Linear Actuator

Abstract

To allow for easy and highly accurate alignment between a motor and a direct-acting conversion mechanism in a direct-acting actuator.SOLUTION: A direct-acting actuator includes a motor 21 and a direct-acting conversion mechanism 23. The direct-acting conversion mechanism 23 includes a screw shaft 24 and a nut 25 screwed onto the screw shaft 24, and is configured such that the nut 25 is direct-acting as the screw shaft 24 rotates. In such a direct-acting actuator, the screw shaft 24 is provided with a hole 35 coaxial with its rotational axis O1. The motor 21 and the direct-acting conversion mechanism 23 are aligned by inserting into the hole 35 a tip portion of a motor shaft 29 that is coaxial with a rotor 28 of the motor 21 and rotates together.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a linear actuator. [Background technology]

[0002] A linear motion actuator including an electric motor, a reduction mechanism, and a linear motion conversion mechanism is known, as seen in Patent Document 1. The linear motion actuator in this document employs a screw mechanism as the linear motion conversion mechanism, which includes a rotatable nut and a screw shaft that moves linearly in the extension direction of the rotation axis of the nut in response to rotation of the nut. The linear motion actuator in this document also employs a planetary gear mechanism as the reduction mechanism. In this linear motion actuator, the sun gear of the planetary gear mechanism is connected to the electric motor, and each planetary gear of the planetary gear mechanism is journaled on the nut of the linear motion conversion mechanism.

[0003] Furthermore, the linear motion actuator in this document is equipped with an alignment plate having holes through which the gear shafts of the sun gear and each planetary gear are inserted, and the plate aligns the revolution shaft of each planetary gear with the rotation shaft of the sun gear. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 10,378,623 Summary of the Invention [Problem to be solved by the invention]

[0005] In the linear motion actuator described above, the rotational axes of the electric motor and the linear motion conversion mechanism may need to be coaxial. In contrast, the axis alignment plate of the conventional linear motion actuator described above aligns the rotational axis of the sun gear of the planetary gear mechanism with the revolution axis of the planetary gear. With such a plate, the axis alignment of the rotational axes of the electric motor and the linear motion conversion mechanism can only be performed indirectly. Therefore, there is a risk that the axis alignment accuracy of the rotational axes of the electric motor and the linear motion conversion mechanism cannot be sufficiently ensured. [Means for solving the problem]

[0006] A linear motion actuator for solving the above problems includes a linear motion conversion mechanism and an electric motor. The linear motion conversion mechanism includes one of a screw shaft and a nut threaded onto the screw shaft as a rotatable rotating member, and the other as a linear motion member that linearly moves in response to rotation of the rotating member. The electric motor includes a rotor that rotates coaxially with the rotating member. The electric motor rotates the rotating member in response to rotation of the rotor. The linear motion actuator also includes a shaft member connected to the rotor so as to rotate integrally with the rotor. The screw shaft of the linear motion actuator has a hole into which the shaft member is inserted to align the rotating axis of the rotating member with the rotating axis of the rotor.

[0007] In the linear motion actuator, the rotation axis of the rotating member is aligned with the rotation axis of the rotor, i.e., the electric motor and the linear motion conversion mechanism are aligned, by inserting a shaft into a hole in the screw shaft. The shaft inserted into the hole in the screw shaft is connected to the rotor of the electric motor so that they rotate together. This enables direct alignment between the electric motor and the linear motion conversion mechanism. Therefore, the linear motion actuator enables easy and highly accurate alignment between the electric motor and the linear motion conversion mechanism. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of an electric cylinder provided with an embodiment of a linear actuator. [Figure 2]FIG. 4 is a cross-sectional view showing the linear motion actuator in an assembled state. [Figure 3] FIG. 10 is a schematic diagram of an axis alignment structure provided in the first modified example of the linear actuator. [Figure 4] FIG. 10 is a schematic diagram of an axis alignment structure provided in a second modified example of the linear actuator. [Figure 5] FIG. 13 is a schematic diagram of an axis alignment structure provided in a third modified example of the linear actuator. [Figure 6] FIG. 11 is a diagram showing the state of the axis alignment structure after the linear motion actuator of the third modified example has started to be used. [Figure 7] FIG. 10 is a cross-sectional view of an electric cylinder provided with a linear actuator according to a fourth modification. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of a linear motion actuator will be described below with reference to Figures 1 and 2. A linear motion actuator 20 of this embodiment is provided in an electric cylinder 10 that generates hydraulic pressure that is converted into braking force in a braking device of a vehicle or the like, for example.

[0010] <Configuration of the electric cylinder 10> First, the configuration of an electric cylinder 10 will be described with reference to FIG. 1. The electric cylinder 10 includes a cylinder 14 and a piston 15 that slides within the cylinder 14. In the following description, the sliding direction of the piston 15 within the cylinder 14 will be referred to as the stroke direction S. Within the cylinder 14, a fluid chamber 16 filled with brake fluid is defined by the piston 15. The electric cylinder 10 generates fluid pressure that is converted into braking force by pressing the brake fluid within the fluid chamber 16 through operation of the piston 15. A groove 17 extending in the sliding direction of the piston 15 is formed on the side wall of the cylinder 14. A protrusion 18 formed on the piston 15 is engaged with the groove 17. The engagement between the groove 17 and the protrusion 18 prevents the piston 15 from rotating within the cylinder 14.

[0011] The electric cylinder 10 is broadly composed of two units, a cylinder unit 11 and a motor unit 12. The electric cylinder 10 is manufactured by assembling the cylinder unit 11 and the motor unit 12 separately and then assembling them together. The above-mentioned cylinder 14 and piston 15 are provided in the cylinder unit 11.

[0012] <Configuration of Linear Actuator 20> The electric cylinder 10 is provided with a linear motion actuator 20 for driving the piston 15. The linear motion actuator 20 includes an electric motor 21, a reduction mechanism 22, and a linear motion conversion mechanism 23. Of these, the reduction mechanism 22 and the linear motion conversion mechanism 23 are provided in the cylinder unit 11, and the electric motor 21 is provided in the motor unit 12. In addition, the motor unit 12 is provided with a control board 36 for controlling the power of the electric motor 21. When the cylinder unit 11 and the motor unit 12 are assembled together, the electric motor 21, the reduction mechanism 22, and the linear motion conversion mechanism 23 are arranged in series in the stroke direction S.

[0013] The linear motion conversion mechanism 23 is a screw mechanism having a screw shaft 24 and a nut 25 screwed onto the screw shaft 24. In this embodiment, a ball screw mechanism with a ball circulation mechanism built into the nut 25 is used as the linear motion conversion mechanism 23. The screw shaft 24 is rotatably supported by a bearing 26 and attached to the case 13 of the cylinder unit 11. Meanwhile, the nut 25 is connected to the piston 15 so as to slide integrally in the stroke direction S. The linear motion conversion mechanism 23 converts the rotation of the screw shaft 24 into linear motion of the nut 25 in the stroke direction S.

[0014] The electric motor 21 includes a stator 27 fixed to the case 19 of the motor unit 12, and a rotor 28 arranged radially inside the stator 27. A motor shaft 29, which is a cylindrical shaft member, is connected to the rotor 28 so as to be coaxial and rotate integrally with the rotor 28. The motor shaft 29 is located on the rotation axis O2 of the rotor 28 and extends along the rotation axis O2 of the rotor 28. The motor shaft 29 is attached to the case 19 of the motor unit 12 in a state where it is rotatably supported by bearings 30 and 31. A portion of the motor shaft 29 protrudes from the case 19 in the direction of the cylinder unit 11 as viewed from the motor unit 12.

[0015] The reduction mechanism 22 reduces the rotational speed of the rotor 28 of the electric motor 21 and transmits it to the screw shaft 24 of the linear motion conversion mechanism 23. In this embodiment, a planetary gear mechanism is used as the reduction mechanism 22. The reduction mechanism 22 includes a sun gear 32, which is an external gear, a ring gear 33, which is an internal gear arranged radially outward of the sun gear 32, and a plurality of planetary gears 34 interposed between the sun gear 32 and the ring gear 33. A motor shaft 29 is inserted through the sun gear 32. The sun gear 32 and the motor shaft 29 are connected to each other by spline engagement or the like so as to rotate integrally. The ring gear 33 is fixed to the case 13 of the cylinder unit 11. Each planetary gear 34 is rotatably attached to the screw shaft 24. That is, in this linear motion actuator 20, the screw shaft 24 also functions as a planetary carrier of the planetary gear mechanism.

[0016] The screw shaft 24 has a hole 35 drilled from the end face on the side where the reduction gear mechanism 22 is located, as viewed from the screw shaft 24. The hole 35 is a circular hole that is located on the rotation axis O1 of the screw shaft 24 and extends along the rotation axis O1 of the screw shaft 24. The inner diameter of the hole 35 is approximately the same as the outer diameter of the motor shaft 29. When the cylinder unit 11 and the motor unit 12 are assembled together, the tip portion of the motor shaft 29 is inserted into this hole 35. As described above, the inner diameter of the hole 35 is approximately the same as the outer diameter of the motor shaft 29. Therefore, the tip portion of the inserted motor shaft 29 is in sliding contact with the hole 35.

[0017] <Actions and Effects of the Embodiment> The operation and effects of the linear motion actuator 20 of this embodiment configured as described above will be described.

[0018] In the linear motion actuator 20, the rotation of the electric motor 21 is reduced by the reduction mechanism 22 and transmitted to the screw shaft 24 of the linear motion conversion mechanism 23. The rotation of the screw shaft 24 is converted into linear motion of the nut 25 in the linear motion conversion mechanism 23. The linear motion of the nut 25 moves the piston 15 in the stroke direction S within the cylinder 14. At this time, a speed difference occurs between the rotation of the motor shaft 29 and the rotation of the screw shaft 24.

[0019] Of the three elements that make up the linear motion actuator 20, the electric motor 21, the reduction mechanism 22, and the linear motion conversion mechanism 23, the electric motor 21 is installed in the motor unit 12. In contrast, the remaining two elements, the reduction mechanism 22 and the linear motion conversion mechanism 23, are installed in the cylinder unit 11. The electric cylinder 10 is manufactured by integrally assembling the cylinder unit 11 and the motor unit 12. In such a linear motion actuator 20, if the rotation axis O1 of the screw shaft 24 of the linear motion conversion mechanism 23 and the rotation axis O2 of the rotor 28 of the electric motor 21 are misaligned, the linear motion actuator 20 will not operate smoothly. Therefore, when assembling the cylinder unit 11 and the motor unit 12 together, it is necessary to align the rotation axis O1 of the screw shaft 24 with the rotation axis O2 of the rotor 28.

[0020] FIG. 2 shows the state in which the cylinder unit 11 and the motor unit 12 of the electric cylinder 10 are assembled. As shown in the figure, when assembling the cylinder unit 11 and the motor unit 12, the tip portion of the motor shaft 29 is inserted through the sun gear 32 and into a hole 35 provided in the threaded shaft 24. The inner diameter of this hole 35 is approximately the same as the outer diameter of the tip portion of the motor shaft 29. Therefore, with the tip portion inserted, the hole 35 and the motor shaft 29 are coaxial. Meanwhile, the hole 35 is formed coaxially with the rotation axis O1 of the threaded shaft 24. The motor shaft 29 is also coupled to the rotor 28 so that they rotate coaxially. Therefore, by inserting the tip portion of the motor shaft 29 into the hole 35, the rotation axis O1 of the threaded shaft 24 and the rotation axis O2 of the rotor 28 are aligned. As described above, in the linear motion actuator 20 of this embodiment, the electric motor 21 and the linear motion conversion mechanism 23 can be aligned easily and with high precision.

[0021] In the linear motion actuator 20 of this embodiment, the linear motion conversion mechanism 23 is configured so that the nut 25 linearly moves in response to the rotation of the screw shaft 24. On the other hand, if the linear motion conversion mechanism 23 is configured so that the screw shaft 24 linearly moves in response to the rotation of the nut 25, there is a risk that the screw shaft 24 will move in the stroke direction S during assembly, causing the tip portion of the motor shaft 29 to come out of the hole 35. If assembly is performed in this state, coaxiality between the rotation axis O1 of the screw shaft 24 and the rotation axis O2 of the rotor 28 will not be achieved. In contrast, in this embodiment, the screw shaft 24 will not move in the stroke direction S during assembly. This allows for more reliable axial alignment between the linear motion conversion mechanism 23 and the electric motor 21.

[0022] Furthermore, when the linear motion conversion mechanism 23 is configured so that the screw shaft 24 linearly moves in response to the rotation of the nut 25, the screw shaft 24 moves in the stroke direction S in response to the operation of the linear motion actuator 20. Then, in response to the movement of the screw shaft 24, the tip portion of the motor shaft 29 moves in and out of the hole 35 of the screw shaft 24. During this movement, the motor shaft 29 may not be properly inserted into the hole 35, and interference may occur between the screw shaft 24 and the motor shaft 29. In this regard, the linear motion conversion mechanism 23 of this embodiment is configured so that the nut 25 linearly moves in response to the rotation of the screw shaft 24. Therefore, the motor shaft 29 remains inserted in the hole 35 even during operation of the linear motion actuator 20. Therefore, the above-mentioned interference between the screw shaft 24 and the motor shaft 29 does not occur during operation of the linear motion actuator 20.

[0023] In this embodiment, the screw shaft 24 corresponds to the rotating member, and the nut 25 corresponds to the linearly moving member. The hole 35 provided in the screw shaft 24 and the motor shaft 29 form an alignment structure for aligning the rotation axis O1 of the screw shaft 24 with the rotation axis O2 of the rotor 28. The motor shaft 29 corresponds to the shaft member in the alignment structure.

[0024] In the linear motion actuator 20 of this embodiment, the motor shaft 29 is connected to the sun gear 32 of the reduction mechanism 22. The screw shaft 24 constitutes the planetary carrier of the reduction mechanism 22. In other words, functionally, the screw shaft 24 has a structure in which the rotating member of the linear motion conversion mechanism 23 and the planetary carrier of the reduction mechanism 22 are integrally connected. In the linear motion actuator 20 of this embodiment, the rotation axis O1 of the screw shaft 24 and the rotation axis of the motor shaft 29 are aligned by inserting the motor shaft 29 into a hole 35 provided in the screw shaft 24. This allows the rotation axis of the sun gear 32 of the reduction mechanism 22 to be aligned with the rotation axis of the planetary carrier, i.e., the orbital axis of the planetary gear 34. As described above, the axis alignment structure of the linear motion actuator 20 of this embodiment not only aligns the rotational axes of the electric motor 21 and the linear motion conversion mechanism 23, but also aligns the rotational axis of the sun gear 32 of the reduction mechanism 22 with the revolution axis of the planetary gear 34.

[0025] <Other embodiments> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0026] <Regarding the axis alignment structure of the linear actuator 20> In the above embodiment, the motor shaft 29, which transmits the rotation of the rotor 28 of the electric motor 21 to the sun gear 32, extends toward the linear motion conversion mechanism 23 beyond the sun gear 32. The electric motor 21 and the linear motion conversion mechanism 23 are aligned by inserting the tip of the extended motor shaft 29 into the hole 35 formed in the threaded shaft 24. That is, the motor shaft 29 is formed as an integrated component including the portion inserted into the hole 35 during alignment. The portion inserted into the hole 35 during alignment may be a separate component from the motor shaft 29. The portion inserted into the hole 35 in this case is a shaft member as follows. That is, this shaft member is positioned on the rotation axis O2 of the rotor 28 and extends along the rotation axis O2 of the rotor 28, and rotates integrally with the rotor 28. For example, the length of the motor shaft 29 is from the rotor 28 to the sun gear 32. A metal pin that can be inserted into hole 35 of screw shaft 24 is connected to the tip of motor shaft 29 so as to rotate integrally with motor shaft 29. In this case as well, the axes of electric motor 21 and linear motion conversion mechanism 23 are aligned in response to the insertion of the metal pin into hole 35.

[0027] In the linear motion actuator 20 of the above embodiment, the reduction mechanism 22 reduces the rotation speed of the rotor 28 of the electric motor 21 and transmits it to the threaded shaft 24. Therefore, when the linear motion actuator 20 is operating, the threaded shaft 24 rotates relative to the motor shaft 29. Meanwhile, in the linear motion actuator 20 of the above embodiment, the electric motor 21 and the linear motion conversion mechanism 23 are aligned with each other by inserting the tip of the motor shaft 29 into a hole 35 formed in the screw shaft 24. To improve the coaxiality, the tip of the motor shaft 29 needs to be tightly fitted into the hole 35. However, doing so increases the sliding resistance between the tip of the motor shaft 29 and the hole 35 during relative rotation, which may hinder smooth operation of the linear motion actuator 20. By adopting the following structures of Modifications 1 to 3 as the alignment structures for the electric motor 21 and the linear motion conversion mechanism 23, it is possible to achieve both improved alignment accuracy and smoother operation of the linear motion actuator 20.

[0028] <Variation 1> The configuration of a first modified example of the shaft alignment structure of the linear actuator 20 will be described with reference to FIG. 3. The screw shaft 40 of the linear actuator 20 of the first modified example includes a metal substrate 41 and a resin bushing 43, which is a resin bushing. The substrate 41 is formed with a bushing mounting hole 42 drilled from the end face on the side where the reduction gear mechanism 22 is located as viewed from the screw shaft 40. The resin bushing 43 is mounted in the bushing mounting hole 42. The resin bushing 43 is formed with a through hole 44 that passes through in the extension direction of the rotation axis O1 of the screw shaft 40. When assembling the cylinder unit 11 and the motor unit 12, the tip portion of the motor shaft 29 is inserted into the through hole 44. The motor shaft 29, the tip portion of which is inserted into the through hole 44 of the resin bushing 43, is made of metal.

[0029] In Modification 1, the rotation axis O1 of the screw shaft 40 and the rotation axis O2 of the rotor 28 are aligned by inserting the motor shaft 29 into the through hole 44. That is, the through hole 44 of the resin bushing 43 provided on the screw shaft 40 corresponds to the hole of the alignment structure, and the motor shaft 29 corresponds to the shaft material of the coaxial alignment structure. In Modification 1, the contact surface of the hole with the shaft material is formed of a resin material, while the contact surface of the shaft material with the hole is formed of a metal material.

[0030] When the shaft alignment structure of Modified Example 1 is adopted, the tip portion of the motor shaft 29 comes into sliding contact with the resin bushing 43. The surface of the through hole 44 of the resin bushing 43, which is made of resin, is less hard than the metal motor shaft 29 and is therefore more susceptible to wear. Therefore, when the linear motion actuator 20 operates and the screw shaft 40 and the motor shaft 29 rotate relative to each other, wear progresses on the surface of the through hole 44 in the resin bushing 43, and the sliding resistance between the motor shaft 29 and the through hole 44 decreases. Therefore, by adopting the shaft alignment structure of Modified Example 1, it is possible to achieve both improved shaft alignment accuracy and smoother operation of the linear motion actuator 20.

[0031] <Variation 2> The configuration of Modified Example 2 of the shaft alignment structure of the linear actuator 20 will be described with reference to FIG. 4. As shown in FIG. 4, the motor shaft 50 constituting the shaft material of the shaft alignment structure in Modified Example 2 is composed of a metal substrate 51 and a resin ring 52 attached to the tip of the substrate 51. The substrate 51 is a cylindrical member extending in the stroke S direction. The resin ring 52 is a hollow cylindrical member made of resin. The outer diameter of the resin ring 52 is approximately the same as the inner diameter of the hole 35 of the screw shaft 24. In Modified Example 2, the contact surface of the hole 35 with the motor shaft 50 is made of a metal material, and the contact surface of the motor shaft 50 with the hole 35 is made of a resin material.

[0032] When the linear motion actuator 20 is operated, the threaded shaft 24 and the motor shaft 50 rotate relative to each other. At this time, the resin ring 52 rotates together with either the threaded shaft 24 or the base material 51 of the motor shaft 50, and rotates relative to the other. As a result, wear progresses on the inner or outer periphery of the resin ring 52, and the sliding resistance between the motor shaft 50 and the hole 35 decreases. Therefore, even when the axis alignment structure of Modification 2 is adopted, it is possible to achieve both improved axis alignment accuracy and smoother operation of the linear motion actuator 20.

[0033] As described above, in Modifications 1 and 2, one of the contact surface of the hole with the shaft material and the contact surface of the shaft material with the hole is formed of metal, while the other is formed of resin, which has a lower hardness than metal. In this way, if one of the two contact surfaces is made harder than the other, the contact surface with a lower hardness and therefore more susceptible to wear will gradually wear, reducing the sliding resistance of both contact surfaces. This makes it possible to achieve both improved axial alignment accuracy and smoother operation of the linear actuator 20. Note that the combination of materials for both contact surfaces may be any combination other than resin and metal, as long as they are made of materials with different hardness.

[0034] <Variation 3> 5 and 6, the configuration of Modified Example 3 of the axis alignment structure of the linear motion actuator 20 will be described. As shown in Fig. 5, in the axis alignment structure of Modified Example 3, a metal pin 61, which is a metal pin attached to the tip of the motor shaft 29, serves as the shaft material to be inserted into the hole 35 of the screw shaft 24 during axis alignment.

[0035] The metal pin 61 is integrated with the motor shaft 60 by resin insert molding. That is, the metal pin 61 is connected to the motor shaft 60 via a resin portion 63, which is a portion formed from resin. The metal pin 61 has a cylindrical portion with the same outer diameter as the motor shaft 60. A spline 62 is formed on the side circumference of the cylindrical portion of the metal pin 61. The extension direction of the spline 62 is the extension direction of the rotation axis of the motor shaft 60, i.e., the extension direction of the rotation axis O2 of the rotor 28 of the electric motor 21, which is connected to the motor shaft 60 so as to rotate coaxially and integrally with the motor shaft 60. Meanwhile, a hole 64 is formed in the threaded shaft 24, into which the metal pin 61 can be inserted. A spline 65 that can engage with the spline 62 of the metal pin 61 is formed on the side wall of the hole 64.

[0036] In the linear motion actuator 20 employing the axis alignment structure of Modified Example 3, the electric motor 21 and the linear motion conversion mechanism 23 are aligned by inserting the metal pin 61 into the hole 64 of the screw shaft 24. In this state, the relative rotation between the screw shaft 24 and the motor shaft 60 is restricted by the engagement of the splines 62, 65.

[0037] As shown in Figure 6, when the electric motor 21 is energized in this state, a torsional torque T is generated between the screw shaft 24 and the motor shaft 60. This torsional torque T causes the joint between the motor shaft 60 or the metal pin 61 and the resin part 63 to break. The joint between the metal pin 61 or the motor shaft 60 and the resin part 63 is a weak part that breaks in response to the relative rotation between the rotor 28 and the screw shaft 24 that occurs when the electric motor 21 is operating. The metal pin 61 that constitutes the shaft material of the shaft alignment structure of Modification 3 is connected to the rotor 28 via this weak part. Note that Figure 6 shows a state in which the joint between the motor shaft 60 and the resin part 63 breaks.

[0038] The fracture of the joint allows relative rotation between the screw shaft 24 and the motor shaft 60. Thereafter, the metal pin 61 rotates integrally with the screw shaft 24. Therefore, the fit between the metal pin 61 and the hole 64 does not affect the operation of the linear motion actuator 20. Therefore, even when the axis alignment structure of Modification 3 is adopted, it is possible to achieve both improved axis alignment accuracy and smoother operation of the linear motion actuator 20.

[0039] <Configuration of the linear motion conversion mechanism 20> The linear motion actuator 20 of the above embodiment employs a linear motion conversion mechanism 23 configured such that the nut 25 linearly moves in response to the rotation of the screw shaft 24. Conversely, a linear motion conversion mechanism may be employed in which the screw shaft linearly moves in response to the rotation of the nut.

[0040] FIG. 7 shows a cross-sectional structure of an electric cylinder 110 provided with a linear motion actuator 120 employing such a linear motion conversion mechanism 123. The configurations of the electric motor 21 and the reduction mechanism 22 in this linear motion actuator 120 are the same as those in the above embodiment. The linear motion conversion mechanism 123 in this linear motion actuator 120 includes a nut 125 rotatably attached to the case 13 by a bearing 26, and a threaded shaft 124 threadedly engaged with the nut 125. In this linear motion conversion mechanism 123, the threaded shaft 124 moves linearly in response to the rotation of the nut 125. The piston 115 in the electric cylinder 110 is connected to the threaded shaft 124 so as to move integrally in the stroke direction S. Furthermore, in this linear motion actuator 120, each planetary gear 34 of the reduction mechanism 22 is rotatably attached to the nut 125, not to the threaded shaft 124. That is, in this linear motion actuator 120, the nut 125 also functions as a planetary carrier of the planetary gear mechanism.

[0041] In this linear motion actuator 120 as well, a hole 35 is formed in the screw shaft 124, into which the tip portion of the motor shaft 60 can be inserted. By inserting the tip portion of the motor shaft 60 into the hole 35, the rotation axis O1 of the nut 125 of the linear motion conversion mechanism 123 and the rotation axis O2 of the rotor 28 of the electric motor 21 are aligned. Therefore, in this linear motion actuator 120 as well, the alignment between the electric motor 21 and the linear motion conversion mechanism 123 can be performed easily and with high precision. Note that in this linear motion actuator 120, movement of the screw shaft 124 is permitted in the extension direction of the rotation axis O1 of the nut 125. Therefore, it is desirable to align the electric motor 21 and the linear motion conversion mechanism 123 with the movement of the screw shaft 124 restricted.

[0042] In the above-described embodiment and modified examples, a ball screw mechanism is used as the linear motion conversion mechanism 23, 123. However, any mechanism other than a ball screw mechanism may be used as the linear motion conversion mechanism 23, 123 as long as it has a screw shaft and a nut threaded onto the screw shaft, and one of them moves linearly in response to rotation of the other. An example of a screw mechanism that can be used as the linear motion conversion mechanism 23, 123 is a feed screw mechanism in which the screw shaft and nut are threaded directly together without using balls.

[0043] <Regarding the reduction mechanism 22> In the above embodiment and modified examples, a planetary gear mechanism is used as the reduction mechanism 22. A mechanism other than a planetary gear mechanism may be used as the reduction mechanism 22. Mechanisms that can be used as the reduction mechanism 22 include a cycloid reducer, a paradox gear, and a worm gear. Furthermore, the linear motion actuators 20, 120 may be configured so that the electric motor 21 and the linear motion conversion mechanism 23, 123 are directly connected without providing the reduction mechanism 22.

[0044] <Applications of Linear Actuators 20 and 120> The linear motion actuators 20, 120 of the above-described embodiment and modified examples have been used as actuators that drive the pistons 15, 115 of the electric cylinders 10, 110. The linear motion actuators 20, 120 of the above-described embodiment and modified examples can also be used for other purposes.

[0045] <Application of the bearing structure of the linear actuator 20> The structures for axial alignment between the screw shaft 24 and the rotor 28 in the above-described embodiment and Modifications 1 to 3 can be used for other axial alignment between two rotating bodies that rotate relative to each other. Here, one of the two rotating bodies is designated as a first rotating body, and the other is designated as a second rotating body. As an axial alignment structure for these two rotating bodies, the following shaft member is provided. This shaft member is located on the rotation axis of the first rotating body and extends along the rotation axis of the first rotating body, and rotates integrally with the first rotating body. In addition, the following hole is provided in the second rotating body. This hole is located on the rotation axis of the second rotating body and extends along the rotation axis of the second rotating body. The tip end of the shaft member can be inserted into this hole, and when inserted, the tip end of the shaft member slides against the hole. When the shaft member is inserted into this hole, the rotation axis of the first rotating body and the rotation axis of the second rotating body are aligned. Therefore, direct axis alignment between the first rotating body and the second rotating body is possible by inserting a shaft into the hole provided in the second rotating body. Therefore, the above-described axis alignment structure enables easy and highly accurate axis alignment between the electric motor and the linear motion conversion mechanism.

[0046] If the base material of the second rotating body is made of metal, the alignment structure can be configured in accordance with the first modification. That is, a resin bushing is attached to the base material of the second rotating body. Then, a hole into which the shaft material is inserted is formed in the resin bushing. In this case, when the two rotating bodies are rotated relative to each other after alignment, the hole in the resin bushing rubs against the shaft material inserted therein, causing wear on the surface of the hole in the resin bushing. This wear then reduces the sliding resistance between the hole and the shaft material. Therefore, even if the fit between the shaft material and the hole is tightened to increase the coaxiality of the two rotating bodies, the relative rotation of the two rotating bodies can be performed smoothly.

[0047] Furthermore, if the second rotating body is made of metal, the alignment structure can be configured in accordance with the second modification. That is, the sliding surface of the shaft material inserted into the hole in the second rotating body is made of resin. In such a case, when the two rotating bodies are rotated relative to each other after alignment, the sliding surface of the resin shaft material wears due to friction with the hole, reducing the sliding resistance between the hole and the shaft material. Therefore, even if the fit between the shaft material and the hole is tightened to increase the coaxiality of the two rotating bodies, the relative rotation of the two rotating bodies can be performed smoothly.

[0048] <Other technical ideas> Next, the technical ideas that can be understood from the above-described embodiment and modified examples will be described. (i) An alignment structure for aligning first and second rotating bodies that are rotatably installed and rotate relative to each other, the structure comprising a shaft member located on the rotation axis of the first rotating body and extending along the rotation axis of the first rotating body, the shaft member rotating integrally with the first rotating body, and the second rotating body having a hole located on the rotation axis of the second rotating body and extending along the rotation axis of the second rotating body, into which the tip portion of the shaft member can be inserted and which comes into sliding contact with the tip portion when inserted.

[0049] (ii) The second rotating body has a metal base and a resin bush attached to the base, and the hole is provided in the bush, in the axial alignment structure of the rotating body described in (i) above.

[0050] (c) The shaft alignment structure of the rotor described in (a) above, wherein the second rotor is made of metal, and the sliding surface of the shaft member that comes into contact with the hole is made of resin. (ii) A linear actuator according to any one of claims 1 to 4, wherein the shaft material is connected to the rotor via a fragile portion that breaks in response to relative rotation between the rotor and the screw shaft resulting from operation of the electric motor. [Explanation of symbols]

[0051] 10...Electric cylinder 11...Cylinder unit 12...Motor unit 13...Case (of cylinder unit 11) 14...Cylinder 15,115...Piston 16...liquid chamber 17...Groove 18...Protrusion 19...(Motor unit 12) case 20,120...Linear Actuator 21...Electric motor 22...Reduction mechanism 23,123...Linear motion conversion mechanism 24, 40...Screw shaft (rotating member) 25...Nut (linear motion component) 26...Bearing 27...Rotor 28...Stator 29, 50, 60...Motor shaft 30, 31...Bearings 32...Sun gear 33...Ring gear 34...Planetary gear 35,64…holes 36...Control board 41...Base material 42...Bushing mounting hole 43...Resin bushing 44...Through hole 51...Base material 52...Resin ring 61...Metal pin 62,65...Spline 123...Screw shaft (linear motion member) 124...Nut (rotating part) O1...rotation axis (of a rotating part) O2...rotation axis (of rotor 27)

Claims

1. a linear motion conversion mechanism including one of a screw shaft and a nut threaded onto the screw shaft as a rotatable rotating member, and the other as a linear motion member that moves linearly in response to the rotation of the rotating member; an electric motor including a rotor that rotates coaxially with the rotating member, and rotates the rotating member in response to rotation of the rotor; a shaft connected to the rotor so as to rotate integrally with the rotor; a speed reduction mechanism that reduces the speed of rotation of the rotor and transmits the reduced speed to the rotating member; and By inserting the shaft material, a hole for aligning the rotation axis of the rotating member and the rotation axis of the rotor is provided in the screw shaft, In order to reduce the sliding resistance between the contact surface of the hole with the shaft material and the contact surface of the shaft material with the hole, the hardness of one of the contact surface of the hole with the shaft material and the contact surface of the shaft material with the hole is higher than the hardness of the other. Linear actuator.

2. 2. The linear motion actuator according to claim 1, wherein the screw shaft is the rotating member, and the nut is the linear motion member.

3. A linear actuator as described in claim 1 or claim 2, wherein the reduction mechanism is a planetary gear mechanism having a sun gear connected to the shaft material and a planetary carrier connected to the rotating member.

4. A linear actuator described in any one of claims 1 to 3, wherein the screw shaft has a metal base material and a resin bush attached to the base material, and the hole is provided in the bush.

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