Ball screw and method for operating a ball screw

Abstract

Ball screw drive (1), with a threaded spindle (2), a spindle nut (3), and rolling elements (5), namely balls, arranged between the threaded spindle (2) and the spindle nut (3) in a thread (4), wherein spring elements (8, 9) are also arranged in the thread (4) between stops (6, 7) on the nut side, characterized in that there is at least one setting of the threaded spindle (2) and spindle nut (3), in which the balls (5) and the spring elements (8, 9) are arranged in the thread (4) with play.

Description

The invention relates to a ball screw suitable for use in an electromechanical actuator according to the preamble of claim 1.A ball screw of the generic type provided for operation in an electromechanically actuatable parking brake of a motor vehicle is known, for example, from EP 2 207 982 B1. The known ball screw has balls as rolling elements, which are arranged to be movable to a limited extent in a thread turn of the ball screw. In this case, a spring element in the form of a helical spring is arranged between a stop and the rolling bodies. In addition, at least one intermediate spring element is provided, which is arranged between rolling bodies and is either likewise designed as a helical compression spring or as an elastomer spring. If a plurality of intermediate spring elements are present, these are distributed uniformly between the rolling bodies in order to reduce frictional forces between the rolling bodies.A further ball screw drive having the features according to the preamble of claim 1 is disclosed in DE 10 2014 223 021 A1. In contrast to the ball screw according to the aforementioned patent specification EP 2 207 982 B1, in this case, which likewise relates to a floating-caliper disc brake, rigid separating bodies are provided instead of intermediate spring elements. With the aid of the separating bodies, the frictional force between the balls of the ball screw is reduced. Nonelastic plastic is mentioned as the material from which the separating bodies can be produced in DE 10 2014 223 021 A1. The separating bodies may be separating balls whose diameter is smaller than the diameter of the balls functioning as rolling bodies.A ball screw which has spring elements which are inserted into a thread turn and which subject the balls of the thread drive to a force is also described in DE 10 2017 127 404 B4. In this case, a stop for a spring element is formed by a molded part which is connected to the threaded nut of the ball screw.The invention is based on the object of further developing a ball screw suitable, inter alia, for a parking brake of a motor vehicle, compared with the aforementioned prior art, to the effect that a particularly favorable relationship is provided between production outlay, installation space required and friction occurring during operation of the ball screw.This object is achieved according to the invention by a ball screw having the features of claim 1. Likewise, the object is achieved by a method for operating a ball screw according to claim 10.The ball screw drive comprises, in a basic construction known per se, a threaded spindle, a spindle nut, and rolling elements, namely balls, arranged in a thread path between the threaded spindle and the spindle nut, wherein spring elements are furthermore arranged in the thread path between stops on the nut side. Either the threaded spindle or the spindle nut, which has the stops for the two spring elements or is firmly connected to such stops, can be provided as the rotating drive element of the ball screw, while the other component of the threaded screw, i.e. the spindle nut or the threaded spindle, functions as a displaceable output element of the ball screw which is secured against rotation.In principle, driving of the ball screw by linear displacement of one of the components mentioned is also possible, wherein the output element of the threaded drive in such a case represents a rotating element, i.e. a linear rotary transmission is formed. Regardless of which component of the ball screw is the drive component, the thread of the ball screw can basically be configured either as a single-start or as a multiple-start thread.According to claim 1, there is at least one setting of the ball screw, i.e. a positioning of the spindle nut relative to the threaded spindle, in which the rolling bodies and the spring elements are arranged in the thread turn with play. The play can occur between different elements of the ball screw, in particular between individual rolling bodies and / or between a spring element and a rolling body. In this case, play can be present at the same time at one, two or more points within the ball-spring strand formed between the two stops, which is also referred to as a ball-spring package or, for simplification, as a ball chain.The ball screw can be operated by, starting from a first positioning in which exactly one of the two spring elements is tensioned, i.e. compressed, first being brought into a middle position in which both spring elements are relaxed and accordingly the balls are arranged with play in the thread turn. The term "center position" does not necessarily mean that this position lies exactly in the center between two stop positions. Rather, the central position can also be located eccentrically between two stop positions. In any case, the ball screw can be brought from the central position into a second setting, in which, in contrast to the first setting, the second spring element is tensioned and the first spring element is relaxed.Due to the arrangement of the balls in the thread path with play, the balls can slide through in the middle section of the adjustment range of the ball screw, which is accepted. Towards the end of the adjustment range, with increasing forces, the balls roll off practically without slipping. There is no ball return with a ball channel closed in an annular manner. As such, the ball screw is designed for applications with small adjustment paths, such as are typical for electromechanical parking brakes. The balls, which are loaded by spring force depending on the operating state, roll off in the thread course in this application both during the tensioning and during the release of the brake, wherein at the same time a hydraulic preload can be provided.The rolling bodies of the ball screw can be combined into a plurality of groups, wherein a spacer element is arranged in each case between individual groups. Suitable spacer elements are, in particular, balls, i.e. separating balls, which are smaller than the rolling bodies, i.e. supporting balls, of the ball screw drive. For example, the diameter of each separating ball, i.e. spacer ball, is at least 80% and at most 95% of the diameter of each supporting ball of the screw drive.As regards the choice of material, the spacer elements can be manufactured either from the same material as the rolling bodies or from a different material, for example from a metallic material composed differently and / or heat-treated in a different manner, or else from a plastic. The rolling elements of the ball screw drive are metallic rolling elements in typical applications. In principle, the use of ceramic rolling bodies is also possible.Regardless of the materials from which the rolling bodies and the spacer elements are manufactured, rolling body groups, which are each formed from at least two and at most four rolling bodies, are each separated from one another by a spacer element, in particular a spacer ball.For example, the group of rolling elements adjacent to the first spring element comprises exactly three rolling elements, while the group of rolling elements adjacent to the second spring element comprises exactly two rolling elements, wherein at least one spacer element is arranged between these outermost rolling element groups, typically a plurality of spacer elements, in particular separating balls, are arranged. The first spring element is also referred to as a readjustment spring, and the second spring element is also referred to as a preliminary compression spring. Both spring elements are typically designed as helical compression springs. According to possible embodiments, the first spring element-measured in the direction of the extension of the thread-is longer than the second spring element.Overall, for example, at least twelve and at most 30 groups of rolling elements are arranged between the pre-compression spring and the adjusting spring, wherein each group of rolling elements comprises no more than four load-bearing balls.The total length of the free space, which is measured in the direction of the extent of the thread turn and is not necessarily formed at only one point and which is equivalent to the existing play in the ball-spring strand, is, for example, at least 0.2% and at most 5% of the total length of the thread turn to be measured between the two nut-side stops when the spring elements are relaxed.The advantage of the invention is in particular that, in the case of a ball screw without ball feedback, particularly high efficiency can be achieved without additional apparatus complexity as compared to conventional threaded screws with prestress in the ball-spring strand as a result of play in the ball-spring package, wherein no practically relevant disadvantages, for example with regard to the wear behavior or the acoustic behavior, are associated therewith.An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. Shown herein are: FIG. 1 shows a ball screw in a sectional view, FIG. 2 shows components of the ball screw according to FIG. 1 in an abstract illustration.A ball screw drive, which is identified overall by the reference symbol 1, is provided for use as a rotary linear gear in a parking brake of a motor vehicle and comprises a threaded spindle 2 and a spindle nut 3. Between the threaded spindle 2 and the spindle nut 3, balls roll as rolling bodies 5 in the thread turn 4. With regard to the basic function of the ball screw 1, reference is made to the prior art cited at the beginning. The parking brake of the motor vehicle is combined in a manner known per se with a hydraulic disc brake.In the spindle nut 3, which is also referred to as a threaded nut, there are a first nut-side stop 6 and a second nut-side stop 7. As such, use of the ball screw 1 is not considered for applications with large travel distances.In the thread turn 4 there are a first spring element 8, which can abut on the first stop 6, and a second spring element 9, which can be supported on the second stop 7. Both spring elements 8, 9 are designed as helical springs.In addition to the rolling bodies 5, that is to say supporting balls, a plurality of spacer elements 10 are located in the thread turn 4. the spacer elements 10 are balls which have a smaller diameter than the rolling bodies 5. As regards the arrangement of the various balls in the thread turn 4, reference is made to the symbolized illustration according to FIG. 2. Directly following the first spring element 8, i.e. the adjusting spring, which is significantly longer than the second spring element 9, i.e. the pre-compression spring, in the arrangement shown there is a ball arrangement 11 which is constructed in a defined manner from groups of rolling bodies 5 and spacer elements 10: Groups of three rolling bodies 5 and individual spacer elements 10 are inserted alternately into the thread thread 4. Overall, the ball arrangement 11 shown only in a detail is constructed from 19 groups of three rolling bodies 5 in each case, that is to say a total of 57 rolling bodies 5, and additionally 19 spacer elements 10 designed as balls. This ball arrangement 11 is adjoined by a terminating ball arrangement 12 which is formed from only two rolling bodies 5. In the exemplary embodiment, the rolling bodies 5 of both the ball arrangement 11 and the closing ball arrangement 12 have a diameter of 2.778 mm and the spacer elements 10 have a diameter of 2.5 mm. The difference in size between the rolling bodies 5 and the spacer elements 10 is shown in an exaggerated manner in both figures. The spacer elements 10 have the purpose of reducing the friction inside the ball screw 1 compared to screw drives which have balls of uniform size only.As a further measure for reducing friction and thus for increasing the efficiency, a free space FR is formed in the thread turn 4, as can be seen from both figures. This is equivalent to the fact that the rolling bodies 5 and spacer elements 10, even in the completely relaxed state of both spring elements 8, 9, are arranged in the thread turn 4 with play, wherein the play relates to the direction of the extension of the thread turn 4. The total length of the free space FR, which is to be measured in the mentioned direction and which can be present in a divided manner, i.e. in the form of individual free sections, amounts to at least 0.2% and at most 5% of the total length of the thread turn 4.In the arrangement according to FIG. 2, the free space FR between the closing ball arrangement 12 and the pre-compression spring 9 is drawn-not to scale. If, starting from this arrangement, the spindle nut 3 is adjusted to the left relative to the threaded spindle 2, this means that the free space FR indicated in FIG. 2 becomes smaller, while both spring elements 8, 9 are relaxed and an additional free space FR is produced between the first spring element 8 and / or-as indicated in FIG. 1-within the ball arrangement 11. The rolling bodies 5 are thus not under spring prestress in this state. In a load-free or low-load state, this can lead to rolling bodies 5 slipping through. If, on the other hand, the rolling bodies 5 are displaced against one of the spring elements 8, 9 in the event of increasing forces within the ball screw 1, wherein either the first spring element 8 contacts the ball arrangement 11 or the second spring element 9 presses against the closing ball arrangement 12, the forces occurring, including the spring forces, ensure a kinematically approximately ideal rolling of the rolling bodies 5.List of reference characters1 Ball screw 2 Threaded spindle 3 Threaded nut 4 Thread of the threaded spindle 5 Rolling bodies, 6 First nut-side stop 7 Second nut-side stop 8 First spring element 9 Second spring element 10 Spacer element, 11 Ball arrangement 12 Terminating ball arrangement FR Free space

Claims

Ball screw (1) having a threaded spindle (2), a spindle nut (3), and also rolling bodies (5) arranged between the threaded spindle (2) and the spindle nut (3) in a thread turn (4), namely balls, wherein spring elements (8, 9) are furthermore arranged in the thread turn (4) between stops (6, 7) on the nut side, characterized in that at least one setting of the threaded spindle (2) and spindle nut (3) exists, in which the balls (5) and the spring elements (8, 9) are arranged in the thread turn (4) with play.Ball screw (1) according to Claim 1, characterized in that spacer elements (10) are arranged between a plurality of groups of rolling bodies (5).Ball screw (1) according to Claim 2, characterized in that balls which are smaller than the rolling bodies (5) are provided as spacer elements (10).Ball screw (1) according to Claim 3, characterized in that the diameter of each spacer ball (10) is at least 80% and at most 95% of the diameter of each ball provided as rolling bodies (5).Ball screw (1) according to one of Claims 2 to 4, characterized in that groups of at least two and at most four rolling bodies (5) are in each case separated from one another by a spacer element (10).Ball screw (1) according to Claim 5, characterized in that the group of rolling bodies (5) adjacent to the first spring element (8) comprises exactly three rolling bodies (5) and the group of rolling bodies (5) adjacent to the second spring element (9) comprises exactly two rolling bodies (5), wherein at least one spacer element (10) is arranged between these outermost rolling body groups.Ball screw (1) according to Claim 6, characterized in that the first spring element (8) - measured in the direction of the extent of the thread (4) - is longer than the second spring element (9).Ball screw (1) according to one of Claims 2 to 7, characterized in that a total of at least twelve and at most 30 groups of rolling bodies (5) are arranged between the two spring elements (8, 9).Ball screw (1) according to one of Claims 1 to 8, characterized in that the total length of a free space (FR) measured in the direction of the extent of the thread (4) is at least 0.2% and at most 5% of the total length of the thread (4) to be measured between the two nut-side stops (6, 7) when the spring elements (8, 9) are relaxed.Method for operating a ball screw (1) which comprises balls (5) as rolling bodies arranged in a thread turn (4) formed between a threaded spindle (2) and a spindle nut (3), and spring elements (8, 9) arranged in the same thread turn (4), characterized in that, starting from a first setting of the ball screw (1) in which exactly one of the spring elements (8, 9) is tensioned, the ball screw (1) is brought beyond a central position of the threaded spindle (2) and spindle nut (3) in which both spring elements (8, 9) are relaxed, into a second setting in which the second spring element (9, 8) is tensioned.

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