Ball screw
The ball screw design addresses high-speed durability issues by using a narrowing tongue and optional split tubes or cushioning to reduce stress and fatigue, enabling faster operation.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- NSK LTD
- Filing Date
- 2022-09-16
- Publication Date
- 2026-07-07
AI Technical Summary
Existing ball screws face challenges in increasing rotational speed due to high stress and fatigue issues in the tang when balls collide with the tongue at high speeds, limiting their durability and speed capabilities.
A ball screw design with a return tube having a tongue that gradually narrows from the base to the tip, reducing the rigidity and stress concentration, and optionally using split return tubes or cushioning materials to mitigate impact forces.
The design enhances durability and allows for higher rotational speeds by reducing stress and fatigue in the tongue, improving the ball screw's performance under high-speed conditions.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to ball screws.
Background Art
[0002] A ball screw includes a nut having a female thread groove formed on its inner peripheral surface, a screw shaft having a male thread groove formed on its outer peripheral surface, balls disposed between the tracks formed by the female thread groove of the nut and the male thread groove of the screw shaft, and a ball return path for returning the balls from the end point to the start point of the track. The nut moves relative to the screw shaft as the balls roll within the track. As a ball return path for the ball screw, a return tube method may be adopted because of advantages such as ease of assembly.
[0003] Patent Document 1 discloses an example of a ball screw of the return tube type. In this ball screw, the tip of the return tube as a circulation component consists of an end face along a baseline perpendicular to the tube axis and a tongue protruding tongue-shaped from the baseline. The tongue protrudes into the track and functions to scoop up the balls from the track into the return tube.
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0005] By the way, the demand for speeding up ball screws is constantly made from the market in order to shorten the cycle time of machines using ball screws. However, in order to achieve the speeding up of ball screws, it is necessary to increase the allowable rotational speed of the screw shaft. However, by increasing the allowable rotational speed of the screw shaft, the balls will roll along the track at high speed.
[0006] In the ball screw described in Patent Document 1, the balls rolling at high speed collide with the tongue when they are scooped up from the track. At that time, a large stress is applied to the base of the tongue, and since the circulating balls successively hit the tongue, the tongue is subjected to repeated loads.
[0007] Generally, the allowable rotational speed of a screw shaft is determined by the fatigue strength due to ball collisions in the circulating components. Fatigue strength is generally determined by the magnitude of the repeated stress and the number of load cycles. Therefore, reducing the stress generated in the tang is a key challenge in increasing the speed of ball screws.
[0008] To address these challenges and accommodate higher speeds, ball screws have been developed that reduce the impact on the tang by aligning it tangentially to the screw groove to scoop up the balls. However, it is difficult to perfectly align a thick tang tangentially, and as the rotational speed increases, the impact on the tang from the balls also increases, so there is a need to further reduce the stress on the tang.
[0009] This invention has been made in view of the above problems, and aims to provide a ball screw that can handle high speeds and has improved durability. [Means for solving the problem]
[0010] The ball screw of the present invention is A screw shaft with an external thread groove formed therein, A nut with an internal thread groove, Multiple balls housed within a rolling path formed by the opposing outer circumferential screw grooves and inner circumferential screw grooves, The system includes a return tube that returns the ball from one end of the rolling path to the other end, The return tube protrudes toward the rolling path and has a tongue for scooping up the ball from the rolling path. The tongue has a shape that gradually narrows from the base to the tip of the tongue. This is the feature.
Effect of the Invention
[0011] According to the present invention, it is possible to provide a ball screw that can cope with high speed and improve durability.
Brief Explanation of Drawings
[0012] [Figure 1] FIG. 1 is a perspective view showing the configuration of a ball screw according to the first embodiment. [Figure 2] FIG. 2 is a partial cross-sectional view taken along the axial direction showing a part of the ball screw according to the first embodiment. [Figure 3] FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2, but only a part is shown. [Figure 4] FIGS. 4(a) and (b) are views showing the ends of the return tube seen from different directions. [Figure 5] FIG. 5 is a graph showing the rigidity ratio of the tongue tip on the vertical axis (with a ratio of the tongue length to the ball diameter of 60% as a reference) and the ratio of the tongue length to the ball diameter on the horizontal axis. [Figure 6] FIGS. 6(a) and (b) are views similar to FIGS. 4(a) and (b) showing the ends of the return tube according to the modified example. [Figure 7] FIGS. 7(a) to (c) are views showing the ends of the return tube of the ball screw according to the second embodiment. [Figure 8] FIG. 8 is a cross-sectional view similar to FIG. 3 showing a ball screw including a return tube according to the first modified example. [Figure 9] FIG. 9 is a cross-sectional view similar to FIG. 8 showing a ball screw according to the second modified example.
Modes for Carrying Out the Invention
[0013] Hereinafter, embodiments of the ball screw according to the present invention will be described with reference to the drawings.
[0014] (First Embodiment) FIG. 1 is a perspective view showing the configuration of a ball screw according to the first embodiment, FIG. 2 is a partial cross-sectional view along the axial direction showing a part of the ball screw according to the first embodiment, and FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2. Only the portion corresponding to the first quadrant when the cross-sectional view seen in the direction of arrow A is regarded as a two-dimensional coordinate system is shown, and the separator is omitted.
[0015] As shown in FIGS. 1 and 2, the ball screw 1 of the present embodiment includes a screw shaft 10 having a helically continuous screw groove (outer peripheral screw groove) 10a formed on its outer peripheral surface, and a cylindrical nut 20 having a helically continuous screw groove (inner peripheral screw groove) 20a formed on its inner peripheral surface.
[0016] The screw shaft 10 is inserted into the nut 20 along the axial direction. The screw groove 10a of the screw shaft 10 and the screw groove 20a of the nut 20 face each other, and a helical rolling path 40 is formed by the space between the two screw grooves 10a and 20a. A plurality of balls (rolling elements) 30 are loaded in the rolling path 40 so as to be freely rotatable, and the nut 20 is coupled to the screw shaft 10 so as to be relatively rotatable via the balls 30. In the present embodiment, a separator 31 is disposed between adjacent balls 30 to suppress contact between the balls 30, but the separator may be omitted. The separator is described, for example, in Japanese Patent Application Laid-Open No. 2002-206617.
[0017] A portion of the outer surface of the nut 20 is machined flat, and three return tubes 50, bent in a roughly U-shape, are fixed to this plane 21 parallel to the axial direction. The inside of these return tubes 50 forms a return path that returns the balls 30 from one end to the other of the rolling path 40. The nut 20 has two through holes 22 for each return tube 50 that open into this plane 21 and communicate with the screw groove 20a of the nut 20. The ends 51 of each return tube 50 are inserted into these through holes 22 from the plane 21 side and are positioned at an angle along the lead angle of the screw groove 10a, as shown in Figure 2. The central portion 52 of each return tube 50 is positioned on the plane 21 and fixed by a fastener 60. In this embodiment, three return tubes 50 are attached to one nut 20, but two or fewer, or four or more return tubes may be attached.
[0018] The return tube 50 is made of metal and has a structure in which the central part 52 and the ends 51 connected to both ends are integrated from the time of manufacture. However, the central part 52 and the ends 51 may be manufactured separately and then joined together to form the return tube 50.
[0019] In Figure 3, the end portion 51 of the return tube 50 has a tongue 51b formed at a certain phase in the circumferential direction, which protrudes in a tongue shape into the rolling path, and a pair of arc-shaped portions 51c formed at the base of the tongue 51b. In this embodiment, a substantially semicircular edge 51a is formed between the pair of arc-shaped portions 51c on the side facing the tongue 51b, and the edge 51a is perpendicular to the center line of the end portion 51. The arc-shaped portions 51c and the substantially semicircular edge 51a intersect to form an angle, but the intersection of the arc-shaped portions 51c and the semicircular edge 51a may be formed as a smooth arc. The inner surface of the tongue 51b (the surface facing the edge 51a) is curved with the same radius of curvature as the inner circumference of the return tube 50 and has a shape that becomes thinner towards the tip. The contour line from the arc-shaped portion 51c to the tip of the tongue 51b is a smooth curve.
[0020] The end portion 51 is installed in the through-hole 22 such that its outer circumferential surface fits all the way around the inner circumferential surface of the through-hole 22, and its end edge 51a abuts against the stepped portion 22a formed inside the through-hole 22. This allows the return tube 50 to be positioned relative to the nut 20.
[0021] Figure 4(a) shows the end portion 51 cut from the central portion 52, viewed along the direction normal to an arbitrary point on the center line of the tongue 51b, and Figure 4(b) shows the end portion 51 viewed from a different direction than that in Figure 4(a).
[0022] The length L of the tongue 51b is preferably 70% or more of the outer diameter of the ball 30. Also, as shown in Figure 4(a), the tongue 51b has a shape that gradually narrows from the base toward the tip P3, that is, the two sides of the tongue 51b that straddle the center line do not have parallel parts. At a distance of L / 2 from the tip P3 of the tongue 51b, the width of the tongue 51b is preferably in the range of 1 / 2 to 2 / 3 of the outer diameter D of the return tube 50. Also, the radius of curvature R at the inner edge of the arc-shaped portion 51c is preferably 1 / 10 to 3 / 10 of the outer diameter D of the return tube 50. The hardness of the tongue 51b is preferably HRC30 or higher.
[0023] Referring to Figure 3, the ball 30 moves within the rolling path 40, rotating multiple times around the screw shaft 10 until it reaches the end of the rolling path 40 (the intersection of the return tube 50 and the rolling path 40), and is scooped up into the return tube 50 via the tongue 51b from one end (opening) of the return tube 50.
[0024] More specifically, the balls 30 that have rolled along the screw groove 10a of the screw shaft 10 are picked up by the tongue 51b at the position of the through hole 22. Also, the balls 30 that have passed through the return tube 50 are returned to the rolling path by the tongue 51b on the opposite side.
[0025] Here, if the ball screw 1 is used at high speed, the energy generated when the balls 30, which have been rolling at high speed, collide with the tongue 51b also increases. If the rigidity of the tongue 51b is too high, the local stress on the tongue 51b will also increase, and stress concentration may cause cracks or other damage at the base of the tongue 51b. In addition, since the balls 30 collide with the tongue 51b intermittently, a so-called repeated load is applied to the tongue 51b, which may cause fatigue failure (e.g., delamination) in the tongue 51b.
[0026] Therefore, in this embodiment, the tongue 51b is shaped to gradually narrow from the base to the tip P3, thereby controlling and reducing the rigidity of the tongue 51b, i.e., making it more flexible, and thus relieving stress. Furthermore, since the strength of the tongue 51b increases towards the base, stress concentration at the base during impact with the ball 30 can be mitigated.
[0027] To avoid stress concentration, it is desirable to gradually increase the second moment of area from the tip P3 of the tongue 51b towards the base (arc-shaped portion 51c), that is, to avoid having any points where the second moment of area increases abruptly or where the second moment of area is equal (where the thickness and width of the tongue are equal). For this reason, the tongue 51b is shaped to spread out in a fan shape from the tip, and there are no parts where both sides are parallel.
[0028] In Figure 4(a), if the angle between the tangents L4 and L5 to points P4 and P5 on the virtual plane passing through the tip P3 of the tongue 51b and any two points P4 and P5 on both sides equidistant from tip P3 is defined as the opening angle θ, then the opening angle θ gradually decreases as the distance x from tip P3 to the two points P4 and P5 along the centerline of the tongue 51b increases. However, it is desirable that the opening angle θ gradually increases from the inflection point in the range of (2L / 3) to L from tip P3 along the centerline of the tongue 51b.
[0029] Here, we consider the relationship between the impact force of the ball 30 and the stiffness of the tongue 51b. Modeling the tongue 51b as a cantilever beam that can bend away from the centerline of the return tube 50, and assuming its spring constant is K and the amount of deflection at the tip of the tongue 51b where the ball 30 collides is σ, the energy U stored in the cantilever beam can be expressed by the following equation (1). U=(K·σ 2 ) / twenty one)
[0030] On the other hand, the kinetic energy W of ball 30 can be expressed by the following equation (2), where v is the velocity and m is the mass when it collides with tongue 51b. W=(m·v 2 ) / twenty two)
[0031] If we assume that when the tongue 51b is at its maximum deflection, the velocity of the ball 30 in the direction that deflects the tongue 51b is 0 m / s, then U=W holds true. Furthermore, σ is the maximum load P at the time of collision. max Expressed as, σ = K·P max Therefore, from the above relationship, the following equation (3) holds true. P max =v√(mK) (3)
[0032] From equation (3), the maximum load P applied to the tongue 51b is max It can be seen that this is proportional to the (1 / 2) power of the deflection stiffness K of the tongue 51b. From this result, it can be seen that reducing the stiffness of the tongue 51b to some extent is effective in mitigating the ball impact force.
[0033] Here, we propose the following two methods to reduce the deflection stiffness of the tongue 51b.
[0034] (First method) The length L of the tongue 51b shall be 70% or more of the diameter of the ball 30. Typically, the tongue 51b is inserted into the thread groove 10a of the screw shaft 10 (which has a depth of approximately 40-45% of the diameter of the ball 30). Therefore, its length is slightly longer than the depth of the thread groove 10a, and is generally set to about 50-60% of the diameter of the ball 30. The longer the tongue 51b, the lower the deflection stiffness, and thus the smaller the impact force.
[0035] The inventors investigated how much the tongue 51b should be stretched to achieve a 20% reduction in impact force. Figure 5 shows the results of calculations showing how much the stiffness decreases when the length L of the tongue 51b is increased, with the stiffness of the tongue 51b having a length L that is 60% of the diameter of the ball 30 being set to 1.
[0036] To reduce the energy stored in the tongue 51b by 20% from 100% to 80% due to impact, the stiffness of the tongue 51b must be reduced to 80% squared, which is 64%, according to equation (1). To achieve this, as shown in the graph in Figure 5, it is preferable to set the length L of the tongue 51b to 70% or more of the diameter of the ball 30, and even more preferable to set it to 75% or more of the diameter of the ball 30, as this reduces the stiffness of the tongue 51b by 50% or more and reduces the stored energy by 30%.
[0037] (Second method) The maximum wall thickness of the tongue 51b shall be no more than 25% of the diameter of the ball 30. By reducing the thickness of the tongue 51b, the rigidity in the bending direction can be decreased. On the other hand, if the thickness of the tongue 51b becomes too thin, the strength will be too low and it will not be able to withstand impact forces, so it is desirable to make it as thin as possible, for example, 10% or more of the diameter of the ball 30. It is also desirable to make the thickness of the tongue 51b as uniform as possible and to avoid creating areas where the thickness increases locally.
[0038] If the return tube 50 is made of metal, the rigidity of the tongue 51b tends to be higher, making this embodiment more suitable for application. When the return tube 50 is made of metal, it can be used at high temperatures, especially above 80°C, compared to when it is made of resin.
[0039] Furthermore, as the diameter of the ball 30 increases, the impact force also increases accordingly. This embodiment can be more suitably applied when the diameter of the ball 30 used in the ball screw 1 subjected to high loads is 12.7 mm or more, and the diameter of the screw shaft 10 is 63 mm or more.
[0040] To further increase strength, it is desirable to use SUS630 or SUS631, which have relatively high durability, for the material of the return tube 50, and to set the hardness of the tang 51b to HRC30 or higher.
[0041] (modified version) Figure 6 is a diagram similar to Figure 4, showing the end portion 51A of the return tube in the modified example. In this modified example, the radius of curvature of the arc-shaped portion 51Ac is made smaller than in the first embodiment. The other configurations are the same as in the first embodiment, so a redundant explanation is omitted.
[0042] In the first embodiment and its modifications, the end of the return tube is integrated from the time of manufacture, but it can be difficult to form the return tube by three-dimensionally bending the tube material. The second embodiment described below can overcome this problem.
[0043] (Second embodiment) Figure 7(a) shows one half of the ball screw return tube according to the second embodiment, the first half-return tube 50B1. Figure 7(b) shows the end portion 51B1 of the first half-return tube 50B1, shown from a different viewing direction. Figure 7(c) shows the end portion 51B where the first half-return tube 50B1 and the second half-return tube 50B2, which is the other half of the return tube, are joined. In the second embodiment, the return tube is formed in half, and the configuration other than the end portion 51B is the same as in the first embodiment, so a redundant explanation is omitted.
[0044] In the second embodiment, as shown in Figure 7(c), a return tube is formed by joining a first split return tube 50B1 and a second split return tube 50B2, which are formed separately, at a mating surface FP. By forming the first split return tube 50B1 and the second split return tube 50B2 in this way, the deflection rigidity can be reduced, and the impact force due to ball collision can be further reduced. However, the first split return tube 50B1 and the second split return tube may be formed by further dividing the central part and both ends.
[0045] In this embodiment, since the return tube is split in half, the tongue 51Bb formed at the end 51B1 of the first split return tube 50B1 can be precisely set to any shape, thus reducing manufacturing limitations. Furthermore, it is not necessary to provide an arc-shaped portion at the base of the tongue 51Bb to avoid stress concentration. Therefore, in this modified example 2, as shown in Figure 7(c), the base of the tongue 51Bb is joined to the end edge 51Ba of the second split return tube 50B2. However, an arc-shaped portion may be formed as in the embodiment described above. In addition, the end 51B may be formed by joining three or more individually manufactured parts.
[0046] (Variation 1) Figure 8 is a cross-sectional view similar to Figure 3, showing a ball screw including a return tube according to Modification 1. The configuration other than the end 51C of the return tube is the same as in the second embodiment, so a redundant explanation is omitted.
[0047] The return tube in this modified example is also formed by joining a first split return tube 50C1 and a second split return tube 50C2, which are manufactured and joined separately. The end 51C consists of the end 51C1 of the first split return tube 50C1 and the end 51C2 of the second split return tube 50C2. The end 51C1 of the first split return tube 50C1 has a shape substantially similar to the end 51B1 of the first split return tube 50B1 shown in Figure 7 and is equipped with a tongue 51Cb. On the other hand, the end 51C2 of the second split return tube 50C2 has a thickened raised portion 51Cd in the upper half 51Ca formed on the side facing the tongue 51Cb relative to the end 51B2 of the second split return tube 50B2. The outer surface of the raised portion 51Cd has a shape substantially consistent with the inner circumference shape of the through hole 22. As a result, the raised portion 51Cd of the end portion 51C of the first half-split return tube 50C1 makes surface contact with the inner circumferential surface of the through hole 22 on the side opposite to the center line of the end portion 51C, where the end portion 51C1 of the first half-split return tube 50C1 makes contact with the inner circumferential surface of the through hole 22 (on the side closer to the center). Therefore, when the end portion 51C is installed in the through hole 22, the retention of the return tube 50C is improved.
[0048] Furthermore, the outer circumferential surface of the end portion 51C opposite to the raised portion 51Cd (the side where the tongue 51Cb is attached) contacts the through hole 22 at contact point P1, but does not contact the nut 20 on the side of contact point P1 that is closer to the center of the nut 20. In other words, a gap CL is formed between the end portion 51C1 of the first split return tube 50C1 and the inner circumferential surface of the through hole 22, on the side of contact point P1 that is closer to the center of the nut in the circumferential phase in which the tongue 51Cb is formed, and this gap CL allows for the bending of the tongue 51Cb.
[0049] (Modification 2) Figure 9 is a cross-sectional view similar to Figure 8, showing a ball screw according to Modification 2. In this modification, the same return tube 50C as in Modification 1 is used, but the difference is that a cushioning material 55 is fixedly placed in the gap CL between the end 51C of the return tube 50C (including the base side of the tongue 51Cb) and the through hole 22 using adhesive or the like. The other configurations are the same as in Modification 1, so a redundant explanation is omitted. The cushioning material 55 should be placed between the base of the tongue 51Cb and the inner circumferential surface of the through hole 22.
[0050] By placing the cushioning material 55 in the gap CL, the flexibility of the tongue 51Cb can be ensured while mitigating the impact force when the ball 30 collides with the tongue 51Cb. The cushioning material 55 can be made of a soft elastic material such as rubber, resin, or a spring.
[0051] The present invention is not limited to the embodiments described above. Within the scope of the present invention, any component of the embodiments described above can be modified. Furthermore, any component can be added to or omitted in the embodiments described above. [Explanation of Symbols]
[0052] 10 Screw shaft 20 nuts 30 balls 31 Separator 50,50C return tube 51,51A,51B,51C End 51Ca upper half 51b, 51Ab, 51Bb, 51Cb tongue 51c, 51Ac Arc-shaped portion 52 Central part 55 Cushioning material 60 fasteners
Claims
1. A screw shaft with an external screw groove formed therein, A nut with an internal thread groove, Multiple balls housed within a rolling path formed by the opposing outer circumferential screw grooves and inner circumferential screw grooves, The system includes a return tube that returns the ball from one end of the rolling path to the other end, The return tube protrudes toward the rolling path and has a tongue for scooping up the ball from the rolling path. The tongue has a shape that gradually narrows from the base of the tongue towards the tip. The end of the return tube is inserted into the through hole formed in the nut, and is in contact with the through hole on both sides of the center line of the end, Between the end of the return tube and the inner circumference of the through hole, a gap is formed on the nut side relative to the contact point between the end and the through hole, in the circumferential phase in which the tongue is formed. A ball screw characterized by the following features.
2. An arc-shaped portion having a predetermined radius of curvature is formed at the base of the tongue. The ball screw according to feature 1.
3. A cushioning material is placed in the gap, The ball screw according to feature 1.
4. The length of the tongue shall be 70% or more of the diameter of the ball, or the maximum wall thickness of the tongue shall be 25% or less of the diameter of the ball. The ball screw according to feature 1.
5. The two sides of the tongue do not have portions that are parallel to each other. The ball screw according to feature 1.
6. The end of the return tube is formed by joining together a plurality of individually manufactured parts. A ball screw according to any one of claims 1 to 5.
7. A screw shaft having an outer screw groove formed thereon, A nut with an internal thread groove, Multiple balls housed within a rolling path formed by the opposing outer circumferential screw grooves and inner circumferential screw grooves, The system includes a return tube that returns the ball from one end of the rolling path to the other end, The return tube protrudes toward the rolling path and has a tongue for scooping up the ball from the rolling path. The end of the return tube is inserted into the through hole formed in the nut. Between the end of the return tube and the inner circumference of the through hole, a gap is formed on the nut side relative to the contact point between the end and the through hole, in the circumferential phase in which the tongue is formed. A ball screw characterized by the following features.
8. The tongue has a shape that gradually narrows from the base of the tongue towards the tip. The ball screw according to feature 7.
9. The two sides of the tongue do not have portions that are parallel to each other. The ball screw according to feature 8.
10. The end of the return tube is in contact with the through hole on both sides of the center line of the end, The ball screw according to feature 7.