BALL SCREW DEVICE

A ball screw device having a screw shaft, a nut, a plurality of balls, a carrier constituting a planetary reduction mechanism, a rolling bearing for rotatably supporting the carrier, where the screw shaft includes a screw portion having a spiral-shaped shaft-side ball screw groove on an outer circumferential surface thereof, and a fitting shaft portion, the carrier is fixed to the fitting shaft portion such that relative rotation is not possible, an inner ring raceway of the rolling bearing is formed directly on an outer circumferential surface of the carrier.

TECHNICAL FIELD

The present disclosure relates to a ball screw device.

BACKGROUND ART

The ball screw device causes balls to roll and move between a screw shaft and a nut, and thus higher efficiency may be obtained compared to a sliding screw device that brings the screw shaft and nut into direct contact. Therefore, ball screw devices are incorporated in various mechanical devices, such as electric brake devices and automatic manual transmissions (AMT) of automobiles, and positioning devices of machine tools, in order to convert a rotational motion of a drive source such as an electric motor into linear motion.

A ball screw device includes a screw shaft having a spiral-shaped shaft-side ball screw groove on an outer circumferential surface thereof, a nut having a spiral-shaped nut-side ball screw groove on an inner circumferential surface thereof, and a plurality of balls arranged between the shaft-side ball screw groove and the nut-side ball screw groove. In a ball screw device, one of the screw shaft and the nut is used as a rotational motion element, and the other of the screw shaft and the nut is used as a linear motion element, depending on the application.

FIG.14illustrates a ball screw device100with a known structure, which is described in JP 2009-286137 A.

The ball screw device100includes a screw shaft101, a nut102, and a plurality of balls (not illustrated).

The screw shaft101has a screw portion103and a fitting shaft portion104that is arranged adjacent to one side in the axial direction of the screw portion103. A spiral-shaped shaft-side ball screw groove105is formed on an outer circumferential surface of the screw portion103. The fitting shaft portion104has a smaller outer diameter than the screw portion103. The screw shaft101is arranged coaxially with the nut102in a state in which the screw portion103is inserted through the nut102.

The nut102has a cylindrical shape. A spiral-shaped nut-side ball screw groove (not illustrated) is formed on an inner circumferential surface of the nut102. The nut102engages with a plurality of guide rods107supported by a housing106. This makes it possible to prevent the nut102from rotating.

The shaft-side ball screw groove105and the nut-side ball screw groove are arranged so as to face each other in the radial direction, and form a spiral-shaped load path. The starting point and ending point of the load path are connected by a circulation means (not illustrated). Therefore, the balls that have reached the end point of the load path are returned to the start point of the load path through the circulation means. Note that the starting point and the ending point of the load path are switched depending on a direction of relative displacement in the axial direction between the screw shaft101and the nut102, that is, the relative rotation direction between the screw shaft101and the nut102.

In the ball screw device100, rotation of an electric motor108, which is a drive source, is transmitted to the screw shaft101at a reduced speed by a pulley device109. For this purpose, a driven pulley110is externally fitted onto the fitting shaft portion104provided at an end portion on the one side in the axial direction of the screw shaft101such that relative rotation is not possible.

In addition, a drive pulley112is externally fitted onto the tip-end portion of a motor shaft111of the electric motor108such that relative rotation is not possible. A belt113is stretched between the drive pulley112and the driven pulley110. Thus, the rotation of the electric motor108is decelerated and transmitted to the screw shaft101.

CITATION LIST

Patent Literature

Patent Literature 1: JP 2009-286137 A

SUMMARY OF INVENTION

Technical Problem

In order to transmit the rotation of the electric motor to the screw shaft of the ball screw device, using a pulley device as in a known construction described in JP 2009-286137 A, or using a spur gear type reduction mechanism is considered to be possible.

The inventors of the present invention considered transmitting the rotation of an electric motor to a screw shaft of a ball screw device by using a planetary reduction mechanism that, when compared to a pulley device and a spur gear type reduction mechanism, offers advantages such as being able to arrange the input and output shafts coaxially and making the device more compact.FIG.15shows an undisclosed ball screw device that the present inventors considered before completing the ball screw device according to the present disclosure.

In this ball screw device, a carrier115of a planetary reduction mechanism114is externally fitted onto an end portion on one side in the axial direction of a screw shaft101aof the ball screw device100asuch that relative rotation is not possible. More specifically, a fitting shaft portion104aformed at the end portion on the one side in the axial direction of the screw shaft101ais spline-fitted into a mounting hole116formed in a center portion in the radial direction of the carrier115.

Moreover, a plurality of planetary gears117are rotatably supported with respect to the carrier115. More specifically, pinion pins119are inserted through and supported by a plurality of support holes118formed in an intermediate portion in the radial direction of the carrier115, and the planetary gears117are rotatably supported around the pinion pins119. In addition, the plurality of planetary gears117are arranged between a sun gear120fixed to a tip-end portion of a motor shaft111aof the electric motor108aand a ring gear121arranged coaxially with the sun gear120and fixed to a housing124. The plurality of planetary gears117mesh with the sun gear120and the ring gear121.

In this ball screw device, when the electric motor108ais energized and the sun gear120is rotated, the planetary gears117revolve around the sun gear120while rotating. The revolution motion of the planetary gears117is transmitted to the screw shaft101athrough the carrier115, and rotationally drives the screw shaft101a.

Moreover, the carrier115fixed to the fitting shaft portion104aof the screw shaft101ais rotatably supported with respect to the housing124using a rolling bearing122. Therefore, the axial force transmitted to the carrier115is supported by the housing124through the rolling bearing122.

With this construction, when the screw shaft101ais rotationally driven, a reaction force in the axial direction that acts on the screw shaft101afrom the nut102athrough the balls can be prevented from being transmitted to an engagement section between the planetary gears117and the sun gear120and an engagement section between the planetary gears117and the ring gear121. In addition, even in a case where a force in the axial direction acts on the carrier115due to the use of a helical gear as the planetary gear117for reasons such as ensuring sound vibration performance, the rolling bearing122is provided, and thus it is possible to prevent such axial force from being transmitted to rolling contact sections between the balls and the shaft-side ball screw groove105aand the nut-side ball screw groove125.

In this ball screw device, in order to efficiently transmit a force in the axial direction from the carrier115to the rolling bearing122, an outward flange-shaped collar portion123is provided on a part of the outer circumferential surface of the carrier115, and the flange-shaped collar portion123is brought into contact with the rolling bearing122.

In this ball screw device, the carrier115is rotatably supported by the housing124using the rolling bearing122that is separate from the carrier115, and thus as the number of parts increases, the ease of assembling the ball screw device100atends to deteriorate.

An object of the technique according to the present disclosure is to provide a ball screw device capable of reducing the number of parts and improving ease of assembly even in a case where construction is adopted in which the screw shaft is rotationally driven using a planetary reduction mechanism.

Solution to Problem

A ball screw device according to one aspect of the present disclosure includes a screw shaft, a nut, a plurality of balls, a carrier, and a rolling bearing.

The screw shaft includes a screw portion having a spiral-shaped shaft-side ball screw groove on an outer circumferential surface thereof, and a fitting shaft portion arranged on one side in an axial direction of the screw portion, and is configured to rotate when in use.

The nut includes a spiral-shaped nut-side ball screw groove on an inner circumferential surface and configured to move linearly when in use.

The plurality of balls are arranged between the shaft-side ball screw groove and the nut-side ball screw groove.

The carrier constitutes a planetary reduction mechanism, is fixed in a relatively unrotatable manner to the fitting shaft portion, and configured to rotationally drive the screw shaft.

The rolling bearing has an outer ring with an outer ring raceway on an inner circumferential surface thereof, an inner ring raceway provided in a portion facing the outer ring raceway in a radial direction, and a plurality of rolling elements rotatably arranged between the outer ring raceway and the inner ring raceway, and is configured to rotatably support the carrier.

The fitting shaft portion has an inner diameter side engaging portion on an outer circumferential surface thereof.

The carrier has a mounting hole in a center portion in the radial direction into which the fitting shaft portion can be inserted, and the mounting hole has an outer diameter side engaging portion on an inner circumferential surface thereof that engages with the inner diameter side engaging portion such that relative rotation is not possible.

In the ball screw device according to an aspect of the present disclosure, the inner ring raceway is directly formed on an outer circumferential surface of the carrier. That is, it can also be said that the inner ring that constitutes the rolling bearing and has the inner raceway is integrally formed with the carrier.

In the ball screw device according to an aspect of the present disclosure, the fitting shaft portion may have an inner diameter side fitting surface portion on a portion of the outer circumferential surface that is separated in the axial direction from the inner diameter side engaging portion, and the mounting hole may have an outer diameter side fitting surface portion on a portion of an inner circumferential surface thereof that is separated in the axial direction from the outer diameter side engaging portion, and the outer diameter side fitting surface portion fits with the inner diameter side fitting surface portion with a spigot fit.

In the ball screw device according to an aspect of the present disclosure, the inner diameter side fitting surface portion may be press-fitted into the outer diameter side fitting surface portion.

Alternatively, the inner diameter side fitting surface portion and the outer diameter side fitting surface portion may be fitted with a clearance fit with a minute gap.

In a case where the inner diameter side fitting surface portion and the outer diameter side fitting surface portion are fitted with a clearance fit with a minute gap, a locking ring, for example, is locked to the outer circumferential surface of the fitting shaft portion or the inner circumferential surface of the mounting hole, and the locking ring is able to prevent the fitting shaft portion from coming out of the mounting hole in the axial direction. Alternatively, a crimped portion is formed on the outer circumferential surface of the fitting shaft portion, and the crimped portion is able to prevent the fitting shaft portion from coming out of the mounting hole in the axial direction.

In the ball screw device according to an aspect of the present disclosure, the carrier may have a symmetrical shape with respect to the axial direction.

In the ball screw device according to an aspect of the present disclosure, an engagement section between the inner diameter side engaging portion and the outer diameter side engaging portion has an interference with regard to the radial direction in a portion in the axial direction.

In the ball screw device according to an aspect of the present disclosure, the carrier has support holes at a plurality of locations in the circumferential direction of an intermediate portion in the radial direction for inserting and supporting pinion pins of the planetary reduction mechanism.

In this case, the support holes may be through holes that pass through the carrier in the axial direction.

In the ball screw device according to an aspect of the present disclosure, the carrier may have a protruding portion extending toward the one side in the axial direction in a portion including openings of the support holes on the one side in the axial direction.

In this case, the protruding portion may have an annular shape.

Effect of Invention

In the ball screw device according to an aspect of the present disclosure, construction is employed in which the screw shaft is rotationally driven through a planetary reduction mechanism, making it possible to reduce the number of parts and improve the ease of assembly.

MODE FOR CARRYING OUT THE INVENTION

First Example

FIGS.1and2illustrate an example of construction in which a ball screw device of a first example of an embodiment according to the present disclosure and a planetary reduction mechanism are combined.

Overall Configuration of Ball Screw Device

A ball screw device1of the present example is incorporated into, for example, an electric brake booster device, and is used for converting the rotational motion of an electric motor, which is a drive source, into linear motion, and operating a piston of a hydraulic cylinder.

The ball screw device1includes a screw shaft2, a nut3, a plurality of balls4, a carrier5, and a rolling bearing6.

The screw shaft2is a rotational motion element that is rotationally driven by an electric motor7as a drive source through a planetary reduction mechanism8and rotates during use. The screw shaft2is inserted through the inside of the nut3and is arranged coaxially with the nut3. The nut3is prevented from rotating with respect to the screw shaft2by a rotation prevention mechanism (not illustrated), and is a linear motion element that moves linearly during use. That is, the ball screw device1of the present example is used in such a manner that the screw shaft2is rotationally driven and the nut3is moved linearly.

A spiral-shaped load path9is provided between the outer circumferential surface of the screw shaft2and the inner circumferential surface of the nut3. A plurality of balls4are arranged in the load path9so as to be able to roll. When the screw shaft2and the nut3are rotated relative to each other, the balls4that have reached the end point of the load path9are returned to the starting point of the load path9through a circulation groove10formed on the inner circumferential surface of the nut3.

Next, the construction of each component of the ball screw device1will be explained. In the present description, the axial direction, radial direction, and circumferential direction refer to the axial direction, radial direction, and circumferential direction with respect to the screw shaft2, unless otherwise specified. Further, one side in the axial direction refers to the right side inFIGS.1and2, and the other side in the axial direction refers to the left side inFIGS.1and2.

Screw Shaft

The screw shaft2is made of metal and includes a screw portion11and a fitting shaft portion12adjacent to the screw portion11on the one side in the axial direction. The screw portion11and the fitting shaft portion12are arranged coaxially and are integrally formed. The fitting shaft portion12has a smaller outer diameter than the screw portion11.

The screw portion11has a spiral-shaped shaft-side ball screw groove13on an outer circumferential surface thereof. The shaft-side ball screw groove13is formed by grinding, cutting, or rolling the outer circumferential surface of the screw portion11. In the present example, the number of shaft-side ball screw grooves13is one. The cross-sectional groove shape (groove bottom shape) of the shaft-side ball screw groove13has a Gothic arch shape or a circular arc shape. The screw portion11has a ring-shaped abutment surface14on an end surface on the one side in the axial direction. The abutment surface14is a flat surface that exists on a virtual plane orthogonal to the central axis of the screw shaft2. The screw portion11has a bottomed first center hole15in a center portion in the radial direction of an end surface on the other side in the axial direction.

The fitting shaft portion12has an inner diameter side engaging portion16at a portion on the one side in the axial direction (tip-end side portion) of the outer circumferential surface, and has an inner diameter side fitting surface portion17at a portion on the other side in the axial direction (base end side portion), which is a portion of the outer circumferential surface that is separated in the axial direction from the inner diameter side engaging portion16. In the present example, a dimension in the axial direction of the inner diameter side engaging portion16is larger than a dimension in the axial direction of the inner diameter side fitting surface portion17.

The inner diameter side engaging portion16has male spline teeth18over the entire circumference. That is, in the present example, the inner diameter side engaging portion16constitutes a male spline portion. In the present example, the male spline teeth18are configured by involute spline teeth; however, the male spline teeth may also be configured by square spline teeth.

The inner diameter side fitting surface portion17has a cylindrical outer circumferential surface, an outer diameter of which does not change in the axial direction. The outer diameter of the inner diameter side fitting surface portion17is larger than the tip circle diameter of the male spline teeth18of the inner diameter side engaging portion16and smaller than the outer diameter of the screw portion11.

The fitting shaft portion12has a bottomed second center hole19in a center portion in the radial direction in an end surface on the one side in the axial direction. The second center hole19and the first center hole15provided in the screw portion11are arranged coaxially with each other. In addition, the bottom portion (inner end portion) of the second center hole19is located at an intermediate portion in the axial direction of the fitting shaft portion12, and at an inner side in the radial direction of the inner diameter side engaging portion16.

The screw shaft2is arranged coaxially with the nut3in a state in which the screw portion11is inserted through the inner side of the nut3. In the present example, the screw shaft2is configured by the screw portion11and the fitting shaft portion12; however, the screw shaft may also include a support shaft portion (second fitting shaft portion) for externally fitting and fixing other members.

The nut3is made of metal and has a cylindrical shape as a whole. The nut3has a spiral nut-side ball screw groove20and a circulation groove10on an inner circumferential surface thereof.

The nut-side ball screw groove20has a spiral shape and is formed by grinding, cutting, rolling tapping, or cutting tapping on the inner circumferential surface of the nut3, for example. The nut-side ball screw groove20has the same lead as the shaft-side ball screw groove13. Therefore, in a state in which the screw portion11of the screw shaft2is inserted through the inner side of the nut3, the shaft-side ball screw groove13and the nut-side ball screw groove20are arranged to face each other in the radial direction, and a spiral-shaped load path9is formed. The number of nut-side ball screw grooves20is one, similar to the shaft-side ball screw groove13. Similar to the shaft-side ball screw groove13, the cross-sectional groove shape of the nut-side ball screw groove20is also a Gothic arch shape or a circular arc shape.

The circulation groove10has a substantially S-shape and is formed on the inner circumferential surface of the nut3by forging, such as cold forging. The circulation groove10smoothly connects portions adjacent in the axial direction of the nut-side ball screw groove20and connects the starting point and the ending point of the load path9. Therefore, the balls4that have reached the end point of the load path9are returned to the starting point of the load path9through the circulation groove10. Note that the starting point and the ending point of the load path9are switched depending on a direction of relative displacement in the axial direction of the screw shaft2and the nut3(the direction of relative rotation between the screw shaft2and the nut3).

The circulation groove10has a substantially semicircular cross-sectional shape. The circulation groove10has a groove width slightly larger than the diameter of the balls4, and has a groove depth that allows the balls4moving in the circulation groove10to get over the screw thread of the shaft-side ball screw groove13.

The nut3has an outward-facing flange-shaped collar portion21at an end portion on the one side in the axial direction of the outer circumferential surface. The collar portion21, at a plurality of locations (three locations in this example) in the circumferential direction, includes engaging grooves23that engage with a rotation prevention member (not illustrated) provided on a fixed member such as the housing22to prevent the nut3from rotating. Note that various conventionally known types of construction may be employed as the nut rotation prevention mechanism. For example, construction may be adopted in which a protruding portion (key) provided on the inner circumferential surface of a fixing member such as a housing is engaged with a concave groove formed in the axial direction on the outer circumferential surface of the nut.

In addition, a small diameter portion having an outer diameter that is smaller than a portion adjacent on the one side in the axial direction may be formed at an end portion on the other side in the axial direction of the outer circumferential surface of the nut3. In this case, for example, a fitting cylinder such as a piston (not illustrated) can be externally fitted and fixed to the small diameter portion.

Balls

Balls4are respectively a steel ball having a predetermined diameter, and are arranged in the load path9and the circulation groove10so as to be able to roll. The balls4placed in the load path9roll while being subjected to a compressive load, whereas the balls4placed in the circulation groove10are pushed by the following balls4and roll and move without being subjected to a compressive load.

Carrier

The carrier5constitutes the planetary reduction mechanism8, and rotates and drives the screw shaft2by transmitting torque input from the electric motor7, which is a drive source, to the screw shaft2.

The carrier5has a circular flat plate shape, and has a mounting hole24that penetrates in the axial direction at a center portion in the radial direction. The mounting hole24has an outer diameter side engaging portion25at a portion on the one side in the axial direction of the inner circumferential surface, and has an outer diameter side fitting surface portion26at a portion on the other side in the direction of the inner circumferential surface that is separated in the axial direction from the outer diameter side engaging portion25.

The outer diameter side engaging portion25has female spline teeth27over the entire circumference. That is, in the present example, the outer diameter side engaging portion25constitutes a female spline portion. In the present example, the female spline teeth27are configured by involute spline teeth; however, the female spline teeth may also be configured by square spline teeth.

The outer diameter side fitting surface portion26has a cylindrical outer circumferential surface, an inner diameter of which does not change in the axial direction. The inner diameter of the outer diameter side fitting surface portion26is larger than the root diameter of the female spline teeth27of the outer diameter side engaging portion25, and is slightly smaller than the outer diameter of the inner diameter side fitting surface portion17.

The carrier5is externally fixed to the fitting shaft portion12by inserting the fitting shaft portion12of the screw shaft2inside the mounting hole24. In addition, by abutting the abutting surface14provided on the screw portion11of the screw shaft2against the side surface on the other side in the axial direction of the carrier5, the screw shaft2and the carrier5may be positioned in the axial direction.

In the present example, by inserting the fitting shaft portion12into the mounting hole24, the outer diameter side engaging portion25and the inner diameter side engagement portion16are engaged with each other so as to be unable to rotate relative to each other. More specifically, the female spline teeth27of the outer diameter side engaging portion25and the male spline teeth18of the inner diameter side engaging portion16are engaged with a spline engagement. As a result, it is possible to rotate and drive the screw shaft2through the carrier5.

In addition, by inserting the fitting shaft portion12into the mounting hole24, the outer diameter side fitting surface portion26and the inner diameter side fitting surface portion17are fitted with a spigot fit. Thus, the degree of coaxiality between the screw shaft2and the carrier5is increased. The inner diameter side fitting surface portion17is press-fitted into the outer diameter side fitting surface portion26, and thus the fitting shaft portion12is prevented from coming out from the mounting hole24toward the other side in the axial direction. However, by making the inner diameter of the outer diameter side fitting surface portion slightly larger than the outer diameter of the inner diameter side fitting surface portion, the outer diameter side fitting surface portion and the inner diameter side fitting surface portion can be fitted with a clearance fit having a small gap that is small enough not to allow looseness.

An inner ring raceway28of the rolling bearing6is directly formed at an intermediate portion in the axial direction (in the present example, the center portion in the axial direction) of the outer circumferential surface of the carrier5. That is, the carrier5not only functions as a component of the planetary reduction mechanism8but also functions as an inner ring of the rolling bearing6. In other words, it can be said that the carrier and the inner ring of the rolling bearing are integrally formed. In the present example, the rolling bearing6is configured by a deep groove ball bearing capable of supporting a radial load and an axial load in both directions, and thus the inner ring raceway28is configured by a deep groove having a concave arc shaped cross section.

In the present example, the portions of the outer circumferential surface of the carrier5that are separated from the inner ring raceway28on both sides in the axial direction are formed into a partially cylindrical surface shape. However, in order to make sliding contact with an inner diameter side end portion of a seal ring, which is an optional element for sealing the rolling bearing, it is also possible to form seal grooves over the entire circumference on both sides in the axial direction of the outer circumferential surface of the carrier.

The carrier5has support holes29for inserting and supporting pinion pins41of the planetary speed reduction mechanism8at a plurality of locations (three locations in this example) in the circumferential direction of an intermediate portion in the radial direction. The plurality of support holes29are arranged at equal intervals in the circumferential direction. In addition, the central axes of the plurality of support holes29are arranged parallel to each other. Each support hole29is configured by a through hole passing through the carrier5in the axial direction. That is, the support hole29is open not only on a side surface on the one side in the axial direction of the carrier5, but also on a surface on the other side in the axial direction of the carrier5. However, the support hole can also be configured as a bottomed hole that is open only on the side surface on the one side in the axial direction of the carrier.

An inner diameter of the support hole29is constant in the axial direction. In the present example, the diameter (inscribed circle diameter) of a virtual circle passing through an end portion on the inside in the radial direction of the plurality of support holes29is approximately the same as the outer diameter of the screw portion11. In addition, the diameter of the virtual circle passing through the end portions of the plurality of support holes29(circumscribed circle diameter) is slightly smaller than the outer diameter of the portion of the nut3separated in the axial direction from the collar portion21.

The carrier5has a protruding portion30in an intermediate portion in the radial direction of the side surface on the one side in the axial direction that includes the opening portions of the plurality of support holes29, and the protruding portion30protrudes farther toward the one side in the axial direction than portions existing on the outer side in the radial direction and the inner side in the radial direction. The protruding portion30has an annular shape. The inner diameter of the protruding portion30is smaller than the diameter of a virtual cylindrical surface passing through the groove bottom portion of the shaft-side ball screw groove13. The outer diameter of the protruding portion30is approximately the same as the outer diameter of a portion of the nut3that is separated in the axial direction from the collar portion21, and is smaller than an outer diameter of the groove bottom portion of the inner ring raceway28. A side surface (tip-end surface) on the one side in the axial direction of the protruding portion30is a flat surface that exists on a virtual plane orthogonal to the central axis of the carrier5.

The protruding portion30and a portion of the side surface on the one side in the axial direction of the carrier5that exists on the outer side in the radial direction of the protruding portion30are connected by an outer diameter side connecting surface31that is inclined in a direction in which the outer diameter becomes larger while going toward the other side in the axial direction. In addition, the protruding portion30and a portion of the side surface on the one side in the axial direction of the carrier5that exists on the inner side in the radial direction of the protruding portion30are connected by an inner diameter side connecting surface32inclined in a direction in which the inner diameter becomes smaller while going toward the other side in the axial direction.

The side surface on the other side in the axial direction of the carrier5is a flat surface that exists on a virtual plane orthogonal to the central axis of the carrier5.

The outer circumferential surface of the carrier5on which the inner ring raceway28is formed is subjected to an induction hardening process and a tempering process to form a heat-treated hardened layer. However, a heat-treated hardened layer is not formed on the side surface on the one side in the axial direction and the side surface on the other side in the axial direction of the carrier5.

Rolling Bearing

The rolling bearing6supports the carrier5that is externally fitted onto the screw shaft2so as to be able to rotate with respect to the housing22, and also supports a force in the axial direction transmitted to the carrier5by the housing22. In the present example, the rolling bearing6is configured by a deep groove ball bearing capable of supporting a radial load and an axial load in both directions. However, multi-point contact ball bearings such as four-point contact ball bearings, double-row deep groove ball bearings, double-row angular contact ball bearings, conical rolling bearings, double-row tapered roller bearings, and the like, regardless of whether they are single or double row, can be used as rolling bearings, and any bearing capable of supporting radial and axial loads can be used.

The rolling bearing6includes an outer ring33, the inner ring raceway28, a plurality of rolling elements34, and a cage35.

The outer ring33has an annular shape and has an outer ring raceway36at a center portion in the axial direction of the inner circumferential surface. The outer ring33is internally fitted and fixed into the housing22and does not rotate during use. In the present example, the outer ring raceway36is configured by a deep groove having a concave arc-shaped cross section. Note that it is possible to provide a retaining ring that is locked to a portion of an inner circumferential surface of the housing22that is separated in the axial direction from a portion to which the outer ring33is internally fitted, and the retaining ring is also able to prevent the outer ring33from coming off.

In the present example, the portions of the inner circumferential surface of the outer ring33that are deviated from the outer ring raceway36on both sides in the axial direction are configured in a partially cylindrical shape. However, in order to lock the outer diameter side end portion of the seal ring, which is an optional element for sealing the rolling bearing, it is also possible to form locking grooves over the entire circumference in both side portions in the axial direction of the inner circumferential surface of the outer ring.

In the present example, the inner ring raceway28of the rolling bearing6is formed directly at an intermediate portion in the axial direction of the outer circumferential surface of the carrier5that faces the outer ring raceway36in the radial direction, and the inner ring is omitted.

The plurality of rolling elements34are made of steel or ceramic, and are arranged between the outer ring raceway36and the inner ring raceway28at equal intervals in the circumferential direction. In the present example, balls are used as the rolling elements34.

The cage35has an annular shape and has pockets37equally spaced in the circumferential direction. Rolling elements34are rotatably held inside the pockets37.

Planetary Reduction Mechanism

In the present example, a planetary reduction mechanism8is used to transmit the rotation of the electric motor7to the screw shaft2of the ball screw device1. The planetary reduction mechanism8includes a sun gear38, a plurality of planetary gears39, a ring gear40, a carrier5, and pinion pins41.

The sun gear38is fixed to the tip-end portion of a motor shaft (sun gear shaft)42of the electric motor7. The ring gear40is arranged coaxially with the sun gear38and is internally fitted and fixed into the housing22. Note that the housing22can be split into two parts, and a portion into which the ring gear40is fitted and a portion into which the outer ring33of the rolling bearing6is fitted may be configured by separate members.

A plurality of (three in the present example) planetary gears39are arranged at equal intervals in the circumferential direction and are rotatably supported by the carrier5. More specifically, a half portion on the other side in the axial direction of the pinion pin41is press-fitted into the support hole29formed in the carrier5, and a half portion on the one side in the axial direction of the pinion pin41is made to protrude in the axial direction from the support hole29. The planetary gear39is rotatably supported around a half portion on the one side in the axial direction of the pinion pin41through a slide bearing or a needle bearing (C&R) (not illustrated).

Note that the method of fixing the pinion pin to the support hole is not particularly limited, and a fixing structure using crimping, a locking pin, or the like may also be adopted. In addition, it is also possible to adopt a construction in which the pinion pins are supported on both sides by supporting end portions on the one side in the axial direction of the pinion pins by a second carrier having an annular shape (not illustrated). Moreover, the number of planetary gears is not limited to three, but may be two, or four or more.

The planetary gears39engage with both the sun gear38and the ring gear40.

Explanation of Operation of Ball Screw Device

The ball screw device1of the present example causes the nut3to move linearly by rotationally driving the screw shaft2through the planetary reduction mechanism8by an electric motor7serving as a drive source. More specifically, when the electric motor7is energized and the sun gear38is rotated in a predetermined direction, the planetary gears39revolve around the sun gear38while rotating. The nut3is caused to move linearly by the revolution motion of the planetary gears39being transmitted to the screw shaft2through the carrier5, and the screw shaft2being rotationally driven in a predetermined direction. For example, in a case where the sun gear38is rotationally driven toward one side in the circumferential direction, the nut3moves toward the one side in the axial direction relative to the screw shaft2, and in a case where the sun gear38is rotationally driven toward the other side in the circumferential direction, the nut3moves to the other side in the axial direction with respect to the screw shaft2.

With the ball screw device1of the present example, the screw shaft2can be rotationally driven through the planetary reduction mechanism8by the electric motor7serving as the drive source. Note that the stroke end related to the relative movement of the nut3to the one side in the axial direction and to the other side in the axial direction with respect to the screw shaft2can be regulated using various conventionally known stroke limiting mechanisms.

In the ball screw device1of the present example, despite using construction in which the screw shaft2is rotationally driven using the planetary reduction mechanism8, the number of parts may be reduced and ease of assembly may be improved.

That is, in the present example, the inner ring raceway28of the rolling bearing6is directly formed on the outer circumferential surface of the carrier5, and thus the inner ring of the rolling bearing6may be omitted. Therefore, compared with the construction illustrated inFIG.15in which an inner ring separate from the carrier is externally fitted and fixed to the carrier, the number of parts and assembly man-hours may be reduced, and the ease of assembly may be improved. In addition, in the present example, there is no need to form a collar portion for transmitting force in the axial direction on the outer circumferential surface of the carrier5, and the number of processing steps may be reduced accordingly. Furthermore, in the present example, the carrier5does not require a collar portion, and thus it is sufficient to form a heat-treated hardened layer only on the outer circumferential surface including the inner ring raceway28. Therefore, when performing a hole drilling process to form the support holes29on side surfaces in the axial direction of the carrier5, there is no need to perform a removal process to remove the heat-treated hardened layer, and the number of processing steps can be reduced accordingly.

In the present example, in a state in which the fitting shaft portion12of the screw shaft2is inserted into the mounting hole24of the carrier5, the inner diameter side fitting surface portion17having a cylindrical outer circumferential surface formed on the fitting shaft portion12, and the outer diameter side fitting surface portion26having a cylindrical inner circumferential surface formed in the mounting hole24of the carrier5are fitted with a spigot fit, and thus the degree of coaxiality between the screw shaft2and the carrier5may be increased. In addition, the inner diameter side fitting surface portion17is press-fitted into the outer diameter side fitting surface portion26, and thus the fitting section between the inner diameter side fitting surface portion17and the outer diameter side fitting surface portion26may also prevent the fitting shaft portion12from coming out from the mounting hole24toward the other side in the axial direction.

In the present invention, by providing the outer diameter side fitting surface portion26at a portion on the other side in the axial direction of the inner circumferential surface of the mounting hole24, the portion where the outer diameter side fitting surface portion26and the inner diameter side fitting surface portion17are fitted with a spigot fit, and the portions where the pinion pins41are press-fitted into the support holes29may be prevented from overlapping in the radial direction. In other words, it is possible to offset the positions in the axial direction of the portion where the outer diameter side fitting surface portion26and the inner diameter side fitting surface portion17are fitted with a spigot fit, and the portions where the pinion pins41are press-fitted into the support holes29.

Therefore, even in a case where the diameter is expanded (material is moved outward in the radial direction) in a portion of the carrier5that exists around the outer diameter side fitting surface portion26as the inner diameter side fitting surface portion17is press-fitted into the outer diameter side fitting surface portion26, an influence on the change in the inner diameters of portions on the one side in the axial direction of the support holes29into which the pinion pins41are press-fitted may be reduced. Moreover, even in a case where the diameter is reduced (material is moved radially inward) in a portion of the carrier5that is exists on inner sides in the radial direction of portions on the one side in the axial direction of the support holes29as the pinion pins41are press-fitted into portions on the one side in the axial direction of the support holes29, an influence on the change in the inner diameter of the outer diameter side fitting surface portion26may be reduced.

In the present example, a side surface on the one side in the axial direction forms the flat-surface shaped protruding portion30in an intermediate portion in the radial direction of the side surface on the one side in the axial direction of the carrier5that includes the opening portions of the support holes29. Therefore, the planetary gears39can be prevented from moving to the other side in the axial direction by using the side surface on the one side in the axial direction of the protruding portion30. In addition, even in a case where the end surface on the other side in the axial direction of the planetary gear39comes into sliding contact with the side surface on the one side in the axial direction of the protruding portion30, it is possible to prevent the sliding resistance from becoming excessive. Note that in the present example, the side surface on the other side in the axial direction of the planetary gear39and the side surface on the one side in the axial direction of the protruding portion30are in direct contact; however, another member such as a sliding washer may be interposed between the side surface on the other side in the axial direction of the planetary gear39and the side surface on the one side in the axial direction of the protruding portion30.

In the present example, the carrier5is rotatably supported by the housing22using a rolling bearing6, and thus axial force transmitted to the carrier5can be supported by the housing22through the rolling bearing6. More specifically, when the screw shaft2is rotationally driven, the reaction force in the axial direction that acts on the screw shaft2from the nut3through the balls4can be prevented from being transmitted to the engagement section between the planetary gears39and the sun gear38and the engagement section between planetary gears39and the ring gear40. In addition, even in a case where a force in the axial direction acts on the carrier5due to the use of helical gears as the planetary gears39for reasons such as ensuring sound vibration performance, the rolling bearing6is provided, and thus it is possible to prevent such axial force from being transmitted to rolling contact sections between the balls and the shaft-side ball screw groove13and the nut-side ball screw groove20.

Second Example

FIG.3illustrates a ball screw device1of a second example of an embodiment according to the present disclosure.

In the present example, the protruding portion30(seeFIG.2, etc.) that the carrier5was provided with in the construction of the first example is not provided on the side surface on the one side in the axial direction of the carrier5a.The side surface on the one side in the axial direction of the carrier5ais configured by a flat surface existing on a virtual plane orthogonal to the central axis of the carrier5a.

In the present example, the width dimension in the axial direction of the carrier5acan be shortened. Therefore, the ball screw device1may be made smaller. The other configurations and effects are the same as in the first example.

Third Example

FIGS.4and5illustrate a ball screw device1of a third example of an embodiment according to the present disclosure.

In the present example, a locking groove43is formed in a part in the axial direction of the outer circumferential surface of a fitting shaft portion12aof the screw shaft2a.More specifically, the locking groove43is arranged over the entire circumference at an end portion on the one side in the axial direction of the inner diameter side engaging portion16of the fitting shaft portion12a.

In addition, an outer diameter side engaging portion25, an outer diameter side fitting surface portion26a,a large diameter portion44, and a step surface45are provided on the inner circumferential surface of a mounting hole24aof the carrier5b.

The outer diameter side engaging portion25is provided at an intermediate portion in the axial direction of the inner circumferential surface of the mounting hole24a,and has female spline teeth27over the entire circumference. In the present example, the dimension in the axial direction of the outer diameter engaging portion25is made shorter than the dimension in the axial direction of the inner diameter side engaging portion16provided on the outer circumferential surface of the fitting shaft portion12a.In a state in which the fitting shaft part12aof the screw shaft2ais inserted inside the mounting hole24aof the carrier5b,and the abutment surface14of the screw portion11abutting against the side surface on the other side in the axial direction of the carrier5b,an end portion on the one side in the axial direction of the inner diameter side engaging portion16(a portion in which the locking groove43is formed) is made to protrude from the outer diameter side engaging portion25toward the one side in the axial direction.

The outer diameter side fitting surface portion26ais provided on the other side in the axial direction of the inner circumferential surface of the mounting hole24a.The inner diameter of the outer diameter side fitting surface portion26ais larger than the root diameter of the female spline teeth27of the outer diameter side engaging portion25, and is slightly larger than the outer diameter of the inner diameter side fitting surface portion17of the outer circumferential surface of the fitting shaft portion12a.

In the present example as well, by fitting the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17with a spigot fit in a state in which the fitting shaft portion12ais inserted into the mounting hole24a,the coaxiality between the screw shaft2aand the carrier5bis improved. However, in the present example, the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17are fitted with a clearance fit with a gap small enough to not allow looseness. By the fitting section between the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17, it is not possible to prevent the fitting shaft portion12afrom coming out from the mounting hole24atoward the other side in the axial direction. Therefore, in the present example, a locking ring46is provided.

A large diameter portion44is provided on the one side in the axial direction of the inner circumferential surface of the mounting hole24a.The inner diameter of the large diameter portion44is larger than the inner diameter of the outer diameter side fitting surface portion26aand larger than the outer diameter of the locking ring46in the free state thereof. The large diameter portion44is a conical cylindrical surface whose inner diameter increases while going toward the one side in the axial direction (opening side).

The step surface45is arranged between the outer diameter side engaging portion25and the large diameter portion44, and radially connects an end portion on the one side in the axial direction of the outer diameter engaging portion25and an end portion on the other side in the axial direction of the large diameter portion44. The step surface45is a flat surface existing on a virtual plane perpendicular to the central axis of the carrier5, and has a circular ring shape.

In the present example, the fitting shaft portion12ais inserted inside the mounting hole24a,and in a state in which the abutting surface14of the screw portion11abuts against the side surface on the other side in the axial direction of the carrier5b,the locking ring46is locked in the locking groove43formed in the outer circumferential surface of the fitting shaft portion12a. Moreover, the side surface on the other side in the axial direction of the locking ring46abuts against the step surface45. This prevents the fitting shaft portion12afrom coming out from the mounting hole24atoward the other side in the axial direction. The locking ring46has a C-shape and has a discontinuous portion in a part in the circumferential direction. The operation of locking the locking ring46to the locking groove43can be easily performed from a space on the one side in the axial direction of the screw shaft2ausing a pliers tool or the like.

As a modification of the present example, it is possible to employ a configuration in which a locking ring is provided so as to span between an outer diameter side locking groove formed on an inner peripheral surface of the mounting hole and an inner diameter side locking groove formed on the outer circumferential surface of the fitting shaft portion. In this case, it is also possible to employ a configuration in which the locking ring is locked to the inner circumferential surface of the mounting hole (outer diameter side locking groove) in advance, and by elastically expanding and then elastically contracting (restoring) the locking ring as the fitting shaft portion is inserted inside the mounting hole, the inner diameter side portion of the locking ring is locked in the inner diameter locking groove formed on the outer circumferential surface of the fitting shaft portion. Alternatively, it is also possible to employ a configuration in which the locking ring is locked to the outer circumferential surface of the fitting shaft portion (inner diameter side locking groove) in advance, and by elastically expanding and then elastically contracting (restoring) the locking ring as the fitting shaft portion is inserted inside the mounting hole, the outer diameter side portion of the locking ring is locked in the outer diameter side locking groove formed on the inner circumferential surface of the mounting hole.

In the present example, since the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17are fitted with a clearance fit, it is possible to easily perform the work of inserting the fitting shaft portion12ainside the mounting hole24a.In addition, the locking ring46that is engaged with the outer circumferential surface of the fitting shaft portion12acan effectively prevent the fitting shaft portion12afrom coming out from the mounting hole24atoward the other side in the axial direction. However, by press-fitting the inner diameter side fitting surface portion into the outer diameter side fitting surface portion and locking the locking ring to the outer circumferential surface of the fitting shaft portion or to the inner circumference surface of the mounting hole, it is also possible to prevent the fitting shaft portion from coming out from the mounting hole toward the other side in the axial direction. The other configurations and effects are the same as in the first and second examples.

Fourth Example

FIGS.6to8illustrate a ball screw device1of a fourth example of an embodiment according to the present disclosure.

In the present example as well, similar to the construction of the third example, an outer diameter side engaging portion25, an outer diameter side fitting surface portion26a,a large diameter portion44, and a step surface45are provided on the inner circumferential surface of the mounting hole24a.The basic construction of the mounting hole24ain the present example is the same as the construction in the third example.

In the present example as well, the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17are fitted with a clearance fit with a small gap that does not allow looseness, and thus by the fitting section between the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17, it is not possible to prevent the fitting shaft portion12bfrom coming out from the mounting hole24atoward the other side in the axial direction. For this reason, in the present example, a crimped portion47is provided.

In the present example, in a state in which the fitting shaft portion12bis inserted inside the mounting hole24a,and the abutment surface14of the screw portion11is abutted against the side surface on the other side in the axial direction of the carrier5b,the crimped portion47is formed on an outer circumferential edge portion of the end portion on the one side in the axial direction of the fitting shaft portion12b(inner diameter side engaging portion16) using a jig (not shown). Then, the crimped portion47is pressed against the step surface45. This prevents the fitting shaft portion12bfrom coming out from the mounting hole24atoward the other side in the axial direction.

In the present example, since the outer diameter side fitting surface portion26aand the inner diameter side fitting surface portion17are fitted with a clearance fit, it is possible to easily perform the work of inserting the fitting shaft portion12binside the mounting hole24a.In addition, the crimped portion47formed on the outer circumferential surface of the fitting shaft portion12bcan effectively prevent the fitting shaft portion12bfrom coming out from the mounting hole24atoward the other side in the axial direction. Moreover, in order to prevent the fitting shaft portion12bfrom coming out, it is not necessary to form a locking groove on the outer circumferential surface of the fitting shaft portion12b,and the locking ring is also not necessary. However, by forming a crimped portion on the outer circumferential surface of the fitting shaft portion and locking the locking ring on the outer circumferential surface of the fitting shaft portion or to the inner circumferential surface of the mounting hole, it is also possible to prevent the fitting shaft portion from coming out from the mounting hole toward the other side in the axial direction. The other configurations and effects are the same as in the first, second, and third examples.

Fifth Example

FIG.9illustrates a ball screw device1of a fifth example of an embodiment according to the present disclosure.

In the present example, the shape of the outer circumferential surface of the fitting shaft portion12cof the screw shaft2cand the shape of the inner circumferential surface of the mounting hole24bof the carrier5care different from the construction of the first to fourth examples.

In the present example, the inner diameter side engaging portion16is formed on the entire outer circumferential surface of the fitting shaft portion12c,and the cylindrical inner diameter side fitting surface portion is omitted from the fitting shaft portion12c.In addition, the outer diameter side engaging portion25is formed entirely on the inner circumferential surface of the mounting hole24c,and the cylindrical outer diameter side fitting surface portion is omitted from the mounting hole24b.

In the present example, by processing the male spline teeth18of the inner diameter side engaging portion16of the screw shaft2cwith reference to the outer diameter surface of the screw shaft2c,the coaxiality of the tooth tip surface (outer diameter surface) or the tooth surface (side surface) with respect to the screw shaft2cis increased. Moreover, by processing the female spline teeth27of the outer diameter side engaging portion25of the carrier5cwith reference to the outer diameter surface of the carrier5c,the coaxiality of the tooth bottom surface (outer diameter surface) or tooth surface (side surface) of the female spline teeth27with respect to the carrier5cis increased. Thus, when externally fitting the carrier5cto the fitting shaft portion12cof the screw shaft2c,it is possible to employ outer diameter surface matching (large diameter matching) in which the tooth tip surfaces of the male spline teeth18and the tooth bottom surfaces (large diameter surfaces) of the female spline teeth27are brought into pressure contact, or tooth surface matching in which the tooth surfaces (side surfaces) of the male spline teeth18and the tooth surfaces (side surfaces) of the female spline teeth27are brought into pressure contact with each other as a fit between the inner diameter side engaging portion16and the outer diameter side engaging portion25. Therefore, in the present example as well, it is possible to ensure a high degree of coaxiality between the screw shaft2cand the carrier5c.Note that, in a case where tooth surface matching is employed, the accuracy of the tooth surfaces of the male spline teeth18and the tooth surfaces of the female spline teeth27affects the coaxiality, and thus it is preferable to adopt outer diameter surface matching.

In the present example, the dimension in the axial direction of the engaging section (spline engagement portion) between the inner diameter side engaging portion16and the outer diameter side engaging portion25can be made longer than in the construction of the first to fourth examples. Therefore, torque that can be transmitted from the carrier5cto the screw shaft2ccan be increased. In addition, in the present example, in a case where tooth surface matching is employed as the fit between the inner diameter side engaging portion16and the outer diameter side engaging portion25, the pressure contact between the tooth surfaces of the male spline teeth18and the tooth surfaces of the female spline teeth27can prevent the carrier5cfrom coming out from the fitting shaft portion12ctoward the one side in the axial direction, and in a case where outer diameter surface matching is employed, the pressure contact between the tooth tip surfaces of the male spline teeth18and the bottom surfaces of the female spline teeth27can prevent the carrier5cfrom coming out from the fitting shaft portion12ctoward the one side in the axial direction. The other configurations and effects are the same as in the first and second examples.

Sixth Example

FIGS.10to12illustrate a ball screw device1of a sixth example of an embodiment according to the present disclosure.

The present example is a modification of the fifth example. In the case of the present example as well, the inner diameter side engaging portion16ais formed entirely on the outer circumferential surface of the fitting shaft portion12c,and the outer diameter side engaging portion25is formed entirely on the inner circumferential surface of the mounting hole24b.

In the present example, the engagement section between the inner diameter side engaging portion16aand the outer diameter side engaging portion17ahas an interference in the radial direction in a portion in the axial direction. In order for this, the root diameter of the male spline teeth18aof the inner diameter side engaging portion16ais not constant in the axial direction. More specifically, the root diameter of the male spline tooth18afrom the end portion on the one side in the axial direction to a portion near the end portion on the other side in the axial direction is made slightly smaller than the tip circle diameter of the female spline teeth27of the outer diameter side engaging portion25, and the root diameter of the male spline teeth18aat the end portion on the other side in the axial direction is slightly larger than the tip circle diameter of the female spline teeth27. In order for this, a protruding portion49that protrudes slightly outward in the radial direction is provided at the end portion on the other side in the axial direction of the tooth root portion48of the male spline teeth18a.

In the present example, in a state in which the fitting shaft part12cis inserted into the mounting hole24b,and the inner diameter side engaging portion16aand the outer diameter side engaging portion25are engaged with a spline engagement, it is possible to provide an interference in the radial direction between the tooth tip surface of the female spline teeth27and the protruding portion49provided on the tooth root portion48of the male spline teeth18a.Therefore, in the present example, it is possible to provide an interference in the radial direction at the end portion on the other side in the axial direction of the spline engagement section between the inner diameter side engaging portion16aand the outer diameter side engaging portion25. In other words, the end portion on the other side in the axial direction of the inner diameter side engaging portion16acan be press-fitted into the end portion on the other side in the axial direction of the outer diameter side engaging portion25. Note that in the present example as well, similar to the fifth example, when the carrier5cis externally fitted onto the fitting shaft portion12cof the screw shaft2c,a high degree of coaxiality between the screw shaft2cand the carrier5ccan be ensured by employing outer diameter surface matching or tooth surface matching as a fit between the inner diameter side engaging portion16aand the outer diameter side engaging portion25.

As a modification of the present example, the root diameter of the male spline teeth is constant in the axial direction, and a protruding portion that protrudes inward in the radial direction may also be provided on a part in the axial direction of the groove bottom portion of the female spline teeth (for example, the end portion on the one side in the axial direction). In addition, protruding portions may be provided on both the groove bottom portions of the male spline teeth and the groove bottom portions of the female spline teeth. Furthermore, protruding portions may be provided on the tooth tip surfaces of the male spline teeth and/or the female spline teeth instead of on the groove bottom portions of the male spline teeth and the female spline teeth. Moreover, the inner diameter side engaging portion and the outer diameter side engaging portion are not limited to spline teeth, and other construction such as serration teeth can also be employed.

In the present example, an interference in the radial direction is provided in a part in the axial direction of the engagement section between the inner diameter side engaging portion16aand the outer diameter side engaging portion25, and thus it is possible to prevent the fitting shaft portion12cfrom coming out from the mounting hole24btoward the other side in the axial direction. The other configurations and effects are the same as in the first, second, and fifth examples.

Seventh Example

FIG.13illustrates a ball screw device1of a seventh example of an embodiment according to the present disclosure.

In the present example, the carrier5dhas a symmetrical shape with respect to the axial direction entirely. For this reason, the outer diameter side engaging portion25and a pair of outer diameter side fitting surface portions26b,26care provided on the inner circumferential surface of the mounting hole24cof the carrier5d.

The outer diameter side engaging portion25is provided at an intermediate portion in the axial direction of the inner circumferential surface of the mounting hole24c.The pair of outer diameter side fitting surface portions26b,26care provided on both sides in the axial direction of the outer diameter engaging portion25on the inner circumferential surface of the mounting hole24c.

In the present example, directionality in the axial direction of the carrier5dcan be eliminated, and thus the work efficiency of assembling the carrier5dto the screw shaft2can be improved. Therefore, the ease of assembling the ball screw device1can be further improved. The other configurations and effects are the same as in the first and second examples.

Embodiments according to the present disclosure have been described above; however, the embodiments according to the present disclosure are not limited thereto and can be modified as appropriate without departing from the technical idea thereof. In addition, the construction of the embodiments according to the present disclosure can be combined as appropriate, as long as no contradiction occurs.

In each of the embodiments according to the present disclosure, construction is employed in which male spline teeth are provided on the outer circumferential surface of the fitting shaft portion of the screw shaft, and the inner circumferential surface of the carrier is configured with a mounting hole having female spline teeth, and the carrier is spline-fitted to the fitting shaft portion. However, according to the present disclosure, the construction for fixing the carrier to the fitting shaft portion is not particularly limited. For example, it is possible to employ construction in which the fitting shaft portion has an oval cross-sectional shape and a width across flats shape (stadium shape) with a pair of flat outer surfaces parallel to each other on the outer peripheral surface, the mounting hole of the carrier has an oval cross-sectional shape and a width across flats shape (stadium shape) with a pair of flat inner surfaces parallel to each other on the inner circumferential surface, and the carrier is fit to the fitting shaft portion with a non-circular fit.

In each of the embodiments according to the present disclosure, construction is described in which the circulation groove is directly formed on the inner circumferential surface of the nut; however, it is also possible to form the circulation groove in a circulation part (for example, a piece) separate from the nut, and to fix the circulation part to the nut.

REFERENCE SIGNS LIST