Ball screw drive for an electromechanical braking system

Abstract

The invention relates to a ball screw drive (10) for an electromechanical braking system, wherein a rotary movement of an electric actuator, which rotates in a braking direction to actuate a brake of the electromechanical braking system and in a relieving direction to relieve the brake, is transmittable to the ball screw drive (10) by means of a reduction gear and can be converted by means of the ball screw drive (10) into a translatory movement, for actuating the brake in a braking direction (58) and for relieving the brake in a relieving direction (52). The ball screw drive (10) comprises a spindle nut (18) and a spindle (14), which together form a helically wound ball channel (22) with balls (26) rolling therein, which transmits a braking force (FB) and a relieving force (FE) between the spindle (14) and the spindle nut (18).In the braking direction (58), a braking contact surface (36) of the ball channel (22) which is in contact with the balls (26) by the spindle nut (18) and spindle (14) and which transmits the braking force (FB) is larger than a relieving contact surface (54) of the ball channel (22) which is in contact with the balls (26) and which transmits the relieving force (FE).

Description

[0001] The present invention relates to a ball screw drive for an electromechanical braking system. Furthermore, the invention relates to a motor vehicle with an electromechanical braking system that includes such a ball screw drive. State of the art

[0002] From DE 10 2013 216 327 A1, for example, an electromechanical braking system with a hydraulically actuated service brake is known. The braking system further comprises an electromechanical actuator, the rotational movement of which is transmitted to the threaded spindle of a ball screw drive. The rotational movement of the threaded spindle is converted into a translational movement of the threaded nut via rolling elements arranged within a helical thread between the threaded spindle and the threaded nut of the ball screw drive. To carry out a braking process, the threaded nut is supported on a brake piston, which moves the piston in a translational manner and brings the brake pads into contact with a brake disc. The rotational movement of the electromechanical actuator is reduced to the threaded spindle via a shaft with the interposition of a 2-stage worm gear. For this purpose, a worm wheel of the worm gear is connected to the shaft orconnected to the threaded spindle.

[0003] The object underlying the invention is to provide a ball screw drive for an electromechanical braking system with which a higher braking force can be transmitted without increasing the weight of the ball screw drive.

[0004] The object is achieved by a ball screw drive for an electromechanical braking system having the subject matter of patent claim 1. Preferred embodiments can be found in the dependent claims. Disclosure of the invention

[0005] The invention specifies a ball screw drive for an electromechanical braking system. A rotary motion of an electric actuator, which rotates in one braking direction to actuate a brake of the electromechanical braking system and in one relieving direction to release the brake, is transmitted to the ball screw drive by means of a reduction gear and converted into a translatory motion by means of the ball screw drive, for actuating the brake in one braking direction and for relieving the brake in one relieving direction. The ball screw drive comprises a spindle nut and a spindle, which together form a helically wound ball channel with balls rolling therein, which transmits a braking force and a relieving force between the spindle and the spindle nut.In the braking direction, a braking contact surface of the ball channel that rests on the balls from the spindle nut and spindle and transmits the braking force is larger than a relieving contact surface of the ball channel that rests on the balls and transmits the unloading force.

[0006] The contact surface of the ball channel is understood to be the surface that rests on the balls due to the acting forces and participates in the force transmission due to its geometric alignment in relation to the acting force. The braking contact surface therefore includes surfaces of the spindle and spindle nut that rest against the balls during a braking process. Accordingly, the unloading contact surface is the surface of the spindle and spindle nut that rests against the balls during movement of the ball screw in a unloading direction. Consequently, according to the invention, the braking contact surface available for transmitting the braking force is larger than the unloading contact surface for the unloading force. Since the unloading force is usually significantly smaller than the braking force in an electromechanical braking system, the ball screw can be better adapted to the loads occurring during use.To absorb the high braking forces, the entire ball screw assembly does not need to be made larger to provide a larger braking contact surface. This allows the overall size of such a ball screw assembly to be reduced. Likewise, higher braking forces can be transmitted without increasing the size and weight of the ball screw assembly.

[0007] This type of ball screw allows for weight savings. It also allows for the use of a cheaper material for the spindle or spindle nut. This type of ball screw is therefore compact and economical to manufacture.

[0008] In a preferred embodiment of the invention, the relief contact surface is point-shaped. Point-shaped contact between the ball channel and the respective balls reduces friction in the ball screw in a relief direction. The ball screw can thus be operated quickly and with low torque in a relief direction.

[0009] In a further preferred embodiment of the invention, a ball return is formed in the spindle nut. With a ball return, the balls are transported via a return channel to an opposite axial end of the ball screw drive. This makes it possible to achieve a larger adjustment range.

[0010] The invention additionally provides a motor vehicle with an electromechanical braking system that includes such a ball screw drive. Such a motor vehicle has the advantages and properties described above.

[0011] Embodiments of the invention are illustrated in the drawing and explained in more detail in the following description. It shows: Fig. 1 sectional view of a ball screw according to an embodiment of the invention, Fig. 2 Enlarged view of the ball channel after Fig. 1, and Fig. 3 Enlarged view of the ball channel according to another embodiment of the invention.

[0012] In Fig. 1 is a sectional view of a ball screw 10 according to an embodiment of the invention. The ball screw 10 is part of an electromechanical braking system and comprises a spindle 14, which is surrounded by a spindle nut 18. The spindle 14 and spindle nut 18 form a helically wound ball channel 22, in which a plurality of balls 26 are arranged. The spindle nut 18 additionally comprises a ball return 30, via which balls 26 can be conveyed to the axial ends of the ball screw 10. In the embodiment shown here, the spindle nut 18 is driven by a reduction gear (not shown). Accordingly, the spindle 14 is axially displaceable, so that a braking force F B can be applied to a brake actuator not shown here.

[0013] Fig. 2 shows an enlarged view of the ball channel 22 according to Fig. 1. This figure shows how the braking force F Bfrom the driven spindle nut 18 via the balls 26 to the spindle 14. Here you can see how spindle nut thread flanks 34 are used to transmit the braking force F B at a brake contact surface 36 with the balls 26, extending inward over an equator 38 of the balls 26 running in the axial spindle direction. In other words, the beginning of a gap 42 between the spindle nut 18 and spindle 14 is arranged between the equator 38 of the balls 26 running in the axial spindle direction and an inner end of the ball 26 in the radial direction of the ball screw 10. The gap 42 is thus not provided centrally above the ball 26. This creates a brake contact surface 36 for transmitting the braking force F B from the spindle nut 18 to the balls 26.

[0014] In the same way, a brake contact surface 36 of the spindle 14 with the ball 26 for transmitting the braking force F BThe spindle 14 also forms spindle thread flanks 50, which are used to transmit the braking force F B the correspondingly enlarged spindle nut thread flanks 34, with balls 26 arranged therebetween, are provided axially opposite one another. Here, the spindle thread flanks 50 project beyond the axially extending equator 38 of the balls 26 in the direction of a radial outer side. Accordingly, here too, a beginning of the gap 42 between spindle 14 and spindle nut 18 is arranged between the axially extending equator 38 of the balls 26 and a radially outer end of the ball 26. Accordingly, the brake contact surface 36 for absorbing the braking force F B on spindle 14.

[0015] The Fig. 2 also shows how a relief force F Ein a relief direction 52 from the spindle nut 18 to the spindle 14. In the relief direction 52, a brake actuator is removed from a brake disc. The relief force F E is therefore much smaller than the braking force F B . Accordingly, a relief contact surface 54 between balls 26 and spindle nut 18 or spindle 14 in a relief direction 52 is smaller than the braking contact surface 36 between balls 26 and spindle nut 18 or spindle 14 in a braking direction 58. Accordingly, a force required for the transmission of the relief force F E relevant spindle nut thread flank 34 already ends before the axial equator 38 of the balls 26 running in the spindle direction. Likewise, a flange for the transmission of the unloading force F E relevant spindle thread flank 50 already in front of the axial equator 38 of the balls 26 running in the spindle direction.

[0016] Fig. Figure 3 shows an enlarged view of the ball channel 22 according to another embodiment of the invention. This figure differs from the one shown in Fig. 2 in that the relief contact surface 54 is formed only as a point in the relief direction 52 between balls 26 and spindle 14 or spindle nut 18. To form a point-like contact, the ball channel 22 is in the area of the relief force F E designed in a straight line. The ball channel 22 therefore does not have the same radius as the ball 26. Accordingly, the ball 26 only rests on the ball channel 22 at a point. Such a design has the advantage that friction between the balls 26 and the ball channel 22 is reduced due to the point-like contact. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2013 216 327 A1

[0002]

Claims

[1] Ball screw drive (10) for an electromechanical braking system, wherein a rotary movement of an electric actuator, which rotates in a braking direction to actuate a brake of the electromechanical braking system and in a relieving direction to relieve the brake, is transferable to the ball screw drive (10) by means of a reduction gear and can be converted by means of the ball screw drive (10) into a translatory movement to actuate the brake in a braking direction (58) and to relieve the brake in a relieving direction (52), which ball screw drive (10) comprises a spindle nut (18) and a spindle (14), which together form a helically wound ball channel (22) with balls (26) rolling therein, which generates a braking force (F B ) and a relief force (F E ) between spindle (14) and spindle nut (18), characterized bythat in the braking direction (58) a force applied by the spindle nut (18) and spindle (14) to the balls (26) and the braking force (F B ) transmitting brake contact surface (36) of the ball channel (22) is larger than a contact surface on the balls (26) and the relief force (F E ) transmitting relief contact surface (54) of the ball channel (22). [2] Ball screw drive (10) according to claim 1, characterized by that the relief contact surface (54) is point-shaped in a relief direction (52). [3] Ball screw drive (10) according to claim 1 or 2, characterized by that a ball return (30) is formed in the spindle nut (18). [4] Motor vehicle comprising an electromechanical braking system with a ball screw drive (10) according to one of the preceding claims.

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