JP2013076462A - Ball screw actuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ball screw actuator, in which a nut has high circularity although a rotor of an electric motor is fitted on the outer circumference of the nut, and which can be manufactured in the fewer number of processes and includes the nut having sufficient strength.SOLUTION: The ball screw actuator includes an electric motor and a ball screw. The ball screw includes: a screw shaft having a helical thread groove on an outer circumferential surface thereof; a nut having, on an inner circumferential surface thereof, the thread groove facing the thread groove of the screw shaft; a plurality of balls loaded, to be rolled freely, to a helical ball rolling path formed by both thread grooves; and a ball circulation path for returning and circulating the balls from the end point of the ball rolling path to the start point thereof. The nut is driven to rotate by the electric motor. The ball circulation path is integrally formed with the nut, and a rotor of the electric motor is press-fitted into the outer circumference of the nut.

Description

  The present invention relates to an actuator having a ball screw, which is used for automobile parts, positioning devices and machines, and particularly relates to an actuator that can be suitably used for a brake device.

2. Description of the Related Art Conventionally, as an actuator using a ball screw, a configuration in which a nut is rotationally driven through a rotation shaft is known (see, for example, Patent Document 1). On the other hand, from the viewpoint of compactness, it is preferable to circulate the balls of the ball screw by an internal circulation method (coma type), and it is preferable to directly attach the rotor of the electric motor to the outer periphery of the nut. It has been.
As for the attachment of the rotor to the nut, the rotor is attached to the coma type ball screw nut by press-fitting, fixing with a lock nut (for example, see Patent Document 2), spline fitting (for example, see Patent Document 3), or the like. It has been done to attach directly to.

  However, in press-fitting and fixing with a lock nut, when the rotor is fixed to the nut, a compressive load is applied to the nut. In the case of the top type ball screw, since the top hole penetrating the inner periphery and the outer periphery of the nut is provided, the deformation due to the load is not uniform in the circumferential direction and the axial direction of the nut. Due to the uneven deformation in the circumferential direction, there is a risk that the roundness of the nut changes and the durability and acoustic properties of the ball screw are impaired. In addition, man-hours are required to avoid this, and design inconveniences such as not providing a rotor on the outer periphery of the frame may occur. On the other hand, in the case of spline fitting, although the above-mentioned concerns are small, there is a problem that the processing steps increase.

Further, as in Patent Document 4, if the nut integrated with the circulation groove is formed by sintering, the compression load due to the coma hole does not become uneven. However, there is a problem that the sintered body is easily deformed as compared with a steel material for a ball screw and is not suitable for press-fitting. In addition, since there are many material defects and impurities, there is a problem that the rolling surface is easy to flake and is not suitable for a brake application requiring a long life and high reliability.
Furthermore, in the manufacturing method of Patent Document 4, in order to manufacture a nut by die cutting, a female screw groove is formed over the entire inner peripheral surface of the nut. That is, there is an internal thread groove where the ball does not roll. Therefore, there is a problem that the strength of the nut is lowered as compared with the case where the female screw groove does not exist. In addition, the same thing as the above can be said also when fitting a bearing to a nut.

Japanese Patent No. 4182726 Japanese Patent Laid-Open No. 11-270641 JP 2007-132493 A JP 2000-297854 A

  In view of the above, the present invention has a sufficient roundness and can be manufactured with a small number of steps and has sufficient strength despite the fact that the rotor of the electric motor is fitted to the outer periphery of the nut. An object is to provide a ball screw actuator including a nut.

The above object of the present invention is achieved by the following configurations.
That is, a ball screw actuator according to an aspect of the present invention is a ball screw actuator including an electric motor and a ball screw, wherein the ball screw includes a screw shaft having a helical thread groove on an outer peripheral surface, and the screw shaft. A nut having a thread groove opposed to the thread groove on the inner peripheral surface, a plurality of balls slidably loaded in a spiral ball rolling path formed by the both thread grooves, and the ball rolling the ball. A ball circulation path that circulates back from the end point of the movement path to the start point, and the nut is rotationally driven by the electric motor, the ball circulation path is formed integrally with the nut, The rotor of the electric motor is press-fitted to the outer periphery. “The ball circulation path is formed integrally with the nut” means that “the ball circulation path is formed of a concave groove formed by recessing a part of the inner peripheral surface of the nut”. means.

The ball screw actuator preferably moves the friction pad of the brake device by the axial movement of the screw shaft.
Furthermore, in this ball screw actuator, it is preferable that the ball circulation path is formed by plastic working.
This plastic working is inserted into a cylindrical nut material and moves along the axial direction thereof, and is disposed between the nut material and the cam driver, and a convex portion corresponding to the ball circulation path. , And a cam slider in which the convex portion moves in the radial direction of the nut by the movement of the cam driver, can be performed using a press method using a mold of a cam mechanism.

Moreover, it is preferable that the nut is formed of any one of chromium steel, chromium molybdenum steel, or carbon steel having a carbon content of 0.4 mass% or more and 0.6 mass% or less.
Moreover, it is preferable that the thread groove of the said nut is formed only in the part which connects from the one end of the said ball circulation path to the other end.
Furthermore, the rotor of the electric motor covers the outer periphery of the nut so as to cover all the portions of the outer peripheral surface of the nut facing the thread groove and the ball circulation path formed on the inner peripheral surface of the nut. It is preferable to be press-fitted into.

  Since the ball screw actuator of the present invention does not have a top hole that penetrates the inner and outer peripheral surfaces of the nut, unlike the top nut, the nut of the nut is in spite of the fact that the rotor of the electric motor is press-fitted to the outer periphery of the nut. A nut having high roundness and capable of being manufactured with a small number of steps and having sufficient strength is provided.

It is an axial sectional view of the ball screw actuator and brake device concerning the present invention. It is a figure explaining the formation method of a ball circuit. It is a figure explaining the metal mold | die used for formation of a ball circulation path. It is a figure explaining the metal mold | die used for formation of a ball circulation path.

Embodiments of the present invention will be described below with reference to the drawings.
(Constitution)
FIG. 1 is a cross-sectional view showing a configuration of a brake device using a ball screw actuator according to the present embodiment. In the figure, reference numeral 1 denotes a carrier as an attachment member provided in a non-rotating portion of the vehicle. The carrier 1 is fixed to, for example, a non-rotating portion (not shown) of a vehicle via a bolt or the like, and a pair of arms (not shown) extending in the axial direction of the disk 2 (from the inner side to the outer side). have.

  Each arm portion of the carrier 1 straddles the outer peripheral side of the disk 2 that rotates integrally with a vehicle wheel (not shown), and slidably supports friction pads 24 described later on both sides in the axial direction of the disk 2. It has a configuration. Further, the carrier 1 is provided with an arch-shaped reinforcing portion 1B that integrally connects the arm portions on the outer side of the disk 2, and the reinforcing portion 1B increases the rigidity of the carrier 1 as a whole.

  The caliper 6 is attached to the carrier 1 so as to be displaceable in the axial direction of the disk 2, and is located on the inner leg 7 located on one side (inner side) of the disk 2 and on the other side (outer side) of the disk 2. And a bridge portion 9 integrally connecting the inner leg 7 to the outer claw 8 across the outer peripheral side of the disk 2, and one end of the inner leg 7. A lid 10 is provided on the side.

The ball screw actuator 11 according to this embodiment is provided in the inner leg portion 7 of the caliper 6, and the ball screw actuator 11 includes an electric motor 12 and a ball screw mechanism 17 described later. The electric motor 12 is composed of, for example, a direct-acting motor (DD motor (direct drive motor)), a stator 13 composed of a winding fixed to the inner wall side of the inner leg 7, and the stator 13. The rotor 14 is provided on the radially inner side.
The rotor 14 of the electric motor 12 includes a magnet 15 provided on the outer peripheral side of a nut cylinder 19 described later. The electric motor 12 drives the rotor 14 to rotate in the forward and reverse directions within the inner leg 7 by energizing the stator 13 via the external wiring 16.

  That is, when the driver of the vehicle depresses a brake pedal (not shown), a signal corresponding to the operation amount at this time is output to a brake control controller (not shown). The controller outputs a control signal to rotate the electric motor 12 with a rotation angle or torque corresponding to the operation amount, and power is supplied to the windings of the stator 13 through the external wiring 16. The stator 13 of the electric motor 12 rotates the rotor 14 by power supply from the controller and displaces the piston 21 in the axial direction of the disk 2.

  The ball screw mechanism 17 is provided in the inner leg 7 as a pressing force generating means together with the electric motor 12. The ball screw mechanism 17 is rotatably provided in the inner leg portion 7 via a bearing 18, and has a stepped cylindrical nut cylinder 19 in which a spiral thread groove 19D is formed on the inner peripheral side. The piston 21 is composed of a cylindrical screw shaft that is screwed into the thread groove 19D of the nut cylinder 19 via a plurality of circulated balls (not shown).

  The nut cylinder 19 in which the rotor 14 is press-fitted to the outer peripheral surface extends in the inner leg portion 7 in the axial direction to a position close to the lid body 10 at one end side. Further, on the other end side of the nut cylinder 19, an annular flange portion 19 </ b> B that protrudes radially outward and has an outer diameter that substantially corresponds to the inner diameter dimension of the stator 13 is provided. A bearing 18 is disposed on the outer peripheral side of the flange portion 19B, and the bearing 18 is press-fitted and fixed between the flange portion 19B and the inner leg portion 7.

  On the other hand, a spiral groove 21A having a constant lead angle corresponding to the thread groove 19D is formed on the outer peripheral surface of the piston 21, and between the spiral groove 21A and the thread groove 19D of the nut cylinder 19, Each ball is accommodated in a rollable manner. When the nut cylinder 19 is rotated by the electric motor 12, the ball screw mechanism 17 converts the rotation of the nut cylinder 19 into the axial displacement of the piston 21, and causes the piston 21 to rotate with respect to the rotation of the nut cylinder 19. A large axial driving force is generated.

  When the axial driving force of the piston 21 is further increased, the lead angle of the spiral groove 21A or the like may be decreased, and thereby the rotational load (torque) of the electric motor 12 can be further decreased. Become. The piston 21 is in contact with the inner friction pad 24 from the back side, and presses the inner friction pad 24 against one side of the disk 2 in accordance with the axial displacement of the piston 21.

  The friction pads 24, 24 are disposed on both sides of the disk 2. When a brake operation is performed, the inner friction pad 24 of each friction pad 24 is pressed toward one side of the disk 2 by the piston 21. The Further, the friction pad 24 on the outer side slides and displaces the entire caliper 6 with respect to the carrier 1 in the direction indicated by the arrow A in FIG. 2 is pressed toward the other side. Each friction pad 24 gives a braking force to the rotating disk 2 by strongly holding the disk 2 from both sides.

Next, features of the ball screw actuator 11 of the brake device having the above-described configuration will be described.
As described above, the nut cylinder 19 of the ball screw actuator 11 has the spiral thread groove 19D opposed to the spiral groove 21A formed on the outer periphery of the piston 21 on the inner peripheral surface of the screw hole 19A. In addition, a ball circulation path 19C is provided that circulates a plurality of balls (not shown) movably loaded between the spiral groove 21A and the screw groove 19D from the end point of the ball rolling region to the start point.

The ball circulation path 19 </ b> C is an S-shaped concave groove when viewed from the inside of the nut cylinder 19, and is formed integrally with the nut cylinder 19. “The ball circulation path 19C is formed integrally with the nut cylinder 19” means that “the ball circulation path 19C is formed by a concave groove formed by recessing a part of the inner peripheral surface of the nut cylinder 19. Means.
The ball moves around the piston 21 while moving in the ball rolling area to reach the end point of the ball rolling area, where it is scooped up from one end of the ball circulation path 19C and passes through the ball circulation path 19C. The ball circulation path 19C is returned to the starting point of the ball rolling area from the other end.

By integrating the ball circulation path 19C with the nut cylinder 19, there is no top hole penetrating the inner and outer peripheral surfaces of the nut cylinder 19 such as a top nut. Therefore, even if the rotor 14 is press-fitted into the nut cylinder 19, the roundness of the nut cylinder 19 is not easily lost, and a configuration that does not require special attention to the change in roundness is possible. Further, since the roundness of the nut cylinder 19 is high, swinging due to rotation of the nut cylinder 19 and the rotor 14 is suppressed, and the durability and acoustic properties of the nut cylinder 19 are not easily impaired.
Furthermore, since the ball circulation path 19C is formed integrally with the nut cylinder 19, the nut cylinder 19 has high rigidity even when it is thin. Therefore, compared with the conventional nut, since it can be made compact and lightweight, a compact ball screw actuator can be obtained easily.

Further, since no piece hole (through hole) is formed in the nut cylinder 19, deformation due to heat treatment described later hardly occurs.
Here, the thread groove 19D formed in the nut cylinder 19 is not provided outside the ball rolling region. That is, the thread groove 19D is formed only in a necessary portion, and the thread groove 19D is not formed in an unnecessary portion (portion where the ball does not roll) as in the case where the thread groove 19D is formed on the entire circumference. Therefore, the nut has sufficient strength.

In the brake device of the type in which the ball screw is built in the caliper 6 as shown in FIG. 1, a ball screw actuator that can be stored in a small space is required. By using, space saving can be realized.
In the ball screw actuator 11, the electric motor is provided so as to cover all the portions of the outer peripheral surface of the nut cylinder 19 facing the screw groove 19 </ b> D and the ball circulation path 19 </ b> C formed on the inner peripheral surface of the nut cylinder 19. 12 rotors 14 may be press-fitted to the outer periphery of the nut cylinder 19.

  The press fit of the rotor 14 may cause a dimensional change (deformation) in the thread groove 19D and the ball circulation path 19C. However, with the above configuration, a dimensional change in all the thread grooves 19D and the ball circulation path 19C. Can be made almost the same. Therefore, the contact state between the screw groove 19D and the ball can be made substantially the same in all the screw grooves 19D, and the contact state between the ball circulation path 19C and the ball can be made almost the same in all the ball circulation paths 19C. Can do. As a result, the operability of the ball screw actuator 11 is deteriorated, the life is shortened, and the sound and vibration are suppressed.

(Production method)
Next, a method for forming the nut cylinder 19 will be described.
First, a plastic material, a removal process, etc. are applied to the cylindrical material to form a nut material having an outer peripheral shape, and then a ball circulation path 19C is formed on the nut material by cold forging by the method shown in FIGS. Form. Hereinafter, this forging method will be described.

  As shown in FIG. 2, the mold used in this method includes a material holder 102 having a recess 121 for holding the nut material 101, and a cylindrical member inserted into the nut material 101 (a cam slider holding member described later). 105, a cam driver 106 inserted into the center hole 151 of the cylindrical member 105, and cam sliders 107 and 108 disposed in the through holes 152 and 153 of the cylindrical member 105.

  As shown in FIG. 3, the cam driver 106 includes a main body portion 161 formed in a long plate shape and a columnar tip portion 162. One side surface of the main body 161 is a first parallel surface 161a parallel to the axial direction, an inclined surface 161b inclined with respect to the axial direction, and a second parallel surface 161c from the distal end 162 side. The other side surface of the main body portion 161 is a slope 161d inclined with respect to the axial direction and a parallel surface 161e parallel to the axial direction from the distal end portion 162 side. The boundary between the inclined surface 161d on the other side surface and the parallel surface 161e is slightly closer to the tip 162 than the boundary between the parallel surface 161a on one side surface and the inclined surface 161b. The diameter of the column that forms the distal end portion 162 is the same as the width of the lower end (the distal end portion 62 side) of the main body portion 161.

The cylindrical member 105 has an outer peripheral surface that is slightly smaller in diameter than the inner peripheral surface of the nut blank 101. As shown in FIGS. 4A and 4B, the center hole 151 of the tubular member 105 has a rectangular cross-sectional shape whose shape changes in the axial direction. Parallel surfaces 151a to 151e parallel to the axial direction are formed as surfaces forming the center hole 151.
The parallel surfaces 151a to 151c are surfaces on which one side surface of the cam driver 106 is disposed, and a step portion 151f is formed between the parallel surface 151a and the parallel surface 151b. The through hole 152 penetrates in the radial direction of the tubular member 105 with the step portion 151f as a lower end. The parallel surfaces 151d and 151e are surfaces on which the other side surface of the cam driver 106 is disposed, and a through hole 153 that penetrates the cylindrical member 105 in the radial direction is formed at a position facing the parallel surface 151c of the parallel surface 151d. ing. A step portion 151 g that forms a boundary between the parallel surface 151 d and the parallel surface 151 e is the lower end of the through hole 153.

  The cam sliders 107 and 108 are substantially trapezoidal columnar members and have slopes 171 and 181 parallel to the slopes 161b and 161d of the cam driver 106, and the opposite sides of the slopes 171 and 181 are as shown in FIG. The circumferential surfaces 172 and 182 correspond to the circle 111 a forming the inner circumferential surface 111 of the nut material 101. S-shaped convex portions 173 and 183 corresponding to the ball circulation path 19 </ b> C are formed on the circumferential surfaces 172 and 182.

The axial dimension of the main body 161 of the cam driver 106 is longer than the axial dimension of the tubular member 105. The slope 171 of the cam slider 107 and the slope 161b of the cam driver 106, and the slope 181 of the cam slider 108 and the slope 161d of the cam driver 106 constitute a mold cam mechanism.
Parallel surface 161c of cam driver 106 and parallel surface 151a of tubular member 105, parallel surface 161a of cam driver 106 and parallel surface 151c of tubular member 105, parallel surface 161e of cam driver 106 and parallel surface 151d of tubular member 105 However, the load receiving surfaces are in contact with each other.

In the center of the bottom plate portion of the material holder 102, a circular hole 122b into which the front end portion 162 of the cam driver 106 enters and a rectangular hole 122c into which the front end portion 162 side of the main body portion 161 of the cam driver 106 enters are formed as continuous through holes. Yes.
Using this mold, the ball circulation path 19C is formed on the inner surface of the nut blank 101 by the following method.

  First, after the nut material 101 is disposed in the concave portion 121 of the material holder 102, the cam sliders 107 and 108 are held in the through holes 152 and 153 with the convex portions 173 and 183 facing outward inside the nut material 101. The member 105 is inserted. Next, the cam driver 106 is inserted into the center hole 151 of the cylindrical member 105 from the front end 162 side, and the front end 162 of the cam driver 106 is inserted into the through holes 122 b and 122 c of the material holder 102. FIG. 2A shows this state.

  Next, when a press pressure is applied to push the cam driver 106 from above, forces are transmitted from the inclined surface 161b of the cam driver 106 to the inclined surface 171 of the cam slider 107, and from the inclined surface 161d of the cam driver 106 to the inclined surface 181 of the cam slider 108, respectively. The Along with this, the downward force of the cam driver 106 is converted into a force for moving the cam sliders 107 and 108 outward in the radial direction, and the S-shaped convex portions 173 and 183 formed on the cam sliders 107 and 108 are converted into the nut material 101. The inner peripheral surface 111 is pressed and plastically deformed. FIG. 2B shows this state.

  As a result, S-shaped concave grooves corresponding to the shapes of the S-shaped convex portions 173 and 183 are formed on the inner peripheral surface 111 of the nut material 101, and two ball circulation paths 19C and 19C are formed. At that time, the front end 162 of the cam driver 106 is guided to the through hole 122 b of the bottom member 122 of the material holder 102. Further, the reaction force of the radial force transmitted to the cam slider 107 is received when the parallel surface 151d of the cylindrical member 105 and the parallel surface 161e of the cam driver 106 come into contact with each other (the reaction force on the extension line of the reaction force). Receive on a vertical surface). Further, the reaction force of the radial force transmitted to the cam slider 108 is obtained by contacting the parallel surface 151c of the cylindrical member 105 with the parallel surface 161a of the cam driver 106, and the parallel surface 151a of the cylindrical member 105 and the cam driver 106. Is received by contact with the parallel surface 161c (received by a surface perpendicular to the reaction force on the extension line of the reaction force). Therefore, the bending moment received by the cam driver 106 can be reduced.

  According to this method, even when a nut having a long axial dimension and a small inner diameter is manufactured, the two ball circulation paths 19C and 19C can be formed simultaneously without causing damage to the cam driver 106. For simplicity, the case where two ball circulation paths are formed has been described as an example. However, three or more ball circulation paths can be formed by using three or more cam sliders. Alternatively, only one cam slider may be used, and a plurality of ball circulation paths may be formed one by one using the mold as described above.

Further, if the inner diameter of the nut material 101 is the same, the cylindrical member 105 and the cam driver 106 can be used as a common part, and the cam sliders 107 and 108 having different shapes of the S-shaped convex portions 173 and 183 are prepared. However, it is also suitable as a method for producing a small variety of products. Further, since the S-shaped convex portion that is most likely to be worn and deformed can be replaced by replacing only the cam sliders 107 and 108, there is also an advantage in terms of maintenance.
Furthermore, not only the method as described above, but also the strength around the ball circulation path can be improved by work hardening by forming the ball circulation path by cold forging. Further, by reducing the roughness of the mold surface, it is possible to form a ball circulation path having a small inner surface roughness. Therefore, the circulation property of the ball can be improved without requiring finishing.

  After the ball circulation path is formed only in the ball rolling region by cold forging by the method as described above, the thread groove 19D is formed on the inner peripheral surface of the nut cylinder 19 by cutting. Thereafter, the nut cylinder 19 is heat treated. When this heat treatment is carburizing or carbonitriding, the material of the nut cylinder 19 is chromium steel or chromium molybdenum steel (for example, SCM420, SCM415) having a carbon content of 0.10% by mass to 0.25% by mass. ) Is preferable. Moreover, when heat processing is induction hardening, it is preferable that carbon content is 0.4 mass% or more and 0.6 mass% or less (for example, S53C, SAE4150). By using such a material, a sufficiently strong nut can be obtained.

In the present invention, since there is no through hole such as a coma hole and the outer periphery of the nut cylinder has a uniform cylindrical shape, the magnetic flux disturbance can be reduced even if a magnetic material is used for the nut cylinder. Further, it can be magnetized and demagnetized as necessary.
The ball screw actuator according to the present invention can be suitably used in addition to the brake device described above. In addition, modifications can be made as appropriate within the range that can be easily conceived by those skilled in the art.

DESCRIPTION OF SYMBOLS 1 Carrier 1B Reinforcement part 2 Disc 24 Friction pad 6 Caliper 7 Inner leg part 8 Claw part 9 Bridge part 10 Lid body 11 Electric actuator 12 Electric motor 13 Stator 14 Rotor 15 Magnet 16 External wiring 17 Ball screw mechanism 18 Bearing 19 Nut Tube 19A Screw hole 19B Buttocks 19C Ball circulation path 19D Screw groove 21 Piston 21A Spiral groove

Claims (7)

In a ball screw actuator comprising an electric motor and a ball screw,
The ball screw includes a screw shaft having a helical thread groove on an outer peripheral surface, a nut having a screw groove facing the screw groove of the screw shaft on an inner peripheral surface, and a spiral shape formed by the both screw grooves. A plurality of balls slidably loaded in the ball rolling path, and a ball circulation path for circulating the balls from the end point of the ball rolling path to the starting point, and the nut is driven to rotate by the electric motor. It is supposed to
The ball screw actuator is characterized in that the ball circulation path is formed integrally with the nut, and a rotor of the electric motor is press-fitted to the outer periphery of the nut.   The ball screw actuator according to claim 1, wherein a friction pad of a brake device is moved by an axial movement of the screw shaft.   The ball screw actuator according to claim 1, wherein the ball circulation path is formed by plastic working. A cam driver that is inserted in a cylindrical nut material and moves along the axial direction;
A cam slider disposed between the nut material and the cam driver, wherein a convex portion corresponding to the ball circulation path is formed, and the convex portion moves in a radial direction of the nut by the movement of the cam driver;
The ball screw actuator according to any one of claims 1 to 3, wherein the ball circulation path is formed by a press method using a mold of a cam mechanism having the following.   The nut according to any one of claims 1 to 4, wherein the nut is formed of chromium steel, chromium molybdenum steel, or carbon steel having a carbon content of 0.4 mass% or more and 0.6 mass% or less. A ball screw actuator according to claim 1.   The ball screw actuator according to any one of claims 1 to 5, wherein a thread groove of the nut is formed only in a portion connecting from one end to the other end of the ball circulation path.   The rotor of the electric motor is press-fitted into the outer periphery of the nut so as to cover all the part of the outer peripheral surface of the nut facing the thread groove and the ball circulation path formed on the inner peripheral surface of the nut. The ball screw actuator according to any one of claims 1 to 6, wherein the ball screw actuator is fitted.

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