Ball screw drive

Abstract

A ball screw drive comprising a screw and a nut at least partially coaxially surrounding the screw and a plurality of balls circulating in helical ball channels between the screw and the nut. At least one ball recirculation device arranged in the screw comprises two ball diverters and a recirculation channel therebetween parallel to the central axis, ensuring a closed circulation of the balls. The balls follow a guide path from the ball channel into the recirculation channel, which always has at least one circular channel with a net width Lw and comprises at least two turning sections with distinguishable curvature. The second turning section in the immediate vicinity of the transition area of the recirculation channel is derived from the basic shape of a quarter circle arc. The transition from the ball diverter to the recirculation channel is offset back by a distance d1 parallel to the termination plane of the ideal quarter circle arc, while the channel Lw is enlarged in height by a distance d2. Wherein d1 ≈ d2.

Description

Technical Field

[0001] This invention relates to a ball screw drive device for electromechanical brakes, and more particularly to the reversal of the balls from a circulating path located between the screw and the nut. Background Technology

[0002] A ball recirculating screw, also known as a ball screw drive (KGT), generally refers to a rolling helical transmission device that uses balls as rolling elements. From a technical perspective, a KGT is a helical transmission device that converts the rotary motion of a drive motor into linear motion.

[0003] The main components of a KGT consist of a lead screw and a nut fitted onto the lead screw. During operation, balls circulate between these two components. The threads of both the lead screw and the nut are constructed as ball raceways with suitable cross-sections and complement each other, so that they (in the assembled state) together form a helical ball channel consisting of two half-shells. Unlike the sliding motion between the threaded sides in a bolt-nut connection, in a KGT, the balls circulating in the threads bear the load transfer between the nut and the lead screw. Therefore, the sliding motion is replaced by (approximate) rolling motion, resulting in reduced friction.

[0004] To achieve a closed ball recirculation path, a ball return device is used. Its task is to remove the balls from the ball channel between the nut and the screw in a first position and reintroduce them in a second position. Therefore, the ball return device constitutes a bypass that crosses one or more threads of the nut-screw system, thus forming a closed recirculation path for the KGT's balls. For this purpose, a ball reversing device guides the balls out of the ball channel and transfers them to a return channel. After flowing through the return channel, the balls are guided back into the ball channel by a second ball reversing device. Therefore, the ball return device consists of two ball reversing devices and a return channel located between them.

[0005] Typically, the balls are guided radially outward from the ball channel, then guided through channels, tubes, or holes inside or outside the nut, before being reintroduced into the ball channel between the screw and the nut (external reversing). However, so-called internal reversing is also known, in which the balls are reversed radially inward into the screw.

[0006] When a ball screw drive operates under load, the balls primarily bear compressive stress in the ball channels. This stress must be released in the ball reversing mechanism before the balls can enter the return channel without load. Re-entering the ball channels requires performing the corresponding tasks in reverse order. Therefore, the challenge lies in defining a ball reversing mechanism that ensures, as precisely as possible, the decompression and reversal of the balls into the return channel (or vice versa). Furthermore, the ball reversing mechanism should be simple in shape and inexpensive to manufacture.

[0007] DE 198 57 581 A1 illustrates a ball screw drive with internal reversing, wherein the reversing of the balls toward the return channel is achieved by reversing elements that form a wide quarter-circle arc and then guide the balls into the return channel at sharp bends.

[0008] US 2015 / 369349 A1 describes a two-piece ball screw drive with reversing mechanism for internal ball reversing. The ball path in the reversing mechanism first describes a semicircle between the lowest point of the ball channel and the return channel, which is located on the central axis of the screw. In contrast, a shorter, horizontally identical reversing mechanism guides the balls into the return channel located on the central axis of the screw.

[0009] Document WO 2015 / 081 131 A1 illustrates a ball screw drive with internal reversing and a return channel located on a central axis. The balls are guided directly to the horizontal level of the return channel via a radial channel. Removal and reversing are achieved via a tubular insert with shovel-shaped tongues that guide the balls in reversal accordingly. Summary of the Invention

[0010] This invention is based on a conventional ball screw drive (KGT) with internal reversing. This KGT includes a screw and a nut that at least partially coaxially surrounds the screw; that is, the sleeve-shaped nut can be manufactured shorter than the screw relative to the central axis of the KGT. During operation, multiple balls circulate in a helical (spiral) ball channel within the gap between the screw and the nut. To ensure a closed circulation path for these balls, at least one ball return device is provided in the screw, which has two ball reversing devices and a return channel located therebetween. These ball reversing devices can be positioned in corresponding shaped openings in the screw, allowing them to guide or exit the circulating balls from the ball channel into the return channel. These openings are preferably elliptical or elongated, and can be easily manufactured by milling. The return channel is configured as a hole in the screw parallel to the longitudinal axis. However, the channel can also be manufactured using equivalent methods.

[0011] The following discussion considers the ball guide path. This term essentially refers to the theoretical path of the balls during KGT operation, defined by the mechanical or structural boundaries of the ball raceways, ball channels, ball reversing mechanisms, and return channels. In the accompanying drawings, this guide path is ideally represented by a line formed by the position of the ball's center of gravity during the KGT cycle. Actual operating paths may deviate from the guide path due to wear, (necessary) clearance, or variations in operating conditions, but this does not affect the concept of the invention.

[0012] It should be noted that the technical implementation of embedding the ball reversing device as a component into the lead screw does not necessarily mean that the technical component "ball reversing device" itself completely surrounds the guide path. It can also be implemented, as shown in the attached figure, as a component that only fulfills the guiding function for the balls after being embedded in the groove of the lead screw. In other words, the wall area of ​​the groove can complete the ball reversing device, so that they together surround the guide path. This does not impair the invention, because for those skilled in the art, the guide path can still be clearly read from the ball reversing device. In this preferred embodiment, the guide path of the balls inside the ball reversing device (embedded in the lead screw) is tubularly surrounded. In this case, the wall of the tube is segmented by the surface area of ​​the lead screw opening and the surface area of ​​the ball reversing device itself. The term "tubular" should not be interpreted as the entire guide path being surrounded by a perfect tube. Those skilled in the art will understand that the design of the tube should optimize the ball steering and guiding function in terms of safety and noise generation.

[0013] However, the guide path for the balls to enter the return channel from the ball channel, i.e., in the ball reversing device, will always ensure at least one circular channel with a net width Lw. This net width is chosen to be slightly larger than the diameter of the circulating balls, typically a few tenths of a millimeter larger.

[0014] A ball reversing mechanism comprises at least two reversing sections. A reversing section is a region or segment on the guide path that has a uniform curvature (Krümmung). For example, a mathematically precise semicircle has a curvature of 180°, and a quarter circle has a curvature of 90°. "Uniform" curvature does not mean that there must be a uniform radius of curvature within a reversing section. The guide path in a reversing section can also follow the shape of an ellipse or other mathematical curves. When referring to a reversing section with a quarter circle, the start and end points of the reversing section have the same circular channel whose (extending) planes intersect each other perpendicularly.

[0015] Therefore, a identifiable (distinguished) change of direction must occur between the two turning sections because the existing bend ends. In this invention, the second of at least two turning sections is considered, which is adjacent to the transition region to the ball channel. It originates from the basic shape of a quarter circle arc, with the improvement that: at the transition (actually) from the ball reversing device to the subsequent return channel, it is offset back by a distance d1 parallel to the terminating plane of the ideal quarter circle arc, while the channel Lw is expanded in height by a distance d2. As clearly shown in the figure, the direction from the ball reversing device to the return channel is therefore no longer a perfect tangential direction, but discontinuous.

[0016] The enlarged portion d2 is typically located on the section of the ball reversing device that is away from the central axis of the ball screw drive. Preferably, d1 ≈ d2, in other words, the offsets are chosen to be essentially equal. This requirement can be easily achieved by those skilled in the art based on the design of the ball reversing device.

[0017] Preferably, one or more ball reversing devices are manufactured as a single piece. They can be manufactured by plastic injection molding, metalluck, or metal injection molding. Injection molding, including metal injection molding, allows for geometries that are not achievable using conventional machining methods or sheet metal bending.

[0018] More preferably, the two offsets d1 and d2 are determined such that: 0.2mm ≤ d1 ≤ 0.3mm and 0.15mm ≤ d2 ≤ 0.25mm.

[0019] To achieve smooth operation, it has been shown that the net width Lw in the ball reversing device should be selected to be 0.2 mm to 0.4 mm larger than the ball diameter set in the ball screw drive. In a practical embodiment of the ball screw drive, balls with a nominal diameter of 3.6 mm are used in operation, and the net width Lw is between 3.8 mm and 4 mm (inclusive).

[0020] One embodiment of a ball screw drive device specifies that d1 ≥ d2. Attached Figure Description

[0021] Figure 1 shows a cross-sectional view of a conventional ball screw drive with internal reversing according to the prior art.

[0022] Figure 2 shows a schematic diagram of various terms or technical features that are important to this invention.

[0023] Figure 3 shows a top view of a ball reversing device designed with a semi-open housing that can be embedded in a lead screw.

[0024] Figure 4 is a perspective view of the component shown in Figure 3.

[0025] Figure 5 is a side view of a ball reversing device with a transition section leading to a return channel, designed according to the present invention. Detailed Implementation

[0026] The present invention will now be described by way of example with reference to the accompanying drawings.

[0027] Figure 1 shows a ball screw drive 100 with internal reversing according to the prior art, intended to illustrate the terminology used. The screw 110 has helically wound, shell-shaped ball raceways 140 on its cylindrical outer surface. The nut 120 has complementary ball raceways on its hollow cylindrical inner surface. These raceways together form a ball channel 130 in which balls 150 can circulate during operation. To obtain a closed circulation path for the balls 150, the ball screw drive 100 has (at least one in each embodiment) a ball return device 160 comprising two ball reversing devices 170, 170' and a return channel 180. The return channel 180 is configured as a longitudinal parallel hole 200 parallel to the central axis 190 of the screw 110. The ball reversing devices guide the balls from the ball channel 130 into the return channel and back again. Typically, ball reversing devices are designed to be identical or mirror-symmetrical so that the rotational behavior of the ball screw drive 100 is independent of the drive direction.

[0028] Figure 2 abstractly illustrates the (not shown) circulation path of the balls from the ball channel into the return channel 280, and the key elements and terms for understanding the invention. The lead screw 210 is shown as a cylinder with a schematic, dashed line representing a single ball raceway 240. At point 251, the ball begins to exit from the ball raceway 240 into the first turning section 271. The path of the ball through the ball return device is represented by a dotted line, the guide path 260, which—virtually—marks the path at the center of the ball. The transition 252 between the first and second turning sections (271-272) is represented by a circular area; the channel of the guide path 260 is marked with a central cross. At point 255, the return channel 280 begins, positioned parallel to the central axis 290. For the purposes of this invention, the second steering section 272 is important and is therefore drawn in more detail than the first steering section 271, which only describes the portion from the ball raceway 240 to the starting point of the second steering section 272 / transition 252.

[0029] The second turning section 272 is classically (not according to the invention) depicted as a quarter-circle arc that cleanly tangentially flows into the return channel 280. In this figure, the plane of transition 252 is represented such that the normal through the central cross of transition 252 extends precisely radially, thus forming a perpendicular line from the central axis 290 to the surface of the lead screw 210. However, this is not mandatory. The quarter-circle arc of the first turning section may also be inclined toward the exit point 251; while the circular area at the end of the quarter-circle arc 255 remains in a fixed position. Nevertheless, the planes of transition 252 and the circular area at the end of the quarter-circle arc 255 will still intersect at 90°.

[0030] The circular areas referred to as "transition 252" and "quarter-circle termination 254" are not arbitrary or illusory references, but rather refer to the cross-section of the guide path, which corresponds to at least one ball diameter plus the tolerance dimension of the corresponding design. In Figure 2, only one ball reversing device is shown for clarity; the corresponding parts will be implemented with the same logic.

[0031] Figure 3 shows a ball reversing device 270 according to the invention, having a first reversing section 271 and a second reversing section 272. The cross-section is configured such that the quarter-circle arc 250 of the second reversing section is cut precisely along the guide path 260'. The planes of the termination 255 and the transition 252 are perpendicular to each other. A clear width of the circular channel with dimension Lw is ensured at both the transition 252 and the termination 255 of the quarter-circle arc (more clearly in Figure 4). A The radius of the quarter-circle arc 250 is described. It can be clearly seen in this figure that although the ball reversing device 270 accommodates the quarter-circle arc 250, the ball path 260' is shortened because the outflow section of the quarter-circle arc 250 is incomplete. The plane at the end 255 of the quarter-circle arc 250 is offset back by a distance d1 in a plane-parallel manner, so the transition to the return channel, or the end 253 of the ball reversing device, is no longer tangential. Because the back offset of the ending plane is plane-parallel, and the quarter-circle arc is therefore not shortened at a specific angle, the end opening of the ball reversing device, indicated by mark 253, is widened by a distance d2. Also at this plane marked 253, where the ball reversing device actually merges into the return channel 280 (not shown), at least the net diameter Lw is ensured. The letter B indicates... Figure 5 The viewing direction is from the ball reversing device 270.

[0032] Figure 4 is a perspective view of the ball reversing device 270 shown in Figure 3. The second reversing section 272 begins at transition 252 and allows the guide path 260' to extend along a quarter-circle arc, which ends at mark 253 at the end of the ball reversing device. The planes of transition 252 and the end of the ball reversing device 253 are perpendicular to each other because, as described above, the backward offset of the terminating plane 255 (not redrawn here) of the outflowing quarter-circle arc 250 towards the plane of mark 253 is planar parallel. The circular areas at transition 252 and the end of the ball reversing device 253 have a net diameter Lw.

[0033] Figure 5 is a view of a ball reversing device 270 as seen from perspective B shown in Figure 3. The transition plane 252 is drawn here as parallel to the bottom edge of the ball reversing device 270. However, as mentioned above, this is not mandatory. The plane may also be inclined, as indicated by the designation 252', but this will not change the teachings of the invention.

Claims

1. A ball screw drive device (100), including A lead screw (110, 210) and a nut (120), the nut being at least partially coaxially surrounding the lead screw (110, 210). and Multiple balls (150) that can circulate in a helical ball channel (130) in the gap between the lead screw and the nut; and At least one ball return device (160) includes two ball reversing devices (170, 170', 270) and a return channel (180, 280) located therebetween. The ball reversing devices (170, 170', 270) can be disposed in openings within the lead screw (110, 210) such that they can guide the balls (150) from the ball channel (130) into the return channel (180, 280) or out of the return channel, and The return channels (180, 280) are configured as holes (200) in the lead screw (110) parallel to the longitudinal axis, thereby establishing a closed circulation path for the balls (150) in the ball screw drive (100); The guide path (260') for the ball (150) to enter the return channel (280) from the ball channel is as follows: It always has at least one circular channel with a clear width of Lw, and It includes at least two turning sections (271, 272) with distinguishable bends, wherein the second turning section (272), which is adjacent to the transition area to the return channel (280), originates from the basic shape of a quarter circle (250). Its features are, The transition point (253) from the ball reversing device (270) to the return channel (280) is offset back by a distance d1 parallel to the termination plane (254) of the ideal quarter circle arc (250), while the channel Lw is expanded in height by a distance d2, where d1 ≈ d2.

2. The ball screw transmission device (100) according to claim 1, characterized in that, Each ball reversing device (270) is manufactured as a single piece.

3. The ball screw drive device (100) according to any one of claims 1-2, characterized in that, It satisfies 0.2mm ≤ d1 ≤ 0.3mm and 0.15mm ≤ d2 ≤ 0.25mm.

4. The ball screw drive device (100) according to any one of claims 1-3, characterized in that, The ball reversing device (270) is manufactured as a plastic injection molded part, a metal printed part, or a metal injection molded part.

5. The ball screw drive device (100) according to any one of claims 1-4, characterized in that, The guide path (260, 260') is surrounded by a tube inside the ball reversing device (170, 170'), wherein the wall of the tube is composed of segments of the surface area of ​​the opening inside the lead screw (110, 210) and the surface area of ​​the ball reversing device (170, 170') itself.

6. The ball screw drive device (100) according to any one of claims 1-5, characterized in that, The net width Lw is selected to be 0.2 mm to 0.4 mm larger than the diameter of the ball (150) set in the ball screw drive.

7. The ball screw drive device (100) according to any one of claims 1-5, characterized in that, The ball screw drive is designed to operate balls with a diameter of 3.6 mm, and Lw is between 3.8 mm and 4 mm (inclusive).

8. The ball screw drive device (100) according to any one of claims 1-7, characterized in that, The condition d1≥d2 is satisfied.

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