Vehicle braking system

Abstract

The invention relates to a vehicle braking system comprising a multi-disc brake or clutch (3) which has an actuator (21) with a disc pair consisting of a stationary disc (25) and a coaxially rotatable disc (27) and at least one interposed ball-ramp unit (23), wherein, in order to apply a braking function, the rotatable disc (27) rotates in a braking rotational direction (DB) starting from a rotational zero position (0), whereby the ball (29) rolls between facing ball tracks (35) of the disc pair, and even with axial expansion of the disc pair, in order to apply a pressure force to the multi-disc brake or clutch (3), and wherein, in order to applying the parking function, the rotatable disc (27) rotates in a parking rotational direction (Dp) (opposite the braking rotational direction (DB)) starting from the rotational zero position (0), whereby facing sliding surfaces (38) of the disc pair are brought into sliding contact, even with axial expansion of the disc pair, in order to apply a pressure force to the multi-disc brake or clutch (3). According to the invention, the ball ramp unit (23) has a positioning aid (42), by means of which the ball (29), which is not in a power flow with the disc pair when the parking function is applied, remains in a predefined ball position without play.

Description

[0001] Vehicle braking system

[0002] DESCRIPTION:

[0003] The invention relates to a vehicle braking system according to the preamble of claim 1.

[0004] A vehicle braking system of this type features, instead of conventional disc or drum brakes, at least one multi-disc brake acting on the vehicle axle. This brake allows for the activation or deactivation of a braking function for vehicle braking during ferry operations or a parking function when the vehicle is parked. For this purpose, the multi-disc brake has two independently operating actuators: a brake actuator and a parking actuator. These are controlled by a control unit via electrical or hydraulic signals. For safety reasons, the brake actuator can be designed to be normally open (non-self-locking), while the parking actuator can be normally closed (self-locking). Integrating these two actuators into the multi-disc brake requires installation space and involves additional design effort.

[0005] From DE 102 52 974 A1, an engagement mechanism with a two-stage ramp angle is known. The engagement mechanism has a first ball ramp disk that interacts with the housing on one side. The first ball ramp disk has first ramps on the side interacting with the housing. The first ball ramp disk includes second ramps with a predetermined angle, which are provided on the side facing away from the housing. The engagement mechanism further comprises a rotatably driven ball ramp disk next to the first ball ramp disk on the side facing away from the housing. The engagement mechanism also has a spring element arranged between the first ball ramp disk and the second ball ramp disk.

[0006] From EP 2 093 450 A1, a ball ramp brake is known which has a disc arrangement within a housing and an annular, rotatable actuating element for actuating a brake by compressing the disc arrangement. The rotatable actuating element comprises a plurality of recesses which become progressively shallower in a first circumferential direction.

[0007] The object of the invention is to provide a vehicle braking system that, compared to the prior art, provides increased functional reliability in a simple manner.

[0008] The problem is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims.

[0009] The invention relates to a vehicle braking system with at least one multi-disc brake or clutch acting on a vehicle axle, by means of which a braking function for vehicle braking during operation and a parking function when the vehicle is parked can be engaged / disengaged. Exactly one electrically controlled actuator is associated with the multi-disc brake or clutch. The actuator can be used to engage / disengage both the braking function and, alternatively, the parking function. For safety reasons, the electrically controlled actuator is normally open (i.e., not self-locking) when the braking function is engaged, while it is normally closed (i.e., self-locking) when the parking function is engaged. In this way, a simplified braking concept is achieved – compared to the prior art – with regard to cost, installation space, and component weight.

[0010] The actuator comprises a pair of discs, consisting of a stationary disc and a coaxially rotating disc, as well as at least one intermediate ball ramp unit. This assembly allows the multi-disc brake to be subjected to contact pressure when the braking or parking function is engaged. Conversely, the multi-disc brake can be relieved of pressure when the braking or parking function is disengaged. The ball rolls between opposing, inclined ball tracks on the two discs. To engage the braking function, the rotating disc turns from a neutral position in a braking direction, causing the ball to roll between the opposing ball tracks of the disc pair while axially spreading the pair. To engage the parking function, the rotating disc turns from a neutral position in a parking direction opposite to the braking direction.This causes the facing sliding surfaces of the disc pair to engage in a sliding position, with axial spreading of the disc pair in order to apply contact pressure to the multi-disc brake. Simultaneously, the ball loses contact with the ball tracks of the disc pair.

[0011] When the braking function is engaged, there is rolling resistance between the ball and the ball tracks of the disc pair. The low rolling resistance compared to a sliding system means that the ball-ramp unit is not self-locking; that is, it remains open when de-energized. Therefore, in the event of a power failure, the actuator's braking effect automatically cancels out, thus fulfilling safety requirements.

[0012] Conversely, the actuator is designed so that when the parking function is engaged, it is self-locking, meaning it remains closed when de-energized. Therefore, in the event of a power failure, the parking function – unlike the braking function – is permanently maintained, thus ensuring that the parking function also meets safety requirements.

[0013] According to the characterizing part of claim 1, the ball ramp unit has a positioning aid with which the ball, which is out of rolling contact with the ball tracks of the disc pair when the parking function is engaged, remains in a predefined ball position without play, in which it is clearly identifiable by an electronic control unit that controls the actuator. The invention solves the problem that, without such a positioning aid, the ball can move with play between the ball tracks when the parking function is engaged. In this case, the ball can assume an inaccurate ball position relative to the disc pair, which is no longer clearly identifiable by the electronic control unit of the actuator.

[0014] In a technical implementation, the positioning aid can have at least one ball movement stop against which the ball tracks terminate. The positioning aid also includes a spring element that biases the ball towards the ball movement stop with a spring force. When the parking function is engaged, the ball is therefore pressed by the spring force into the predefined ball position against the ball movement stop without any play.

[0015] With the ball track geometry described above, the ball can assume the predefined ball position as soon as the rotatable disc, which rotates from a brake setting range in the parking direction, reaches the zero rotation position, at which the disc pair exerts no pressure on the lamellar brake.

[0016] The ball tracks of the disc pair are therefore only in rolling contact with the ball when the braking function is engaged. In contrast, when the parking function is engaged, the ball is out of rolling contact with the ball tracks. This means that immediately upon the discs starting to rotate from the neutral position to the parking position, the sliding ramps engage and, simultaneously, the ball is released from rolling contact. For example, the spring element can be supported between a ball cage, in which the ball is guided, and the rotationally fixed disc of the disc pair.

[0017] The actuator features an electric motor, controlled by the electronic control unit, which is mechanically connected to the rotating disc. The electronic control unit also includes an evaluation module that performs a calibration step before the vehicle starts operating. During the calibration step, the electronic control unit rotates the disc in different directions, thereby activating and deactivating the braking function and, alternately, the parking function. The evaluation module of the electronic control unit records the current draw of the electric motor during the calibration step. Based on the current draw of the electric motor across the rotation of the disc, the evaluation module can identify the zero position, i.e., a corresponding angle of rotation of the disc, and store this information in a database.

[0018] To ensure unambiguous identification of the zero rotation position, the geometry of the sliding ramps is designed as follows: The sliding ramps can have steep calibration contours. When the rotating disc is turned in the park direction, the calibration contours of the sliding ramps come into sliding contact with each other, creating a pressure point. This pressure point can be overcome by applying increased force, which corresponds to a higher current draw from the electric motor. The calibration contours of the sliding ramps are positioned so that the pressure point is established as soon as the rotating disc reaches its zero rotation position. The evaluation module thus identifies the zero rotation position of the rotating disc as soon as the pressure point is detected.

[0019] A comparative example not covered by the invention and an embodiment of the invention are described below with reference to the accompanying figures.

[0020] They show:

[0021] Fig. 1 shows an electrified vehicle axle;

[0022] Figs. 2 to 3c show different views of a multi-disc brake with actuator installed in the vehicle axle; Figs. 4a to 8b show the actuator according to the comparative example in different operating states; and

[0023] Figs. 9a to 12b show the actuator according to the invention in different operating states.

[0024] Figure 1 shows an electrified vehicle axle with an electric motor EM and a gearbox. The electric motor EM is connected to a high-voltage battery (not shown). Conventional wheel disc or drum brakes have been omitted from the vehicle axle. Instead, the vehicle axle has multi-disc brakes 3, which enable vehicle braking.

[0025] The electric motor EM is connected via its rotor shaft 5 and a reduction gear 7 to the input side of an axle differential 9. The output sides of the differential are connected to the vehicle wheels via output shafts 11. In Figure 1, the electric motor EM is mounted transversely in the vehicle axle. Accordingly, the rotor shaft 5 and the output shafts 11 are parallel to each other. Likewise, the multi-disc brakes 3 mounted in the vehicle axle are aligned parallel to each other in the transverse direction y of the vehicle.

[0026] Viewed in the transverse direction y of the vehicle, the axle has one of the multi-disc brakes 3 on each side of the vehicle. These can be controlled by an electronic control unit (not shown) to perform uniform or uneven braking at both vehicle wheels.

[0027] The reduction gear 7 is in close connection with an input-side axle differential gear 13. The axle differential gear 13 is fixedly connected to a rotating differential housing 15. As shown in Figure 1, with the multi-disc brakes 3 open, the axle differential 9 drives in the transverse direction y of the vehicle in a 50 / 50 distribution to both output shafts 11 leading to the vehicle wheels.

[0028] In Figure 1, the two multi-disc brakes 3 act directly on the output shafts 11. This means that the inner disc carrier 18 of the multi-disc brake 3 is connected to the respective output shaft 11, while the outer disc carrier 17 is fixed to a gearbox housing wall 19. The disc assembly located between the outer disc carrier 17 and the inner disc carrier 18 can be subjected to contact pressure via an actuator 21. The actuator 21 can be controlled by the control unit using electrical signals.

[0029] The actuator 21 consists of a spindle drive 16 with an electric motor 22, indicated in Figure 2, which is driven by a toothed section 31 of a rotatable disc 27. The rotatable disc 27, together with a non-rotating disc 25, forms part of a clamping mechanism. The non-rotating disc 25 can be supported circumferentially by a torque support 14, which is indicated. The non-rotating disc 25 is also supported axially on a counter support 12. The clamping mechanism has a total of four circumferentially distributed ball-ramp units 23, as shown in Figures 3a to 3b. Each ball-ramp unit 23 has a ball 29 that rolls between the two discs 25 and 27.

[0030] The lamellar assembly located between the outer lamella carrier 39 and the inner lamella carrier can be subjected to contact pressure by means of the actuator 21. Depending on the control of the actuator 21, a braking function for vehicle braking during ferry operation, or alternatively a parking function when the vehicle is parked, can be engaged or disengaged. A key aspect of the invention is that both the braking function and the parking function can be engaged / disengaged by means of the actuator 21. For safety reasons, the electrically controlled actuator 21 is normally open (i.e., not self-locking) when the braking function is engaged. In contrast, the electrically controlled actuator 21 is normally closed (i.e., self-locking) when the parking function is engaged. As can be seen from Figures 3a to 3c, each of the ball-ramp units 23 has ball ramps 34 and spaced-apart sliding ramps 38 that act between the disks 25, 27.The balls 29 of the four ball ramp units 23 are guided in a cage 30 according to figure 3b.

[0031] For the sake of a simpler understanding of the invention, a comparative example not encompassed by the invention is described below with reference to Figures 4a to 8b. Accordingly, Figures 4a and 4b each show one of the ball-ramp units 23 in its developed state. Figure 4a shows two corresponding sliding ramps 38 of the disc pair, while Figure 4b shows two corresponding ball ramps 34 of the disc pair with an intermediate ball 29. Each of the ball ramps 34 consists of a braking ball track 35 and a parking ball track 37, which merge into one another at a ramp recess 39. In Figure 4b, the ball 29 is located in the ramp recess 39. The ramp recess 39 defines a rotational zero position 0, in which the disc pair exerts no contact pressure on the lamella pack.The braking ball tracks 35 of the two discs 25, 27 are point-symmetrical with respect to the ball 29 (which is in the zero rotation position 0). Furthermore, the parking ball track 37 and the braking ball track 35 of each ball ramp 34 are symmetrical with respect to an axis of symmetry passing through the ramp recess 39, with equal angles of inclination.

[0032] Depending on the direction of rotation DB, DP of the rotating disc 27, either a braking function for vehicle braking during ferry operation or, alternatively, a parking function for when the vehicle is parked can be engaged / disengaged. When the braking function is engaged, the disc pair of actuator 21 is normally open, i.e., not self-locking. In contrast, when the parking function is engaged, the disc pair of actuator 21 is normally closed, i.e., self-locking.

[0033] Figures 4a and 4b show the pair of disks in their zero-rotation position 0. In the zero-rotation position 0, the two sliding ramps 38 are out of contact (unlike in Figure 4a). According to Figure 4b, the ball 29 is located in the ramp recesses 39 of the ball ramps 34 of the two disks 25, 27.

[0034] The engagement of the brake function is described below with reference to Figures 5a to 6b. Starting from the zero position 0 shown in Figures 4a and 4b, the rotatable disc 27 is moved to the left in a braking direction DB via a braking rotation angle into a braking adjustment range SB. This causes the ball 29 to roll on the two brake ball tracks 35 of the disc pair, with axial spreading of the disc pair via an axial spreading path s. In this way, the disc pack of the multi-disc brake 3 is subjected to contact pressure. The contact pressure varies depending on the size of the braking rotation angle. Immediately after leaving the zero position 0, the rotatable disc 27 is moved by an axial spreading path s, which eliminates any play in the disc pack.Further rotation of the rotatable disc 27 within the brake adjustment range SB results in a kiss point KP (Figure 5b) at which the multi-disc brake 3 transmits a measurable, predefined torque. At the kiss point KP, the rotatable disc 27 (starting from the zero rotation position 0) is rotated by a first axial displacement Asi. In Figures 6a and 6b, the rotatable disc 27 is adjusted to an end position in which it is rotated by a second axial displacement AS2.

[0035] When the braking function is engaged, a comparatively low rolling resistance acts between the ball 29 and the brake ball tracks 35 of the disc pair. This low rolling resistance means that the ball-ramp units 23 are not self-locking, i.e., they remain open when de-energized. Accordingly, in the event of a power failure, the braking effect of the actuated actuator 21 would decrease, thus fulfilling safety requirements.

[0036] To activate the brake function, the rotatable disc 27 (starting from its end position (Figures 6a and 6b) is moved back to the zero position 0 (Figures 4a and 4b) in the opposite direction to the brake rotation DB.

[0037] During the adjustment movement in the brake adjustment range SB, the corresponding, inclined sliding ramps 38 remain out of contact with each other, so that smooth rotary adjustment of the rotatable disc 27 is ensured.

[0038] The engagement of the parking function is described below with reference to Figures 7a to 8b. Starting from the zero position 0 shown in Figures 4a and 4b, the rotatable disc 27 is rotated to the right in a parking direction Dp, opposite to the brake direction DB, via a parking angle into a parking position SP.

[0039] According to Figure 7b, the parking area SP is divided into a first sub-area SPI and a second sub-area SP2. In the first sub-area SPI, the ball 29 rolls between the facing parking ball tracks 37 of the disc pair, while the corresponding sliding ramps 38 are still out of sliding contact. In contrast, in the second sub-area SP2, the sliding ramps 38 are in sliding contact with each other, while the ball 29 is out of rolling contact with the disc pair. To engage the parking function, the rotatable disc 27 is moved from the zero rotation position 0 in the parking rotation direction Dp by a parking rotation angle into the first sub-area SPI until a transfer point U is reached, as indicated in Figures 7a and 7b. According to Figures 7a and 7b, the disc pair generates a third axial spreading path Ass. at the transfer point U.

[0040] During the adjustment movement in the first section SPI, the ball 29 rolls on the parking ball tracks 37 of the disc pair, with axial expansion of the disc pair, thus eliminating any play in the lamellar brake 3. Upon passing the transfer point U, the rotatable disc 27 is rotated into the second section SP2 with a further increasing parking rotation angle until the parking function is engaged. For reliable engagement of the parking function, the transfer point U in the parking adjustment section SP is positioned between the zero rotation position 0 and the kiss point KP. Therefore, when the parking function is engaged, the transfer point U is passed shortly before reaching the kiss point KP. Upon reaching the kiss point KP, the two corresponding sliding ramps 38 are already in contact with each other, while the ball 29 is no longer in rolling contact with the two parking ball tracks 37.

[0041] In Figures 8a and 8b, the rotatable disc 27 is rotated to its parked end position, in which the parking function is fully engaged. With the parking function engaged, the ball 29 is in a predefined ball-freewheel position P (Figure 8b). The ball-freewheel position P is defined in Figure 8b by means of ball track exits 41 of the parking ball tracks 37. With the parking function engaged, the ball track exits 41 are spaced apart in the circumferential direction of the discs by a ball-freewheel f, in which the ball 29 rolls with play, i.e., without rolling contact, between the parking ball tracks 37 of the disc pair. The contact pressure of the disc pair is therefore generated exclusively by means of the sliding ramps 38 in sliding contact.

[0042] To engage the parking function, the rotatable disc 27 is turned back in the opposite direction of rotation Dp to the zero rotation position 0.

[0043] In the comparative example shown in Figures 4a to 8b, the ball freewheel f (Figures 7b and 8b) defines a ball dwell area in which the ball 29 can move with some play. Due to this play in the ball's movement, when the parking function is engaged, i.e., when the rotatable disc 27 rotates back towards its zero position 0, the ball 29's position relative to the ball tracks 37 is imprecise, meaning it can no longer be uniquely identified by an electronic control unit 41 of the electric motor 22. Therefore, in this comparative example, the problem arises that the ball 29 may not be in the ramp recess 39 as soon as the rotatable disc 27 reaches its zero position 0, which could impair operational reliability.

[0044] The following measures are described to keep the ball 29 in a defined ball position when the parking function is engaged, which is uniquely identifiable by the electronic control unit 41 of the electric motor 22.

[0045] Reference is made to Figures 9a to 12b, which describe the embodiment of the actuator 21 according to the invention. The basic structure of the actuator according to the invention is identical to that of the actuator 21 described with reference to Figures 4a to 8b, so reference is made to the preliminary description. In contrast to the comparative example, the actuator 21 according to the invention has a positioning aid 42 for the ball 29. With the aid of the positioning aid 42, the ball 29, which is out of rolling contact with the ball tracks 35 of the pair of discs when the parking function is engaged (Figures 11a, 11b), remains backlash-free in a predefined ball position. When the parking function is engaged (Figures 11a, 11b), the ball 29 is out of force transmission with the pair of discs, i.e., the ball 29 lifts off a limiting contour 28 of the rotatable disc 27.

[0046] As shown in Figures 9a to 12b, the parking ball tracks 37 are omitted according to the invention. Instead, the braking ball tracks 35 end at the ramp recess 39 with a ball movement stop 43, which is part of the positioning aid 42. Furthermore, the positioning aid 42 has a spring element 45, which is supported between the rotationally fixed disc 25 and the ball cage 30 and biases the ball 29 with a spring force towards the ball movement stop 43.

[0047] In Figures 9a to 12b, the ball ramp unit 23 of the actuator 21 is shown in its developed state, analogous to the preceding Figures 4a to 8b. Figure 9a shows two corresponding sliding ramps 38 of the disc pair, while Figure 9b shows two corresponding ball ramps 34 of the disc pair with an intermediate ball 29. The brake ball track 35 of the stationary disc 25 terminates at the ramp recess 39 with the ball movement stop 43. The ramp recess 39 defines—as in the comparative example—the zero rotation position 0, in which the disc pair exerts no contact pressure on the lamellar assembly of the lamellar brake 3. The braking function is engaged by means of the ball-ramp unit 23 according to the invention analogously to the comparative example, with the difference that the rotatable disc 27 is adjusted in the braking direction of rotation DB against the spring force exerted by the spring element 45.Otherwise, engaging the brake function is identical to the comparison example.

[0048] Figures 10a and 10b describe a calibration step that can be performed before the vehicle starts operating. In the calibration step, the electronic control unit 41 identifies the zero position 0 and the corresponding angle of rotation of the rotatable disc 27, based on the current consumption I of the electric motor 22 of the actuator 21, as measured by a current meter 51 (Figure 1), during rotation of the rotatable disc 27. This angle is then stored in a database of the electronic control unit 41.

[0049] For unambiguous identification of the zero rotation position 0, the two sliding ramps 38, in addition to the shallow flat sections 46, have steep calibration contours 47. When the disk rotates from the zero rotation position 0 shown in Figures 9a and 9b over a travel distance wi in the parking direction Dp, the opposing calibration contours 47 come into sliding contact and define a pressure point. This pressure point can be overcome by increasing the force applied, or correspondingly by increasing the current I of the electric motor 22. As soon as an evaluation module 49 (Figure 1) of the electronic control unit 41 detects the pressure point with the ammeter 51, the corresponding rotation angle of the rotatable disk 27 is stored in the control unit 41 as the zero rotation position 0.

[0050] In the calibration step, the pressure point is overcome by rotating the rotatable disc 27. After overcoming the pressure point, the flat sections 46 of the two sliding ramps 38 come into sliding contact with each other, so that the current consumption I is abruptly reduced. The transition from the pressure point to the flat sections 46 of the sliding ramps 38 can be defined, for example, as the point in time at which the rotatable disc 27 reaches its zero rotation position 0.

[0051] The following describes the engagement of the parking function according to the invention with reference to Figures 9a to 12b. Starting from Figures 9a and 9b (where the rotatable disc 27 is in its zero rotation position 0), the rotatable disc 27 is moved in the parking rotation direction Dp. Immediately after leaving the zero rotation position 0, the rotatable disc 27 is moved by an axial spreading path s (Figure 11a), by means of which any play is eliminated and the kiss point KP is reached. Immediately after leaving the zero rotation position 0, the ball 29 is disengaged from the disc pair, i.e., it is no longer in rolling contact with the ball tracks 35. By means of the spring element 45, the ball 29 is pressed to the right against the ball movement stop 43 in Figure 11b. Thus, the ball position remains clearly defined even after the parking function is engaged.By further rotating the rotatable disc 27 (Figures 12a and 12b) the pair of discs spreads further by a further axial path s to a maximum position, while the ball 29 remains in contact with the ball movement stop 43.

[0052] As can be seen, for example, in Figure 11b, the spring 45 is supported against the cage 30 at a slight angle. The cage 30, together with the ball 29, is therefore subjected to a spring force component in the direction of the rotatable disk 27 (i.e., in the axial direction). In this case, the limiting contour 28 of the rotatable disk 27 acts as an axial stop, limiting the axial movement of the ball 29 and the cage 30.

[0053] To activate the parking function, the rotatable disc 27 is turned back in the opposite direction of rotation Dp to its zero-rotation position 0. The ball 29 remaining in the predefined ball position ensures that, upon reaching the zero-rotation position 0, it is again in the ramp recess 39 and thus comes back into rolling contact with the ball tracks 35.

[0054] REFERENCE MARK LIST:

[0055] 3-disc brake

[0056] 5 Rotor shaft

[0057] 7 reduction gears

[0058] 9 axle differential

[0059] 11 Output shaft

[0060] 12 Counterholds

[0061] 13 Axle differential gear

[0062] 14 Torque support

[0063] 15 Differential housings

[0064] 16 Spindle drive

[0065] 17 outer slat carriers

[0066] 18 internal slat carriers

[0067] 19 Gearbox housings

[0068] 21 Actuator

[0069] 23 Ball Ramp Unit

[0070] 25 fixed disc

[0071] 27 rotating disc

[0072] 28 Boundary contour of the rotating disc 27

[0073] 29 balls

[0074] 30 cage

[0075] 31 Gearing

[0076] 34 Ball Ramp

[0077] 35 Brake ball track

[0078] 37 Park Ball Track

[0079] 38 sliding ramps

[0080] 39 Ramp pen recess

[0081] 41 electronic control unit

[0082] 42 Positioning aid

[0083] 43 Ball movement stop

[0084] 45 spring element

[0085] 46 Flat section of the sliding slope 38

[0086] 47 steep calibration contour of the sliding slope 38 49 evaluation module

[0087] 51 Current meter

[0088] 0 Rotation zero position

[0089] U Transfer Point KP Kisspoint

[0090] P Ball freewheel position s Axial spreading path

[0091] SB brake adjustment range

[0092] SP Park parking area SPi, SP2 sub-areas

[0093] DB Brake Direction of Rotation

[0094] DP Park direction of rotation

[0095] Current consumption f ball freewheel W1 , W2, W3 Travel of the rotatable disc 27 in the park / position range SP

Claims

PATENT CLAIMS:

1. Vehicle braking system with a multi-disc brake or clutch (3) comprising an actuator (21) with a pair of discs consisting of a stationary disc (25) and a coaxially rotatable disc (27) and at least one intermediate ball-ramp unit (23), wherein, to engage a braking function, the rotatable disc (27) rotates from a zero position (0) in a braking direction (DB), causing the ball (29) to roll between opposing ball tracks (35) of the disc pair, thereby axially spreading the disc pair to apply pressure to the multi-disc brake or clutch (3), and wherein, to engage the parking function, the rotatable disc (27) rotates from the zero position (0) in a parking direction (Dp) opposite to the braking direction (DB), thereby bringing opposing sliding ramps (38) of the disc pair into a sliding position. are, namely under axial spreading of the disk pair,to apply contact pressure to the lamellar brake or clutch (3), characterized in that the ball-ramp unit (23) has a positioning aid (42) with which the ball (29), which is out of force transmission with the disc pair when the parking function is engaged, remains free of play in a predefined ball position.

2. Vehicle braking system according to claim 1, characterized in that the positioning aid (42) has at least one ball movement stop (43) at which the ball tracks (35) terminate, and that the positioning aid (42) has a spring element (45) which biases the ball (29) with a spring force towards the ball movement stop (43), and that, in particular, when the parking function is engaged, the ball (29) is pressed by the spring force into the predefined ball position against the ball movement stop (43) without play, and that, in particular, the positioning aid (42) also has a rotating disc (27) has a formed boundary contour (28) which, when the parking function is engaged, forms an axial stop for the ball (29) which is no longer in the force flow.

3. Vehicle braking system according to claim 2, characterized in that the spring element (45) is supported between a ball cage (30) in which the ball (29) is guided and the non-rotatable disc (25).

4. Vehicle braking system according to one of the preceding claims, characterized in that the ball (29) assumes the predefined ball position as soon as the rotatable disc (27) coming from a brake positioning range (SB) and rotating in the parking direction (Dp) reaches the zero rotation position (0).

5. Vehicle braking system according to one of the preceding claims, characterized in that immediately upon starting the disc rotation from the zero rotation position (0) into the park position (SP), i.e., when the park function is engaged, the sliding ramps (38) come into sliding contact and simultaneously the ball (29) is disengaged from rolling contact or force flow with the disc pair, and / or that immediately upon starting the disc rotation from the zero rotation position (0) into the brake position (SB), i.e., when the brake function is engaged, the sliding ramps (38) come out of sliding contact and simultaneously the ball (29) comes into rolling contact or force flow with the disc pair from its predefined ball position.

6. Vehicle braking system according to one of the preceding claims, characterized in that the actuator (21) has an electric motor (22) that can be controlled by an electronic control unit (41) and that is in drive connection with the rotatable disc (27), and that the electronic The control unit (41) has an evaluation module (49) with which a calibration step can be performed in which the evaluation module (49) identifies the zero rotation position (0) of the rotatable disk (27) based on the current consumption (I) of the electric motor (22) during a rotation of the rotatable disk (27).

7. Vehicle braking system according to claim 6, characterized in that the sliding ramps (38) have steep calibration contours (47) which define a pressure point upon sliding contact, which can be overcome with increased force or correspondingly increased current consumption (I) of the electric motor (22), and that, in particular, the evaluation module (49) identifies the zero rotation position (0) on the basis of a pressure point detection.

8. Vehicle according to one of the preceding claims, characterized in that when the braking function is engaged, a rolling resistance acts between the ball (29) and the ball tracks (35) of the disc pair, and that in particular the rolling resistance causes the ball-ramp unit (23) to not be self-locking, i.e., to be open when de-energized, so that in particular the braking effect of the actuator (21) is canceled out in the event of a power failure.

9. Vehicle according to one of the preceding claims, characterized in that, when the parking function is engaged, a sliding resistance acts between the corresponding sliding ramps (46) which is significantly greater than the rolling resistance between the ball (29) and the ball tracks (35) of the disc pair, so that when the parking function is engaged, the ball-ramp unit (23) is self-locking, i.e., closed when de-energized, whereby, particularly when the parking function is engaged, the braking effect is maintained permanently despite a power failure, and in particular in contrast to the engaged braking function, in which In the event of a power failure, the braking effect of the actuator (21) is cancelled.

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