Ball screw device

Abstract

The present invention relates to a ball screw device (17) comprising a screw shaft (18) having a first helical groove (29) on an outer periphery, a nut (19) having a second helical groove (30) on an inner periphery, a plurality of balls (20), a stopper (26) provided at an end of the second helical groove (30), and a spring body (31) interposed between an end ball (20a) closest to the stopper (26) and the stopper (26). The spring body (31) is composed of a plurality of coil springs (32) arranged along the first helical groove (29) and the second helical groove (30). Adjacent coil springs (32) each have a spring end portion (34) having a rigidity greater than a rigidity of a spring intermediate portion (33).

Description

TECHNICAL FIELD

[0001] The present application relates to a ball screw device. BACKGROUND

[0002] Japanese Unexamined Patent Application Publication No. 2016-35289 (JP 2016-35289 A) discloses a ball screw device for a brake device for a vehicle. The ball screw device has a screw shaft having a helical groove formed on an outer periphery, a nut having a helical groove formed on an inner periphery, and a plurality of balls disposed between the helical groove of the screw shaft and the helical groove of the nut. The nut moves along an axial direction of the screw shaft by rotation of the screw shaft. The ball screw device disclosed in JP 2016-35289 A is not a type of device in which the balls circulate when the nut moves, but is a non-circulating type of device in which the balls roll while being retained in the helical groove of the nut when the nut moves. SUMMARY

[0003] Figure 9 is an explanatory view in which a portion of the helical groove of the screw shaft and the helical groove of the nut, which the non-circulating type of ball screw device has, is observed from an axial direction. A stopper (stopper ball) 92 is provided at an end portion of the helical groove 90a of the nut 90. An end portion ball 99a closest to the stopper 92 among the plurality of balls 99 is provided between the stopper 92 and the coil spring 93.

[0004] When the nut 90 moves due to rotation of the screw shaft 94, the balls 99 move in the direction indicated by the arrow J along the helical groove 94a of the screw shaft 94 and the helical groove 90a of the nut 90. Thus, the end portion ball 99a presses the coil spring 93, thereby compressing the coil spring 93. The portion 93a of the coil spring 93 at the side of the end portion ball 99a is easily compressed, whereas the portion 93b at the side of the stopper 92 is not easily compressed. This is because, due to the frictional resistance between the coil spring 93 and the helical grooves 90a and 94a, the force with which the end portion ball 99a presses the coil spring 93 is difficult to easily further transmit toward the side of the stopper 92.

[0005] Thus, the coil spring 93 as a whole does not exhibit uniform compression deformation between the end portion ball 99a and the stopper 92, and fatigue develops at the portion where the deformation is large, that is, at the portion 93a at the side of the end portion ball 99a. As a result, the life of the coil spring 93 can be shortened compared to when the coil spring 93 as a whole exhibits uniform compression deformation. In addition, by achieving the compression deformation of the coil spring 93 as a whole, it is possible to make the moving stroke of the nut 90 large.

[0006] Thus, the present disclosure provides a ball screw device in which the compression deformation of the coil spring as a whole can be achieved between the end portion ball and the stopper.

[0007] A ball screw device according to an aspect of the present disclosure includes a screw shaft having a first helical groove on an outer periphery; a nut provided on an outer periphery side of the screw shaft and having a second helical groove on an inner periphery; a plurality of balls provided between the first helical groove and the second helical groove; a stopper provided at an end portion of the second helical groove; and a spring body interposed between an end ball, which is closest to the stopper among the plurality of balls, and the stopper. The spring body is composed of a plurality of coil springs arranged along the first helical groove and the second helical groove.

[0008] According to the above-described ball screw device, the coil springs are divided into a plurality and arranged in a row between the end ball and the stopper. Thus, the coil springs can be subjected to compressive deformation as a whole between the end ball and the stopper, for example, by changing the properties of each coil spring. As a result, the life of the coil springs can be extended.

[0009] In the ball screw device according to the above-described aspect, adjacent coil springs contact each other, and each of the adjacent coil springs has a spring end portion having a rigidity greater than that of a spring intermediate portion. When the coil springs are divided into a plurality, the spring end portions come into contact with each other. When the outer shape of the spring end portion is the same as that of the spring intermediate portion, that is, when the rigidity of the spring end portion is low, there is a possibility that the centers of the spring end portions will be misaligned in a state in which the spring end portions contact each other, resulting in unpredictable behavior. In such a case, the function of the ball screw device will deteriorate. However, according to the ball screw device of the present disclosure, the rigidity of the spring end portion is high. As a result, the orientation and behavior of the adjacent spring end portions can be stabilized.

[0010] In the above-described aspect, since the pitch at the spring end portion is narrower than the pitch at the spring intermediate portion, the spring end portion can be allowed to become crimped when the spring end portion is compressed under a load at which the spring intermediate portion exhibits elastic compressive deformation. In such a case, when the coil springs are compressed, the spring intermediate portion exhibits elastic compressive deformation, but the spring end portion is allowed to become crimped. By allowing the coil springs to be compressed and become crimped at the spring helical portion, the rigidity of the spring end portion is higher than that of the spring intermediate portion.

[0011] Alternatively, in the above-described aspect, the spring end portion can be in a crimped state. According to this configuration, the rigidity of the spring end portion is higher than that of the spring intermediate portion.

[0012] Alternatively, in the above aspect, the spring constant of the spring end portion can be high because the pitch at the spring end portion is wider than the pitch at the spring middle portion. In the coil spring, according to the above configuration, the pitch of the coil spring wire is widened and the number of coils is reduced, which increases the spring constant, and thus the rigidity of the spring end portion is higher than the rigidity of the spring middle portion. In addition, in the case of this configuration, the spring end portion can also exhibit elastic compression deformation, and thus the effective length of the spring body constituted by the coil spring disposed between the end portion ball and the stopper is longer. Thus, the moving stroke of the nut can be made even larger.

[0013] In addition, in the above aspect, the total dimension of two of the adjacent spring end portions in the spring longitudinal direction can be smaller than the average diameter of the coils of the coil spring. In an arrangement in which spacer balls are interposed between the coil springs, the diameter of the spacer ball is approximately the same as the average diameter of the coils of the coil spring. Thus, according to the above configuration, the spring end portion is short, and the effective length of the coil spring can be made longer compared to when a spacer ball is used.

[0014] When the nut moves in the axial direction of the screw due to rotation of the screw, the ball also moves along the first helical groove and the second helical groove. The coil spring compressed by the movement of the end portion ball included in the ball is apt to be compressed at the end portion ball side, but is not apt to be compressed at the stopper side. Thus, in the above aspect, in the coil spring disposed between the end portion ball and the stopper, the spring constant of the coil spring at the stopper side can be smaller than the spring constant of the coil spring at the end portion ball side. According to the above configuration, the coil spring at the stopper side is apt to exhibit compression deformation. Thus, the spring body constituted by the coil spring as a whole can be apt to exhibit compression deformation. As a result, the moving stroke of the nut can be increased.

[0015] According to the present disclosure, compression deformation of the coil spring as a whole can be achieved between the end portion ball and the stopper. BRIEF DESCRIPTION OF DRAWINGS

[0016] Features, advantages, and technical and industrial significance of exemplary embodiments of the application will be described below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements, and wherein:

[0017] Figure 1 is a sectional view showing an example of a brake device provided with a ball screw device;

[0018] Figure 2 is an exploded perspective view of the ball screw device;

[0019] Figure 3 is a sectional view of the ball screw device;

[0020] Figure 4 is an explanatory view showing the first helical groove and the second helical groove in an unfolded state on a plane;

[0021] Figure 5 is an explanatory view of the first spring body viewed from the axial direction of the ball screw device;

[0022] Figure 6 is an explanatory view of the spring end portions of the adjacent coil springs (first arrangement);

[0023] Figure 7 is an explanatory view of the spring end portions of the adjacent coil springs (second arrangement);

[0024] Figure 8 is an explanatory view of the spring end portions of the adjacent coil springs (third arrangement); and

[0025] Figure 9 is an explanatory view of a part of the helical groove of the screw and the helical groove of the nut possessed by a non-circulating type ball screw device (prior art). DETAILED DESCRIPTION

[0026] Figure 1 is a cross-sectional view showing an example of a brake device provided with a ball screw device. For example, Figure 1 The ball screw device 17 shown in FIG. 1 is used in a brake device 5 in a vehicle (an automobile). The brake device 5 applies a braking force from friction to a brake disc 6 that rotates integrally with a wheel of the automobile. The brake device 5 is provided with the ball screw device 17 to generate such a braking force. The brake device 5 is in an unbraked state in FIG. 1. Figure 1

[0027] The brake device 5 is provided with a floating caliper 7 supported by a knuckle or the like not shown and a pair of brake pads 8 that sandwich the brake disc 6 therebetween. The caliper 7 is provided with a first main body 9 and a second main body 10 that is provided integrally with the first main body 9.

[0028] One (right side in FIG. 1) of the brake pads 8 is attached to a housing 21 possessed by the ball screw device 17 via a first backup brake plate 12 described below. The other (left side in FIG. 1) of the brake pads 8 is attached to the second main body 10 via a second backup brake plate 13. Figure 1 Figure 1

[0029] ​​​The first body 9 has a cylindrical shape (cylindrical outer shape with a bottom) including a cylindrical body portion 14 and a bottom brake plate portion 15, and is open toward the side of the brake disc 6. A ball screw device 17 is provided on the inner side of the cylindrical body portion 14. The ball screw device 17 is provided with a screw shaft 18, a nut 19 provided on the outer peripheral side of the screw shaft 18, and a plurality of balls 20. A housing 21 is attached to the nut 19. The center line C of the screw shaft 18 is the center line of the ball screw device 17. In the present disclosure, the direction parallel to this center line C will be referred to as the "axial direction".

[0030] A through-hole 16 is formed on the bottom brake plate portion 15 of the first body 9. A bearing 22 is attached to this through-hole 16. The screw shaft 18 is rotatably supported by the bearing 22. A key 24 is provided between the housing 21 and the cylindrical body portion 14. The housing 21 is arranged so as to be able to reciprocate in the axial direction with respect to the cylindrical body portion 14, but not to be able to rotate in the circumferential direction about the center line C.

[0031] The nut 19 and the housing 21 are integrated. When the screw shaft 18 is rotated in one direction (forward rotation) about the center line C, the nut 19 and the housing 21 move along the screw shaft 18 from one side (right side in Figure 1 Figure 1 When the screw shaft 18 is rotated in the other direction (reverse rotation) about the center line C, the nut 19 and the housing 21 move along the screw shaft 18 from the other side (left side in

[0032] A motor (electric motor) 51 and a speed reducer 23 are provided outside the cylindrical body portion 14. Command signals from a control unit 52 are input to the motor 51, and the output shaft of the motor 51 performs forward rotation, reverse rotation, and stop based on these command signals. The speed reducer 23 is constituted, for example, by a plurality of gears, and reduces the rotation of the output shaft from the motor 51, and rotates the screw shaft 18. Thus, when the motor 51 rotates, the nut 19 and the housing 21 move in the axial direction. That is, the rotational motion of the screw shaft 18 transmitted from the motor 51 via the speed reducer 23 is converted by the ball screw device 17 into linear motion of the nut 19 and the housing 21 in the axial direction. Thus, the brake pad 8 clamps the brake disc 6, thereby generating a braking force.

[0033] Figure 2 is an exploded perspective view of the ball screw device 17. Figure 3 is a cross-sectional view of the ball screw device 17. A first helical groove 29 is formed on the outer periphery of the screw shaft 18. A second helical groove 30 is formed on the inner periphery of the nut 19. A ball row 25 constituted by the balls 20 is provided between the first helical groove 29 and the second helical groove 30.

[0034] ​Figure 4 is an explanatory view showing the first helical groove 29 and the second helical groove 30 in a state of being spread on a plane. All the balls 20 (ball row 25) are in a state of being accommodated at the inner peripheral side of the nut 19. The ball screw device 17 is further provided with stoppers 26 and 27 which are provided at respective end portions of the second helical groove 30. The stoppers 26 and 27 are provided on respective sides in the axial direction, on the inner peripheral side of the nut 19 (second helical groove 30). The first stopper 26 on one side is constituted by a wall portion at the end portion on one side of the second helical groove 30. The wall portion is a portion of the nut 19. The second stopper 27 on the other side is constituted by a wall portion at the end portion on the other side of the second helical groove 30. It should be noted that one or both of the first stopper 26 and the second stopper 27 can be constituted, for example, by a ball provided in a non-movable manner to the nut 19, or by a pin member fixed to the nut 19.

[0035] Among the balls 20 included in the ball row 25, the ball 20 closest to the first stopper 26, that is, the ball 20 at the rightmost side in Figure 4 will be referred to as a "first end portion ball 20a". A first spring body 31 constituted by a coil spring is provided between the first end portion ball 20a and the first stopper 26. The first spring body 31 is in a compressed state.

[0036] Among the balls 20 included in the ball row 25, the ball 20 closest to the second stopper 27, that is, the ball 20 at the leftmost side in Figure 4 will be referred to as a "second end portion ball 20b". A second spring body 37 constituted by a coil spring is provided between the second end portion ball 20b and the second stopper 27. The second spring body 37 is in a compressed state.

[0037] The ball screw device 17 having the above-described configuration is a non-circulating ball screw device in which the balls 20 roll while being seated in the second helical groove 30 when the nut 19 moves in the axial direction of the screw shaft 18. The nut 19 moves through a predetermined stroke from a state of the initial position. When the nut 19 moves from the initial position, the moving direction of the ball row 25 is a direction toward the first stopper 26. That is, when the nut 19 moves from the initial position, the moving direction of the ball row 25 is a direction in which the first spring body 31 is further compressed.

[0038] Figure 5is an explanatory view of the first spring body 31 as viewed from the axial direction of the ball screw device 17. As described previously, the first spring body 31 is provided between the first end ball 20a and the first stopper 26. The first spring body 31 includes a plurality of coil springs 32. The coil springs 32 are arranged in series along the first helical groove 29 and the second helical groove 30. Adjacent coil springs 32 directly contact each other at their end portions. In the present disclosure, three coil springs 32a, 32b, and 32c are included in the first spring body 31. Note that the number of coil springs 32 included in the first spring body 31 can be freely changed depending on the model of the ball screw device 17 or the like.

[0039] The coil springs 32 each have an intermediate portion, and end portions on both sides of the intermediate portion. In the present disclosure, the intermediate portion is referred to as a "spring intermediate portion 33", and the end portions are referred to as "spring end portions 34". The spring end portions 34 of adjacent coil springs 32 are in a state of contacting each other. In the case of the present disclosure, the spring end portion 34a-2 of the first coil spring 32a and one spring end portion 34b-1 of the second coil spring 32b are in a state of contacting each other, and the other spring end portion 34b-2 of the second coil spring 32b and the spring end portion 34c-1 of the third coil spring 32c are in a state of contacting each other.

[0040] In the first spring body 31, the properties (spring constants) of the coil springs 32a, 32b, and 32c are different from each other. The third coil spring 32c at the first stopper 26 side has a smaller spring constant than the second coil spring 32b, and the second coil spring 32b has a smaller spring constant than the first coil spring 32a at the end ball 20a side. Note that the spring constant referred to here is the value at the spring intermediate portion 33. That is, in the first spring body 31, the spring constant of the spring intermediate portion 33 of the coil spring 32 at the first stopper 26 side is smaller than the spring constant of the spring intermediate portion 33 of the coil spring 32 at the end ball 20a side. Note that the coil average diameter D (see FIG. 2) is the same for each of the coil springs 32a, 32b, and 32c. The diameter of the wire (helical spring wire) of the coil springs 32a, 32b, and 32c can be the same or can be different. The length of each of the coil springs 32a, 32b, and 32c can be the same or can be different. Figure 6 ) are the same for each of the coil springs 32a, 32b, and 32c. The diameter of the wire (helical spring wire) of the coil springs 32a, 32b, and 32c can be the same or can be different. The length of each of the coil springs 32a, 32b, and 32c can be the same or can be different.

[0041] The first coil spring 32a that directly contacts the end ball 20a will be described. One spring end portion 34a-1 of the first coil spring 32a contacts the end ball 20a. The spring end portion 34a-1 has the same property (same rigidity) as the spring intermediate portion 33a. The other spring end portion 34a-2 of the first coil spring 32a has a configuration in which its rigidity is higher than that of the spring intermediate portion 33a of the first coil spring 32a. In the present disclosure, the spring end portion 34a-2 is compressed, as shown in FIG. 4. Figure 6The interval between the spiral spring wires constituting the first coil spring 32a is zero at the spring end portion 34a-2. Note that other configurations can be made to improve rigidity, which will be described later. The boundary between the first coil spring 32a and the second coil spring 32b is indicated by a long dashed line in FIG. 6. Figure 6

[0042] The second coil spring 32b will be described. One spring end portion 34b-1 of the second coil spring 32b contacts the spring end portion 34a-2 of the first coil spring 32a. This one spring end portion 34b-1 of the second coil spring 32b has a configuration in which its rigidity is higher than that of the spring intermediate portion 33b of the second coil spring 32b. In the present disclosure, the spring end portion 34b-1 is crimped, as indicated in FIG. 7. Figure 6 The other spring end portion 34b-2 (see FIG. 8) of the second coil spring 32b contacts the spring end portion 34c-1 of the third coil spring 32c. This other spring end portion 34b-2 of the second coil spring 32b has a configuration in which its rigidity is higher than that of the spring intermediate portion 33b of the second coil spring 32b. In the present disclosure, the spring end portion 34b-2 is crimped. Figure 5 The third coil spring 32c will be described. One spring end portion 34c-1 of the third coil spring 32c contacts the spring end portion 34b-2 of the second coil spring 32b. This one spring end portion 34c-1 of the third coil spring 32c has a configuration in which its rigidity is higher than that of the spring intermediate portion 33c of the third coil spring 32c. In the present disclosure, the spring end portion 34c-1 is crimped. The other spring end portion 34c-2 of the third coil spring 32c contacts the first stopper 26. The spring end portion 34a-2 has the same properties (the same rigidity) as the spring intermediate portion 33c.

[0043] As described above, the adjacent first coil spring 32a and the second coil spring 32b have the spring end portions 34a-2 and 34b-1, respectively, which have higher rigidity than that of the spring intermediate portions 33a and 33b. The adjacent second coil spring 32b and the third coil spring 32c have the spring end portions 34b-2 and 34c-1, respectively, which have higher rigidity than that of the spring intermediate portions 33b and 33c.

[0044] Configuration for improving the rigidity of the spring end portion 34 (first arrangement)

[0045] As described above, the spring end portions 34 are in a crimped state so as to make the rigidity of the spring end portions 34 higher than that of the spring intermediate portions 33. According to this configuration, the orientation and behavior of each of the adjacent spring end portions 34 can be stabilized. In addition, the spring wires of the adjacent coil springs 32 are not easily displaced into the spring end portions 34. For example, in the case where the spring end portions 34 are not crimped, the spring wires of the adjacent coil springs 32 can be displaced into the spring end portions 34, and the rigidity of the spring end portions 34 can be reduced. In the present disclosure, the spring end portions 34 are crimped, and the rigidity of the spring end portions 34 is made higher than that of the spring intermediate portions 33.

[0046] Figure 6 ​​In the first coil spring 32b, the spring wire of the spring end 34b-1, which contacts the spring end 34a-2 of the first coil spring 32a, is not easily displaced into the spring end 34a-2 because the spring end 34a-2 is compressed. Similarly, in the same manner, the spring wire of the spring end 34a-2 of the first coil spring 32a, which contacts the spring end 34b-1 of the second coil spring 32b, is not easily displaced into the spring end 34b-1 because the spring end 34b-1 is compressed.

[0047] As described above, the three coil springs 32 have the same average coil diameter D. One spring end 34a-2 and another spring end 34b-1 are in contact. The dimension of the spring end 34a-2 in the longitudinal direction of the spring is L1, and the dimension of the spring end 34b-1 in the longitudinal direction of the spring is L2. It should be noted that the longitudinal direction of the spring is the length along the centerline of the coil spring 32. Figure 6 In the arrangement shown, dimensions L1 and L2 are the dimensions of the compressed portion in the longitudinal direction of the spring. The sum (L1+L2) of the dimension L1 of the spring end 34a-2 in the longitudinal direction of the spring and the dimension L2 of the spring end 34b-1 in the longitudinal direction of the spring is less than the average diameter D of the coil of the coil spring 32 in this disclosure, i.e. (L1+L2<D).

[0048] Additionally, the spring end 34b-2 of the second coil spring 32b and the spring end 34c-1 of the third coil spring 32c (see...) Figure 5 They have the same construction. That is, the sum of the dimensions of one spring end 34b-2 in the longitudinal direction of the spring and the dimensions of the other spring end 34c-1 in the longitudinal direction of the spring is less than the average diameter of the coil springs 32b and 32c.

[0049] As described above (see Figure 6 The total dimension (L1+L2) of the two adjacent spring ends 34a-2 and 34b-1 in the longitudinal direction of the spring is smaller than the average diameter D of the coil springs 32a and 32b, i.e. (L1+L2<D).

[0050] exist Figure 6 In this embodiment, assuming a spacer ball 38 (indicated by a double-dotted line) is inserted between the first coil spring 32a and the second coil spring 32b, the diameter d of the spacer ball 38 will be approximately the same as the average diameter D of the coil springs 32a (32b) (d = D). Therefore, according to the above-described structure where L1 + L2 < D, the spring ends 34a-2 and 34b-1 are shorter in the longitudinal direction of the spring, allowing the effective length of the coil springs 32a and 32b to be longer compared to when the spacer ball 38 is used. Furthermore, the spacer ball 38 is not required in the ball screw device 17 according to this disclosure, and the number of parts in the ball screw device 17 can be reduced.

[0051] Configuration for increasing the rigidity of the spring end portion 34 (second arrangement)

[0052] In order to make the rigidity of the spring end portion 34 higher than the rigidity of the spring intermediate portion 33, the spring end portion 34 can be configured as follows. That is, by making the pitch at the spring end portion 34a-2 of the first coil spring 32a narrower than the pitch at the spring intermediate portion 33a (P2 < P1 in the case of the first coil spring 32a) in the longitudinal direction of the spring, the spring end portion 34a-2 is allowed to become crimped when the spring end portion 34a-2 is compressed under a load at which the spring intermediate portion 33a exhibits elastic compression deformation. In the same manner, by making the pitch at the spring end portion 34b-1 of the second coil spring 32b narrower than the pitch at the spring intermediate portion 33b, the spring end portion 34b-1 is allowed to become crimped when the spring end portion 34b-1 is compressed under a load at which the spring intermediate portion 33b exhibits elastic compression deformation. Figure 7 Figure 7 In the case of the first coil spring 32a, P2 < P1, the spring end portion 34a-2 is allowed to become crimped when the spring end portion 34a-2 is compressed under a load at which the spring intermediate portion 33a exhibits elastic compression deformation. In the same manner, by making the pitch at the spring end portion 34b-1 of the second coil spring 32b narrower than the pitch at the spring intermediate portion 33b, the spring end portion 34b-1 is allowed to become crimped when the spring end portion 34b-1 is compressed under a load at which the spring intermediate portion 33b exhibits elastic compression deformation.

[0053] According to the configuration shown in Figure 7 , when the coil springs 32a and 32b are compressed, the spring intermediate portions 33a and 33b exhibit elastic compression deformation, but the spring end portions 34a-2 and 34b-1 are allowed to become crimped. By compressing the coil springs 32a and 32b, and each of the spring end portions 34a-2 and 34b-1 becoming crimped, the rigidity of each of the spring end portions 34a-2 and 34b-1 becomes higher than the rigidity of the spring intermediate portions 33a and 33b. Thus, the orientation and behavior of each of the adjacent spring end portions 34a-2 and 34b-1 can be stabilized. In addition, in the same manner as in Figure 6 , the spring wire of the coil spring 32b (32a) is not easily displaced into the spring end portion 34a-2 (34b-1) because the spring end portion 34a-2 (34b-1) becomes crimped.

[0054] The spring end portion 34b-2 and the spring end portion 34c-1 have the same configuration as the configuration shown in Figure 7 , in which the spring end portion 34b-2 and the spring end portion 34c-1 form a contact portion in which the second coil spring 32b and the third coil spring 32c contact. Thus, the orientation and behavior of the adjacent spring end portions 34b-2 and 34c-1 can be stabilized.

[0055] When the two adjacent spring end portions 34a-2 and 34b-1 (34b-2 and 34c-1) become crimped, in the same manner as in the arrangement in Figure 6 , the total dimension of these spring end portions 34a-2 and 34b-1 (34b-2 and 34c-1) in the longitudinal direction of the spring is also preferably smaller than the average diameter of the coil of the coil spring 32a and 32b in the arrangement in Figure 7 .

[0056] ​Configuration for increasing rigidity of spring end portion 34 (third arrangement)

[0057] In order to make the rigidity of the spring end portion 34 higher than the rigidity of the spring middle portion 33, the spring end portion 34 has a high spring constant due to the pitch of the spring end portion 34 being wider than the pitch of the spring middle portion 33. To describe this in detail, in the first coil spring 32a, the pitch at the spring end portion 34a-2 is wider than the pitch at the spring middle portion 33a, as shown in FIG. 6 (in the Figure 8 middle portion 33b. According to this configuration, the rigidity of the spring end portion 34b-1 is higher than the rigidity of the spring middle portion 33b in the second coil spring 32b. Figure 8 Without needing to describe, in a coil spring, widening the pitch of the helical spring wire (spring wire) and reducing the number of coils increases the spring constant. Thus, according to this configuration, the rigidity of the spring end portion 34a-2 is higher than the rigidity of the spring middle portion 33a in the first coil spring 32a.

[0058] In the second coil spring 32b, the pitch at the spring end portion 34b-1 is wider than the pitch at the spring middle portion 33b. According to this configuration, the rigidity of the spring end portion 34b-1 is higher than the rigidity of the spring middle portion 33b in the second coil spring 32b.

[0059] Further, according to this configuration (third arrangement), the spring end portion 34a-2 and the spring end portion 34b-1 can also exhibit elastic compression deformation. Thus, the effective length of the first spring body 31 is longer, and it is possible to make the moving stroke of the nut 19 larger.

[0060] Regarding the ball screw device 17 according to the present disclosure

[0061] As described above, the ball screw device 17 (see Figure 5 ) according to the present disclosure is provided with: a screw shaft 18; a nut 19; a ball 20 that is disposed between a first helical groove 29 of the screw shaft 18 and a second helical groove 30 of the nut 19; a first stopper 26 that is disposed at an end portion of the second helical groove 30; and a first spring body 31 that is disposed between an end ball 20a and the first stopper 26. The first spring body 31 is composed of coil springs 32 (32a, 32b, 32c) that are arranged in a row along the first helical groove 29 and the second helical groove 30.

[0062] According to the ball screw device 17 having the above-described configuration, the coil springs 32 (32a, 32b, 32c) are divided into a plurality and arranged in a row between the end ball 20a and the first stopper 26. Thus, the first spring body 31, i.e., the coil springs 32 (32a, 32b, 32c) can be subjected to compressive deformation as a whole between the end ball 20a and the first stopper 26 by changing the properties (spring constants) of each of the coil springs 32 (32a, 32b, 32c). As a result, the life of the coil springs 32 can be extended against fatigue. Further, by setting the properties (spring constants) of the coil springs 32 (32a, 32b, 32c) to exhibit compressive deformation most uniformly as a whole, it is possible to further increase the moving stroke of the nut 19, which will be described below.

[0063] Further, in the ball screw device 17 according to the present disclosure, adjacent coil springs 32 contact each other at the spring end portions 34. The adjacent first coil spring 32a and the second coil spring 32b each have a spring end portion 34a-2 and 34b-1 that is more rigid than the spring intermediate portions 33a and 33b. The adjacent second coil spring 32b and the third coil spring 32c each have a spring end portion 34b-2 and 34c-1 that is more rigid than the spring intermediate portions 33b and 33c.

[0064] As described above, when the coil springs 32 (32a, 32b, 32c) are divided into a plurality, the spring end portions 34 come into contact with each other. When the outer shape of the spring end portions 34 is the same as that of the spring intermediate portions 33, i.e., when the spring end portions 34 are low in rigidity, there is a possibility that, for example, in a state in which the spring end portions 34 contact each other, the centers of the spring end portions 34 will be misaligned, resulting in unpredictable behavior. In such a case, the function of the ball screw device 17 will be deteriorated. However, according to the ball screw device 17 of the present disclosure, the spring end portions 34 are high in rigidity. As a result, it is possible to stabilize the orientation and behavior of the adjacent spring end portions 34.

[0065] In the ball screw device 17, the nut 19 moves along the axial direction of the screw shaft 18 due to the rotation of the screw shaft 18, and the balls 20 also move along the first helical groove 29 and the second helical groove 30. When the nut 19 moves through a predetermined stroke from a state in which it is at an initial position, the moving direction is the direction indicated by the arrow J in FIG. 6. The movement of the end ball 20a included in the balls 20 compresses the coil springs 32a, 32b, and 32c. The coil springs 32a, 32b, and 32c that are compressed in this way are apt to be compressed at the side of the end ball 20a, but are not apt to be compressed at the side of the first stopper 26. Figure 4 and Figure 5 The movement of the end ball 20a included in the balls 20 compresses the coil springs 32a, 32b, and 32c. The coil springs 32a, 32b, and 32c that are compressed in this way are apt to be compressed at the side of the end ball 20a, but are not apt to be compressed at the side of the first stopper 26.

[0066] Thus, in the present disclosure, in the first spring body 31, as described above, the spring constant of the side of the coil spring 32 (spring intermediate portion 33) toward the first stopper 26 is set to be smaller than the spring constant of the side of the coil spring 32 (spring intermediate portion 33) toward the end ball 20a. According to this configuration, it is easier to cause the coil spring 32 at the first stopper 26 side to exhibit compression deformation than the coil spring 32 at the end ball 20a side. Thus, it is possible to easily cause the first spring body 31 composed of the coil springs 32a, 32b, and 32c as a whole to maximally and uniformly exhibit compression deformation. As a result, the moving stroke of the nut 19 can be further increased.

[0067] In addition, the rigidity of the spring end portions 34 contacting the adjacent coil springs 32 is higher than that of the spring intermediate portion 33, and thus, the springs better seat at both of the adjacent spring end portions 34. That is, the centers of the spring end portions 34 easily align, and it is possible to cause the first spring body 31 to behave like a single coil spring 32.

[0068] The embodiments disclosed herein are exemplary in all respects and are not limited to the embodiments described above. The scope of the present application is not limited to the embodiments described above, and includes all modifications made within the scope equivalent to the configurations set forth in the claims.

Claims

1. A ball screw device (17) characterized by Comprising: a screw shaft (18) having a first helical groove (29) on an outer periphery; a nut (19) provided on an outer periphery side of the screw shaft (18) and having a second helical groove (30) on an inner periphery; a plurality of balls (20) provided between the first helical groove (29) and the second helical groove (30); a stopper (26) provided at an end of the second helical groove (30); and a spring body (31) interposed between an end ball (20a) of the plurality of balls (20) closest to the stopper (26) and the stopper (26), wherein the spring body (31) is composed of a plurality of coil springs (32) arranged along the first helical groove (29) and the second helical groove (30), the plurality of coil springs (32) having different spring constants from each other, adjacent coil springs (32) are in contact with each other, and adjacent coil springs (32) each have a spring end portion (34) having a rigidity greater than a rigidity of a spring intermediate portion (33), and wherein, among the coil springs (32) interposed between the end ball (20a) and the stopper (26), a spring constant of a coil spring (32c) on the stopper (26) side is smaller than a spring constant of a coil spring (32a) on the end ball (20a) side.

2. Ball screw device (17) according to claim 1, characterized in that Since the pitch at the spring end portion (34) is narrower than the pitch at the spring intermediate portion (33), when the spring end portion (34) is compressed under a load at which the spring intermediate portion (33) exhibits elastic compression deformation, the spring end portion (34) is allowed to become compressed.

3. The ball screw device (17) according to claim 1, characterized in that The spring end portion (34) is in a compressed state.

4. The ball screw device (17) according to claim 1, characterized in that Since the pitch at the spring end portion (34) is wider than the pitch at the spring intermediate portion (33) and the number of coils is reduced, the spring constant of the spring end portion (34) is high.

5. Ball screw device (17) according to any one of claims 1 to 3, characterized in that The total size of two of the adjacent spring end portions (34) in a spring longitudinal direction is smaller than the average diameter of the coils of the coil spring (32).

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