Electric power steering system, ball screw

Abstract

To provide a ball screw that allows the ball to move smoothly. [Solution] A ball screw comprising a screw shaft and a nut that engages the screw shaft via a plurality of balls and has a housing portion formed therein for housing an end deflector, wherein the housing portion has a first wall surface and a second wall surface formed facing each other from the inner circumferential surface to the outer circumferential surface of the nut, and the end deflector has a first side surface facing the first wall surface, a second side surface facing the second wall surface, and a convex portion projecting from the first side surface toward the first wall surface, wherein the convex portion is provided at a position that generates a moment in which the second side surface rotates toward the second wall surface.

Description

[Technical Field]

[0001] The present invention relates to an electric power steering apparatus and a ball screw. [Background Art]

[0002] For example, the ball screw described in Patent Document 1 includes an end deflector and a nut formed with an accommodating portion that accommodates the end deflector. The end deflector is divided into two or more members, at least one of the divided members has a convex portion that contacts a wall surface forming the accommodating portion, and the divided members include a first member and a second member. The wall surface includes a first side wall surface and a second side wall surface formed from the inner peripheral surface toward the outer peripheral surface of the nut, and spans between an end portion of the first side wall surface closer to the outer peripheral surface of the nut and an end portion of the second side wall surface closer to the outer peripheral surface of the nut, and has a bottom wall surface formed therebetween. The first member has a first side surface facing the first side wall surface, an inner surface facing the screw shaft, and a first divided surface in contact with the second member. The second member has a second side surface facing the second side wall surface, an outer surface facing the bottom wall surface, and a second divided surface in contact with the first member. The convex portion is formed on the first side surface or the second side surface. [Prior Art Documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent No. 6151161 [Summary of the Invention] [Problems to be Solved by the Invention]

[0004] In the ball screw described in Patent Document 1, a second semi-passage is formed on the split surface of the second member of the end deflector, which, when assembled with the first member, forms a passage for the ball. Therefore, when the end deflector is press-fitted into the housing along the nut axis, a step may occur at the connection between the second semi-passage of the second member and the helical groove of the nut. If a step occurs and the connection between the second semi-passage of the second member and the helical groove of the nut is not smooth, the ball may not be able to move smoothly. The present invention aims to provide a ball screw or the like that can move a ball smoothly. [Means for solving the problem]

[0005] To this end, the present invention provides a ball screw comprising a screw shaft and a nut that engages the screw shaft via a plurality of balls and has a housing portion formed therein for housing an end deflector, wherein the housing portion has a first wall surface and a second wall surface formed from the inner circumferential surface of the nut toward the outer circumferential surface and facing each other, and the end deflector has a first side surface facing the first wall surface, a second side surface facing the second wall surface, and a convex portion projecting from the first side surface toward the first wall surface, wherein the convex portion is provided at a position that generates a moment that causes the second side surface to rotate toward the second wall surface. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a ball screw or the like that can move a ball smoothly. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic diagram of the electric power steering system. [Figure 2] This figure shows an example of a partial cross-section illustrating the general structure of a ball screw. [Figure 3] This is an example of a view of the nut and end deflector in direction III of Figure 2. [Figure 4] This figure shows an example of a cross-section of the nut, end deflector, and circlip in section IV-IV of Figure 3. [Figure 5] This figure shows an example of a cross-section of a ball screw in the VV section of Figure 4. [Figure 6] This is an example of a perspective view showing the schematic configuration of the first and second members that make up the end deflector. [Figure 7] This figure shows an example of a view of an end deflector from the first axial side in the axial direction. [Figure 8] This is an example of a perspective view of an end deflector seen from the inside. [Figure 9] This is an example of a perspective view of an end deflector seen from the outside. [Figure 10] This is an example of a view of the nut and end deflector of a ball screw according to the second embodiment, as seen in direction III of Figure 2. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described in detail below with reference to the attached drawings. Figure 1 is a schematic diagram of the electric power steering system 100. As shown in Figure 1, the electric power steering system 100 (hereinafter sometimes referred to as "steering system 100") is a steering device for arbitrarily changing the direction of travel of the vehicle. The steering system 100 is a rack-assist type power steering system.

[0009] The steering device 100 includes a steering wheel 101 operated by the driver to change the direction of travel of the vehicle, and a steering shaft 102 integrally provided with the steering wheel 101. The steering device 100 also includes a first connecting shaft 103 connected to the steering shaft 102 via a universal joint 103a, and a second connecting shaft 104 connected to the first connecting shaft 103 via a universal joint 103b. The second connecting shaft 104 rotates in conjunction with the rotation of the steering wheel 101.

[0010] The steering device 100 also includes tie rods 105 connected to each of the left and right wheels (for example, front wheels) 150, which act as rolling wheels, and a rack shaft 106 connected to the tie rods 105. The steering device 100 also includes a pinion 107a that, together with the rack teeth 106a formed on the rack shaft 106, constitutes a rack and pinion mechanism. The pinion 107a is formed at the lower end of the pinion shaft 107. The pinion shaft 107 applies a driving force to the rack shaft 106, which rotates the wheels 150, thereby causing the wheels 150 to roll. The pinion shaft 107 is connected to a second connecting shaft 104 via a torsion bar 108 within the steering gearbox.

[0011] Furthermore, the steering device 100 includes an electric motor 121 and an assist unit 120 that transmits the driving force of the electric motor 121 to the rack shaft 106 as steering assist force to assist the movement of the rack shaft 106. The assist unit 120 comprises an electric motor 121 and a drive pulley 122 mounted on the output shaft of the electric motor 121. The assist unit 120 also comprises a ball screw 1 that converts the rotational driving force of the electric motor 121 into axial movement force of the rack shaft 106 (hereinafter, the axial direction of the rack shaft 106 may be simply referred to as "axial direction"). The assist unit 120 also comprises a driven pulley 125 that rotates together with a nut 7 (described later) of the ball screw 1, and a lock nut 126 that fixes the driven pulley 125 to the outer circumference of the nut 7. The assist unit 120 also comprises an endless belt 127 stretched between the drive pulley 122 and the driven pulley 125.

[0012] <First Embodiment> Next, the ball screw 1 according to the first embodiment will be described. Figure 2 shows an example of a partial cross-section illustrating the schematic configuration of the ball screw 1. Figure 3 is an example of a view of the nut 7 and end deflector 8 in direction III of Figure 2. FIG. 4 is a diagram illustrating an example of a cross-section of the nut 7, the end deflector 8, and the circlip 9 taken along line IV-IV in FIG. 3. FIG. 5 is a diagram illustrating an example of a cross-section of the ball screw 1 taken along line V-V in FIG. 4. FIG. 6 is an example of a perspective diagram illustrating a schematic configuration of a first member 10 and a second member 20 that constitute the end deflector 8. FIG. 7 is a diagram illustrating an example of a view of the end deflector 8 as seen axially from a first axial side. FIG. 8 is an example of a perspective diagram of the end deflector 8 as seen from the inner side. FIG. 9 is an example of a perspective diagram of the end deflector 8 as seen from the outer side.

[0013] The ball screw 1 includes a screw shaft 5, a plurality of balls 6, a nut 7 attached to the screw shaft 5 via the balls 6, an end deflector 8 described in detail later, and a circlip 9 that suppresses movement of the end deflector 8 relative to the nut 7. The screw shaft 5 is provided so as to be integrated with the rack shaft 106 between the rack shaft 106 and one tie rod 105. A helical groove 51 is formed on the outer circumferential surface of the screw shaft 5.

[0014] (Nut 7) A helical groove 71 for accommodating the balls 6 together with the helical groove 51 of the screw shaft 5 is formed on the inner circumferential surface 70 of the nut 7. The nut 7 is also formed with an accommodation portion 80 that accommodates the end deflector 8, and a clip groove 72 into which the circlip 9 is fitted. In the ball screw 1 according to the present embodiment, the accommodation portion 80 and the clip groove 72 are formed on both axial sides of the helical groove 71, respectively, so as to sandwich the helical groove 71. Hereinafter, the side closer to the axial center portion of the helical groove 71 may be referred to as the "first side", and the side farther from the axial center portion may be referred to as the "second side". Further, the direction from the axial center CL (see FIG. 2) of the screw shaft 5 toward the inner circumferential surface 70 may be referred to as the "radial direction", and in the radial direction, the side closer to the axial center CL may be referred to as the "inner side", and the side farther from the axial center CL may be referred to as the "outer side".

[0015] The housing portion 80 is a part of the nut 7 that is recessed toward the first side from the axial end face 73 and recessed toward the outward side from the inner circumferential surface 70. Furthermore, the nut 7 has a through hole 74 formed in the outer part of the helical groove 71, which penetrates axially to allow two housing portions 80 to pass through. The through hole 74 functions as a circulation path for moving the ball 6 axially. An opening recess 75 is formed in the opening of the through hole 74 into which the first fitting portion 17 and the second fitting portion 27 of the end deflector 8, which will be described later, fit.Hereinafter, when viewing the housing portion 80 axially from the second side to the first side, the direction of the tangential line from the center O of the through hole 74 to the inner circumferential surface 70 may be referred to as the "tangential direction," and the direction perpendicular to the tangential direction may be referred to as the "orthogonal direction."In the orthogonal direction, the side from the center O of the through hole 74 to the inner circumferential surface 70 may be referred to as the "third side," and the side from the center O of the through hole 74 to the outer circumferential surface 76 may be referred to as the "fourth side."

[0016] The housing section 80 has a first wall surface 81 and a second wall surface 82 formed tangentially from the inner circumferential surface 70 side to the through hole 74 side. The housing section 80 also has an outer circumferential wall surface 83 formed in an arc shape centered on the center O of the through hole 74 so as to connect the end of the first wall surface 81 on the through hole 74 side and the end of the second wall surface 82 on the through hole 74 side. The outer circumferential wall surface 83 is the deepest part in the tangential direction. The housing section 80 is also the deepest part in the axial direction and has abutment surface 84 against which the end deflector 8 abuts.

[0017] (End deflector 8) As shown in Figure 2, the end deflector 8 forms a passage for the ball 6 between the helical groove 51 of the screw shaft 5 and the through hole 74 of the nut 7, and is a component that switches the passage of the ball 6 from the helical groove 51 to the through hole 74, or from the through hole 74 to the helical groove 51.

[0018] The end deflector 8 comprises a first member 10 and a second member 20 fitted into the first member 10. The second member 20 is fitted into the first member 10 by moving axially from the first side to the second side relative to the first member 10.

[0019] The first member 10 is a member in which a passage recess 11 constituting a passage for the ball 6 is formed. The first member 10 has a first side portion 12 provided on the first side of the passage recess 11, a second side portion 13 provided on the second side of the passage recess 11, and a first outer peripheral portion 14 provided on the outer peripheral side of the passage recess 11. Furthermore, the first member 10 is provided at the lower part of the passage recess 11 and has a facing portion 15 that protrudes toward the axis CL side from the inner circumferential surface 70 of the nut 7 and faces the helical groove 51 of the screw shaft 5.

[0020] Furthermore, the first member 10 has a first fitting portion 17 that protrudes to the first side from the first end face 16, which is the first end face of the first side portion 12, and fits into the opening recess 75 of the through hole 74 of the nut 7. Furthermore, the first side portion 12 has a protruding recess 18 into which a protruding portion 28 of the second member 20, which will be described later, fits. The protruding recess 18 is recessed in a rectangular parallelepiped shape from the first end face 16 toward the second side. The first member 10 has a second end face 19, which is the second end face of the second side portion 13, that contacts the circlip 9.

[0021] The second member 20 is a thin plate-shaped member provided around the first member 10. The second member 20 covers the opening in the passage recess 11 and, together with the passage recess 11, constitutes the passage for the ball 6. The second member 20 has a third side portion 21 provided on the third side of the first member 10, a fourth side portion 22 provided on the fourth side of the first member 10, and a second outer peripheral portion 23 provided on the outer peripheral side of the first outer peripheral portion 14. The surface of the third side portion 21 facing the first wall surface 81 is the first side surface 24, the surface of the fourth side portion 22 facing the second wall surface 82 is the second side surface 25, and the surface of the second outer peripheral portion 23 facing the outer peripheral wall surface 83 is the outer peripheral side surface 26.

[0022] Furthermore, the second member 20 has a second fitting portion 27 that protrudes to the first side from the first end face of at least one of the fourth side portion 22 and the second outer peripheral portion 23 and fits into the opening recess 75 of the through hole 74. Furthermore, the second member 20 has a projection 28 at the first end of the fourth side portion 22 that protrudes in a rectangular parallelepiped shape from the third side surface of the fourth side portion 22 toward the third side.

[0023] Furthermore, the second member 20 has a first protrusion 31 that projects from the first side surface 24 toward the third side. As shown in Figure 3, the first protrusion 31 is located at the end of the first side surface 24 on the tangential side toward the inner circumferential surface 70, and is also provided at the end on the second side. As shown in Figures 7 and 8, there may be multiple first protrusions 31 provided in the tangential direction. Furthermore, the second member 20 has a second protrusion 32 that projects from the second side surface 25 toward the fourth side. As shown in Figure 3, the second protrusion 32 is located at the end of the second side surface 25 on the tangential through hole 74 side and is provided at the end on the second side. The second protrusion 32 may also be provided so as to project from the outer peripheral side surface 26 toward the outer peripheral wall surface 83 side. Furthermore, as shown in Figures 3 and 6, the second member 20 has a third protrusion 33 that projects from the second side surface 25 toward the fourth side. The third protrusion 33 is provided at the end of the second side surface 25 on the tangential inner circumferential surface 70 side. The third protrusion 33 is provided over the entire axial area of ​​the second member 20.

[0024] In the end deflector 8 configured as described above, the second member 20 is fitted into the first member 10 by moving axially from the first side to the second side relative to the first member 10. At this time, the protruding portion 28 of the second member 20 is fitted into the protruding recess 18 formed in the first member 10. Then, the movement of the second member 20 toward the second side relative to the first member 10 is suppressed when the protruding portion 28 abuts against the first side portion 12.

[0025] Then, the end deflector 8, with the first member 10 and the second member 20 integrated, is fitted into the housing portion 80 of the nut 7. In other words, as shown in Figure 4, the end deflector 8 is fitted so that the first fitting portion 17 of the first member 10 and the second fitting portion 27 of the second member 20 are fitted into the opening recess 75 of the through hole 74, and the first end face 16 of the first member 10 abuts against the abutment surface 84 of the nut 7.

[0026] Then, the circlip 9 is fitted into the clip groove 72 of the nut 7. The clip groove 72 is formed so that the circlip 9 is positioned on the second side of the end deflector 8 fitted into the housing portion 80 of the nut 7, and the circlip 9 is fitted into the second side of the end deflector 8. This suppresses the movement of the end deflector 8 toward the second side relative to the nut 7. Here, the second member 20 is prevented from moving toward the second side relative to the first member 10 by the projection 28 abutting against the first side portion 12 of the first member 10. Also, as shown in Figure 4, the axial size W1 of the first member 10 is larger than the axial size W2 of the second member 20. Therefore, the second end face 19 of the first member 10 contacts the circlip 9, while the second member 20 does not contact the circlip 9.

[0027] Furthermore, the end deflector 8 increases the adhesion force between the first member 10 and the second member 20 as the first protrusion 31 of the second member 20 receives force from the first wall surface 81, and the second protrusion 32 of the second member 20 receives force from the second wall surface 82. Furthermore, in the end deflector 8, as shown in Figure 3, the first protrusion 31 is positioned to generate a moment M1 that causes the second side surface 25 to rotate toward the second wall surface 82. In other words, as shown in Figure 3, the first protrusion 31 is positioned on the first side surface 24 at a distance L1 tangentially from the center O of the through hole 74 toward the inner circumferential surface 70. Therefore, if the force that the end deflector 8 receives from the nut 7 at the first protrusion 31 is F1, a moment of F1 × L1 = M1 is generated in the end deflector 8.

[0028] As described above, the ball screw 1 comprises an end deflector 8 and a nut 7 having a housing portion 80 for housing the end deflector 8. The housing portion 80 has a first wall surface 81 and a second wall surface 82 formed from the inner circumferential surface 70 of the nut 7 toward the outer circumferential surface 76 and facing each other. The end deflector 8 has a first side surface 24 facing the first wall surface 81, a second side surface 25 facing the second wall surface 82, and a first protrusion 31 as an example of a protrusion projecting from the first side surface 24 toward the first wall surface 81. The first protrusion 31 is positioned to generate a moment M1 that causes the second side surface 25 to rotate toward the second wall surface 82.

[0029] In the ball screw 1 configured as described above, the moment M1 causes the second side surface 25 of the end deflector 8 to move toward the second wall surface 82 of the nut 7. Therefore, as shown in Figure 5, the occurrence of a step between the inner surface 29, which is the third side surface of the fourth side portion 22 of the end deflector 8, and the helical groove 71 of the nut 7 is suppressed. As a result, the ball 6 can move smoothly from between the helical groove 71 of the nut 7 and the helical groove 51 of the screw shaft 5 into the end deflector 8. Thus, the ball screw 1 can improve the sliding properties and quietness of the ball 6.

[0030] Furthermore, the end deflector 8 has a second protrusion 32 that projects from the second side surface 25 toward the second wall surface 82. This makes it easier for the end deflector 8 to be positioned at the center between the first wall surface 81 and the second wall surface 82. Furthermore, the nut 7 has a through hole 74 formed between its inner circumferential surface 70 and outer circumferential surface 76, which serves as an example of a circulation path for moving the ball 6 in the axial direction of the screw shaft 5, and the first protrusion 31 is positioned further from the through hole 74 than the second protrusion 32. For example, as shown in Figure 3, when the second protrusion 32 is positioned on the second side surface 25 at a distance L2 tangentially from the center O of the through hole 74 toward the inner circumferential surface 70, the distance L1 is greater than the distance L2. Therefore, if the force that the end deflector 8 receives from the nut 7 at the second protrusion 32 is F2, a moment of F2 × L2 = M2 is generated on the end deflector 8, but if F1 = F2, then M1 is greater than M2. As a result, a moment acts on the end deflector 8 that causes the second side surface 25 to rotate toward the second wall surface 82, and the occurrence of a step between the inner surface 29 of the end deflector 8 and the helical groove 71 of the nut 7 is suppressed. Therefore, the ball 6 moves smoothly into the end deflector 8 from between the helical groove 71 of the nut 7 and the helical groove 51 of the screw shaft 5.

[0031] Here, the end deflector 8 according to this embodiment comprises a first member 10 having an opposing portion 15 facing the screw shaft 5, and a second member 20 provided around the first member 10 and having a first side surface 24 and a second side surface 25. However, the end deflector 8 does not have to be composed of two parts, the first member 10 and the second member 20, and may be composed of one part. When the end deflector 8 is composed of one part, it can be manufactured, for example, by machining a molded product formed using a mold from metal or resin. In contrast, when it is composed of two parts, the first member 10 and the second member 20, it can be manufactured more easily than when it is composed of one part, for example, by forming the passage recess 11 of the second member 20 without machining. The materials of the first member 10 and the second member 20 can be exemplified as metal or resin. Also, either the first member 10 or the second member 20 may be metal and the other may be resin. For example, if the first member 10 is made of metal and the second member 20 is made of resin, wear on the second end face 19 that the circlip 9 contacts can be suppressed, and the end deflector 8 can be easily press-fitted into the housing portion 80 of the nut 7.

[0032] The end deflector 8 is press-fitted into the housing 80 along the axial direction of the screw shaft 5, and the first protrusion 31 is formed further outward (in other words, closer to the second side) than the axial center of the first side surface 24. This reduces the distance the end deflector 8 must be moved while keeping the first protrusion 31, which faces the first wall surface 81 of the housing 80, in contact with the first wall surface 81 when assembling the end deflector 8 into the housing 80, thus making it easier to assemble the end deflector 8. However, the position where the first protrusion 31 is provided is not limited. Furthermore, since the second protrusion 32 is formed closer to the outside (in other words, closer to the second side) than the axial center of the first side surface 24, the end deflector 8 can be easily assembled to the nut 7. However, the position where the second protrusion 32 is provided is not limited.

[0033] Here, the first protrusion 31 is integrally molded together with the first side surface 24. For example, when molding the second member 20 using a mold, the first side surface 24 and the first protrusion 31 are integrally molded by the mold. Also, the second protrusion 32 is integrally molded together with the second side surface 25. For example, when molding the second member 20 using a mold, the second side surface 25 and the second protrusion 32 are integrally molded by the mold. This makes it easy to mold the second member 20.

[0034] However, at least one of the first protrusion 31 and the second protrusion 32 may be molded separately from the first side surface 24. For example, the first protrusion 31 may be an elastic member (e.g., rubber) attached to the first side surface 24 which is molded from resin. The second protrusion 32 may be an elastic member (e.g., rubber) attached to the second side surface 25 which is molded from resin. By having at least one of the first protrusion 31 and the second protrusion 32 be an elastic member, the end deflector 8 can be easily assembled to the nut 7, and the adhesion force between the first member 10 and the second member 20 can be increased.

[0035] <Second Embodiment> Figure 10 is an example of a view of the nut 207 and end deflector 208 of the ball screw 2 according to the second embodiment, as seen in direction III of Figure 2. The ball screw 2 according to the second embodiment differs from the ball screw 1 according to the first embodiment in the shape of the end deflector 208 corresponding to the end deflector 8 and the nut 207 corresponding to the nut 7. The differences from the ball screw 1 according to the first embodiment will be described below. Parts having the same shape and function in the ball screw 1 according to the first embodiment and the ball screw 2 according to the second embodiment are denoted by the same reference numerals, and their detailed descriptions will be omitted.

[0036] The end deflector 208 differs from the end deflector 8 according to the first embodiment in that the third side portion 221 of the second member 220, which corresponds to the second member 20, is molded to be parallel to the radial direction. The second member 220 also has a projection 229 that protrudes from the inner end of the third side portion 221 toward the through hole 74. The end deflector 208 also differs from the end deflector 8 in that the first outer peripheral portion 214 of the first member 210, which corresponds to the first member 10, has a radial portion 215 that is parallel to the radial direction so as to correspond to the third side portion 221. The projection 229 of the second member 220 is positioned on the inner side of the inner end face of the radial portion 215.

[0037] The nut 207 differs from the nut 7 according to the first embodiment in that the housing portion 280, which corresponds to the housing portion 80, has a radial wall surface 285 that is parallel to the radial direction. The radial wall surface 285 is formed outside the first wall surface 81 and at a position further from the through hole 74 than the first wall surface 81. The second member 220 has a first protrusion 231 that projects radially toward the wall surface 285 from the first side surface 224, which is the side of the third side portion 221 opposite to the first member 210. The first protrusion 231 is located at the inner end of the first side surface 224 and is provided at the end on the second side.

[0038] In the end deflector 208, the adhesion force between the first member 210 and the second member 220 increases when the first protrusion 231 of the second member 220 receives a force from the radial wall surface 285, and when the second protrusion 32 of the second member 220 receives a force from the second wall surface 82. Furthermore, as shown in Figure 10, the first protrusion 231 is positioned to generate a moment M3 that causes the second side surface 25 to rotate in the direction toward the second wall surface 82. In other words, as shown in Figure 10, the first protrusion 231 is positioned on the first side surface 224 at a distance L3 radially inward from the center O of the through hole 74. Therefore, if the force that the end deflector 208 receives from the nut 207 at the first protrusion 231 is F3, a moment of F3 × L3 = M3 is generated in the end deflector 208.

[0039] In the ball screw 2 configured as described above, the moment M3 causes the second side surface 25 of the end deflector 208 to move toward the second wall surface 82 of the nut 207. Therefore, the occurrence of a step between the inner surface 29, which is the third side surface of the fourth side portion 22 of the end deflector 208, and the helical groove 71 of the nut 207 is suppressed. As a result, the ball 6 can move smoothly from between the helical groove 71 of the nut 207 and the helical groove 51 of the screw shaft 5 into the end deflector 208. Thus, the ball screw 2 can improve the sliding properties and quietness of the ball 6.

[0040] Furthermore, with respect to the ball screw 2, when the ball 6 contacts the opposing portion 15, the first side surface 224 of the second member 220 contacts the radial wall surface 285, thereby suppressing the rotation of the end deflector 208 in a direction away from the second wall surface 82 of the nut 207 by the second side surface 25. [Explanation of Symbols]

[0041] 1,2…Ball screw, 5…Screw shaft, 6…Ball, 7,207…Nut, 8,208…End deflector, 10,210…First member, 15…Opposite part, 20,220…Second member, 24,224…First side surface, 25…Second side surface, 26…Outer peripheral side surface, 31,231…First protrusion (example of protrusion), 32…Second protrusion, 70…Inner peripheral surface, 74…Through hole, 76…Outer peripheral surface, 80,280…Housing section, 81…First wall surface, 82…Second wall surface, 83…Outer peripheral wall surface, 100…Electric power steering device, 106…Rack shaft, 121…Electric motor, 285…Radial wall surface (example of first wall surface)

Claims

1. Electric motor and, A ball screw that transmits the rotational driving force of the electric motor to a nut and converts it into a moving force of a rack shaft on which a screw shaft is formed that engages with the nut via a plurality of balls, An electric power steering system comprising, The nut has a housing portion for housing the end deflector, The housing portion has a first wall surface and a second wall surface that are formed facing each other and extending from the inner circumferential surface to the outer circumferential surface of the nut, The end deflector has a first side surface facing the first wall surface, a second side surface facing the second wall surface, and a protrusion projecting from the first side surface toward the first wall surface. The protrusion is positioned such that it generates a moment that causes the second side surface to rotate toward the second wall surface. An electric power steering system characterized by the following features.

2. A screw shaft, a nut having a housing portion formed therein that engages the screw shaft via a plurality of balls and houses an end deflector, A ball screw equipped with, The housing portion has a first wall surface and a second wall surface that are formed facing each other and extending from the inner circumferential surface to the outer circumferential surface of the nut, The end deflector has a first side surface facing the first wall surface, a second side surface facing the second wall surface, and a protrusion projecting from the first side surface toward the first wall surface. The protrusion is positioned such that it generates a moment that causes the second side surface to rotate toward the second wall surface. A ball screw characterized by the following features.

3. The housing portion has an outer peripheral wall surface formed between the end of the first wall surface on the outer peripheral surface side and the end of the second wall surface on the outer peripheral surface side. The end deflector has an outer peripheral surface facing the outer peripheral wall surface between the first and second side surfaces, and a second protrusion that protrudes from the outer peripheral surface toward the outer peripheral wall surface or from the second side surface toward the second wall surface. The ball screw according to claim 2.

4. The nut has a circulation path formed between the inner surface and the outer surface that moves the ball in the axial direction of the screw shaft. The aforementioned protrusion is located further from the circulation path than the second protrusion. The ball screw according to claim 3.

5. The end deflector comprises a first member having an opposing portion facing the screw shaft, and a second member provided around the first member and having the first side surface and the second side surface. The ball screw according to claim 2.

6. The end deflector is press-fitted into the housing along the axial direction of the screw shaft, The aforementioned protrusion is formed further outward than the central portion in the axial direction on the first side surface. The ball screw according to claim 5.

7. The aforementioned protrusion is integrally molded with the first side surface. The ball screw according to claim 6.

8. The aforementioned protrusion is an elastic member attached to the first side surface. The ball screw according to claim 6.

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