Ball screw

Abstract

A ball screw is provided with: a threaded shaft in which an outer peripheral threaded groove is formed; a nut in which an inner circumferential thread groove is formed; a plurality of balls accommodated in a rolling path formed by the outer peripheral thread groove and the inner peripheral thread groove which face each other; and a return pipe for returning the ball from one end of the rolling path to the other end, the return pipe having a tongue piece protruding toward the rolling path and picking up the ball from the rolling path, the tongue piece having a shape in which the width gradually becomes narrower from the base of the tongue piece toward the tip thereof.

Description

Technical Field

[0001] The invention relates to a ball screw. Background Art

[0002] The ball screw has: a nut with an internal thread groove formed on the inner circumference; a threaded shaft with an external thread groove formed on the outer circumference; balls arranged between the raceways formed by the internal thread groove of the nut and the external thread groove of the threaded shaft; and a ball return path for returning the balls from the end point of the raceway to the starting point, and the balls roll in the raceway, thereby causing the nut to move relative to the threaded shaft. As the ball return path of the ball screw, a return tube method is sometimes used due to advantages such as easy assembly.

[0003] An example of a return tube type ball screw is disclosed in Patent Document 1. In this ball screw, the end portion of the return tube as a circulation element is composed of an end surface along a base line perpendicular to the tube axis and a tongue protruding from the base line in a tongue shape, and the tongue protrudes into the raceway to function in a manner of scooping up balls from the raceway into the return tube.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2007-285379 Summary of the invention

[0007] Technical problem that the invention aims to solve

[0008] However, in order to shorten the cycle time of machines using ball screws, the market has been demanding the high speed of ball screws, but in order to achieve the high speed of ball screws, the allowable rotation speed of the screw shaft needs to be increased. However, due to the increase in the allowable rotation speed of the screw shaft, the balls will roll along the raceway at high speed.

[0009] In the ball screw of Patent Document 1, the ball rolling at high speed collides with the tongue when being picked up from the raceway. At this time, a large stress is applied to the root of the tongue, and the circulating ball contacts the tongue in turn, so the load is repeatedly applied to the tongue.

[0010] Generally, the permissible speed of the threaded shaft is determined by the fatigue strength of the circulating element against the ball. Fatigue strength is usually determined by the magnitude of repeated stress and the number of load repetitions. Therefore, how to reduce the stress generated on the tongue has become a problem in the high-speed ball screw.

[0011] In response to this problem, a ball screw has been developed. In order to cope with high speed, the ball screw is raised by making the tongue piece face the tangent direction of the thread groove to reduce the impact of the ball on the tongue piece. However, it is difficult to make the tongue piece with a thick wall completely consistent with the tangent direction. As the rotation speed increases, the impact of the ball on the tongue piece also increases. Therefore, it is expected to further reduce the stress of the tongue piece.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a ball screw that can cope with high speeds and has improved durability.

[0013] Technical solutions to the problem

[0014] The ball screw of the present invention is characterized in that:

[0015] A threaded shaft, wherein the threaded shaft is formed with an outer peripheral thread groove;

[0016] A nut having an inner circumferential thread groove;

[0017] a plurality of balls, the plurality of balls being accommodated in a rolling path formed by the outer circumferential thread groove and the inner circumferential thread groove that are opposed to each other; and

[0018] a return pipe, the return pipe returning the balls from one end of the rolling path to the other end,

[0019] The return pipe has a tongue piece, which protrudes toward the rolling path and picks up the ball from the rolling path.

[0020] The tongue piece has a shape in which the width gradually narrows from the root portion toward the tip portion of the tongue piece.

[0021] Effects of the Invention

[0022] According to the present invention, it is possible to provide a ball screw that can cope with increased speed and has improved durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a perspective view showing the structure of the ball screw according to the first embodiment.

[0024] Figure 2 This is a partial cross-sectional view along the axial direction showing a part of the ball screw according to the first embodiment.

[0025] Figure 3 It is along Figure 2 The cross-sectional view along the line AA only shows a portion of the figure.

[0026] Figure 4 (a) and (b) are diagrams showing the end of the return pipe viewed from different directions.

[0027] Figure 5 The vertical axis is a graph showing the rigidity ratio of the tongue tip (the ratio of the tongue length to the ball diameter is based on 60%), and the horizontal axis is a graph showing the ratio of the tongue length to the ball diameter.

[0028] Figure 6 (a) and (b) show the end of the return pipe according to the modification. Figure 4 (a) and (b) are the same figures.

[0029] Figure 7 (a) to (c) are views showing the end portion of the return pipe of the ball screw according to the second embodiment.

[0030] Figure 8 The ball screw including the return pipe according to the first modification is shown in FIG. Figure 3 Same cross-section view.

[0031] Fig. 9 is a diagram showing the ball screw involved in Modification Example 2. Figure 8 Same cross-section view. DETAILED DESCRIPTION

[0032] Hereinafter, embodiments of a ball screw according to the present invention will be described with reference to the drawings.

[0033] (First Embodiment)

[0034] Figure 1 is a perspective view showing the structure of a ball screw according to the first embodiment. Figure 2 is a partial cross-sectional view along the axial direction showing a part of the ball screw involved in the first embodiment, Figure 3 It is along Figure 2 The cross-sectional view along the AA line is regarded as a two-dimensional coordinate system, and only the portion corresponding to the first quadrant is illustrated, and the partition is omitted.

[0035] like Figure 1 , Figure 2 As shown, the ball screw 1 of this embodiment includes: a threaded shaft 10 having a spirally continuous thread groove (outer circumferential thread groove) 10a formed on its outer circumferential surface; and a cylindrical nut 20 having a spirally continuous thread groove (inner circumferential thread groove) 20a formed on its inner circumferential surface.

[0036] The threaded shaft 10 is inserted into the nut 20 along the axial direction, and the thread groove 10a of the threaded shaft 10 is opposite to the thread groove 20a of the nut 20, and the space between the two thread grooves 10a and 20a forms a spiral rolling path 40. A plurality of balls (rolling elements) 30 are loaded in the rolling path 40 so as to roll freely, and the nut 20 is connected to the threaded shaft 10 via the balls 30 in a relatively rotatable manner. In this embodiment, a partition 31 is provided between adjacent balls 30 to suppress contact between the balls 30, but there may be no partition. The partition is described in, for example, Japanese Patent Publication No. 2002-206617.

[0037] A portion of the outer surface of the nut 20 is cut flat, and three return pipes 50 bent into a substantially U-shape are fixed on a plane 21 parallel to the axial direction thereof. The inner side of the return pipe 50 forms a return path for returning the balls 30 from one end of the rolling path 40 to the other end. In the nut 20, two through holes 22 are provided for each return pipe 50, which are open on the plane 21 and communicate with the thread groove 20a of the nut 20. The end 51 of each return pipe 50 is inserted into the two through holes 22 from the plane 21 side, and as shown in FIG. Figure 2 As shown, the return pipes 50 are arranged obliquely along the lead angle of the thread groove 10a. The central portion 52 of each return pipe 50 is arranged on the plane 21 and fixed by the fixing member 60. In this embodiment, three return pipes 50 are installed on one nut 20, but two or less or four or more return pipes may be installed.

[0038] The return pipe 50 is made of metal and has a structure in which a central portion 52 and end portions 51 connected to both ends thereof are integrated during manufacturing. However, the central portion 52 and the end portions 51 may be manufactured separately and then joined to form the return pipe 50 .

[0039] exist Figure 3 In the embodiment, the end 51 of the return pipe 50 has: a tongue piece 51b, which is formed at a phase of a part of the circumferential direction and protrudes in a tongue shape into the rolling path; and a pair of arc-shaped portions 51c, which are formed at the root of the tongue piece 51b. In the present embodiment, a substantially semicircular end edge 51a is formed on the side opposite to the tongue piece 51b and between the pair of arc-shaped portions 51c, and the end edge 51a is orthogonal to the center line of the end 51. The arc-shaped portion 51c intersects with the substantially semicircular end edge 51a in a manner of forming a corner, but the intersection of the arc-shaped portion 51c and the semicircular end edge 51a may also be formed into a smooth arc shape. The inner side surface of the tongue piece 51b (the surface opposite to the end edge 51a) has a shape that is curved with the same curvature radius as the inner circumference of the return pipe 50 and becomes thinner toward the end. The contour line from the arc-shaped portion 51c to the end of the tongue piece 51b draws a smooth curve.

[0040] The end portion 51 is provided in the through hole 22 such that the outer peripheral surface of the end portion 51 is fitted to the inner peripheral surface of the through hole 22 over the entire circumference and the end edge 51a contacts the step portion 22a formed in the through hole 22. Thus, the return pipe 50 can be positioned relative to the nut 20.

[0041] Figure 4 (a) is a diagram showing the end portion 51 cut off from the central portion 52 along the normal direction of an arbitrary point on the center line of the tongue piece 51b. Figure 4 (b) is from Figure 4 (a) is a diagram of the end portion 51 observed from different directions.

[0042] The length L of the tongue piece 51b is preferably at least 70% of the outer diameter of the ball 30. Figure 4 As shown in (a), the tongue piece 51b has a shape in which the width gradually narrows from the root toward the end P3, that is, the two side surfaces of the tongue piece 51b sandwiching the center line do not have parallel parts. At a distance of L / 2 from the end P3 of the tongue piece 51b, the width of the tongue piece 51b is preferably within a range of 1 / 2 to 2 / 3 of the outer diameter D of the return pipe 50. In addition, the curvature radius R of the inner edge of the arc-shaped portion 51c is preferably 1 / 10 to 3 / 10 of the outer diameter D of the return pipe 50. At least the hardness of the tongue piece 51b is preferably HRC30 or above.

[0043] Reference Figure 3 The ball 30 moves in the rolling path 40 while rolling multiple times around the threaded shaft 10 to reach the end of the rolling path 40 (the intersection of the return pipe 50 and the rolling path 40), and is scooped into the return pipe 50 from one end (opening) of the return pipe 50 via the tongue 51b.

[0044] More specifically, the ball 30 rolling along the thread groove 10a of the threaded shaft 10 is scooped up by the tongue piece 51b at the position of the through hole 22. In addition, the ball 30 passing through the return pipe 50 returns to the rolling path from the tongue piece 51b on the opposite side.

[0045] Here, when the ball screw 1 is used at high speed, the energy generated by the collision between the ball 30 rolling at high speed and the tongue 51b also increases. If the rigidity of the tongue 51b is too high, the local stress on the tongue 51b also increases, and cracks may occur at the root of the tongue 51b due to stress concentration. In addition, since the ball 30 intermittently collides with the tongue 51b, so-called repeated loads are applied to the tongue 51b, which may cause fatigue damage (such as peeling, etc.) to the tongue 51b.

[0046] Therefore, in this embodiment, the tongue piece 51b is formed into a shape whose width gradually narrows from the root toward the end P3, thereby controlling and reducing the rigidity of the tongue piece 51b, that is, stress relaxation is achieved by making it easy to bend. In addition, since the strength of the tongue piece 51b increases toward the root, the stress concentration at the root when the ball 30 collides can be alleviated.

[0047] In order to avoid stress concentration, it is preferred to gradually increase the cross-sectional inertia moment from the end P3 of the tongue piece 51b toward the root (arc-shaped portion 51c), that is, there may be no portion where the cross-sectional inertia moment increases sharply and no portion where the cross-sectional inertia moment is equal (a portion where the wall thickness and width of the tongue piece are equal). Therefore, the tongue piece 51b is formed into a fan-shaped shape extending from the end, and the two side surfaces do not have parallel portions.

[0048] exist Figure 4 In (a), on an imaginary plane passing through the end P3 of the tongue piece 51b and any two points P4 and P5 on two sides at equal distances from the end P3 of the tongue piece 51b, when the angle formed by the tangent lines L4 and L5 of the points P4 and P5 is defined as the opening angle θ, the opening angle θ gradually decreases as the distance x from the end P3 to the two points P4 and P5 along the center line of the tongue piece 51b increases. However, it is preferred that the opening angle θ gradually increases with the inflection point as the starting point within a range of (2L / 3) to L from the end P3 along the center line of the tongue piece 51b.

[0049] Here, the relationship between the impact force of the ball 30 and the rigidity of the tongue 51b is examined. The tongue 51b is modeled as a cantilever beam that can bend in a direction away from the center line of the return pipe 50. If its spring constant is set to K and the deflection amount of the end of the tongue 51b where the ball 30 collides is set to σ, the energy U accumulated in the cantilever beam is expressed by the following formula (1).

[0050] U=(K·σ 2 ) / twenty one)

[0051] On the other hand, assuming that the velocity when colliding with the tongue piece 51b is v and the mass thereof is m, the kinetic energy W of the ball 30 is expressed by the following equation (2).

[0052] W=(m·v 2 ) / twenty two)

[0053] When the tongue 51b is bent to the maximum extent, if the speed of the ball 30 in the direction of bending the tongue 51b is 0 m / s, then U=W holds. max Denotes σ, then σ=K·P max According to the above relationship, the following formula (3) holds.

[0054] Pmax =v√(mK) (3)

[0055] From equation (3), we can see that the maximum load P applied to the tongue 51b is max It is proportional to the (1 / 2)th power of the flexural rigidity K of the tongue piece 51b. From this result, it is known that in order to alleviate the ball impact force, it is effective to reduce the rigidity of the tongue piece 51b to a certain extent.

[0056] Here, in order to reduce the flexural rigidity of the tongue piece 51 b , the following two methods are proposed.

[0057] (First method)

[0058] The length L of the tongue piece 51 b is equal to or greater than 70% of the diameter of the ball 30 .

[0059] Usually, the tongue piece 51b is inserted into the thread groove 10a (depth is about 40% to 45% of the diameter of the ball 30) of the threaded shaft 10, so its length is slightly longer than the depth of the thread groove 10a, and is usually set to about 50% to 60% of the diameter of the ball 30. The longer the tongue piece 51b, the smaller the flexural rigidity, and thus the smaller the collision force.

[0060] The present inventors studied how much the tongue piece 51 b needs to be stretched in order to reduce the impact force by 20%. Figure 5 The following is a result of calculating how much the rigidity would decrease if the length L was increased, assuming that the rigidity of the tongue piece 51 b having a length L that is 60% of the diameter of the ball 30 is 1.

[0061] The energy accumulated in the existing tongue piece 51b due to the impact is set to 100%, and then reduced by 20% to 80%. Therefore, according to equation (1), the rigidity of the tongue piece 51b needs to be reduced to 64%, which is 80% squared. Figure 5 According to the diagram, the length L of the tongue piece 51b can be made more than 70% of the diameter of the ball 30. If it is set to more than 75% of the diameter of the ball 30, the rigidity of the tongue piece 51b is reduced by more than 50% and the stored energy is reduced by 30%, so it is more preferred.

[0062] (Second method)

[0063] The maximum thickness of the tongue piece 51 b is set within 25% of the diameter of the ball 30 .

[0064] By reducing the thickness of the tongue piece 51b, the rigidity in the bending direction can be reduced. On the other hand, if the wall thickness of the tongue piece 51b becomes too thin, the strength becomes too low to withstand the impact force, so it is preferably made thinner within a possible range, for example, it can be 10% or more of the diameter of the ball 30. In addition, it is preferred that the wall thickness of the tongue piece 51b is as uniform as possible, and no part with locally increased wall thickness is manufactured.

[0065] When the return pipe 50 is made of metal, the tongue piece 51b tends to have high rigidity, so the present embodiment can be more preferably applied. When the return pipe 50 is made of metal, it can be used at high temperatures, particularly 80° C. or higher, compared to when it is made of resin.

[0066] In addition, as the diameter of the ball 30 increases, the impact force increases accordingly. This embodiment can be more appropriately applied when the diameter of the ball 30 used in the ball screw 1 subjected to a high load is 12.7 mm or more and the diameter of the screw shaft 10 is 63 mm or more.

[0067] In order to further improve the strength, it is preferable that the material of the return pipe 50 be SUS630 or SUS631 having relatively high durability, and the hardness of the tongue piece 51 b be HRC30 or higher.

[0068] (Variation Example)

[0069] Figure 6 The end portion 51A of the return pipe according to the modification example is shown in FIG. Figure 4 In the present modification, the radius of curvature of the arc-shaped portion 51Ac is made smaller than that of the first embodiment. The other structures are the same as those of the first embodiment, and therefore, duplicate descriptions are omitted.

[0070] In the first embodiment and the modified example, the end of the return pipe is integrated from the time of manufacturing, but it is sometimes difficult to bend the pipe material three-dimensionally to form the return pipe. According to the following second embodiment, this problem can be overcome.

[0071] (Second Embodiment)

[0072] Figure 7 (a) is a diagram showing a first bisected return tube 50B1 when the return tube of the ball screw according to the second embodiment is bisected. Figure 7 (b) is a diagram showing the end portion 51B1 of the first bisected return pipe 50B1, shown with the viewing direction changed. Figure 7 (c) is a diagram showing the end 51B of the first bisected return pipe 50B1 and the other bisected return pipe, namely the second bisected return pipe 50B2. In the second embodiment, the structure other than the bisected return pipe and the end 51B is the same as that of the first embodiment, and therefore repeated description is omitted.

[0073] In a second embodiment, if Figure 7 As shown in (c), the return pipe is formed by joining the first bisected return pipe 50B1 and the second bisected return pipe 50B2, which are formed separately, at the mating surface FP. In this way, by forming the first bisected return pipe 50B1 and the second bisected return pipe 50B2 separately, the flexural rigidity can be reduced, and the impact force generated by the ball collision can be further reduced. However, the first bisected return pipe 50B1 and the second bisected return pipe can also be formed by further dividing the central part and the two end sides.

[0074] In this embodiment, by making the return pipe into a bisection, the tongue piece 51Bb formed at the end 51B1 of the first bisection return pipe 50B1 can be set to any shape with high precision, and the manufacturing restrictions are reduced. In addition, it is not necessary to provide an arc-shaped portion at the root of the tongue piece 51Bb to avoid stress concentration. Therefore, in this modification example 2, as shown in FIG. Figure 7 As shown in (c), the root of the tongue piece 51Bb is joined to the end edge 51Ba of the second bisected return pipe 50B2. However, an arc-shaped portion may be formed as in the above-mentioned embodiment. In addition, three or more components manufactured separately may be joined to form the end 51B.

[0075] (Variant 1)

[0076] Figure 8 The ball screw including the return pipe according to the first modification is shown in FIG. Figure 3 The structure other than the end portion 51C of the return pipe is the same as that of the second embodiment, and therefore, duplicate description will be omitted.

[0077] The return pipe of this modified example is also formed by joining the first split return pipe 50C1 and the second split return pipe 50C2 which are manufactured and joined separately, and the end portion 51C is composed of the end portion 51C1 of the first split return pipe 50C1 and the end portion 51C2 of the second split return pipe 50C2. Figure 7The end 51B1 of the first bisected return pipe 50B1 shown in the figure has a substantially identical shape and is provided with a tongue piece 51Cb. On the other hand, the end 51C2 of the second bisected return pipe 50C2 has a bulge 51Cd with increased wall thickness formed in the upper half 51Ca on the side opposite to the tongue piece 51Cb relative to the end 51B2 of the second bisected return pipe 50B2. The outer surface of the bulge 51Cd has a shape substantially consistent with the inner peripheral shape of the through hole 22. Thus, relative to the contact point (the side close to the center) P1 where the end 51C1 of the first bisected return pipe 50C1 contacts the inner peripheral surface of the through hole 22, the bulge 51Cd makes surface contact with the inner peripheral surface of the through hole 22 on the side opposite to the center line of the end 51C. Therefore, when the end 51C is set in the through hole 22, the retention of the return pipe 50C is improved.

[0078] In addition, the outer peripheral surface of the end portion 51C on the side opposite to the raised portion 51Cd (the root side of the tongue piece 51Cb) contacts the through hole 22 at the contact point P1, but does not contact the through hole 22 on the central side of the nut 20 relative to the contact point P1. That is, between the end portion 51C1 of the first bisected return pipe 50C1 and the inner peripheral surface of the through hole 22, a gap CL is formed on the central side of the nut relative to the contact point P1 in the phase in the circumferential direction of the tongue piece 51Cb, and the flexure of the tongue piece 51Cb is allowed by the gap CL.

[0079] (Variant 2)

[0080] Fig. 9 is a diagram showing the ball screw involved in Modification Example 2. Figure 8 The same cross-sectional view. In this modification, the same return pipe 50C as in the modification 1 is used, and the buffer material 55 is fixedly arranged in the gap CL between the end 51C of the return pipe 50C (including the root side of the tongue piece 51Cb) and the through hole 22 using an adhesive or the like. The other structures are the same as those in the modification 1, so repeated descriptions are omitted. The buffer material 55 can be arranged between the root of the tongue piece 51Cb and the inner peripheral surface of the through hole 22.

[0081] By arranging the buffer material 55 in the gap CL, the flexibility of the tongue piece 51Cb can be ensured, and the impact force when the ball 30 collides with the tongue piece 51Cb can be mitigated. As a raw material of the buffer material 55, a soft elastic material such as rubber, resin, and spring can be used.

[0082] The present invention is not limited to the above-described embodiment. Any constituent elements of the above-described embodiment may be modified within the scope of the present invention. In addition, any constituent element may be added or omitted in the above-described embodiment.

[0083] Various embodiments have been described above, but the present invention is certainly not limited to these examples. As long as one skilled in the art can think of various variations or modifications within the scope described in the claims, it is obvious that these variations or modifications also belong to the technical scope of the present invention. In addition, within the scope of not departing from the gist of the invention, the various constituent elements in the above-mentioned embodiments may also be arbitrarily combined.

[0084] In addition, this application is based on the Japanese patent application (Japanese Patent Application No. 2022-148014) filed on September 16, 2022, and the contents are incorporated herein by reference.

[0085] Description of Reference Numerals

[0086] 10 Threaded shaft

[0087] 20 Nut

[0088] 30 Ball

[0089] 31 Bulkhead

[0090] 50, 50C return pipe

[0091] 51, 51A, 51B, 51C end

[0092] 51Ca upper part

[0093] 51b, 51Ab, 51Bb, 51Cb tongue

[0094] 51c, 51Ac arc-shaped portion

[0095] 52 Central

[0096] 55 Cushioning material

[0097] 60 Fixing parts

Claims

1. A ball screw, characterized in that: have: A threaded shaft, wherein the threaded shaft is formed with an outer peripheral thread groove; A nut having an inner circumferential thread groove; A plurality of balls, wherein the plurality of balls are accommodated in a rolling path formed by the outer peripheral thread groove and the inner peripheral thread groove that are opposed to each other; as well as a return pipe, the return pipe allowing the balls to return from one end of the rolling path to the other end, The return pipe has a tongue, which protrudes toward the rolling path and picks up the ball from the rolling path. The tongue piece has a shape in which the width gradually narrows from the root portion toward the tip portion of the tongue piece.

2. The ball screw according to claim 1, characterized in that: An arc-shaped portion having a predetermined curvature radius is formed at a base of the tongue piece.

3. The ball screw according to claim 1, characterized in that: The end of the return pipe is inserted into the through hole formed in the nut, and contacts the through hole on both sides sandwiching the center line of the end.

4. The ball screw according to claim 3, characterized in that: Between the end of the return pipe and the inner periphery of the through hole, a gap is formed on the center side of the nut with respect to a contact point between the end and the through hole in a phase in the circumferential direction forming the tongue.

5. The ball screw according to claim 4, characterized in that: A cushioning material is disposed in the gap.

6. The ball screw according to claim 1, characterized in that: The length of the tongue piece is set to be greater than 70% of the diameter of the ball, or the maximum wall thickness of the tongue piece is set to be within 25% of the diameter of the ball.

7. The ball screw according to any one of claims 1 to 6, characterized in that: The end portion of the return pipe is formed by joining a plurality of members that are manufactured separately.

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