阅读说明：本技术 运动转换机构及包括运动转换机构的电动制动执行机构 (Motion conversion mechanism and electric brake actuator including the same ) 是由 七原正辉 于 2020-01-23 设计创作，主要内容包括：本发明涉及运动转换机构及包括运动转换机构的电动制动执行机构。运动转换机构包括：轴,该轴具有外螺纹；以及筒体,该筒体具有与所述外螺纹进行螺纹连接的内螺纹。运动转换机构将轴和筒体中的一个的旋转运动转换为轴和筒体中的另一个的线性运动。筒体设置有两个内螺纹部,使得无螺纹部在轴向方向上介于两个内螺纹部之间,该两个内螺纹部每个均具有内螺纹。(The present invention relates to a motion conversion mechanism and an electric brake actuator including the motion conversion mechanism. The motion conversion mechanism includes: a shaft having an external thread; and a cylinder body having an internal thread screw-coupled with the external thread. The motion conversion mechanism converts a rotational motion of one of the shaft and the cylinder into a linear motion of the other of the shaft and the cylinder. The cylinder is provided with two internal threaded portions such that the unthreaded portion is interposed between two internal threaded portions each having an internal thread in the axial direction.)

1. A motion conversion mechanism characterized by comprising:

a shaft having an external thread; and

a barrel having an internal thread in threaded connection with the external thread,

wherein the motion conversion mechanism converts a rotational motion of one of the shaft and the cylinder into a linear motion of the other of the shaft and the cylinder, and

wherein the cylinder is provided with two internal thread portions so that a non-threaded portion having no internal thread is interposed between the two internal thread portions in an axial direction, the two internal thread portions each having an internal thread.

2. The motion conversion mechanism according to claim 1, wherein an axial length of the unthreaded portion is longer than an axial length of one of the two internal threaded portions that is shorter in axial length.

3. The motion conversion mechanism according to claim 1 or 2, characterized in that when one direction along the axial direction is defined as a first direction and a direction opposite to the first direction is defined as a second direction, a rotational moment and an axial force act on a portion of the shaft on the first direction side and a portion of the cylinder on the second direction side.

4. The motion conversion mechanism according to claim 3, characterized in that, of the two internal thread portions, an axial length of the internal thread portion on the first direction side is longer than an axial length of the internal thread portion on the second direction side.

5. The motion conversion mechanism according to claim 3 or 4, characterized in that a flange is provided on an outer periphery of the portion of the cylinder on the second direction side.

6. A motion conversion mechanism as claimed in any one of claims 1 to 5, characterized in that the external thread and the internal thread are each a multi-start thread.

7. The motion conversion mechanism according to any one of claims 1 to 6, wherein the external thread and the internal thread are each a trapezoidal thread.

8. An electric brake actuator configured to press a friction member against a rotating body that rotates together with a wheel, characterized by comprising:

a plunger advanced to press the friction member toward the rotating body;

an electric motor; and

the motion conversion mechanism according to any one of claims 1 to 7, configured such that one of the shaft and the cylinder is rotated by the motor, and the other of the shaft and the cylinder is engaged with the plunger to advance and retreat the plunger.

9. The electric brake actuator of claim 8, further comprising a plunger tilt allowing mechanism that is provided at a portion where the other of the shaft and the cylinder is engaged with the plunger, and that allows the plunger to tilt with respect to the other of the shaft and the cylinder.

10. The electric brake actuator of claim 9, wherein the plunger tilt permission mechanism is formed by engaging the other of the shaft and the cylinder with the plunger such that a convex spherical surface and a concave spherical surface contact each other, the convex spherical surface being provided on one of: the other of the shaft and the barrel; and the plunger, and the concave spherical surface is provided on the other of: the other of the shaft and the barrel; and the plunger.

Technical Field

The present invention relates to a motion conversion mechanism that converts rotary motion into linear motion. The present invention also relates to an electric brake actuator configured such that a motion conversion mechanism converts a rotational motion of an electric motor into an advancing-retreating motion of a plunger, and a friction member is pressed against a rotating body that rotates together with a wheel (e.g., a brake disc) as the plunger advances.

Background

There are various devices equipped with a motion conversion mechanism that converts a rotational motion of one member into a linear motion of another member. For example, an electric brake actuator described in japanese unexamined patent application publication No. 2009-197958 (JP2009-197958A) includes a mechanism that converts a rotational motion of a motor into a linear motion of a plunger serving as a linearly-moving member.

Disclosure of Invention

The electric brake actuator described in JP2009-197958A includes a motion conversion mechanism formed of a ball screw mechanism. Ball screw mechanisms are relatively expensive. In view of this, the use of a motion conversion mechanism formed of a screw mechanism that does not use rolling elements is being studied. The motion conversion mechanism generally comprises: a shaft on which an external thread is formed; and a cylinder on which an internal thread screwed with the external thread is formed. In view of the reduction in efficiency and strength due to the inclination between the shaft and the cylinder, it is desirable to increase the length of the cylinder. However, generally, the internal thread is formed over the entire length of the cylinder. Therefore, when the length of the cylinder is increased, the process for forming the internal thread becomes relatively difficult. Therefore, the manufacturing cost of the motion conversion mechanism becomes high. This results in an increase in the manufacturing cost of the electric brake actuator including this motion conversion mechanism. The present invention provides a motion conversion mechanism that has relatively high efficiency and high strength and can be realized at relatively low cost, and by using the motion conversion mechanism, an electric brake actuator that has relatively high efficiency and high strength and can be realized at relatively low cost is provided.

A first aspect of the invention relates to a motion conversion mechanism. The motion conversion mechanism includes: a shaft having an external thread; and the cylinder body is provided with an internal thread in threaded connection with the external thread. The motion conversion mechanism converts a rotational motion of one of the shaft and the cylinder into a linear motion of the other of the shaft and the cylinder. The cylinder is provided with two internal thread portions such that a non-threaded portion having no internal thread is interposed between the two internal thread portions each having an internal thread in the axial direction.

In the above aspect, the unthreaded portion may have a certain length in the axial direction. Specifically, the axial length of the unthreaded portion may be longer than the axial length of one of the two internal thread portions that is shorter in axial length.

In the above-described aspect, when one direction along the axial direction is defined as a first direction and a direction opposite to the first direction is defined as a second direction, the rotational moment and the axial force may act on the portion of the shaft on the first direction side and the portion of the cylinder on the second direction side. In other words, at a portion of the shaft protruding from the cylinder in one direction (hereinafter also referred to as "torque/axial force acting portion of the shaft"), it is possible to apply a torque for rotating the shaft and receive an axial force to prevent the shaft from moving in the axial direction. At an end of the cylinder remote from the torque/axial force application part of the shaft (hereinafter also referred to as "torque/axial force application part of the cylinder"), a torque for preventing rotation of the cylinder may be received and an axial force may be applied to the outside. Further, at the torque/axial force acting portion of the cylinder, a torque for rotating the cylinder may be applied and an axial force may be received to prevent movement of the cylinder in the axial direction. At the torque/axial force action portion of the shaft, a torque for preventing rotation of the shaft may be received and an axial force may be applied to the outside. Hereinafter, this configuration may be referred to as a "torque/axial force relative position action configuration".

When the torque/axial force relative position acting configuration is adopted, in the above-described aspect, of the two internal thread portions, the axial length of the internal thread portion on the first direction side may be longer than the axial length of the internal thread portion on the second direction side. The axial force acting between the external thread of the shaft and the internal thread of the cylinder is highest at the end of the cylinder on the first direction side and decreases from the end toward the second direction side, as described below. This is evident, for example, when the shaft is made thin for the purpose of downsizing and improving the efficiency of the motion conversion mechanism. In other words, it can be considered that the internal thread at the portion of the cylinder on the first direction side receives the most of the action of the axial force, and the internal thread at the portion of the cylinder on the second direction side only needs to mainly perform the function of suppressing the relative inclination of the shaft and the cylinder. In view of the above, the number of the engagement ridges of the internal thread portion on the first direction side may be relatively large, and the number of the engagement ridges of the internal thread portion on the second direction side may be relatively small. In the above aspect, the flange may be provided on an outer periphery of a portion of the cylinder on the second direction side. With this configuration, it is possible to make the rotational moment act on the cylinder more effectively.

In the above aspect, the external thread and the internal thread may each be a multiple start thread or a trapezoidal thread. The machining load of the multi-start thread and the trapezoidal thread is high. When the present invention is applied to a motion conversion mechanism including a multiple start thread or a trapezoidal thread, the effect of reducing the processing cost will be fully exerted.

A second aspect of the invention relates to an electric brake actuator configured to press a friction member against a rotating body that rotates together with a wheel. The electric brake actuator includes: a plunger that advances to press the friction member toward the rotating body; an electric motor; and the motion conversion mechanism according to the above-described first aspect, configured such that one of the shaft and the cylinder is rotated by the motor, and the other of the shaft and the cylinder is engaged with the plunger to advance and retreat the plunger.

In the above aspect, the electric brake actuator may further include a plunger tilt allowing mechanism that is provided at a portion where the other of the shaft and the barrel is engaged with the plunger, and that allows the plunger to tilt with respect to the other of the shaft and the barrel. By the plunger tilt allowing mechanism, for example, a radial force acting on the plunger due to uneven wear of the friction member or the like can be appropriately solved. In the above-described aspect, the plunger tilt permission mechanism may be formed by engaging the other of the shaft and the barrel with the plunger such that the convex spherical surface and the concave spherical surface are in contact with each other, the convex spherical surface being provided on one of: the other of the shaft and the barrel; and a plunger, and the concave spherical surface is provided on the other of: the other of the shaft and the barrel; and a plunger.

Here, the number of ridges or roots of the internal thread of the barrel which engage with the roots or ridges of the external thread of the shaft in the axial direction is defined as the number of engaging ridges. In the motion conversion mechanism according to the present invention, even when the number of the engaging ridges is relatively small, the axial distance between two ridges most spaced apart in the axial direction among the ridges of the internal thread of the cylinder that engage with the roots of the external thread of the shaft can be set. Therefore, the inclination between the shaft and the cylinder can be made relatively small without increasing the difficulty of the processing of forming the internal thread of the cylinder. Therefore, according to the present invention, a motion conversion mechanism that has relatively high efficiency and strength and can be realized at relatively low manufacturing cost can be provided. Further, according to the present invention, an electric brake actuator having relatively high efficiency and strength and being realized at relatively low cost can be provided by employing the motion conversion mechanism according to the present invention.

Drawings

Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, wherein like reference numerals denote like elements, and in which:

fig. 1 shows an electric brake device including an electric brake actuator according to an embodiment, the electric brake actuator including a motion conversion mechanism according to the embodiment;

fig. 2 is a sectional view showing an electric brake actuator according to the embodiment;

fig. 3A is a diagram for illustrating a speed reduction mechanism included in an electric brake actuator according to the embodiment;

fig. 3B is a diagram for illustrating a speed reduction mechanism included in the electric brake actuator according to the embodiment;

fig. 4 is a perspective view showing a nut used as a cylinder included in the motion conversion mechanism according to the embodiment;

fig. 5 is a diagram for illustrating a plunger tilt permission mechanism of an electric brake actuator according to the embodiment; and is

Fig. 6 is a partial sectional view illustrating a motion conversion mechanism according to an embodiment.

Detailed Description

Hereinafter, a motion conversion mechanism and an electric brake actuator as embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may be carried out in various forms other than the following embodiments, wherein various modifications and improvements are made based on knowledge of those skilled in the art.

Electric brake device comprising an electric brake actuator

As shown in fig. 1, an electric brake actuator 10 (hereinafter sometimes simply referred to as "actuator 10") of the embodiment is a main component in an electric brake device. The electric brake device includes: a brake caliper 12 (hereinafter sometimes simply referred to as "caliper 12"), the brake caliper 12 holding the actuator 10; a disk-shaped rotor 14, the disk-shaped rotor 14 serving as a rotating body that rotates together with a wheel; a pair of brake pads (hereinafter sometimes simply referred to as "pads") 16a, 16 b; and an electronic control unit (hereinafter sometimes referred to as "ECU") 18, the electronic control unit 18 serving as a controller.

The forceps 12 are held by a mounting member (not shown) so as to extend across the disc rotor 14 and be movable in the axial direction (left-right direction in fig. 1). The mounting is provided on a bracket (not shown) that holds the wheel so that the wheel can rotate. The head blocks 16a, 16b are held by the mounts such that the head blocks 16a, 16b are movable in the axial direction, and the disc rotor 14 is sandwiched between the head blocks 16a and 16 b. Each of the head blocks 16a, 16b includes a friction member 26 on one side of the disc-shaped rotor 14 to contact the disc-shaped rotor 14 and a backup plate 28 supporting the friction member 26. The friction members 26 of each of the pads 16a, 16b are pressed against the disc-shaped rotor 14. The pads 16a, 16b may themselves be referred to as friction members.

For convenience, description will be made on the assumption that the left side of fig. 1 represents the front side and the right side of fig. 1 represents the rear side. The front pad 16a is supported by a claw 32, which claw 32 is a front end portion of the caliper body 30. The actuator 10 is held by a rear portion of the caliper body 30 such that the housing 40 of the actuator 10 is secured to the rear portion. The actuator 10 has a plunger 42, which plunger 42 is held by the housing 40 so as to advance and retreat. As the plunger 42 advances, a tip (specifically, a front end) of the plunger 42 engages the rear pad 16b, specifically, the fence 28 of the pad 16 b. When the plunger 42 in the engaged state is further advanced, the pair of head blocks 16a, 16b hold the disc-shaped rotor 14. In other words, the friction member 26 of each pad 16a, 16b is pressed against the disc rotor 14. This pressing generates a braking force on the rotation of the wheel, which depends on the frictional force between the disc rotor 14 and the friction member 26, i.e., the braking force for decelerating and stopping the vehicle.

Basic structure of electric brake actuator

As shown in fig. 2, the actuator 10 according to the embodiment of the present invention includes a motor (three-phase Direct Current (DC) brushless motor) 44 serving as a driving source, a speed reduction mechanism 46 for reducing the rotation transmitted from the motor 44, a motion conversion mechanism 50, and the like, in addition to the plunger 42 and the housing 40 serving as the main body of the actuator 10. The motion conversion mechanism 50 has a rotary shaft 48 (an example of a shaft) that rotates in accordance with the rotation of the motor 44, which is transmitted via the speed reduction mechanism 46. The motion conversion mechanism 50 converts the rotational motion of the rotary shaft 48 into forward-backward motion (forward and backward motion) of the plunger 42. The motion conversion mechanism 50 is a motion conversion mechanism according to the embodiment, and simply speaking, can be considered to convert the rotational motion of the motor 44 into the forward-backward motion of the plunger 42. In the following description, for convenience, the left side in fig. 2 will be referred to as the front side, and the right side in fig. 2 will be referred to as the rear side.

Specifically, the housing 40 includes a front case 40a, a rear case 40b, an inner cylinder 40c, a support wall 40d, a support plate 40e, and the like. The front case 40a and the rear case 40b each have a substantially cylindrical shape. The inner cylinder 40c has a front end supported by the front housing 40a, and the plunger 42 is disposed inside the inner cylinder 40 c. A support wall 40d having a substantially annular shape is disposed inside the front housing 40a and is supported by the front end of the rear housing 40 b. The support plate 40e is fixed and held by the rear end of the rear case 40 b.

The plunger 42 includes a plunger head 42a and a hollow plunger barrel 42 b. The plunger 42 is engaged with the friction member 26 of the brake pad 16b via the backup plate 28 at the front end of the plunger head 42a serving as the tip end of the plunger 42. The actuator 10 has a hollow shaft 52, the hollow shaft 52 having a cylindrical shape. The front portion of the hollow shaft 52 mainly serves as a motor shaft (rotor) that is a rotational drive shaft of the motor 44, and the rear portion of the hollow shaft 52 mainly serves as an input shaft of the speed reduction mechanism 46 described in detail later. That is, the motor 44 may be regarded as one type of motor in which a hollow motor shaft rotates. In the following description, the hollow shaft 52 may be regarded as a shaft formed by integrating a motor shaft of the motor 44 and an input shaft of the reduction mechanism 46 rotated by the motor 44. In short, the hollow shaft 52 itself may be regarded as an input shaft of the reduction mechanism 46, or the hollow shaft 52 itself may be regarded as a motor shaft of the motor 44. The motor 44 has a coil 44a and a magnet 44 b. The coil 44a is fixed and held by the front housing 40a of the housing 40 so as to be located inside the front housing 40 a. The magnet 44b is disposed on the outer periphery of the front portion of the hollow shaft 52 so as to face the coil 44 a.

The hollow shaft 52 is arranged such that the inner cylinder 40c is located inside the front portion of the hollow shaft 52. The hollow shaft 52 is supported by the housing 40 via two radial ball bearings 58, 60 so as to be rotatable about an axis L as a central axis of the actuator 10 and immovable in an axial direction in which the axis L extends. With regard to the positional relationship between the hollow shaft 52 serving as the motor shaft and the plunger 42, the rear end of the plunger 42 is disposed inside the hollow shaft 52. The inner cylinder 40c has a pair of slots 40f extending in the axial direction, and a pair of keys 42c attached to the plunger 42 engage with the pair of slots 40 f. Thus, the plunger 42 is allowed to move in the axial direction while being prevented from rotating about the axis L relative to the housing 40.

The rotating shaft 48 is arranged inside a hollow shaft 52 serving as a motor shaft so as to be coaxial with the hollow shaft 52. The rotating shaft 48 includes three portions integrated together, that is, a shaft portion 48a serving as an output shaft of the reduction mechanism 46, an externally threaded portion 48b provided on a front side of the shaft portion 48a, and a flange portion 48c provided on a rear end of the shaft portion 48 a. The rotary shaft 48 is supported at its shaft portion 48a inside the hollow shaft 52 via rollers (also referred to as "needle rollers") 62 so as to be rotatable about the axis L.

In addition to the hollow shaft 52 serving as an input shaft and the rotary shaft 48 of which the shaft portion 48a serves as an output shaft, the reduction mechanism 46 includes a planetary gear body 66 that is supported by the rear portion of the hollow shaft 52 via a radial ball bearing 64 so as to be rotatable but immovable in the axial direction. The rear end of the hollow shaft 52 (hereinafter sometimes also referred to as "eccentric shaft portion 52 a") that supports the planetary gear body 66 at the outer periphery via the radial ball bearing 64 has an axis L '(hereinafter sometimes referred to as "eccentric axis L'") defined by the outer peripheral surface. The axis L' is eccentric with respect to the axis L by an eccentric amount Δ L. Thus, the planetary gear body 66 rotates about the eccentric axis L' and revolves about the axis L together with the rotation of the hollow shaft 52 about the axis L.

The reduction mechanism 46 further includes a ring gear body 68 fixedly supported by the support wall 40d of the housing 40. As shown in fig. 3A, the ring gear body 68 has a first internal gear 70, and the outer periphery of the planetary gear body 66 is provided with a first external gear 72, a portion of which 72 meshes with a portion of the first internal gear 70. Further, as shown in fig. 3B, a second internal gear 74 is provided on the inner periphery of the planetary gear main body 66 such that the first external gear 72 and the second internal gear 74 are arranged side by side in the axial direction. The outer periphery of the flange portion 48c of the rotary shaft 48 is provided with a second external gear 76, and a part of the second external gear 76 meshes with a part of the second internal gear 74.

The center of the first internal gear 70 is positioned on the axis L, the center of the first external gear 72 is positioned on the eccentric axis L ', the center of the second internal gear 74 is positioned on the eccentric axis L', and the center of the second external gear 76 is positioned on the axis L. The meshing points of the first internal gear 70 and the first external gear 72 and the meshing points of the second internal gear 74 and the second external gear 76 are positioned on opposite sides of the axis L or the eccentric axis L', that is, at positions (phases) shifted from each other by 180 degrees in the circumferential direction. That is, the speed reduction mechanism 46 is a differential speed reduction gear including a first inner planetary gear mechanism and a second inner planetary gear mechanism. The first internal planetary gear mechanism has a first internal gear 70 and a first external gear 72 that contacts the first internal gear 70 from the inside to mesh with the first internal gear 70. The second inner planetary gear mechanism has a second inner gear 74 and a second outer gear 76 contacting the second inner gear 74 from the inside to mesh with the second inner gear 74.

The first inner gear 70 has a circular arc tooth profile, and the first outer gear 72 has an epitrochoidal parallel curved tooth profile. Similarly, the second internal gear 74 has a circular arc tooth profile, and the second external gear 76 has an epitrochoidal parallel-curved tooth profile. Therefore, the speed reduction mechanism 46 is configured as a cycloid speed reducer (sometimes referred to as a "Cyclo (registered trademark) speed reducer"). Therefore, in the speed reduction mechanism 46, the number of teeth of the first internal gear 70 and the number of teeth of the first external gear 72 differ by only one, and the number of teeth of the second internal gear 74 and the number of teeth of the second external gear 76 differ by only one. Therefore, the speed reduction mechanism 46 is a speed reduction mechanism having a high speed reduction ratio, that is, the speed reduction mechanism 46 has a considerably small ratio of the rotational speed of the rotating shaft 48 serving as the output shaft to the rotational speed of the hollow shaft 52 serving as the input shaft, thus providing smooth speed reduction.

As shown in fig. 2, the motion conversion mechanism 50 includes the rotary shaft 48 (more specifically, the externally threaded portion 48b of the rotary shaft 48) and a nut 78 serving as a cylinder that is screwed with the externally threaded portion 48 b. The external threads of the external threaded portion 48b and the internal threads of the nut 78 are trapezoidal threads and multi-start threads (three-start threads in the actuator 10). A flange 78a is provided on the outer periphery of the front end portion of the nut 78. As shown in fig. 4, the flange 78a of the nut 78 has a hexagonal outer periphery. The flange 78a is inserted into a bottomed hexagonal hole 42d formed in a rear portion of the plunger cylinder 42b of the plunger 42, and is prevented from falling out rearward by the ring 84. As described above, because the plunger 42 is prevented from rotating relative to the housing 40, rotation of the nut 78 relative to the housing 40 is also prevented by the engagement of the flange 78a with the bottomed hexagonal hole 42 d. The motion conversion mechanism 50 according to the embodiment will be described in detail below.

The front end face 86 of the nut 78 (specifically, the front end face of the flange 78 a) is in contact with a receiving face 88 serving as the bottom face of the bottomed hexagonal hole 42d of the plunger cylinder 42 b. The advancing force of the nut 78 is transmitted as the advancing force of the plunger 42 via the front end face 86 and the receiving face 88 as contact faces that contact each other. The advancing force of the plunger 42 serves as a pressing force by which the plunger 42 presses the friction members 26 of the brake pads 16a, 16b against the disc rotor 14.

Referring also to fig. 5, the front end face 86 of the nut 78 and the receiving face 88 of the plunger barrel 42b are configured to mate with one another, and the front end face 86 and the receiving face 88 form a portion of a spherical surface centered on the point O on the axis L. Specifically, the front face 86 of the nut 78 is a convex spherical surface and the receiving surface 88 of the plunger barrel 42b is a concave spherical surface. Therefore, when a radial force acts on the plunger 42 that receives the reaction force of the pressing force, the plunger 42 is inclined such that the receiving surface 88 slides along the front end surface 86 of the nut 78, as shown in fig. 5. As a mechanism for providing this function, the actuator 10 is provided with a plunger tilt allowing mechanism including a front end face 86 and a receiving face 88.

The plunger 42 may receive a radial force when braking force is generated, due to uneven wear of the friction member 26, inclination of the disc rotor 14 due to rotation of the vehicle, or the like. In this condition, the plunger tilt allowing mechanism allows smooth tilting of the plunger 42, thus reducing excessive load or burden on the actuator 10. In the actuator 10, the plunger tilt allowing mechanism is formed by engaging a convex spherical surface provided on the nut 78 and a concave spherical surface provided on the plunger 42 so as to contact each other. However, the plunger tilt allowing mechanism may be formed by providing the nut 78 with a concave spherical surface and providing the plunger 42 with a convex spherical surface.

The rotary shaft 48 is supported by the housing 40 via a thrust bearing (specifically, a thrust ball bearing 90) at a flange portion 48c provided at a rear end of the rotary shaft 48. More specifically, a pressing force sensor 92 for detecting a pressing force (axial force) is disposed between the thrust ball bearing 90 and the support plate 40 e. The rotary shaft 48 is also supported by the support plate 40e of the housing 40 via the pressing force sensor 92. The pressing force sensor 92 is a so-called dynamometer, and its detailed structure is omitted in the drawings. More specifically, the actuator 10 includes a reverse torque applying mechanism 102, the reverse torque applying mechanism 102 including a stator 96, a rotor 98, and a torsion coil spring 100 (the torsion coil spring 100 is a torsion spring). The rotor 98 is provided between the thrust ball bearing 90 and the flange portion 48c of the rotary shaft 48. A small clearance CL (shown exaggerated in fig. 2) exists between the rotor 98 and the flange portion 48 c. When the plunger 42 advances and presses the friction member 26 against the disc-shaped rotor 14, the rotary shaft 48 is slightly retreated by the reaction force of the pressing force, and the flange portion 48c and the front end surface 98a of the rotor 98 contact each other to eliminate CL. Therefore, the rotary shaft 48 is supported by the housing 40 via the thrust ball bearing 90 at the rear end of the rotary shaft 48 (i.e., at the flange portion 48 c).

The rotary shaft 48 and the rotor 98 rotate together while the pressing force is being applied. Although the detailed description is omitted, one end of the torsion coil spring 100 is connected to the stator 96, and the other end is connected to the rotor 98. When the plunger 42 advances to increase the braking force, the torsion coil spring 100 is further twisted with the increase. Due to the elastic torque of the torsion coil spring 100, a retreating force as a torque in a direction in which the plunger 42 retreats is applied to the rotary shaft 48. For example, when an electric power failure that prevents the operation of the motor 44 occurs while braking force is being generated, there may be a situation where the plunger 42 cannot be retracted and drag cannot be eliminated depending on the operation of the motor 44. In view of this, the actuator 10 is provided with a reverse torque applying mechanism 102.

In addition to the pressing force sensor 92, the actuator 10 includes a rotation angle sensor 104 for detecting a rotation angle (rotation phase) of the hollow shaft 52 as the motor shaft. The rotation angle sensor 104 is a resolver.

As shown in fig. 1, the ECU 18 as the control device includes a computer 110 (the computer 110 has a Central Processing Unit (CPU), a Random Access Memory (RAM), a Read Only Memory (ROM), and the like) and an inverter 112 serving as a drive circuit (driver) of the motor 44. The pressing force FS detected by the pressing force sensor 92 and the rotation angle θ of the hollow shaft 52 detected by the rotation angle sensor 104 are transmitted to the computer 110 and the inverter 112.

The control of the actuator 10 will be briefly described. The computer 110 determines, for example, a necessary braking force that is a braking force to be generated by the electric brake device according to the degree of operation of a brake operating member such as a brake pedal. Based on the necessary braking force, the computer 110 determines a target pressing force that is a target of the pressing force FS. Next, the computer 110 determines a target supply current, which is a current I to be supplied to the motor 44 so that the detected pressing force FS matches the target pressing force. The inverter 112 controls the motor 44 based on the detected rotation angle θ according to the target supply current.

Motion conversion mechanism

As described above, the actuator 10 includes the motion conversion mechanism 50, the motion conversion mechanism 50 has the rotary shaft 48 serving as a shaft having an external thread and the nut 78 serving as a cylinder having an internal thread screwed with the external thread, and converts the rotary motion of the rotary shaft 48 into the linear motion of the nut 78.

Referring to fig. 6, in the motion conversion mechanism 50, internal threads are formed at two portions spaced apart from each other in the axial direction in the nut 78. When a rear portion of the nut 78 (i.e., a portion including an end where the flange 78a is not formed) is referred to as a first internal threaded portion 78b and a front portion of the nut 78 (i.e., a portion including an end where the flange 78a is formed) is referred to as a second internal threaded portion 78c, the first internal threaded portion 78b and the second internal threaded portion 78c are provided such that an unthreaded portion 78d having no internal thread intervenes therebetween in the axial direction.

Here, the backward direction is defined as a first direction that is one direction along the axial direction, and the forward direction is defined as a second direction opposite to the first direction, as shown in fig. 6. In the motion conversion mechanism 50, as indicated by a white arrow in fig. 6, a rotational moment for rotating the rotary shaft 48 acts on the flange portion 48c, the flange portion 48c being a portion of the rotary shaft 48 on the first direction side, and an axial force as a reaction force against the pressing force from the support plate 40e of the housing 40 also acts on the flange portion 48 c. At the flange 78a, which is a portion of the nut 78 on the second direction side, a reaction force of the rotational moment and the pressing force for suppressing the rotation of the nut 78 acts. That is, the flange portion 48c functions as a torque/axial force application portion of the rotating shaft 48, and the flange 78a functions as a torque/axial force application portion of the nut 78. The flange portion 48c is located on the first direction side of the rotation shaft 48, and the flange 78a is located on the second direction side of the nut 78. Therefore, the motion conversion mechanism 50 adopts a torque/axial force relative position acting configuration.

In consideration of reduction in efficiency, strength, and the like of the motion conversion mechanism 50 due to inclination between the rotary shaft 48 and the nut 78 (more specifically, relative inclination between respective axes of the rotary shaft 48 and the nut 78), the length of the nut 78 in the axial direction (hereinafter sometimes referred to as "axial length") is desirably long so as to reduce the inclination. Here, it is assumed that the internal thread is provided over the entire axial length of the nut like a general nut. When the axial length is increased, the number of ridges of the internal thread (the number of ridges in the axial direction) is increased, so that the process of forming the internal thread is relatively difficult. Since the motion converting mechanism 50 employs a trapezoidal thread and a multiple start thread, the processing load is large and the processing is rather difficult.

In view of the foregoing, the motion conversion mechanism 50 includes a nut 78 having two internal threaded portions, i.e., a first internal threaded portion 78b and a second internal threaded portion 78c, with an unthreaded portion 78d interposed between the first and second internal threaded portions 78b and 78 c. Therefore, the number of ridges to be formed is considerably small, and the process of forming the internal thread can be performed relatively easily. The axial length of the nut 78 is long, that is, the distance between the end of the internal thread on the first direction side that meshes with the external thread of the rotary shaft 48 and the end of the internal thread on the second direction side is large. Therefore, the inclination between the nut 78 and the rotation shaft 48 is suppressed. In other words, the motion conversion mechanism 50 is a mechanism that has relatively high efficiency and strength and can be realized at relatively low cost, and therefore, the actuator 10 including the motion conversion mechanism 50 is an actuator that has relatively high efficiency and strength and can be realized at relatively low cost.

When the number of ridges or roots of the internal thread of the nut 78 that engage with the roots or ridges of the external thread of the rotary shaft 48 in the axial direction is defined as the number of engagement ridges, the number of engagement ridges of the first internal thread portion 78b is substantially "6", and the number of engagement ridges of the second internal thread portion 78c is substantially "3", specifically, as shown in fig. 6. Further, the unthreaded portion 78d has an axial length such that the number of engaging ridges is substantially "4". That is, when the first internal threaded portion 78b, the second internal threaded portion 78c, and the unthreaded portion 78d are Lb, Lc, and Ld, respectively, as shown in fig. 6, Lb > Ld > Lc is satisfied in the motion conversion mechanism 50.

Assuming that the internal thread is formed over the entire axial length of the nut 78, when a rotational moment and an axial force act as shown in fig. 6, the axial force F acting on each ridge is as shown in the graph of fig. 6. Specifically, because the diameter of the externally threaded portion 48b of the rotary shaft 48 is relatively small, that is, the torsional rigidity of the rotary shaft 48 is relatively low, the axial force F is greatest on the ridges of the internal thread at the first-direction end. The axial force F acting on the ridge gradually decreases toward the second direction side. In short, the axial force acting on the nut 78 is received mainly by the ridges on the first direction side and rarely by the ridges on the second direction side. Based on this axial force distribution, the axial length Lb of the first internal thread portion 78b on the first direction side is made longer. In other words, the axial length Lc of the second internal thread portion 78c on the second direction side is maintained to a length sufficient to suppress the inclination between the rotation shaft 48 and the nut 78.

The motion conversion mechanism 50 of the embodiment has been described above. The electric brake actuator may be configured such that a nut having an internal thread is rotated by the motor, and a shaft having an external thread is advanced and retreated to cause a plunger to press the friction member against the disc rotor. In this case, a motion conversion mechanism that converts the rotational motion of the nut into the linear motion of the shaft may be employed. When such an electric brake actuator includes a plunger tilt permitting mechanism, a mechanism that permits tilting of the plunger with respect to the shaft may be employed.