Ball screw

Abstract

In a ball screw, ensuring the balls are reliably unloaded at the introduction point of the circulation groove improves the durability of the ball screw and ensures smooth operation. [Solution] In the cross-section in the contact angle direction between the female screw groove 8 and the male screw groove 7 and the ball 4, the introduction portion 9a of the circulation groove 9 is positioned on the outer diameter side of the rolling surface of the female screw groove 8. Let Δ be the distance in the contact angle direction between the introduction portion 9a and the rolling surface of the female screw groove 8 at the boundary between the female screw groove 8 and the circulation groove 9. When the maximum allowable load is applied to the ball screw 1, let δM be the sum of the elastic deformation amounts in the contact angle direction between the rolling surface of the female screw groove 8 and the ball 4 at the contact portion M, and let δS be the sum of the elastic deformation amounts in the contact angle direction between the rolling surface of the male screw groove 7 and the ball 4 at the contact portion S, then Δ > Δ min The equation = δM + δS is satisfied.

Description

Technical Field

[0001] The present invention relates to ball screws.

Background Art

[0002] A ball screw is a motion conversion mechanism that mutually converts rotational motion and linear motion, and is used, for example, to convert the rotational motion of an electric motor into linear motion. A ball screw includes a screw shaft and a nut that rotate relative to each other via a plurality of balls, and a circulation member provided on the nut. When the screw shaft and the nut rotate relative to each other, the balls circulate (orbit) along a loop-shaped ball passage including a rolling path formed by a male screw groove formed on the outer peripheral surface of the screw shaft and a female screw groove formed on the inner peripheral surface of the nut, and a circulation portion formed by the circulation member. Examples of the circulation member include a circulation ball, an end cap, a return tube, a guide plate, etc., which are selectively used according to the application of the ball screw (see Patent Document 1 below).

[0003] In a ball-type ball screw that uses a circulation ball as the circulation member, a female screw groove is formed on the inner peripheral surface of the nut, and a circulation groove is formed on the surface on the inner diameter side (the side of the nut axis) of the circulation ball. For example, in the ball screw shown in Patent Document 2 below, the circulation groove formed in the circulation ball is composed of a pair of introduction straight portions, a pair of transition curve portions, and an intermediate connection portion. Among these, in the introduction straight portion, by guiding the ball to a no-load state without changing the traveling direction of the ball, it is possible to suppress the occurrence of ball jamming in the circulation path and smooth the relative rotation of the screw shaft with respect to the nut (paragraph 0009 of the same document).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

[0005] As described above, in order to transition the ball from a loaded state to an unloaded state in the straight section of the circulation groove, for example, the straight section of the circulation groove formed in the circulating spindle can be moved outward, gradually increasing the radial distance between the straight section and the male screw groove of the screw shaft. In other words, if the distance from the screw shaft axis to the male screw groove surface is constant in both the rolling path and the circulation path, the groove surface of the straight section can be gradually made radially larger than the female screw groove surface. However, in order to appropriately transition the load from the rolling path before entering the straight section to the straight section, it is necessary to position the straight section radially with high precision relative to the female screw groove. For example, if the groove surface of the straight section is radially inward from the female screw groove surface, the ball load in the straight section will be greater than the ball load in the rolling path, inducing torque fluctuations, vibration and noise, and further negatively affecting durability. Conversely, if the groove surface of the introduction straight section is radially outward from the nut female thread groove surface, and the distance between the two is large, no load will be applied to the ball in the introduction straight section, and the transition from a loaded state to an unloaded state in the introduction straight section cannot be achieved. Furthermore, if the distance between the two is excessive, it can cause abnormal noise and vibration due to the ball colliding with the circulation groove wall.

[0006] In ball screws, individually manufactured nuts and circulating balls are generally integrated into a nut assembly by press-fitting, crimping, or screw fastening. While this manufacturing method is effective in suppressing increases in manufacturing costs, it makes it difficult to achieve high-precision radial positioning of the straight section that enters the female screw groove, as described above.

[0007] Therefore, the present invention aims to improve the durability of a ball screw and ensure smooth operation by ensuring that the balls are reliably unloaded at the introduction section of the circulation groove. [Means for solving the problem]

[0008] To solve the above problems, the present invention provides a ball screw comprising: a screw shaft having a helical male screw groove formed on its outer circumference; a nut body having a helical female screw groove formed on its inner circumference opposite to the male screw groove; a circulating ball attached to the nut body and having a circulating groove connecting the end and start of the female screw groove; and a plurality of balls arranged in a loop-shaped ball passage having a rolling path formed by the male screw groove of the screw shaft and the female screw groove of the nut body, and a circulating path formed by the circulating groove of the circulating ball, wherein The circulation groove has a pair of introduction portions connected to the female screw groove and extending in the same direction as the female screw groove, and a connecting portion extending in a direction different from the female screw groove and connecting the pair of introduction portions. In the cross-section in the direction of the contact angle between the female screw groove and the male screw groove and the ball, the introduction portion of the circulation groove is positioned on the outer diameter side of the rolling surface of the female screw groove, and the distance in the direction of the contact angle between the introduction portion and the rolling surface of the female screw groove at the boundary between the female screw groove and the circulation groove is Δ. When the maximum allowable load is applied to the ball screw, the sum of the elastic deformation amounts in the direction of the contact angle between the rolling surface of the female screw groove and the ball at the contact point is δM, and the sum of the elastic deformation amounts in the direction of the contact angle between the rolling surface of the male screw groove and the ball at the contact point is δS, Δ>Δ min =δM +δS We provide ball screws that meet the requirements.

[0009] Thus, in this invention, based on the elastic deformation amounts δM and δS of the ball and both screw grooves when the maximum allowable load is applied to the ball screw, the minimum allowable value Δ of the step difference Δ between the introduction portion of the circulation groove and the rolling surface of the female screw groove is determined. min This setting was established. In this way, instead of defining the distance between the radially moving circulation groove and the male screw groove, the step difference between the rolling surface of the female screw groove, which does not move relative to it, and the introduction of the circulation groove can be defined, ensuring that the balls are in an unloaded state at the introduction of the circulation groove.

[0010] When the diameter of the ball is Da, the above step Δ is 5·Δ minIt is preferable to make it smaller than the smaller of 0.07·Da. This suppresses the generation of abnormal noise and vibration caused by the ball's collision with the wall surface of the circulation groove when the ball enters the introduction section of the circulation groove.

[0011] In the ball screw described above, the cross-section of the female screw groove perpendicular to the ball's center trajectory can be a Gothic arch shape. On the other hand, the cross-section of the circulating groove perpendicular to the ball's center trajectory can be a single circular arc shape or a Gothic arch shape. [Effects of the Invention]

[0012] As described above, the ball screw according to the present invention ensures that the ball is reliably unloaded at the introduction point of the circulation groove. This prevents excessive contact between the introduction point of the circulation groove and the ball, improving the durability of the ball screw and ensuring smooth operation. [Brief explanation of the drawing]

[0013] [Figure 1] This is a plan view of a ball screw according to one embodiment of the present invention. [Figure 2] This is a cross-sectional view taken along the line II-II in Figure 1. [Figure 3] This is a schematic plan view showing the screw shaft and circulating spool of the ball screw described above. [Figure 4] This is a view of the circulating valve from the inner diameter side. [Figure 5] This is a radial cross-sectional view along the trajectory of the ball's center in the spinning top. [Figure 6] This is a schematic diagram of a circulation channel unfolded in plan. [Figure 7] This is a cross-sectional view of the contact area between the male and female screw grooves and the ball, taken in a direction perpendicular to the ball's center trajectory. [Figure 8] This is a cross-sectional view taken near the boundary between the rolling path and the circulation path, parallel to the ball's center trajectory. [Figure 9] This is a cross-sectional view of a ball screw according to another embodiment, taken in a direction parallel to the ball center trajectory, near the boundary between the rolling path and the circulation path.

Best Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0015] As shown in FIGS. 1 and 2, a ball screw 1 according to an embodiment of the present invention includes a screw shaft 2, a nut 3 externally fitted to the screw shaft 2, and a plurality of balls 4 disposed between the screw shaft 2 and the nut 3. The nut 3 has a cylindrical nut body 5 and a circulating ball 6 attached to a window 5a penetrating the nut body 5 in the radial direction. In the illustrated example, the windows 5a and the circulating balls 6 are provided at two locations spaced apart in the axial direction of the nut body 5.

[0016] A spiral male thread groove 7 is continuously formed on the outer peripheral surface of the screw shaft 2. A spiral female thread groove 8 of one turn or less that faces the male thread groove 7 in the radial direction is formed on the inner peripheral surface of the nut body 5. A circulation groove 9 that connects the end and the start of the female thread groove (10) is formed on the surface on the inner diameter side of the circulating ball 6. A loop-shaped ball passage is formed by the rolling path formed by the male thread groove 7 and the female thread groove 8 and the circulation path formed by the circulation groove 9, and a large number of balls 4 are filled in this ball passage. When the screw shaft 2 and the nut 3 rotate relative to each other, the balls 4 circulate in the loop-shaped ball passage. That is, when the screw shaft 2 and the nut 3 rotate relative to each other, a large number of balls 4 roll along the rolling path, and when they reach the end of the rolling path, they are guided by the circulation path, cross over the thread crest 2a of the screw shaft 2, return to the start of the rolling path, and roll along the rolling path again.

[0017] FIG. 3 schematically shows the screw shaft 2 and the circulating ball 6. In the figure, C1 is the ball center locus of the rolling path (male thread groove 7 and female thread groove 8), C2 is the ball center locus of the circulation groove 9, L is the lead of the ball screw 1, and β is the lead angle of the ball screw 1. The ball center locus C1 of the rolling path has a constant radial distance from the axis. As shown in the figure, the ball center locus C2 of the circulation groove 9 is formed to curve in an S shape and is smoothly connected to the ball center locus C1 of the female thread groove 8.

[0018] Figure 4 shows a view of the circulating ball 6 from the inner diameter side. In the illustrated example, the circulating groove 9 has a pair of inlet sections 9a provided at both ends and a connecting section 9b connecting the pair of inlet sections 9a. The connecting section 9b has a pair of curved sections 9b1 connected to the pair of inlet sections 9a and an intermediate connecting section 9b2 connecting them. In the cross-section shown in Figure 5, the groove depth of the connecting section 9b increases towards the center. The groove depth of the connecting section 9b is set so that the ball 4 passing through the circulating path can pass over the threads 2a of the screw shaft 2.

[0019] Figure 6 is a plan view of the circulation groove 9. More specifically, it is a plan view of the cylindrical surface formed by projecting the ball center trajectory C2 of the circulation groove 9 onto a cylindrical surface provided on the outer circumference of the nut 3 from the axis of the nut 3. The vertical direction (y' direction) in the figure is parallel to the ball center trajectory C1 of the female screw groove 8. The lines connecting points A to B and F to G are the ball center trajectory C2a of the introduction section 9a, which is a straight line provided on the extension of the ball center trajectory C1 of the female screw groove 8. The line connecting points B to F is the ball center trajectory of the connecting section 9b, which extends in a different direction from the ball center trajectory C1 of the female screw groove 8. Of these, the lines connecting points B to C and E to F are the ball center trajectory C2b of the curved section 9b1, which consists of a single circular arc with a radius of curvature r. Furthermore, the line connecting points C and E is the ball center trajectory C2c of the intermediate connecting section 9b2, and is a straight line that smoothly connects the ball center trajectories C2b of the pair of curved sections 9b1. In the illustrated example, the ball center trajectory C2 of the circulation groove 9 is point-symmetric with respect to the midpoint of the path length, and in the illustrated example, with respect to the midpoint D of the ball center trajectory C2c of the intermediate connecting section 9b2. Note that the shape of the curved section 9b1 is not limited to the above, and for example, the curved section 9b1 may be a non-arc curve (for example, a curve formed by connecting multiple arcs, or a curve in which the radius of curvature changes continuously).

[0020] The ball center trajectory C1 of the female screw groove 8 and the ball center trajectory C2a of the introduction portion 9a of the circulation groove 9 extend in the same helical direction (linear direction in Figure 6). The female screw groove 8 has a load region where the ball 4 is clamped between it and the male screw groove 7 and receives an axial load. In this embodiment, the entire rolling path is a load region where the contact angular distance between the female screw groove 8 and the male screw groove 7 is constant. The entire circulation path is a non-load region where the ball 4 is not clamped between the male screw groove 7 and the circulation groove 9.

[0021] As shown in Figure 7, the male screw groove 7 and female screw groove 8 have a Gothic arch shape in a cross section perpendicular to the ball center trajectory. The cross section of the circulation groove 9 has a single circular arc shape. Note that the cross-sectional shape of each groove is not limited to the above; for example, the cross section of the circulation groove 9 may also be a Gothic arch shape.

[0022] The male screw groove 7 and the ball 4 make angular contact at two points S and S', and the female screw groove 8 and the ball 4 make angular contact at two points M and M'. When the ball screw 1 is in operation, the male screw groove 7 and the female screw groove 8 and the ball 4 make contact at at least two opposing points S, M or S', M' in the diametrical direction of the ball 4. Depending on the operating conditions of the ball screw 1, the contact angles α at each contact point S, S', M, M' may differ slightly, but in the following, we will assume that they are equivalent. The contact angle α is the angle formed by the line l connecting the contact points S, M (or S', M') between the male screw groove 7 and the female screw groove 8 and the ball 4 to the ball center O, and the radial line K of the ball screw 1.

[0023] Figure 8 is a cross-sectional view in the direction of the contact angle (the straight line l in Figure 7) near the connection between the rolling path and the circulation path. In this figure, the spiral rolling path is shown unfolded in a straight line. In this figure, the introduction portion 9a of the circulation groove 9 is located on the outer diameter side of the rolling surface of the female screw groove 8, and a step Δ in the direction of the contact angle is formed at the boundary between them.

[0024] Here, we will explain a method for setting the distance in the contact angle direction between the female screw groove 8 and the introduction portion 9a of the circulation groove 9, that is, the size of the step difference Δ mentioned above.

[0025] The magnitude of the step Δ described above is set based on the amount of elastic deformation of the male screw groove 7, female screw groove 8, and ball 4 in the rolling path (load region) when the maximum allowable load is applied to the ball screw 1. Specifically, when the ball screw 1 is operating, the ball 4 is clamped from both sides in the direction of the contact angle (straight line l in Figure 7) between the male screw groove 7 and the female screw groove 8 in the rolling path. At this time, the contact portion S between the male screw groove 7 and the ball 4, and the contact portion M between the female screw groove 8 and the ball 4, undergo elastic deformation. The lower cross-hatched area in Figure 8 schematically shows the state in which the male screw groove 7 and the ball 4 are pressed against each other at the contact portion S and both undergo elastic deformation, and the upper cross-hatched area in Figure 8 schematically shows the state in which the female screw groove 8 and the ball 4 are pressed against each other at the contact portion M and both undergo elastic deformation.

[0026] Then, when the maximum allowable load is applied to the ball screw 1, the sum of the elastic deformation amounts δS in the contact direction (up and down in Figure 8) at the contact point S between the rolling surface of the male screw groove 7 and the ball 4, and the sum of the elastic deformation amounts δM in the contact direction at the contact point M between the rolling surface of the female screw groove 8 and the ball 4 are calculated. The sum of δS and δM is the sum of the elastic deformation amount (indentation) in the contact direction of the male screw groove 7, the elastic deformation amount (indentation) in the contact direction of the female screw groove 8, and the elastic deformation amount (decrease in diameter) of the ball 4 in the contact direction when the maximum allowable load is applied to the ball screw 1. The maximum allowable load is, for example, the load at which the higher value of the maximum contact pressure at the contact point S between the male screw groove 7 and the ball 4, and the maximum contact pressure at the contact point M between the female screw groove 8 and the ball 4, becomes 4200 MPa.

[0027] The amount of elastic deformation can be calculated using Hertzian elasticity theory, and the normal load F and the amount of elastic deformation δ are: δ = C·F 2 / 3 The following relationship exists. Here, C is a coefficient determined by the radius of curvature, the Young's modulus of the member, and Poisson's ratio.

[0028] The distance Δ in the contact angle direction between the female screw groove 8 and the introduction portion 9a of the circulation groove 9 is set to satisfy the following equation. Δ>Δ min =δM +δS

[0029] As a result, as shown by the dotted line in Figure 8, when the ball 4 enters the introduction section 9a of the circulation groove 9 from the female screw groove 8, the elastically restored ball 4 is reliably in an unloaded state. This prevents excessive contact between the ball 4 and the introduction section 9a of the circulation groove 9, improving the durability of the ball screw 1 and ensuring smooth operation of the ball screw 1.

[0030] Incidentally, in order to keep the ball 4 that has entered the introduction section 9a of the circulation groove 9 unloaded, it is thought that the width W between the introduction section 9a and the male screw groove 7 should be set to be larger than the diameter Da of the ball 4 in the unloaded state. However, since the nut 3 and the screw shaft 2 are relatively movable and their relative positions fluctuate depending on the operating conditions, it is difficult to set the width W between the introduction section 9a and the male screw groove 7 with high precision.

[0031] In contrast, in the present invention, as described above, by defining the positional relationship between the nut body 5 and the circulating ball 6 which are fixed to each other, specifically the step difference (distance in the contact angle direction) Δ between the rolling surface of the female screw groove 8 formed in the nut body 5 and the introduction portion 9a of the circulating groove 9 formed in the circulating ball 6, it becomes possible to set this step difference Δ with high precision, thereby ensuring that the ball 4 is in an unloaded state at the introduction portion 9a.

[0032] On the other hand, if the step difference Δ between the female screw groove 8 and the introduction portion 9a of the circulation groove 9 is too large, the collision of the ball 4 with the wall surface of the introduction portion 9a when the ball 4 enters the introduction portion 9a of the circulation groove 9 will increase. Therefore, the above distance Δ should be set to the allowable upper limit Δ. max It is preferable to make it smaller than, for example, 5·Δ min It is preferable to make it smaller than the smaller of 0.07·Da. Note that Da is the diameter of ball 4 under no-load conditions.

[0033] The present invention is not limited to the embodiments described above. Other embodiments of the present invention will be described below, but redundant explanations of points similar to those in the embodiments described above will be omitted.

[0034] In the embodiment shown in Figure 9, the female thread groove 8 of the nut body 5 has inclined portions 8b connected to both sides of the rolling surface 8a. The inclined portions 8b smoothly connect to the end of the rolling surface 8a and are inclined so as they approach the circulation groove 9, they are displaced towards the outer diameter side, i.e., the side away from the male thread groove 7 (upper side in Figure 9). In this case, the rolling surface 8a becomes a load region that clamps the ball 4 between itself and the male thread groove 7, and as the ball 4 rolls on the inclined portions 8b, the clamping force (δS + δM) between the ball 4 and the male thread groove 7 and the female thread groove 8 gradually decreases. The introduction portion 9a of the circulation groove 9 is provided on the outer diameter side of the rolling surface 8a of the female thread groove 8, and in the illustrated example, it is provided on the outer diameter side of the end portion 8c of the female thread groove 8. In this case, the distance in the contact angle direction between the end of the rolling surface 8a of the female screw groove 8 on the introduction portion 9a side (right side in Figure 9) and the end of the introduction portion 9a on the female screw groove 8 side (left side in Figure 9) becomes "the distance Δ in the contact angle direction between the introduction portion 9a and the rolling surface 8a of the female screw groove 8 at the boundary between the female screw groove 8 and the circulation groove 9". The shapes of the female screw groove 8 and the circulation groove 9 are set so that this distance Δ is greater than δM + δS on the rolling surface 8a.

[0035] The present invention is applicable to both a screw shaft rotation type ball screw, which rotates the screw shaft to move the nut linearly, and a nut rotation type ball screw, which rotates the nut to move the screw shaft linearly. Furthermore, the present invention is applicable to both a ball screw, which takes rotation as input and outputs linear motion, and a ball screw, which takes linear motion as input and outputs rotation. [Explanation of Symbols]

[0036] 1 Ball screw 2 Screw shaft 3 nuts 4 balls 5. Nut body 6 Circulating spinning top 7 Male screw groove 8 Female thread grooves 9 Circulation groove 9a Introduction 9b Connecting part 9b1 Curved section 9b2 Intermediate connection C1 Ball center trajectory of the rolling path (male screw groove and female screw groove) C2 Ball center trajectory in the circulation groove C2a Introductory section ball center trajectory C2b Ball center trajectory of the curved section C2c Ball center trajectory of the intermediate connecting section

Claims

1. A screw shaft having a helical male screw groove formed on its outer surface, A nut body having a spiral female thread groove formed on its inner circumferential surface opposite to the male thread groove, A circulating spindle is attached to the nut body and has a circulating groove formed therein that connects the end and start of the female screw groove, A ball screw comprising a plurality of balls arranged in a loop-shaped ball passage having a rolling path formed by the male thread groove of the screw shaft and the female thread groove of the nut body, and a circulation path formed by the circulation groove of the circulating spindle, The circulation groove has a pair of introduction portions connected to the female screw groove and extending in the same direction as the female screw groove, and a connecting portion extending in a direction different from the female screw groove and connecting the pair of introduction portions. In the cross-section in the direction of the contact angle between the female screw groove and the male screw groove and the ball, the introduction portion of the circulation groove is positioned on the outer diameter side of the rolling surface of the female screw groove, and the distance in the direction of the contact angle between the introduction portion and the rolling surface of the female screw groove at the boundary between the female screw groove and the circulation groove is Δ. When the maximum allowable load is applied to the ball screw, let δM be the sum of the elastic deformation amounts in the direction of the contact angle between the rolling surface of the female screw groove and the ball at the contact point, and let δS be the sum of the elastic deformation amounts in the direction of the contact angle between the rolling surface of the male screw groove and the ball at the contact point, D>D ｍｉｎ =δM+δS A ball screw that satisfies the requirements.

2. When the diameter of the ball is Da, then Δ is 5 * Δ ｍｉｎ The ball screw according to claim 1, which is smaller than the smaller of 0.07・Da.

3. The cross-section of the female screw groove perpendicular to the ball center trajectory is in the shape of a Gothic arch. The ball screw according to claim 1 or 2, wherein the cross section of the circulation groove perpendicular to the ball center trajectory has a single arc shape.

4. The ball screw according to claim 1 or 2, wherein the cross-section of the female screw groove and the circulation groove perpendicular to the ball center trajectory is both in the shape of a Gothic arch.

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