Electric actuator

Abstract

To increase the strength or durability of electric actuators. [Solution] The electric actuator 1 comprises an electric motor 2, a shaft-shaped member 32, and a motion conversion mechanism 3 that converts the rotational motion output from the electric motor 2 into linear motion of the shaft-shaped member 32. The shaft-shaped member 32 is provided with a pin hole 7 for inserting a pin 8 used to connect the shaft-shaped member to other members. The pin hole 7 opens on the outer circumferential surface of the shaft-shaped member 32. Crowning is provided on the inner circumferential surface 7a of the pin hole 7.

Description

Technical Field

[0001] The present invention relates to an electric actuator.

Background Art

[0002] In recent years, electrification of automobiles has progressed for purposes such as labor saving and low fuel consumption. For example, systems that use the force of an electric motor to operate automatic transmissions, brakes, steering and other components of automobiles have been developed and put on the market. Electric actuators including an electric motor and a motion conversion mechanism that converts the rotational motion of the electric motor into linear motion are widely used in such systems.

[0003] For example, the following Patent Document 1 discloses a motion conversion mechanism (ball screw mechanism) including: a rotation-side nut having an internal thread formed on an inner peripheral surface thereof; a linearly-moving screw shaft having an external thread formed on an outer peripheral surface thereof; and a plurality of balls disposed between the internal thread and the external thread. A connecting shaft is fixed coaxially with the screw shaft to a tip end of the screw shaft. The connecting shaft is a shaft-shaped member for connecting the screw shaft and an operated component, and by inserting a pin into a pin hole provided at an end portion on one axial side of the connecting shaft, another member as the operated component is attached to one end of the connecting shaft.

Prior Art Literature

Patent Literature

[0004]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0005] Pins are typically inserted into pinholes using a clearance fit, resulting in a gap between the outer surface of the pin and the inner surface of the pinhole. Therefore, due to dimensional tolerances of the pin and pinhole, the pin may be inserted at an angle to the pinhole. In this state, when an electric motor is driven, particularly in a pulling direction, the pin may contact the corners at both ends of the pinhole, increasing stress at the contact point. This could potentially damage the component with the pinhole or the pin itself.

[0006] Therefore, the present invention aims to improve the strength or durability of electric actuators. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides an electric actuator comprising an electric motor, a shaft-shaped member, and a motion conversion mechanism that converts rotational motion output from the electric motor into linear motion of the shaft-shaped member, wherein the shaft-shaped member is provided with a pin hole for inserting a pin used to connect the shaft-shaped member to another member, and the pin hole is opened on the outer circumferential surface of the shaft-shaped member, characterized in that crowning is provided on the inner circumferential surface of the pin hole.

[0008] By providing crowning on the inner surface of the pinhole in this way, even if the pin is inserted into the pinhole at an angle due to machining errors in the pinhole or pin, the inner surface of the pinhole will not come into contact with the pin's edge. Therefore, stress generated at the contact point between the corner of the inner surface of the pinhole and the pin can be suppressed, making it possible to avoid damage to the screw shaft and pin.

[0009] Furthermore, the present invention relates to an electric actuator comprising an electric motor, a shaft-shaped member, a motion conversion mechanism that converts rotational motion output from the electric motor into linear motion of the shaft-shaped member, and a pin used to connect the shaft-shaped member to another member, wherein the pin is provided on the shaft-shaped member and inserted into a pin hole opening on the outer circumferential surface of the shaft-shaped member, and the pin is characterized in that crowning is provided on the outer circumferential surface of the pin. The effects and advantages in this case are the same as described above, so the explanation will be omitted.

[0010] The crowning is preferably formed by an arc. The crowning may also be a full crowning formed by a convex arc with a radius of curvature R. [Effects of the Invention]

[0011] According to the present invention, the strength or durability of an electric actuator can be increased. [Brief explanation of the drawing]

[0012] [Figure 1] This is a longitudinal cross-sectional view of the electric actuator in the axial direction. [Figure 2] This is a cross-sectional view showing the II-II section in Figure 1. [Figure 3] This is a perspective view showing the screw shaft before the joint member is installed. [Figure 4] This is a perspective view showing a screw shaft with a joint component attached. [Figure 5] This is a cross-sectional view of the connecting shaft along the axial direction of the pin hole. [Figure 6] This is an axial cross-sectional view showing a screw shaft to which a joint member is attached. [Figure 7] This is a cross-sectional view showing an enlarged view of region A in Figure 6. [Figure 8] This graph shows the relationship between coordinates along the axial direction of the inner surface of a pinhole and stress. [Figure 9] This is a cross-sectional view of a pin with crowning on its outer surface. [Figure 10] This is a cross-sectional view showing another example of crowning. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings (FIG. 1 to FIG. 5). For explaining directions, the terms "axial direction" and "radial direction" are used. When simply referred to as "axial direction", it means the direction along the axis of the nut 31 constituting the motion conversion mechanism 3 shown in FIG. 1 and other figures (the left-right direction on the paper surface of FIG. 1), and when simply referred to as "radial direction", it means the radial direction of a circle centered on said axis (the up-down direction on the paper surface of FIG. 1). Furthermore, "one axial side" and "the other axial side" refer respectively to the left side and the right side on the paper surface of FIG. 1 (the forward side and the reverse side of the screw shaft 32 constituting the motion conversion mechanism 3).

[0014] FIG. 1 is a schematic longitudinal sectional view of an electric actuator 1 according to an embodiment of the present invention, and FIG. 2 is a sectional view taken along the line II-II in FIG. 1. This electric actuator 1 includes an electric motor 2 as a drive source, a motion conversion mechanism 3 (screw mechanism) that converts rotational motion output from the electric motor into linear motion for output, a power transmission mechanism 4, and a housing 5 that accommodates these components.

[0015] The electric motor 2 includes a motor main body containing a stator core and stator coils, and an output shaft 2a. The stator coils are electrically connected to an unillustrated power supply. For the electric motor 2, a motor capable of detecting and controlling the rotation angle of the output shaft 2a, such as a three-phase brushless motor, is used.

[0016] The housing 5 is divided at a plurality of positions. In the present embodiment, the housing 5 is divided into three parts: a first housing 5a, a second housing 5b, and a third housing 5c. The first housing 5a holds the electric motor 2. The second housing 5b covers the linear motion member (the screw shaft 32) of the motion conversion mechanism 3. The third housing covers the motion conversion mechanism 3 and the power transmission mechanism 4 from the other axial side. The second housing 5b and the third housing 5c are respectively attached to one axial side and the other axial side of the first housing 5a using fastening members such as unillustrated bolts.

[0017] The motion converting mechanism 3 of the present embodiment is a so-called ball screw mechanism including: a nut 31 having a female screw 31a formed on an inner circumferential surface thereof; a screw shaft 32 disposed parallel to an output shaft 2a of an electric motor 2, the screw shaft having a male screw 32a opposite the female screw 31a formed on an outer circumferential surface thereof; and a large number of balls interposed in a ball passage defined between the mutually opposing female screw 31a and male screw 32a. Although not shown in the drawings, the motion converting mechanism 3 is provided with a circulation member for circulating the balls 33 within the ball passage.

[0018] The screw shaft 32 includes a screw shaft portion 32b having the male screw 32a, and a connecting shaft portion 32c fixed to an end portion on one axial side of the screw shaft portion 32b. A pin hole 7 is formed in an end portion (tip end) on one axial end side of the connecting shaft portion 32c. The pin hole 7 penetrates the connecting shaft portion 32c in a radial direction, and both ends of the pin hole 7 open to an outer circumferential surface of the connecting shaft portion 32c. As shown in FIG. 1, the connecting shaft portion 32c may be formed separately from the screw shaft portion 32b, or may be formed integrally with the screw shaft portion 32b.

[0019] The nut 31 is rotatably supported relative to a housing 5 by a rolling bearing 6. The rolling bearing 6 in the illustrated example is a ball bearing including: an inner ring 6a fixed to an outer circumferential surface of the nut 31; an outer ring 6b fixed to an inner circumferential surface of a first housing 5a; a plurality of balls 6c rollably disposed between the inner ring 6a and the outer ring 6b; and a retainer (not shown) that holds the plurality of balls 6c at intervals in a circumferential direction. Although the inner ring 6a of the rolling bearing 6 in the illustrated example is a separate body from the nut 31, the inner ring 6a may be integrated with the nut 31.

[0020] The power transmission mechanism 4 transmits the rotational motion output from the electric motor 2 to the nut 31, which is a rotating member of the motion conversion mechanism 3. The power transmission mechanism 4 includes a drive gear 41 fixed to the output shaft 2a of the electric motor 2, and a driven gear 42 fixed to the nut 31 and meshing with the drive gear 41. The rotational motion of the electric motor 2 input to the power transmission mechanism 4 is reduced in speed by the power transmission mechanism 4 and transmitted to the nut 31. By driving the electric motor 2 in forward and reverse directions, the nut 31 rotates in forward and reverse directions, and consequently, the screw shaft 32 performs linear motion in one axial direction (forward direction) and the other axial direction (reverse direction).

[0021] A rotation-preventing member 35 is fixed to the screw shaft 32 in a region on one axial side of the nut 31. As shown in Figure 2, the rotation-preventing member 35 has a radially extending projection 35a. The outer diameter end of the projection 35a is inserted into an axially extending guide groove 5e provided on the inner circumferential surface of the housing 5. By inserting the outer diameter end of the projection 35a into the guide groove 5e, the rotation of the screw shaft 32 that follows the rotation of the nut 31 is restricted. When the screw shaft 32 performs linear motion in the axial direction, the outer diameter end of the projection 35a is guided by the guide groove 5e, and the rotation-preventing member 35 moves axially together with the screw shaft 32. In this embodiment, the case in which the projection 35a and the guide groove 5e are provided at two locations equally spaced in the circumferential direction is illustrated, but the projection 35a and the guide groove 5e can also be provided at one or three or more locations in the circumferential direction.

[0022] A stopper 34 is fixed to the screw shaft 32. The stopper 34 is positioned closer to the nut 31 than the anti-rotation member 35. Like the anti-rotation member 35, the stopper 34 has a radially projecting projection. The projection of the stopper 34 has the same shape as the projection 35a of the anti-rotation member 35 when viewed from the axial direction, and is positioned at the same location as the projection 35a in the circumferential direction. In this embodiment, the stopper 34 is fixed to the screw shaft 32 in a state where it is superimposed on the anti-rotation member 35 and in contact with each other. A projection-like engaging portion 31b is provided in a circumferential region of the end face on one axial side of the nut 31, projecting toward the one axial side.

[0023] As the screw shaft 32 retracts due to the rotation of the nut 31, the stopper 34 contacts the engaging portion 31b in the circumferential direction when it reaches a certain position in the axial direction. This contact restricts further retraction of the screw shaft 32, thereby defining the stroke end of the screw shaft 32 in the retraction direction. The stroke end of the screw shaft 32 can also be defined by detecting the position of the screw shaft 32 with a sensor.

[0024] Figures 3 and 4 show other components 9 attached to the tip of the screw shaft 32 of the electric actuator 1 described above. Figure 3 shows the other component 9 before attachment, and Figure 4 shows the other component 9 after attachment. This embodiment exemplifies a case where, for example, a coupling member that transmits the linear motion of the screw shaft 32 to the object to be operated is used as the other component 9.

[0025] As shown in Figure 3, the joint member 9 has a base portion 9a and bifurcated arm portions 9b extending from the base portion 9a. The arm portions 9b are provided with engagement holes 9c that penetrate radially through the arm portions 9b. To avoid interference with the arm portions 9b, a flat portion 32c1 is formed at the tip of the connecting shaft portion 32c by chamfering. As shown in Figure 4, the tip of the connecting shaft portion 32c is inserted between the two arm portions 9b so that the flat portion 32c1 faces the inner surface of the arm portions 9b, and the pin hole 7 and the engagement hole 9c are aligned. Then, the pin 8 is inserted into the pin hole 7 and the two engagement holes 9c, thereby attaching the joint member 9 to the tip of the screw shaft 32 via the pin 8. By attaching retaining rings to both ends of the pin 8, it is possible to prevent the pin 8 from falling out of the pin hole 7.

[0026] The fit between the outer surface of pin 8 and the inner surface 7a of pin hole 7, and the fit between the outer surface of pin 8 and the inner surface of engagement hole 8c, are both clearance fits. Because there is a gap between the outer surface of pin 8 and the inner surface 7a of pin hole 7, as shown in Figures 6 and 7, pin 8 may be inserted at a slight inclination (angle θ) relative to pin hole 7 due to dimensional tolerances of pin 8 and pin hole 7. In this state, when the electric motor 2 is driven to pull the object being operated, the corner 7a1 of the inner surface 7a of pin hole 7 comes into contact with pin 8, and the stress at the contact point increases. According to the analysis results, as shown in Figure 8, this stress is small in the center of the connecting shaft 32c but is greatest at the corner 7a1 of the inner surface 7a of pin hole 7. Therefore, there is a risk of damage to the connecting shaft 32c and pin 8.

[0027] In this way, the inventors investigated the cause of damage to the pin 8 and the connecting shaft portion 32c and found that the cause was the inclined insertion of the pin 8 into the pin hole 7 due to dimensional tolerances, etc. Therefore, based on this finding, the present invention decided to provide crowning on the inner circumferential surface 7a of the pin hole 7 provided in the connecting shaft portion 32c as a countermeasure, as shown in Figure 5. Crowning refers to a shape in which the surface of the member is raised in the middle. In Figure 5, as an example of crowning, a so-called full crowning is illustrated, in which the generatrix of the inner circumferential surface 7a of the pin hole 7 is formed by a convex circular arc with a radius of curvature R, thereby providing crowning to the entire inner circumferential surface 7a. The inner circumferential surface 7a has a symmetrical shape in the axial direction of the pin hole 7. In Figure 5, for ease of understanding, the maximum drop amount δ of the crowning is exaggerated, but the maximum drop amount δ of the crowning is about 100 μm to 200 μm.

[0028] By providing crowning on the inner circumferential surface 7a of the pin hole 7 in this way, even if the pin 8 is inserted into the pin hole 7 at an angle due to machining errors in the pin hole 7 or the pin 8, the corner 7a1 of the inner circumferential surface 7a of the pin hole will not come into contact with the pin 8. Therefore, the stress generated at the contact point between the corner 7a1 and the pin 8 can be suppressed, and damage to the screw shaft 32 and the pin 8 can be avoided.

[0029] Figure 10 shows the case where partial crowning is provided on the inner circumferential surface 7a of the pin hole 7. In this partial crowning, the generatrix of the inner circumferential surface 7a is formed by a straight section S extending parallel to the axial direction of the pin hole 7 and circular arc sections with a radius of curvature R connected to both ends of the straight section. The straight section S extends in the tangential direction of the circular arc section. Even when partial crowning is formed on the inner circumferential surface 7a in this way, just like with full crowning, it is possible to suppress the generation of excessive stress at the contact point between the corner 7a1 and the pin 8, and to avoid damage to the screw shaft 32 and the pin 8 due to such stress.

[0030] Figures 5 and 10 illustrate the case where the gensha arc is formed with a single radius of curvature, but each arc can also be formed as a continuous curve by combining multiple arcs with different radii of curvature. Alternatively, each arc of the gensha can be formed with one or more tapered surfaces, and the boundaries between the tapered surfaces or the boundaries between the tapered surfaces and the straight sections S can be smoothly connected with arcs.

[0031] In the embodiments described above, crowning is provided on the inner circumferential surface 7a of the pin hole 7. However, as shown in Figure 9, similar effects can be obtained by providing crowning on the outer circumferential surface of the pin 8. The crowning provided on the outer circumferential surface of the pin 8 can be either full crowning or partial crowning as described above. Furthermore, crowning may be provided on both the inner circumferential surface 7a of the pin hole 7 and the outer circumferential surface of the pin 8.

[0032] Furthermore, while the above explanation exemplified the use of a ball screw mechanism as the motion conversion mechanism 3, a sliding screw mechanism that does not use balls can also be used.

[0033] The present invention is not limited in any way to the embodiments described above, and can be implemented in various other forms without departing from the spirit of the invention. The scope of the present invention is indicated by the claims, and includes all modifications within the meaning and scope of equivalents set forth in the claims. [Explanation of Symbols]

[0034] 1 Electric Actuator 2 Electric motor 3. Motion conversion mechanism 4 Power transmission mechanism 5 Housing 7 pin holes 7a Inner surface 8 pins 31 nuts 32 Shaft-shaped member (screw shaft)

Claims

1. An electric actuator comprising an electric motor, a shaft-shaped member, and a motion conversion mechanism that converts rotational motion output from the electric motor into linear motion of the shaft-shaped member, wherein the shaft-shaped member is provided with a pin hole for inserting a pin used to connect the shaft-shaped member to another member, and the pin hole is opened on the outer circumferential surface of the shaft-shaped member, An electric actuator characterized by having crowning on the inner circumferential surface of the aforementioned pin hole.

2. An electric actuator comprising an electric motor, a shaft-shaped member, a motion conversion mechanism that converts rotational motion output from the electric motor into linear motion of the shaft-shaped member, and a pin used for connecting the shaft-shaped member to other members, wherein the pin is provided on the shaft-shaped member and inserted into a pin hole opening on the outer circumferential surface of the shaft-shaped member, An electric actuator characterized by having crowning on the outer surface of the aforementioned pin.

3. The electric actuator according to claim 1 or 2, wherein the crowning is formed in the shape of an arc.

4. The electric actuator according to claim 1 or 2, wherein the crowning is a full crowning formed by a convex circular arc with a radius of curvature R.

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