Linear actuator
The linear actuator addresses the challenge of generating large thrust by using a reduction gear with adjustable rollers and a wave-shaped inner surface, enabling compact and cost-effective operation without multi-staging.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- NTN CORP
- Filing Date
- 2025-12-11
- Publication Date
- 2026-07-16
AI Technical Summary
Existing linear actuators face challenges in generating large thrust without increasing size or cost, particularly due to limitations in increasing the reduction ratio of planetary reduction gears, which often require multi-staging, leading to increased axial dimension and component count.
A linear actuator design incorporating a reduction gear with an input member, rollers on an eccentric outer surface, and a wave-shaped inner surface on the reduction gear outer ring, allowing for adjustable reduction ratios without multi-staging, using a screw mechanism with a ball or sliding screw configuration.
The design enables a compact and cost-effective actuator capable of generating high thrust without increasing size or part count, achieving reduction ratios of 5 to 60 through adjustable roller placement.
Smart Images
Figure JP2025043196_16072026_PF_FP_ABST
Abstract
Description
Linear actuator
[0004]
[0001] The present invention relates to a linear actuator.
[0002] In recent years, the electrification of automobiles has been progressing for the purposes of labor saving, fuel consumption reduction, function improvement, etc. For example, systems that utilize the power of an electric motor for operations such as an automatic transmission, brakes, and steering of an automobile have been developed and marketed. In such systems, a linear (electric) actuator equipped with an electric motor and a motion conversion mechanism that converts the rotational output thereof into linear motion is widely used.
[0003] For example, Patent Document 1 below describes a linear actuator incorporated in an electric cylinder for generating hydraulic pressure that is converted into braking force in an automobile braking system. In this actuator, an electric motor, a planetary reduction gear that decelerates the rotational output of the electric motor, and a screw mechanism as a motion conversion mechanism are arranged coaxially in an axial direction, and a nut that is a linear member of the screw mechanism constitutes a part of a piston that linearly moves in the axial direction within the cylinder.
[0004] Japanese Patent Application Laid-Open No. 2023-7796
[0005] In the linear actuator of Patent Document 1 that employs a planetary reduction gear (planetary gear reduction gear) as a reduction gear, if a large thrust is to be generated in the linear member of the screw mechanism without increasing the size or output of the electric motor, it is necessary to increase the reduction ratio of the planetary reduction gear. However, it is difficult to sufficiently increase the reduction ratio with a single-stage planetary reduction gear, so it is common to increase the reduction ratio of the planetary reduction gear by multi-staging. However, when the planetary reduction gear is multi-staged, there are concerns such as an increase in the axial dimension (size) of the actuator and an increase in cost due to an increase in the number of parts.
[0006] In view of such circumstances, an object of the present invention is to provide a linear actuator that is compact and inexpensive and can generate a large thrust in the linear member of a motion conversion mechanism that functions as a final output member.
[0007] The present invention, devised to achieve the above objective, comprises a coaxially arranged electric motor and motion conversion mechanism, and a reduction gear positioned between the axial directions of the electric motor and the motion conversion mechanism, which reduces the rotational output of the electric motor and outputs it to the motion conversion mechanism, wherein the motion conversion mechanism is a linear actuator composed of a screw mechanism in which, as the output of the reduction gear is input to a rotating member composed of either a screw shaft or a nut fitted around its outer circumference, a linear member composed of the other of the screw shaft and nut moves linearly in the axial direction, the reduction gear comprises an input member rotatably connected integrally with the output shaft of the electric motor, an output member rotatably connected integrally with the rotating member of the motion conversion mechanism, a rolling bearing mounted on the eccentric outer surface of the input member, a plurality of rollers arranged to roll on an inner raceway eccentric with respect to the rotation center of the input member, formed on the outer surface of the outer ring of the rolling bearing by the eccentric outer surface, and a reduction gear outer ring having a wave-shaped inner surface in which the rotation of the input member and output member is restricted and the number of recesses into which the rollers can be fitted is greater than the number of rollers arranged, The output member is characterized by having multiple columnar sections arranged at intervals in the circumferential direction, with a roller held between two adjacent columnar sections in the circumferential direction.
[0008] With a reduction gear (roller reduction gear) having the above configuration, the reduction ratio can be set in the range of approximately 5 to 60 by adjusting the number of recesses on the wave-shaped inner surface formed on the outer ring of the reduction gear according to the number of rollers. Therefore, when setting a high reduction ratio of the reduction gear to increase the thrust of the linear motion member of the motion conversion mechanism, which is the final output member of the actuator, it is not necessary to increase the number of stages or the number of parts as in a planetary gear reduction gear. This makes it possible to provide a linear actuator that is compact and inexpensive, yet capable of generating a large thrust for the linear motion member of the motion conversion mechanism.
[0009] The screw mechanism, which serves as a motion conversion mechanism for the linear actuator of the present invention, may be a so-called ball screw mechanism further comprising a plurality of balls that are rotatably interposed between a male screw groove formed on the outer circumferential surface of a screw shaft and a female screw groove formed on the inner circumferential surface of a nut, or it may be a so-called sliding screw mechanism in which a male screw groove formed on the outer circumferential surface of a screw shaft and a female screw groove formed on the inner circumferential surface of a nut are screwed together.
[0010] Based on the above, the present invention provides a linear actuator that is compact and inexpensive, yet capable of generating a large thrust in the linear member of a motion conversion mechanism (screw mechanism).
[0011] This is a longitudinal cross-sectional view of a linear actuator according to an embodiment of the present invention. This is a partially enlarged view of Figure 1. This is a cross-sectional view taken along the line A-A in Figure 2. This is a partially enlarged cross-sectional view of a linear actuator according to another embodiment of the present invention.
[0012] Embodiments of the present invention will be described below with reference to the drawings. In the following description, "axial direction," "radial direction," and "circumferential direction" refer to the direction along the central axis X of the electric motor 2 shown in Figure 1, the radial direction of the circle centered on the central axis X, and the circumferential direction of the circle centered on the central axis X, respectively.
[0013] Figure 1 is a longitudinal cross-sectional view of a linear actuator 1 according to an embodiment of the present invention, Figure 2 is a partially enlarged view of Figure 1, and Figure 3 is a cross-sectional view taken along the line A-A in Figure 2. This linear actuator 1 is used, for example, as an electric cylinder to generate hydraulic pressure that is converted into braking force in an automobile brake system, or to generate hydraulic pressure that is converted into vibration damping force in an automobile suspension system, and comprises an electric motor 2, a reduction gear 3, a screw mechanism 4 as a motion conversion mechanism, and a casing 5 housing these. The electric motor 2 and the screw mechanism 4 are arranged coaxially, and the reduction gear 3 is positioned between the axial directions of the electric motor 2 and the screw mechanism 4.
[0014] The electric motor 2 comprises a motor body 21 including a stator core and stator coils, and a motor output shaft 22, the stator coils being electrically connected to a power supply (not shown). The electric motor 2 is provided with a detection unit for detecting the rotation angle of the motor output shaft 22. The detection unit comprises a magnet 23 attached to the motor output shaft 22 and a rotation sensor 24 positioned opposite the magnet 23 with an axial gap between them. Therefore, the electric motor 2 uses a motor in which the rotation of the rotor (motor output shaft 22) is controlled by the value detected by the rotation sensor 24, such as a three-phase brushless motor.
[0015] The casing 5 comprises a first case 51, a second case 52, and a third case 53 that are coupled in the axial direction. Here, the casing 5 is formed by attaching a bottomed cylindrical first case 51, which houses the electric motor 1, to one axial side (the side where the electric motor 2 is located, the right side of Figure 1; the same applies hereinafter) of a cylindrical second case 52 that houses the reduction gear 3, and attaching a bottomed cylindrical third case 53, which houses the screw mechanism 4, to the other axial side (the side where the screw mechanism 4 is located, the left side of Figure 1; the same applies hereinafter) of the second case 52.
[0016] The screw mechanism 4 converts the rotational output of the motor output shaft 22, which is input via the reduction gear 3, into linear motion. It comprises a screw shaft 41 with a male screw groove 41a formed on its outer circumference, a cylindrical nut 42 fitted to the outer circumference of the screw shaft 41 and with a female screw groove 42a formed on its inner circumference, a plurality of balls 43 arranged to roll freely in a spiral ball rolling path formed between the opposing male screw groove 41a and female screw groove 42a, and a spindle 44 as a circulating member for circulating the balls 43 within the nut 42. Therefore, the screw mechanism 4 of this embodiment is a so-called spindle-type ball screw (ball screw mechanism).
[0017] In this embodiment, the screw mechanism 4 consists of a screw shaft 41 which is a rotating member that rotates in response to the rotational output of a motor output shaft 22 input via a reduction gear 3, and a nut 42 which is a linear motion member that moves linearly in the axial direction as the screw shaft 41 rotates. Therefore, the screw mechanism 4 is provided with an anti-rotation structure to restrict the rotation of the nut 42, which is a linear motion member, around the central axis of the screw shaft 41. The anti-rotation structure in the illustrated example is formed by fitting an anti-rotation member 46, which is attached to the nut 42 so as to protrude radially outward from the nut 42, into an axial groove 53a formed on the inner circumferential surface of the third case 53. A bottomed cylindrical piston 45 is attached to the other axial end of the nut 42 to close the other end opening of the nut 42.
[0018] The reduction gear 3 reduces the rotational output of the electric motor 2 and outputs it to the screw mechanism 4. It comprises an input member 31, first rolling bearings 32 to third rolling bearings 34, a plurality of rollers 35, a reduction gear outer ring 36, and an output member 37. The input member 31 and the output member 37 are arranged coaxially with the motor output shaft 22, and the first rolling bearings 32 to third rolling bearings 34 are all ball bearings with an inner ring and an outer ring that rotate relative to each other via balls acting as rolling elements.
[0019] The input member 31 is a cylindrical (hollow) member having a central hole that extends along the central axis X of the electric motor 2, and is connected to the motor output shaft 22 so as to be able to rotate integrally with it by press-fitting the motor output shaft 22 into the central hole.
[0020] The outer circumferential surface of the input member 31 has a cylindrical first bearing mounting surface 31a and a second bearing mounting surface 31b, which are spaced apart in the axial direction. The center of the first bearing mounting surface 31a is on the central axis X of the electric motor 2, and the inner ring of the first rolling bearing 32 is mounted on this first bearing mounting surface 31a. The outer ring of the first rolling bearing 32 is mounted on the inner circumference of the motor bracket 54 that holds the electric motor 2. The motor bracket 54 is supported by the second case 52 of the casing 5 using a plurality of anti-rotation pins 6 and mounting bolts 7. Therefore, the input member 31 of the reduction gear 3 is rotatably supported relative to the casing 5 via the first rolling bearing 32 and the like.
[0021] The center of the first bearing mounting surface 31a lies on the central axis X of the electric motor 2, while, as shown in Figures 2 and 3, the center O' of the second bearing mounting surface 31b is eccentric by a predetermined amount δ with respect to the central axis X of the electric motor 2. Therefore, the second bearing mounting surface 31b corresponds to the "eccentric outer surface" as defined in this invention. Because the inner ring 33a of the second rolling bearing 33 is mounted on the second bearing mounting surface 31b, which is an eccentric outer surface, an "inner raceway" is formed on the outer surface of the outer ring 33b of the second rolling bearing 33, which is eccentric by a predetermined amount δ with respect to the central axis X of the electric motor 2 (the rotation center of the input member 31).
[0022] Multiple rollers 35 are arranged circumferentially at intervals on the radially outer side of the second rolling bearing 33, which has an inner ring 33a mounted on the second bearing mounting surface 31b, which is an "eccentric outer surface." When the electric motor 2 is driven, each roller 35 rolls on an inner raceway formed on the outer surface of the outer ring 33b of the second rolling bearing 33.
[0023] The gearbox outer ring 36 is fixed to the casing 5 (the second case 52) in a state where it is prevented from rotating by a plurality of anti-rotation pins 6 and mounting bolts 7, together with the motor bracket 54 that holds the electric motor 2. The inner circumferential surface 36a of the gearbox outer ring 36 is formed in a corrugated shape with more recesses (arc-shaped recesses) into which rollers 35 can be fitted than the number of rollers 35. In the illustrated example, there are a total of 15 rollers 35, while there are a total of 29 recesses on the corrugated inner circumferential surface 36a. Not all of the plurality of rollers 35 interposed between the second rolling bearing 33 (outer ring 33b) and the gearbox outer ring 36 are fitted into the recesses of the corrugated inner circumferential surface 36a; some are fitted into the recesses, while others ride up on the protruding parts between the recesses. With this configuration, when the motor output shaft 22 rotates and its rotational output is input to the reduction gear 3, the speed at which the roller 35 rolls along the inner track (moves circumferentially along the inner track) is slower than the rotational speed of the motor output shaft 22 and the input member 31.
[0024] As shown in Figure 2, the output member 37 integrally comprises a hollow connecting portion 37a to which the screw shaft 41 of the screw mechanism 4 is connected in a torque-transmitting manner, a flange portion 37b extending radially outward from one axial end of the connecting portion 37a, and a plurality of column portions 37c extending axially from the outer diameter end of the flange portion 37b and spaced apart in the circumferential direction. The output member 37 is rotatably supported relative to the casing 5 (the second case 52) by a third rolling bearing 34 mounted on the outer circumference of the connecting portion 37a.
[0025] A spline (female spline) is formed on the inner circumferential surface of the connecting portion 37a of the output member 37, and this female spline is spline-fitted with a male spline formed on the outer circumference of one end of the screw shaft 41. In this embodiment, a bolt member 8 is screwed into a bolt hole provided on one end face of the screw shaft 41 via the output member 37. With this configuration, the output member 37 of the reduction gear 3 and the screw shaft 41, which is also a rotating member of the screw mechanism 4, rotate together.
[0026] The output member 37 houses and holds a roller 35 so that it can roll freely between two adjacent column portions 37c in the circumferential direction. Therefore, as the electric motor 2 is rotated, the roller 35 rolls on the inner track (the outer surface of the outer ring 33b of the second rolling bearing 33), and the output member 37 rotates around the central axis X of the electric motor 2 as its center of rotation. At this time, the rotational speed of the output member 37 (and the screw shaft 41 which is rotatably connected to it) is slower than the rotational speed of the motor output shaft 22 because the speed at which the roller 35 rolls on the inner track is slower than the rotational speed of the motor output shaft 22. Consequently, the rotation of the motor output shaft 22 is transmitted to the screw shaft 41 after being decelerated.
[0027] In the reduction gear 3 provided in the linear actuator 1 of this embodiment described above, the reduction ratio can be set in the range of 5 to 60 by adjusting the number of recesses on the wave-shaped inner surface formed on the outer ring according to the number of rollers 35 arranged. Therefore, in order to set a high reduction ratio of the reduction gear 3 in order to increase the thrust of the linear member (nut 42) of the screw mechanism 4, which is the final output member of the actuator 1, it is not necessary to increase the number of stages or the number of parts as in a planetary gear reducer. As a result, a linear actuator 1 can be realized that is compact and inexpensive, yet capable of generating a large thrust in the nut 42, which is the linear member of the screw mechanism 4.
[0028] Although a linear actuator 1 according to one embodiment of the present invention has been described above, the linear actuator 1 can be modified as appropriate without departing from the spirit of the present invention.
[0029] For example, in the linear actuator 1 described above, the screw mechanism 4 as a motion conversion mechanism is made of a ball screw. However, the screw mechanism 4 can also be replaced with a so-called sliding screw, in which the numerous balls 43 interposed between the radially opposing male screw grooves 41a and female screw grooves 42a are omitted, and the male screw grooves 41a and female screw grooves 452a are directly screwed together.
[0030] Furthermore, in the linear actuator 1 described above, when connecting the output member 37 of the reduction gear 3 and the screw shaft 41 of the screw mechanism 4 so that they can rotate as a single unit, a bolt member 8 screwed into the screw shaft 41 via the output member 37 is used to restrict the axial separation of the output member 37 and the screw shaft 41. However, as shown in Figure 4, a retaining ring 9 placed in the annular space defined between an annular groove 37d formed on the inner circumferential surface of the connecting portion 37a of the output member 37 and an annular groove 41c formed on the outer circumferential surface of the screw shaft 41 opposite to it may also be used to restrict the axial separation of the output shaft 37 of the reduction gear 3 and the screw shaft 41 of the screw mechanism 4.
[0031] Furthermore, while the screw mechanism 4 of the linear actuator 1 described above uses the screw shaft 41 as the rotating member (rotating side) and the nut 42 as the linear member (linear side), the present invention can also be applied to a linear actuator 1 employing a screw mechanism 4 in which the nut 42 is the rotating member and the screw shaft 41 is the linear member.
[0032] The present invention is not limited in any way to the embodiments described above, and can be implemented in various other forms without departing from the spirit of the invention.
[0033] 1 Linear actuator 2 Electric motor 3 Reducer 4 Screw mechanism (motion conversion mechanism) 22 Motor output shaft 31 Input member 31b Second bearing mounting surface (eccentric outer surface) 33 Second rolling bearing 33b Outer ring 35 Roller 36 Reducer outer ring 37 Output member 37c Column 41 Screw shaft (rotating member) 42 Nut (linear member) 43 Ball
Claims
1. A linear actuator comprising a coaxially arranged electric motor and motion conversion mechanism, and a reduction gear positioned between the axial directions of the electric motor and the motion conversion mechanism, which reduces the rotational output of the electric motor and outputs it to the motion conversion mechanism, wherein the motion conversion mechanism is a screw mechanism in which a linear motion member, composed of the other of the screw shaft and the nut, moves linearly in the axial direction as the output of the reduction gear is input to a rotating member composed of either a screw shaft or a nut fitted around its outer circumference, wherein the reduction gear comprises an input member rotatably connected integrally with the output shaft of the electric motor, an output member rotatably connected integrally with the rotating member of the motion conversion mechanism, a rolling bearing mounted on the eccentric outer surface of the input member, and a plurality of rollers arranged to roll on an inner raceway eccentric with respect to the rotation center of the input member, formed on the outer surface of the outer ring of the rolling bearing by the eccentric outer surface, A linear actuator comprising: a reduction gear outer ring having a wave-shaped inner circumferential surface in which rotation of the input member and the output member is restricted and the number of recesses into which the rollers can be fitted is greater than the number of rollers arranged, wherein the output member has a plurality of columnar portions arranged at intervals in the circumferential direction, and the rollers are held between two adjacent columnar portions in the circumferential direction.
2. The linear actuator according to claim 1, further comprising a plurality of balls that are rotatably interposed between a male screw groove formed on the outer circumferential surface of the screw shaft and a female screw groove formed on the inner circumferential surface of the nut.
3. The linear actuator according to claim 1, wherein the screw mechanism comprises a male screw groove formed on the outer circumferential surface of the screw shaft and a female screw groove formed on the inner circumferential surface of the nut, which are screwed together.