Patent ID: 12222023

FIRST EXAMPLE

FIG.1toFIG.4illustrate a ball screw device1of a first example of an embodiment of the present disclosure and an example of a structure in which the ball screw device1and a planetary speed-reducing mechanism8are combined.

[Overall Configuration of Ball Screw Device]

The ball screw device1of this example can be incorporated in, for example, an electric brake booster device and used for converting rotational motion of an electric motor, which is a drive source, into linear motion of a piston of a hydraulic cylinder.

The ball screw device1includes a screw shaft2, a nut3, a plurality of balls4, and a rolling bearing5.

The screw shaft2is a rotational motion element that is rotationally driven by an electric motor6, which is a drive source, through a planetary speed-reducing mechanism7, and undergoes rotational motion during use. The screw shaft2is inserted inside the nut3and arranged coaxially with the nut3. The nut3is a linear motion element that is prevented from co-rotating with respect to the screw shaft2by a rotation-locking mechanism (not illustrated), and undergoes linear motion during use. That is, the ball screw device1of this example is used in an aspect in which the screw shaft2is rotationally driven and the nut3is linearly moved.

A load path8having a spiral shape is provided between an outer peripheral surface of the screw shaft2and an inner peripheral surface of the nut3. The plurality of balls4is rotatably arranged in the load path8. When the screw shaft2and the nut3are relatively rotated, the balls4that have reached an end point of the load path8are returned to a start point of the load path8through a circulation groove9formed on the inner peripheral surface of the nut3.

Next, the structure of each component of the ball screw device1will be described below. In the following description, 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. In addition, one side in the axial direction refers to the right side inFIGS.1to3, and the other side in the axial direction refers to the left side inFIGS.1to3.

<Screw Shaft>

The screw shaft2is made of metal, and has a screw portion10and a carrier11of the planetary speed-reducing mechanism7adjacently arranged on the one side in the axial direction of the screw portion10. The screw portion10and the carrier11are coaxially arranged and integrally formed with each other. The carrier11has a substantially disk shape and has an outer diameter larger than the screw portion10having a substantially column shape. Therefore, the screw shaft2has a substantially T-shaped cross-sectional shape with respect to the axial direction.

The screw portion10has a shaft-side ball screw groove12having a spiral shape on an outer peripheral surface thereof. The shaft-side ball screw groove12is formed by an infeed rolling process. In this example, the number of threads of the shaft-side ball screw groove12is one. The cross-sectional groove shape (groove bottom shape) of the shaft-side ball screw groove12is a gothic arch groove or a circular arc groove. The screw portion10has a bottomed screw-side center hole13in the central portion in the radial direction of an end surface on the other side in the axial direction thereof.

In this example, the screw portion10has an incomplete screw portion38at the end portion on the one side in the axial direction of the outer peripheral surface thereof, in which the shaft-side ball screw groove12with a complete groove bottom shape is not formed. The incomplete screw portion38has a groove depth which is shallower than that of the shaft-side ball screw groove12. Specifically, the groove depth of the incomplete screw portion38gradually becomes shallower as going toward the one side in the axial direction. The space around the incomplete screw portion38can be used as a storage space for peripheral components.

The carrier11has an inner ring raceway14of the rolling bearing5. Specifically, the inner ring raceway14is formed directly in the intermediate section in the axial direction (central portion in this example) of the outer peripheral surface of the carrier11. As a result, the carrier11functions not only as a component of the planetary speed-reducing mechanism7but also as an inner ring of the rolling bearing5. In other words, in this example, the carrier and the inner ring of the rolling bearing are integrally formed. In this example, since the rolling bearing5is constructed by a four-point contact ball bearing capable of supporting a radial load and an axial load in both directions, the inner ring raceway14is constructed by a compound curved surface having a gothic arch shape in cross section.

In this example, portions of the outer peripheral surface of the carrier11that are deviated from the inner ring raceway14on both sides in the axial direction are formed into a partially cylindrical surface shape. However, it is also possible to form seal concave grooves over the entire circumference in portions on both sides in the axial direction of the outer peripheral surface of the carrier with which an end portion on the inner diameter side of a seal ring, which is an optional element for sealing the rolling bearing, is brought into sliding contact.

The carrier11has support holes15for the pinion pins33of the planetary speed-reducing mechanism7to be inserted through and supported by at a plurality of locations (three locations in this example) in the circumferential direction of the intermediate portion in the radial direction. The plurality of support holes15is arranged so as to be uniformly spaced in the circumferential direction. Further, the center axes of the plurality of support holes15are arranged parallel to each other. Each of the support holes15is configured by a through hole that penetrates the carrier11in the axial direction. That is, the support holes15are open not only on the side surface on the one side in the axial direction of the carrier11, but also on the side surface on the other side in the axial direction of the carrier11. However, the support holes can also be configured by bottomed holes that are open only on the side surface on the one side in the axial direction of the carrier.

The inner diameter of the support holes15is constant along the axial direction. In this example, the diameter of an imaginary circle (diameter of the inscribed circle) passing through end portions on the inside in the radial direction of the plurality of support holes15is approximately the same as the outer diameter of the screw portion10. Also, the diameter of an imaginary circle passing through end portions on the outside in the radial direction of the plurality of support holes15is slightly smaller than the outer diameter of the nut3.

The carrier11has a projecting portion16in the intermediate portion in the radial direction including opening portions of the plurality of support holes15of the side surface on the one side in the axial direction thereof, the projecting portion16protruding toward the one side in the axial direction more than portions which are located on the outer and inner sides thereof. The projecting portion16has an annular shape that is continuous in the circumferential direction. The inner diameter of the projecting portion16is smaller than the diameter of the diameter of an imaginary cylindrical surface passing through a groove bottom portion of the shaft-side ball screw groove12. The outer diameter of the projecting portion16is larger than the outer diameter of the nut3and is smaller than the outer diameter of a groove bottom portion of the inner ring raceway14. A side surface (tip end surface) on the one side in the axial direction of the projecting portion16is a flat surface that exists on an imaginary flat plane orthogonal to the center axis of the carrier11.

The projecting portion16and the portion of the side surface on the one side in the axial direction of the carrier11that is located on the outer side in the radial direction of the projecting portion16are connected by an outer diameter side connecting surface17inclined in a direction in which the outer diameter increases as going toward the other side in the axial direction. Also, the projecting portion16and the portion of the side surface on the one side in the axial direction of the carrier11that is located on the inner side in the radial direction of the projecting portion16are connected by an inner diameter side connecting surface18inclined in a direction in which the inner diameter decreases as going toward the other side in the axial direction.

The side surface on the other side in the axial direction of the carrier11is a flat surface existing on an imaginary flat plane orthogonal to the center axis of the carrier11.

The carrier11has a bottomed carrier-side center hole19in the central portion in the radial direction of the side surface on the one side in the axial direction. The carrier-side center hole19and the screw-side center hole13provided in the screw portion10are arranged so as to be coaxial with each other.

The carrier11is subjected to induction hardening treatment and tempering treatment on the outer peripheral surface on which the inner ring raceway14is formed, and a heat-treated hardened layer is formed thereon. However, the 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 carrier11.

The screw shaft2is arranged coaxially with the nut3in a state where the screw portion10is inserted through the inside of the nut3. In this example, the screw shaft2is configured by the screw portion10and the carrier11, but the screw shaft can also be provided with a fitting shaft portion and the like for externally fitting and fixing other members.

<Nut>

The nut3is made of a metal and has a cylindrical shape as a whole. The nut3has a nut-side ball screw groove20having a spiral shape and a circulation groove9on the inner peripheral surface thereof.

The nut-side ball screw groove20is formed by subjecting the inner peripheral surface of the nut3to, for example, a grinding process, a cutting process, a rolling process, or a cutting and tapping process. The nut-side ball screw groove20has the same lead as the shaft-side ball screw groove12. Therefore, in a state in which the screw portion10of the screw shaft2is inserted inside the nut3, the shaft-side ball screw groove12and the nut-side ball screw groove20are arranged so as to face each other in the radial direction to form the spiral-shaped load path8. The number of threads of the nut-side ball screw groove20is one, similar to the shaft-side ball screw groove12. The cross-sectional groove shape of the nut-side ball screw groove20is also a gothic arch groove or a circular arc groove, similar to that of the shaft-side ball screw groove12.

The circulation groove9has a substantially S-shape and is formed on the inner peripheral surface of the nut3by cold forging, for example. The circulation groove9smoothly connects axially adjacent portions of the nut-side ball screw groove20, and connects the start point and the end point of the load path8. Therefore, the balls4that have reached the end point of the load path8are returned to the start point of the load path8through the circulation groove9. Therefore, the balls4that have reached the end point of the load path8are returned to the start point of the load path8through the circulation means9. Note that the start point and end point of the load path8are interchanged according to the relative displacement direction (relative rotation direction between the screw shaft2and the nut3) in the axial direction between the screw shaft2and the nut3.

The circulation groove9has a substantially semicircular cross-sectional shape. The circulation groove9has a groove width slightly larger than the diameter of the balls4, and has a groove depth that allows the balls4moving in the circulation groove9to climb over the thread peaks of the shaft-side ball screw groove12.

The nut3has a cylindrical surface portion43in which the nut-side ball screw groove20is not formed at an end portion on the one side in the axial direction of the inner peripheral surface. Therefore, in this example, as illustrated inFIG.3, the cylindrical surface portion43can be arranged around the incomplete screw portion38of the screw shaft2when the nut3is moved to the one side in the axial direction relative to the screw shaft2. As a result, the balls4are prevented from being caught between the incomplete screw portion38and the nut-side ball screw groove20, thereby increase in the driving torque of the screw shaft2is prevented.

In this example, as illustrated inFIG.2, the distance La from an end portion on the one side in the axial direction of the nut-side ball screw groove20to a side surface on the one side in the axial direction of the nut3is made larger than the distance Lb from the boundary between the incomplete screw portion38and the shaft-side ball screw groove12to the side surface on the other side in the axial direction of the carrier11(La>Lb). As a result, even when the nut3is displaced to the one side in the axial direction relatively to the screw shaft2until the side surface on the one side in the axial direction of the nut3comes close to the side surface on the other side in the axial direction of the carrier11, the balls4are securely positioned between the nut-side ball screw groove20and the shaft-side ball screw groove12, and thereby the balls4are prevented from being caught between the incomplete screw portion38and the nut-side ball screw groove20.

The nut3has an outward flange portion21at an end portion on the one side in the axial direction of the outer peripheral surface located on the outside in the radial direction of the cylindrical surface portion43. The flange portion21is provided with engagement grooves22at a plurality of locations in the circumferential direction (three locations in this example) that engage with rotation-locking members (not illustrated) provided on fixed members such as the housing23to prevent the nut3from co-rotating. However, as a rotation-locking mechanism for preventing the nut from rotating, various types of conventionally known structures can be adopted. For example, a structure in which protruding portions (keys) provided on the inner peripheral surface of fixed members such as the housing are engaged with concave grooves formed on the outer peripheral surface of the nut in the axial direction can be adopted.

Further, it is also possible to form a small-diameter portion at the end portion on the other side in the axial direction of the outer peripheral surface of the nut3which has an outer diameter smaller than that of the portion adjacent to the one side in the axial direction. In this case, a fitting cylinder such as a piston (not illustrated) can be fitted on and fixed to the small-diameter portion.

<Balls>

The balls4are steel balls having a predetermined diameter, and are arranged in the load path8and the circulation groove9so as to be able to roll. The balls4arranged in the load path8roll while being subjected to a compressive load, whereas the balls4arranged in the circulation groove9are pushed by the succeeding balls4and roll without being subjected to a compressive load.

<Rolling Bearing>

The rolling bearing5supports the carrier11of the screw shaft2so as to be able to rotate freely with respect to the housing23, and supports the axial force transmitted to the carrier11by the housing23. In this example, the rolling bearing5is configured by a four-point contact ball bearing capable of supporting a radial load and an axial load in both directions. However, as a rolling bearing, it is possible to use any single or double row bearing capable of supporting radial load and axial load. For example, multi-contact ball bearings other than four-point contact ball bearings such as three-contact ball bearings, single-row deep groove ball bearings, double-row deep groove ball bearings, double-row angular contact ball bearings, tapered rolling bearings, and double-row tapered roller bearings can be used.

The rolling bearing5includes an outer ring25, an inner ring raceway14, a plurality of rolling bodies26, and a cage27.

The outer ring25has an annular shape, and has an outer ring raceway28in the middle section in the axial direction of the inner peripheral surface. The outer ring25is fitted and fixed inside the housing23and does not rotate during use. In this example, by forming the projecting portion16on the side surface on the one side in the axial direction of the carrier11, a side surface on the one side in the axial direction of the outer ring25is offset from the side surface on the one side in the axial direction of the carrier11(side surface on the one side in the axial direction of the projecting portion16) toward the other side in the axial direction. Further, the width dimension in the axial direction of the outer ring25is made smaller than the width dimension in the axial direction of the carrier11. Furthermore, since the rolling bearing5is constructed by a four-point contact ball bearing, the outer ring raceway36is constructed by a compound curved surface having a gothic arch shape in cross section. Here, it is also possible to provide a retaining ring that is engaged with a portion of the inner peripheral surface of the housing23that is deviated in the axial direction from the portion where the outer ring25is internally fitted so as to prevent the outer ring25from coming off. Further, the side surface on the one side in the axial direction of the outer ring25can be arranged on the same plane as the side surface on the one side in the axial direction of the carrier11, or can be offset to the one side in the axial direction from the side surface on the one side in the axial direction of the carrier11.

In this example, portions of the inner peripheral surface of the outer ring25that are deviated from the outer ring raceway28in the axial direction are formed in a partially cylindrical surface shape. However, it is also possible to form locking grooves over the entire circumference in the portions on both sides in the axial direction of the inner peripheral surface of the outer ring to which an end portion on the outer diameter side of a seal ring for sealing the rolling bearing is locked.

In this example, the inner ring raceway14of the rolling bearing5is formed directly in the intermediate section in the axial direction of the outer peripheral surface of the carrier11that face the outer ring raceway28in the radial direction, and the inner ring is omitted.

The plurality of rolling bodies26are made of steel or ceramics, and are arranged so as to be uniformly spaced in the circumferential direction between the outer ring raceway28and the inner ring raceway14. In this example, balls are used as the rolling bodies26.

The cage27has an annular shape, and has pockets29at equal intervals in the circumferential direction. The rolling bodies26are held inside the pockets29so as to be able to roll freely.

[Planetary Speed-Reducing Mechanism]

In this example, a planetary speed-reducing mechanism7is used in order to transmit the rotation of the electric motor6to the screw shaft2of the ball screw device1. The planetary speed-reducing mechanism7includes a sun gear30, a plurality of planetary gears31, a ring gear32, a carrier11, and pinion pins33.

The sun gear30is fixed to a tip end portion of the motor shaft (sun gear shaft)34of the electric motor6. The ring gear32is arranged so as to be coaxial with the sun gear30and is fitted and fixed inside the housing23. The housing23may have a split structure, and a portion into which the ring gear32is fitted and a portion into which the outer ring25of the rolling bearing5is fitted may be configured by separate members.

A plurality of (three in this example) planetary gears31are arranged so as to be uniformly spaced in the circumferential direction and supported so as to be able to rotate freely with respect to the carrier11. Specifically, half portions on the other side in the axial direction of the pinion pins33are press-fitted into the support holes15formed in the carrier11, and half portions on the one side in the axial direction of the pinion pins33protrude from the support holes15to the one side in the axial direction. The planetary gears31are supported around the half portions on the one side in the axial direction of the pinion pins33through a sliding bearing or needle bearing (C&R) (not illustrated) so as to be able to rotate freely.

A method for fixing the pinion pins to the support holes is not particularly limited, and a fixing structure using swaging, locking pins, or the like may be adopted. Also, as a configuration in which the end portions on the one side in the axial direction of the pinion pins are supported by a second carrier having an annular shape (not illustrated), it is possible to adopt a structure that supports the pinon pins on both sides. Further, the number of planetary gears is not limited to three, and may be two or four or more.

The planetary gears31engage with the sun gear30and the ring gear32respectively.

[Explanation of Operation of Ball Screw Device]

In the ball screw device1of this example, the nut3is linearly moved by rotationally driving the screw shaft2through the planetary speed-reducing mechanism7by the electric motor6which is the drive source. Specifically, when the electric motor6is energized and the sun gear30is rotated in a predetermined direction, the planetary gears31revolve around the sun gear30while rotating. Then, the revolving motion of the planetary gears31is transmitted to the screw shaft2through the carrier11and rotationally drives the screw shaft2in a predetermined direction, so that the nut3is linearly moved. For example, when the sun gear30is rotationally driven toward one side in the circumferential direction, the nut3moves to the one side in the axial direction relative to the screw shaft2, and when the sun gear30is rotationally driven toward the other side in the circumferential direction, the nut3is moved to the other side in the axial direction relative to the screw shaft2.

With the ball screw device1of this example, it is possible to rotationally drive the screw shaft2through the planetary speed-reducing mechanism7by the electric motor6which is the drive source. A stroke end associated with the nut3moving to the one side in the axial direction and to the other side in the axial direction relative to the screw shaft2can be restricted using various conventionally known stroke limiting mechanisms.

[Manufacturing Method of Screw Shaft]

The screw shaft2of the ball screw device1of this example can be manufactured, for example, by a manufacturing method including the following processes. In this example, in addition to the forging process and the rolling process, a first cutting/grinding process, a raceway groove cutting process, a heat treatment process, and a second cutting/grinding process are included as optional and additional processes.

<Forging Process>

A forging process is a process of forming an intermediate material integrally comprising a disk portion and a shaft-shaped portion and having a T-shaped cross-sectional shape with respect to the axial direction by performing a forging process to a raw material. In this example, a raw material (billet) made of metal (not illustrated) is subjected to a hot forging process to form an intermediate material35as illustrated inFIG.4(A). The intermediate material35integrally comprises a shaft-shaped portion36having a column shape that will be processed into the screw portion10and a disk portion37that will be processed into the carrier11, and has a T-shaped cross-sectional shape with respect to the axial direction. Here, in the forging process, it is also possible to roughly form a concave groove having a shape similar to the inner ring raceway14on the outer peripheral surface of the disk portion37.

<First Cutting Process>

In this example, after the forging process and before the rolling process, a process is provided to form the carrier-side center hole19on a side surface on the one side in the axial direction of the disk portion37and the screw-side center hole13on a side surface on the other side in the axial direction of the shaft-shaped portion36so as to be coaxial with the carrier-side center hole19. In this process, a cutting process is performed on the intermediate material35. Specifically, a cutting process is performed on the outer peripheral surface and the end surface on the other side in the axial direction of the shaft-shaped portion36, and the side surface on the one side in the axial direction of the disk portion37. As a result, oxide films formed on the outer peripheral surface and the end surface on the other side in the axial direction of the shaft-shaped portion36, and on the side surface on the one side in the axial direction of the disk portion37are removed, and the shape of each portion is adjusted. In addition, a grinding process can be performed on these surfaces as necessary. Further, as illustrated inFIG.4(B), a small-diameter portion44having an outer diameter smaller than the outer diameter of a portion of the shaft-shaped portion36deviating from the end portion on the one side in the axial direction is formed at the end portion on the one side in the axial direction of the shaft-shaped portion36, and a screw-side center hole13is formed in the end surface on the other side in the axial direction of the shaft-shaped portion36. The outer peripheral surface of the small-diameter portion44becomes the incomplete screw portion38in the state where the screw shaft2is completed.

Further, the projecting portion16and the carrier-side center hole19are formed on a side surface on the one side in the axial direction of the disk portion37. In this example, the screw-side center hole13and the carrier-side center hole19are formed so as to be coaxial with each other. The small-diameter portion44can also be formed at an end portion on the one side in the axial direction of the outer peripheral surface of the shaft-shaped portion36in the forging process.

<Rolling Process>

A rolling process is a process of forming the shaft-side ball screw groove12on the outer peripheral surface of the shaft-shaped portion36by performing an infeed rolling process to the intermediate material35. In this example, as illustrated inFIG.4(C), the shaft-side ball screw groove12having a spiral shape is formed on a portion of the outer peripheral surface of the shaft-shaped portion36that is deviated from the small-diameter portion44by performing an infeed rolling process to the intermediate material35. Since the cross-sectional shape of the intermediate material35in the axial direction is T-shaped, it is difficult to perform a through-feed type rolling process to the intermediate material35. In this example, the shaft-side ball screw groove12is formed on the outer peripheral surface of the shaft-shaped portion36by bringing a pair of rolling dies close to each other with respect to the shaft-shaped portion36by the infeed rolling process.

In this example, such an infeed rolling process is performed in a state where the intermediate material35is centered using the screw-side center hole13and the carrier-side center hole19. Further, in this process, the incomplete screw portion38is formed on the outer peripheral surface of the small-diameter portion44of the shaft-shaped portion36. In this example, since the small-diameter portion44is provided at an end portion on the one side in the axial direction of the outer peripheral surface of the shaft-shaped portion36, excess thickness can be released to the small-diameter portion44side when the shaft-side ball screw groove12is formed by performing an infeed rolling process. As a result, the shape and dimensions of the shaft-side ball screw groove12can be stabilized.

<Raceway Groove Cutting Process>

In this example, a raceway groove cutting process is provided after the rolling process. The raceway groove cutting process is a process of performing a cutting process on the outer peripheral surface of the disk portion37to form the inner ring raceway14. In this example, the inner ring raceway14is formed in the middle section in the axial direction of the outer peripheral surface of the disk portion37as illustrated inFIG.4(D)by performing a cutting process on the outer peripheral surface of the disk portion37. Further, the cutting process for forming the inner ring raceway14is performed with the intermediate material35centered using the screw-side center hole13and the carrier-side center hole19. When a concave groove is roughly formed on the outer peripheral surface of the disk portion37in the forging process, the inner ring raceway14is formed by adjusting the shape of the concave groove in the raceway groove cutting process. Note that the rolling process may be performed after the raceway groove cutting process.

<Heat Treatment Process>

In this example, a heat treatment process is provided after the rolling process and the raceway groove cutting process. The heat treatment process is a step of forming a heat-treated hardened layer in a range including the outer peripheral surface of the disk portion37and the outer peripheral surface of the shaft-shaped portion36. In this example, by performing a heat treatment on the intermediate material35, a heat-treated hardened layer is formed at least on a portion of the outer peripheral surface of the shaft-shaped portion36where the shaft-side ball screw groove12is formed and on a portion of the outer peripheral surface of the disk portion37where the inner ring raceway14is formed.

Specifically, the outer peripheral surface of the shaft-shaped portion36and the outer peripheral surface of the disk portion37are subjected to induction hardening treatment and tempering treatment, and the side surface in the axial direction of the disk portion37(the side surface on the one side in the axial direction and the side surface on the other side in the axial direction thereof) is not subjected to induction hardening treatment and tempering treatment. As a result, a heat-treated hardened layer is formed only the portion of the outer peripheral surface of the shaft-shaped portion36where the shaft-side ball screw groove12is formed and the portion of the outer peripheral surface of the disk portion37where the inner ring raceway14is formed, and the side surface in the axial direction of the disk portion37is prevented from being deformed by heat treatment. As a quenching treatment, sub-quenching, carburizing quenching, or the like can be employed other than induction hardening.

<Second Cutting/Grinding Process>

In this example, after the rolling process, the raceway groove cutting process, and the heat treatment process, a step of forming the support holes15at a plurality of locations in the circumferential direction of the central portion in the radial direction of the disk portion37is provided for inserting and supporting the pinion pins33of the planetary speed-reducing mechanism7. Specifically, as illustrated inFIG.4(E), the support holes15are formed by performing a drilling process at a plurality of locations in the circumferential direction of the intermediate portion in the radial direction of the disk portion37. In this example, since induction hardening treatment and tempering treatment are not performed on the side surface in the axial direction of the disk portion37in the heat treatment process as a previous step, no heat treatment deformation occurs on the side surface in the axial direction of the disk portion37. As a result, it is possible to perform a drilling process to the intermediate portion in the radial direction of the disk portion37without performing a removal process to remove deformed portions due to the heat treatment. Further, since the intermediate portion in the radial direction of the disk portion37has the same hardness as the raw material, the drilling process can be easily performed.

In this step, a grinding process is performed on the inner ring raceway14formed on the outer peripheral surface of the disk portion37. In this example, the cutting process (drilling process) for forming the support holes15and the grinding process to the inner ring raceway14are respectively performed in a state where the intermediate material35is centered by using the screw-side center hole13and the carrier-side center hole19. In the heat treatment process, when the intermediate material35is subjected to sub-quenching or carburizing quenching, the hardness of the side surface in the axial direction of the disk portion37also increases, but it is possible to perform a drilling process to the intermediate portion in the radial direction of the disk portion37.

In this example, the screw shaft2is obtained from the intermediate material35by forming the screw portion10from the shaft-shaped portion36and forming the carrier11from the disk portion37through the manufacturing processes described above.

In the ball screw device1of this example, in spite of adopting a structure in which the screw shaft2is rotationally driven using the planetary speed-reducing mechanism7, the number of components is suppressed and the assembling efficiency is improved.

That is, in this example, since the inner ring raceway14constituting the rolling bearing5is directly formed on the outer peripheral surface of the carrier11, it is possible to omit the inner ring constituting the rolling bearing5. As a result, compared to a structure in which an inner ring, which is separated from the carrier, is externally fitted and fixed to the carrier, as the structure illustrated inFIG.12, the number of components is suppressed, the number of assembling steps is reduced, and the assembling efficiency is improved. Also, in this example, it is not necessary to form a flange portion for transmitting axial force to the outer peripheral surface of the carrier11, and the number of processing steps can be reduced accordingly. Further, in this example, since the carrier11does not require a flange portion, it is sufficient to form a heat-treated hardened layer only on the outer peripheral surface including the inner ring raceway14. Accordingly, when performing a drilling process to the side surface in the axial direction of the carrier11to form support holes15, it is not necessary to perform a removal process to remove the heat-treated hardened layer, and the number of processing steps can be reduced accordingly.

In this example, since the screw portion10and the carrier11are also integrally configured, the number of components can be reduced and the number of assembling steps can be reduced compared to a case of adopting a structure in which a carrier separate from a screw shaft is fixed to the screw shaft. Further, since the coaxiality between the screw portion10and the carrier11can be increased, it is possible to improve quietness of the ball screw device1during operation.

In this example, since the side surface on the one side in the axial direction of the outer ring25is offset from the side surface on the one side in the axial direction of the carrier11toward the other side in the axial direction, it is possible to prevent interference between the side surface on the one side in the axial direction of the outer ring25and an end surface on the other side in the axial direction of the planetary gears31. Further, a projecting portion16having a flat-surface shaped side surface on the one side in the axial direction is formed in the intermediate portion in the radial direction of the side surface on the one side in the axial direction of the carrier11including the opening portions of the support holes15. Therefore, it is possible to prevent the planetary gears31from moving to the other side in the axial direction by using the side surface on the one side in the axial direction of the projecting portion16. Further, even when side surfaces on the other side in the axial direction of the planetary gears31are brought into sliding contact with the side surface on the one side in the axial direction of the projecting portion16, it is possible to prevent sliding resistance from becoming excessive. In this example, although the side surfaces on the other side in the axial direction of the planetary gears31and the side surface on the one side in the axial direction of the projecting portion16are in direct contact with each other, other members such as a sliding washer may be interposed between the side surfaces on the other side in the axial direction of the planetary gears31and the side surface on the one side in the axial direction of the projecting portion16.

In this example, when manufacturing the screw shaft2, the shaft-side ball screw groove12, the inner ring raceway14, and the support holes15can be machined using the same (common) reference such as the screw-side center hole13and the carrier-side center hole19. As a result, the dimensional accuracy of the screw shaft2can be improved. Accordingly, the mechanical efficiency of the ball screw device1can be improved, and the meshing accuracy between the planetary gears31and the sun gear30and the meshing accuracy between the planetary gears31and the ring gear32can be improved respectively.

In this example, since the carrier11is supported by using the rolling bearing5so as to be able to rotate freely with respect to the housing23, it is possible to support the axial force transmitted to the carrier11by the housing23through the rolling bearing5. Specifically, it is possible to prevent an axial reaction force acting on the screw shaft2from the nut3through the balls4from being transmitted to an engaging portion between the planetary gears31and the sun gear30and an engaging portion between the planetary gears31and the ring gear32. Further, even if an axial force acts on the carrier11due to the fact that helical gears are used as the planetary gears31for reasons such as securing performance reducing noise and vibration, since the rolling bearing5is provided, it is possible to prevent such axial force from being transmitted to rolling contact portions between the balls4and the shaft-side ball screw groove12and the nut-side ball screw groove20.

SECOND EXAMPLE

FIG.5andFIG.6illustrate a ball screw device of a second example of an embodiment of the present disclosure.

A nut-side ball screw groove20is formed on the inner peripheral surface of a nut3aused in this example to an end portion on the one side in the axial direction. That is, the nut3adoes not include the cylindrical surface portion43that the nut3includes in the first example.

On the other hand, in this example, a screw shaft2dincludes a tapered concave portion (recessed portion)45of which the generatrix shape is inclined in a direction in which the outer diameter becomes smaller as going toward the one side in the axial direction. That is, the screw shaft2dis provided with the concave portion45by removing the incomplete screw portion38included in the screw shaft2in the first example by a cutting process.

In this example, it is possible to more effectively prevent the balls4from being caught between the nut-side ball screw groove20and the incomplete screw portion, thereby preventing an increase in the driving torque of the screw shaft2d. Other configurations and operational effects are the same as in the first example.

THIRD EXAMPLE

FIG.7illustrates a ball screw device1of a third example of an embodiment of the present disclosure.

In this example, a side surface on the one side in the axial direction of a carrier11aof a screw shaft2ais not provided with a projecting portion16(seeFIG.2, etc.) provided in the screw shaft2in the first example. The side surface on the one side in the axial direction of the carrier11ais made to be a flat surface that exists on an imaginary flat plane orthogonal to a center axis of the carrier11a(screw shaft2a). In this example as well, the side surface on the one side in the axial direction of the outer ring25is offset from the side surface on the one side in the axial direction of the carrier11atoward the other side in the axial direction.

In this example, the width dimension in the axial direction of the carrier11a(screw shaft2a) can be shortened. As a result, the size of the ball screw device1can be reduced. Other configurations and operational effects are the same as in the first example.

FOURTH EXAMPLE

FIG.8illustrates a ball screw device1of a fourth example of an embodiment of the present disclosure.

In this example, a carrier11bof a screw shaft2bhas a carrier hollow portion39that is open to a side surface on the one side in the axial direction in a central portion (inner portion) in the radial direction thereof. The carrier hollow portion39has a column-shaped inner space. The inner diameter of the carrier hollow portion39is constant along the axial direction, and it is smaller than the diameter of an imaginary circle passing through end portions on the inside in the radial direction of the plurality of support holes15. A bottom surface40of the carrier hollow portion39is located on the one side in the axial direction of a side surface on the other side in the axial direction of the carrier11b. However, the inner diameter of the carrier hollow portion may also be varied depending on a position in the axial direction. In other words, the carrier hollow portion may also be configured by a stepped hole.

Further, a screw portion10aof the screw shaft2bhas a screw hollow portion41which is elongated in the axial direction and open to an end surface on the other side in the axial direction in a central portion (inner portion) in the radial direction. The screw hollow portion41is arranged coaxially with the carrier hollow portion39and has a column-shaped inner space. The inner diameter of the screw hollow portion41is constant along the axial direction, and it is smaller than the inner diameter of the carrier hollow portion39. However, the inner diameter of the screw hollow portion may also be varied depending on a position in the axial direction. In other words, the screw hollow portion may be configured by a stepped hole.

The screw hollow portion41is formed over the entire length of the screw portion10a, and an end portion on the one side in the axial direction of the screw hollow portion41reaches a side portion on the other side in the axial direction of the carrier11b. In this example, the screw hollow portion41is open to the bottom surface40of the carrier hollow portion39. As a result, the carrier hollow portion39and the screw hollow portion41communicate in the axial direction. However, it is also possible to adopt a structure in which the carrier hollow portion and the screw hollow portion do not communicate in the axial direction.

In this example, since the carrier hollow portion39is formed in the central portion in the radial direction of the carrier11band the screw hollow portion41is formed in the central portion in the radial direction of the screw portion10a, the weight of the screw shaft2bcan be reduced. Further, since the carrier hollow portion39and the screw hollow portion41communicate in the axial direction, the carrier hollow portion39and the screw hollow portion41can be used as a passage for passing lubricating oil, air, and the like. Other configurations and operational effects are the same as in the first example.

As in the first example, the carrier hollow portion39and the screw hollow portion41can be machined after the shaft-side ball screw groove12, the inner ring raceway14, and the support holes15are machined using the screw-side center hole13and the carrier-side center hole19. In other words, when manufacturing the screw shaft2b, the carrier hollow portion39and the screw hollow portion41can be formed after a forging process and before a rolling process by performing a cutting process and a grinding process to the intermediate material35. However, it is also possible to form the carrier hollow portion39and the screw hollow portion41by forming a concave portion having a shape similar to that of the carrier hollow portion39and a recessed portion having a shape similar to that of the screw hollow portion41in the forging process and then adjusting the shapes of the concave portion and the recessed portion by a cutting process.

FIFTH EXAMPLE

FIG.9illustrates an example of a structure in which a ball screw device1of a fifth example an embodiment of the present disclosure and a planetary speed-reducing mechanism8are combined.

In this example, as in the fourth example, a carrier hollow portion39is formed in a carrier11b. However, unlike the fourth example, the screw hollow portion41(seeFIG.8) is not formed in the screw portion10in this example.

In this example, a tip end portion (end portion on the other side in the axial direction) of the motor shaft34aof the electric motor6is inserted inside the carrier hollow portion39. Specifically, of the motor shaft34a, a portion protruding to the other side in the axial direction from the sun gear30is inserted inside the carrier hollow portion39. Further, a radial needle bearing (C&R)42is arranged between the outer peripheral surface of the tip end portion of the motor shaft34aand the inner peripheral surface of the carrier hollow portion39. As a result, the tip end portion of the motor shaft34ais supported so as to be able to rotate freely with respect to the carrier11aby using the radial needle bearing42. However, instead of the radial needle bearing, it is also possible to use a radial roller bearing, a sliding bearing, or the like.

In this example, the tip end portion of the motor shaft34ais supported so as to be able to rotate freely with respect to the carrier11a, so that the motor shaft34acan be supported on both sides. As a result, coaxiality between the motor shaft34aand the screw shaft2ccan be increased, and the meshing accuracy between the sun gear30and the planetary gears31can be increased. Other configurations and operational effects are the same as in the first and the fourth examples.

SIXTH EXAMPLE

FIG.10illustrates a ball screw device1of a sixth example of an embodiment of the present disclosure.

In this example, a side surface on the one side in the axial direction of a carrier11cof a screw shaft2eis a flat surface existing on an imaginary flat plane orthogonal to the center axis of the carrier11c(screw shaft2e) similar to the carrier11aof the third example. Further, in this example, the position in the axial direction of the side surface on the one side in the axial direction of the carrier11cand the position in the axial direction of the side surface on the one side in the axial direction of the outer ring25are aligned. In other words, in this example, the side surface on the one side in the axial direction of the carrier11cand the side surface on the one side in the axial direction of the outer ring25are arranged on the same imaginary flat plane orthogonal to the center axis of the carrier11c(screw shaft2e).

Further, in this example, the position in the axial direction of the side surface on the other side in the axial direction of the carrier11cand the position in the axial direction of the side surface on the other side in the axial direction of the outer ring25are aligned. However, the side surface on the other side in the axial direction of the carrier11ccan be offset from the side surface on the other side in the axial direction of the outer ring25toward the other side in the axial direction or toward the one side in the axial direction. Other configurations and operational effects are the same as in the first and the third examples.

Embodiments according to the present disclosure have been described above, however, the content of the present disclosure is not limited to this, and can be changed as appropriate without departing from the technical idea of the present disclosure. In addition, the structures of each of the examples of an embodiment of the present disclosure can be combined as appropriate and implemented as long as there is no contradiction.

Although a structure is adopted in which the circulation groove is directly formed on the inner peripheral surface of the nut in each example of an embodiment of the present disclosure, it is also possible to form the circulation groove in a circulation component (for example, a top) separate from the nut and fix the circulation component to the nut.

REFERENCE SIGNS LIST

1Ball screw device2,2a,2b,2c,2d,2eScrew shaft3,3aNut4Balls5Rolling bearing6Electric motor7Planetary speed-reducing mechanism8Load path9Circulation groove10,10aScrew portion11,11a,11b,11cCarrier12Shaft-side ball screw groove13Screw-side center hole14Inner ring raceway15Support holes16Projecting portion17Outer diameter side connecting surface18Inner diameter side connecting surface19Carrier-side center hole20Nut-side ball screw groove21Flange portion22Engagement grooves23Housing25Outer ring26Rolling bodies27Cage28Outer ring raceway29Pockets30Sun gear31Planetary gears32Ring gear33Pinion pins34,34aMotor shaft35Intermediate material36Shaft-shaped portion37Disk portion38Incomplete screw portion39Carrier hollow portion40Bottom surface41Screw hollow portion42Radial needle bearing43Cylindrical surface portion44Small-diameter portion45Concave portion100,100aBall screw device101,101aScrew shaft102,102aNut103Screw portion104,104aFitting shaft portion105,105aShaft-side ball screw groove106Housing107Guide rods108Electric motor109Pulley device110Driven pulley111Motor shaft112Drive pulley113Belt114Planetary speed-reducing mechanism115Carrier116Mounting hole117Planetary gears118Support holes119Pinion pins120Sun gear121Ring gear122Rolling bearing123Flange portion124Housing125Nut-side ball screw groove