Speed reduction device and brake actuator including ihe same

A speed reduction device including a first internally meshing planetary gear mechanism and a second internally meshing planetary gear mechanism, including: a housing; an input shaft rotatably supported by the housing at its first shaft portion through a first bearing; a planetary gear member rotatably supported by an eccentric shaft portion of the input shaft through a second bearing; and an output shaft rotatably supported by a second shaft portion of the input shaft through a third bearing, wherein a distance in an axial direction between a first support position and a second support position is equal to a distance in the axial direction between the second support position and a third support position, and a distance in the axial direction between the second support position and an input-side meshing position is equal to a distance in the axial direction between the second support position and an output-side meshing position.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2018-218541, which was filed on Nov. 21, 2018, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND

Technical Field

The following disclosure relates to a speed reduction device including an internally meshing planetary gear mechanism and also relates to a brake actuator including the speed reduction device and configured to stop rotation of a wheel.

Description of Related Art

In recent years, a brake actuator as disclosed in Japanese Patent Application Publication No. 2014-109315, for instance, has been actively developed. The brake actuator includes an electric motor as a drive source and is configured to give, to a wheel, a braking force whose magnitude corresponds to a force generated by the electric motor. For achieving the brake actuator that is compact in size, it is desirable to employ a speed reduction device whose reduction ratio is relatively high, such as the one as disclosed in Japanese Patent Application Publication No. 5-321990.

SUMMARY

Though the speed reduction device disclosed in Japanese Patent Application Publication No. 5-321990 is considerably compact in size, it includes bearings used exclusively for the device, undesirably pushing up the cost of the device. Thus, there is much room for improvement in the speed reduction device, and some modifications enable achievement of a speed reduction device having high utility. Accordingly, one aspect of the present disclosure is directed to a speed reduction device having high utility. Another aspect of the present disclosure is directed to a brake actuator having high utility owing to employment of the speed reduction device.

In a first aspect of the present disclosure, a speed reduction device including (i) a first internally meshing planetary gear mechanism constituted by a first internally toothed gear and a first externally toothed gear that is meshing internally with the first internally toothed gear and (ii) a second internally meshing planetary gear mechanism constituted by a second internally toothed gear and a second externally toothed gear that is meshing internally with the second internally toothed gear, comprising:

a housing;

an input shaft including an eccentric shaft portion located intermediate in an axial direction and a first shaft portion and a second shaft portion respectively located on opposite sides of the eccentric shaft portion in the axial direction, the input shaft being rotatably supported by the housing at the first shaft portion through a first bearing;

a planetary gear member which is rotatably supported by the eccentric shaft portion of the input shaft through a second bearing and on which one of the first internally toothed gear and the first externally toothed gear and one of the second internally toothed gear and the second externally toothed gear are provided so as to be arranged in the axial direction; and

an output shaft which is rotatably supported by the second shaft portion of the input shaft through a third bearing and on which the other of the second internally toothed gear and the second externally toothed gear is provided,

wherein the other of the first internally toothed gear and the first externally toothed gear is provided on the housing,

wherein, where a position at which the input shaft is supported through the first bearing, a position at which the planetary gear member is supported through the second bearing, and a position at which the output shaft is supported through the third bearing are respectively defined as a first support position, a second support position, and a third support position, a distance (hereinafter referred to as “input-side supporting distance” where appropriate) in the axial direction between the first support position and the second support position is equal to a distance (hereinafter referred to as “output-side supporting distance” where appropriate) in the axial direction between the second support position and the third support position and; and

wherein a distance (hereinafter referred to as “input-side meshing distance” where appropriate) in the axial direction between the second support position and a position at which the first internally toothed gear and the first externally toothed gear are in mesh with each other (hereinafter referred to as “input-side meshing position” where appropriate) is equal to a distance (hereinafter referred to as “output-side meshing distance” where appropriate) in the axial direction between the second support position and a position at which the second internally toothed gear and the second externally toothed gear are in mesh with each other (hereinafter referred to as “output-side meshing position” where appropriate).

In a second aspect of the present disclosure, a brake actuator including the speed reduction according to the first aspect, including:

an electric motor configured to rotate the input shaft;

a piston configured to push a friction member onto a rotary body that rotates with a wheel: and

a motion converting mechanism configured to convert a rotating motion of the output shaft into an advancing and retracting movement of the piston.

The speed reduction device according to the present disclosure may be referred to as a differential speed reduction device that employs internally meshing planetary gear mechanisms. The present speed reduction device is relatively compact in size and has a high reduction ratio. The high reduction ratio means that a ratio of the rotation speed of the output shaft with respect to the rotation speed of the input shaft is small. As will be later explained in detail, the speed reduction device of the present disclosure has the following features: (a) The input-side meshing distance and the output-side meshing distance are the same; (b) The output shaft is supported by the input shaft; and (c) The input-side supporting distance and the output-side supporting distance are the same. These features enable support loads with respect to the input shaft to be well balanced, resulting in efficient deceleration. Further, the present speed reduction device has fewer restrictions on bearings, and general-purpose bearings are available in the present speed reduction device, thus obviating a cost increase. Accordingly, owing to these advantages, the speed reduction device of the present disclosure is excellent in utility.

The brake actuator according to the present disclosure including the speed reduction device constructed as described above is excellent in utility.

In the input shaft of the present speed reduction device, the axis of the first shaft portion and the axis of the second shaft portion coincide with the axis of the input shaft, and the axis of the eccentric shaft portion shifts from the axis of the input shaft. In this case, the axis of the first shaft portion can be considered to correspond to the center axis of its circumferential surface at which the first shaft portion is supported by the housing, the axis of the second shaft portion can be considered to correspond to the center axis of its circumferential surface that supports the output shaft, and the axis of the eccentric shaft portion can be considered to correspond to the center axis of its circumferential surface that supports the planetary gear member. In a case where the output shaft is supported by the outer circumferential surface of the second shaft portion, the axis of the second shaft portion is the center axis of the outer circumferential surface. On the other hand, in a case where the second shaft portion has a hollow shape and the output shaft is supported by the inner circumferential surface of the second shaft portion, the axis of the second shaft portion is the center axis of the inner circumferential surface. In this case, the axis of the second shaft portion is not defined by the outer circumferential surface thereof.

The layout of the first internally toothed gear, the first externally toothed gear, the second internally toothed gear, and the second externally toothed gear in the present speed reduction device is not limited to any particular one. For instance, the following four configurations are employable: (A) a configuration in which the first internally toothed gear is provided on the housing, the first externally toothed gear is provided on the planetary gear member, the second internally toothed gear is provided on the planetary gear member, and the second externally toothed gear is provided on the output shaft; (B) a configuration in which the first internally toothed gear is provided on the housing, the first externally toothed gear is provided on the planetary gear member, the second internally toothed gear is provided on the output shaft, and the second externally toothed gear is provided on the planetary gear member; (C) a configuration in which the first internally toothed gear is provided on the planetary gear member, the first externally toothed gear is provided on the housing, the second internally toothed gear is provided on the planetary gear member, and the second externally toothed gear is provided on the output shaft; and (D) a configuration in which the first internally toothed gear is provided on the planetary gear member, the first externally toothed gear is provided on the housing, the second internally toothed gear is provided on the output shaft, and the second externally toothed gear is provided on the planetary gear member.

More specifically, the speed reduction device according to the present disclosure may be configured as follows. That is, the first internally toothed gear is fixedly supported by the housing, the first externally toothed gear and the second internally toothed gear are formed at a radially outer portion of the planetary gear member, and the second externally toothed gear is formed on a flange of the output shaft. In the thus constructed speed reduction device, the input-side meshing position and the output-side meshing position are different from each other in phase by 180° in the circumferential direction, resulting in good balance in the loads that act on the planetary gear member.

The speed reduction device according to the present disclosure may be configured such that each of the first internally toothed gear and the second internally toothed gear has a circular arc tooth profile and each of the first externally toothed gear and the second externally toothed gear has an epitrochoid parallel curve tooth profile. According to this configuration, the present speed reduction device functions as what is called cycloid speed reducer, so that smooth rotational deceleration is achieved.

The speed reduction device according to the present disclosure may be configured such that the first shaft portion of the input shaft is supported by the housing also through a fourth bearing at a fourth support position located opposite to the second support position with respect to the first support position in the axial direction. This configuration enables the input shaft to be securely supported by the housing in a state in which the support load in the radial direction does not substantially act on the fourth bearing.

The brake actuator according to the present disclosure may be configured such that the input shaft is a cylindrical member that functions as a rotor of the electric motor and the output shaft and the motion converting mechanism are disposed in the input shaft. This configuration achieves the brake actuator that is more compact in size.

DETAILED DESCRIPTION OF THE EMBODIMENT

Referring to the drawings, there will be explained below in detail a speed reduction device according to one embodiment of the present disclosure and a brake actuator according to one embodiment of the present disclosure including the speed reduction device. It is to be understood that the present disclosure is not limited to the details of the following embodiment but may be embodied with other changes and modifications based on the knowledge of those skilled in the art.

A. Electric Brake Device Including Brake Actuator

As shown inFIG. 1, a brake actuator10(hereinafter simply referred to as “actuator10” where appropriate) according to the present embodiment is employed in an electric brake device as a major constituent element. The electric brake device includes: a brake caliper12(hereinafter simply referred to as “caliper12” where appropriate) that holds the actuator10; a disc rotor14, as a rotary body, configured to rotate with a wheel; a pair of brake pads16a,16b(hereinafter simply referred to as “pads16a,16b”, “pad16a” or “pad16b” where appropriate); and an electronic control unit (ECU)18, as a controller, which will be explained in detail.

The caliper12is held by a mount (not shown) provided on a carrier (not shown) that rotatably holds the wheel, such that the caliper12is movable in the axial direction, i.e., in the right-left direction inFIG. 1, and such that the caliper12straddles the disc rotor14. The pads16a,16bare held by the mount so as to sandwich the disc rotor14therebetween in a state in which the pads16a,16bare movable in the axial direction. Each of the pads16a,16bincludes: a friction member26disposed on one side thereof on which the pad16a,16bcomes into contact with the disc rotor14; and a backup plate28supporting the friction member26. The friction member26of each pad16a,16bis configured to be pushed onto the disc rotor14.

For the sake of convenience, the left side and the right side inFIG. 1are defined as a front side and a rear side, respectively. The pad16alocated on the front side is supported by a front end portion (claw portion)32of a caliper main body30. The actuator10is held by a rear-side portion of the caliper main body30such that a housing40of the actuator10is fixed to the rear-side portion of the caliper main body30. The actuator10includes a piston42configured to advance and retract relative to the housing40. When the piston42advances, a front end portion, specifically, a front end, of the piston42comes into engagement with the pad16blocated on the rear side, specifically, the backup plate28of the rear-side pad16b. When the piston42further advances while being kept engaged with the backup plate28of the rear-side pad16b, the pads16a,16bsandwich or nip the disc rotor14therebetween. In other words, the friction members26of the pads16a,16bare pushed onto the disc rotor14. Owing to the pushing by the pads16a,16b, there is generated a braking force for stopping rotation of the wheel that depends on a friction force between the disc rotor14and the friction members26, in other words, there is generated a braking force for reducing the speed of the vehicle or stopping the vehicle.

B. Basic Structure of Brake Actuator

As shown inFIG. 2, the actuator10includes the housing40, the piston42, an electric motor (three-phase DC brushless motor)44as a drive source, a speed reduction mechanism46for decelerating rotation of the electric motor44, a rotational shaft48configured to be rotated by the rotation of the electric motor44decelerated by the speed reduction mechanism46, and a motion converting mechanism50configured to convert the rotational motion of the rotational shaft48into an advancing and retracting movement (forward and backward movement) of the piston42. In the following description, the left side and the right side inFIG. 2are defined as a front side and a rear side, respectively, for the sake of convenience. It is noted that the speed reduction mechanism46is one example of a speed reduction device according to the present disclosure, and its structure will be later explained in detail.

The housing40is constituted by: a front-side casing40aand a rear-side casing40beach having a generally cylindrical shape; an inner sleeve40cwhich is supported at a front end portion thereof by the front-side casing40aand in which the piston42is disposed; a generally annular support wall40ddisposed radially inward of the front-side casing40aand supported by a front end of the rear-side casing40b; and a support plate40efixedly held by a rear end portion of the rear-side casing40b.

The piston42includes a piston head42aand a hollow piston cylinder42b. The actuator10includes a cylindrically shaped hollow shaft52. A front-side portion of the hollow shaft52functions as a motor shaft (rotor) that is a rotary drive shaft of the electric motor44, and a rear-side portion of the hollow shaft52functions as an input shaft of the speed reduction mechanism46that will be later explained in detail. In other words, the cylindrically shaped hollow shaft52is considered as being formed by integrating the motor shaft of the electric motor44and the input shaft of the speed reduction mechanism46that is configured to be rotated by the electric motor44. In short, the hollow shaft52itself is regarded as the input shaft of the speed reduction mechanism46, and the hollow shaft52itself is regarded as the motor shaft of the electric motor44. The electric motor44is constituted by coils44aheld by the front-side casing40aof the housing40so as to be fixed to an inner circumference of the front-side casing40aand magnets44bprovided on an outer circumference of the front-side portion of the hollow shaft52so as to be opposed to the coils44a.

The hollow shaft52is disposed such that the front-side portion thereof incorporates the inner sleeve40c. Further, the hollow shaft52is supported by the housing40through two radial ball bearings58,60so as to be rotatable about an axis L that is a center axis of the actuator10and so as to be immovable in an axial direction that is a direction of extension of the axis L. Specifically, the hollow shaft52is supported at a front end portion thereof by the front-side casing40athrough the radial ball bearing58and is supported at a rear-side portion thereof by the support wall40dthrough the radial ball bearing60. More specifically, the hollow shaft52functioning as the motor shaft is rotatably supported at its outer circumferential surface by the housing40.

The rotational shaft48is constituted by integrally formed four portions, i.e., an output shaft portion48afunctioning as an output shaft of the speed reduction mechanism46, an external thread portion48bwhich is located on the front side of the output shaft portion48aand is externally threaded, a flange portion48cdisposed at a rear end of the output shaft portion48a, and an outer cylindrical portion48dgenerally cylindrically shaped and extending from an outer peripheral end of the flange portion48cin the axial direction. The outer cylindrical portion48dmay be regarded as a part of the flange portion48c. The rotational shaft48is supported at the output shaft portion48athereof by an inner circumferential portion of the hollow shaft52through rollers (that are also referred to as needles)62, such that the rotational shaft48is rotatable about the axis L. That is, the rotational shaft48is rotatably supported at its outer circumferential surface by an inner circumferential surface of the hollow shaft52as the motor shaft through rollers62. It is noted that the rollers62constitute a radial bearing.

In addition to the hollow shaft52that functions as the input shaft and the rotational shaft48whose output shaft portion48afunctions as the output shaft, the speed reduction mechanism46includes a planetary gear member66supported by an intermediate portion of the hollow shaft52through a radial ball bearing64so as to be rotatable and immovable in the axial direction. The intermediate portion of the hollow shaft52that supports, on its outer circumference, the planetary gear member66through the radial ball bearing64has an axis L′ defined by its outer circumferential surface, the axis L′ being eccentric with respect to the axis L by an eccentric amount ΔL. This intermediate portion will be hereinafter referred to as an eccentric shaft portion52a, and the axis L′ will be hereinafter referred to as an eccentric axis L′. In this configuration, the planetary gear member66is configured to not only rotate about the eccentric axis L′, but also revolve about the axis L in conjunction with rotation of the hollow shaft52about the axis L.

The hollow shaft52is divided, in the axial direction, into three portions including the eccentric shaft portion52a. Specifically, the hollow shaft52includes the eccentric shaft portion52a, a first shaft portion52blocated on the front side of the eccentric shaft portion52aand supported by the housing40through the two radial ball bearings58,60, and a second shaft portion52clocated on the rear side of the eccentric shaft portion52aand supporting, at its inner circumferential surface, the rotational shaft48through the rollers62. The inner circumferential surface of the eccentric shaft portion52aand the inner circumferential surface of the second shaft portion52care continuous to each other without any step, and the outer circumferential surface of the eccentric shaft portion52aand the outer circumferential surface of the second shaft portion52care continuous to each other without any step. In terms of supporting of the planetary gear member66and the rotational shaft48, however, it can be considered that the eccentric shaft portion52ais a portion eccentric with respect to the axis L, and the second shaft portion52cis a portion not eccentric with respective to the axis L, namely, a portion coaxial with the axis L, as well as the first shaft portion52b.

The speed reduction mechanism46includes a ring gear member68that is fixedly supported by the support wall40dof the housing40. As also shown inFIG. 3A, a first internally toothed gear70is formed on the ring gear member68. Further, a first externally toothed gear72, a part of which is in mesh with a part of the first internally toothed gear70, is formed at a radially outer portion of the planetary gear member66. As also shown inFIG. 3B, a second internally toothed gear74is formed at the radially outer portion of the planetary gear member66so as to be arranged side by side with the first externally toothed gear72in the axial direction. Further, a second externally toothed gear76, a part of which is in mesh with a part of the second internally toothed gear74, is formed at a radially outer portion at a front end of the outer cylindrical portion48dof the rotational shaft48. In this respect, if the outer cylindrical portion48dis regarded as a part of the flange portion48c, it can be considered that the second externally toothed gear76is formed at a radially outer portion of a flange of the rotational shaft48.

The center of the first internally toothed gear70lies on the axis L while the center of the first externally toothed gear72lies on the eccentric axis L′. The center of the second internally toothed gear74lies on the eccentric axis L′ while the center of the second externally toothed gear76lies on the axis L. The meshing position of the first internally toothed gear70and the first externally toothed gear72is located opposite to the meshing position of the second internally toothed gear74and the second externally toothed gear76with respect to the axis L or the eccentric axis L′. That is, those meshing positions are different from each other in phase by 180° in the circumferential direction. In other words, the speed reduction mechanism46is a differential speed reduction device including: a first internally meshing planetary gear mechanism constituted by the first internally toothed gear70and the first externally toothed gear72that is meshing internally with the first internally toothed gear70; and a second internally meshing planetary gear mechanism constituted by the second internally toothed gear74and the second externally toothed gear76that is meshing internally with the second internally toothed gear74.

The first internally toothed gear70has a circular arc tooth profile, and the first externally toothed gear72has an epitrochoid parallel curve tooth profile. Similarly, the second internally toothed gear74has a circular arc tooth profile, and the second externally toothed gear76has an epitrochoid parallel curve tooth profile. Thus, the speed reduction mechanism46is constructed as a cycloid speed reducer. The thus constructed speed reduction mechanism46achieves a mechanism in which the number of teeth of the first internally toothed gear70and the number of teeth of the first externally toothed gear72differ from each other only by one and the number of teeth of the second internally toothed gear74and the number of teeth of the second externally toothed gear76differ from each other only by one. Accordingly, the speed reduction mechanism46is constructed as a speed reduction mechanism which has a high reduction ratio (i.e., a considerably small ratio of the rotation speed of the rotational shaft48as the output shaft with respect to the rotation speed of the hollow shaft52as the input shaft) and which is capable of performing smooth deceleration.

As shown inFIG. 2, the motion converting mechanism50is constituted by the rotational shaft48, specifically, the external thread portion48bof the rotational shaft48, and a nut78that is threadedly engaged with the external thread portion48band functions as a movable member. Each of an external thread of the external thread portion48band an internal thread of the nut78is a trapezoidal thread and is a multiple thread, specifically, a triple thread in the present actuator10. Two protrusions80each functioning as a key are formed on an outer circumference of the nut78. The two protrusions80are respectively held in engagement with two slots82formed on the inner sleeve40cof the housing40so as to extend in the axial direction. Owing to the engagement of the protrusions80and the slots82, the nut78is movable in the axial direction while being prohibited from rotating about the axis L. In this respect, an internal thread may be formed on the rotational shaft48, and there may be provided a movable member which has an external thread threadedly engaged with the internal thread and which is configured to advance and retract by the rotation of the rotational shaft48.

A front-side portion of the nut78as the movable member is disposed in a rear-side portion of the piston cylinder42bof the piston42, and the piston42is prohibited from being withdrawn from the nut78by a stopper ring84. A distal end face86of the nut78is held in contact with a contact surface88formed in the piston cylinder42b. A forward force of the nut78is transmitted as a forward force of the piston42via the mutually contacting distal end face86and contact surface88. The forward force of the piston42functions as a force by which the piston42pushes the friction members26of the brake pads16a,16bonto the disc rotor14, i.e., a pushing force. A force that causes the piston42to be inclined in the radial direction may act on the piston42when the piston42is pushing the friction members26, due to uneven wear of the friction members26of the brake pads16a,16b, inclination of the disc rotor14in turning of the vehicle, or the like. In such a case, the distal end face86and the contact surface88are allowed to be shifted or moved relative to each other in the radial direction, so that the piston42is allowed to be inclined to some extent.

The rotational shaft48is supported, at the flange portion48cformed at its rear end, by the housing40through a thrust bearing, namely, a thrust ball bearing90. Specifically, a pushing-force sensor92is disposed between the thrust ball bearing90and the support plate40efor detecting the pushing force (axial force). The rotational shaft48is supported by the support plate40eof the housing40also through the pushing-force sensor92. In this respect, the structure of the pushing-force sensor92and the supporting structure through the pushing-force sensor92are not illustrated inFIG. 2. An inner race96, which is a constituent component of a biasing mechanism94, is disposed between the thrust ball bearing90and the flange portion48cof the rotational shaft48, and a slight clearance is formed between the inner race96and the flange portion48c. (The clearance is exaggeratedly illustrated inFIG. 2.) When the piston42advances and pushes the friction member26onto the disc rotor14, the rotational shaft48is retracted by a reaction force of the pushing force and the clearance is removed by contact of the flange portion48cand the inner race96, so that the rotational shaft48is supported, at its rear end, namely, at the flange portion48cformed at the rear end, by the housing40through the thrust ball bearing90.

The biasing mechanism94is constituted by the inner race96explained above and a torsion coil spring98disposed in the rear-side casing40bof the housing40. One end100of the torsion coil spring98is retained by the rear-side casing40bwhile the other end thereof (not shown) is retained by the inner race96. When the piston42advances and pushes the friction member26onto the disc rotor14, in other words, when the braking force is generated, the clearance is removed, so that the inner race96starts rotating together with the rotational shaft48. With an increase in the braking force, namely, with further rotation of the rotational shaft48, the inner race96is further rotated to cause the torsion coil spring98to be twisted. An elastic reaction force of the torsion coil spring98acts on the rotational shaft48as a rotational biasing force in a direction in which the piston42is retracted. Even in a case where the electric motor44fails to generate the rotational driving force when the braking force is being generated, the piston42is retracted to a set rearward position, namely, to substantially the position of the piston42shown inFIG. 2, by the rotational biasing force, thus preventing a phenomenon in which the disc rotor14keeps rotating with the friction members26pushed onto the disc rotor14, i.e., what is called drag phenomenon.

While not illustrated in detail, the pushing-force sensor92is constituted mainly by a load cell. The actuator10includes a rotation angle sensor102for detecting a rotation angle (rotational phase) of the hollow shaft52as the motor shaft, in addition to the pushing-force sensor92. The rotation angle sensor102is a resolver.

As shown inFIG. 1, the ECU18as the controller includes a computer110constituted by a CPU, a RAM, a ROM, etc., and an inverter112that is a drive circuit (driver) of the electric motor44. The pushing force FS detected by the pushing-force sensor92and the rotation angle θ of the hollow shaft52detected by the rotation angle sensor102are transmitted to the computer110and the inverter112. The control of the actuator will be briefly explained. The computer110determines a required braking force that is a braking force to be generated by the electric brake device based on a degree of the operation of the brake operation member such as a brake pedal, and determines, based on the required braking force, a target pushing force that is a target of the pushing force FS. The computer then determines a target supply current that is an electric current I to be supplied to the electric motor44, such that the pushing force FS detected by the pushing-force sensor92becomes equal to the target pushing force. The inverter112controls the electric motor44based on the detected rotation angle θ according to the target supply current.

In the actuator10, the rotational shaft48, the piston42, and the electric motor44are disposed roughly coaxially so as to be arranged in this order from the center toward the radially outer side. Thus, the actuator10has a reduced axial dimension. Accordingly, the actuator10is compact in size, and the electric brake device that employs the actuator10is also compact in size.

C. Characteristic Structure of Brake Actuator

The actuator10according to the present embodiment is characterized by the structure of the speed reduction mechanism46as the speed reduction device of the present disclosure. For easier understanding of the characteristic structure, a modification of the speed reduction mechanism46, namely, a typical example of the speed reduction device, will be explained.

A speed reduction device120according to the modification schematically illustrated inFIG. 4Aincludes a housing130, an input shaft132, an output shaft134, a planetary gear member136, and a ring gear member138. The input shaft132is divided, in the axial direction in which the axis L extends, into an eccentric shaft portion132a, a first shaft portion132b, and a second shaft portion132c. The first and second shaft portions132b,132care respectively located on opposite sides of the eccentric shaft portion132ain the axial direction. The center axis of the eccentric shaft portion132adefined by its outer circumferential surface is eccentric with respect to the axis L by an eccentric amount ΔL. The center axis of the eccentric shaft portion132awill be hereinafter referred to as an eccentric axis L′. In this configuration, the planetary gear member136is configured to not only rotate about the eccentric axis L′, but also revolve about the axis L in conjunction with the rotation of the input shaft132about the axis L. The output shaft134includes a flange portion134aand an outer cylindrical portion134b.

The input shaft132is rotatably supported, at the first shaft portion132bthereof, by the housing130through a first bearing140that is a radial bearing. The planetary gear member136is rotatably supported by the eccentric shaft portion132aof the input shaft132through a second bearing142that is a radial bearing. The output shaft134is rotatably supported, at the inner circumferential surface of the outer cylindrical portion134bthereof, by the second shaft portion132cof the input shaft132through a third bearing144that is a radial bearing. It is noted that the input shaft132is supported also at a position different from the position at which the input shaft132is supported by the first bearing140. Specifically, the input shaft132is rotatably supported at a position distant from the position supported by the first bearing140in a direction away from the eccentric shaft portion132a, by the housing130through a fourth bearing146that is a radial bearing.

As in the speed reduction mechanism46, in the speed reduction device120, a first internally toothed gear150is formed on the ring gear member138that is fixedly supported by the housing130, and a first externally toothed gear152, a part of which is in mesh with a part of the first internally toothed gear150, is formed at a radially outer portion of the planetary gear member136. Further, a second internally toothed gear154is formed at the radially outer portion of the planetary gear member136so as to be arranged side by side with the first externally toothed gear152in the axial direction, and a second externally toothed gear156, a part of which is in mesh with a part of the second internally toothed gear154, is formed at a radially outer portion of the front end of the outer cylindrical portion134bof the output shaft134.

As in the speed reduction mechanism46, in the speed reduction device120, the center of the first internally toothed gear150lies on the axis L while the center of the first externally toothed gear152lies on the eccentric axis L′. The center of the second internally toothed gear154lies on the eccentric axis L′ while the center of the second externally toothed gear156lies on the axis L. The meshing position of the first internally toothed gear150and the first externally toothed gear152is located opposite to the meshing position of the second internally toothed gear154and the second externally toothed gear156with respect to the axis L or the eccentric axis L′, in other words, those meshing positions are different from each other in phase by 180° in the circumferential direction. Like the speed reduction mechanism46, the speed reduction device120is constructed as a differential speed reduction device including: a first internally meshing planetary gear mechanism constituted by the first internally toothed gear150and the first externally toothed gear152that is meshing internally with the first internally toothed gear150; and a second internally meshing planetary gear mechanism constituted by the second internally toothed gear154and the second externally toothed gear156that is meshing internally with the second internally toothed gear154. Further, as in the speed reduction mechanism46, the first internally toothed gear150has a circular arc tooth profile, the first externally toothed gear152has an epitrochoid parallel curve tooth profile, the second internally toothed gear154has a circular arc tooth profile, and the second externally toothed gear156has an epitrochoid parallel curve tooth profile. Thus, the speed reduction device120operates in the same manner as the speed reduction mechanism46in terms of deceleration of the rotation.

Here, the position in the axial direction at which the input shaft132is supported through the first bearing140is defined as a first support position P1, the position in the axial direction at which the planetary gear member136is supported through the second bearing142is defined as a second support position P2, the position in the axial direction at which the output shaft134is supported through the third bearing144is defined as a third support position P3, and the position in the axial direction at which the input shaft132is supported through the fourth bearing146is defined as a fourth support position P4. In the speed reduction device120, a distance between the first support position P1and the second support position P2, i.e., an input-side supporting distance Di, is equal to a distance between the second support position P2and the third support position P3, i.e., an output-side supporting distance Do. Further, the position in the axial direction at which the first internally toothed gear150and the first externally toothed gear152are in mesh with each other is defined as an input-side meshing position Gi, and the position in the axial direction at which the second internally toothed gear154and the second externally toothed gear156are in mesh with each other is defined as an output-side meshing position Go. In the speed reduction device120, a distance between the second support position P2and the input-side meshing position Gi, i.e., an input-side meshing distance Hi, is equal to a distance between the second support position P2and the output-side meshing position Go, i.e., an output-side meshing distance Ho. It is noted that supporting by each bearing and meshing of the gears are each established in a certain area (that is a concept including a distance, a width or the like) in the axial direction. Accordingly, the center of the area is regarded as each support position or each meshing position for convenience sake in the present disclosure. However, each support position and each meshing position may be set suitably within the area in the actual speed reduction device120.

When the speed reduction device120operates, the planetary gear member136receives, at the meshing position of the first internally toothed gear150and the first externally toothed gear152, a force (that may be referred to as a contact force) Fi from the ring gear member138supported by the housing130while the ring gear member138receives, at the meshing position, the same magnitude of the force Fi as reaction from the planetary gear member136, as shown inFIG. 4. Similarly, the planetary gear member136receives, at the meshing position of the second internally toothed gear154and the second externally toothed gear156, a force (that may be referred to as a contact force) Fo from the output shaft134while the output shaft134receives, at the meshing position, the same magnitude of the force Fo as reaction from the planetary gear member136. Though each of the force Fi and the force Fo that the planetary gear member136receives is represented as a force in the radial direction inFIG. 4Afor convenience sake, the force Fi and the force Fo actually act in the circumferential direction as illustrated inFIG. 4B. It can be considered that the force Fi and the force Fo that the planetary gear member136receives are substantially identical to each other in direction and magnitude. The force Fi and the force Fo act on the input shaft132from the planetary gear member136. Because the input-side meshing distance Hi and the output-side meshing distance Ho are equal to each other in the speed reduction device120, moment M shown inFIG. 4A, namely, moment M that causes the input shaft132to rotate, hardly acts on the input shaft132.

Because the moment M need not be taken into consideration, a force F2that acts on the input shaft132at the second support position P2and a force F3that acts on the input shaft132at the third support position P3can be represented as follows, as apparent fromFIGS. 4A and 4B:
F2=−Fi−Fo, F3=Fo
When a force F1that acts on the input shaft132at the first support position P1and a force F4that acts on the input shaft132at the fourth support position P4are taken into consideration, the forces that act on the input shaft132are in a balanced state as represented as follows:
F1+F2+F3+F4=0
Substitution of F2represented by the above equation and F3represented by the above equation cancels the force Fo, and the balanced state is accordingly represented as follows:
F1−Fi−Fo+Fo+F4=F1−Fi+F4=0
According to the above equation, F4is equal to 0 (F4=0) when F1is equal to Fi (F1=Fi). Because the input-side supporting distance Di is equal to the output-side supporting distance Do (Di=Do) in the speed reduction device120, F4is almost equal to 0 (F4≈0). Thus, the moments that depend respectively on the force F1, the force F2, and the force F3are balanced. The forces F1, F2, F3, F4can be regarded as the support loads of the respective bearings140,142,144,146to support the input shaft132. The support loads are almost minimal.

As understood from the description above, the support loads to support the input shaft132are well balanced in the speed reduction device120, enabling efficient deceleration. Further, the present speed reduction device120has fewer restrictions on bearings, and general-purpose bearings are available in the present speed reduction device120, thus obviating a cost increase. In other words, the present speed reduction device120is excellent in utility.

The speed reduction device120has the configuration (A) in which the first internally toothed gear is provided on the housing, the first externally toothed gear is provided on the planetary gear member, the second internally toothed gear is provided on the planetary gear member, and the second externally toothed gear is provided on the output shaft. The speed reduction device120may have the following configurations (B)-(D) each as a modification: (B) a configuration in which the first internally toothed gear is provided on the housing, the first externally toothed gear is provided on the planetary gear member, the second internally toothed gear is provided on the output shaft, and the second externally toothed gear is provided on the planetary gear member; (C) a configuration in which the first internally toothed gear is provided on the planetary gear member, the first externally toothed gear is provided on the housing, the second internally toothed gear is provided on the planetary gear member, and the second externally toothed gear is provided on the output shaft; and (D) a configuration in which the first internally toothed gear is provided on the planetary gear member, the first externally toothed gear is provided on the housing, the second internally toothed gear is provided on the output shaft, and the second externally toothed gear is provided on the planetary gear member. Though the force Fi and the force Fo act on the planetary gear member as illustrated inFIG. 4Cin the speed reduction device that employs the configuration (B) or (C), the speed reduction device can enjoy the advantages described above that the support loads can be made almost minimal.

Structural differences between the speed reduction mechanism46and the speed reduction device120are explained. In the speed reduction mechanism46, the hollow shaft52functions as the input shaft, and the rotational shaft48functioning as the output shaft is disposed so as to pass through the hollow shaft52. The speed reduction mechanism46differs from the speed reduction device120in that the inner circumferential surface of the hollow shaft52supports the rotational shaft48at its outer circumferential surface through the rollers62as the radial bearing. It is noted that the radial ball bearing60, the radial ball bearing64, the rollers62, and the radial ball bearing58correspond to the first bearing, the second bearing, the third bearing, and the fourth bearing, respectively.

In spite of the differences described above, the speed reduction device120and the speed reduction mechanism46are constructed based on the same concept and have the same characteristics. Specifically, as shown inFIG. 2, the input-side meshing distance Hi is equal to the output-side meshing distance Ho, the rotational shaft48as the output shaft is supported by the hollow shaft52as the input shaft so as to be rotatable, and the input-side supporting distance Di is equal to the output-side supporting distance Do. Thus, the speed reduction mechanism46employed in the actuator10of the illustrated embodiment also enjoys the advantages that the support loads can be made almost minimal, and efficient deceleration can be performed. Further, the speed reduction mechanism46has fewer restrictions on bearings, and general-purpose bearings are available therein, thus obviating a cost increase. As a result, the actuator10of the illustrated embodiment is also excellent in utility.