Patent Publication Number: US-2022235862-A1

Title: Device for Operating a Parking Lock of a Motor Vehicle Transmission

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related and has right of priority to German Patent Application No. 102021200774.0 filed in the German Patent Office on Jan. 28, 2021, which is incorporated by reference in its entirety for all purposes. 
     FIELD OF THE INVENTION 
     The invention relates to a device for operating a parking lock of a transmission in a motor vehicle. 
     BACKGROUND 
     Automatic transmissions for motor vehicles usually include a parking lock, in the interlocked position of which a locking pawl engages into a toothing of a parking interlock gear connected to the output of the automatic transmission and, thereby, acting on an axle of the motor vehicle. In modern automatic transmissions, electrohydraulic systems, which are also referred to as “electronic shifting” or “shift-by-wire systems”, have prevailed as the operative connection between the parking lock and an operating unit in the interior space of the motor vehicle. An electrical operative connection between the operating unit of the automatic transmission in the interior space of the vehicle and the electrohydraulic transmission control unit requires a conversion of the electrical signal for actuating the parking lock into a mechanical movement of the locking pawl. A hydraulically actuatable actuator can be provided for this purpose, the hydraulic supply of which is an integral part of a transmission hydraulic system. Here, pressure is usually applied at a piston of the actuator, which is axially displaceably arranged in a cylinder cavity and is operatively connected to the locking pawl, in order to bring the parking lock out of the interlocked position, counter to the spring force of an engagement spring provided for engaging the parking lock. 
     This type of hydraulically actuatable actuator for actuating a transmission parking lock is known, for example, from DE 41 27 991 A1. Here, the spring force of the engagement spring of the parking lock acts, in the engagement direction of the parking lock, upon the actuator piston of this actuator. In the disengagement direction of the parking lock, this actuator piston can be acted upon by system pressure of an electrohydraulic transmission control unit. For this purpose, an intake line of an actuator pressure chamber, within which the actuator piston is axially displaceably arranged, is connectable to the pressure side of an oil pump of the transmission by an electrically actuatable solenoid valve. In a first switching position of this solenoid valve, the actuator pressure chamber is vented, and so the spring force of the engagement spring acting upon the actuator piston brings about an engagement of the parking lock. In a second switching position of this solenoid valve, the actuator pressure chamber is pressurized with the system pressure delivered by the oil pump, and so the actuator piston actuates the parking lock counter to the spring force of the engagement spring in the disengagement direction of the parking lock. 
     In order not to need to constantly maintain the pressure acting upon the actuator piston in the disengaged condition of the parking lock at a level sufficient for holding the parking lock in this condition, the actuator can additionally include an electromagnetically actuatable locking device, by which the actuator piston can be mechanically fixed. DE 10 2012 013 373 A1, for example, describes a parking lock actuator including a locking device, which can mechanically interlock the actuator piston in a piston position associated with the engaged condition of the parking lock as well as in a piston position associated with the disengaged condition of the parking lock. Such a locking device is therefore also referred to as “bistable piston interlock”. In order to disengage the parking lock and upon disengagement of the parking lock, the mechanical piston interlock must be deactivated—i.e., released—and the actuator piston must be acted upon by clutch pressure, and so the clutch pressure brings the parking lock into the disengaged condition counter to the spring force of the engagement spring. In the disengaged condition of the parking lock, the mechanical piston interlock is reactivated and, in this way, prevents an unintentional movement of the actuator piston. 
     In the case of the actuator described in DE 10 2012 0133 373 A1, the interlock of the actuator piston takes place via balls, which are radially displaceably mounted in a stationary ball cage and are bringable into a released position or an interlocked position, depending on the switching condition of the electromagnet, by a cone, which is fixedly connected to the armature rod of the electromagnet of the locking device. In the interlocked position, the balls, which have then been displaced radially outward into a corresponding inner contour of the actuator piston, block the actuator piston from moving axially. Alternatively, systems are also known for interlocking the actuator piston that include a pin arranged laterally with respect to the piston rod of the actuator piston, which engages into one of two circumferential grooves of this piston rod, depending on the engagement position of the actuator piston, in order to mechanically fix the actuator piston. 
     If the actuator piston is mechanically fixed by the locking device in the piston position associated with the disengaged condition of the parking lock and is simultaneously acted upon by the system pressure acting counter to the spring force of the engagement spring, pressure fluctuations—in particular, brief pressure drops and brief pressure peaks—in the system pressure can result in wear at the mechanical piston interlock. If the actuator piston is mechanically fixed by the locking device in the piston position associated with the disengaged condition of the parking lock and, simultaneously, the system pressure previously acting upon the actuator piston is separated, with respect to control, from the pressure chamber of the actuator, and so now only the spring force of the engagement spring acts upon the actuator piston, brief pressure peaks can occur during the reconnection of the system pressure to the pressure chamber of the actuator, which can also result in wear at the mechanical piston interlock. 
     SUMMARY OF THE INVENTION 
     On the basis thereof, example aspects of the invention provide an alternative device for operating a transmission parking lock of the generic type, the parking lock actuator of which includes an actuator piston, which is acted upon by a system pressure provided by a pump of the transmission for an electrohydraulic control unit of the transmission in order to disengage the parking lock counter to the spring force of an engagement spring of the parking lock and can be mechanically interlocked in a piston position associated with the engaged condition of the parking lock as well as in a piston position associated with the disengaged condition of the parking lock. Primarily, the mechanical piston interlock is to be better protected against wear caused by pressure fluctuations. 
     Accordingly, example aspects of the invention are directed to a device for operating a parking lock of a motor vehicle transmission, including an engagement spring provided for engaging the parking lock, an actuator that is hydraulically actuatable in order to disengage the parking lock, an electrohydraulic control unit, and an electronic control unit. The electrohydraulic control unit hydraulically actuates, depending on the situation, gear-forming shift elements of the transmission as well as the actuator, via electromagnetically actuatable hydraulic valves, with pressure that is provided by a pump of the transmission. For this purpose, the electronic control unit electrically actuates the electromagnetically actuatable hydraulic valves in order to specify various shift positions and gears in the transmission. Here, the electrohydraulic control unit generates, by one of the electromagnetically actuatable hydraulic valves, a system pressure predefined by the electronic control unit depending on the situation, which ensures the pressure supply of the gear-forming shift elements and of the actuator that is necessary depending on the situation. 
     The actuator includes a hydraulic piston, which is operatively connected to the parking lock, is axially displaceably mounted in a housing of the actuator and, together with the housing, forms a pressure chamber, which, upon disengagement of the parking lock, is acted upon by system pressure from the electromagnetically actuatable hydraulic valve generating the system pressure via a pressure line and, upon engagement of the parking lock, is emptied via the pressure line. Additionally, the hydraulic piston of the actuator is mechanically interlockable by an interlocking device, which is actuatable by the electronic control unit, in a piston position associated with the engaged condition of the parking lock as well as in a piston position associated with the disengaged condition of the parking lock. 
     According to example aspects of the invention, a choke unit, which includes an orifice and a non-return valve, is installed in the pressure line leading to the pressure chamber of the actuator, in the area between the hydraulic valve generating the system pressure and the pressure chamber. The orifice acts in a flow-limiting manner in the feed direction to as well as in the return direction from the pressure chamber of the actuator, and so the flow routed to the pressure chamber of the actuator upon disengagement of the parking lock is limited to a predefined amount, which reduces, in an advantageous way, the installation space of the actuator itself and also the installation space of hydraulic components optionally provided for protecting the actuator. The non-return valve of the choke unit is closed in the feed direction to the pressure chamber of the actuator and is open in the return direction from the pressure chamber, and so a predefined emptying time of the pressure chamber upon engagement of the parking lock is ensured. 
     In one preferred example embodiment of the choke unit, the orifice and non-return valve are fluidically connected in parallel. This enables a structurally simple, large bandwidth for the individual adaptation of the filling speed as well as of the emptying speed of the actuator pressure chamber to different applications. 
     In an alternative example embodiment of the choke unit, the orifice and non-return valve are fluidically connected in series, which provides advantages related to installation space as compared to the parallel connection of the orifice and the non-return valve. 
     The non-return valve of the choke unit can include, as the closing element, for example, a ball, but also a plate, wherein the ball or the plate is then preloaded in the closing direction via a spring counter to the system pressure. Preferably, the through-flow cross-section and the spring characteristics of the non-return valve are structurally dimensioned in such a way that the flow resistance of the non-return valve is as low as possible during the emptying of the actuator compression chamber, i.e., during the engagement of the parking lock. 
     The inner diameter of the orifice of the choke unit is preferably structurally dimensioned in such a way that due to a flow resistance of the choke, on the one hand, the filling time of the actuator pressure chamber upon disengagement of the parking lock is not adversely affected to an excessive extent also at low operating temperatures and, on the other hand, however, a hydraulic damping effect arises that is sufficiently high for ensuring that the mechanical loading of the actuator piston interlock is mechanically gentle. 
     Viewed spatially, the choke unit can be an integral part of the electrohydraulic control unit of the transmission, alternatively, however, also an integral part of the actuator. 
     In order to effectively protect the actuator against damage or destruction by overpressure, it is provided in one example refinement of the invention to additionally fluidically connect a pressure limiting valve to the pressure line leading to the pressure chamber of the actuator in the area—i.e., in the direction of flow—between the choke unit and the pressure chamber. A pressure limiting valve of this type can be designed, for example, structurally simply as a ball or plate valve that is spring-loaded counter to the system pressure prevailing in the pressure line. Viewed spatially, the ball or plate valve is integrated in the electrohydraulic control unit of the transmission or, alternatively, in the actuator. Since the pressure limiting valve is arranged in the direction of flow between the choke unit and the pressure compartment, the flow limitation in the pressure line leading to the actuator pressure chamber generated by the choke unit upon disengagement of the parking lock has a load-reducing effect on the pressure limiting valve, and so the pressure limiting valve can be designed having comparatively small dimensions. 
     In order to effectively protect the actuator against damage or destruction by overpressure, it is provided in one example refinement of the invention to additionally fluidically connect a pressure limiting valve to the pressure line leading to the pressure chamber of the actuator in the area—i.e., in the direction of flow—between the choke unit and the pressure chamber. Preferably, a hydraulic damper of this type is designed as an integral part of the electrohydraulic control unit of the transmission, although, alternatively, the hydraulic damper can also be an integral part of the actuator. 
     Structurally, a hydraulic damper of this type is preferably designed as a piston that is axially displaceably arranged in a housing bore vented toward the transmission interior and is spring-loaded counter to the system pressure prevailing in the pressure line leading to the pressure chamber of the actuator. As previously indicated, the housing bore can be arranged in the electrohydraulic control unit of the transmission or in the actuator housing. Alternatively, the hydraulic damper can also be designed as an elastomer element that is deformable under pressure and is placed into a branch of the pressure line leading to the pressure chamber of the actuator that is closed toward the transmission interior. In both cases, the elasticity of the hydraulic damper for damping the amplitudes of dynamic and highly dynamic pressure fluctuations, pressure peaks, and pressure drops arising depending to the situation can be structurally attuned to the particular application, i.e., to the present parking lock system. This concept according to example aspects of the invention advantageously provides a passive damping of the amplitude of dynamic and highly dynamic pressure irregularities—arising depending on the situation—in the pressure feed of a hydraulically actuatable parking lock actuator of any type. 
     The passive damping of pressure fluctuations—arising depending on the situation—in the pressurization of the actuator piston therefore enables, in a particularly advantageous way, a significant reduction of wear at a mechanical interlock of the actuator piston, in particular while the parking lock is held in the disengaged condition. Disengagement-based component tolerances permit a certain small axial movement of the actuator piston, namely also when the piston interlock is activated, and so pressure fluctuations and pressure peaks of the system pressure acting upon the actuator piston can be transmitted as highly dynamic axial loads from the actuator piston onto the mechanism of the piston interlock, also when the piston interlock is activated. These types of highly dynamic impacts have a wear-promoting effect, as is known. By the pressurization of the actuator piston, which is damped according to example aspects of the invention, such impact-like loads at the piston interlock of the interlocking device can be significantly reduced, which advantageously increases the reliability and service life of the actuator. 
     In one example embodiment of the invention, it is provided to combine the pressure limiting valve provided as a first example refinement of the invention with the hydraulic damper provided as a second example refinement of the invention. In one preferred example embodiment, the pressure limiting valve is integrated in the hydraulic damper in an installation space-saving manner. The maximum pressure level to be safeguarded by way of the pressure limiting valve is always higher, in this case, with respect to the absolute value thereof, than the pressure fluctuations and pressure peaks to be damped by the hydraulic damper. 
     A pressure limiting valve integrated in the hydraulic damper can be formed or implemented, for example, by way of an interaction of the spring force of the damper spring, which is already present, with a predefined leading-edge dimension. If the damper piston is now displaced by the leading-edge dimension along the central axis, the existing feed-in port of the hydraulic damper fluidically connects to an appropriately positioned drain hole of the hydraulic damper leading to the interior space of the transmission. For this purpose, the damper spring can have progressive spring characteristics, and so the feed-in port of the hydraulic damper is fluidically connected to the drain hole of the hydraulic damper only above a predefined clutch pressure level. The “soft” portion of the progressive spring characteristics then carries out the desired damping of the high-frequency pressure fluctuations and pressure peaks. Alternatively, the damper spring can also be formed, however, by a mechanical interconnection of two springs—which are preferably mechanically connected in series—having different spring characteristics, wherein the first of these two springs has flat spring characteristics designed for damping the damper piston, whereas the second of these two springs has steep spring characteristics designed for opening the pressure limiting valve. 
     A pressure limiting valve integrated in the hydraulic damper can also be designed, for example, as a spring-loaded valve, which is integrated in the damper piston in such a way that the existing feed-in port of the hydraulic damper is fluidically connected, above a predefined system pressure level, to a drain hole of the hydraulic damper leading to the interior space of the transmission. This type of pressure limiting valve can be structurally simply designed as a ball valve preloaded counter to the system pressure by a pressure limiting spring or as a plate valve preloaded counter to the system pressure by a pressure limiting spring. The pressure limiting spring can be arranged, in an installation space-saving manner, concentrically within the damper spring, which is always acting upon the damper piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is explained in greater detail with reference to the drawings, wherein 
         FIG. 1  shows a schematic of a motor vehicle including a transmission having a parking lock; 
         FIG. 2  shows a schematic of a first exemplary embodiment of a device according to the invention for actuating the parking lock according to  FIG. 1 ; 
         FIG. 3  shows a schematic of a second exemplary embodiment of a device according to the invention for actuating the parking lock according to  FIG. 1 ; 
         FIG. 4  shows a schematic of a third exemplary embodiment of a device according to the invention for actuating the parking lock according to  FIG. 1 ; 
         FIG. 5  shows a schematic of a first exemplary design of a damper piston having an integrated pressure limiting piston; 
         FIG. 6  shows a schematic of a second exemplary design of a damper piston having an integrated pressure limiting piston; and 
         FIG. 7  shows a schematic of a third exemplary design of a damper piston having an integrated pressure limiting piston. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to embodiments of the invention, one or more examples of which are shown in the drawings. Each embodiment is provided by way of explanation of the invention, and not as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be combined with another embodiment to yield still another embodiment. It is intended that the present invention include these and other modifications and variations to the embodiments described herein. 
       FIG. 1  shows a schematic of a motor vehicle  1  including an automatic transmission  3 , which has multiple gear-forming shift elements  33  and is drivable by a prime mover  2  via a starting component  30 . In this way, the drive power of the prime mover  2  is transmittable in preferably multiple different gears or gear steps from an input shaft  31  onto an output shaft  32  of the automatic transmission  3 . The output shaft  32  is operatively connected to a drive axle  4  of the motor vehicle  1  via further motor vehicle components merely indicated in  FIG. 1 . 
     In addition, the automatic transmission  3  includes a parking lock  34 , by which the output shaft  32  and, thereby, also the drive axle  4  of the motor vehicle  1  are fixable. A combination of an electrohydraulic control unit  35  and an electronic control unit  36  is provided for the open-loop control of the automatic transmission  3 . The electrohydraulic control unit  35  carries out, on the one hand, the hydraulic actuation of the starting component  30 , which is designed as a clutch in this case, by way of example, in order to establish the frictional connection between a crankshaft  20  of the prime mover  2  and the input shaft  31  of the automatic transmission  3 . On the other hand, the electrohydraulic control unit  35  carries out the hydraulic actuation of the transmission-internal, gear-forming shift elements  33  in order to implement the gears in the automatic transmission  3  that are appropriate for the situation. In addition, the electrohydraulic control unit  35  also carries out the hydraulic actuation of an actuator  340 , which must be supplied with hydraulic fluid in order to disengage the parking lock  34 . Multiple electromagnetically actuatable hydraulic valves are provided in the electrohydraulic control unit  35  for actuating the starting component  30 , the shift elements  33 , and the actuator  340 . Of the multiple electromagnetically actuatable hydraulic valves, the hydraulic valves associated with the individual shift elements  33  are labeled with  350 , and the hydraulic valve that is provided for generating a system pressure ensuring the pressure supply of the gear-forming shift elements  33  and of the actuator  340  that is necessary depending on the situation is labeled with reference character  351 . Accordingly, the electromagnetically actuatable hydraulic valve  351  can also be referred to as a system pressure control valve. 
     The electronic control unit  36  determines the shift commands necessary depending on the situation as well as open-loop and closed-loop control-related specifications for the electromagnetically actuatable hydraulic valves  350  and  351  and appropriately actuates the electromagnetically actuatable hydraulic valves  350  and  351 . The electronic control unit  36  processes, among other things, signals of a selector device  5  arranged in the motor vehicle  1 , by which a driver of the motor vehicle  1  can specify individual shift positions for the automatic transmission, in particular the shift positions “Park” (P), “Neutral” (N), “Drive” (D), and “Reverse” (R). 
     The pressure medium necessary for actuating the starting component  30 , the shift elements  33 , and the actuator  340  is provided by a pump  37  of the transmission  3 . The area of the transmission interior of the transmission  3  that acts as a reservoir for the hydraulic fluid scavenged by the pump  37  and into which excess pressure medium is returned, forms a tank labeled with  38 . 
     In the following and with reference to the schematic in  FIG. 2 , a first exemplary embodiment of a device according to example aspects of the invention for operating the parking lock  34  shown in  FIG. 1  is explained in greater detail. 
     The hydraulic actuator  340  shown in  FIG. 2  for actuating the parking lock  34  is known, per se, from the prior art. The actuator  340  includes a hydraulic piston  341 , which is operatively connected, in a suitable way, to a blocking element (not represented, for the sake of simplicity) of the parking lock  34  and, in order to disengage the parking lock  34 , is acted upon by system pressure P_sys of the hydraulic control unit  35  that is sufficiently high for the pressure supply of the gear-forming shift elements  33  and of the actuator  340  that is necessary depending on the situation. For this purpose, the hydraulic piston  341  forms, together with a housing part of the actuator  340 , a pressure chamber  346 , which can be filled with hydraulic fluid at system pressure P_sys via a pressure line  347 . The actuator  340  is hydraulically actuated by system pressure P_sys via the electromagnetically actuatable system pressure control valve  351 , which generates the system pressure P_sys from the pump pressure P_p provided by the transmission-side pump  37  as specified by the electronic control unit  36 . 
     The shift elements  33 , however, are hydraulically actuated in order to form gears via electromagnetically actuatable hydraulic valves  350  of the electrohydraulic control unit  35 , wherein these hydraulic valves  350  themselves are supplied with system pressure P_sys from the system pressure control valve  351  via a hydraulic line  357  and, from the system pressure P_sys, generate a demand-oriented clutch pressure P_k for the particular gear-forming shift element  33  as specified by the electronic control unit  36 . Preferably, a separate hydraulic valve  350  is associated with each shift element  33 . 
     The operative connection between the actuator  340  and the parking lock  34  is designed in such a way that, for the case in which the hydraulic piston  341  is in the engaged position E, the operative connection blocks the parking lock  34  and, for the case in which the hydraulic piston  341  is in the disengaged position A, the operative connection does not block the parking lock  34 . If the hydraulic piston  341  is acted upon by pressure, it moves into the disengaged position A, counter to the spring force of an engagement spring  345 . Due to the spring force of the engagement spring  345 , the hydraulic piston  341  moves in the direction of the engaged position E when the pressurization of the hydraulic piston  341  is switched off, with the result that the parking lock  34  is mechanically engaged. This actuation logic, provided here, for the engagement and disengagement of the parking lock  34  is to be understood as an example. Correspondingly, in an alternative example embodiment of the parking lock, an inverted actuation logic can also be provided, according to which the parking lock is engaged by hydraulic pressure and is disengaged by spring pressure. 
     Additionally, the actuator  340  includes an interlocking device  342  for mechanically fixing the hydraulic piston  341 . The interlocking device  342  includes, by way of example, a pin  344  and an electromagnet  343  provided for actuating the pin  344 , wherein, preferably in the non-energized condition of the electromagnet  343 , the pin  344  arrests the hydraulic piston  341  either in the engaged position E or in the disengaged position A, i.e., secures the hydraulic piston  341  against an undesired axial displacement. 
     In order to improve the hydraulic actuation of the pressure chamber  346  of the actuator  340 , a choke unit  352  is provided, which is installed in the pressure line  347  in the area between the system pressure control valve  351  (which generates the system pressure P_sys) and the pressure chamber  346  and includes an orifice  353  and a non-return valve  354 . The orifice  353  acts in a flow-limiting manner in the feed direction to as well as in the return direction from the pressure chamber  346 . The non-return valve  354 , however, is closed in the feed direction to the pressure chamber  346  and is open in the return direction from the pressure chamber  346 . 
     In the exemplary embodiment represented in  FIG. 2 , the orifice  353  and the non-return valve  354  are fluidically connected in parallel, which gives the design engineer a certain amount of freedom when adapting the choke unit  352  to different application-specific requirements in combination with an otherwise identical basic design. 
     In the exemplary embodiment represented in  FIG. 2 , the non-return valve  354  is designed, by way of example, as a spring-loaded ball valve including a ball as a closing body  3540 , an internally open truncated cone as a closing body seat  3541 , and a spring  3542  preloaded counter to the system pressure P_sys for closing the closing body seat  3541  with the closing body  3540 . The flow possible through the non-return valve  354  is greater, many times over, than the flow through the orifice  353 . While the open inner diameter of the orifice  353  is structurally attuned to the application-specifically desired filling time of the actuator pressure chamber  346  upon disengagement of the parking lock  34 , i.e., to the application-specifically desired disengagement speed of the parking lock  34 , the open inner diameter of the closing body seat  3541  is attuned to the application-specifically desired emptying time of the actuator pressure chamber  346  upon engagement of the parking lock, i.e., to the application-specifically desired engagement speed of the parking lock  34 . A numerical example of a structural design of the choke unit  352  illustrates these relationships: Through-flow diameter one and two-tenths millimeter (1.2 mm) for the orifice  353 ; through-flow diameter three millimeters (3 mm) and closing pressure one-tenth of a bar (0.1 bar) for the non-return valve  354 . 
     As mentioned above, measures directed against damage to or destruction of the actuator  340  by an excessively high system pressure P_sys can be meaningful. For this purpose, it is provided in the second exemplary embodiment—represented in  FIG. 3 —of a device according to example aspects of the invention, which is based on the parking lock actuating system shown in  FIG. 2 , to fluidically connect the pressure line  347  leading to the pressure chamber  346  of the actuator  340  to a pressure limiting valve  355 . In the exemplary design represented here, the pressure limiting valve  355  is designed as a plate valve spring-loaded counter to the system pressure P_sys, including a piston-like closing body  3550 , a ring-shaped closing body seat  3551 , a pressure limiting spring  3552  mounted between the closing body  3550  and a housing section, a cylindrical feed-in port  3553  fluidically connected to the pressure line  347  and situated on the end face of the closing body  3550  on the side opposite the pressure limiting spring  3552 , a drain hole  3554  leading to the tank  38  and situated laterally with respect to the closing body  3550 , and an air escape  3555  in the spring chamber of the closing body  3550  leading to the tank  38 . 
     In order to be able to utilize the advantages of the choke unit  352  also for the dimensioning of the pressure limiting valve  355 , the pressure limiting valve  355  is fluidically connected to a line section of the pressure line  347  leading to the actuator pressure chamber  346 , which is located between the choke unit  352  and the pressure chamber  346 . Here, the pressure limiting valve  355  in  FIG. 3 —which is additional as compared to  FIG. 2 —is an integral part of the actuator  340 , by way of example. The choke unit  352  acquired from  FIG. 2 , however, is an integral part of the electrohydraulic control unit  35 . A numerical example of a structural design of the pressure limiting valve  355  illustrates the aforementioned advantage: If the pressure limiting valve  355  is to open at a pressure threshold of twenty-two bar (22 bar), a spring force of approximately twenty-seven and six-tenths newtons (27.6 N) is calculated for the pressure limiting spring  3552  at an effective diameter of the closing body seat  3551  of four millimeters (4 mm); the installation space needed here is also correspondingly small. 
     As mentioned above, pressure pulsation, temporary pressure peaks, and temporary pressure drops of the system pressure P_sys, by which the hydraulic piston  341  of the actuator  340  can be acted upon, can also result in damage of the actuator  340 , in particular in undesired wear at the mechanism of the interlocking device  342  when the interlocking device  342  is in the interlocked position, i.e., in the present example, wear at the pin  344  of the interlocking device  342  and at the piston rod groove, into which the pin  344  engages in position E of the hydraulic piston  341 . The obligatory component tolerances permit, namely also in the case of an interlocked pin  344 , a certain small axial movement of the hydraulic piston  341 , and so highly dynamic axial loads acting upon the hydraulic piston  341 , which can be produced by the aforementioned highly dynamic irregularities in the system pressure P_sys, act as highly dynamic impacts upon the piston rod/pin contact point. Disruptive pressure peaks and disruptive pressure drops can arise, for example, during gear ratio changes in the transmission ( 3 ). 
     In order to protect the interlocking device  342  against mechanical damage that can be caused by such pressure peaks and pressure drops in the supply pressure of the pressure chamber  346  of the actuator  340 , in the third exemplary embodiment of a device according to example aspects of the invention represented in  FIG. 4 , which is based on the parking lock actuating system shown in  FIG. 2 , the pressure line  347  leading to the pressure chamber  346  of the actuator  340  is fluidically connected to a hydraulic damper  356 . In order to be able to utilize the advantages of the choke unit  352  also for the dimensioning of the hydraulic damper  356 , this connection is located at a line section of the pressure line  347  situated between the choke unit  352  and the pressure chamber  346 . Here, the choke unit  352  acquired from  FIG. 2  and the hydraulic damper  356 , which is additional as compared to  FIG. 2 , are integral parts of the electrohydraulic control unit  35 , by way of example, in  FIG. 4 . 
     In the exemplary design shown in  FIG. 4 , the hydraulic damper  356  is designed as a piston/spring damper. Here, the piston of the damper  356  is axially displaceably arranged in a bore of a housing of the electrohydraulic control unit  35  of the transmission ( 3 ), wherein the spring of the damper  356  preloads the piston of the damper  356  counter to the system pressure P_sys prevailing in the pressure line  347 . Correspondingly, the spring chamber of the damper  356  is vented to the tank  38 , which is formed, for example, by an oil pan of the transmission ( 3 ). By way of example, an inlet orifice is additionally provided in the fluid flow between the pressure line  347  and the piston chamber of the damper  356 . 
     In  FIG. 4 , the parking lock ( 34 ) (not represented here in greater detail) of the transmission ( 3 ) is in the engaged condition. Correspondingly, the hydraulic piston  341  of the parking lock actuator  340  is in the engagement position E and, in the engagement position E, is fixed in the axial direction by the pin  344  of the interlocking device  342  now form-lockingly engaging into a circumferential groove of the piston rod of the hydraulic piston  341 . 
     If the parking lock ( 34 ) is disengaged starting from the engaged condition, the pressure chamber  346  of the actuator  340  is acted upon by system pressure P_sys from the electromagnetically actuatable system pressure control valve  351  via the orifice  353  of the choke unit  352  and the pressure line  347 , and so, with the pin  344  of the interlocking device  342  released, the hydraulic piston  341  of the actuator  340  moves axially from the piston position E to the piston position A before the pin  344  once again fixes the hydraulic piston  341  in the axial direction. The fluidic connection, which is now provided, of the pressure line  346  leading to the actuator pressure chamber  346  at the hydraulic damper  356  effectively and reliably prevents highly dynamic pressure fluctuations and pressure peaks in the system pressure P_sys from reaching a level that is disruptive to the interlocking device  342 . In other words, the hydraulic damper  356  in the pressure feed to the hydraulic piston  341  of the actuator  340  prevents excessive wear at the mechanical interlock of the hydraulic piston  341 . 
     If the parking lock ( 34 ) is engaged starting from the disengaged condition, the pressure chamber  346  of the actuator  340  is vented to the tank  38  via the pressure line  347 , the non-return valve  354  of the choke unit  352 , and the system pressure control valve  351 , and so, with the pin  344  of the interlocking device  342  released, the hydraulic piston  341  of the actuator  340  moves axially from the piston position A to the piston position E due to the spring force of the engagement spring  345  of the parking lock ( 34 ) before the pin  344  once again fixes the hydraulic position  341  in the axial direction. 
     In the following and with reference to  FIG. 5 through 7 , three exemplary designs are explained in greater detail, in which a pressure limiting valve  355  as well as a hydraulic damper  356  are provided for protecting the actuator  340 . In these three exemplary designs, the pressure limiting valve  355  is integrated in the hydraulic damper  356  in an installation space-saving manner. Similarly to the third exemplary embodiment of a device according to example aspects of the invention for hydraulically actuating a parking lock actuator shown in  FIG. 4 , the hydraulic damper  356  in all three of these exemplary designs includes a damper piston  3561 , which is preloaded by spring force of a damper spring  3562  counter to the system pressure P_sys provided by the system pressure control valve ( 351 ) and is arranged displaceably along the central axis in a bore of a housing  3560  that is vented toward an interior space of the transmission ( 3 ). The maximum pressure level to be safeguarded by way of the pressure limiting valve  355  is higher, in any case, with respect to the absolute value thereof, than the pressure fluctuations and pressure peaks to be damped by the hydraulic damper  356 . 
     In the first exemplary design represented in  FIG. 6 , the pressure limiting valve labeled with position number  355  is formed by an interaction of the spring force of the damper spring  3562  with a predefined leading-edge dimension  3569 , by which the damper piston  3561  must be displaced along the central axis in order to fluidically connect a feed-in port  3567  of the hydraulic damper  356  supplied with hydraulic fluid under system pressure P_sys to a drain hole  3568  of the hydraulic damper  356  leading to the interior space or to the tank ( 38 ) of the transmission ( 3 ). 
     Preferably, multiple lateral drain openings  3568  are provided, since at least three contact surfaces aligned in the shape of a star, i.e., arranged symmetrically distributed at the circumference, are necessary for a well functioning guidance of the damper piston  3561 . The intermediate spaces between these contact surfaces can be utilized in a problem-free manner as lateral drain openings  3568 . 
     In the upper part of  FIG. 5 , the damper piston  3561  is in the basic or normal position, in which the system pressure P_sys has no pressure fluctuations and pressure peaks that necessitate a hydraulic damping, and in which the maximum pressure to be safeguarded by the pressure limiting valve  355  has also not yet been reached, by far. In the lower part of  FIG. 5 , the system pressure P_sys applied at the feed-in port  3567  has exceeded the permissible maximum pressure, and so the damper piston  3561  releases a drainage edge, defined by the leading-edge dimension  3569 , of the drain hole  3568  arranged laterally with respect to the damper piston  3561 , with the result that hydraulic fluid is now discharged into the transmission interior or into the tank ( 38 ) to such an extent that the system pressure P_sys at the feed-in port  3567  is limited to the permissible maximum pressure. 
     In the exemplary design represented in  FIG. 5 , the damper spring  3562  has progressive spring characteristics, wherein the “soft” part of these spring characteristics ensures the desired damping function, whereas the switching point of the desired safeguard against overpressure is located in the area of the “hard” part thereof. 
     Alternatively, a series connection of two springs having different spring characteristics can also be provided, wherein the first spring is then designed as a damper spring having flat spring characteristics designed for damping the damper piston  3561 , whereas the second of these two springs then has steep spring characteristics designed for opening the drainage edge of the drain hole  3568  defined by the leading-edge dimension  3569 . 
     In another alternative, two concentrically nested springs having different spring characteristics can also be provided, wherein the first spring is then designed as a damper spring having flat spring characteristics designed for damping the damper piston  3561 , whereas the second of these two springs then has steep spring characteristics designed for opening the drainage edge of the drain hole  3568  defined by the leading-edge dimension  3569 . In this case, the first spring must have a greater length than the second spring, so that the “hard” second spring does not impede the travel of the “soft” first spring needed for damping. Therefore, it is also useful when the shorter of the two springs, i.e., the “hard” second spring, is attached either at the damper piston  3561  or at the base of the damper housing  3560  facing away from the feed-in port  3567  or, however, at the “soft” first spring. Provided the hydraulic damper  356  designed in such a way is situated in the damping area, only the “soft” damper spring is functioning. Only beyond the damping area does a parallel connection of the spring forces of the two springs arise, as the result of which the damper  356  now operates as a pressure limiting valve  355 . 
     In the second exemplary design represented in  FIG. 6 , the pressure limiting valve  355  is designed as a ball valve preloaded by a pressure limiting spring  3552 , which is integrated in the damper piston  3561  preloaded by spring force of the damper spring  3562  in such a way that the existing feed-in port  3567  of the hydraulic damper  356 , via which system pressure P_sys is applicable or applied at the damper piston  3561 , is fluidically connected, above a predefined system pressure level, to the drain hole  3568  of the hydraulic damper  356  leading to the interior space or to the tank ( 38 ) of the transmission ( 3 ). The damper piston  3561  forms a housing element for the pressure limiting valve  355 , which is displaceable within a housing  3560  of the hydraulic damper  356 . A feed-in port  3553  is provided in the damper piston  3561 , which is always fluidically connected to the feed-in port  3567  of the hydraulic damper  356 . On a back side, i.e., on the side facing away from the feed-in port  3567 , a closing body seat  3551  is formed for the closing body  3550 —designed as a ball in this case, by way of example, —of the pressure limiting valve  355 , against which the pressure limiting spring  3552  presses the closing body  3550 . The spring force of the pressure limiting spring  3552  is selected in such a way that the system pressure P_sys pushes the closing body  3550  out of the closing body seat  3551  once a maximum pressure predefined for the actuator ( 340 ) is exceeded and, as a result, enables the inflow of hydraulic fluid from the feed-in port  3567  via the feed-in port  3553  into a hollow chamber  3565  of the damping piston  3561 . The closing body  3550  and the pressure limiting spring  3552  of the pressure limiting valve  355  are located within the hollow chamber  3565 . 
     On the side facing away from the feed-in port  3567 , i.e., in the spring chamber of the damper spring  3562 , the hydraulic damper  356  or the damper housing  3560  is vented via a lateral drain hole  3568  to the transmission interior or to the tank ( 38 ). The hollow chamber  3565  of the damper piston  3561  also has a lateral drain hole  3554 . The damper piston  3561  itself is axially displaceably arranged, in a known way, in a bore of the housing  3560  of the hydraulic damper  356 . Since the pressure limiting spring  3552  is arranged, as viewed spatially, within the hollow chamber  3565  and the damper spring  3562  is arranged, as viewed spatially, above the hollow chamber  3565  of the damper piston  3561 , the damper spring  3562  and the pressure limiting spring  3552  are connected in series with respect to force. 
     In the upper part of  FIG. 6 , the damper piston  3561  is in the basic position or normal, in which the system pressure P_sys has no pressure fluctuations and pressure peaks that necessitate a hydraulic damping, and in which the maximum pressure to be safeguarded by the pressure limiting valve  355  has also not yet been reached, by far. The spring characteristics of the damper spring  3562  preloading the damper piston  3561  are attuned to the pressure fluctuations and pressure peaks to be damped and are comparatively flat. The spring characteristics of the pressure limiting spring  3552  preloading the closing body  3550 , however, are attuned to the maximum pressure to be safeguarded and, therefore, are comparatively steep. 
     In the lower part of  FIG. 6 , the system pressure P_sys is at a level that is so high that the damper piston  3561  has compressed the damper spring  3562  to the fully compressed size, with the result that the hollow chamber  3565  of the damper piston  3561  is now fluidically connected via the drain hole  3554  of the hollow chamber  3565  to the drain hole  3568  of the damper housing  3560  and, as a result, is also vented to the transmission interior or to the tank ( 38 ). In addition, in the lower part of  FIG. 6 , the system pressure P_sys applied at the feed-in port  3567  has exceeded the permissible maximum pressure, and so the pressure limiting valve  355  is open, with the result that hydraulic fluid is now discharged into the transmission interior or into the tank ( 38 ) to such an extent that the system pressure P_sys at the feed-in port  3567  is limited to the permissible maximum pressure. 
     The third exemplary design represented in  FIG. 7  is a technically simplified and installation space-saving variant of the pressure limiting valve  355  shown in  FIG. 6  and is also integrated into the damper piston  3561  of the hydraulic damper  356 . The damper piston  3561 —similarly to  FIG. 6 —is axially displaceably arranged in a bore of a housing  3560  of the hydraulic damper  356 , wherein system pressure P_sys is applicable or applied at the top-side end face of the damper piston  3561  via a feed-in port  3567  provided in the housing  3560 . The system pressure P_sys also acts upon the hydraulic piston ( 341 ) (not represented here) of the parking lock actuator ( 340 ). On the lower side facing away from the feed-in port  3567 , the damper piston  3561  forms a spring chamber for a damper spring  3562 , which preloads the damper piston  3561  with respect to the housing  3560 . The spring chamber is vented via a drain hole  3568  to the interior space or to the tank ( 38 ) of the transmission ( 3 ). As in  FIG. 6 , the spring characteristics of the damper spring  3562  are attuned to the pressure fluctuations and pressure peaks to be damped. 
     In contrast to  FIG. 6 , the pressure limiting valve  355  is now designed as a spring-loaded plate valve, which is arranged within a longitudinal bore  3563  of the damper piston  3561 . A pressure limiting spring  3552  provided for preloading the pressure limiting valve  355  is also arranged within the longitudinal bore  3563  and presses a closing body  3550 —which is now designed as a ring-shaped plate—of the pressure limiting valve  355  against a closing body seat  3551 —which is now flat—at the damper piston  3561 . The pressure limiting spring  3552  is supported via a disk  3556  at a securing or snap ring  3558 , which is placed into an annular groove  3557  of the longitudinal bore  3563 , at the damper piston  3561 , and so the closing body  3550  is axially mounted between the closing body seat  3551  and the annular groove  3557 . Viewed spatially, the pressure limiting spring  3552  is concentrically arranged within the damper spring  3562 , at least partially depending on the length of the guidance of the damper piston  3561  in the bore of the housing  3560 . 
     Hydraulic fluid is applied at the closing body  3550  of the pressure limiting valve  355  on the side opposite the pressure limiting spring  3552  via a feed-in port  3553 , which is provided in the damper piston  3561  and is constantly fluidically connected to the feed-in port  3567  provided in the housing  3560  of the hydraulic damper  356 . The spring characteristics of the pressure limiting spring  3552  are designed in such a way that the pressure limiting valve  355  opens as soon as the system pressure P_sys has exceeded a predefined maximum value. If the pressure limiting valve  355  is open, the longitudinal bore  3563  of the damper piston  3561  simultaneously functions as a drain hole for the excess hydraulic fluid resulting from overpressure, which is then discharged via the drain hole  3568  of the damper housing  3560  to the interior space or to the tank ( 38 ) of the transmission ( 3 ). 
     Contrary to the representation in  FIG. 7 , it can also be provided that the plate-shaped closing body  3550  of the pressure-limiting valve  355  is laterally guided in the longitudinal bore  3563  of the damping piston  3561 . In this case, the closing body  3550  needs at least three contact surfaces, which are aligned in the shape of a star, i.e., arranged symmetrically distributed at the circumference. The intermediate spaces between these contact surfaces then function as lateral overflow ducts for the fluid transport from the feed-in port  3553  via the longitudinal bore  3563  to the drain hole  3568  when the pressure limiting valve  355  is open. 
     Modifications and variations can be made to the embodiments illustrated or described herein without departing from the scope and spirit of the invention as set forth in the appended claims. In the claims, reference characters corresponding to elements recited in the detailed description and the drawings may be recited. Such reference characters are enclosed within parentheses and are provided as an aid for reference to example embodiments described in the detailed description and the drawings. Such reference characters are provided for convenience only and have no effect on the scope of the claims. In particular, such reference characters are not intended to limit the claims to the particular example embodiments described in the detailed description and the drawings. 
     REFERENCE CHARACTERS 
     
         
           1  motor vehicle 
           2  prime mover of the motor vehicle 
           20  crankshaft of the prime mover 
           3  transmission of the motor vehicle 
           30  starting component between the prime mover and the transmission 
           31  input shaft of the transmission 
           32  output shaft of the transmission 
           33  shift elements of the transmission 
           34  parking lock of the transmission 
           340  actuator of the parking lock 
           341  hydraulic piston of the actuator 
           342  interlocking device 
           343  electronic component of the interlocking device 
           344  pin of the interlocking device 
           345  engagement spring of the parking lock 
           346  pressure chamber of the actuator 
           347  pressure line to the pressure chamber 
           35  electrohydraulic control unit of the transmission 
           350  electromagnetically actuatable hydraulic valve of the electrohydraulic control unit for actuating a shift element 
           351  electromagnetically actuatable hydraulic valve of the electrohydraulic control unit for generating the system pressure 
           352  choke unit 
           353  orifice of the choke unit 
           354  non-return valve of the choke unit 
           3540  closing body of the non-return valve 
           3541  closing body seat of the non-return valve 
           3542  spring of the non-return valve 
           355  pressure limiting valve 
           3550  closing body of the pressure limiting valve 
           3551  closing body seat of the pressure limiting valve 
           3552  pressure limiting spring 
           3553  feed-in port of the pressure limiting valve 
           3554  drain hole of the pressure limiting valve 
           3555  air escape of the pressure limiting valve 
           3556  disk 
           3557  annular groove 
           3558  securing ring 
           356  hydraulic damper 
           3560  housing of the damper 
           3561  damper piston 
           3562  damper spring 
           3563  longitudinal bore in the damper piston 
           3564  hollow chamber in the damper piston 
           3566  drain hole of the hollow chamber 
           3567  feed-in port of the damper 
           3568  drain hole of the damper 
           3569  leading-edge dimension 
           357  hydraulic line 
           36  electronic control unit of the transmission 
           37  pump of the transmission 
           38  tank; oil sump 
           4  drive axle of the motor vehicle 
           5  operating unit for the transmission 
         A position of the hydraulic piston in the disengaged condition of the parking lock 
         E position of the hydraulic piston in the engaged condition of the parking lock 
         P_k clutch pressure 
         P_p pump pressure 
         P_sys system pressure