Ball screw spindle mechanism with asymmetrical angled thread

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the field of ball screws, and more specifically to the threads of such a screw mechanism. PREVIOUS STATE OF THE ART

[0002] Document DE102007017214 discloses a ball screw mechanism comprising a screw defining a reference axis, a nut, and at least two balls rolling on a screw thread formed on the screw and a nut thread formed on the nut. This mechanism has threads whose flanks have different radii of curvature. When the mechanism is subjected to small-amplitude forces in a given axial direction corresponding to normal operation, the balls are in contact with a flank of large radius of curvature of the screw thread and with a flank of large radius of curvature of the nut thread, in order to minimize friction.When the mechanism at rest is subjected to axial loads in a direction opposite to its normal operating direction, the balls bear against a small radius flank of the screw thread and a small radius flank of the nut thread, enabling them to withstand large static loads, albeit with increased friction. However, this mechanism is not suitable for applications where the loads acting on the moving mechanism have a large magnitude and a constant axial direction, regardless of the mechanism's direction of rotation. DESCRIPTION OF THE INVENTION

[0003] The invention aims to remedy the drawbacks of the prior art and to propose a ball screw mechanism that can rotate without excessive friction in both directions of rotation, while being subjected to high amplitude axial forces that are always applied in the same axial direction.

[0004] To achieve this, according to a first aspect of the invention, a ball screw mechanism is proposed, comprising a screw defining a reference axis, a nut and at least two balls of radius R positioned so as to roll on a screw thread formed on the screw and a nut thread formed on the nut, the screw thread and the nut thread having, in an active portion of the screw and the nut, a helical pitch P, characterized in thatin a cutting plane containing the reference axis and passing through the center of a first ball among the at least two balls, the first ball being positioned in the active portion of the screw and nut, any segment perpendicular to the reference axis, located at a distance x from the center of the first ball, and having a first endpoint that belongs to the screw thread and a second endpoint that belongs to the nut thread, has a center which, in an orthonormal coordinate system having an abscissa axis coinciding with the reference axis and a ordinate axis that passes through the center of the first ball, has an abscissa equal to x and an ordinate y defining a function x → y = f ( x ) which, when x varies between 0 and P, passes through a minimum Y0 reached for an abscissa X0 and through a maximum Y1 reached for an abscissa X1, such that the following inequalities are respected: 9 10 R < X 0 < P − 9 10 R 9 10 R < X 1 < P − 9 10 R Y 1 − Y 0 > 0 , 10 mm

[0005] Furthermore, any segment perpendicular to the reference axis, located at a distance x from the center of the first ball, and having a first endpoint belonging to the screw thread and a second endpoint belonging to the nut thread, has a length L defining a function x → L = g ( x ) which, when the abscissa x of the center of the segment varies between 0 and the helix pitch P, is always greater than 0.25 mm, and preferably greater than 0.40 mm.

[0006] The screw thread is radially raised in the area near the maximum x-coordinate X1, and recessed in the area near the minimum x-coordinate X0. Conversely, the nut thread is radially raised in the area near the minimum x-coordinate X0, and recessed in the area near the maximum x-coordinate X1. This allows, when a force is applied to the nut in a preferred axial direction from x-coordinate X1 to x-coordinate X0, a large contact area between the balls and the threads of the screw and nut, thus giving the mechanism a high capacity. In the opposite axial direction, the available contact areas are smaller, which is not a disadvantage since the expected forces remain low. The minimum distance L maintained between screw thread and nut thread ensures good lubrication of the mechanism.It also prevents any contact between the screw and nut, even under high loads and deformations.

[0007] Preferably: Y 1 − Y 0 > 0 , 20 mm

[0008] According to one implementation method: The screw thread forms an internal helical bearing raceway rotated radially in the opposite direction to the reference axis and delimited by two opposite flanks of the screw thread located on either side of a root of the screw thread; the nut thread forms an external helical bearing raceway rotated radially towards the reference axis and delimited by two opposite flanks of the nut thread located on either side of a root of the nut thread; one extended flank among the two flanks of the nut thread is extended towards the reference axis relative to the other of the two flanks of the nut thread; one extended flank among the two flanks of the screw thread is extended in a radial direction opposite to the reference axis relative to the other of the two flanks of the screw thread; and the extended flank of the nut thread is radially opposite the other flank of the screw thread.the extended flank of the nut thread and the extended flank of the screw thread being axially oriented towards each other.

[0009] In a preferred axial force direction corresponding to contact between the balls and the extended flanks of the screw and nut threads, the contact ellipse can extend over a larger area of ​​the extended flanks of both threads, including the areas corresponding to the flank extensions. This asymmetry increases the mechanism's capacity in this direction, while maintaining sufficient space between the screw and nut, in the area between two successive turns, for proper lubrication. The flank asymmetry results in a smaller available surface area on the non-extended flanks for contact with the balls, leading to a reduced mechanism capacity in the force direction opposite to the preferred direction. However, this is not a problem since, in the intended application, the forces in this direction are small or even negligible.

[0010] Preferably, the two flanks of the screw thread have, in a cutting plane perpendicular to the root of the screw thread, a radius of curvature at every point greater than the radius R of the balls. Similarly, the two flanks of the nut thread preferably have, in a cutting plane perpendicular to the root of the nut thread, a radius of curvature at every point greater than the radius R of the balls. This ensures good control of the quasi-point contact interface between the balls and the flanks of the screw and nut threads.

[0011] Preferably, both flanks of the screw thread have a circular arc curvature in a cutting plane perpendicular to the root of the screw thread, and both flanks of the nut thread have a circular arc curvature in a cutting plane perpendicular to the root of the nut thread. Preferably, one or more of the following conditions are met: both sides of the screw thread have an identical radius of curvature; and / or both sides of the nut thread have an identical radius of curvature; and / or at least one of the two sides of the screw thread has an identical radius of curvature to at least one of the two sides of the nut thread.

[0012] Thus, the coefficient of friction between balls and raceways is independent of the direction of application of the axial force (in the preferred direction or the opposite direction), for a given magnitude of the resultant axial force.

[0013] According to one embodiment, the values ​​X0 and X1 satisfy the following inequalities: R < X 0 < P − R et R < X 1 < P − R

[0014] According to one embodiment, the two extremes satisfy the following double inequality, where D is the distance D between the center of the first ball and the reference axis: Y 0 < D < Y 1

[0015] According to one embodiment, D - Y 0 - ε <Y 1 - D < D - Y 0 + ε ; ε denoting a distortion margin such that 0 < ε < 0.05 mm .

[0016] According to one embodiment, the function the function x → y = f ( x ) is such that, for any value of z between 0 and P / 2 - R, where R is the radius of at least two balls, we observe: 2 f P 2 − ε 1 < f P 2 + z + f P 2 − z < 2 f P 2 + ε 1 avec ε 1 < 0 , 05 mm

[0017] These inequalities reflect an approximate symmetry of the function x → y = f ( x ) , with a distortion margin ε1, around and near a center of symmetry located at the abscissa P / 2.

[0018] According to one implementation method: D − ε 1 < f P 2 < D + ε 1

[0019] According to one implementation method: X 0 + X 1 2 = P 2

[0020] According to one embodiment, the function x → y = f ( x) is continuous.

[0021] According to one embodiment, the function x → y = f ( x ) is strictly increasing or strictly decreasing between X0 and X1.

[0022] Near the center of the marbles, the function x → y = f ( x ) is relatively constant. Thus, D denotes the distance between the center of the first ball and the reference axis, and ε denotes a distortion margin such that 0 < ε < 0.05 mm We observe: pour 0 < x < 9 R 10 ′ D − ε < f x < D + ε pour P − 9 R 10 < x < P , D − ε < f x < D + ε BRIEF DESCRIPTION OF THE FIGURES

[0023] Other features and advantages of the invention will become apparent from the following description, with reference to the attached figures which illustrate: [ Fig. 1 ] there figure 1, a ball screw mechanism in axial section, illustrating in particular balls moving on the raceways of a nut and screw of the ball screw mechanism; Fig. 2 ] There figure 2 , a detailed view of a nut thread and a ball screw thread. Fig. 3 ] There figure 3 illustrates a curve of evolution of the radial distance between the midpoint of an imaginary segment separating the two threads of the ball screw mechanism and a reference axis, as well as a curve of evolution of the length of said imaginary segment, as a function of the axial distance to the center of a ball of the mechanism.

[0024] For clarity, identical or similar elements are identified by identical reference symbols across all figures. DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0025] On the figure 1 A ball screw mechanism is illustrated 1 comprising two threaded components, namely a screw 10and a nut 12, and marbles 2. The screw 10 is preferably metallic, for example steel, and has a screw thread 34 which forms an internal helical rolling path 14 around a reference axis 200 of the ball screw mechanism 1 defined by the screw 10 the inner helical rolling path 14 being rotated radially in the opposite direction to the reference axis 200. The nut 12 is preferably metallic, for example steel, and has a nut thread 134 which forms an external helical rolling path 114 around the reference axis 200, and rotated radially towards the reference axis 200. One of the two threaded components, namely the screw 10 or the nut 12,It also forms a recirculation channel, not shown in the figures, which preferably spans several turns of the rolling tracks 14, 114 of the screw 10 and the nut 12. The marbles 2 They can, for example, be made of steel or ceramic, and are sized and positioned to circulate in a closed circuit between the external helical raceway 114 of the nut 12 and the internal helical raceway 14 of the screw 10, as well as in the recirculation channel, preferably without separators between the balls 2.

[0026] The inner helical raceway 14 of the screw 10 presents, on at least one active part, a helical envelope of constant pitch P around the reference axis 200, with a path background 16, as well as a first flank 18 and a second side 20facing each other across the end of the path 16, the first side 18 being rotated in a first axial direction 210 of the reference axis 200, the second side 20 being rotated in a second axial direction 220 of the reference axis 200.

[0027] In the rest of the description, we will refer to Figures 1 And 2 where the ball screw mechanism 1 is represented in a cross-sectional view. The cross-section of the figure 2 is performed in a cutting plane containing the reference axis 200 and passing through the center of one of the marbles.

[0028] The flanks 18, 20 of the helical rolling path 14 of the screw 10 preferably have a concave section, in any plane containing the reference axis 200. These concave sections have a non-constant radius of curvature in the plane of the figure 2However, observed in any cutting plane perpendicular to a tangent to the bottom of path 16, therefore in a plane inclined at the value of the helix angle with respect to the plane of the figure 2 the flanks 18, 20 are preferably in the shape of a circular arc, and preferably with equal radii of curvature for both sides, so as to form an ogive extending on either side of the bottom of the path 16, or a continuous circular arc. The helix angle is, however, sufficiently small that, in the cutting plane of the figure 2 , the variations in the radius of curvature and the center of curvature of the flanks 18, 20 be small, for example less than 0.1 mm.

[0029] The first flank 18 presents an end 38 opposite the end of the road 16. The second side 20 presents an end 40 opposite the end of the road 16.

[0030] The end 38of the first flank 18 of the screw 10 is radially less distant from the reference axis 200 that the end 40 of the second flank 20 of the screw 10 .

[0031] The rolling path 14 helical screw 10 consists of several successive turns separated by bonding portions 36 protruding screw threads 34. Each connecting portion 36 links the ends 38, 40 flanks 18, 20 of the screw 10 The connecting portion 36 forms a straight curve 46 in the vicinity of the end 38 of the first flank 18 of the rolling track 14 of the screw 10 the end 38 forming an inflection point with two half-tangents forming an obtuse angle with each other. The connecting portion 36 presents a convex curvature 45in the vicinity of the end 40 of the second flank 20 of the rolling track 14 of the screw 10 this end 40 constituting a point of inflection, the surface of the screw thread 34 crossing its tangent at this point of inflection 40.

[0032] The outer helical raceway 114 of the nut 12 presents, on at least one active part, a helical envelope of constant pitch P around the reference axis 200, with a path background 116, as well as a first flank 118 and a second side 120 facing each other across the end of the path 116, the first side 118 being rotated in the second axial direction 220 of the reference axis 200, axially opposite the first flank 18 of the rolling track 14 of the screw 10 and radially opposite the second flank 20of the rolling track 14 of the screw 10 the second flank 120 being rotated in a first axial direction 210 of the reference axis 200, axially opposite the second flank 20 of the rolling track 14 of the screw 10 and radially opposite the first flank 18 of the rolling track 14 of the screw 10 .

[0033] The flanks 118, 120 of the helical rolling path 114 of the nut 12 preferably have a concave section in any plane containing the reference axis 200. These concave sections have a non-constant radius of curvature in the plane of the figure 2 However, observed in any cross-section perpendicular to a tangent at the bottom of the path 116, therefore in an inclined plane of the value of the helix angle with respect to the plane of the figure 2 the flanks 118, 120are preferably in the shape of a circular arc, and preferably with equal radii of curvature for both sides, so as to form an ogive extending on either side of the bottom of the path 116, or a continuous circular arc. The helix angle is, however, sufficiently small that, in the cutting plane of the figure 2 , the variations in the radius of curvature and the center of curvature of the flanks 118, 120 be small, for example less than 0.1 mm.

[0034] The first flank 118 presents an end 138 opposite the end of the road 116. The second side 120 presents an end 140 opposite the end of the road 116. The end 138 of the first flank 118 of the nut 12 is radially further from the reference axis 200 that the end 140 of the second flank 120 of the nut 12. The rolling path 114helical nut 12 comprises several successive turns separated by protruding bonding portions 136 of the nut thread 134. Each connecting portion 136 links the ends 138, 140 flanks 118, 120 of the nut 12. The connecting portion 136 forms a straight curve 146 in the vicinity of the end 138 of the first flank 18 of the rolling track 114 of the nut 12, the end 138 forming an inflection point with two half-tangents forming an obtuse angle with each other. The connecting portion 136 presents a convex curvature 145 in the vicinity of the end 140 of the second flank 120 of the rolling track 114 of the nut 12, this end 140 constituting a point of inflection, the surface of the nut thread 134 crossing its tangent at this inflection point140.

[0035] Thus, the second flan 120 of the nut thread 134 is extended towards the reference axis 200 compared to the first flank 118 of the nut thread 134. Similarly, the second flank 20 screw thread 34 is extended in the radial direction opposite to the reference axis 200 compared to the first flank 18 screw thread 34. The extended flank 120 of the nut thread 134 is located radially opposite the non-extended flank 18 screw thread 34 and, similarly, the extended flank 20 screw thread 34 is located radially opposite the non-extended flank 118 of the nut thread 134. The extended flanks 120 And 20 are opposite each other on either side of the center of ball 2.

[0036] To describe the volume delimited by the screw thread 34 and the nut thread 134 in the plan of the figure 1 and of the figure 2 , we define an orthonormal coordinate system R having an x-axis coinciding with the reference axis 200 and a y-axis that passes through the center C1 of the first marble 2a. We can then observe a segment 240 any imaginary object perpendicular to the reference axis 200 located at a distance x from a center C1 of a first marble 2a, the distance x varying from 0 to P, P being the value of the helix pitch of the screw thread 34 and the nut thread 134. The segment 240 presents a first end belonging to the screw thread 34 as well as a second end belonging to the nut thread 134.

[0037] The segment 240 presents a center C2which has coordinates x on the abscissa and y on the ordinate, defining a function x → y = f ( x ) whose curve F is illustrated schematically by superimposing it on the figure 3 , and with reference to the left-hand scale on the graph of the figure 3 The x-axis is graduated in units, each corresponding to 1 / 5th of P. The numerical values ​​provided are purely illustrative. In practice,

[0038] The function x → y = f ( x ) passes through a minimum Y0 reached for an abscissa X0 and a maximum Y1 reached for an abscissa X1, where the values ​​X0 and X1 are each greater than the radius of the balls 2. The function x → y = f ( x ) is continuously increasing or continuously decreasing between X0 and X1, and is more generally continuous over the entire length of the screw 10 .

[0039] Inside the rolling tracks 14, 114, the center of the segment 240 remains at a distance from the reference axis that is close to the value D, which can be expressed as a function of an ε denoting a distortion margin such that 0 < ε < 0.05 mm , by : pour 0 < x < 9 R 10 , D − ε < f x < D + ε pour P − 9 R 10 < x < P , D − ε < f x < D + ε

[0040] The center C1 of the first marble 2a is distant from the reference axis 200 of a distance D which corresponds to half the primitive diameter of the mechanism. The minimum Y0 is less than the distance D, while Y1 is greater than the distance D. The minimum Y0 is reached for an abscissa X0 such that R < X 0 < P − R

[0041] The maximum Y1 is reached for an abscissa X1 such that: R < X 1 < P − R

[0042] The ball screw mechanism 1 preferably exhibits an approximate symmetry in the cutting plane of the Figures 1 And2 , such as D − Y 0 − ε < Y 1 − D < D − Y 0 + ε ; ε denotes a distortion margin such that 0 < ε < 0.05 mm. More generally, the central symmetry exhibited by the ball mechanism 1 at the center of symmetry located at the abscissa XS=(X1+X0) / 2 or XS=P / 2, and at the ordinate YS=D, can be written 2 f P 2 − ε 1 < f P 2 + z + f P 2 − z < 2 f P 2 + ε 1 ε1 denotes a distortion margin such that 0 < ε 1 < 0.05 mm .

[0043] In these inequalities, ε and ε1 account for the inclination of the helical threads with respect to the axial cutting plane of the Figures 1 And 2 , therefore of the helix angle and the 2D pitch diameter of the mechanism.

[0044] The distortion margins ε and ε1 are related to the inclination of the helical threads relative to the axial cutting plane of the Figures 1 And 2 , and are a function of the helix angle and the 2D pitch diameter of the mechanism. The values ​​of ε and ε1 are less than D / 10, and preferably less than D / 50.

[0045] In practice, the difference Y 1 - Y 0 is significant, and satisfies the following inequality: Y 1 − Y 0 > 0 , 10 mm

[0046] Preferably: Y 1 − Y 0 > 0 , 20 mm

[0047] Furthermore, the length L of the segment 240 varies according to the x-coordinate of segment 240, defining a function x → g ( x ) = L, whose curve G is illustrated on the graph of the figure 3 , with reference to the right-hand scale for the y-axis. The function x → L = g ( x ), passes through a minimum value Lmin,itself greater than or equal to 0.25 mm, preferably greater than 0.40 mm, for example greater than 0.45 mm or 0.50 mm, firstly to ensure satisfactory grease circulation between the windings of the raceways and secondly to prevent impact between the threads of the screw and nut under high loads and high deformations, particularly in the area corresponding to the connecting portions 36, 136 or this minimum value is eventually reached.

[0048] The axial direction 210 is a preferred direction for the forces applied by the nut 12 on the marbles 2 and for the forces applied by the balls 2 and by the screw 10 : when the nut applies pressure to the balls 2 an effort whose resultant has an axial component in the axial direction 210, the contact ellipse between each ball 2 and the rolling path 114 of the nut12 is formed on the second flank 120, and can extend over a significant area of ​​the second flank, including the area corresponding to the extension of this second flank 120. Similarly, the contact ellipse between each ball 2 and the rolling path 14 of the screw 10 is formed on the second flank 20, and can extend over a significant area of ​​the second flank 20, including in the area corresponding to the extension of this second flank 20.

[0049] This asymmetry increases the mechanism's capacity in this direction, while maintaining sufficient space between the screw and nut in the radial area between the connecting portions. 36, 136, for proper lubrication of the mechanism.

[0050] The asymmetry of the flanks also means that the available surface area at the level of the non-extended flanks 18, 118is less to constitute a contact interface with the balls, so the capacity of the mechanism in the direction of forces 220 opposed to the preferred direction 210, is significantly lower than the capacity in the preferred direction 210, However, this is not a problem as long as, in the application under consideration, the axial forces are unidirectional or essentially unidirectional.

Claims

1. A ball screw mechanism (1), comprising a screw (10) defining a reference axis (200), a nut (12) and at least two balls (2) of radius R positioned to roll on a screw thread (34) formed on the screw (10) and a nut thread (134) formed on the nut (12), the screw thread (34) and nut thread (134) having, in an active portion of the screw and nut, a helical pitch P, characterized in that in a sectional plane containing the reference axis (200) and passing through a center (C1) of a first ball (2a) of the at least two balls (2), the first ball being positioned in the active portion of the screw and nut, any segment (240) perpendicular to the reference axis (200), located at a distance x from the center (C1) of the first ball (2a), and having a first end which belongs to the screw thread (34) and a second end which belongs to the nut thread (134), has: - a center (C2) which, in an orthonormal reference frame having an abscissa axis coincident with the reference axis (200) and an ordinate axis passing through the center (C1) of the first ball (2a), has an abscissa equal to x and an ordinate y, defining a function x → y = f(x) which, when x varies between 0 and P, passes through a minimum Y0 reached for an abscissa X0 and through a maximum Y1 reached for an abscissa X1, such that the following inequalities are respected: 9 10 R < X 0 < P − 9 10 R 9 10 R < X 1 < P − 9 10 R Y 1 − Y 0 > 0.10 mm and - a length L defining a function x → L = g(x) which, when the abscissa x of the center of the segment (C2) varies between 0 and the helical pitch P, is always greater than 0.25 mm, and preferably greater than 0.40 mm.

2. The ball screw mechanism (1) according to claim 1, characterized in that - the screw thread (34) forms a helical inner raceway (14) facing radially away from the reference axis (200) and delimited by two opposite flanks (18, 20) of the screw thread (34) located on either side of a bottom (16) of the screw thread (34), - the nut thread (134) forms a helical outer raceway (114) facing radially towards the reference axis (200) and delimited by two opposite flanks (118, 120) of the nut thread (134) located on either side of a bottom (116) of the nut thread (134), - one extended flank (120) of the two flanks of the nut thread (134) is extended towards the reference axis (200) with respect to the other of the two flanks (118) of the nut thread (134), - one extended flank (20) of the two flanks of the screw thread (34) is extended in a radial direction opposite to the reference axis (200) with respect to the other of the two flanks (18) of the screw thread (34), - the extended flank (120) of the nut thread (134) lies radially facing the other flank (18) of the screw thread (34) and the extended flank (20) of the screw thread (34) lies radially facing the other flank (118) of the nut thread (134), the extended flank (120) of the nut thread (134) and the extended flank (20) of the screw thread (34) facing one another axially.

3. The ball screw mechanism (1) according to claim 2, characterized in that the two flanks (18, 20) of the screw thread (34) have an arcuate curvature in a sectional plane perpendicular to the base (16) of the screw thread (34) and the two flanks (118, 120) of the nut thread (134) have an arcuate curvature in a sectional plane perpendicular to the bottom (118) of the nut thread (134), and, preferably, - the two flanks (18, 20) of the screw thread (34) have an identical radius of curvature; and / or - the two flanks (118, 120) of the nut thread (134) have an identical radius of curvature; and / or - at least one of the two flanks (18, 20) of the screw thread (12) has an identical radius of curvature as at least one of the two flanks (118, 120) of the nut thread (134).

4. The ball screw mechanism (1) according to any of the preceding claims, characterized in that D - Y0 - ε < Y1 - D < D - Y0 + ε ; D designating the distance between the center of the first ball (2a) and the reference axis (200), ε designating a margin of distortion such that 0 < ε < 0.05 mm.

5. The ball screw mechanism (1) according to any of the preceding claims, characterized in that the function x → y = f(x) is such that, for any value z between 0 and P / 2 - R, where R is the radius of the at least two balls (2): 2 f P 2 − ε 1 < f P 2 + z + f P 2 − z < 2 f P 2 + ε 1 with ε 1 < 0.05 mm 6. The ball screw mechanism (1) according to any of the preceding claims, characterized in that: X 0 + X 1 2 = P 2 7. The ball screw mechanism (1) according to any of the preceding claims, characterized in that the function x → y = f(x) is continuous.

8. The ball screw mechanism (1) according to any of the preceding claims, characterized in that the function x → y = f(x) is strictly increasing or strictly decreasing between X0 and X1.

9. The ball screw mechanism (1) according to any of the preceding claims, characterized in that: Y 1 − Y 0 > 0.20 mm10. The ball screw mechanism (1) according to any of the preceding claims, characterized in that: - for 0 < x < 9 10 R , D − ε < f x < D + ε - for P − 9 10 R < x < P , D − ε < f x < D + ε D is the distance between the center of the first ball (2a) and the reference axis (200), ε is a margin of distortion such that 0 < ε < 0.05 mm.