Ball screw drive unit and spindle nut for it

Abstract

To provide a spindle nut having a ball direction changing portion, which improves load release of balls with respect to an NVH standard and at the same time enables efficient manufacture.SOLUTION: A spindle nut S for a ball screw drive unit includes a hollow cylindrical workpiece having a jacket surface MA and a central longitudinal axis Z, and a ball groove K disposed in a cylinder inner surface in a screw shape for receiving balls. The spindle nut includes cylindrical relief holes B provided in a pair form, in which the relief holes are provided as an introduction passage and a lead-out passage for the balls. In the spindle nut, a tangential line T extends parallel to a jacket line L at a spacing a>0 on a cylindrical surface G, in which the lines T and also the line L are perpendicular to a common perpendicular line R to the central longitudinal axis Z.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to ball screw drives and in particular to spindle nuts for ball screw drives. [Background technology]

[0002] A roller screw drive with balls as rolling elements is usually called a ball recirculating spindle or ball screw drive. The main components of a ball screw drive include a threaded spindle with an external thread and a spindle nut with an internal thread that surrounds the spindle. The threads of the threaded spindle and spindle nut are formed as ball grooves with appropriate outer shapes and are matched to each other so that, when installed, they form a common tunnel-like ball passage or ball guide. Unlike a screw-nut connection in which the thread flanks slide flat against each other, in a ball screw drive, the balls that circulate in the thread are responsible for the load transmission between the nut and the spindle. Therefore, the flat sliding motion is replaced by a rolling motion, which involves less friction.

[0003] To achieve a closed circulation for the balls, a ball feedback mechanism is used. The ball feedback mechanism has the role of removing balls from the ball passage between the spindle nut and the threaded spindle at a first location and supplying them again to a second location. The ball feedback mechanism is thus a bypass that bridges one or more thread paths of the nut-spindle system, thereby enabling a closed circulation for the balls of the ball screw drive. Typically, the ball feedback mechanism is composed of a first ball diverter (from the ball passage), a transitional passage, and a second ball diverter (to the ball passage). Typically, the balls are guided in a transitional passage (e.g., a groove or a tube) inside or outside the spindle nut before being removed radially outward from the ball passage and reintroduced into the ball passage between the threaded spindle and the spindle nut through the second ball diverter at a location provided for this purpose.

[0004] Technically, a ball screw drive operates as a screw transmission that can convert rotary motion into longitudinal motion and vice versa. The reduction or conversion ratio is determined by the dimensions of the threaded spindle and the pitch of the thread. Ball screw drives can operate in two basic types: In one, the spindle nut is supported in a fixed position but rotatable, and the threaded spindle is supported in a non-rotatable but longitudinally displaceable manner, so that the threaded spindle moves along its longitudinal axis when the nut is driven. In the second case, the threaded spindle is supported in a fixed position but rotatable, and the spindle nut is supported in a non-rotatable manner so that it can move longitudinally. This results in a linear movement of the spindle nut along the threaded spindle when the threaded spindle is driven.

[0005] Ball screw drives are used in many technical applications, particularly in mechanical engineering and preferably in tool machines. However, they are also increasingly being used as longitudinal drives in areas where hydraulic or pneumatic systems were previously used, such as presses, injection molding machines, and servo steering. In addition, ball screw drives are increasingly playing a role in electromechanical and electrohydraulic braking systems, where they are used as a replacement for hydraulic brake cylinders or in parallel with known brake systems in brake assist systems. Here, the ball screw drive is driven by an electric motor to amplify the driver's braking force or to help initiate or assist (emergency) braking processes as part of a safety system. This also makes possible purely electrically operated brake systems with ball screw drives as a replacement for brake cylinders at each wheel.

[0006] During operation, various forces act on the balls. Therefore, depending on the drive direction and load, the ball screw drive is pressed against various sides of the ball passage, thus undergoing compressive stress. This compressive stress is then released when the balls are removed from the ball passage between the spindle and the nut. In practice, it is difficult to continuously configure this release process so that the balls can easily roll or slide into the ball feedback section. Rather, a somewhat sudden or stepwise release of the load generates a force that causes the balls to suddenly eject into the ball deflector or transition passage. This characteristic can lead to excessive wear on the balls or ball deflector, or to unpleasant operating noise and / or undesirable vibrations. This characteristic is summarized by the term NVH, which stands for noise, vibration, and harshness. Particularly in the automotive field, NVH is considered a problem because it can create an impression of lack of quality or (premature) failure in users, even when no technical issues have yet been identified.

[0007] In the prior art, various approaches are known to improve the unloading of the balls in the ball diverter. One approach is to configure the ball diverter so that the passage or tube that is adapted to the ball passage of the spindle nut that it surrounds begins exactly tangentially. However, manufacturing the necessary holes through the spindle nut is technically challenging.

[0008] Similarly, Patent Document 1 describes that the ball groove and the ball introduction portion in the spindle nut are provided with the same cross-sectional shape.

[0009] Patent Document 2 proposes configuring a feedback passage externally by connecting the spindle nut to the through-portion so that the center line of the feedback passage describes a special three-dimensional space curve.

[0010] In DE 10 200 04 143 A1 it is proposed to extend the inlet or outlet area for the balls from the threaded path. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] German Patent Application Publication No. 102004025683 [Patent Document 2] DE 3635212 [Patent Document 3] German Patent Application Publication No. 102011076438 Summary of the Invention [Problem to be solved by the invention]

[0012] All of the prior art aspects have in common that they are technically complex and / or expensive to manufacture. Therefore, there is a need for a spindle nut with a ball diverter that improves ball unloading with respect to NVH criteria and at the same time allows for efficient manufacturing. [Means for solving the problem]

[0013] This problem is solved by a spindle nut having the features of the independent claims. Advantageous embodiments of the invention are set forth in the respective dependent claims.

[0014] The spindle nut for the ball screw drive has an outer jacket surface M AThe workpiece comprises an essentially hollow cylindrical workpiece made of metal, having a central longitudinal axis Z of symmetry with respect to the center, and mounting elements of the spindle nut, such as flanges, brackets, and drive elements, not being taken into account. The (hollow) cylindrical inner surface has a threaded ball groove K, which is configured to receive a ball of diameter d, and the helical thread baseline of the ball groove K defines an inner cylindrical surface G. Since the cylindrical inner surface with the ball groove has a three-dimensional surface structure, the cylindrical surface G is considered as a reference surface for the following discussion.

[0015] As mentioned above, the ball grooves K together with the complementary ball grooves of the spindle nut form a ball passage (in the mounted state) in the spindle nut.

[0016] The spindle nut also has pairs of linear, essentially cylindrical relief holes B, which are configured as inlet and outlet passages for the balls to and from the ball passage through the spindle nut wall. Each relief hole B has an intersection with the thread baseline of the ball groove K and is characterized (geometrically or technically) by a jacket line which is an extension of the path of travel of the balls from the ball passage or ball groove K to the relief hole B. For this purpose, it should be noted that the jacket line L has a linear course at least initially, so that the position of the jacket line L can be geometrically determined unambiguously from the intersection of L with G and the subsequent characterization is unambiguous.

[0017] Preferably, the relief holes having all the settings described herein are drilled directly into the spindle nut and / or milled, thus directly fulfilling the introduction / export function. Technically equivalently, the above geometries can also be realized by a shaped workpiece made of synthetic resin, metal (cast, 3D printed) or a similar shape-stable material, which workpiece is fitted into a suitably dimensioned recess of the spindle nut. The advantages of the above geometries are not reduced thereby. Also, although the relief holes are realized inside at the wall as described above, it is also conceivable that they transition on the outside of the spindle nut to a shaped member which can be manufactured, for example, integrally with a transfer passage. Here too, it is possible to use synthetic resin, metal or combinations thereof.

[0018] At this time, the relief holes should not be understood as purely cylindrical holes and can also have a non-circular or circular cross-section. The setting of L and the intersection with K are not important.

[0019] According to the geometry, a tangent T can be defined, which on the one hand is in close contact with the cylindrical surface G and furthermore extends parallel to the jacket line L at a distance a > 0. Both T and L are perpendicular to the central longitudinal axis Z in a common perpendicular R (are arranged perpendicularly).

[0020] In another embodiment, both T and L are perpendicular to a plane E. The plane E is complementary to the central longitudinal axis Z of the spindle nut, and the perpendicular R is likewise located in this plane. In other words, R and Z define the plane E.

[0021] The distance a between both parallel T and L is between 1 / 5 and 1 / 3 of the ball diameter d. The ball diameter d is selected as a reference or comparison for the dimensioning of the present invention. This is because thereby a wide range of applications can be described independently of the dimensions of the spindle nut or the screw helix. It has been found that 1 / 5d < a < 1 / 3 is preferred for a. An a which is 25% ± 3% of the ball diameter d is particularly preferred.

[0022] Therefore, a value of a>0 clearly means that the path of travel of the ball to the relief hole B does not extend perfectly tangentially (and therefore without folds), but that a certain point C marks the transition as the intersection between K and L. The direction of the relief hole, or more precisely the angle of the hole in the spindle nut wall, then corresponds to the tangent T to the cylindrical surface G, but is displaced radially inwards towards the central longitudinal axis Z by a distance a (jacket line L).

[0023] It is important to note that the condition "the tangent (T) is parallel to the jacket line (L) on the cylindrical surface (G) with a distance a>0, and both T and L are perpendicular to the central longitudinal axis Z at the common perpendicular line R" does not automatically mean that the hole or its projection onto Z is perpendicular to the central longitudinal axis Z. Rather, this condition can also be met by hole B, which accepts the pitch angle of the helix, for example. Only the additional condition "T and L are perpendicular to the plane E defined by the perpendicular line R and the central longitudinal axis Z" ensures that hole B or the projection of its central axis is perpendicular to the central longitudinal axis.

[0024] Thus, the present invention can also be described as a design in which a step or edge of predetermined geometry is intentionally provided where ball relief effectively terminates at the transition to the relief hole, but the relief hole is shaped or the transition is selected such that NVH conditions are improved compared to known designs.

[0025] In another embodiment, the travel path of the ball in the ball passage or ball groove (K) is reduced at a travel length l in front of the intersection C of K with L, and as a result, the radial spacing of the balls increases from the central axis Z at the passage of l. For the path portion W having the travel length l, 2 / 5d < l < 2 / 3d holds. In other words, the ball groove K is flattened, or expanded, or distributed such that the ball moves away from the central longitudinal axis Z in a short path section W in front of the above-described step / edge portion C. The expression "flattened, or expanded, or distributed" is selected to account for different designs of the ball groove. Thus, the "Gothic arch" often used as the ball groove cross-section can be corrected by expansion such that the relative position of the circulating ball with respect to the central longitudinal axis of the spindle nut increases. According to the present invention, the effect of the spacing change is important, and the method for causing this (technically meaningful and possible) is not so important. In FIG. 2, this is shown as a reduction of the travel path without being limitedly intended.

[0026] Preferably, the radial spacing t of the balls increases from the central axis by 1 / 33 to 1 / 25 of the ball diameter d when passing through W. The said correction of the ball groove can be achieved by milling operation or by electromechanical cutting in terms of manufacturing technology. In addition, by this cutting, it is possible to simultaneously achieve a plurality of travel path reductions in the spindle nut having a plurality of relief holes B.

[0027] The transition range W having the travel length l from the ball groove (K) to the point C is technically included in the helix of the female thread of the spindle nut and not included in the relief hole B. Here lies one of the advantages of the present invention. Because thereby, the screw spindle can be used without being changed in the transition range W as a counter member to the spindle nut, and the relief (load relief) of the balls in the portion W is achieved by only a slight additional processing step of the spindle nut.

[0028] In all descriptions herein, the intersection point C is always intended as the resulting intersection point of K and L in the state of the fully machined spindle nut. Point C may be located elsewhere (in the otherwise same spindle nut) depending on whether and how long a reduction in the path of travel of the balls in the ball groove K is realized. However, this does not present a problem for the skilled person in the art when setting up, who will usually select point C, specify the distance t and configure the reduction of the path of travel I therefrom.

[0029] As mentioned at the beginning, there are always two relief holes B forming the ball feedback section as an inlet passage and an outlet passage with a transition passage arranged therebetween. If the singular number is used, described or shown in the specification or drawings, this should be understood as an example and not as a deficiency.

[0030] It has proven advantageous from a manufacturing perspective to manufacture the two relief holes B so that they are oriented parallel to one another. This means that the balls always experience the same conditions when unloading or when being introduced into the ball passage, regardless of the direction of rotation of the ball screw drive. In addition, drilling or milling operations can therefore be carried out using one fixture.

[0031] The proposed design and measures make it possible to avoid the complexity of milled transitions or curved lines. The provision of a calculated transition (bend, step) from the ball groove to the relief hole allows for better control of manufacturing tolerances and NVH issues than a perfectly tangential derivation or complex transitions and radii.

[0032] In the sense of the present invention, a ball screw drive, in its basic configuration as described above and exemplarily shown in the drawings, comprises a threaded spindle and a spindle nut. The threaded spindle is at least partially coaxially surrounded by the spindle nut (the threaded spindle usually has a longer length than the spindle nut). A large number of balls circulate in a helical ball path in the intermediate space between the threaded spindle and the spindle nut. A ball diverter with a transition passage arranged in or on the jacket surface of the spindle nut results in a closed ball circulation. The ball screw drive can be equipped with one or more ball diverters with independent ball circulation sections.

[0033] In one exemplary configuration, a spindle nut with ball grooves set for balls of d=2.4 mm was realized. The radius of the ball groove was approximately 7.5 mm. A drop of t=0.08 mm was realized over a travel length l of 1.2 mm. The spacing a was approximately 0.56 mm. [Brief explanation of the drawings]

[0034]

Figure 1

Figure 2

Figure 3

[0035] FIG. 1 shows a schematic cross section of a spindle nut as a single hollow cylindrical part with an outer surface MA. The central longitudinal axis Z lies entirely in a plane E, which intersects the spindle nut S in half in accordance with the longitudinal axis Z. The inner surface of the hollow cylindrical part shows the spiral of the ball groove K. The baseline of the thread defines a (cylindrical) surface G. The wall of the spindle nut S is punched with a relief hole B. The jacket line L (dash-dot line) that characterizes the relief hole B is defined by its intersection with K. The hole is therefore technically defined, in simplified terms, to continue the ball groove K (dashed line) at B along L. In the example shown here, it can be seen that the jacket line L is perpendicular to the plane E. The tangent line T (dotted line) touches G and runs parallel to it with a distance a. T is also perpendicular to E. Similarly, the normal line R of T is located at Z, where L is also perpendicular.

[0036] The case where T and L are perpendicular to R but not perpendicular at E is not shown. This means that the relief hole has some inclination in the plane or deviation from the right angle. The relationship "T and L are perpendicular to R" is maintained in all cases. The spacing A as the distance between T and L is also shown.

[0037] 2 shows an enlarged section of the ball displacement from the ball groove K with a reduction in the path of travel W of length l, which is realized as an embodiment of the invention. Looking at the path of travel of the ball (in the figure) from the right, the ball is guided under load in the ball groove from the point indicated by A until it transitions into the reduced path of travel W. At the end of section W, at C, the ball transitions into relief hole B and follows path of travel L, which, like W, is configured in the wall of the spindle nut S. For illustrative purposes, point C' is also included in FIG. 2, which indicates the (imaginary) intersection of L with the unreduced ball groove K without W.

[0038] As can be seen from the above explanation, the lowered path portion W is relatively short. This has therefore proved to be relatively unproblematic in terms of NVH and manufacturing costs. This concerns the curvature of W. W can be configured as a single straight line or as a curve with a constant or variable radius. This means that, depending on the setting, the path of the ball from K via W to L will have at least a bend or step (at least at C) also at A, depending on the shape of the transition from K to W.

[0039] The tangent T (dotted line) at the distance a to L (dashed line) and the perpendicular R to the central longitudinal axis Z are shown accordingly.

[0040] 3 shows a plan view of a spindle nut S having two ball deflectors of the type described herein. The outer jacket of the spindle nut S has two pairs of relief holes B 11 ,B 12 and B21,B 22 are provided, which are used as the outlet passage and the inlet passage. 11 ,B 12 are connected by a transition passage V1, and B 21 ,B 22 are connected by V2. In Figure 3, the transition passage V is configured as a milled groove in the material of the spindle nut. It can be seen from this figure that the relief holes B all run parallel to one another. The present invention may also include the following aspects: 1. An essentially hollow cylindrical workpiece made of metal, having an outer jacket surface M A and a central longitudinal axis Z that is centrosymmetric, and a ball nut (S) for a ball screw drive unit, including a ball groove (K) arranged in a helical shape on the inner cylindrical surface so as to accommodate a ball with a diameter d, wherein the inner cylindrical surface (G) is defined by the helical base line of the ball groove (K), and includes linear, essentially cylindrical relief holes (B) provided in pairs, the relief holes being provided as an introduction passage and an outlet passage for the balls from or to the ball passage (K) through the ball nut wall portion, each relief hole (B) being characterized by a jacket line (L), the jacket line having an intersection point (C) with the helical base line of the ball passage (K) and being an extension of the movement path of the balls from the ball passage (K) to the relief hole (B), in the spindle nut, a tangent (T) extends parallel to the jacket line (L) at an interval a>0 on the cylindrical surface (G), and both T and L are arranged perpendicular to a common perpendicular (R) to the central longitudinal axis Z, a spindle nut characterized by this. 2. The spindle nut (S) according to 1. above, characterized in that both the tangent (T) and the jacket line (L) are arranged perpendicular to a plane (E) defined by the central longitudinal axis Z and the perpendicular (R) of the spindle nut. 3. The spindle nut (S) according to 1. or 2. above, characterized in that for the interval a, 1 / 5d < a < 1 / 3d holds. 4. The spindle nut (S) according to any one of 1. to 3. above, characterized in that the interval a is preferably 25% ± 3% of the ball diameter d. 5. The movement path of the balls in the ball groove (K) is reduced at the movement length l in front of the intersection point of K with L, and as a result, the radial interval of the balls increases by t from the central axis Z while traveling l, and for l, 2 / 5d < l < 2 / 3d holds, the spindle nut (S) according to any one of 1. to 4. above, characterized by this. 6. The spindle nut (S) according to 5. above, characterized in that the radial interval t of the balls increases from the central axis by 1 / 33 to 1 / 25 of the ball diameter d. 7. A spindle nut (S) according to any one of 1. to 6. above, characterized in that the central axes of the two relief holes (B), which together form components of the ball feedback section as an inlet passage and an outlet passage, are oriented parallel to each other. 8. A ball screw drive including a threaded spindle, a spindle nut coaxially surrounding the threaded spindle at least partially, a plurality of balls circulating in a helical ball passage in an intermediate space between the threaded spindle and the spindle nut, and a ball direction changer arranged on a jacket surface of the spindle nut, A ball screw drive unit characterized in that the spindle nut is configured according to any one of 1. to 7. above.

Claims

1. Outer jacket surface M Ａ 1. A spindle nut (S) for a ball screw drive, comprising: a hollow cylindrical workpiece made of metal having a central longitudinal axis Z and a central symmetry; and a ball groove (K) arranged in a threaded manner on its inner cylindrical surface, configured to receive a ball of diameter d, wherein an inner cylindrical surface (G) is defined by a helical thread base line of the ball groove (K), and the spindle nut comprises a pair of linear cylindrical relief holes (B), which are provided as inlet and outlet passages for the ball from and to the ball passage (K) through the spindle nut wall, each relief hole (B) being characterized by a jacket line (L), which has an intersection (C) with the thread base line of the ball passage (K) and which is an extension of the path of travel of the ball from the ball passage (K) to the relief hole (B). A spindle nut characterized in that a tangent (T) extends parallel to a jacket line (L) on a cylindrical surface (G) with an interval a>0, and both T and L are arranged perpendicular to a common perpendicular line R to a central longitudinal axis Z, and the movement path of the balls in the ball groove (K) is lowered over a movement length l before the intersection of K with L, so that the radial spacing of the balls increases by t from the central axis Z while traveling through l, and 2 / 5d<l<2 / 3d holds for l.

2. 2. The spindle nut (S) according to claim 1, characterized in that both the tangent line (T) and the jacket line (L) are arranged perpendicular to a plane E defined by the central longitudinal axis Z of the spindle nut and the perpendicular line R.

3. 3. The spindle nut (S) according to claim 1 or 2, characterized in that the interval a satisfies the relationship 1 / 5d<a<1 / 3d.

4. The spindle nut (S) according to any one of claims 1 to 3, characterized in that the interval a is 25% ± 3% of the ball diameter d.

5. 5. Spindle nut (S) according to any one of claims 1 to 4, characterized in that the radial spacing t of the balls increases from the central axis by 1 / 33 to 1 / 25 of the ball diameter d.

6. The spindle nut (S) according to any one of claims 1 to 5, characterized in that the central axes of the two relief holes (B), which together form components of the ball feedback portion as an inlet passage and an outlet passage, are oriented parallel to each other.

7. A ball screw drive unit including a threaded spindle, a spindle nut coaxially surrounding the threaded spindle at least partially, a plurality of balls circulating in a helical ball passage in an intermediate space between the threaded spindle and the spindle nut, and a ball direction changing portion disposed on a jacket surface of the spindle nut, A ball screw drive unit, characterized in that the spindle nut is constructed according to any one of claims 1 to 6.

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