Ball screw device

Abstract

To provide a ball screw device that enables an increased number of balls to be arranged in a nut load range without complicating its structure, and has an elongated life at low cost.SOLUTION: A ball screw device 100 comprises: a screw shaft 11 having a first screw groove 17; a nut 13 having a second screw groove 19; and a plurality of balls 15 accommodated in a rolling path 21. The screw shaft 11 has small-diameter parts 23 at both ends of the first screw groove, and the nut 13 has nut unloaded range parts A2 at both ends of a nut loaded range part A1. Also, extension screw grooves 33 are formed in the nut unloaded range parts A2 while extending from the second screw groove 19. In an annular space between the nut unloaded range parts A2 and the small-diameter parts 23, there are coil springs 37A, 37B with one axial end fixed to the screw shaft 11 and the other end arranged toward the first screw groove 17. The coil springs 37A, 37B generate elastic forces for returning the balls 15 being pushed out of the rolling path 21 into the rolling path 21.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a ball screw device. [Background technology]

[0002] A ball screw device that converts rotational motion into linear motion is widely used. This ball screw device includes a nut with a helical screw groove formed on its inner circumferential surface, a screw shaft with a helical screw groove formed on its outer circumferential surface, a plurality of balls arranged in a rolling path formed between the screw grooves of the nut and the screw shaft, and a circulation unit including a circulation part that circulates the balls from one end of the rolling groove in the load area to the other end. Among the above-mentioned components, the circulation part in particular has a complex shape and processing, and the assembly work is complicated, and it is also a costly and relatively prone to failure. For this reason, in recent years, a ball screw device (e.g., Patent Documents 1 and 2) has been proposed that can achieve high-efficiency power transmission without a circulation unit, assuming use in an environment with a short operating stroke and low demand for positional accuracy.

[0003] In the ball screw device described in Patent Document 1, rod members are housed at both ends of the nut, and springs are arranged in a spiral position between each rod member and one end of the ball groove portion. The springs are fixed in the nut by each rod member, so that the balls can roll within a marginal stroke that occurs when the springs are compressed. With this configuration, there is no need to circulate the balls, and no need to provide a return unit.

[0004] In the technology described in Patent Document 2, the balls that roll through a helical member and a disk member arranged along a screw groove are pressed by a spring arranged at the end of the shaft, and the balls are allowed to roll within a marginal stroke generated by the compression of the spring. This configuration also does not require the balls to roll circulatingly, and does not require a return unit. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 1,905,039 [Patent Document 2] JP 2020-76486 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the structure of Patent Document 1, a long and thin spring is housed in a spiral thread groove. In such a structure in which a long and thin spring is arranged in a spiral shape, the spring does not shrink evenly over its entire length, and there is a problem that early fatigue failure occurs partially due to the variation in deformation. In addition, since the base of the spring is fixed and the rod member is fixed to the nut, it is necessary to process a long and thin hole in the nut, which increases the processing cost.

[0007] In addition, in the structure of Patent Document 2, the balls in the rolling path must be pushed through a helical member (coil spring) inserted along the helical screw groove, which reduces the number of loaded balls in the rolling path and reduces the load capacity. In addition, friction caused by the sliding of the helical member is large, which may lead to abnormal heating of the ball screw device depending on the usage conditions, and the large number of parts makes it disadvantageous in terms of cost.

[0008] Furthermore, if the screw grooves of the structures shown in Patent Documents 1 and 2 above are changed to, for example, two helical grooves, there is a risk of malfunction due to variations in the expansion and contraction of the springs arranged in the two circuits of the double helix, or variations in the sliding of the helical members.

[0009] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a non-circulating ball screw device that can increase the number of balls placed in the nut load zone without complicating the structure, and that is low-cost and has a long life. [Means for solving the problem]

[0010] The present invention comprises the following configurations. A ball screw device comprising: a screw shaft having a first helical screw groove on an outer peripheral surface; a nut having a second helical screw groove on an inner peripheral surface and fitted onto the screw shaft; and a plurality of balls accommodated in a rolling path formed by the first screw groove and the second screw groove, The screw shaft has small diameter portions at both ends of the first screw groove, each having the same outer diameter as the groove bottom of the first screw groove, The nut has a nut non-loaded area portion at both ends of the nut loaded area portion in which the second thread groove is formed, and an inner circumferential surface having an inner diameter shallower than the groove depth of the second thread groove is formed at each end of the nut loaded area portion in which the second thread groove is formed. The nut non-loaded area portion further has an extension thread groove having the same inner diameter as the groove bottom of the second thread groove, which extends from the second thread groove. A coil spring is provided in each of the annular spaces between the nut non-load area portion and the small diameter portion at both axial ends of the first screw groove, the coil spring having one axial end fixed to the screw shaft and the other end disposed toward the first screw groove, The coil spring generates an elastic force that pushes back, into the rolling path, the ball that is pushed out of the rolling path when the screw shaft and the nut rotate relative to each other. Ball screw device. Effect of the Invention

[0011] According to the present invention, it is possible to provide a non-circulating ball screw device that can increase the number of balls arranged in the nut load zone without complicating the structure, and that is low-cost and has a long life. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic configuration diagram showing, in partial cross section, a main part of a ball screw device according to a first embodiment. [Diagram 2] FIG. 2 is a side view seen from a direction V1 shown in FIG. [Diagram 3] FIG. 3 is a perspective view of the screw shaft. [Figure 4] FIG. 4 is a cross-sectional perspective view of the nut, showing the nut cut in the axial direction. [Diagram 5] FIG. 5 is an enlarged cross-sectional view of a portion P shown in FIG. [Figure 6] FIG. 6 is a perspective view of the annular member. [Figure 7] 7 is a cross-sectional view of the annular member taken along line VII-VII shown in FIG. [Figure 8] FIG. 8 is an explanatory diagram showing, in comparison, axial cross sections of the first thread groove and the second thread groove in the nut loaded zone and the small diameter portion and the extended thread groove in the nut non-loaded zone. [Figure 9] FIG. 9 is a schematic explanatory view showing, in an enlarged axial cross section, a state in which balls are in contact with the screw shaft and the nut in the nut loaded zone and the nut non-loaded zone shown in FIG. [Figure 10] FIG. 10 is an explanatory diagram showing how the screw shaft is rotated to move the nut to the right and left from the initial state of the ball screw device. [Figure 11] FIG. 11 is an explanatory diagram showing how the screw shaft is rotated to move the nut to the right and left from the initial state of the ball screw device. [Figure 12] FIG. 12 is a schematic configuration diagram showing, in partial cross section, a main part of a ball screw device according to a second embodiment. [Figure 13] FIG. 13 is an explanatory diagram showing a contact position between the annular member and the ball in a cross section taken along line VIII-VIII shown in FIG. [Figure 14] FIG. 14 is an explanatory diagram showing a state in which two balls come into contact with an annular member. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. First Embodiment FIG. 1 is a schematic diagram showing a main part of a ball screw device 100 of a first embodiment in a partial cross section. FIG. 2 is a side view seen from a V1 direction shown in FIG. 1. As shown in FIG. 1, the ball screw device 100 includes a screw shaft 11, a nut 13, and a plurality of balls 15. The screw shaft 11 has a first helical screw groove 17 on its outer circumferential surface. The nut 13 has a second helical screw groove 19 on its inner circumferential surface and is fitted onto the screw shaft 11. The plurality of balls 15 are accommodated in a rolling path 21 formed by the first screw groove 17 and the second screw groove 19. As also shown in FIG. 2, the nut 13 has a cylindrical portion 13a and a flange portion 13b provided at one axial end of the cylindrical portion 13a.

[0014] 3 is a perspective view of the screw shaft 11. The screw shaft 11 has small diameter portions 23 on both ends of the first screw groove 17. The outer diameter of the small diameter portions 23 is the same as or slightly smaller than the groove bottom of the first screw groove 17. In addition, the small diameter portion 23 is formed with a locking groove 27 for fixing a C-shaped retaining ring 25 shown in FIGS. 1 and 2.

[0015] Fig. 4 is a cross-sectional perspective view of the nut 13 cut in the axial direction. Fig. 5 is an enlarged cross-sectional view of the P portion shown in Fig. 4. In the nut 13, the region of the central inner circumferential surface 29 where the second screw groove 19 is formed is the nut loaded area A1, and the regions at both ends of the nut loaded area A1 where the end inner circumferential surfaces 31 having an inner diameter shallower than the groove depth of the second screw groove 19 are formed are the nut non-loaded area A2. In other words, the end inner circumferential surfaces 31 in the nut non-loaded area A2 have a larger diameter than the central inner circumferential surface 29 in the nut loaded area A1.

[0016] In the nut non-loaded zone A2, an extended thread groove 33 having a groove bottom with the same inner diameter as the groove bottom of the second thread groove 19 is formed by further extending from the second thread groove 19. That is, the second thread groove 19 and the extended thread groove 33 have different groove wall heights from the groove bottom, but are continuously connected by the same spiral. As a result, the ball 15 pushed out from the nut loaded zone A1 is guided between the second thread groove 19 and the extended thread groove 33 while smoothly rolling.

[0017] Specifically, as shown in Fig. 5, the groove bottom height, which is the radial length from the groove bottom of the second screw groove 19 (in the example of Fig. 5, the groove bottom of the extended screw groove 33 is also the same) to the axis Axb passing through the center of the ball 15 shown by the dashed line, is defined as H, and the groove wall height, which is the radial length from the groove bottom of the extended screw groove 33 to the end inner circumferential surface 31 of the nut non-load zone portion A2, is defined as h. In this case, it is preferable that the relationship between H and h satisfies formula (1). h ≧ H / 3 (1) By making the groove wall height h satisfy the above formula (1), the extended screw groove 33 can smoothly guide the ball 15 along the extended screw groove 33 without applying load to the ball 15 from any source other than the coil springs 37A and 37B.

[0018] 1, coil springs 37A and 37B are provided in annular spaces 35 between the small diameter portions 23 of the screw shaft 11 and the end inner circumferential surface 31 of the nut 13 in the pair of nut non-load zones A2. Annular members 39 are disposed between the ends of the first screw groove 17, i.e., the ends of the rolling path 21, and the coil springs 37A and 37B.

[0019] 1, the coil springs 37A and 37B are disposed in a natural length or in a slightly compressed state between the retaining ring 25 engaged in the retaining groove 27 of the small diameter portion 23 and the annular member 39. The coil springs 37A and 37B, the retaining ring 25, and the annular member 39 are disposed coaxially with the end inner circumferential surface 31 of the nut 13 without interfering with each other.

[0020] FIG. 6 is a perspective view of the annular member 39, and FIG. 7 is a cross-sectional view of the annular member 39 taken along line VII-VII in FIG. 6. The annular member 39 is preferably made of a material having rigidity, such as metal or resin, and the ball 15 abuts against one side surface 39a, and the coil springs 37A and 37B abut against the other side surface 39b. The side surface 39a, which the ball 15 abuts against, is formed as a concave curved surface along the curved surface of the ball 15, for example, a concave spherical surface. The other side surface 39b is a flat surface. The inner peripheral surface of the annular member 39 desirably has an inner diameter that is larger in the radial direction by 10 μm to 100 μm than the outer peripheral surface of the opposing screw shaft 11. In this case, the force received from the ball 15 prevents the annular member 39 from tilting from the axis, and the sliding resistance of the axial movement is reduced. In addition, the above-mentioned tilt can also be prevented by widening the axial width of the inner peripheral surface of the annular member 39.

[0021] When the screw shaft 11 and the nut 13 rotate relative to each other, the screw shaft 11 and the nut 13 move relative to each other in the axial direction, and the balls 15 in the rolling path 21 are pushed out from one end of the rolling path 21. The pushed-out balls 15 urge the annular member 39 toward the outside of the nut 13. The coil springs 37A and 37B, which are pressed by the annular member 39 and contract, generate an elastic force that pushes the balls 15 pushed out of the rolling path 21 back into the rolling path 21.

[0022] FIG. 8 is an explanatory diagram showing the axial cross section of the first screw groove 17 and the second screw groove 19 in the nut loaded area A1, and the small diameter portion 23 and the extended screw groove 33 in the nut non-loaded area A2 in comparison. FIG. 9 is a schematic explanatory diagram showing an enlarged axial cross section of the state in which the ball 15 contacts the screw shaft 11 and the nut 13 in each of the nut loaded area A1 and the nut non-loaded area A2 shown in FIG. 8. Focusing on the area Q including the contact position of the ball 15 and the screw shaft 11 shown in FIG. 8, as shown in FIG. 9, the first screw groove 17 and the second screw groove 19 in the nut loaded area A1 are both formed as V grooves. In that case, in the nut loaded area A1, the ball 15 rolls sandwiched between the V groove of the first screw groove 17 and the V groove of the second screw groove 19, and the ball 15 is disposed in a position slightly above the groove bottom P1 of the first screw groove 17 without contacting the groove bottom P1. In that case, the ball 15 rolls while maintaining a constant contact angle.

[0023] On the other hand, in the nut non-load zone A2, the outer circumferential surface of the small diameter portion 23 is formed to be equal to or slightly smaller than the radial height of the groove bottom P1 of the first screw groove 17. Therefore, the ball 15 either contacts the small diameter portion 23 or contacts the extended screw groove 33 in which the groove bottom P3 is formed at the same radial height as the groove bottom P2 of the second screw groove 19. That is, a radial gap is generated between the ball 15 and the screw shaft 11, or between the ball 15 and the nut 13. FIG. 9 shows the latter case in which a gap δ is generated between the ball 15 and the nut 13. In that case, the ball 15 rolls while being guided by the extended screw groove 33, and no load on the ball screw device 100 other than the axial force from the coil springs 37A and 37B is applied to the ball 15.

[0024] Next, the operation of the ball screw device 100 having the above configuration will be described. 10 is an explanatory diagram showing how the screw shaft 11 is rotated to move the nut 13 to the right and left from the initial state of the ball screw device 100. The initial state here refers to a state in which the ball screw 11 and the nut 13 are disposed in the center of the axial stroke in which they move relative to each other, and this state is defined as the initial position of the nut 13. The operation of moving the nut 13 to the right in FIG. 10 is called the right stroke, and the operation of moving the nut 13 to the left is called the left stroke.

[0025] In the initial position of the nut 13, the coil springs 37A, 37B are assembled in a natural length state or a slightly compressed state. When the screw shaft 11 is rotated clockwise R1 from this state to perform a right stroke, the nut 13 moves a distance L1 to the right relative to the screw shaft 11, and the ball 15 is pushed out from the rolling path 21 toward the coil spring 37B in the left nut non-load zone A2. The pushed-out ball 15 moves along the spiral of the extended screw groove 33 and compresses the coil spring 37B in the axial direction via the annular member 39. Meanwhile, the right coil spring 37A is maintained in its initial position.

[0026] Furthermore, when the screw shaft 11 is rotated in the counterclockwise direction R2 to perform a left stroke operation, the nut 13 moves a distance L2 (L2=2L1) to the left relative to the screw shaft 11, and the ball 15 is pushed out from the rolling path 21 toward the coil spring 37A in the right-side nut non-load zone A2. The pushed-out ball 15 moves along the helix of the extended screw groove 33 and compresses the coil spring 37A in the axial direction via the annular member 39. Meanwhile, the left-side coil spring 37A has the ball 15 pushed back into the rolling path 21 by the elastic force of the compressed coil spring 37B, returning it to its initial position.

[0027] In this way, according to the ball screw device 100 of this configuration, the nut 13 can be moved to the right and left from the initial position to the compression limit of the coil springs 37A and 37B. This movement is not limited to the movement of the nut 13, and may be the movement of the screw shaft 11 in the axial direction while the nut 13 is fixed. In addition, during the movement, the ball 15 is pushed out along the extended screw groove 33 and compresses one of the coil springs 37A and 37B via the annular member 39. The restoring force (elastic force) generated by the contracted coil spring at this time is transmitted to the ball 15 via the annular member 39, and becomes a force that pushes the ball 15 back into the rolling path 21 when the nut 13 moves in the opposite direction.

[0028] In this way, the ball 15 is guided while rolling to the axial end of the nut non-load zone A2, whereby the ball 15 pushes the annular member 39 in the axial direction to contract the coil springs 37A, 37B, thereby reducing the number of parts required for movement and making the device configuration simpler and less costly. The annular member 39 described above suppresses the transmission loss of the axial force between the ball 15 and the coil springs 37A, 37B, allowing the ball 15 to move and the springs to expand and contract more smoothly.

[0029] Moreover, in the nut non-loaded zone A2, the ball screw 11 does not have a screw groove. Therefore, the ball 15 is not subjected to load from the ball screw 11 and the nut 13, and is subjected to only the elastic force from the coil spring. As a result, the sliding resistance is small, and the movement of the ball 15 from the rolling path 21 and reinsertion is smooth. Furthermore, since the number of balls 15 arranged in the nut loaded zone A1 can be maintained large, the load resistance of the ball screw device 100 can be suppressed from decreasing. In this way, it is possible to realize a non-circulating ball screw device 100 that is low-cost, has a long life, and is highly reliable and efficient.

[0030] (Examples of other uses) In the above-described ball screw device 100, both coil springs 37A, 37B are arranged in the same state, either in a natural length state or in a slightly compressed state, in the initial state, but may be arranged in different states. 11 is an explanatory diagram showing how the screw shaft 11 is rotated to move the nut 13 to the right and left from the initial state of the ball screw device 100. In this state, when the ball screw 11 and the nut 13 are disposed at either end of their axial strokes, the coil spring 37A at either end is assembled in a natural length state or a slightly compressed state, and the coil spring 37B at the other end is assembled in a compressed state. In this case, the ball 15 is pushed out of the rolling path 21 toward the left coil spring 37B, causing the coil spring 37B to contract via the annular member 39.

[0031] When the screw shaft 11 is rotated in the counterclockwise direction R2 to perform a left stroke operation, the nut 13 moves a distance L1 to the left relative to the screw shaft 11, and the ball 15 pushed out of the rolling path 21 is pushed back into the rolling path 21 by the elastic force of the coil spring 37B. Meanwhile, the ball 15 in the rolling path 21 is pushed out toward the right coil spring 37B. The ball 15 pushed to the right compresses the coil spring 37A via the annular member 39.

[0032] When the screw shaft 11 is rotated in the clockwise direction R1 to perform a right stroke, the nut 13 moves a distance L1 to the right and returns to the initial position. In this case, the coil springs 37A and 37B can also be moved to their compression limits.

[0033] In the case of the usage form shown in Figure 11, the stroke length L1 over which the ball screw 11 and the nut 13 move relative to each other is shortened, but the number of balls 15 arranged within the nut load area A1 can be increased, thereby improving the load-bearing capacity.

[0034] <Second embodiment> Next, a ball screw device according to a second embodiment will be described. Fig. 12 is a schematic configuration diagram showing a partial cross-section of a main part of a ball screw device 200 of a second embodiment. The balls 15 are omitted in Fig. 12. Fig. 13 is an explanatory diagram showing the contact position between an annular member 39 and the balls 15 in a cross section taken along line VIII-VIII shown in Fig. 12.

[0035] In the ball screw device 200 of this configuration, the first screw grooves 17A, 17B of the screw shaft 11A and the second screw grooves 19A, 19B of the nut 13A are formed by two helical grooves. In other words, the first screw groove 17A and the second screw groove 19A form a rolling path 21A, and the first screw groove 17B and the second screw groove 19B form a rolling path 21B. The rolling paths 21A, 21B are arranged adjacent to each other with an equal pitch.

[0036] 13, at the ends of the rolling paths 21A, 21B, the balls 15 from each of the rolling paths 21A, 21B come into contact with the annular member 39 simultaneously. In the case of two rolling paths 21A, 21B, the balls 15 come into contact with the annular member 39 at intervals of 180° in the circumferential direction of the annular member 39.

[0037] FIG. 14 is an explanatory diagram showing the state in which two balls 15 contact the annular member 39. When the annular member 39 is pressed by the balls 15 at positions evenly spaced in the circumferential direction, the inclination from the axial direction is suppressed, and the annular member 39 can move smoothly in the axial direction with small sliding resistance. Furthermore, if the rolling path is made three-threaded, three balls 15 will contact the annular member 39 at equal intervals of 120° in the circumferential direction. In that case, the annular member 39 is made even less likely to incline, and can move more smoothly in the axial direction. By making the rolling paths into multiple-thread rolling paths 21A and 21B in this way, the relative movement between the screw shaft 11A and the nut 13A can be made smoother.

[0038] As such, the present invention is not limited to the above-described embodiments, and the present invention also contemplates mutual combinations of the various components of the embodiments, as well as modifications and applications by those skilled in the art based on the descriptions in the specification and well-known technologies, and these are included in the scope of protection sought. For example, the coil springs 37A, 37B may be directly pressed in the axial direction by omitting the annular member 39. In this case, the number of parts can be further reduced.

[0039] As described above, the present specification discloses the following: (1) A ball screw device comprising: a screw shaft having a first helical screw groove on an outer peripheral surface; a nut having a second helical screw groove on an inner peripheral surface and fitted onto the screw shaft; and a plurality of balls accommodated in a rolling path formed by the first screw groove and the second screw groove, The screw shaft has small diameter portions at both ends of the first screw groove, each having the same outer diameter as the groove bottom of the first screw groove, The nut has a nut non-loaded area portion at both ends of the nut loaded area portion in which the second thread groove is formed, and an inner circumferential surface having an inner diameter shallower than the groove depth of the second thread groove is formed at each end of the nut loaded area portion in which the second thread groove is formed. The nut non-loaded area portion further has an extension thread groove having the same inner diameter as the groove bottom of the second thread groove, which extends from the second thread groove. A coil spring is provided in each of the annular spaces between the nut non-load area portion and the small diameter portion at both axial ends of the first screw groove, the coil spring having one axial end fixed to the screw shaft and the other end disposed toward the first screw groove, The coil spring generates an elastic force that pushes back, into the rolling path, the ball that is pushed out of the rolling path when the screw shaft and the nut rotate relative to each other. Ball screw device. According to this ball screw device, when the screw shaft and the nut are rotated relative to each other to move the nut relative to the screw shaft, the balls pushed out from the rolling path in the loaded area of ​​the nut roll along the extended screw groove formed in the non-loaded area of ​​the nut and compress the coil spring. The elastic force of the contracted coil spring pushes the pushed-out balls back into the rolling path along the extended screw groove. This reduces the number of parts required for movement, resulting in a simple and low-cost device configuration.

[0040] (2) An annular member is provided between each end of the rolling path and the other end of the coil spring, with the ball abutting on one surface and the coil spring abutting on the other surface. 1. A ball screw device as described in (1). According to this ball screw device, the ball expands and contracts the coil spring via the annular member, thereby suppressing the transmission loss of axial force between the ball and the coil spring, thereby enabling smoother ball movement and spring expansion and contraction.

[0041] (3) A ball screw device as described in (1) or (2), in which a radial groove bottom height H from a groove bottom of the second screw groove to a center of the ball and a radial groove wall height h of the extended screw groove have a relationship of h ≧ H / 3. According to this ball screw device, the extended screw groove can guide the ball along the extended screw groove without applying a load to the ball from any source other than the coil spring.

[0042] (4) The ball screw device according to any one of (1) to (3), wherein the rolling path is a multiple-thread rolling path. According to this ball screw device, the multiple balls can press the coil spring in the axial direction evenly, making the relative movement between the screw shaft and the nut smoother. [Explanation of symbols]

[0043] 11,11A Screw shaft 13,13A Nut 13a Cylindrical part 13b Flange part 15 Ball 17,17A First Thread Groove 19,19A Second screw groove 21,21A,21B Rolling path 23 Small diameter section 25 Retaining ring 27 Locking groove 29 Center inner circumferential surface 31 End inner peripheral surface 33 Extended Thread Groove 35 Annular Space 37A, 37B Coil spring 39 Circular members 39a side 39b side 100 Ball screw device A1 Nut load area A2 Nut non-load area

Claims

1. A ball screw device comprising: a screw shaft having a first helical screw groove on an outer peripheral surface; a nut having a second helical screw groove on an inner peripheral surface and fitted onto the screw shaft; and a plurality of balls accommodated in a rolling path formed by the first screw groove and the second screw groove, The screw shaft has small diameter portions at both ends of the first screw groove, each having the same outer diameter as the groove bottom of the first screw groove, The nut has a nut non-loaded area portion at both ends of the nut loaded area portion in which the second thread groove is formed, and an inner circumferential surface having an inner diameter shallower than the groove depth of the second thread groove is formed at each end of the nut loaded area portion in which the second thread groove is formed. The nut non-loaded area portion further has an extension thread groove having the same inner diameter as the groove bottom of the second thread groove, which extends from the second thread groove. A coil spring is provided in each of the annular spaces between the nut non-load area portion and the small diameter portion at both axial ends of the first screw groove, the coil spring having one axial end fixed to the screw shaft and the other end disposed toward the first screw groove, The coil spring generates an elastic force that pushes back, into the rolling path, the ball that is pushed out of the rolling path when the screw shaft and the nut rotate relative to each other. Ball screw device.

2. annular members each disposed between both ends of the rolling path and the other end of the coil spring, the balls abutting on one surface and the coil spring abutting on the other surface; The ball screw device according to claim 1 .

3. The radial groove bottom height H from the groove bottom of the second screw groove to the center of the ball and the radial groove wall height h of the extended screw groove have a relationship of h ≧ H / 3. The ball screw device according to claim 1 .

4. The rolling path is a multiple-thread rolling path. The ball screw device according to any one of claims 1 to 3.

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