposite each other within the guide housing 61.

[0020] 3A is an external view of the guide housing 61, FIGS. 3B to 3D are a front view, a right side view (symmetrical with the left side view), and a bottom view (symmetrical with the plan view) of the guide housing 61, and FIGS. 3E to 3F are a B-B cross-sectional view (symmetrical with the C-C cross-sectional view) of FIG. 3B and a D-D cross-sectional view of FIG. 3C.

[0021] As shown in the figure, the guide housing 61 is a block having a hollow cylindrical shape into which the screw shaft 30 can be inserted in the Z direction. Because the screw shaft 30 is supported by the two guide members 62 while being aligned with the guide housing 61, hereinafter, the axis of the guide housing 61 will also be referred to as axis O. The inner surface of the guide housing 61 includes two pairs of inner wall surfaces facing each other across the axis O, namely, two inner wall surfaces (hereinafter, rotation preventing member mounting surfaces) 612A, 612B on which the rotation preventing members 64 are disposed, and two inner wall surfaces (hereinafter, guide member mounting surfaces) 613A, 613B on which the guide members 62 are disposed.

[0022] The two rotation-preventing member mounting surfaces 612A, 612B are parallel flat surfaces provided along the XZ plane and are formed at positions spaced an equal distance D1 in opposite directions from the axis O. The distance D1 from the axis O to each rotation-preventing member mounting surface 612A, 612B is set to be greater than the sum of the distance d from the axis O to the abutment surface 33 of the screw shaft 30 and the thickness t (see FIG. 4) of a plate 641 (described later) provided on the rotation-preventing member 64 so that a clearance 70 of a predetermined thickness G (see FIG. 6) is formed between a rotation-preventing surface 644 of a plate 641 (described later) provided on the rotation-preventing member 64 arranged on the rotation-preventing member mounting surfaces 612A, 612B and the abutment surface 33 of the screw shaft 30.

[0023] A through hole (hereinafter referred to as a boss press-fit hole) 614 is formed at approximately the center of each of the two rotation-preventing member mounting surfaces 612A, 612B, into which a boss 643 (described later) provided on the rotation-preventing member 64 is press-fitted. On the other hand, the two guide member mounting surfaces 613A, 613B are parallel flat surfaces provided along the YZ plane, and are formed at positions spaced an equal distance D2 in opposite directions from the axis O. The distance 2 × D2 between the guide member mounting surfaces 613A, 613B is set to be larger than the diameter of the screw shaft 30.

[0024] A stepped through-hole (hereinafter referred to as a snap-fit ​​insertion hole) 615 is formed at approximately the center of each of the two guide member mounting surfaces 613A, 613B, into which a snap-fit ​​insertion portion 624 (described later) of the guide member 62 is inserted. Each snap-fit ​​insertion hole 615 has two sections with different inner diameters, namely, a first section that opens at the guide member mounting surfaces 613A, 613B, and a second section that is located closer to the outer periphery of the guide housing 61 than the first section and has a larger inner diameter than the first section, and these sections are continuously provided in the X direction. A step surface 616 formed between the inner circumferential surfaces of the first section and the second section engages with a latch portion (described later) provided on the snap-fit ​​insertion portion 624 of the guide member 62.

[0025] Furthermore, an annular O-ring mounting groove 617 for mounting an O-ring 63 is formed on at least one of the two guide member mounting surfaces 613A, 613B so as to surround the snap-fit ​​insertion hole 615. In this embodiment, in order to bias each of the two guide members 62 with the O-ring 63, an annular mounting groove 617 is formed on each of the two guide member mounting surfaces 613A, 613B.

[0026] The guide housing 61 having such a shape can be formed, for example, as an assembly of two symmetrical semi-cylindrical parts.

[0027] The O-ring 63 has a wire diameter larger than the sum of the thickness of the clearance provided between the guide member mounting surfaces 613A, 613B and the bottom surface of the guide member 62 and the depth of the O-ring mounting groove 617. Therefore, the O-ring 63 is compressed between the back surface of the guide member 62 and the groove bottom of the O-ring mounting groove 617, and the guide member 62 is urged in the X direction toward the sliding surface 34 of the screw shaft 30 by the O-ring 63 in the compressed state.

[0028] In this embodiment, an O-ring 63 is interposed between the guide housing 61 and the guide member 62 to bias the guide member 62 in the X direction toward the sliding surface 34 of the screw shaft 30, but other elastic members may be used instead of the O-ring 63. For example, an annular elastic member having a non-circular cross-sectional shape, such as an X-ring, a D-ring, or a T-ring, may be used. Furthermore, plate-shaped elastic members made of an elastomer having rubber elasticity, such as urethane rubber or silicone rubber, may be arranged around the snap-fit ​​insertion hole 615 in an appropriate layout, such as at equal angular intervals.

[0029] 4(A) to 4(C) are a front view, a right side view (symmetrical to the left side view), a rear view, and a bottom view (symmetrical to the plan view) of the rotation-preventing member 64, and FIG. 4(E) is a cross-sectional view taken along line E-E of FIG. 4(A).

[0030] As shown in the figure, the rotation-preventing member 64 is formed of a durable and wear-resistant material such as iron, a composite material, or a sintered alloy. The rotation-preventing member 64 includes a plate 641 and a boss 643 provided on the plate 641 so as to protrude in the Y direction from one surface (back surface) 642 of the plate 641. The other surface (the surface opposite the back surface; i.e., the front surface) of the plate 641 includes a rotation-preventing surface 644 for preventing rotation of the screw shaft 30 about the axis O. The outer diameter of the boss 643 is set to be larger than the inner diameter of the boss press-fit hole 614 of the guide housing 61 by a predetermined interference. The two rotation-preventing members 64 are fixed to the guide housing 61 by press-fitting the boss 643 into the boss press-fit hole 614 until the entire back surface 642 of the plate 641 contacts the rotation-preventing member mounting surfaces 612A, 612B of the guide housing 61. In this state, the plate 641 has a thickness t that is smaller than the difference (D1-d) between the distance D1 from the axis O of the guide housing 61 to the rotation preventing member mounting surfaces 612A, 612B and the distance d from the axis O of the screw shaft 30 to the abutment surfaces 33, so that a gap larger than the flat width (the distance between the two abutment surfaces 33) of the screw shaft 30 is formed between the rotation preventing surfaces 644 of the two rotation preventing members 64 fixed to the guide housing 61 and the two abutment surfaces 33 of the screw shaft 30 supported by the two guide members 62. Therefore, a clearance 70 (see FIG. 6) of a predetermined thickness G (D1-d-t) is formed between the rotation preventing surfaces 644 of the two rotation preventing members 64 fixed to the guide housing 61 and the two abutment surfaces 33 of the screw shaft 30 supported by the two guide members 62.

[0031] 5(A) to 5(D) are a front view, a bottom view, a back view (symmetrical to the plan view), and a right side view (symmetrical to the left side view) of the guide member 62, FIG. 5(E) is a cross-sectional view taken along line F-F of FIG. 5(A), and FIG. 5(F) is a cross-sectional view taken along line G-G of FIG. 5(A).

[0032] As shown in the figure, the guide member 62 has a guide main body 621 and a snap-fit ​​insertion portion 624 provided on the guide main body 621 so as to protrude in the X direction from one surface (back surface) 623 of the guide main body 621. Such a guide member 62 can be integrally molded from a resin with excellent sliding properties, such as polyethylene terephthalate, polybutylene terephthalate, polyamide, polyphenylene sulfide, or polyacetal, reinforced with fiber such as glass.

[0033] The snap-fit ​​insertion portion 624 has a cylindrical portion 624A protruding from the back surface 623 of the guide body 621 and a latch portion 624B provided on the end surface of the cylindrical portion 624A. The cylindrical portion 624A has an outer diameter smaller than the inner diameter of the first section so that it can be placed in the first section of the snap-fit ​​insertion hole 615. The length of the cylindrical portion 624A (the distance from the back surface 623 of the guide member 62 to the end surface of the cylindrical portion 624A) is greater than the length of the first section within the snap-fit ​​insertion hole 615 on the guide member mounting surfaces 613A, 613B. Meanwhile, the latch portion 624B has a maximum diameter greater than the inner diameter of the first section within the snap-fit ​​insertion hole 615 but smaller than the inner diameter of the second section, and has a tapered shape (e.g., a conical or truncated conical shape) such that its tip is narrower than the first section of the snap-fit ​​insertion hole 615.

[0034] When such snap-fit ​​insertion portion 624 is inserted into snap-fit ​​insertion hole 615 of guide member mounting surfaces 613A, 613B, latch portion 624B passes through the first section while elastically deforming due to contact with the inner circumferential surface of the first section, and then returns to its original shape and is housed in the second section, engaging with stepped surface 616 formed at the boundary between the first and second sections. Guide member 62 is elastically supported by O-rings 63 on guide member mounting surfaces 613A, 613B, with movement permitted a distance equivalent to the difference between the length of the first section within snap-fit ​​insertion hole 615 and the length of cylindrical portion 624A.

[0035] Furthermore, a guide groove 625 into which the outer periphery of the screw shaft 30 is fitted is formed in the Z direction on the other surface of the guide body 621 (the surface opposite the back surface: the front surface). A curved guide surface 622 that slidably supports the sliding surface 34 of the screw shaft 30 is formed on the inner wall of the guide groove 625. This guide surface 622 may have an inverted shape of the sliding surface 34 of the screw shaft 30 so as to be in surface contact with the sliding surface 34 of the screw shaft 30, or may include two cylindrical surface regions (cylindrical surface regions with a diameter larger than that of the screw shaft 30) that make line contact with the sliding surface 34 of the screw shaft 30 at positions symmetrical with respect to the XZ plane including the axis O. Alternatively, a V-groove may be formed in the Z direction as the guide groove 625, and flat guide surfaces that make line contact with the sliding surface 34 of the screw shaft 30 at positions symmetrical with respect to the XZ plane including the axis O may be formed on its inner walls (two opposing side walls).

[0036] In this configuration, the screw shaft 30 is supported so as to be movable in the Z direction while being prevented from rotating around the axis O by more than a predetermined angle, as will be described below.

[0037] FIG. 6 is a diagram showing the change in the positional relationship between the contact surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64 in response to the rotation of the screw shaft 30.

[0038] Although omitted in Figure 6, the guide surfaces 622 of the two guide members 62 arranged opposite each other across the screw shaft 30 are pressed against the sliding surface 34 of the screw shaft 30 by the force of the O-ring 63, supporting the screw shaft 30 so that it can rotate around the axis O and move in the Z direction.

[0039] A clearance 70 is formed between each of the rotation preventing surfaces 644 of the two rotation preventing members 64 arranged opposite each other with the screw shaft 30 therebetween and the contact surface 33 of the screw shaft 30 .

[0040] As described above, the screw shaft 30 is supported by the guide surfaces 622 of the two guide members 62 so as to be rotatable about the axis O, but as shown by the dotted lines in Figure 6, when the screw shaft 30 rotates about the axis O by a predetermined angle determined according to the thickness G of the clearance 70, further rotation about the axis O is prevented by interference between the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64. Therefore, when a large moment acts about the axis O of the screw shaft 30 due to rotation of the nut 20, the screw shaft 30 can be prevented from rotating by more than a predetermined angle.

[0041] On the other hand, when the rotation angle of the screw shaft 30 about the axis O remains less than a predetermined angle, as shown by the dashed line in Fig. 6, no interference occurs between the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64. Therefore, when a relatively small moment acts about the axis O of the screw shaft 30 due to the rotation of the nut 20, the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64 are maintained in a non-contact state, so that energy loss associated with the movement of the screw shaft 30 can be reduced.

[0042] As described above, according to this embodiment, a clearance 70 is provided between the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64. Therefore, when a large moment about the axis O acts on the screw shaft 30 due to the rotation of the nut 20, the screw shaft 30 can be prevented from rotating more than a predetermined angle. Furthermore, when the rotation angle of the screw shaft 30 about the axis O remains less than the predetermined angle, a non-contact state is maintained between the abutment surface 33 of the screw shaft 30 and the rotation preventing surface 644 of the rotation preventing member 64 while the screw shaft 30 is moving, thereby reducing energy loss associated with the movement of the screw shaft 30.

[0043] Furthermore, the guide surface 622 of the guide member 62 supports the screw shaft 30 rotatably around the axis O, and does not have the function of preventing rotation of the screw shaft around the axis, so the load in the rotational direction received from the screw shaft 30 is relatively small. For this reason, the guide member 62 does not need to be made of a hard, high-friction material that is more durable than necessary, and can be made of a synthetic resin with a small coefficient of friction with respect to the screw shaft 30. Therefore, the energy loss accompanying the reciprocating movement of the screw shaft 30 can be further reduced.

[0044] Furthermore, in the above-described conventional technology, a gap is provided between the inner wall surface of the rotation restricting member accommodating portion of the steering housing and the rotation preventing member, providing a movable space for the rotation preventing member. The rotation preventing member swings due to the expansion and contraction of the spring that biases the rotation preventing member, causing fluctuations in the allowable rotation angle range around the axis of the steering shaft. In contrast, the rotation preventing member 64 according to this embodiment is fixed to the guide housing 61 with its entire bottom surface (the back surface of the plate 641) 642 in close contact with the rotation preventing member mounting surfaces 612A, 612B, limiting its swing relative to the guide housing 61. Therefore, according to this embodiment, fluctuations in the allowable rotation angle range of the screw shaft 30 can be prevented.

[0045] The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the present invention.

[0046] In the above embodiment, the rotation-blocking member 64 is coupled to the guide housing 61 by press-fitting the boss 643 of the rotation-blocking member 64 into the boss press-fit hole 614 of the guide housing 61. However, other methods of coupling the rotation-blocking member 64 and the guide housing 61 may also be used. For example, as shown in FIG. 7 , when using a boss-less rotation-blocking member 64A that is wider in the X direction than the rotation-blocking member mounting surfaces 612A, 612B, slits 618 for inserting both sides of the rotation-blocking member 64A may be provided parallel to the rotation-blocking member 64A at the boundary between the rotation-blocking member mounting surfaces 612A, 612B and the guide member mounting surfaces 613A, 613B of the guide housing 61A. In this case, the boss press-fit hole 614 may be omitted. Each slit 618 has a thickness (width in the Y direction) greater than the thickness of the rotation-preventing member 64A, but the distance between one of the two opposing wall surfaces 618A, 618B, the wall surface 618A facing the rotation-preventing member mounting surfaces 612A, 612B, and the rotation-preventing member mounting surfaces 612A, 612B, is set to be slightly smaller than the thickness of the rotation-preventing member 64A. Therefore, the rotation-preventing member 64A can be joined to the guide housing 61A by press-fitting both sides of the rotation-preventing member 64A in the Z direction into the two slits 618 that face each other across the YZ plane that includes the axis O.

[0047] Furthermore, in the above embodiment, rotation-preventing members 64, 64A having rotation-preventing surfaces 644 are attached to the guide housings 61, 61A, but instead of using the rotation-preventing members 64, a rotation-preventing layer made of a material with excellent durability and wear resistance, such as a sintered metal layer or a metal coating layer, may be provided on a pair of opposing inner wall surfaces 612A, 612B of the guide housing, and the surface of this rotation-preventing layer may function as a rotation-preventing surface.

[0048] In the above embodiment, a flat surface is formed as an abutment surface on the outer periphery of the screw shaft 30, but such an abutment surface does not need to be flat as long as it can interfere with the rotation preventing surface 644 when the screw shaft 30 rotates a predetermined angle around the axis O, and may be curved as shown in Fig. 8. In this case, the rotation preventing surface 644 may be flat or may be curved to follow the shape of the abutment surface of the screw shaft 30.

[0049] In addition, in the above embodiment, two abutment surfaces 33 are formed on the outer periphery of the screw shaft 30, which interfere with the rotation prevention surface 644 when the screw shaft 30 rotates a predetermined angle around the axis O, but it is sufficient that at least one such abutment surface 33 is provided on the outer periphery of the screw shaft 30.

[0050] In the above embodiment, an example of application to a ball screw actuator that can be used as a steering actuator in a steer-by-wire system has been given. However, the present invention is not limited to steering actuators in steer-by-wire systems, and can be widely applied to devices that use a feed mechanism, such as a nut-rotating ball screw mechanism that converts the rotational motion of a nut into linear motion of a screw shaft.

[0051] 10: Ball screw mechanism; 20: Nut; 30: Screw shaft; 31: Outer periphery of screw shaft; 32: Rolling groove; 33: Contact surface; 34: Sliding surface; 40: Bearing; 50: Housing; 60: Ball screw guide unit; 61, 61A: Guide housing; 62: Guide member; 63: O-ring; 64, 64A: Rotation preventing member; 70: Clearance between the rotation preventing surface of the rotation preventing member and the contact surface of the screw shaft; 612A, 612B: Rotation preventing member mounting surface; 613A, 613B: Guide member mounting surface; 614: Boss press-fit hole; 615: Snap-fit ​​insertion hole; 616: Step surface; 617: O-ring mounting groove; 618: Slit; 621: Guide member main body; 622: Guide surface; 623: Back surface of guide member main body; 624: Snap-fit ​​insertion portion; 624A: Cylindrical portion; 624B: Latch portion; 625: Guide groove; 641: Plate; 642: Back surface of plate; 643: Boss; 644: Rotation prevention surface

Claims

1. A screw shaft guide unit for supporting a screw shaft in an actuator that converts the rotational motion of a nut into the linear motion of a screw shaft, comprising: a guide housing into which the screw shaft, which has on its outer periphery a sliding surface with a cylindrical surface shape and an abutment surface having a surface shape different from that of the sliding surface, is inserted in the direction of the axis of the screw shaft; two resin guide members having guide surfaces for contacting the sliding surfaces of the screw shaft, and disposed opposite each other on either side of the axis within the guide housing so that the screw shaft is supported by the guide surfaces so that it can rotate around the axis and move in the direction of the axis; and a rotation prevention means, which is disposed opposite the abutment surfaces with a clearance between them and the abutment surfaces of the screw shaft, and has a rotation prevention surface that comes into contact with the abutment surface of the screw shaft when the screw shaft rotates around the axis by a predetermined rotation angle or more determined by the thickness of the clearance, and which prevents rotation of the screw shaft by the predetermined rotation angle or more due to contact between the rotation prevention surface and the abutment surface of the screw shaft.

2. A screw shaft guide unit as claimed in claim 1, characterized in that the rotation prevention means comprises a rotation prevention member having the rotation prevention surface on one side facing the abutment surface of the screw shaft, and the other side located opposite the rotation prevention surface being fixed to the guide housing with the other side being in surface contact with the guide housing.

3. A screw shaft guide unit as claimed in claim 1, characterized in that the rotation prevention means comprises a rotation prevention layer formed on the inner wall surface of the guide housing and having the rotation prevention surface on the surface facing the abutment surface of the screw shaft.

4. A ball screw actuator comprising: a screw shaft guide unit as defined in any one of claims 1 to 3; and a ball screw having the nut and the screw shaft and converting the rotational motion of the nut into linear motion of the screw shaft, wherein the screw shaft has the sliding surface and the abutment surface on its outer periphery, and is supported by the guide surfaces of the two guide members so as to be rotatable around the axis and movable in the direction of the axis, with the clearance provided between the abutment surface and the rotation prevention surface.

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