Ball screw drive with adjustable preload

Abstract

Ball screw drive with adjustable preload, with a double nut consisting of two separate nut parts 6a and 6b, wherein the distance A between the two nut parts 6a, 6b defines the preload V(A) of the double nut, characterized in that the two nut parts 6a, 6b are connected to each other via an axial fixing element and the nut parts 6a, 6b each have claws Ka, Kb at their ends which interlock similarly to a claw coupling, wherein a rotation of the two nut parts 6a, 6b relative to each other is possible, since the claws Ka, Kb do not interlock positively and that a radial adjustment device is provided, which consists of the claws Ka, Kb and several force elements Ki, which act on the claws in such a way that a torque can be generated which generates a preload.

Description

The invention relates to a ball screw drive with adjustable preload according to the preamble of claim 1. The preload of the ball screw drive is responsible for the stiffness and thus for the precision of the machining process. Ball screw drives are widely used in machine tools to move loads quickly and linearly or to position them precisely. The rotary motion of a motor-driven spindle is converted into a linear motion, for example, of a flange to which a worktable is bolted or of a spindle housing, by means of a nut. The balls are guided in a helical raceway within the spindle nut. There are also spindles with a double nut that is split into two parts to achieve preload via an intermediate spring assembly or an adjustable thread. This preload is essentially independent of manufacturing tolerances and wear. Increasingly, however, individual nuts are also being used, in which the preload is generated by a corresponding oversize of the balls, just a few micrometers. Since the balls in this design bear contact at four points (4-point contact), they are forced to slide during operation, which naturally leads to a certain amount of wear. This reduces the preload over the service life. The reduced preload can lead to unacceptable results on the machined surfaces in critical machining processes, such as when used in a machine tool. One example is the grinding of gears, where the requirements for accuracy and surface finish of the flanks are extremely high. Defects such as pitting or chatter marks in the spindle drive, which can be caused by material fatigue or overstressing, are also disruptive in high-precision machining processes, as they cause a discontinuous feed on the machine tool. A ball screw drive is a sensitive component and always represents a compromise between high stiffness and accuracy through high preload or fast processing after machining at lower accuracy and low preload. In applications requiring both high positioning speed and high rigidity, or that are subject to uncontrolled overloads, it cannot be ruled out that the ball screw drive will fail prematurely and unexpectedly due to high wear or overloading. Particularly in specialized machines, so-called bottleneck machines or those with high availability requirements, a sudden failure can lead to significant financial losses. From DE 10 2013 225 467 A1, a preload monitoring system for systems with a 4-point ball bearing is known. The systems currently in use for preload monitoring by indirect force measurements via the current of the drive motor are very inaccurate and, due to high moments of inertia, not dynamic enough to detect damage to the ball screws at an early stage. From EP 0 569 595 A1, a ball screw drive with adjustable preload is known. An actuator is provided between the two nut parts, by means of which the preload is adjusted. Piezoelectric elements are mentioned as an example of suitable actuators. Manufacturing piezoelectric elements with the necessary precision for such gap dimensions is only possible with extremely high effort. The object of the invention is to provide a ball screw drive with adjustable preload that does not have the aforementioned disadvantages, is suitable for continuous use in harsh machine environments, and is easy and inexpensive to manufacture. This problem is solved by the features specified in the characterizing portion of claim 1. Advantageous embodiments of the invention are specified in the dependent claims. The invention is explained in more detail below with reference to exemplary embodiments and the drawings. In the following description of preferred embodiments, the same reference numerals denote identical or comparable components. The figures show schematically: Fig. 1 conventional ball screw drive without adjustable preload with one nut; Fig. 2 first ball screw drive with a double nut in longitudinal section; Fig. 3 second ball screw drive with a double nut in longitudinal section; Fig. 4 first ball screw drive according to Fig. 2 in a highly schematic representation; Fig. 5 second ball screw drive according to Fig. 2 in a highly schematic representation; Fig. 6 schematic representation of a ball screw drive with two separately controllable piezoelectric units; Fig. 7a ball screw drive with a double nut and radial force adjustment according to the invention; Fig. 7b section in the area of ​​the jaws according to Fig. 7a Figure 1 shows a conventional ball screw drive 1, which is a special type of roller screw drive, with a spindle 5 and a nut 6. The spindle 5, with a typical diameter of 30 mm–60 mm, has a raceway 7a which, together with a raceway (not shown) provided in the nut 6, forms a raceway 7. An end cap 14 with a seal is provided on the flange side of the nut 6. The actual motor for the spindle drive is not shown. A flange or a spindle housing for a workpiece can be provided on the nut 6. Several balls 12 serve as rolling elements, which circulate continuously in a return system with a return channel 8. The rotational movement of the spindle 5 allows the nut 6 to be moved linearly. There are also ball screw drives with a double nut. The embodiments shown in Figs. 2, 3, 4, 5 to 6 serve to illustrate different technical approaches to adjusting the preload of a ball screw drive. These figures show exemplary embodiments with an axial adjustment device and are not part of the subject matter of the invention, which relates to a radial adjustment device via claws. Figure 2 shows a longitudinal section of a first ball screw drive with a double nut. The double nut consists of two nut parts 6a, 6b, which are screwed together via a thread GW as a mechanical adjusting device EV2. Between the two mother parts 6a, 6b, a ring stack of piezoelectric elements P1-Pn is provided as a piezoelectric adjusting device EV1, with which the preload can be adjusted during operation of the ball screw drive. The piezoelectric elements are electrically controlled in a known manner. A fixing screw FS allows the relative position of the two threaded elements, in this case the two nut parts 6a and 6b, and thus the mechanical adjusting device EV2, to be fixed. The piezoelectric adjusting device EV1 allows the two nut parts to be pushed apart, which increases the preload. The direction of the clamping force on the individual balls is indicated by lines. The spindle is under tensile stress (O-arrangement). The preload can be detected using a strain gauge DMS. A close-up shows the thread play on a threaded cone. The piezoelectric adjusting device EV1 utilizes this play to generate a change in distance. Figure 3 shows a longitudinal section of a second ball screw drive with a double nut. The double nut also consists of two nut parts 6a and 6b. Additionally, there is a separate second threaded element GE2 which is screwed to nut part 6b by means of the thread GW. Here too, the thread GW serves as a mechanical adjusting device EV2. The nut part 6a is supported on the threaded element GE2. Between the threaded element GE and the nut part 6a, a ring stack of piezoelectric elements P1-Pn is provided as a piezoelectric adjustment device EV1, with which the preload can be adjusted during operation. The mechanical adjusting device EV2 is also fixed via the fixing screw FS. The two nut parts can be pressed together using this piezoelectric adjusting device EV1, which increases the preload. The direction of the clamping force on the individual balls is indicated by lines. The spindle is under compressive stress (X-arrangement). The preload can be detected using the strain gauge DMS. The thread play on a threaded cone is shown as a close-up. Figures 4 and 5 show, corresponding to Figures 2 and 3, the respective ball screw drives in a very schematic representation. Fig. 4 shows a schematic section view of a ball screw drive, where the preload is generated by a compressive stress produced via the piezoelectric assembly. The relative change in distance between the two nut parts is exaggerated for clarity. Fig. 5 shows a schematic section view of a ball screw drive, where the preload is generated by a tensile stress produced via the piezoelectric assembly. The relative change in distance between the two nut parts is exaggerated for clarity. Fig. 6 shows a schematic representation of a ball screw drive with two piezoelectric assemblies PP1 and PP2, where the two assemblies can be controlled separately for two different load situations. Three separately controllable piezoelectric assemblies are also conceivable. Figure 7a shows a ball screw drive with a double nut and a radial adjustment device according to the invention. The distance A between the two nut parts 6a, 6b can be variably adjusted via an axial fixing element, e.g., a cap nut Ü with opposing threads (left-hand / right-hand thread). The cap nut can be fixed to one nut part 6a with a locking element (grub screw). The nut parts 6a, 6b each have claws Ka, Kb at their ends, which interlock similarly to a claw coupling, whereby a rotation of the two nut parts 6a, 6b relative to each other is still possible because the claws Ka, Kb do not engage in a positive-locking manner. The radial adjustment device consists of the claws Ka, Kb and several force elements that act on the claws in such a way as to generate a torque that creates a preload. The force elements could be, for example, piezoelectric elements or hydraulic elements. Here too, there is a mechanical adjustment device EV2 and a piezoelectric adjustment device EV1, whereby hydraulic adjustment is also possible due to the special design of the radial adjustment device. Fig. 7b shows a radial section in the area of ​​the claws. Only one hydraulic element HE is shown as an example of the force element. In reality, several hydraulic elements are provided. Piezoelectric elements can also be used as force elements instead of hydraulic elements. The function of the invention is explained in more detail below. The essential idea of ​​the invention is that, in addition to the piezoelectric adjustment device EV1, a second mechanical adjustment device EV2 is provided, which allows for the adjustment of an initial distance A0 between the two nut parts 6a, 6b. At this distance A0, no significant force acts between the nut parts, and at the same time, the piezo stack is minimally clamped. This second mechanical adjustment device EV2 is locked by means of a locking screw FS after the initial distance A0 has been set, so that the two nut parts 6a, 6b can no longer rotate relative to each other. At the initial distance A0, there is usually no or only a small preload. Let V(A) denote the preload as a function of the distance A. Therefore, V(A0) < Vg, where Vg corresponds to a small preload value. In this state, a fast ball screw drive operation with low wear is possible. The piezoelectric adjustment device EV1 typically consists of a stack (package) of individual piezoelectric elements P1-Pn. The piezoelectric elements can be stacked as solid rings, segments, or blocks of multiple layers to generate the necessary force-displacement ratio. The required power supply is generated outside the spindle nut and supplied via cable connections. The mechanical adjusting device EV2 consists of two threaded elements. In one case, the two nut parts 6a and 6b are screwed directly together via the thread GW, while in the other case, nut part 6b is screwed to the second threaded element GE2. Nut part 6b, as the first threaded element, forms a chamber together with the second threaded element GE2, in which the piezoelectric adjusting device EV1 and the second nut part 6a are enclosed. By activating the first piezoelectric adjusting device EV1, which causes the piezoelectric elements to expand, the distance A between the two nut parts 6a and 6b can be reduced, thereby increasing the preload V. The strain gauge (DMS) is used to measure the preload force. The ball screw drive, and in particular the piezoelectric elements, are centrally controlled by a control unit. The control unit also receives information about the current preload. The invention offers a multitude of advantages. A high preload is required to ensure backlash-free operation and high rigidity of the ball screw drive. Due to wear, the preload decreases over the ball screw drive's service life. An incorrect preload can lead to increased friction and heat generation, which can accelerate wear and negatively impact precision. According to a service life calculation based on DIN ISO 3408-5, the preload force is inversely proportional to the cube of the ball screw drive's service life. By adjusting the preload during operation, the service life of the ball screw drive can be significantly increased. Furthermore, friction can negatively affect both positioning accuracy and the preload itself. In the present invention, a piezo stack is used in the force flow between the two individual nuts. The invention is very suitable for practical use, as it meets the high requirements for a rigid and cost-effectively integrable actuator system and also exhibits high dynamics.

Claims

Ball screw drive with adjustable preload, with a double nut consisting of two separate nut parts 6a and 6b, wherein the distance A between the two nut parts 6a, 6b defines the preload V(A) of the double nut, characterized in that the two nut parts 6a, 6b are connected to each other via an axial fixing element and the nut parts 6a, 6b each have claws Ka, Kb at their ends which interlock similarly to a claw coupling, wherein a rotation of the two nut parts 6a, 6b relative to each other is possible, since the claws Ka, Kb do not interlock positively and that a radial adjustment device is provided, which consists of the claws Ka, Kb and several force elements Ki, which act on the claws in such a way that a torque can be generated which generates a preload. Ball screw drive according to claim 1, characterized in that the axial fixing element is designed as a union nut Ü with a counter-rotating thread. Ball screw drive according to claim 1 or 2, characterized in that the force elements are designed as hydraulic elements. Ball screw drive according to claim 1 or 2 characterized in that the force elements are designed as piezoelectric elements.

Images