Electric Actuator

Abstract

To provide an electric actuator capable of keeping smooth sliding of a piston.SOLUTION: An electric actuator according to an embodiment includes a motor, a ball screw device, a piston, and a housing. A wall surface has a sliding surface for slidably supporting an outer peripheral surface of the piston. A screw shaft includes a screw shaft body, and a power transmission portion. A nut has a nut body and a fitting portion. The nut body has a pressing surface which is an end surface facing in a second direction so as to press the piston in the second direction. The piston has a piston body which is disposed in the second direction with respect to the nut body and has an outer peripheral surface supported by the sliding surface, a cylindrical extension portion which extends in a first direction from an outer peripheral side portion of an end in the first direction of the piston body so that the nut body is arranged in an inner peripheral side, an end surface which is an end surface in the first direction of the piston body and is brought into contact with the pressing surface, and a cylindrical fitted portion which extends in the first direction from the extension portion so that the fitting portion is fitted in the inner peripheral side.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an electric actuator. [Background technology]

[0002] An electric actuator includes a motor that generates rotational motion, a ball screw device that converts the rotational motion into linear motion, a piston that exerts a pressing force, and a housing. Such electric actuators are used, for example, in brake devices. In the brake device disclosed in the following patent document, the piston is fitted into a nut. The outer peripheral surface of the piston is slidably supported on the sliding surface of the housing. When the ball screw device is operated, the piston presses the brake pad, pressing the brake pad against the brake disc. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-65923 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the above patent document, the piston is fitted only to the tip of the nut. Therefore, when the piston travels a large distance (protrusion), the piston and nut are positioned on the inner periphery of the sliding surface. In other words, the nut faces a portion of the sliding surface, and the engagement between the piston and the sliding surface (the axial range of the opposing surfaces) is short. As a result, the piston is prone to tilting, which may impede smooth sliding of the piston. For these reasons, there is a need for a ball screw device that maintains smooth sliding of the piston even when the piston travels a large distance.

[0005] The present disclosure has been made in view of the above, and has an object to provide an electric actuator in which smooth sliding of a piston is maintained. [Means for solving the problem]

[0006] To achieve the above object, an electric actuator according to one aspect of the present disclosure includes a motor that generates rotational motion, a ball screw device having a screw shaft, a nut, and balls and converting the rotational motion into linear motion, a piston that fits into the nut and moves in an axial direction parallel to the screw shaft, and a housing that has an accommodation space extending in the axial direction and a cylindrical wall surface surrounding the accommodation space, with the ball screw device and the piston disposed in the accommodation space. The wall surface has a cylindrical sliding surface that slidably supports the outer circumferential surface of the piston. The screw shaft includes a screw shaft main body having an outer circumferential raceway surface on its outer circumferential surface, and a power transmission part that is disposed relative to the screw shaft main body in a first direction, which is one of the axial directions, and is rotatably supported by the housing. The nut includes a nut main body having an inner circumferential raceway surface on its inner circumferential surface, and a fitting part that is disposed relative to the nut main body in the first direction and has an outer diameter larger than the outer diameter of the nut main body. The nut body has an end face facing a second direction opposite to the first direction and a pressing surface that presses the piston in the second direction. The piston is arranged in the second direction with respect to the nut body, has an outer diameter larger than that of the nut body, and has an outer peripheral surface supported by the sliding surface, a cylindrical extension portion that extends in the first direction from an outer peripheral part of an end of the piston body in the first direction and has the nut body arranged on its inner peripheral side, an end face that is an end face of the piston body in the first direction and is arranged on the inner peripheral side of the extension portion and abuts against the pressing surface, and a cylindrical fitted portion that extends in the first direction from the extension portion and has an inner peripheral side on which the fitting portion fits.

[0007] According to the above-described invention, the outer peripheral surface of the nut body is covered by the expansion portion of the piston. Therefore, even if the piston moves a large distance in the second direction, the outer peripheral surface of the piston body and the outer peripheral surface of the expansion portion are supported by the sliding surface. In other words, according to the present disclosure, the sliding surface and the nut body do not face each other, making the piston less likely to tilt and improving piston operability. Furthermore, when the piston moves in the second direction, the piston receives a reaction force from the pressing object. This causes a compressive load to act on the ball screw device, elastically deforming the screw shaft body and the nut body. In other words, the inner peripheral raceway surface and the outer peripheral raceway surface are displaced in the axial direction, increasing the load on the balls. In the present disclosure, the load acting in the first direction from the piston to the nut acts on the pressing surface of the nut. Therefore, the displacement amount of each thread of the inner peripheral raceway surface is greater the closer it is to the end (pressing surface) in the second direction. Meanwhile, the power transmission unit of the screw shaft is supported by the housing. Therefore, the displacement amount of each thread of the outer peripheral raceway surface is greater the closer it is to the end (power transmission unit) in the first direction. As a result, the areas where the load increases among the multiple balls are the balls that roll closer to the first direction in the raceway and the balls that roll closer to the second direction. In other words, the areas where the load increases are distributed to both sides in the axial direction. This makes it possible to even out the load distribution on the balls.

[0008] In a preferred embodiment of the electric actuator, a spiral raceway is provided between the inner raceway surface and the outer raceway surface of the nut body, and the balls roll on the raceway.

[0009] In the present disclosure, the fitting portion is deformed when fitted into the fitted portion. Therefore, if an inner circumferential raceway surface or a circulation portion (such as an S-shaped groove) is provided in the fitting portion, the inner circumferential raceway surface or the circulation portion will be deformed, and the balls will not be able to roll smoothly. On the other hand, with the above configuration, the balls do not roll on the inner circumferential side of the fitting portion. Therefore, smooth rolling of the balls is ensured, and the operability of the ball screw device is excellent.

[0010] In a preferred embodiment of the electric actuator, the inner diameter of the expansion portion is larger than the outer diameter of the nut body. A cylindrical gap is provided between the inner peripheral surface of the expansion portion and the outer peripheral surface of the nut body.

[0011] If the nut body were to fit into the expansion portion, the inner raceway surface of the nut body would be deformed, making it impossible to ensure smooth rolling of the balls. Furthermore, the fitting would cause the expansion portion to expand in diameter, potentially impairing smooth sliding of the piston. On the other hand, with the above configuration, deformation of the inner raceway surface is avoided, allowing the balls to roll smoothly. Furthermore, expansion of the expansion portion is avoided, allowing the piston to slide smoothly.

[0012] In a preferred aspect of the electric actuator, an end portion of the inner circumferential surface of the expansion portion in the second direction abuts against an outer circumferential surface of the nut body.

[0013] According to this configuration, the end of the nut in the second direction is supported by the inner circumferential surface of the expansion portion, making the nut less likely to tilt and allowing the nut to move smoothly in the axial direction.

[0014] In a preferred embodiment of the electric actuator, an end portion of the inner circumferential surface of the expansion portion in the second direction is fitted into an outer circumferential surface of the nut body.

[0015] According to the above configuration, the end of the nut in the second direction is supported by the inner circumferential surface of the expansion portion. This makes the nut less likely to tilt and allows the nut to move smoothly in the axial direction. In addition, the number of fitting points increases, so the nut and piston are more firmly fixed together. This makes it difficult for the nut and piston to separate.

[0016] In a preferred embodiment of the electric actuator, the wall surface of the housing has a cylindrical large-diameter surface that is disposed in the first direction relative to the sliding surface and has a diameter larger than that of the sliding surface, and the axial movement range of the fitted portion is limited to an inner peripheral side of the large-diameter surface within the accommodation space.

[0017] The mated portion may expand in diameter as the mating portion is mated with it. In other words, the mated portion may get caught on the wall surface of the housing, hindering the smooth sliding of the piston. On the other hand, with the above configuration, the mated portion does not move toward the inner periphery of the sliding surface. In other words, the mated portion does not get caught on the sliding surface. Therefore, the smooth sliding of the piston is ensured.

[0018] In a preferred embodiment of the electric actuator, the outer diameter of the fitted portion is larger than the diameter of the sliding surface. A first concave surface recessed toward the inner periphery is provided on the outer circumferential surface of the fitted portion. A second concave surface facing the first concave surface and extending in the axial direction is provided on the large diameter surface. A pin extending in the axial direction is housed in the second concave surface. At least a portion of the pin protrudes toward the inner periphery beyond the large diameter surface and penetrates the interior of the first concave surface in the axial direction.

[0019] According to the above configuration, the fitted portion has an outer diameter larger than the diameter of the sliding surface, so it does not penetrate into the inner periphery of the sliding surface. Therefore, the fitted portion does not get caught on the sliding surface. Furthermore, because the fitted portion gets caught on the pin, the piston is supported non-rotatably but axially movable. Therefore, the nut fixed to the piston is also supported non-rotatably but axially movable.

[0020] In a preferred embodiment of the electric actuator, the nut is disposed in the first direction relative to the fitting portion and has a protruding portion that protrudes radially outward beyond the fitted portion. A third concave surface recessed toward the inner periphery is provided on the outer circumferential surface of the protruding portion. A second concave surface facing the third concave surface and extending in the axial direction is provided on the large diameter surface. A pin extending in the axial direction is housed in the second concave surface. At least a portion of the pin protrudes toward the inner periphery beyond the large diameter surface and penetrates the interior of the third concave surface in the axial direction.

[0021] According to this configuration, the protrusion catches on the pin, and the nut is supported so as to be non-rotatable but movable in the axial direction.

[0022] In a preferred embodiment of the electric actuator, the nut is disposed in the first direction relative to the fitting portion and has a protruding portion with an outer diameter equal to the outer diameter of the fitted portion. The outer peripheral surface of the protruding portion is provided with a third concave surface recessed toward the inner peripheral side. The pin passes through the interior of the first concave surface and the interior of the third concave surface in the axial direction.

[0023] According to this configuration, the protrusion catches on the pin, and the nut is supported so as to be non-rotatable but movable in the axial direction. [Effects of the Invention]

[0024] According to the electric actuator of the present disclosure, when the piston moves a large distance, the expansion portion faces the sliding surface, thereby maintaining smooth sliding of the piston. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view of a brake caliper according to a first embodiment, taken along an axial direction, in a state prior to actuation. [Figure 2] FIG. 2 is an enlarged view of a part of FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. [Figure 5] FIG. 5 is a cross-sectional view of the brake caliper of the first embodiment taken along the axial direction in a state after actuation. [Figure 6] FIG. 6 is a cross-sectional view of the electric actuator of the second embodiment taken along the axial direction in a state before operation. [Figure 7] FIG. 7 is a cross-sectional view of the electric actuator of the second embodiment taken along the axial direction in a state after actuation. [Figure 8] FIG. 8 is a cross-sectional view of the electric actuator of the third embodiment taken along the axial direction in a state before operation. [Figure 9]FIG. 9 is an enlarged view of a cross section taken along the axial direction of the electric actuator of the fourth embodiment in a state before operation. [Figure 10] FIG. 10 is an enlarged view of a cross section cut in the axial direction of the electric actuator of the fifth embodiment in a state before operation. [Figure 11] FIG. 11 is a cross-sectional view of the electric actuator of the modified example taken along the axial direction. DETAILED DESCRIPTION OF THE INVENTION

[0026] The embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the contents described below. Furthermore, the components described below include those that can be easily imagined by a person skilled in the art and those that are substantially the same. Furthermore, the components described below can be combined as appropriate.

[0027] (Embodiment 1) Fig. 1 is a cross-sectional view of a brake caliper according to a first embodiment taken along an axial direction before actuation. As shown in Fig. 1, the brake device according to the first embodiment is a brake caliper 100. The brake caliper 100 is a device that applies braking force to a wheel (not shown) by sandwiching a brake disc 101, which rotates together with the wheel, between two brake pads 102 and 103. The brake caliper 100 includes the brake disc 101, two brake pads 102 and 103, and an electric actuator 104 that actuates the brake pad 102.

[0028] The electric actuator 104 includes a motor (not shown) that generates rotational motion, a reduction gear 110 that decelerates the rotational motion, a ball screw device 1 that converts the rotational motion into linear motion, a piston 40 that presses the brake pad 102, and a housing 120.

[0029] In the following description, the direction parallel to the axis O of the screw shaft 2 of the ball screw device 1 is referred to as the axial direction. In addition, within the axial direction, the direction in which the ball screw device 1 is disposed as viewed from the brake disc 101 is referred to as the first direction X1, and the direction opposite to the first direction X1 is referred to as the second direction X2.

[0030] The housing 120 has a brake space S10 in which the brake disc 101 and brake pads 102, 103 are arranged, a first accommodating space S1 communicating with the brake space S10, and a second accommodating space S2 arranged in the first direction X1 relative to the first accommodating space S1.

[0031] The first accommodating space S1 and the second accommodating space S2 are cylindrical spaces centered on the axis O. The diameter of the first accommodating space S1 is smaller than the diameter of the second accommodating space S2. The housing 120 has a cylindrical first wall surface 121 that surrounds the first accommodating space S1 and a cylindrical second wall surface 122 that surrounds the second accommodating space S2.

[0032] The reduction gear 110 is a planetary gear mechanism and is housed in the second housing space S2. The reduction gear 110 includes an input shaft 111, a sun gear 112, a ring gear 113, a plurality of planetary gears 114, a plurality of transmission shafts 115, and a carrier 116.

[0033] The rotational motion of the motor is input to the input shaft 111. The input shaft 111 is arranged coaxially with the axis O. The sun gear 112 passes through the input shaft 111 and is fixed to the input shaft 111 so as not to rotate. The ring gear 113 is an internal gear centered on the input shaft 111. The outer peripheral surface of the ring gear 113 is fitted into a second wall surface 122 of the housing 120. In this way, the ring gear 113 is fixed to the housing 120.

[0034] The planetary gear 114 is disposed between the sun gear 112 and the ring gear 113. The planetary gear 114 is meshed with the sun gear 112 and the ring gear 113. A transmission shaft 115 passes through the planetary gear 114. The planetary gear 114 is supported so as to be rotatable about the transmission shaft 115.

[0035] The carrier 116 is an annular part centered on the axis O. The outer peripheral surface of the carrier 116 is fitted into the bearing 117. Therefore, the carrier 116 is rotatably supported by the housing 120. The outer ring of the bearing 117 is sandwiched between a stepped surface of the housing 120 and a retaining ring. Therefore, the bearing 117 is fixed so as not to move in the axial direction. The carrier 116 has a flange that abuts against the inner ring of the bearing 117 from the second direction. Therefore, the carrier 116 is fixed so as not to move in the first direction X1. The screw shaft 2 passes through the center of the carrier 116. The carrier 116 and the screw shaft 2 are spline-fitted. Therefore, the carrier 116 and the screw shaft 2 are connected so as not to rotate relative to each other. The transmission shaft 115 passes through the carrier 116 at a position eccentric radially outward from the center of the carrier 116.

[0036] As described above, when rotational motion is input to the input shaft 111, the sun gear 112 rotates around the axis O. The planetary gear 114 rotates (revolves) around the axis O while rotating (spinning) around the transmission shaft 115. This causes the carrier 116 and the screw shaft 2 to rotate around the axis O. The rotational speed of the screw shaft 2 is reduced below the rotational speed of the input shaft 111.

[0037] The ball screw device 1 includes a screw shaft 2, a nut 3, and balls 4. The screw shaft 2 includes a power transmission unit 10 fitted into a carrier 116, and a screw shaft main body 11 arranged in the second direction X2 relative to the power transmission unit 10. An outer peripheral raceway surface 12 extending in the spiral direction is provided on the outer peripheral surface of the screw shaft main body 11. The power transmission unit 10 has a smaller diameter than the screw shaft main body 11. As a result, a stepped surface facing the first direction X1 is provided between the power transmission unit 10 and the screw shaft main body 11. This stepped surface abuts against the side surface of the carrier 116. Therefore, the screw shaft 2 is fixed so as not to move in the first direction X1.

[0038] The nut 3 has a cylindrical shape. An inner peripheral surface of the nut 3 is provided with an inner peripheral raceway surface 3a that faces the outer peripheral raceway surface 12. A spiral raceway 8 is formed between the outer peripheral raceway surface 12 and the inner peripheral raceway surface 3a. A plurality of balls 4 are arranged in the raceway 8. The piston 40 is a cylindrical component with a bottom and a lid portion 41 that closes the end in the second direction X2. The piston 40 is fixed to the nut 3, as will be described in detail later. The lid portion 41 of the piston 405 abuts against the brake pad 102.

[0039] Therefore, when the nut 3 moves in the second direction X2 due to rotation of the screw shaft 2, the piston 40 also moves in the second direction X2. The lid portion 41 of the piston 40 presses the brake pad 102 in the first direction X1, and the brake pad 102 abuts against the brake disc 101. When the piston 5 further moves in the second direction X2, the brake disc 101 is pressed in the second direction X2 and comes into contact with the brake pad 103. As a result, the brake disc 101 is sandwiched between the brake pads 102 and 103, and rotation of the wheel (not shown) is restricted. The electric actuator 104 of embodiment 1 will be described in detail below.

[0040] 2 is an enlarged view of a portion of FIG. 1. As shown in FIG. 2, the nut 3 has a nut body 20 and a fitting portion 30 arranged in a first direction X1 relative to the nut body 20. The outer peripheral surface 31 of the fitting portion 30 has a circular cross section. Note that in FIG. 2, an imaginary line H1 is drawn to make the boundary between the nut body 20 and the fitting portion 30 easier to see. The outer diameter of the fitting portion 30 is larger than the outer diameter of the nut body 20. Therefore, the outer peripheral surface 31 of the fitting portion 30 protrudes radially outward beyond the outer peripheral surface 21 of the nut body 20. The amount of radial outward protrusion of the fitting portion 30 is such that there is an interference with the inner peripheral surface of the piston 40 (fitted portion 45).

[0041] The end face of the nut body 20 in the second direction X2 is a pressing surface 22. A plurality of S-shaped grooves 23 (only one is shown in FIG. 2 ) are provided on the inner peripheral surface of the nut body 20. The S-shaped groove 23 is a circulation section that returns the balls 4 that have moved one lead by one lead. Note that in this embodiment, the balls 4 are circulated by the S-shaped groove 23, but the present disclosure is not limited to the S-shaped groove 23. For example, the balls 4 may be circulated by a top. Alternatively, an axial return hole may be provided in the nut 3, and the balls 4 may be circulated by an end deflector or a middle deflector. Alternatively, the balls 4 may be circulated by a tube, and there are no particular limitations on the circulation method.

[0042] On the other hand, no S-groove 23 is provided in the fitting portion 30. In other words, the balls 4 do not roll on the inner raceway surface 3a of the fitting portion 30. In this embodiment, the inner raceway surface 3a is provided on the entire inner surface of the nut 3, but the range in which the balls 4 roll is only the inner raceway surface 3a provided on the nut body 20. Therefore, the raceway 8 is composed of the inner raceway surface 3a and the outer raceway surface 12 of the nut body 20.

[0043] The piston 40 includes a piston body 42, an extension portion 44 arranged in the first direction X1 of the piston body 42, and a fitted portion 45 arranged in the first direction X1 of the extension portion 44.

[0044] The piston body 42 includes a cylindrical portion 43 and a lid portion 41 that closes the second direction X2 of the cylindrical portion 43. The piston body 42 is disposed in the second direction X2 relative to the nut body 20. The inner diameter of the cylindrical portion 43 is approximately the same as the inner diameter of the nut body 20. The outer diameter of the cylindrical portion 43 is larger than the outer diameter of the nut body 20. Therefore, the outer peripheral surface 42a of the piston body 42 protrudes radially outward beyond the nut body 20.

[0045] An end face 42b facing the first direction X1 is provided at an end of the piston body 42 in the first direction X1. The end face 42b is disposed on the inner peripheral side of the expansion portion 44. The end face 42b abuts against the pressing surface 22 of the nut body 20. Therefore, when the nut 3 moves in the second direction X2, the end face 42b is pressed in the second direction X2 by the pressing surface 22.

[0046] The expansion portion 44 is cylindrical and extends in the first direction X1 from the outer circumferential portion of the end of the piston body 42 in the first direction X1. The outer diameter of the expansion portion 44 is the same as the outer diameter of the piston body 42 (cylindrical portion 43). Therefore, the outer circumferential surface 42a of the piston body 42 and the outer circumferential surface 44a of the expansion portion 44 form a cylindrical surface that is continuous in the axial direction.

[0047] The nut body 20 is disposed on the inner peripheral side of the expansion portion 44. The axial length of the expansion portion 44 is substantially the same as that of the nut body 20. Therefore, the fitted portion 45, which is disposed in the first direction X1 relative to the expansion portion 44, and the fitting portion 30, which is disposed in the first direction X1 relative to the nut body 20, overlap in the radial direction.

[0048] Fig. 3 is a cross-sectional view taken along the line III-III in Fig. 2. The inner diameter of the expansion portion 44 is slightly larger than the outer diameter of the nut body 20. Therefore, as shown in Fig. 3, an annular gap S11 is provided between the inner peripheral surface 44b of the expansion portion 44 and the outer peripheral surface 21 of the piston body 42. In other words, the nut body 20 is not fitted into the expansion portion 44.

[0049] As shown in FIG. 2, the fitted portion 45 has a cylindrical shape. Therefore, the inner circumferential surface 45a of the fitted portion 45 has a circular cross section. The inner diameter of the fitted portion 45 is the same as the inner diameter of the expansion portion 44. The fitting portion 30 is disposed on the inner circumferential side of the fitted portion 45. The fitting portion 30 is fitted into the inner circumferential surface 45a of the fitted portion 45. This fixes the piston 40 to the nut 3.

[0050] The outer diameter of the fitted portion 45 is larger than the outer diameters of the piston body 42 and the expanding portion 44. Therefore, the outer peripheral surface 45b of the fitted portion 45 protrudes radially outward beyond the outer peripheral surface 42a of the piston body 42 and the outer peripheral surface 44a of the expanding portion 44.

[0051] Fig. 4 is a cross-sectional view taken along line IV-IV in Fig. 2. As shown in Fig. 4, a first concave surface 46 that is recessed radially inward and opens in the axial direction is provided on the outer peripheral surface 45b of the fitted portion 45. The first concave surface 46 has an arc shape when viewed from the axial direction. In this embodiment, two first concave surfaces 46 are provided at an interval of 180°.

[0052] 2, the first wall surface 121 of the housing 120 has a sliding surface 123 and a large-diameter surface 124 disposed in the first direction X1 relative to the sliding surface 123. The diameter of the sliding surface 123 is slightly larger than the outer diameter of the piston body 42 (cylindrical portion 43). Therefore, a small gap (not shown) is provided between the outer peripheral surface 42a of the piston body 42 and the sliding surface 123. As a result, the piston body 42 is supported on the sliding surface 123 so as to be slidable in the axial direction.

[0053] A recess 130 and a seal portion 131 are provided near the end of the sliding surface 123 in the second direction X2, thereby preventing liquid or foreign matter from entering the first housing space S1 from the brake space S10.

[0054] The large diameter surface 124 is a cylindrical surface having a larger diameter than the sliding surface 123. Therefore, a step surface 125 facing the first direction X1 is provided between the large diameter surface 124 and the sliding surface 123.

[0055] The large diameter surface 124 is formed to have a diameter larger than the outer diameter of the fitted portion 45. Therefore, as shown in FIG. 4, an annular gap S12 is provided between the large diameter surface 124 and the outer peripheral surface 45b of the fitted portion 45. Note that the fitted portion 45 may expand in diameter as the fitting portion 30 fits into it. Therefore, if the fitted portion 45 expands in diameter, this is absorbed by the gap S12. Therefore, the fitted portion 45 does not come into contact with the large diameter surface 124.

[0056] 3, the gap S12 also extends between the large diameter surface 124 and the expanded portion 44. The radial gap S12 between the large diameter surface 124 and the expanded portion 44 is larger than the radial gap between the large diameter surface 124 and the outer peripheral surface 45b of the fitted portion 45.

[0057] As shown in FIG. 4, the large diameter surface 124 is provided with a second concave surface 126 that faces the first concave surface 46. The second concave surface 126 has an arc shape when viewed from the axial direction. Two second concave surfaces 126 are provided at 180° intervals to correspond to the first concave surfaces 46. The second concave surfaces 126 extend in the axial direction (see FIG. 2). A cylindrical pin 60 is disposed in this second concave surface 126. The pin 60 is restricted by a C-ring (not shown) to prevent it from slipping out in the first direction X1.

[0058] A portion of the pin 60 protrudes radially inward beyond the large diameter surface 124. A portion of the pin 60 is disposed inside the first concave surface 46 of the piston 40 and passes through the fitted portion 45 in the axial direction. A small gap (not shown) is provided between the pin 60 and the first concave surface 46. Therefore, the piston 40 does not rotate even when a load is applied in the rotational direction about the axis O. When a load is applied in the axial direction, the piston 40 moves in the axial direction while being guided by the pin 60. As described above, the piston 40 and the nut 3 are supported by the housing 120 so as to be movable in the axial direction but not rotatable in the axial direction.

[0059] Next, details of each component will be described when the brake caliper 100 is operated. First, details of the fitted portion 45 of the piston 40 will be described.

[0060] FIG. 5 is an axial cross-sectional view of the brake caliper of the first embodiment in an actuated state. As shown in FIG. 1, before the brake caliper 100 is actuated, the fitted portion 45 is caught on the end of the pin 60 in the first direction X1. When the motor is driven, the piston 40 (the fitted portion 45) moves in the second direction X2. As shown in FIG. 5, when the fitted portion 45 is caught on the end of the pin in the second direction X2, the fitted portion 45 abuts against the stepped surface 125. Furthermore, the piston 40 does not move further in the second direction X2 from the state in which the fitted portion 45 is caught on the stepped surface 125. In other words, the fitted portion 45 does not move toward the inner periphery of the sliding surface 123. Therefore, the axial movement range of the fitted portion 45 is limited to the inner periphery of the large diameter surface 124 in the first housing space S1. Moreover, the case where the movement amount of the piston 40 in the second direction X2 is maximum is when the fitted portion 45 abuts against the step surface 125.

[0061] Next, the engagement between the piston 40 and the sliding surface 123 (the range of opposing axial movements) will be described. As shown in FIG. 2, before the brake caliper 100 is actuated, the outer peripheral surface 42a of the piston body 42 faces the entire sliding surface 123. When the piston 40 moves in the second direction X2, the outer peripheral surface 42a of the piston body 42 and the outer peripheral surface 44a of the extension portion 44 face the sliding surface 123. As shown in FIG. 5, when the movement amount of the piston 40 in the second direction X2 is maximum, the outer peripheral surface 42a of the piston body 42 and the outer peripheral surface 44a of the extension portion 44 face the sliding surface 123. In this way, the outer peripheral surface of the piston 40 always faces the entire sliding surface 123. Therefore, the engagement between the piston 40 and the sliding surface 123 is constant before and after actuation.

[0062] Next, the reaction force acting on the piston 40 from the brake pad 102 will be described. The piston 40 receives a reaction force (load in the first direction X1) from the pressing brake pad 102. As a result, a compressive load acts on the ball screw device 1, compressing it in the axial direction. In other words, the screw shaft 2 and the nut 3 each elastically deform in the axial direction. Therefore, the outer peripheral raceway surface 12 and the inner peripheral raceway surface 3a are displaced in the axial direction, and the load acting on the balls 4 increases.

[0063] As shown in Figure 5, the load F1 in the first direction X1 acting on the nut 3 from the piston 40 acts on the pressing surface 22 of the nut body 20. Therefore, the stress acting on each groove of the inner raceway surface 3a (see F11 and F12) increases the closer it is to the end (pressing surface 22) in the second direction X2 (F11 > F12). In other words, the closer each groove of the inner raceway surface 3a is to the end (pressing surface 22) in the second direction X2, the greater the amount of displacement in the first direction X1. For the above reasons, of the balls 4 arranged in the raceway 8, the balls 4 located closer to the end in the second direction X2 receive an increased load from the inner raceway surface 3a, and therefore experience an increased load.

[0064] On the other hand, the power transmission unit 10 of the screw shaft 2 is supported by the housing 120 via a carrier 116 and a bearing 117. A load F2 in the second direction X2 acts on the power transmission unit 10, counteracting the reaction force acting on the piston 40. Therefore, the stress acting on each groove of the outer circumferential raceway surface 12 (see F21 and F22) increases the closer it is to the end (power transmission unit 10) in the first direction X1 (F21>F22). In other words, the closer each groove of the outer circumferential raceway surface 12 is to the end (power transmission unit 10) in the first direction X1, the greater the amount of displacement in the first direction X1. For this reason, of the balls 4 arranged in the raceway 8, the balls 4 located near the end (power transmission unit 10) in the first direction X1 receive an increased load from the outer circumferential raceway surface 12, resulting in an increased load.

[0065] Next, the effects of the brake caliper 100 of embodiment 1 will be described. According to this embodiment, the engagement between the piston 40 and the sliding surface 123 does not decrease before and after operation of the brake caliper 100. Therefore, the piston 40 is less likely to tilt, and the operability of the piston 40 is excellent.

[0066] When the brake caliper 100 is operated, the balls 4 receive a reaction force from the brake pads 102, increasing the load on the balls 4. However, the increased load on the balls 4 due to the displacement of the inner peripheral raceway surface 3a is borne by the balls 4 located closer to the end of the raceway 8 in the second direction X2. On the other hand, the increased load on the balls 4 due to the displacement of the outer peripheral raceway surface 12 is borne by the balls 4 located closer to the end of the raceway 8 in the first direction X1. As a result, the increased load is distributed to the balls 4 located on both sides of the raceway 8 in the axial direction. In other words, the load distribution on the balls 4 is made uniform, and the life of the ball screw device 1 is extended.

[0067] The inner circumferential raceway surface 3a is provided on both the nut body 20 and the fitting portion 30. However, the inner circumferential raceway surface 3a of the fitting portion 30 does not constitute the raceway 8. This is because the inner circumferential raceway surface 3a of the fitting portion 30 may be deformed by fitting into the fitted portion 45, which may prevent the balls 4 from rolling smoothly. On the other hand, the nut body 20 is not fitted into the expansion portion 44. In other words, the inner circumferential raceway surface 3a and the S-groove 23 of the nut body 20 are not deformed. For these reasons, in this embodiment, the portion of the inner circumferential raceway surface 3a that constitutes the raceway 8 is limited to the portion provided on the nut body 20. As a result, the balls 4 roll smoothly on the raceway 8, and the operability of the ball screw device 1 is excellent.

[0068] The axial movement range of the fitted portion 45 is limited to the inner circumferential side of the large diameter surface 124 in the first housing space S1. Here, the fitted portion 45 may have a larger diameter than at the time of manufacture because the fitting portion 30 is fitted thereto. In other words, even if the fitted portion 45 is manufactured so that its outer diameter is the same as the outer diameter of the expansion portion 44, the fitted portion 45 may expand in diameter and get caught on the sliding surface 123. Therefore, in this embodiment, to ensure smooth sliding of the piston 40, the fitted portion 45 gets caught on the stepped surface 125 and does not move toward the inner circumferential side of the sliding surface 123.

[0069] As described above, the electric actuator 104 of the first embodiment includes a motor that generates rotational motion, a ball screw device 1 that has a screw shaft 2, a nut 3, and balls 4 and converts the rotational motion into linear motion, a piston 40 that fits into the nut 3 and moves in an axial direction parallel to the screw shaft 2, and a housing 120 that has an accommodation space (first accommodation space S1) extending in the axial direction and a cylindrical wall surface (first wall surface 121) surrounding the accommodation space, and in which the ball screw device 1 and the piston 40 are disposed. The wall surface has a cylindrical sliding surface 123 that slidably supports the outer peripheral surface of the piston 40. The screw shaft 2 includes a screw shaft main body 11 that has an outer peripheral raceway surface 12 provided on its outer peripheral surface, and a power transmission unit 10 that is disposed relative to the screw shaft main body 11 in a first direction X1, which is one axial direction, and is rotatably supported by the housing 120. The nut 3 has a nut body 20 having an inner circumferential raceway surface 3a on its inner circumferential surface, and a fitting portion 30 that is disposed in a first direction X1 relative to the nut body 20 and has an outer diameter larger than that of the nut body 20. The nut body 20 has an end face that faces a second direction X2 opposite to the first direction X1 and has a pressing surface 22 that presses the piston 40 in the second direction X2. The piston 40 has a piston body 42 arranged in the second direction X1 relative to the nut body 20, having an outer diameter larger than the outer diameter of the nut body 20, and an outer peripheral surface 42a supported by the sliding surface 123, a cylindrical extension portion 44 extending in the first direction X1 from the outer peripheral part of the end of the piston body 42 in the first direction X1 and with the nut body 20 arranged on its inner peripheral side, an end surface 42b which is the end face of the piston body 42 in the first direction X and is arranged on the inner peripheral side of the extension portion 44 and abuts the pressing surface 22, and a cylindrical fitted portion 45 extending in the first direction X1 from the extension portion 44 and with the fitting portion 30 fitted on its inner peripheral side.

[0070] According to the electric actuator 104 of the first embodiment, the piston 40 is less likely to tilt, and the operability of the piston 40 is excellent. The load distribution on the balls 4 is made uniform, and the life of the ball screw device 1 is long.

[0071] In the electric actuator 104 of the first embodiment, a spiral raceway 8 is provided between the inner raceway surface 3a and the outer raceway surface 12 of the nut body 20. The balls 4 roll on the raceway 8.

[0072] The balls 4 do not roll on the inner peripheral raceway surface 3a of the fitting portion 30. Therefore, the balls 4 roll smoothly on the raceway 8, and the operability of the ball screw device 1 is excellent.

[0073] In the electric actuator 104 of the first embodiment, the inner diameter of the expansion portion 44 is larger than the outer diameter of the nut body 20. A cylindrical gap S11 is provided between the inner circumferential surface 44b of the expansion portion 44 and the outer circumferential surface 21 of the nut body 20.

[0074] The inner circumferential raceway surface 3a and the S-shaped groove 23 provided on the nut body 20 are not deformed. This ensures smooth rolling of the balls 4. In addition, expansion of the diameter of the expansion portion 44 due to fitting is avoided. This allows the piston 40 to slide smoothly.

[0075] A wall surface (first wall surface 121) of the housing 120 of the electric actuator 104 of the first embodiment is disposed in the first direction X1 with respect to the sliding surface 123 and has a cylindrical large-diameter surface 124 that is larger in diameter than the sliding surface 123. The axial movement range of the fitted portion 45 is limited to the inner peripheral side of the large-diameter surface 124 within the accommodation space (first accommodation space S1).

[0076] The fitted portion 45 faces the large diameter surface 124. Therefore, the fitted portion 45 does not get caught on the sliding surface 123. This ensures smooth sliding of the piston 40.

[0077] Next, a description will be given of another embodiment of the electric actuator 104. In the following description, the differences from the first embodiment will be focused on.

[0078] (Embodiment 2) Fig. 6 is a cross-sectional view of the electric actuator of embodiment 2 taken in the axial direction before operation. Fig. 7 is a cross-sectional view of the electric actuator of embodiment 2 taken in the axial direction after operation. As shown in Fig. 6, the electric actuator 104A of embodiment 2 differs from the electric actuator 104 of embodiment 1 in that the piston 40 has a fitted portion 45A instead of the fitted portion 45. The electric actuator 104A of embodiment 2 also differs from the electric actuator 104 of embodiment 1 in that the nut 3 has a protrusion 35.

[0079] The outer diameter of the fitted portion 45A is manufactured to be the same as the outer diameters of the piston body 42 and the expansion portion 44. However, the fitted portion 30 may be fitted to the inner periphery of the fitted portion 45A, expanding the diameter. Therefore, in this embodiment, the axial movement range of the fitted portion 45A is limited only to the inner periphery of the large-diameter surface 124. That is, as shown in FIG. 7 , even when the piston 40 moves the fitted portion 45A in the second direction X2 to the maximum extent, the fitted portion 45A is not positioned on the inner periphery of the sliding surface 123. Note that the movement range of the fitted portion 45A can be controlled by controlling the rotation of the motor or by making the fitted portion 45A larger in diameter than the sliding surface 123 and abutting it against the step surface 125.

[0080] The protruding portion 35 is disposed in the first direction X1 relative to the fitting portion 30. The outer diameter of the protruding portion 35 is larger than the outer diameter of the fitted portion 45A. In other words, the protruding portion 35 protrudes radially outward from the fitted portion 45A. The outer diameter of the protruding portion 35 is smaller than the large diameter surface 124. The outer peripheral surface 36 of the protruding portion 35 is provided with a third concave surface 37 that is recessed radially inward and opens in the axial direction. The third concave surface 37 has an arc shape when viewed from the axial direction. A portion of the pin 60 is disposed inside the third concave surface 37 and penetrates the protruding portion 35 in the axial direction.

[0081] For the above reasons, the nut 3 will not rotate even when a load is applied in the rotational direction around the axis O. Furthermore, when an axial load is applied, the nut 3 moves in the axial direction while being guided by the pin 60. For the above reasons, the nut 3 and the piston 40 are supported by the housing 120 so as to be movable in the axial direction but not rotatable in the axial direction.

[0082] (Embodiment 3) FIG. 8 is an axial cross-sectional view of the electric actuator of the third embodiment before operation. The electric actuator 104B of the third embodiment differs from the electric actuator 104 of the first embodiment in that the nut 3 is provided with a protrusion 35. Therefore, in the third embodiment, the first concave surface 46 of the piston 40 and the third concave surface 37 of the nut 3 are caught on the pin 60. According to the third embodiment, the nut 3 and the piston 40 are reliably prevented from rotating. Furthermore, the fitted portion 45 abuts against the stepped surface 125. Therefore, the range of movement of the fitted portion 45 is limited to the inner peripheral side of the large diameter surface 124.

[0083] (Embodiment 4) 9 is an enlarged view of an axial cross section of the electric actuator of embodiment 4 in a state before operation. The electric actuator 104C of embodiment 4 differs from the electric actuator 104 of embodiment 1 in that a concave surface 27 is provided on the outer peripheral surface 21 of the nut body 20. The electric actuator 104C of embodiment 4 differs from the electric actuator 104 of embodiment 1 in that a convex surface 48 is provided on the inner peripheral surface 44b of the expansion portion 44.

[0084] The concave surface 27 is located at the end of the outer circumferential surface 21 of the nut body 20 in the second direction X2. The concave surface 27 is recessed radially inward from the outer circumferential surface 21. The concave surface 27 extends in the circumferential direction and is annular. The convex surface 48 is located at the end of the inner circumferential surface 44b of the expansion portion 44 in the second direction X2. The convex surface 48 protrudes radially inward from the inner circumferential surface 44b of the expansion portion 44. The convex surface 48 extends in the circumferential direction and is annular. The concave surface 27 and the convex surface 48 have the same diameter. Therefore, the concave surface 27 and the convex surface 48 abut against each other. In other words, the end of the inner circumferential surface 44b of the expansion portion 44 in the second direction X2 abuts against the outer circumferential surface 21 of the nut body 20.

[0085] As a result, the nut 3 is supported by the piston 40 without tilting. This allows the nut 3 to move smoothly in the axial direction. Furthermore, since the concave surface 27 and the convex surface 48 have the same diameter, the inner raceway surface 3a of the nut 3 is not deformed, ensuring smooth rolling of the balls 4. Furthermore, the diameter of the expanded portion 44 is also prevented from expanding, so the expanded portion 44 does not get caught on the sliding surface 123.

[0086] The nut 3 also has an inner peripheral stepped surface 28 between the outer peripheral surface 21 and the concave surface 27. The piston 40 also has an outer peripheral stepped surface 49 between the inner peripheral surface 44b and the convex surface 48. The inner peripheral stepped surface 28 and the outer peripheral stepped surface 49 are spaced apart in the axial direction and do not abut against each other. Therefore, the reaction force that the piston 40 receives from the brake pad 102 is input to the pressing surface 22 of the nut. In other words, in the fourth embodiment as well, the increased load on the balls 4 due to the displacement of the inner peripheral raceway surface 3a is borne by the balls 4 located near the end of the raceway 8 in the second direction X2. This results in a more uniform load distribution on the balls 4.

[0087] (Embodiment 5) 10 is an enlarged view of an axial cross section of the electric actuator of embodiment 5 in a state before operation. The electric actuator 104D of embodiment 5 differs from the electric actuator 104 of embodiment 1 in that a convex surface 29 is provided on the nut body 20.

[0088] The convex surface 29 is disposed at the end of the outer peripheral surface 21 of the nut body 20 in the second direction X2. The convex surface 29 protrudes radially outward from the outer peripheral surface 21. The convex surface 29 extends in the circumferential direction and is annular. The convex surface 29 has the same diameter as the inner peripheral surface 44b of the expansion portion 44. Therefore, the nut 3 is supported by the piston 40 so as not to tilt. This allows smooth axial movement of the nut 3.

[0089] Although each embodiment has been described above, the present disclosure is not limited to the examples shown in each embodiment. For example, in embodiment 4, the concave surface 27 and the convex surface 48 have the same diameter, but the concave surface 27 may have a larger diameter than the convex surface 48. In other words, the concave surface 27 may have an interference with the convex surface 48. In this way, the concave surface 27 fits into the convex surface 48, improving the fixing strength between the nut 3 and the piston 40. Similarly, in embodiment 5, the convex surface 29 may fit into the inner circumferential surface 44b of the expansion portion 44.

[0090] FIG. 11 is a cross-sectional view of a modified electric actuator taken in the axial direction. Although the electric actuator of this embodiment is used in the brake caliper 100, it may also be used in other devices. In the present disclosure, as shown in FIG. 11, the housing 120 of the electric actuator 104E may have a cylinder 128. In this modified example, a liquid (not shown) is sealed inside the cylinder 128. When the piston 40 moves in the second direction X2, the hydraulic pressure of the liquid increases. The hydraulic pressure of the liquid is transmitted to the brake system via the through-hole 129a.

[0091] In addition, in this embodiment, an annular gap S11 is provided between the inner circumferential surface 44b of the extension portion 44 and the outer circumferential surface 21 of the piston body 42, but the present disclosure does not particularly limit the radial gap size (radial distance). However, in the present disclosure, a small gap size, such as 50 μm or less, is preferable. This is because the tilt angle of the nut 3 is thereby kept small. In addition, in the present disclosure, the radial gap size of the gap S11 does not need to be constant in the axial direction. For example, the gap size between the end of the outer circumferential surface 21 of the nut body 20 in the second direction X2 and the inner circumferential surface of the extension portion 44 may be the smallest among all axial portions of the gap S11, and the radial gap size may be 50 μm or less.

[0092] Furthermore, in the present embodiment, the first concave surface 46, the second concave surface 126, and the third concave surface 37 have an arc-shaped cross section when viewed in the axial direction. However, as long as they can restrict the circumferential movement of the pin 60, the present disclosure may also have a rectangular groove cross section when viewed in the axial direction. Furthermore, in the present disclosure, the pin 60 is not limited to a cylindrical shape and may be a prism shape, and is not particularly limited. Furthermore, in the present embodiment, the housing 120 supports the piston 40 or the nut 3 non-rotatably using the pin 60. However, in the present disclosure, a separate key (a separate component) may be used instead of the pin 60 so that the housing 120 supports the piston 40 or the nut 3 non-rotatably. Alternatively, the key may be integrally formed on the housing 120, or the key may be integrally formed on the piston 40 or the nut 3, rather than being a separate component (a separate component). Furthermore, the present disclosure is not particularly limited and may employ configurations other than those described above regarding the anti-rotation structure for the piston 40 or the nut 3.

[0093] Furthermore, if the frictional force between the fitting portion 30 and the fitted portion 45 is small, there is a possibility that the nut 3 will rotate together with the screw shaft 2 (the nut 3 will spin relative to the piston 40). Therefore, to improve the frictional force, at least one of the outer peripheral surface 31 of the fitting portion 30 and the inner peripheral surface 45a of the fitted portion 45 may be subjected to fine unevenness processing or a coating of a high-friction material. Alternatively, the outer peripheral surface 31 of the fitting portion 30 and the inner peripheral surface 45a of the fitted portion 45 may be provided with protrusions and recesses that can be caught in the circumferential direction. Furthermore, the outer peripheral surface 31 of the fitting portion 30 and the inner peripheral surface 45a of the fitted portion 45 may be fitted with protrusions and recesses provided on the outer peripheral surface 31 of the fitting portion 30. This also prevents the piston 40 from slipping off the nut 3.

[0094] Furthermore, in order to prevent the nut 3 from spinning relative to the piston 40, at least one of the pressing surface 22 of the nut 3 and the end face 42b of the piston 40 may be provided with fine irregularities or coated with a high-friction material. Alternatively, the pressing surface 22 of the nut 3 and the end face 42b of the piston 40 may be provided with protrusions and recesses that catch in the circumferential direction. [Explanation of symbols]

[0095] 1. Ball screw device 2 screw shaft 3 nuts 3a Inner raceway surface 4 balls 8 orbits 10 Power transmission section 11 Screw shaft body 12 Outer raceway surface 20 Nut body 21 Outer surface 22 Pressing surface 23 S groove 27 Concave 29 Convex 30 Fitting part 31 Outer surface 35 Protrusion 37 Third concave surface 40 pistons 41 Lid 42 Piston body 43 Cylindrical part 44 Extension 45, 45A Mated part 46 1st concave surface 48 Convex 60 pins 100 brake caliper 102, 103 brake pads 104, 104A, 104B, 104C, 104D, 104E Electric Actuators 120 Housing 121 First Wall (Wall) 122 Second wall 123 Sliding surface 124 Large diameter surface 125 Step surface 126 Second concave surface 128 cylinders S1 First storage space (storage space) S2 Second storage space

Claims

1. a motor for generating rotational motion; a ball screw device having a screw shaft, a nut, and balls, and converting the rotational motion into linear motion; a piston fitted to the nut and moving in an axial direction parallel to the screw shaft; a housing having an axially extending accommodation space and a cylindrical wall surface surrounding the accommodation space, the housing having the ball screw device and the piston disposed in the accommodation space; Equipped with the wall surface has a cylindrical sliding surface that slidably supports an outer circumferential surface of the piston, The screw shaft is a screw shaft body having an outer circumferential raceway surface on its outer circumferential surface; a power transmission unit disposed in a first direction, which is one of the axial directions, relative to the screw shaft body and rotatably supported by the housing; and The nut is a nut body having an inner raceway surface on its inner circumferential surface; a fitting portion disposed in the first direction relative to the nut body and having an outer diameter larger than the outer diameter of the nut body; and The nut body has an end surface facing a second direction opposite to the first direction, the end surface having a pressing surface that presses the piston in the second direction, The piston is a piston body disposed in the second direction relative to the nut body, having an outer diameter larger than the outer diameter of the nut body, and an outer peripheral surface supported by the sliding surface; a cylindrical expansion portion that extends in the first direction from an outer peripheral portion of an end portion of the piston body in the first direction and has the nut body disposed on an inner peripheral side thereof; an end surface of the piston body in the first direction, the end surface being disposed on an inner circumferential side of the expansion portion and abutting against the pressing surface; a cylindrical fitted portion extending from the expansion portion in the first direction and having an inner circumferential side into which the fitting portion is fitted; and A spiral raceway is provided between the inner raceway surface and the outer raceway surface of the nut body, The ball rolls on the raceway, The inner diameter of the expansion portion is larger than the outer diameter of the nut body, A cylindrical gap is provided between the inner circumferential surface of the expansion portion and the outer circumferential surface of the nut body, The entire inner circumferential surface of the expansion portion in the axial direction is spaced apart from the outer circumferential surface of the nut body. Electric actuator.

2. a motor for generating rotational motion; a ball screw device having a screw shaft, a nut, and balls, and converting the rotational motion into linear motion; a piston fitted to the nut and moving in an axial direction parallel to the screw shaft; a housing having an axially extending accommodation space and a cylindrical wall surface surrounding the accommodation space, the housing having the ball screw device and the piston disposed in the accommodation space; Equipped with the wall surface has a cylindrical sliding surface that slidably supports an outer circumferential surface of the piston, The screw shaft is a screw shaft body having an outer circumferential raceway surface on its outer circumferential surface; a power transmission unit disposed in a first direction, which is one of the axial directions, relative to the screw shaft body and rotatably supported by the housing; and The nut is a nut body having an inner raceway surface on its inner circumferential surface; a fitting portion disposed in the first direction relative to the nut body and having an outer diameter larger than the outer diameter of the nut body; and The nut body has an end surface facing a second direction opposite to the first direction, the end surface having a pressing surface that presses the piston in the second direction, The piston is a piston body disposed in the second direction relative to the nut body, having an outer diameter larger than the outer diameter of the nut body, and an outer peripheral surface supported by the sliding surface; a cylindrical expansion portion that extends in the first direction from an outer peripheral portion of an end portion of the piston body in the first direction and has the nut body disposed on an inner peripheral side thereof; an end surface of the piston body in the first direction, the end surface being disposed on an inner circumferential side of the expansion portion and abutting against the pressing surface; a cylindrical fitted portion extending from the expansion portion in the first direction and having an inner circumferential side into which the fitting portion is fitted; and The inner diameter of the expansion portion is larger than the outer diameter of the nut body, A cylindrical gap is provided between the inner circumferential surface of the expansion portion and the outer circumferential surface of the nut body, The entire inner circumferential surface of the expansion portion in the axial direction is spaced apart from the outer circumferential surface of the nut body. Electric actuator.

3. a motor for generating rotational motion; a ball screw device having a screw shaft, a nut, and balls, and converting the rotational motion into linear motion; a piston fitted to the nut and moving in an axial direction parallel to the screw shaft; a housing having an axially extending accommodation space and a cylindrical wall surface surrounding the accommodation space, the housing having the ball screw device and the piston disposed in the accommodation space; Equipped with the wall surface has a cylindrical sliding surface that slidably supports an outer circumferential surface of the piston, The screw shaft is a screw shaft body having an outer circumferential raceway surface on its outer circumferential surface; a power transmission unit disposed in a first direction, which is one of the axial directions, relative to the screw shaft body and rotatably supported by the housing; and The nut is a nut body having an inner raceway surface on its inner circumferential surface; a fitting portion disposed in the first direction relative to the nut body and having an outer diameter larger than the outer diameter of the nut body; and The nut body has an end surface facing a second direction opposite to the first direction, the end surface having a pressing surface that presses the piston in the second direction, The piston is a piston body disposed in the second direction relative to the nut body, having an outer diameter larger than the outer diameter of the nut body, and an outer peripheral surface supported by the sliding surface; a cylindrical expansion portion that extends in the first direction from an outer peripheral portion of an end portion of the piston body in the first direction and has the nut body disposed on an inner peripheral side thereof; an end surface of the piston body in the first direction, the end surface being disposed on an inner circumferential side of the expansion portion and abutting against the pressing surface; a cylindrical fitted portion extending from the expansion portion in the first direction and having an inner circumferential side into which the fitting portion is fitted; and the wall surface of the housing is disposed in the first direction relative to the sliding surface and has a cylindrical large-diameter surface having a diameter larger than that of the sliding surface, The range of movement of the fitted portion in the axial direction is limited to the inner peripheral side of the large diameter surface in the accommodation space. Electric actuator.

4. A spiral raceway is provided between the inner raceway surface and the outer raceway surface of the nut body, The balls roll on the raceways. The electric actuator according to claim 3 .

5. The inner diameter of the expansion portion is larger than the outer diameter of the nut body, A cylindrical gap is provided between the inner peripheral surface of the expansion portion and the outer peripheral surface of the nut body. The electric actuator according to claim 3 .

6. The electric actuator according to claim 5 , wherein an end portion of the inner circumferential surface of the expansion portion in the second direction abuts against an outer circumferential surface of the nut body.

7. The electric actuator according to claim 5 , wherein an end portion of the inner circumferential surface of the expansion portion in the second direction is fitted into an outer circumferential surface of the nut body.

8. The inner diameter of the expansion portion is larger than the outer diameter of the nut body, A cylindrical gap is provided between the inner peripheral surface of the expansion portion and the outer peripheral surface of the nut body. The electric actuator according to claim 4.

9. The electric actuator according to claim 8 , wherein an end portion of the inner circumferential surface of the expansion portion in the second direction abuts against an outer circumferential surface of the nut body.

10. The electric actuator according to claim 8 , wherein an end portion of the inner circumferential surface of the expansion portion in the second direction is fitted into an outer circumferential surface of the nut body.

11. The fitted portion has an outer diameter larger than the diameter of the sliding surface, a first concave surface recessed toward an inner periphery of the fitted portion is provided on an outer periphery of the fitted portion; the large diameter surface is provided with a second concave surface that faces the first concave surface and extends in the axial direction, The second concave surface accommodates a pin extending in the axial direction, At least a portion of the pin protrudes radially inward beyond the large diameter surface and penetrates the inside of the first concave surface in the axial direction. The electric actuator according to any one of claims 3 to 10.

12. the nut is disposed in the first direction relative to the fitting portion and has a protruding portion that protrudes radially outward beyond the fitted portion, a third concave surface recessed toward an inner periphery is provided on an outer periphery of the protruding portion; the large diameter surface is provided with a second concave surface that faces the third concave surface and extends in the axial direction, The second concave surface accommodates a pin extending in the axial direction, At least a portion of the pin protrudes radially inward beyond the large diameter surface and penetrates the third concave surface in the axial direction. The electric actuator according to any one of claims 3 to 10.

13. the nut is disposed in the first direction relative to the fitting portion and has a protruding portion having an outer diameter equal to an outer diameter of the fitted portion, a third concave surface recessed toward an inner periphery is provided on an outer periphery of the protruding portion; The pin passes through the inside of the first concave surface and the inside of the third concave surface in the axial direction. The electric actuator according to claim 11.

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