JP7491722B2 - Multi-stage ball screw unit and electric actuator using the same - Google Patents

Description

The present invention relates to a multi-stage ball screw unit configured by stacking a first ball screw device and a second ball screw device that are in a reverse screw relationship with each other, and an electric actuator configured using the same.

Conventionally, actuators that output linear motion according to the amount of rotation of an electric motor are well known, and in such actuators, a ball screw device is used as a mechanism for converting the rotational motion of the electric motor into axial motion of a first screw shaft. In addition, a multi-stage actuator is also known in which multiple ball screw devices that are in a reverse screw relationship are stacked together to increase the axial stroke amount of the first screw shaft (see, for example, Patent Document 1).

These multi-stage actuators are constructed using ball screw units in which ball screw devices, each consisting of a screw shaft and a nut member screwed thereto, are stacked in a radial direction, and the nut member of the first ball screw device located on the inside is integrated with the screw shaft of the second ball screw device located on the outside to form a compound shaft. In addition, the first ball screw device and the second ball screw device constituting such a multi-stage ball screw unit are in a reverse screw relationship with each other. Therefore, when the rotational motion of the electric motor is transmitted to the compound shaft screwed with the nut member of the second ball screw device in a fixed state, the compound shaft moves in the axial direction while rotating, guided by the nut member. At this time, if the screw shaft of the first ball screw device is held non-rotating, the screw shaft will move in the axial direction in accordance with the rotation of the compound shaft. In other words, the axial movement of the compound shaft and the axial movement of the screw shaft of the first ball screw device relative to the compound shaft overlap, and in this ball screw unit, it is possible to move the screw shaft of the first ball screw device in the axial direction with a large stroke amount relative to the nut member of the second ball screw device.

Special table 2018-511754

In general, the load capacity of a ball screw device for axial loads is determined by the diameter and number of balls arranged in a spiral shape between the nut member and the screw shaft. In this regard, in the multi-stage ball screw unit described above, the first ball screw device and the second ball screw device are radially stacked, so that the diameter of the screw shaft of the first ball screw device located on the inside is naturally smaller than that of the screw shaft of the second ball screw device located on the outside, and as a result, the load capacity of the first ball screw device tends to be smaller than that of the second ball screw device.

In other words, the load capacity of the multi-stage electric actuator is dominated by that of the first ball screw device, and in order to increase the load capacity of the actuator itself, it was necessary to increase the shaft diameter of the screw shaft of the first ball screw device. However, if the shaft diameter of the screw shaft of the first ball screw device located on the inside was increased, the screw shaft of the second ball screw device located on the outside also had a larger diameter, which resulted in an increase in the size of the actuator.

The present invention was developed in consideration of these problems, and its purpose is to provide a multi-stage ball screw unit and an actuator using the same that can reduce the size of the entire device while sufficiently increasing the load capacity of the axial load.

The multi-stage ball screw unit to which the present invention is applied comprises a first screw shaft having a first helical groove on its outer circumferential surface along which the rolling elements roll and a first spline groove formed along the axial direction; a second screw shaft formed in a hollow cylindrical shape covering the first screw shaft from the outside, having a second helical groove on its outer circumferential surface along which the rolling elements roll, the second spline groove formed in the axial direction and wound in the opposite direction to the first helical groove; an inner screw nut provided in the hollow portion of the second screw shaft and screwed into the first helical groove of the first screw shaft via a number of balls; and an outer ball screw nut screwed into the second helical groove of the second screw shaft via a number of balls.

In the first invention that achieves the above object, the number of threads of the first helical groove formed on the first screw shaft is set to be greater than the number of threads of the second helical groove formed on the second screw shaft.

In addition, in a second invention that achieves the above object, the axial pitch of the first spiral groove formed on the first screw shaft is set smaller than the axial pitch of the second spiral groove formed on the second screw shaft.

According to the multi-stage ball screw unit of the present invention, it is possible to increase the number of rolling elements arranged between the first screw shaft and the inner screw nut screwed thereto without increasing the shaft diameter of the first screw shaft, and it is possible to increase the load capacity of the first ball screw device located on the inside to a level comparable to that of the second ball screw device located on the outside. Therefore, by constructing an electric actuator using the ball screw unit of the present invention, it is possible to reduce the size of the entire device while sufficiently increasing the axial load capacity of the electric actuator.

1 is a perspective cross-sectional view showing an example of an embodiment of an electric actuator configured using a ball screw unit of the present invention. FIG. 2 is a perspective cross-sectional view showing a state in which the electric actuator shown in FIG. 1 is used. FIG. 1 is a perspective view showing an example of a ball screw device that can be used as a first ball screw device of an electric actuator. FIG. 4 is a cross-sectional view of a nut member of the ball screw device shown in FIG. 3 . FIG. 11 is a perspective view showing an example of a ball screw device that can be used as a second ball screw device of an electric actuator. 6 is a perspective view showing the inside of a nut member of the ball screw device shown in FIG. 5 .

The multi-stage ball screw unit of the present invention and the electric actuator using the same will be described in detail below with reference to the attached drawings.

Figures 1 and 2 are perspective views showing an electric actuator constructed using the multi-stage ball screw unit (hereinafter referred to as the "ball screw unit") of the present invention. Figure 1 is a perspective cross-sectional view showing the state in which the first screw shaft and the second screw shaft of the ball screw unit are housed in the housing of the electric actuator, and Figure 2 is a perspective cross-sectional view showing the state in which the first screw shaft is given the maximum stroke amount and protrudes from the housing.

The electric actuator has a housing 1 having a base plate 1a and a cylindrical cover member 1b fixed to the base plate 1a, and the ball screw unit and electric motor 7 are housed inside the housing 1.

The ball screw unit includes a first screw shaft 3 that is cylindrical with a hollow portion and has a first helical groove on its outer circumferential surface along which balls roll; a second screw shaft 4 that is cylindrical with a hollow portion that accommodates the first screw shaft 3 and has a second helical groove on its outer circumferential surface along which balls roll; an inner screw nut 5 that is provided in the hollow portion of the second screw shaft 4 and screws onto the first screw shaft 3 via a number of balls; and an outer screw nut 6 that is fixed to the cover member 1b of the housing 1 and screws onto the second screw shaft 4 via a number of balls.

The cover member 1b of the housing 1 has a two-stage cylindrical shape in which a large diameter cylindrical portion 11 and a small diameter cylindrical portion 12 are connected, and a chamber for accommodating the electric motor 7 is formed in the housing 1 by fixing the base plate 1a to the open end of the large diameter cylindrical portion 11. In addition, each part constituting the ball screw unit is stored in the small diameter cylindrical portion 12 of the cover member 1b. The inner diameter of the small diameter cylindrical portion 12 is set slightly larger than the outer diameter of the second screw shaft 4, and the second screw shaft 4 can move in the axial direction while freely rotating inside the small diameter cylindrical portion 12 without interfering with the housing 1. An opening 13 is provided at the tip of the small diameter cylindrical portion 12 opposite the large diameter cylindrical portion 11, and the second screw shaft 4 and the first screw shaft 3 are configured to advance from the housing 1 to the outside through the opening 13.

A rod-shaped first joint member 2 is fixed to the base plate 1a of the housing 1. One axial end of this first joint member 2 is fixed to the base plate 1a, and the other end is inserted into the hollow portion of the first screw shaft 3 and is positioned at the center of the cylindrically formed cover member 1b. The first joint member 2 also penetrates the housing 1, and the end opposite the base plate 1a approximately matches the opening 13 of the cover member 1b.

The first joint member 2 and the first screw shaft 3 located outside thereof constitute a ball spline device. A plurality of spline grooves are provided along the axial direction on the outer peripheral surface of the first joint member 2 and the inner peripheral surface of the first screw shaft 3, and a large number of balls are arranged in these spline grooves. That is, the first joint member 2 and the first screw shaft 3 are spline-coupled via a large number of balls that roll in the spline grooves, and the first screw shaft 2 is freely movable in the axial direction of the first joint member 2 relative to the first joint member 2 fixed to the housing 1, but cannot rotate in the circumferential direction of the first joint member 2. That is, the first joint member 2 functions as a rotation stopper for the first screw shaft 3. In the present embodiment shown in FIG. 1, balls are interposed between the first joint member 2 and the first screw shaft 3 as rolling bodies, but rollers may be used instead of balls. Also, the first joint member 2 and the first screw shaft 3 may be spline-coupled by sliding contact.

The first screw shaft 3 and the inner screw nut 5 constitute a first ball screw device. Specifically, a first helical groove 30 in which balls roll is formed on the outer circumferential surface of the first screw shaft 3, and the inner screw nut 5 is screwed onto the first screw shaft 3 via a large number of balls arranged in the first helical groove 30. Since the first screw shaft 3 is prevented from rotating by the first joint member 2, when the inner screw nut 5 is rotated, the first screw shaft 3 moves in the axial direction relative to the inner screw nut 5.

Figure 3 shows an example of a ball screw device that can be used as the first ball screw device, and is composed of a screw shaft 3A having a rolling groove formed as a first spiral groove 30 on its outer circumferential surface, and a cylindrical nut member 3B that is screwed onto the screw shaft 3A via a number of balls. The screw shaft 3A corresponds to the first screw shaft 3, and the nut member 3B corresponds to the internally threaded nut 5.

The screw shaft 3A is a so-called multiple thread, and three rolling grooves 30a, 30b, and 30c are formed on the outer circumferential surface of the screw shaft 3A, and these rolling grooves 30a, 30b, and 30c have the same lead. Therefore, the rolling grooves 30a, 30b, and 30c are evenly spaced at equal intervals along the axial direction on the outer circumferential surface of the screw shaft 3A with a pitch of 1/3 of the lead. In other words, the pitch of the rolling groove in the axial direction is the value obtained by dividing the lead of the rolling groove by the number of threads of the rolling groove. In addition, between adjacent rolling grooves 30a, 30b, and 30c is a thread portion 31, and the thread portion 31 indicates the outer diameter of the screw shaft 3A. Note that the number of threads of the rolling grooves formed on the screw shaft 3A may be appropriately changed as long as it is two or more.

The nut member 3B is formed in a cylindrical shape with a through hole through which the screw shaft 3A passes, and an infinite circulation path for the balls is provided on the inner peripheral surface of the through hole. The balls are interposed between the screw shaft 3A and the nut member 3B, and by rotating the nut member 3B relative to the screw shaft 3A, the screw shaft 3A moves in the axial direction of the nut member 3B. The infinite circulation path goes around the screw shaft 3A, and in the example shown in FIG. 3, three infinite circulation paths are provided for the nut member 3B. The number of circuits of the infinite circulation path can be appropriately designed depending on the axial length of the nut member 3B and the required load bearing capacity.

Figure 4 shows a cross-sectional view of the nut member cut along one of the infinite circulation paths. An infinite circulation path 32 that goes around the screw shaft 3A is provided on the inner peripheral surface of the nut member 3B, and a large number of balls 3C are arranged in the infinite circulation path 32. The infinite circulation path 32 corresponds to the multiple rolling grooves 30a, 30b, and 30c provided on the screw shaft 3A. As described above, if three rolling grooves 30a, 30b, and 30c are provided on the screw shaft 3A, each infinite circulation path 32 is composed of three loaded parts 33 and three unloaded parts 34. Each loaded part 33 faces one of the rolling grooves 30a, 30b, and 30c of the screw shaft 3A, and the balls 3c roll between the loaded part 33 and the rolling grooves 30a, 30b, and 30c of the screw shaft 3A while applying a load. On the other hand, the unloaded portions 34 connect the loaded portions 33 to each other and face the threaded portion 31 of the screw shaft 3A, and the balls 3c rolling on the unloaded portions 34 are released from the load and overcome the threaded portion 31 of the screw shaft 3A. The loaded portions 33 and the unloaded portions 34 are alternately arranged along the circumferential direction of the nut member 3B, and as a result, the infinite circulation path 32 forms a ring that goes around the periphery of the screw shaft 3A. In the embodiment shown in FIG. 4, the unloaded portions 34 are present at three locations within the infinite circulation path 32, and therefore the unloaded portions 34 are arranged at equal intervals in the circumferential direction of the nut member 3B.

The rolling grooves 30a, 30b, and 30c of the screw shaft 3A advance one lead in the axial direction when going around the screw shaft 3A once, but the infinite circulation path 32 includes three combinations of the loaded section 33 and the unloaded section 34, which are arranged in three equal parts of 120 degrees each around the screw shaft 2, so that the ball 3c that rolls in one loaded section 33 advances 1/3 of the lead in the axial direction of the screw shaft 3A.

Therefore, in the ball screw device shown in Figures 3 and 4, the axial length of each infinite circulation path 32 can be set small relative to the lead of each rolling groove 30a, 30b, 30c, i.e., the width of the infinite circulation path 32 along the axial direction of the screw shaft 3A. In a multiple-thread type ball screw device in which the lead of the rolling grooves 30a, 30b, 30c is set large, the axial length of the nut member 3B can be shortened, making it possible to reduce the size of the device.

The unloaded portion 34 is formed in a block-shaped deflector 35. A through hole for mounting the deflector 35 is formed in the nut member 3B from the outer peripheral surface to the inner peripheral surface, and the unloaded portion 34 is provided on the inner peripheral surface of the nut member 3B by fitting the deflector 35 into the through hole, completing the infinite circulation path 32 for the balls 3C.

The infinite circulation path 32 of the balls 3C is completed simply by fitting the deflector 35 to the nut member 3B, and since there are no protrusions on the outer peripheral surface of the nut member 3B, it is possible to keep the outer diameter of the nut member 3B small. Therefore, if the structure of the nut member 3B is applied to the internally threaded nut 5, it is possible to keep the outer diameter of the second threaded shaft 4 with the internally threaded nut 5 housed in the hollow portion small.

On the other hand, the hollow portion of the second screw shaft 4 is divided into two in the axial direction, one of which is used as a housing for the inner threaded nut, and the other is used as a housing for the second joint member 8 described later. The housing for the second joint member 8 is located on the large diameter cylindrical portion side of the cover member 1b, and the housing for the inner threaded nut 5 is located on the opening 13 side of the cover member 1b.

A second joint member 8 for transmitting the rotation of the electric motor 7 to the second screw shaft 4 is provided inside the housing 1. The second joint member 8 is composed of a base end portion 81 held in the housing 1 via a pair of angular contact bearings 80, and a rotation transmission portion 82 formed in a cylindrical shape with a thinner wall than the base end portion 81 and inserted into the gap between the outer circumferential surface of the first screw shaft 3 and the inner circumferential surface of the second screw shaft 4. The rotor of the electric motor 7 is fixed to the outer circumferential surface of the base end portion 81, and when electricity is applied to the electric motor 7, the second joint member 8 rotates relative to the housing 1.

The second joint member 8 and the second screw shaft 4 located outside thereof constitute a ball spline device. A plurality of spline grooves are provided along the axial direction on the outer peripheral surface of the rotation transmission part 82 of the second joint member 8 and the inner peripheral surface of the second screw shaft 4, and a large number of balls are arranged in these spline grooves. That is, the second joint member 8 and the second screw shaft 4 are spline-coupled via a large number of balls rolling in the spline grooves, and the second screw shaft 4 is freely movable in the axial direction of the second joint member 8 relative to the rotation transmission part 82 of the second joint member 8, but cannot rotate in the circumferential direction of the second joint member 8. In the present embodiment shown in FIG. 1, balls are interposed between the second joint member 8 and the second screw shaft 4 as rolling bodies, but rollers may be used instead of balls. The second joint member 8 and the second screw shaft 4 may also be spline-coupled by sliding contact.

Therefore, when the electric motor 7 rotates the second joint member 8, the second screw shaft 4 is rotated by the second joint member 8 and rotates relative to the housing 1, and the inner screw nut 5 fixed to the second screw shaft 4 also rotates relative to the housing 1.

On the other hand, the outer screw nut 6 is fixed to the inner peripheral surface side of the small diameter cylindrical portion 12 of the housing 1. This outer screw nut 6 cooperates with the second screw shaft 4 to form a second ball screw device. Specifically, a second helical groove 40 in which balls roll is formed on the outer peripheral surface of the second screw shaft 4, and the outer screw nut 6 is screwed onto the second screw shaft 4 via a large number of balls arranged in the second helical groove 40. The second helical groove formed on the outer peripheral surface of the second screw shaft 4 has a reverse screw relationship with the first helical groove formed on the outer peripheral surface of the first screw shaft 3.

In the embodiment shown in Figures 1 and 2, the outer threaded nut 6 is fixed to the inner circumferential surface of the small diameter cylindrical portion 12 of the housing 1, but the outer threaded nut 6 may be formed integrally with the small diameter cylindrical portion 12.

As described above, the second screw shaft 4 rotates together with the second joint member 8 relative to the housing 1, so that when rotation is applied to the second screw shaft 4, the second screw shaft 4 moves in the axial direction relative to the outer screw nut 6 fixed to the housing 1. The second screw shaft 4 is supported by the second joint member 8, and such axial movement is permitted. Therefore, when rotation is applied to the second screw shaft 4 by the electric motor 7, the second screw shaft 4 moves in the axial direction while rotating relative to the housing 1, and as a result, the second screw shaft 4 moves in a spiral shape relative to the housing 1.

Figure 5 shows an example of a ball screw device that can be used as the second ball screw device, and is composed of a screw shaft 4A having a ball rolling groove formed in a spiral shape as a first helical groove 40 on the outer circumferential surface, and a cylindrical nut member 4B screwed onto the screw shaft 4A via a large number of balls. The screw shaft 4A corresponds to the second screw shaft 4, and the nut member 4B corresponds to the outer ball screw nut 6.

A ball rolling groove 40 is formed on the outer circumferential surface of the screw shaft 4A with a predetermined lead. In addition, between adjacent rolling grooves 40 is a thread portion 41, and the thread portion 41 indicates the outer diameter of the screw shaft 4A.

The nut member 4B is formed in a cylindrical shape with a through hole through which the screw shaft 4A is inserted, and an infinite circulation path that goes around the screw shaft 4A is provided on the inner peripheral surface of the through hole, and a large number of balls 4C are arranged in the infinite circulation path. The nut member 4B has three infinite circulation paths for the balls 4C, and cover plates 43 for constructing unloaded paths for the balls 4C are attached to the outer peripheral surface of the nut member 4B in three places. The number of circuits of the infinite circulation path may be appropriately selected according to the load-bearing capacity required for the nut member 4B. In addition, in order to clearly show that the balls 4C are arranged in the infinite circulation path, FIG. 5 shows the state in which the cover plate 43 is removed from the second infinite circulation path of the three infinite circulation paths provided in the nut member 4B.

The nut member 4B is formed with a ball path 44 that forms part of the infinite circulation path, and the ball path 44 can be seen from the outside by removing the cover plate 43 from the nut member 4B. The ball path 44 extends in the axial direction of the nut member 4B and penetrates the nut member 4B from the outer peripheral surface to the inner peripheral surface, and can be easily formed by cutting using an end mill, for example. The cover plate 43 can be formed by injection molding of synthetic resin. Then, the ball path 44 is closed from the outside with the cover plate 43, completing the infinite circulation path for the balls 4C.

Figure 6 shows the balls 4C arranged in the endless circulation path of the nut member 4B, as viewed from the through hole of the nut member 4B. A spiral load rolling groove 45 is formed on the inner peripheral surface of the nut member 4B, facing the rolling groove 40 of the screw shaft 4A, and the balls 4C roll while applying a load between the rolling groove 40 of the screw shaft 4A and the load rolling groove 45 of the nut member 4B. When the balls 4C roll in the load rolling groove 45 of the nut member 4B and reach the formation position of the ball transfer path 44, they gradually sink into the ball transfer path 44, and roll in the ball transfer path 44 in an unloaded state.

As shown in the figure, all the balls 4C arranged in the ball path 44 are exposed to the inner peripheral surface of the nut member 4B, and the balls 4C rolling in the ball path 44 are always in contact with the screw shaft 4A. In this way, the balls 4C rolling in the ball path 44 under no load always face the second screw shaft 4, so the outer diameter of the nut member 4B can be kept small, and by applying the structure of the nut member 4B to the outer screw nut 6, the outer diameter of the outer screw nut 6 can be kept small, and as a result, the outer diameter of the small diameter cylindrical portion 12 of the housing 1 that houses the outer screw nut 6 can also be kept small.

Next, we will explain the operation of the electric actuator shown in Figure 1.

In the state shown in FIG. 1, i.e., when the second screw shaft 4 and the first screw shaft 3 are housed in the housing 1, when the electric motor 7 is energized and the electric motor 7 is rotated, the second joint member 8 rotatably supported relative to the housing 1 is rotated. When the second joint member 8 rotates, the second screw shaft 4 is rotated by the second joint member 8 and rotates inside the cover member 1b of the housing 1. In addition, the outer screw nut 6 is fixed to the cover member 1b, and the outer screw nut 6 is screwed onto the second screw shaft 4, so that the second screw shaft 4 moves in the axial direction relative to the outer screw nut 6 as it rotates. In other words, when the electric motor 7 is energized, the second screw shaft 4 advances out of the housing 1 from the opening 13 of the cover member 1b while moving in a spiral manner, as shown in FIG. 2.

On the other hand, when the second screw shaft 4 rotates, the inner screw nut 5 fixed to the second screw shaft 4 also rotates. At this time, the first screw shaft 3 constituting the first ball screw together with the inner screw nut 5 is prevented from rotating in the circumferential direction by the first joint member 2, so when the inner screw nut 5 rotates, the first screw shaft 3 moves in the axial direction relative to the inner screw nut 5. In other words, when the second screw shaft 4 rotates, the first screw shaft 3 moves in the axial direction relative to the second screw shaft 4.

The second rolling groove 40 formed on the outer peripheral surface of the second screw shaft 4 and the first rolling groove 30 formed on the outer peripheral surface of the first screw shaft 3 have opposite spiral winding directions, so when the second screw shaft 4 is rotated by the electric motor 7, the axial movement direction of the second screw shaft 4 and the axial movement direction of the first screw shaft 3 are the same. Therefore, when the electric motor 7 is driven, as shown in FIG. 2, the first screw shaft 3 advances further from the tip of the second screw shaft 4 that has advanced from the housing 1, and it becomes possible to move the first screw shaft 3 in the axial direction within a range in which the first joint member 2 does not come out of the hollow portion of the first screw shaft 3, and the stroke amount of the first screw shaft 3 relative to the housing 1 can be set large. In other words, the electric actuator of this embodiment is a multi-stage type.

On the other hand, when the rotation direction of the electric motor 7 is reversed, the axial movement direction of the second screw shaft 4 and the first screw shaft 3 is reversed, and as shown in FIG. 1, the first screw shaft 3 is pulled into the hollow portion of the second screw shaft 4, and the second screw shaft 4 is further pulled into the inside of the cover member 1b of the housing 1, and the first screw shaft 3 and the second screw shaft 4 are stored inside the housing 1 in a stacked state.

In the electric actuator of this embodiment configured as described above, in order to increase the speed at which the second screw shaft 4 advances from the housing 1 and is accommodated in the housing 1, the lead of the second spiral groove 40 formed on the outer peripheral surface of the second screw shaft 4 is set to be larger than the shaft diameter of the second screw shaft 4. In addition, in order to increase the advancement speed of the first screw shaft 3 relative to the second screw shaft 4, the lead of the first spiral groove 30 formed on the outer peripheral surface of the first screw shaft 3 is set to be approximately the same as the lead of the second spiral groove 40.

On the other hand, if the lead of the first spiral groove 30 and the lead of the second spiral groove 40 are set to be approximately the same, the shaft diameter of the first screw shaft 3 is smaller than that of the second screw shaft 4, so that the load capacity of the first ball screw device formed by the first screw shaft 3 and the inner screw nut 5 becomes smaller than that of the second ball screw device formed by the second screw shaft 4 and the outer screw nut 6, and there is a concern that the load capacity of the electric actuator as a whole will decrease. This is more noticeable the more the lead of the first spiral groove 30 formed on the outer peripheral surface of the first screw shaft 3 is set to be larger in order to increase the advance speed of the first screw shaft 3.

In this regard, in the electric actuator of this embodiment, the first spiral groove 30 is formed as a multiple-thread groove on the first screw shaft 3. Therefore, even if the lead of the first spiral groove 30 is substantially the same as the lead of the second spiral groove 40 of the second screw shaft 4, the number of balls arranged between the first screw shaft 3 and the inner screw nut 5 can be increased to the same extent as the number of balls arranged between the second screw shaft 4 and the outer screw nut 6.

As a result, the load capacity of the first ball screw device located on the radially inner side can be increased to the same level as that of the second ball screw device located on the outer side, making it possible to increase the load capacity of the entire electric actuator without increasing the shaft diameter of the first screw shaft 3.

In addition, since the first spiral groove 30 is formed as a multiple thread on the first screw shaft 3, even if the lead of the first spiral groove 30 is approximately the same as the lead of the second spiral groove 40 of the second screw shaft 4, the axial pitch of the first spiral groove 30 on the first screw shaft 3 is smaller than the axial pitch of the second spiral groove 40 on the second screw shaft 4. In this respect, the number of balls arranged between the first screw shaft 3 and the inner screw nut 5 can be increased to the same extent as the number of balls arranged between the second screw shaft 4 and the outer screw nut 6, and the load capacity of the first ball screw device located on the radial inside can be increased to the same extent as that of the second ball screw device located on the outside.

As explained above, by constructing an electric actuator using the multi-stage ball screw unit of the present invention, it is possible to increase the load capacity of the first ball screw to the same level as that of the second ball screw device located on the outside, without setting a large shaft diameter for the screw shaft 3 of the first ball screw device located on the inside, with respect to the first and second ball screw devices stacked radially, and it is possible to realize a small electric actuator with a large stroke amount and fast expansion/contraction speed.

In the ball screw unit of the present invention, the lead of the first spiral groove 30 and the lead of the second spiral groove 40 formed on the first screw shaft 3 may be changed as appropriate depending on the stroke amount and moving speed required for the first screw shaft 3 as an output shaft and the load bearing capacity required for the first screw shaft 3. In addition, the lead of the first spiral groove 30 and the lead of the second spiral groove 40 do not have to be approximately the same.

1...Housing, 2...First joint member, 3...First screw shaft, 4...Second screw shaft, 5...Inner screw nut, 6...Outer screw nut, 7...Electric motor, 8...Second joint member

Claims (3)

a first screw shaft having a first spiral groove in which a rolling element rolls formed on an outer peripheral surface and a spline groove formed along an axial direction;
A second screw shaft formed in a hollow cylindrical shape covering the first screw shaft from the outside, and having a second spiral groove in which a rolling element rolls formed in a counter-winding manner to the first spiral groove on an outer circumferential surface thereof and having a spline groove formed along an axial direction;
An internally threaded nut provided in a hollow portion of the second threaded shaft and threadedly engaged with a first spiral groove of the first threaded shaft via a plurality of balls;
an outer ball screw nut that is screwed into the second spiral groove of the second screw shaft via a number of balls;
Equipped with
A multi-stage ball screw unit, characterized in that the number of threads of the first spiral groove formed on the first screw shaft is greater than the number of threads of the second spiral groove formed on the second screw shaft. 2. The multi-stage ball screw unit according to claim 1, wherein an axial pitch of the first spiral groove formed in the first screw shaft is smaller than an axial pitch of the second spiral groove formed in the second screw shaft. 3. An electric actuator using the multi-stage ball screw unit according to claim 1 or 2 ,
a housing to which the outer threaded nut is fixed;
A first joint member that is fixed to the housing and is spline-coupled to the first screw shaft to prevent rotation of the first screw shaft around its axis and allow movement of the first screw shaft in its axial direction;
an electric motor that rotates the second screw shaft;
A second joint member that is spline-coupled to the second screw shaft, transmits rotation of the electric motor to the second screw shaft, and allows movement of the second screw shaft in the axial direction;
An electric actuator comprising:

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