Ball screw drive, screw nut and method for manufacturing a screw nut

Abstract

The invention relates to a ball screw drive, a screw nut and a method for manufacturing a screw nut, in particular to a method for manufacturing a metal screw nut for a ball screw drive, comprising the following steps: providing a substantially cylindrical metal workpiece having a first end face and a second end face and a peripheral face; modifying the workpiece by pressing an axis-symmetrical central bore and by simultaneously pressing at least a first channel and a second channel in axis-parallel orientation in the peripheral face of the workpiece, wherein the central bore and the channels are open towards the first end face; introducing an internal thread into the central bore; forming the channels into transition channels for accommodating a diverting insert by introducing at least two radially extending through-holes into at least the first channel. The invention further relates to a screw nut manufactured with the method and to a ball screw drive having such a screw nut.

Description

Technical Field

[0001] This invention relates to a ball screw drive mechanism, a ball screw nut, and a method for manufacturing the ball screw nut. Background Technology

[0002] A ball screw drive mechanism, also known as a ball recirculating screw, typically refers to a rolling helical transmission mechanism that uses balls as rolling elements. The main components of a ball screw drive mechanism include a screw and a screw nut with a ring. During operation, the balls move between these two components. The threads of both the screw and the screw nut are designed with ball grooves of suitable cross-sectional shape and complement each other, thus forming a ball channel or ball guide groove in the installed state. Unlike the screw-nut connection where the threaded sides slide relative to each other in a surface-like manner, in a ball screw drive mechanism, the balls, which rotate and move within the threads, are responsible for the load transfer between the nut and the screw. The surface-like sliding motion is thus replaced by rolling motion, resulting in reduced friction.

[0003] To achieve a complete ball circulation path, a ball steering mechanism is employed. Its task is to elevate the balls from the ball guide grooves between the screw nut and the screw at a first position and return them at a second position. Thus, ball return is a bypass that traverses one or more threads of the nut-screw system, thereby completing a full circulation path for the balls in a ball screw drive mechanism. Typically, the balls are radially moved out of the ball grooves within the screw nut and guided within or outside the screw nut in a channel or tube, subsequently being fed back into the ball channel between the screw and the screw nut at a predetermined position.

[0004] Technically, a ball screw drive mechanism functions as a helical drive mechanism, converting rotary motion into longitudinal motion or vice versa. The speed of acceleration or deceleration is determined by the screw's dimensions and thread lead. Ball screw drives operate in principle in two ways. In the first case, if the screw nut is fixed in position but mounted to be rotatable, while the screw is non-rotatable but longitudinally movable, then once the nut is driven, the screw moves along its longitudinal axis. In the second case, the screw is fixed in position but mounted to be rotatable, while the screw nut is mounted to be longitudinally movable but non-rotatable. This causes the screw nut to move linearly along the screw when driven.

[0005] Ball screw drives are used in many technical applications, especially in machine structures, and are particularly preferred in machine tools. However, ball screw drives are also increasingly used as longitudinal drive mechanisms in areas that have previously employed hydraulic or pneumatic systems, such as presses, casting machines, and power steering systems. Furthermore, ball screw drives also enhance electromechanical and hydraulic-electric braking systems, serving as a replacement for hydraulic brake cylinders or used in parallel with known braking systems in brake assist systems. Their role there is to enhance the driver's braking force under electric motor drive, or to initiate or support (emergency) braking processes as part of a safety system. Therefore, a fully electric braking system with ball screw drives at each wheel as a replacement for brake cylinders is also feasible.

[0006] The configuration of the lead screw nut will be considered in detail below. The lead screw nut, in principle, comprises a hollow cylindrical body with an axial internal thread, which is designed as the upper shell of the ball circulation channel. This body could be manufactured entirely as a turned / milled part, which is time-consuming. Therefore, for some time, the lead screw nut has been essentially made as a pressed part from a metal wire segment, which, for dimensional accuracy reasons, must be machined again at only a few points. Additional work (besides the internal thread) specifically involves the ball steering mechanism, which in the prior art is typically constructed as an elongated hole parallel to its longitudinal axis, concealed within the wall of the lead screw nut. The elongated hole is usually constructed as a through hole along its entire length, as this is the only way to clean it and also allows the ball steering component to be directly mounted on the end face of the lead screw nut. The ball channel can be drilled or top-milled radially from the outside in at specified locations, where the balls should be removed from or returned to the circulation channel between the lead screw nut and the lead screw. It is known that removal or return is achieved here by inserts (mounted parts), which are made of metal, plastic, or a combination thereof. The complex geometry of inserts or ball bearing steering components is easier to shape as castings or sheet metal parts than as geometry that would be milled within the screw nut wall.

[0007] Traditionally, this eliminates the significant expense of additional work on a true ball guide channel between the exit and return points. It is typically drilled as a channel or milled as a groove. In the latter implementation, the channel must be covered after the ball screw drive is installed and the balls are inserted. This can be achieved by a sleeve or a cup to be pressed, or by bearings, flange seats, or sleeves that are to be installed therein.

[0008] In this respect, ball screw drives are relatively complex and time-consuming components, and new approaches are needed for mass production at a reasonable cost. Summary of the Invention

[0009] This invention aims to provide a method for manufacturing lead screw nuts that is simplified and therefore cost-effective compared to existing technologies. The lead screw nuts manufactured using this method, and consequently, the ball screw drive mechanisms incorporating the lead screw nuts, should be precisely manufactured, reliable, and durable under any load.

[0010] This invention relates to a method for manufacturing a metal screw nut for a ball screw drive mechanism, comprising the following steps:

[0011] - Provides a basic cylindrical metal workpiece having a first end face, a second end face, and a circumferential surface;

[0012] The workpiece is modified by pressing out an axisymmetric central hole and simultaneously pressing out at least a first channel and a second channel (axis-parallel orientation) on the circumferential surface of the workpiece, wherein the central hole and the channel are open toward the first end face;

[0013] - Introduce the internal thread into the center hole; and

[0014] - By introducing at least two radially extending through holes into at least the first channel, the channel is formed as a transition channel for accommodating the steering insert.

[0015] Therefore, in particular, the channel is not milled, but pressed into the circumferential surface of the workpiece. This pressing is achieved simultaneously with the remaining basic shape of the lead screw nut. If, for example, the center hole is formed first in order to subsequently form the channel, the second forming step will damage the shape of the center hole. Conversely, when the channel is formed first and the center hole is formed later, the formation of the center hole does not preserve the channel. The channel extends in an axially parallel manner, determined by the axial feed of the forming tool. Because the channels are also pressed out from one end face, they are open toward that end face. The term "channel" here literally means that the channel is formed in the form of a groove, recess, or recess by pressing into the circumferential surface, which is radially outwardly open (viewed from the longitudinal central axis of the workpiece).

[0016] To ensure compensatory material extrusion, two channels are always pressed out. The channels in the circumferential surface are preferably mirrored at the central longitudinal axis of the workpiece, thus offset by 180 degrees about the workpiece's cylindrical axis. This provides the additional advantage that imbalance is minimized during nut rotation due to symmetry about the longitudinal axis. Therefore, in this way, the intermediate product for the lead screw nut is made from a cylindrical workpiece of solid material, which can be achieved through subtractive processing.

[0017] For example, when the nut is rigidly installed during use and the resulting imbalance due to rotation is minimal, it is not necessary to arrange it 180° off-center from the channel on the circumference. Studies have shown that angular arrangements offset between 100° and 180° on the circumference are technically feasible, with values ​​of 120° and 180° being preferred.

[0018] Furthermore, when only the first channel should be designed as a transition channel and the second channel therefore need not be provided as a transition channel, a second channel with a simplified cross-section can be manufactured. This might, for example, mean that the channel is designed with a gentler and more gradual slope, which improves the lifespan of the pressing tool. Importantly, to achieve the aforementioned effects of symmetrical material extrusion, the longitudinal indentation depth of the second channel and the volume of the forming material also substantially correspond to the indentation depth and the volume of the forming material of the first channel.

[0019] For example, one might think of the first channel being made into a U-shaped cross-section with a rounded cross-section, serving as a transition channel, while the second channel is realized as a simple V-shaped cross-section. The above considerations regarding material extrusion still apply, thus requiring the forming to be comparable in volume (within + / - 10% to 15%, depending on the lead screw nut geometry).

[0020] The method of "introducing the internal thread into the center hole" can be achieved through a (internal thread) forming process, through thread milling, or through a combination of both. Typically, a pre-formed thread is first cut, followed by the forming step. This has the advantage that the resulting surface has a relatively high surface hardness due to the compression during the forming process. The final grinding process can be optimized, but it is not necessary.

[0021] The method defined herein is not only significantly simpler and cheaper than existing methods, but also provides precision-manufactured products with long service lives. Unlike subtractive processing methods, the material's cohesion is completely preserved during forming. Plastic changes in the workpiece shape occur due to flow along crystallographically preferred slip planes. The metal regions between these slip planes remain virtually unchanged; therefore, the forming process does not cause macroscopic changes in material properties. Material quality is thus virtually entirely determined by the original workpiece. Under proper method control, no singularities that would lead to failure during use will appear within the workpiece, thus high durability can be expected. Therefore, a simple and inexpensive method can be obtained by combining this method, which is essentially based on forming. Cost savings are primarily based on avoiding the time-consuming and tool-intensive subtractive processing steps of conventional machining and the absence of material loss during forming. Furthermore, this method provides a lead screw nut suitable for meeting high requirements.

[0022] It is useful to specify that the pressing of the channel ends before reaching the second end face, so that the channel is closed toward the second end face. Of course, it is also entirely feasible for the transition channel to be pressed through to the second end face. However, this may result in wear on the second end face, requiring further machining. This may need to be advantageously avoided, so that the pressing ends before reaching the second end face.

[0023] This method is thus improved so that the through hole is milled and / or drilled. Therefore, a subtractive method is preferred for through holes. However, this can also be achieved without much effort, since milling and / or drilling to construct simple through holes is a known task.

[0024] Alternatively, it can be specified that the central hole is pressed all the way to the second end face. Thus, a hollow column with open sides appears as the central hole is pressed in. However, as already described regarding pressing the transition channel to the second end face, this may require further processing, as pressing to the second end face may create irregular material boundaries.

[0025] In this regard, it may also be useful to finish pressing the center hole before reaching the second end face, and to remove the second end face after pressing, so that the center hole opens toward the newly formed second end face. This avoids the formation of irregular material boundaries. Since further subtractive processing is almost unavoidable even if the center hole is pressed through, it can be done immediately without pressing through to the second end face.

[0026] Alternatively, it can be specified that the first end face is removed by subtraction after forming, thereby generating a new first end face. Both end faces of the lead screw nut can therefore be subtractively machined to achieve the required dimensions.

[0027] This is effectively optimized so that the material removal at the end face is achieved by turning.

[0028] It is also specified that the internal thread is introduced into the center hole after forming. The milling process, along with the subsequent forming process, is an optional means of forming threads in the lead screw nut.

[0029] It can also be specified that an axially parallel truncated section is formed on the circumferential surface during workpiece forming. This truncated section is particularly useful for workpiece centering during subtractive processing.

[0030] The present invention also relates to a screw nut for a ball screw drive mechanism, which is obtained in particular by the aforementioned method, wherein the screw nut has at least one first channel and a second channel parallel to the axis in its circumferential surface, which are open toward one end face of the screw nut.

[0031] The present invention also relates to a ball screw drive mechanism having the aforementioned screw nut, screw, a plurality of balls, at least two steering inserts in each transition channel, and at least one transition channel cover. Attached Figure Description

[0032] Figure 1 A three-dimensional view of the workpiece is shown.

[0033] Figure 2 A perspective view of the lead screw nut according to the present invention is shown.

[0034] Figure 3 A perspective view of the steering insert is shown.

[0035] Figure 4 A cross-sectional view of the ball screw drive mechanism according to the present invention is shown.

[0036] Figure 5 A perspective view of the ball screw drive mechanism according to the present invention is shown.

[0037] Figure 6 A perspective view of the ball screw drive mechanism according to the present invention is shown, wherein the screw nut is omitted.

[0038] Figure 7 A front view showing the angular arrangement of the lead screw nut and two pressed-out channels according to the present invention is shown.

[0039] Figure 8 A flowchart of the method according to the present invention is shown. Detailed Implementation

[0040] Figure 1 A perspective view of workpiece 50 is shown. The workpiece 50, upon which the method according to the invention is based for manufacturing the lead screw nut, is cylindrical and is made of a solid material, particularly metal or a metal alloy. The choice of a cylindrical shape for the workpiece is not absolute, as lead screw nuts can also be made from non-cylindrical workpieces in subsequent forming processes. However, the cylindrical shape of the workpiece is almost the only option, since workpiece 50 is undoubtedly a metal wire in most cases. Workpiece 50 is also not absolutely made of a solid material. For example, the workpiece may have a central hole to reduce the amount of material to be expelled in subsequent forming processes. Workpiece 50 has a circumferential surface 14, a first end face 46, and a second end face 44.

[0041] Figure 2 A perspective view of a lead screw nut 10 according to the present invention is shown. The lead screw nut 10 has a circumferential surface 14 and two end faces 30, 32. Two opposing, staggered channels 18, 20 are pressed into the circumferential surface 14, where the first channel 18 is configured as a transition channel. Both channels open toward the first end face 32. A plurality of radially extending through holes 22, 24 are provided within the first channel 18. A central hole 16 pressed into the lead screw nut 10 is provided with an internal thread 34. The circumferential surface 14 of the lead screw nut 10 has truncated portions 36, 38.

[0042] According to Figure 1 The workpiece is manufactured according to 50 Figure 2 The screw nut 10 is formed by first pressing out a center hole 16 and transition channels 18, 20 simultaneously. Here, the center hole 16 is pressed through, thus opening it towards the two end faces 44, 46 of the workpiece 50. The transition channels 18, 20 are not pressed through to the second end face 44, therefore, they are open towards the first end face 46. Cut-off portions 36, 38 are also formed during the forming process. Several subtractive processing steps are performed after forming. These can be performed in different sequences. In particular, the radially extending through holes 32, 34 within the transition channels 18, 20 are milled and / or drilled. End faces 44 and 46 are finished and dimensionally machined into the end faces 30, 32 of the screw nut 10 by a turning process. The internal thread 34 is milled. It is difficult to avoid turning the end face 44, which is reached by pressing through the center hole 16, because irregular edges are almost inevitably produced when the forming tool passes through the center hole 16. The turning of the first end face 46 can be considered redundant, but here, there may still be irregularities due to the forming tool pressing against it, which can be removed by effective means.

[0043] Figure 3 A perspective view of the steering insert 26 is shown. This steering insert 26 is fitted into the through holes 22, 24 of the lead screw nut 10. To remove the balls from the threads of the lead screw or lead screw nut, the balls, for example, enter a groove 52, are radially moved outward, and are conveyed through a transfer zone 56 to the transition channel of the lead screw nut. The balls are removed from the transition channel in the reverse order: from the transition channel into the transfer zone 56, the balls are conveyed into the groove 52, and then the balls are conveyed into the threads of the lead screw or lead screw nut. The steering insert 26 is, in this respect, a radially symmetrical component, and can be particularly manufactured as a one-piece casting.

[0044] Figure 4 A schematic diagram of a ball screw drive mechanism 12 according to the present invention is shown. The orbital path of the balls 42 in the ball screw drive mechanism 12 can be seen. The screw nut 10 is screwed onto the screw 40. Steering inserts 26, 28 are inserted into the screw nut 10, and the balls 42 move along their orbital path.

[0045] Figure 5 A perspective view of a ball screw drive mechanism 12 according to the present invention is shown. The ball screw drive mechanism 12 includes a screw 40, balls 42, steering inserts 26 and 28, and a screw nut 10, wherein the latter has a... Figure 2The aforementioned features. Typically, the ball screw drive mechanism 12 also has one or more (transition) channel covers for completeness. These are used to house the steering inserts 26, 28 within the through holes 22, 24 with as little clearance as possible and, in particular, to prevent them from falling out of the through holes 22, 24.

[0046] Figure 6 A perspective view of the ball screw drive mechanism 12 according to the present invention is shown, wherein the screw nut is omitted. The arrangement shown here is theoretical, as it cannot be shown in practice. It only illustrates the movement path of the balls 42 within the screw 40 and the steering inserts 26, 28, wherein, in particular, the axial parallel movement path of the balls 42 between the steering inserts 26, 28 can be seen, which is formed by a transition channel not shown.

[0047] Combination Figures 1 to 6 The lead screw nut 10 according to the invention is described with reference to an embodiment, wherein two transition channels 18 and 20, i.e., a pair of transition channels, are provided. It is also possible to press out more than one pair of transition channels. Furthermore, the invention will be described in conjunction with an embodiment in which each transition channel 18, 20 is provided with two steering inserts 26, 28. If the transition channel is long enough, more than one pair of such steering inserts can also be provided. Thus, each transition channel obtains more ball movement paths. To use more than two steering inserts 26, 28 within a single transition channel 18, 20, a corresponding number of through holes must be provided.

[0048] Figure 7 A front view of the end face 32 of the lead screw nut 10 according to the invention, viewed axially, is shown. The figure shows a central hole 16 with the shown internal thread 34. The pressed channels 18 and 20 (which have the same design in cross-section) are arranged at an angle α = 120° within the circumferential surface 14 relative to the central longitudinal axis of the lead screw nut 10. The arrangement shown is exemplary, and the technically meaningful angle range is between 100° and 180° (including the endpoints). At α = 180°, channels 18 and 20 are radially completely opposed.

[0049] Figure 8 A flowchart of the method according to the invention is shown. In step S01, the method provides a cylindrical workpiece having a first end face, a second end face, and a circumferential surface. In step S02, the workpiece is machined by simultaneously pressing a central hole and a pair of axially parallel channels into the circumferential surface. The central hole and channels are then pressed into the workpiece from the first end face. Therefore, the transition channels are always open towards the first end face. As a further method step S03, an internal thread is introduced into the central hole. Next, a subtractive material method step is performed in step S04. A radially extending through hole is introduced into at least the first channel.

[0050] The inventive features disclosed in the foregoing description, drawings, and claims are essential for carrying out the invention, either individually or in any combination.

Claims

1. A method for manufacturing a metal screw nut (10) for a ball screw drive mechanism (12), the method comprising the following steps: - Provide a metal workpiece (50) that is substantially cylindrical, the workpiece having a first end face (46) and a second end face (44) and a circumferential surface (14). - The workpiece (50) is modified by pressing out an axisymmetric center hole (16) and by simultaneously pressing out at least a first channel (18) and a second channel (20) of axial parallel orientation in the circumferential surface (14) of the workpiece (50), wherein the center hole (16) and the channels (18, 20) are open toward the first end face (46), and - Introduce the internal thread (34) into the center hole (16), and - By introducing at least two radially extending through holes (22, 24) into at least the first channel (18) to form at least the first channel (18) as a transition channel for receiving the steering insert (26, 28).

2. The method according to claim 1, characterized in that, The second channel (20) is manufactured with a simplified cross-section when it is not formed as a transition channel, wherein the longitudinal indentation depth of the second channel (20) and the volume of the modified processing material substantially correspond to the longitudinal indentation depth of the first channel (18) and the volume of the modified processing material.

3. The method according to claim 1 or 2, characterized in that, The pressing of the channels (18, 20) ends before reaching the second end face, so that the channels (18, 20) are closed toward the second end face (44).

4. The method according to claim 1, characterized in that, The through holes (22, 24) are milled and / or drilled.

5. The method according to claim 1, characterized in that, The central hole (16) is pressed into the second end face (44).

6. The method according to claim 1, characterized in that, The pressing of the center hole (16) ends before reaching the second end face (44), and the second end face (44) is removed by subtractive material after the pressing, so that the center hole (16) opens toward the new second end face (30) thus formed.

7. The method according to claim 1, characterized in that, The first end face (46) is removed by subtractive processing after the modification process, thereby forming a new first end face (32).

8. The method according to claim 7, characterized in that, The material removal of the end faces (44, 46) is achieved by turning.

9. The method according to claim 1, characterized in that, When the workpiece (50) is modified, a truncated section (36, 38) parallel to the axis is formed in the peripheral surface (14).

10. The method according to claim 1, characterized in that, The channel in the circumferential surface is arranged at an angle between 100° and 180° relative to the central longitudinal axis of the workpiece.

11. The method according to claim 10, characterized in that, The channel in the circumferential surface is arranged at an angle of 120° or 180° relative to the central longitudinal axis of the workpiece.

12. The method according to claim 1, characterized in that, The pressing of the axisymmetric center hole (16) and the pressing of at least the first channel (18) and the second channel (20) of the axis parallel orientation in the circumferential surface (14) of the workpiece (50) are achieved by cold modification machining.

13. A screw nut (10) for a ball screw drive mechanism (12), the screw nut being manufactured by the method according to any one of claims 1 to 12, wherein, The lead screw nut (10) has at least a pair of axially parallel channels (18, 20) in its circumferential surface (14), the channels being open toward one end face (32) of the lead screw nut (10) and at least one of the channels being designed as a transition channel.

14. A ball screw transmission mechanism (12), the ball screw transmission mechanism having: - The lead screw nut (10) according to claim 13. - Lead screw (40) - Multiple balls (42) - At least two steering inserts (26, 28) within at least one channel (18) that is formed as a transition channel, and - At least one transition channel cover.

Images