Electric actuator

Provided is an electric actuator, comprising: a driving motor (2); a motion conversion mechanism (6) configured to convert a rotary motion of the driving motor (2) to a linear motion in an axial direction parallel with an output shaft (2a) of the driving motor (2); and a transmission gear mechanism (5) configured to transmit a driving force from the driving motor (2) to the motion conversion mechanism (6), wherein a double-row bearing (24) configured to support the motion conversion mechanism (6) is arranged on an opposite side of the driving motor (2) with respect to the transmission gear mechanism (5), and wherein a relationship of L<Dm/2+Db/2 is satisfied, where Dm is an outer diameter of the driving motor (2), Db is an outer diameter of the double-row bearing (24), and L is an axis-to-axis distance (L) between the driving motor (2) and the motion conversion mechanism (6).

TECHNICAL FIELD

The present invention relates to an electric actuator.

BACKGROUND ART

In recent years, electrification of automobiles and the like has been promoted for power saving and reduction in fuel consumption. For example, a system for operating an automatic transmission, a brake, a steering wheel, and the like of the automobile with use of power of an electric motor has been developed and brought to the market. As an electric actuator for use in such an application, there has been known an electric linear actuator employing a ball screw mechanism configured to convert a rotary motion of a motor into a motion in a linear direction.

For example, as illustrated inFIG. 8, in Patent Literature 1, there is disclosed an electric linear actuator mainly including an electric motor100, a ball screw200, and a gear speed reduction mechanism500. The gear speed reduction mechanism500is configured to transmit a rotary motion of the electric motor100to the ball screw200. The ball screw200includes a screw shaft201, a nut202, and a large number of balls203. The screw shaft201has a spiral screw groove formed in an outer peripheral surface. The nut202is externally fitted to the screw shaft201, and has a spiral screw groove formed in an inner peripheral surface. The balls203are received in both of the screw grooves. The gear speed reduction mechanism500includes a first gear501and a second gear502. The first gear501has a small diameter, and is fixed to a motor shaft100aof the electric motor100. The second gear502has a large diameter, is formed on an outer periphery of the nut202, and meshes with the first gear501. When a driving force of the electric motor100is transmitted to the nut202thorough intermediation of the gears501and502, the nut202rotates, and the screw shaft201performs a linear motion.

CITATION LIST

SUMMARY OF INVENTION

Technical Problem

Incidentally, in the electric linear actuator disclosed in Patent Literature 1, in order to support the nut202, two rolling bearings600and700are arranged on both sides of the nut202while sandwiching the second gear502. However, in the case in which the rolling bearings600and700are arranged at such positions, when other component parts are to be arranged around the ball screw200, such a restriction in terms of layout that interference with the respective rolling bearing600and700needs to be prevented on the both sides of the second gear502is imposed, which may pose a problem in downsizing.

The present invention has been made in view of the above-mentioned problem, and therefore has an object to provide an electric actuator capable of achieving downsizing.

Solution to Problem

As a technical measure to attain the above-mentioned object, according to one embodiment of the present invention, there is provided an electric actuator, comprising: a driving motor; a motion conversion mechanism configured to convert a rotary motion of the driving motor to a linear motion in an axial direction parallel with an output shaft of the driving motor; and a transmission gear mechanism configured to transmit a driving force from the driving motor to the motion conversion mechanism, wherein a double-row bearing configured to support the motion conversion mechanism is arranged on an opposite side of the driving motor with respect to the transmission gear mechanism, and wherein a relationship of L1<Dm/2+Db/2 is satisfied, where Dm is an outer diameter of the driving motor, Db is an outer diameter of the double-row bearing, and L1 is an axis-to-axis distance between the driving motor and the motion conversion mechanism.

In such a manner, the motion conversion mechanism can be stably supported in a cantilever state through use of the double-row bearing as the support bearing configured to support the motion conversion mechanism. That is, there can be provided such a configuration that the double-row bearing is arranged only on the opposite side of the driving motor with respect to the transmission gear mechanism, and any bearing configured to support the motion conversion mechanism is not arranged on a driving motor side. As a result, it is not require that interference between the driving motor and a bearing be considered on the driving motor side, and the driving motor can be arranged close to the motion conversion mechanism in a radial direction orthogonal to the axial direction. Specifically, the driving motor and the motion conversion mechanism can be arranged so close to each other that the relationship of L1<Dm/2+Db/2 is satisfied, where Dm is the outer diameter of the driving motor, Db is the outer diameter of the double-row bearing, and L1 is the axis-to-axis distance between the driving motor and the motion conversion mechanism. As a result, the axis-to-axis distance between the driving motor and the motion conversion mechanism can be reduced, and downsizing of the electric actuator in the radial direction can thus be achieved.

Moreover, in such a configuration that the electric actuator comprises a speed reduction mechanism configured to reduce the speed of the rotary motion of the driving motor and then output the rotary motion reduced in speed to the transmission gear mechanism, when the speed reduction mechanism is provided between the driving motor and the transmission gear mechanism, the speed reduction mechanism can be arranged at a position that does not interfere with the double-row bearing. As a result, the speed reduction mechanism can be arranged close to the motion conversion mechanism in the radial direction orthogonal to the axial direction, and downsizing of the electric actuator in the radial direction can thus be achieved. Specifically, there can be provided a configuration in which a relationship of L2<Dr/2+Db/2 is satisfied, where Dr is an outer diameter of the speed reduction mechanism, Db is the outer diameter of the double-row bearing, and L2 is an axis-to-axis distance between the speed reduction mechanism and the motion conversion mechanism. Moreover, in the case of the configuration comprising the speed reduction mechanism, a small-sized motor can be employed, and further downsizing of the electric actuator can thus be achieved.

Further, the downsizing of the electric actuator is further promoted through employment of a planetary-gear speed reduction mechanism as the speed reduction mechanism.

Moreover, the motion conversion mechanism can stably and reliably be supported in the cantilever state against loads in various directions through employment of a double-row angular contact ball bearing as the double-row bearing. A reduction in operation efficiency and increases in noise and vibration due to an occurrence of a run out in a shaft of the motion conversion mechanism can thus be prevented.

Advantageous Effects of Invention

According to the present invention, the axis-to-axis distance between the driving motor and the motion conversion mechanism can be reduced, and downsizing of the electric actuator in the radial direction can thus be achieved.

DESCRIPTION OF EMBODIMENTS

Now, description is made of the present invention with reference to the accompanying drawings. In the respective drawings for illustrating the present invention, components such as members and component parts having the same functions or shapes are denoted by the same reference symbols as long as the components can be distinguished, and description thereof is therefore omitted after the description is made once.

FIG. 1is a vertical sectional view of an electric actuator according to one embodiment of the present invention.FIG. 2is an exploded perspective view of the electric actuator.

As illustrated inFIG. 1andFIG. 2, the electric actuator1according to this embodiment mainly comprises a motor section4and a drive transmission/conversion section7. The motor section4comprises a driving motor2and a speed reduction mechanism3. The drive transmission/conversion section7comprises a transmission gear mechanism5and a motion conversion mechanism6. As described later, it is not always required that the motor section4comprise the speed reduction mechanism3.

The sections forming the electric actuator1comprise respective exterior cases. Components are accommodated or supported in the respective exterior cases. Specifically, the motor section4comprises a motor case8configured to accommodate the driving motor2and the speed reduction mechanism3. The drive transmission/conversion section7comprises an actuator case9configured to support the transmission gear mechanism5and the motion conversion mechanism6. Moreover, the motor case8comprises a motor-case main body69and a cap member32. The motor-case main body69is configured to accommodate the driving motor2. The cap member32is formed independently of the motor-case main body69. The motor-case main body69is mounted to the actuator case9so as to be coupled and decoupled in an axial direction of the driving motor2. The driving motor2and the speed reduction mechanism3are also mounted to the actuator case9so as to be coupled and decoupled in the axial direction. Further, a shaft case10configured to accommodate apart of the motion conversion mechanism6is mounted to the actuator case9on an opposite side of a motor case8side so as to be coupled and decoupled in the axial direction. These exterior cases are assembled to one another through fastening with bolts. Now, description is made of detailed configurations of the respective parts forming the electric actuator1.

FIG. 3is a view of the speed reduction mechanism as seen in the axial direction.

The speed reduction mechanism3comprises a planetary-gear speed reduction mechanism11formed of a plurality of gears and the like. As illustrated inFIG. 3, the planetary-gear speed reduction mechanism11is formed of a ring gear12, a sun gear13, a plurality of planetary gears14, and a planetary gear carrier15.

The sun gear13is arranged at the center of the ring gear12. An output shaft2aof the driving motor2is press-fitted to the sun gear13. Moreover, the respective planetary gears14are arranged between the ring gear12and the sun gear13so as to mesh with the ring gear12and the sun gear13. The respective planetary gears14are rotatably held by the planetary gear carrier15.

In the planetary-gear speed reduction mechanism11, when the driving motor2performs the rotational drive, the sun gear13coupled to the output shaft2aof the driving motor2rotates, and, along with this rotation, the respective planetary gears14revolve along the ring gear12while rotating. Then, the planetary gear carrier15is rotated by the revolution motion of the planetary gears14. With this, the speed of the rotation of the driving motor2is reduced, the rotation reduced in speed is transmitted, and a rotation torque increases. Through the transmission of the driving force via the planetary-gear speed reduction mechanism11in such a manner, a high output of the electric actuator1is thus obtained, and downsizing of the driving motor2can thus be achieved. In this embodiment, although an inexpensive (brush) DC motor is used as the driving motor2, other motor such as a brushless motor may be used.

Next, as illustrated inFIG. 1andFIG. 2, the transmission gear mechanism5is formed of a drive gear16and a driven gear17. The drive gear16has a small diameter, and serves as a first gear with a rotation shaft arranged coaxially with the output shaft2aof the driving motor2. The driven gear17has a large diameter, and serves as a second gear which meshes with the drive gear16. A gear boss18(seeFIG. 1) serving as a rotation shaft is press-fitted to a rotation center portion of the drive gear16. One end portion (right end portion inFIG. 1) of the gear boss18is rotatably supported by a rolling bearing19mounted to the actuator case9. The drive gear16and the gear boss18may be integrally formed through sintering. Meanwhile, an end portion (left end portion inFIG. 1) of the gear boss on an opposite side is supported through insertion of the output shaft2aof the driving motor2into a shaft hole18aopened on a side of this end portion. That is, the output shaft2aof the driving motor2is inserted into the gear boss18so as to constitute a relationship of a slide bearing rotatable relatively to the gear boss18.

The gear boss18is so coupled to the planetary gear carrier15as to integrally rotate. In detail, the planetary gear carrier15has a cylindrical portion15a(seeFIG. 1) at a center portion thereof, and the cylindrical portion15ais press-fitted over an outer peripheral surface of the gear boss18. The planetary gear carrier15may be made of resin, and the gear boss18may be molded integrally with the planetary gear carrier15by insert molding. As a result, when the driving motor2performs rotary drive, and the planetary gear carrier15rotates accordingly, the drive gear16rotates integrally with the planetary gear carrier15, and the driven gear17thus rotates. This embodiment is so configured that the rotation is reduced in speed (increased in torque) from the drive gear16having a small diameter to the driven gear17having a large diameter, but the rotation may be transmitted at a constant speed from the drive gear16to the driven gear17.

Now, description is made of the motion conversion mechanism.

The motion conversion mechanism6is formed of a ball screw20arranged on an axis parallel with the output shaft2aof the driving motor2. The motion conversion mechanism6is not limited to the ball screw20, and may be a lead screw device. However, in terms of reducing the rotation torque and downsizing the driving motor2, the ball screw20is more preferred.

The ball screw20comprises a ball screw nut21, a ball screw shaft22, a large number of balls23, and a circulation member (not shown). Spiral grooves are formed in each of an inner peripheral surface of the ball screw nut21and an outer peripheral surface of the ball screw shaft22. Two rows of the balls23are received between both of the spiral grooves.

The ball screw nut21is rotatably supported by the double-row bearing24mounted to the actuator case9. The double-row bearing24is fixed through press-fit on a rear end side (right side ofFIG. 1) of the ball screw shaft22with respect to a portion on the outer peripheral surface of the ball screw nut21to which the driven gear17is fixed. Meanwhile, a rotation of the ball screw shaft22is restricted through insertion of a pin25serving as a rotation restriction member provided on a rear end portion (right end portion inFIG. 1) of the ball screw shaft22into guide grooves10ain an axial direction formed in an inner peripheral surface of the shaft case10.

When the ball screw nut21rotates, the plurality of balls23accordingly circulate through the circulation member while moving along the spiral grooves, and the ball screw shaft22advances/retreats along the guide grooves10aof the shaft case10. The rotary motion from the driving motor2is converted to a linear motion in the axial direction parallel with the output shaft2aof the driving motor2through the advance/retreat of the ball screw shaft22in such a manner. A distal end portion (left end portion inFIG. 1) of the ball screw shaft22in the advance direction functions as an operation part (actuator head) configured to operate a device of an object to be operated.FIG. 1is a view for illustrating a state in which the ball screw shaft22is arranged at an initial position when the ball screw shaft22retreats most toward the right side inFIG. 1.

Moreover, the electric actuator1according to this embodiment comprises a lock mechanism26(seeFIG. 2) configured to prevent an unintended advance/retreat of the ball screw shaft22. The lock mechanism26is mounted to the shaft case10, and is configured to be capable of engaging with/disengaging from a plurality of engagement holes16a(seeFIG. 2) formed over the drive gear16in a circumferential direction. Even when an external force is input from a side of the object to be operated to a side of the ball screw shaft22, an unintended advance/retreat of the ball screw shaft22is prevented, and a position of the ball screw shaft22in an advance/retreat direction can be maintained at a predetermined position by the lock mechanism26engaging with one of the engagement holes16a, to thereby restrict the rotation of the drive gear16. The configuration comprising such a lock mechanism26is particularly preferred for a case in which the electric actuator is applied to an application that requires maintenance of a position.

A boot27configured to prevent entry of foreign substances into the ball screw nut21is mounted on a distal end portion side of the ball screw shaft22. The boot27is formed of a large-diameter end portion27a, a small-diameter end portion27b, and a bellows27c. The bellows27cis configured to connect the large-diameter end portion27aand the small-diameter end portion27bto each other, and extend/contract in the axial direction. The small-diameter end portion27bis fixed to an outer peripheral surface of the ball screw shaft22through tightening a boot band28. The large-diameter end portion27aof the boot27is fixed to an outer peripheral surface of a boot mounting member30having a cylindrical shape mounted to the motor-case main body69through tightening a boot band29.

Moreover, a boot cover31having a cylindrical shape configured to protect the boot27is provided on an outer side of the boot27. A cylindrical mounting part31a(seeFIG. 1) is provided on an inner side of the boot cover31. The boot mounting member30is mounted to the mounting part31a. Both the boot cover31and the mounting part31aare provided integrally with the motor-case main body69.

Moreover, the cap member32is mounted to the motor-case main body69on an opposite side of an actuator case9side. An insertion hole32a(seeFIG. 2) configured to insert a bus bar33configured to supply power from a power source (not shown) to the driving motor2is formed in the cap member32. Further, a sensor case34(seeFIG. 2) configured to accommodate a stroke sensor configured to detect a stroke of the ball screw shaft22is provided integrally on the outer peripheral surface of the motor-case main body69.

Next, with reference toFIG. 4, description is made of feedback control for the electric actuator using the stroke sensor.

As illustrated inFIG. 4, when a target value is transmitted to a control device80, a control signal is transmitted from a controller81of the control device80to the driving motor2. The target value is, for example, a stroke value calculated by an ECU provided at an upper position of a vehicle based on an operation amount when the operation amount is input to the ECU.

When the driving motor2receives the control signal, the driving motor2starts the rotational drive, and the driving force thereof is transmitted to the ball screw shaft22through intermediation of the planetary-gear speed reduction mechanism11, the drive gear16, the driven gear17, and the ball screw nut21, and the ball screw shaft22thus advances. With this, the object device to be operated arranged on the distal end portion side (actuator head side) of the ball screw shaft22is operated.

At this time, the stroke value (position in the axial direction) of the ball screw shaft22is detected by the stroke sensor70. The detection value detected by the stroke sensor70is transmitted to a comparison portion82of the control device80, and a difference between the detection value and the target value is calculated. Then, the driving motor2is driven until the detection value matches the target value. When the electric actuator1of this embodiment is applied to, for example, a shift-by-wire system, a shift position can reliably be controlled by feeding back the stroke value detected by the stroke sensor70to control the position of the ball screw shaft22in such a manner.

Next, with reference toFIG. 5, description is made of feedback control in a case in which a pressure sensor83is used in place of the stroke sensor70.

As illustrated inFIG. 5, in this case, the pressure sensor83is provided for the object device to be operated. When the operation amount is input to the ECU provided at the upper position of the vehicle, the ECU calculates a required target value (pressure command value). When the target value is transmitted to the control device80, and the control signal is transmitted from the controller81to the driving motor2, the driving motor2starts the rotational drive. With this, the ball screw shaft22advances, and the object device to be operated arranged on the distal end portion side (actuator head side) of the ball screw shaft22is operated to pressurize.

An operation pressure of the ball screw shaft22at this time is detected by the pressure sensor83, and the position of the ball screw shaft22is subjected to the feedback control based on the detection value and the target value as in the case of the use of the stroke sensor70. When the electric actuator1of this embodiment is applied to, for example, a brake-by-wire system, a hydraulic pressure of a brake can reliably be controlled by feeding back the pressure value detected by the pressure sensor83to control the position of the ball screw shaft22in such a manner.

The overall configuration and the operation of the electric actuator1according to this embodiment are as described above. Now, description is made of components suitable for downsizing relating to the electric actuator1of this embodiment.

As illustrated inFIG. 1, the double-row bearing24is used as a support bearing configured to support the ball screw20in the electric actuator1according to this embodiment. As a result, the ball screw20can be stably supported in a cantilever state, and there can be provided such a configuration that the double-row bearing24is arranged only on one side (right side) with respect to the driven gear17, and any bearing configured to support the ball screw20is not arranged on an opposite side (left side). Need for consideration of interference between a support bearing and other components on the side on which the support bearing is eliminated by arranging the support bearing configured to support the ball screw20only on the one side with respect to the driven gear17, and downsizing can thus be achieved.

Specifically, in this embodiment, interference between the driving motor2and the double-row bearing24is avoided by arranging the double-row bearing24on the opposite side of the driving motor2with respect to the driven gear17. As a result, the driving motor2can be arranged close to the ball screw20in a radial direction orthogonal to the axial direction by an amount saved by removing the double-row bearing24on a driving motor2side. That is, the driving motor2can be arranged so that the driving motor2and the double-raw bearing24overlap each other in the radial direction as seen in the axial direction. On this occasion, the driving motor2and the ball screw20can be arranged close to each other so that a relationship of L1<Dm/2+Db/2 is satisfied, where Dm is an outer diameter of the driving motor2, Db is an outer diameter of the double-row bearing24, and L1 is an axis-to-axis distance between the driving motor2and the ball screw20in the radial direction.

As described above, with the configuration of the electric actuator1in this embodiment, the axis-to-axis distance L1 between the driving motor2and the motion conversion mechanism6(ball screw20) can be reduced compared with a related-art configuration illustrated inFIG. 8by arranging the double-row bearing24configured to support the motion conversion mechanism6(ball screw20) on the opposite side of the driving motor2with respect to the transmission gear mechanism5(the drive gear16and the driven gear17), and downsizing of the electric actuator in the radial direction can thus be achieved.

Moreover, in the case of this embodiment, the planetary-gear speed reduction mechanism11is similarly arranged on the opposite side of the double-row bearing24with respect to the transmission gear mechanism5(drive gear16and the driven gear17), and an axis-to-axis distance L2 between the planetary-gear speed reduction mechanism11and the ball screw20can be reduced. That is, there can be provided a configuration in which a relationship of L2<Dr/2+Db/2 is satisfied, where Dr is an outer diameter of the planetary-gear speed reduction mechanism11(an outer diameter of the ring gear12in this embodiment), Db is the outer diameter of the double-row bearing24, and L2 is the axis-to-axis distance between the planetary-gear speed reduction mechanism11and the ball screw20as the above-mentioned distance. Moreover, in the case of the configuration comprising the speed reduction mechanism as the electric actuator1according to this embodiment, a small-sized motor can be employed, and hence further downsizing of the electric actuator can be achieved. In particular, the planetary-gear speed reduction mechanism11is preferred for the downsizing of the electric actuator.

Moreover, in this embodiment, a double-row angular contact ball bearing is used as the double-row bearing24to stably support the ball screw20. As illustrated inFIG. 1, any of balls40in two rows interposed between an outer ring38and an inner ring39are in contact with raceway surfaces of the outer ring38and raceway surfaces of the inner ring39at contact angles, and the double-row angular contact ball bearing can thus support a radial load as well as axial loads in both directions, thereby being capable of stably and reliably supporting the ball screw20performing the linear motion. The contact angle is an angle formed between a plane (radial plane) perpendicular to a center axis of the bearing and a line of action (long dashed short dashed line passing through the center of each of the balls40illustrated inFIG. 1) of a resultant force of forces transmitted from the raceway surface to the ball40. Further, in this embodiment, there is employed a so-called back-to-back configuration in which the lines of actions of the respective balls44are arranged so as to cross each other on the radially outer side, which is also advantageous with respect to a moment load. In such a manner, in this embodiment, the ball screw20can stably and reliably be supported in the cantilever state against the loads in the various directions through employment of the back-to-back double-row angular contact ball bearing as the double-row bearing24. Therefore, a reduction in operation efficiency and increases in noise and vibration due to an occurrence of a run out in the shaft of the ball screw20can be prevented.

The double-row bearing24configured to support the motion conversion mechanism6is not limited to the double-row angular contact ball bearing, and a pair of single-row angular contact ball bearings may be combined for use. In addition to the angular contact ball bearing, other double-row bearing using, for example, a deep groove ball bearing can be applied.

FIG. 6is a vertical sectional view of the electric actuator according to another embodiment of the present invention.FIG. 7is an exploded perspective view of the electric actuator according to the another embodiment.

The electric actuator illustrated inFIG. 6andFIG. 7does not comprise the planetary-gear speed reduction mechanism11and the lock mechanism26provided for the electric actuator illustrated inFIG. 1toFIG. 5. Therefore, the length of the motor case8(motor-case main body69) is reduced a little in the axial direction, and the shaft case10has a shape without a portion configured to accommodate the lock mechanism26. Moreover, in this case, the output shaft2aof the driving motor2is coupled through press-fit to the shaft hole18aof the gear boss18, the driving force of the driving motor2is directly transmitted to the drive gear16(without intermediation of the planetary-gear speed reduction mechanism11), and is transmitted from the drive gear16to the ball screw20through the driven gear17.

In such a manner, the electric actuator adapted to other applications and specifications can be formed by only omitting the planetary-gear speed reduction mechanism11and the lock mechanism26and replacing the motor case8(motor-case main body69) and the shaft case10with other cases, without changing many common parts. Thus, with the configuration of the electric actuator according to this embodiment, an electric actuator which is low in cost and excellent in versatility can be provided also for deployment as multiple types to an electric parking brake mechanism for vehicles including two-wheeled vehicles, an electric/hydraulic brake mechanism, an electric shift change mechanism, and an electric power steering as well as a 2WD/4WD electric switching mechanism and an electric shift change mechanism for an outboard engine (for a vessel propulsion engine), and the like.

The electric actuator according to the another embodiment is configured as in the embodiment illustrated inFIG. 1toFIG. 5in points other than those described above. Thus, the electric actuator according to the another embodiment is also so configured that the ball screw20is supported by the double-row bearing24in the cantilever state, and any bearing configured to support the ball screw20is not arranged on the driving motor2side as in the embodiment illustrated inFIG. 1toFIG. 5. Therefore, also in the another embodiment, the driving motor2can be arranged close to the ball screw20in the radial direction, and there can be provided the configuration in which the relationship of L1<Dm/2+Db/2 is satisfied, where Dm is the outer diameter of the driving motor, Db is the outer diameter of the double-row bearing, and L1 is the axis-to-axis distance between the driving motor and the motion conversion mechanism.

Description is made of the embodiments of the present invention. However, the present invention is not limited to the above-mentioned embodiments. As a matter of course, the present invention may be carried out in various modes without departing from the spirit of the present invention.

REFERENCE SIGNS LIST