Ball screw drive and its screw nut

Abstract

The invention relates to a ball screw drive and a screw nut therefor. The screw nut (S) for a ball screw drive is generally designed as a hollow cylinder having an outer peripheral surface (M A ) and a centrally symmetrical central longitudinal axis (Z). The screw nut has an internal thread designed as a ball channel (K) arranged on the inner surface of the cylinder. A pair of unloading holes (B) as inlet and outlet passages for the ball passages (K) of the balls passing through the wall of the screw nut substantially in the form of a cylinder. Each unloading hole (B) is characterized by having an outer contour line (L) having an intersection point (C) with the thread base line of the ball passage (K) and is a geometric continuation of the path of movement of the balls from the ball passage (K) to the unloading hole (B). A tangent (T) at the cylinder surface (G) extends parallel to the outer contour line (L) at a distance a > 0, both (T) and (L) being perpendicular to the same plumb line (R) of the central longitudinal axis (Z).

Description

Technical Field

[0001] This invention relates to ball screw drives and, in particular, the screw nut of a ball screw drive. Background Technology

[0002] Typically, a ball screw drive system using balls as rolling elements is called a recirculating ball screw or ball screw transmission. The main components of a ball screw transmission include a screw with external threads and a screw nut with internal threads surrounding the screw. The threads of the screw and screw nut are designed with appropriately profiled ball channels that complement each other, so that they together form a tunnel-shaped ball channel or ball guide mechanism in the assembled state. Unlike bolt-nut connections where the threaded surfaces slide against each other, in a ball screw transmission, the recirculating balls in the threads bear the load transferred between the nut and the screw. The sliding motion in the form of surfaces is thus replaced by rolling motion, resulting in reduced friction.

[0003] To achieve a closed-loop circulation path for the balls, a ball return mechanism is used. Its task is to eject the balls at a first point in the ball channel between the screw nut and the screw, and then return them at a second point. Therefore, the ball return mechanism is a bypass that crosses one or more threads of the nut-screw system, thus achieving a closed-loop circulation path for the balls used in a ball screw drive. Typically, the ball return mechanism consists of a first ball deflection mechanism (leaving the ball channel), a transfer channel, and a second ball deflection mechanism (entering the ball channel). Generally, the balls are removed radially outward from the ball channel and guided within or outside the screw nut in the transfer channel (e.g., a channel or pipe), and then guided back into the ball channel between the screw and the screw nut at a designated position via the second ball return mechanism.

[0004] Technically, a ball screw drive functions similarly to a staggered helical gear drive, converting rotary motion into longitudinal motion or vice versa. The deceleration or acceleration is determined by the screw's dimensions and thread lift. In principle, a ball screw drive can operate in two ways. First, if the screw nut is fixed in position but rotatable, while the screw is non-rotatable but longitudinally movable, then once the nut is driven, the screw moves along its longitudinal axis. Second, in the second case, the screw is fixed in position but rotatable, while the screw nut is longitudinally movable and non-rotatable. When the screw is driven, this causes the screw nut to move linearly along the screw.

[0005] Ball screw drives are used in many technical applications, especially in mechanical engineering and machine tools. However, they are also increasingly used as longitudinal drive systems in areas that have previously relied on hydraulic or pneumatic systems, such as in presses, injection molding machines, and servo steering systems. Furthermore, ball screw drives are playing an increasingly important role in electromechanical and electro-hydraulic braking systems, where they are used as replacements for hydraulic brake cylinders or in parallel with known braking systems in brake assist systems. There, they enhance the driver's braking force under electric motor drive, or initiate or support (emergency) braking processes as part of a safety system. Therefore, it is also possible to implement a fully electric braking system with ball screw drives as a replacement for brake cylinders on each wheel.

[0006] During operation, various forces act on the balls. Therefore, depending on the drive direction and load, they are pressed against different sides of the ball channel, resulting in compressive stress. This leads to the release of compressive stress at the point where the balls are ejected from the ball channel between the screw and nut. It is practically impossible to arrange this unloading process continuously so that the balls simply roll into or slide into the ball return mechanism. Instead, the combined force from more or less sudden or gradual unloading ejects the balls into the ball steering mechanism or transfer channel. This behavior can lead to excessive wear of the balls and / or the ball steering mechanism, or the generation of annoying operating noise and / or undesirable vibrations. This behavior is categorized under the term NVH (Noise, Vibration, and Harshness). Especially in the automotive industry, NVH is considered a problem because it can leave the user with the impression of poor quality or (initial) defects, even without technical issues.

[0007] Various methods are known in the prior art for improving ball unloading during ball reversal. One method involves designing the ball reversal so that it begins as a channel or tube that precisely fits tangentially into the ball passage surrounding the lead screw nut. However, forming the necessary hole through the lead screw nut for this purpose is technically challenging.

[0008] DE102004025683 similarly describes making the ball channels and ball inlets in the lead screw nut have the same cross-sectional profile.

[0009] The literature DE OS3635212 proposes that a return channel is set on the outside, which is grounded to the channel of the lead screw nut, that is, its center line depicts a special three-dimensional spatial curve.

[0010] DE102011076438 proposes to widen the inlet or outlet area of ​​the self-threaded ball bearings.

[0011] All existing variations share the common characteristic of being complex and / or expensive in terms of manufacturing technology. Therefore, there is a need for a ball screw nut with a ball steering mechanism that improves ball unloading in terms of NVH standards while allowing for reasonable manufacturing. Summary of the Invention

[0012] This task is accomplished using a lead screw and nut.

[0013] Screw nuts for ball screw drives comprise a generally hollow cylindrical workpiece made of metal, having an outer contour surface M. A The central longitudinal axis Z is symmetrical to the center. Accessories on the lead screw nut, such as flanges, brackets, and drive components, are not considered here. A ball groove K, arranged in a thread-like pattern, is located on the inner surface of the (hollow) cylinder, designed to accommodate balls of diameter d. The inner surface G of the cylinder is defined by the helical thread baseline of the ball groove K. Because the inner surface of the cylinder with the ball groove has a three-dimensional surface structure, the cylindrical surface G is considered as a reference surface for subsequent considerations.

[0014] As described above, the ball groove K in the lead screw nut, together with the ball groove designed to complement the shape of the lead screw nut, forms the ball channel (in the assembled state).

[0015] Furthermore, the lead screw nut has paired, straight, and generally cylindrical unloading holes B, designed as inlet and outlet channels for the balls to enter and exit the ball channel passing through the wall of the lead screw nut. Each unloading hole B is characterized by an outer contour line L (Mantellinie) that intersects with the thread baseline of the ball channel K and represents (geometrically or technically) the path of movement of the balls from the ball channel or ball channel K into the unloading hole B. It is important to note that the outer contour line L has a straight orientation at least initially, thus clearly defining the position of the outer contour line L geometrically from the intersection of L and G, and explicitly defining subsequent characterization.

[0016] The unloading holes of all designs described herein are preferably drilled and / or milled directly into the lead screw nut, thus directly fulfilling the input / output function. Technically equivalently, the geometry can also be achieved by a molded part made of plastic, metal (cast, 3D printed), or similar dimensionally stable material, fitted into a recess in the lead screw nut wall with correspondingly defined dimensions. This will not diminish the advantages of the geometry. It is also conceivable that the unloading holes on the inner side are realized within the wall as described, but transition into the molded part on the outer side of the lead screw nut, which can also be integrally manufactured, for example, with the transfer channel. Plastic, metal, or combinations thereof can also be used here.

[0017] These relief holes should not be understood herein as purely cylindrical holes, but may also have an elliptical or non-circular cross-section. This is irrelevant for the determination of the intersection point of L and K.

[0018] If these geometric configuration conditions are followed, a tangent T can now be defined, which on the one hand bears closely against the cylindrical surface G and also extends parallel to the outer contour line L at a distance a>0, wherein both T and L are perpendicular to the same plumb line R of the central longitudinal axis Z.

[0019] In another embodiment, both T and L are perpendicular to a plane E. The plane E is coplanar with the central longitudinal axis Z of the screw nut, and the plumb line R is also located in this plane. In other words, R and Z define the plane E.

[0020] The distance a between the two parallel lines T and L is between one fifth and one third of the ball diameter d. The ball diameter d is selected as the scale or reference for the dimensions of the present invention, since this allows a wide range of applicability to be depicted independently of the dimensional configuration of the screw nut or the thread helix. It has proven that 1 / 5d<a<1 / 3d is a preferred relationship for a. A value of a equal to 25%+ / -3% of the ball diameter d is particularly preferred.

[0021] Thus, the value a>0 obviously means that the moving raceway of the balls does not extend exactly tangentially and thus without an inflection point into the relief hole B, but a defined point C marks the transition portion as the intersection point between K and L. The direction of the relief hole, specifically the angle of the hole in the wall of the screw nut, corresponds here to a tangent T on the cylindrical surface G, but is offset radially inwards towards the central longitudinal axis Z by this distance a (outer contour line L).

[0022] It shall be considered herein that the condition "the tangent (T) at the cylindrical surface (G) is parallel to the outer contour line (L) at a distance a>0, wherein both T and L are perpendicular to the same plumb line R of the central longitudinal axis Z" does not automatically mean that the hole or its projection onto Z is also perpendicular to the central axis Z. On the contrary, this condition can also be satisfied by a hole B which, for example, assumes the helix angle (Steigungswinkel) of the thread helix. Only the additional condition "T and L are perpendicular to a plane E defined by the plumb line R and the central longitudinal axis Z" ensures that the projection of the hole B or its central axis is perpendicular to the central longitudinal axis Z.

[0023] Thus, the present invention can also be described as deliberately providing a step or edge with a defined geometry, at which step or edge the ball relief effectively ends when the balls transition into the relief hole. However, this is configured or the transition is selected such that the NVH conditions are improved compared to known designs.

[0024] In another embodiment, the movement path of the balls in the ball channel or ball race K is lowered (absenken) over a movement length l before the intersection point C of K and L, so that the radial distance of the balls from the central axis Z increases when passing through l. For the path section W with the movement length l, 2 / 5d<l<2 / 3d applies. In other words, the ball race K is flattened or widened or deepened over the short path section W before the aforementioned step / edge C, so that the balls move away from the central longitudinal axis Z. The expression "flattened or widened or deepened" is chosen to take into account different designs of the ball race. Thus, the commonly used "Gothic arch" can be modified as the cross-section of the ball race by widening, so that the circulating balls increase their position relative to the central longitudinal axis of the screw nut. According to the invention, this depends on the distance variation effect, and has little to do with the (technically meaningful and possible) method of achieving this. In the Figure 2 , this is shown as a lowering of the movement path, without implying that this is restrictive.

[0025] When passing through W, the radial distance t of the balls from the central axis preferably increases by 1 / 33 to 1 / 25 of the ball diameter d. In terms of manufacturing technology, this modification of the ball race can be obtained by milling or by electrochemical removal. Furthermore, in the case of using electrochemical removal, a greater movement path lowering can be obtained on a single screw nut with a plurality of unloading holes B.

[0026] From a technical point of view, the transition region W with the movement length l from the ball race K to the point C belongs to the helix of the internal thread of the screw nut rather than to the unloading hole B. One of the advantages of the present invention lies in this, because thereby the screw, which is the mating part of the screw nut, is also available unchanged in the transition portion W, and ball unloading in this portion W is obtained only through one small additional machining step of the screw nut.

[0027] In all these set conditions, the intersection point C always refers to the resulting intersection of K and L; in the finished machined state of the screw nut. The point C will be located at a different point in an otherwise identical screw nut, depending on whether the lowering of the movement path of the balls in the ball race K is realized and over what length it is lowered. However, for those skilled in the art, this is not a problem in the described design, and he will usually select the point C, determine the distance t, and design the lowering of the movement path l from there.

[0028] As mentioned at the beginning, there are always two unloading holes B serving as an inlet channel and an outlet channel together with a transfer channel arranged therebetween to form a ball return mechanism. If the singular is used, described or shown in the specification or the drawings, it shall be understood as an example rather than a limitation.

[0029] A proven advantage in manufacturing technology is the production of two unloading holes B in a parallel alignment. This means that regardless of the rotation direction of the ball screw drive, the same conditions always exist for the balls during unloading or introduction into the ball channel. Furthermore, drilling or milling can be performed in a single fixture.

[0030] The structural design and measures proposed in this invention allow for the abandonment of complex milling transitions or curves. Considering manufacturing tolerances and NVH issues, the calculated transition (inflection points, steps) from the ball groove to the unloading hole is better controlled than perfectly tangential turning or complex transitions and radii.

[0031] In the context of this invention, the ball screw drive, in its basic construction as described so far, includes a screw and a screw nut, and is illustrated by way of example in the figures. The screw is at least partially coaxially surrounded by the screw nut (the screw length is typically greater than the screw nut). A large number of balls travel in a helical ball trajectory within the gap between the screw and the screw nut. A ball steering mechanism with transfer channels arranged in or above the outer contour surface of the screw nut creates a closed ball cycle. The ball screw drive may have one or more ball steering mechanisms with independent ball cycles.

[0032] In one embodiment, a lead screw nut is implemented with ball channels designed for balls with a diameter of d = 2.4 mm. The ball channel radius is approximately 7.5 mm. A reduction of t = 0.08 mm is achieved over a 1.2 mm travel path l. The distance a is approximately 0.56 mm. Attached Figure Description

[0033] Figure 1 The functional cross-section of the lead screw nut, along with details regarding its geometry, is shown.

[0034] Figure 2 The unloading section of the ball bearing channel is shown in detail, and

[0035] Figure 3 A top view of a lead screw nut with two pairs of unloading holes and a connecting channel located therebetween is shown. Detailed Implementation

[0036] Figure 1A schematic cross-section of a lead screw nut S, in the form of a simple hollow cylinder with an outer surface MA, is shown. The central longitudinal axis Z lies entirely in a plane E, which logically intersects the lead screw nut S at halfway along the longitudinal axis Z. The helix of the ball groove K is marked on the inner surface of the hollow cylinder. The baseline of this thread defines the (cylindrical) surface G. The wall of the lead screw nut S is penetrated by an unloading hole B. The outer contour line L (dot-dot-dash line) representing the unloading hole B is determined by its intersection with K. Thus, technically, the hole is designed so that it simply continues the ball groove K (dashed line) in B along L. In the example shown here, it can be seen that the outer contour line L is perpendicular to the plane E. The tangent T (dashed line) rests on G and extends parallel to it at a distance a. T is also perpendicular to E. The orientation of the vertical line R from T to Z is also shown, which is also perpendicular to L.

[0037] Not shown is a variation where T and L are perpendicular to R but not to E. This means the unloading hole will have some tilt or deviation from a right angle perpendicular to the plane. In all cases, the relationship "T and L are perpendicular to R" is obtained. The distance A, which is the distance between T and L, is also marked.

[0038] Figure 2 This is a partially enlarged view showing the ball exiting the ball groove K along with the reduced movement path W of length l, as implemented in this embodiment of the invention. Looking at the movement path of the ball (in the figure) coming from the right, it is guided by force into the ball groove K until it transitions from point A into the reduced movement path W. At the end of path W, at point C, the ball enters the unloading hole B and moves along the movement path L, which, like W, is formed in the wall of the lead screw nut S. Figure 2 In the illustration, point C' is also inserted, its marker L being the (virtual) intersection of the unreduced ball groove K, i.e., without local W.

[0039] As explained above, the reduced path length W is relatively short. Therefore, from an NVH and manufacturing cost perspective, the curvature of W proves to be relatively unimportant. W can be designed as a simple straight line, a curve with a constant radius, or a variable radius. Depending on the design, this may result in the ball moving from K to L via W having at least one inflection point or step (at least at C), depending on the design of the transition from K to W also at A.

[0040] Note that the tangent T (dashed line) is at a distance a from the distance L (dot-dot-dash line) and the vertical line R is perpendicular to the central longitudinal axis Z.

[0041] Figure 3 A top view of a lead screw nut S having two ball steering mechanisms of the type described herein is shown. Two pairs of unloading holes B. 11 and B 12 And B 21and B 22 The sleeves are cut into the outer casing of the lead screw nut S, serving as both outlet and inlet channels. B 11 and B 12 Connected via transition channel V1, while B 21 and B 22 Connected via V2. In Figure 3 In the diagram, the transition channel V is designed as a milled groove in the lead screw nut housing. The figure clearly shows that all of these unloading holes B extend in parallel.

Claims

1. A screw nut (S) for a ball screw drive, the screw nut comprising a workpiece made of metal and substantially hollow cylindrical in shape, the workpiece having an outer peripheral surface (M) A The screw nut also has ball channels (K) arranged like threads on the inner surface of the cylinder, and the ball channels (K) are designed to accommodate balls of diameter d, wherein... The inner cylindrical surface (G) is defined by the helical thread baseline of the ball raceway (K); the ball screw nut further has straight, substantially cylindrical unloading holes (B) arranged in respective pairs, said unloading holes (B) are designed as inlet channels and outlet channels for the balls to pass through the wall of the ball screw nut to enter and exit the ball raceway (K), wherein each unloading hole (B) is characterized by - an outer contour line (L), which has an intersection point (C) intersecting with the thread baseline of the ball raceway (K) and represents the continuation of the movement path of the balls from the ball raceway (K) into the unloading hole (B); and - a tangent line (T) of the cylindrical surface (G), said tangent line (T) extending parallel to the outer contour line (L) at a distance a, wherein both the tangent line (T) and the outer contour line (L) are perpendicular to the same plumb line (R) of the central longitudinal axis (Z); wherein 1 / 5d < a < 1 / 3d applies to said distance (a); and at the same time - both the tangent line (T) and the outer contour line (L) are perpendicular to the plane (E) defined by the central longitudinal axis (Z) of the ball screw nut and the plumb line (R); The characteristic is that the movement path (W) of the ball in the ball channel (K) is the length of movement before the intersection (C) of the ball channel (K) and the outer contour line (L). l The upper part is lowered, thereby reducing the radial distance of the balls after passing through l Time increases by t from the central longitudinal axis (Z), where, for l Applicable to 2 / 5d< l <2 / 3d.

2. The lead screw nut (S) according to claim 1, characterized in that, The radial distance t of the balls from the central longitudinal axis (Z) is increased by 1 / 33 to 1 / 25 of the ball diameter d.

3. The lead screw nut (S) according to claim 1 or 2, characterized in that, The central axes of the two unloading holes (B) are oriented parallel to each other, and the two unloading holes jointly serve as an inlet channel and an outlet channel to form a component of the ball return mechanism.

4. A ball screw drive device, the ball screw drive device comprising a screw and a screw nut at least partially coaxially surrounding the screw, a plurality of balls capable of cyclic movement on a helical ball track within the gap between the screw and the screw nut, and a ball deflection mechanism disposed on the circumferential surface of the screw nut, characterized in that, The ball screw nut is designed according to any one of claims 1 to 3.

Images