Device for operating a parking lock of a motor vehicle transmission

A device for operating a parking lock (34) of a transmission (3) includes an engagement spring (345) for engaging the parking lock (34), a hydraulic actuator (340) for disengaging the parking lock (34), an electrohydraulic control unit (35) for hydraulically actuating the actuator (340), and an electronic control unit (36) for electrically actuating the actuator (340) and the electrohydraulic control unit (35). The actuator (340) includes a hydraulic piston (341) operatively connected to the parking lock (34), is actuatable by system pressure (P_sys) of the electrohydraulic control unit (35) via a pressure line (347) upon disengagement of the parking lock (34), and is mechanically interlockable by an interlocking device (342). A choke unit (353) includes an orifice (353) and a non-return valve (354) and is installed in the pressure line (347) downstream from the hydraulic piston (341).

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.

DETAILED DESCRIPTION

FIG.1shows a schematic of a motor vehicle1including an automatic transmission3, which has multiple gear-forming shift elements33and is drivable by a prime mover2via a starting component30. In this way, the drive power of the prime mover2is transmittable in preferably multiple different gears or gear steps from an input shaft31onto an output shaft32of the automatic transmission3. The output shaft32is operatively connected to a drive axle4of the motor vehicle1via further motor vehicle components merely indicated inFIG.1.

In addition, the automatic transmission3includes a parking lock34, by which the output shaft32and, thereby, also the drive axle4of the motor vehicle1are fixable. A combination of an electrohydraulic control unit35and an electronic control unit36is provided for the open-loop control of the automatic transmission3. The electrohydraulic control unit35carries out, on the one hand, the hydraulic actuation of the starting component30, which is designed as a clutch in this case, by way of example, in order to establish the frictional connection between a crankshaft20of the prime mover2and the input shaft31of the automatic transmission3. On the other hand, the electrohydraulic control unit35carries out the hydraulic actuation of the transmission-internal, gear-forming shift elements33in order to implement the gears in the automatic transmission3that are appropriate for the situation. In addition, the electrohydraulic control unit35also carries out the hydraulic actuation of an actuator340, which must be supplied with hydraulic fluid in order to disengage the parking lock34. Multiple electromagnetically actuatable hydraulic valves are provided in the electrohydraulic control unit35for actuating the starting component30, the shift elements33, and the actuator340. Of the multiple electromagnetically actuatable hydraulic valves, the hydraulic valves associated with the individual shift elements33are labeled with350, and the hydraulic valve that is provided for generating a system pressure ensuring the pressure supply of the gear-forming shift elements33and of the actuator340that is necessary depending on the situation is labeled with reference character351. Accordingly, the electromagnetically actuatable hydraulic valve351can also be referred to as a system pressure control valve.

The electronic control unit36determines the shift commands necessary depending on the situation as well as open-loop and closed-loop control-related specifications for the electromagnetically actuatable hydraulic valves350and351and appropriately actuates the electromagnetically actuatable hydraulic valves350and351. The electronic control unit36processes, among other things, signals of a selector device5arranged in the motor vehicle1, by which a driver of the motor vehicle1can 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 component30, the shift elements33, and the actuator340is provided by a pump37of the transmission3. The area of the transmission interior of the transmission3that acts as a reservoir for the hydraulic fluid scavenged by the pump37and into which excess pressure medium is returned, forms a tank labeled with38.

In the following and with reference to the schematic inFIG.2, a first exemplary embodiment of a device according to example aspects of the invention for operating the parking lock34shown inFIG.1is explained in greater detail.

The hydraulic actuator340shown inFIG.2for actuating the parking lock34is known, per se, from the prior art. The actuator340includes a hydraulic piston341, which is operatively connected, in a suitable way, to a blocking element (not represented, for the sake of simplicity) of the parking lock34and, in order to disengage the parking lock34, is acted upon by system pressure P_sys of the hydraulic control unit35that is sufficiently high for the pressure supply of the gear-forming shift elements33and of the actuator340that is necessary depending on the situation. For this purpose, the hydraulic piston341forms, together with a housing part of the actuator340, a pressure chamber346, which can be filled with hydraulic fluid at system pressure P_sys via a pressure line347. The actuator340is hydraulically actuated by system pressure P_sys via the electromagnetically actuatable system pressure control valve351, which generates the system pressure P_sys from the pump pressure P_p provided by the transmission-side pump37as specified by the electronic control unit36.

The shift elements33, however, are hydraulically actuated in order to form gears via electromagnetically actuatable hydraulic valves350of the electrohydraulic control unit35, wherein these hydraulic valves350themselves are supplied with system pressure P_sys from the system pressure control valve351via a hydraulic line357and, from the system pressure P_sys, generate a demand-oriented clutch pressure P_k for the particular gear-forming shift element33as specified by the electronic control unit36. Preferably, a separate hydraulic valve350is associated with each shift element33.

The operative connection between the actuator340and the parking lock34is designed in such a way that, for the case in which the hydraulic piston341is in the engaged position E, the operative connection blocks the parking lock34and, for the case in which the hydraulic piston341is in the disengaged position A, the operative connection does not block the parking lock34. If the hydraulic piston341is acted upon by pressure, it moves into the disengaged position A, counter to the spring force of an engagement spring345. Due to the spring force of the engagement spring345, the hydraulic piston341moves in the direction of the engaged position E when the pressurization of the hydraulic piston341is switched off, with the result that the parking lock34is mechanically engaged. This actuation logic, provided here, for the engagement and disengagement of the parking lock34is 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 actuator340includes an interlocking device342for mechanically fixing the hydraulic piston341. The interlocking device342includes, by way of example, a pin344and an electromagnet343provided for actuating the pin344, wherein, preferably in the non-energized condition of the electromagnet343, the pin344arrests the hydraulic piston341either in the engaged position E or in the disengaged position A, i.e., secures the hydraulic piston341against an undesired axial displacement.

In order to improve the hydraulic actuation of the pressure chamber346of the actuator340, a choke unit352is provided, which is installed in the pressure line347in the area between the system pressure control valve351(which generates the system pressure P_sys) and the pressure chamber346and includes an orifice353and a non-return valve354. The orifice353acts in a flow-limiting manner in the feed direction to as well as in the return direction from the pressure chamber346. The non-return valve354, however, is closed in the feed direction to the pressure chamber346and is open in the return direction from the pressure chamber346.

In the exemplary embodiment represented inFIG.2, the orifice353and the non-return valve354are fluidically connected in parallel, which gives the design engineer a certain amount of freedom when adapting the choke unit352to different application-specific requirements in combination with an otherwise identical basic design.

In the exemplary embodiment represented inFIG.2, the non-return valve354is designed, by way of example, as a spring-loaded ball valve including a ball as a closing body3540, an internally open truncated cone as a closing body seat3541, and a spring3542preloaded counter to the system pressure P_sys for closing the closing body seat3541with the closing body3540. The flow possible through the non-return valve354is greater, many times over, than the flow through the orifice353. While the open inner diameter of the orifice353is structurally attuned to the application-specifically desired filling time of the actuator pressure chamber346upon disengagement of the parking lock34, i.e., to the application-specifically desired disengagement speed of the parking lock34, the open inner diameter of the closing body seat3541is attuned to the application-specifically desired emptying time of the actuator pressure chamber346upon engagement of the parking lock, i.e., to the application-specifically desired engagement speed of the parking lock34. A numerical example of a structural design of the choke unit352illustrates these relationships: Through-flow diameter one and two-tenths millimeter (1.2 mm) for the orifice353; through-flow diameter three millimeters (3 mm) and closing pressure one-tenth of a bar (0.1 bar) for the non-return valve354.

As mentioned above, measures directed against damage to or destruction of the actuator340by an excessively high system pressure P_sys can be meaningful. For this purpose, it is provided in the second exemplary embodiment—represented inFIG.3—of a device according to example aspects of the invention, which is based on the parking lock actuating system shown inFIG.2, to fluidically connect the pressure line347leading to the pressure chamber346of the actuator340to a pressure limiting valve355. In the exemplary design represented here, the pressure limiting valve355is designed as a plate valve spring-loaded counter to the system pressure P_sys, including a piston-like closing body3550, a ring-shaped closing body seat3551, a pressure limiting spring3552mounted between the closing body3550and a housing section, a cylindrical feed-in port3553fluidically connected to the pressure line347and situated on the end face of the closing body3550on the side opposite the pressure limiting spring3552, a drain hole3554leading to the tank38and situated laterally with respect to the closing body3550, and an air escape3555in the spring chamber of the closing body3550leading to the tank38.

In order to be able to utilize the advantages of the choke unit352also for the dimensioning of the pressure limiting valve355, the pressure limiting valve355is fluidically connected to a line section of the pressure line347leading to the actuator pressure chamber346, which is located between the choke unit352and the pressure chamber346. Here, the pressure limiting valve355inFIG.3—which is additional as compared toFIG.2—is an integral part of the actuator340, by way of example. The choke unit352acquired fromFIG.2, however, is an integral part of the electrohydraulic control unit35. A numerical example of a structural design of the pressure limiting valve355illustrates the aforementioned advantage: If the pressure limiting valve355is 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 spring3552at an effective diameter of the closing body seat3551of 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 piston341of the actuator340can be acted upon, can also result in damage of the actuator340, in particular in undesired wear at the mechanism of the interlocking device342when the interlocking device342is in the interlocked position, i.e., in the present example, wear at the pin344of the interlocking device342and at the piston rod groove, into which the pin344engages in position E of the hydraulic piston341. The obligatory component tolerances permit, namely also in the case of an interlocked pin344, a certain small axial movement of the hydraulic piston341, and so highly dynamic axial loads acting upon the hydraulic piston341, 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 device342against mechanical damage that can be caused by such pressure peaks and pressure drops in the supply pressure of the pressure chamber346of the actuator340, in the third exemplary embodiment of a device according to example aspects of the invention represented inFIG.4, which is based on the parking lock actuating system shown inFIG.2, the pressure line347leading to the pressure chamber346of the actuator340is fluidically connected to a hydraulic damper356. In order to be able to utilize the advantages of the choke unit352also for the dimensioning of the hydraulic damper356, this connection is located at a line section of the pressure line347situated between the choke unit352and the pressure chamber346. Here, the choke unit352acquired fromFIG.2and the hydraulic damper356, which is additional as compared toFIG.2, are integral parts of the electrohydraulic control unit35, by way of example, inFIG.4.

In the exemplary design shown inFIG.4, the hydraulic damper356is designed as a piston/spring damper. Here, the piston of the damper356is axially displaceably arranged in a bore of a housing of the electrohydraulic control unit35of the transmission (3), wherein the spring of the damper356preloads the piston of the damper356counter to the system pressure P_sys prevailing in the pressure line347. Correspondingly, the spring chamber of the damper356is vented to the tank38, 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 line347and the piston chamber of the damper356.

InFIG.4, the parking lock (34) (not represented here in greater detail) of the transmission (3) is in the engaged condition. Correspondingly, the hydraulic piston341of the parking lock actuator340is in the engagement position E and, in the engagement position E, is fixed in the axial direction by the pin344of the interlocking device342now form-lockingly engaging into a circumferential groove of the piston rod of the hydraulic piston341.

If the parking lock (34) is disengaged starting from the engaged condition, the pressure chamber346of the actuator340is acted upon by system pressure P_sys from the electromagnetically actuatable system pressure control valve351via the orifice353of the choke unit352and the pressure line347, and so, with the pin344of the interlocking device342released, the hydraulic piston341of the actuator340moves axially from the piston position E to the piston position A before the pin344once again fixes the hydraulic piston341in the axial direction. The fluidic connection, which is now provided, of the pressure line346leading to the actuator pressure chamber346at the hydraulic damper356effectively 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 device342. In other words, the hydraulic damper356in the pressure feed to the hydraulic piston341of the actuator340prevents excessive wear at the mechanical interlock of the hydraulic piston341.

If the parking lock (34) is engaged starting from the disengaged condition, the pressure chamber346of the actuator340is vented to the tank38via the pressure line347, the non-return valve354of the choke unit352, and the system pressure control valve351, and so, with the pin344of the interlocking device342released, the hydraulic piston341of the actuator340moves axially from the piston position A to the piston position E due to the spring force of the engagement spring345of the parking lock (34) before the pin344once again fixes the hydraulic position341in the axial direction.

In the following and with reference toFIG.5through7, three exemplary designs are explained in greater detail, in which a pressure limiting valve355as well as a hydraulic damper356are provided for protecting the actuator340. In these three exemplary designs, the pressure limiting valve355is integrated in the hydraulic damper356in 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 inFIG.4, the hydraulic damper356in all three of these exemplary designs includes a damper piston3561, which is preloaded by spring force of a damper spring3562counter 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 housing3560that is vented toward an interior space of the transmission (3). The maximum pressure level to be safeguarded by way of the pressure limiting valve355is higher, in any case, with respect to the absolute value thereof, than the pressure fluctuations and pressure peaks to be damped by the hydraulic damper356.

In the first exemplary design represented inFIG.6, the pressure limiting valve labeled with position number355is formed by an interaction of the spring force of the damper spring3562with a predefined leading-edge dimension3569, by which the damper piston3561must be displaced along the central axis in order to fluidically connect a feed-in port3567of the hydraulic damper356supplied with hydraulic fluid under system pressure P_sys to a drain hole3568of the hydraulic damper356leading to the interior space or to the tank (38) of the transmission (3).

Preferably, multiple lateral drain openings3568are 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 piston3561. The intermediate spaces between these contact surfaces can be utilized in a problem-free manner as lateral drain openings3568.

In the upper part ofFIG.5, the damper piston3561is 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 valve355has also not yet been reached, by far. In the lower part ofFIG.5, the system pressure P_sys applied at the feed-in port3567has exceeded the permissible maximum pressure, and so the damper piston3561releases a drainage edge, defined by the leading-edge dimension3569, of the drain hole3568arranged laterally with respect to the damper piston3561, 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 port3567is limited to the permissible maximum pressure.

In the exemplary design represented inFIG.5, the damper spring3562has 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 piston3561, whereas the second of these two springs then has steep spring characteristics designed for opening the drainage edge of the drain hole3568defined by the leading-edge dimension3569.

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 piston3561, whereas the second of these two springs then has steep spring characteristics designed for opening the drainage edge of the drain hole3568defined by the leading-edge dimension3569. 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 piston3561or at the base of the damper housing3560facing away from the feed-in port3567or, however, at the “soft” first spring. Provided the hydraulic damper356designed 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 damper356now operates as a pressure limiting valve355.

In the second exemplary design represented inFIG.6, the pressure limiting valve355is designed as a ball valve preloaded by a pressure limiting spring3552, which is integrated in the damper piston3561preloaded by spring force of the damper spring3562in such a way that the existing feed-in port3567of the hydraulic damper356, via which system pressure P_sys is applicable or applied at the damper piston3561, is fluidically connected, above a predefined system pressure level, to the drain hole3568of the hydraulic damper356leading to the interior space or to the tank (38) of the transmission (3). The damper piston3561forms a housing element for the pressure limiting valve355, which is displaceable within a housing3560of the hydraulic damper356. A feed-in port3553is provided in the damper piston3561, which is always fluidically connected to the feed-in port3567of the hydraulic damper356. On a back side, i.e., on the side facing away from the feed-in port3567, a closing body seat3551is formed for the closing body3550—designed as a ball in this case, by way of example, —of the pressure limiting valve355, against which the pressure limiting spring3552presses the closing body3550. The spring force of the pressure limiting spring3552is selected in such a way that the system pressure P_sys pushes the closing body3550out of the closing body seat3551once a maximum pressure predefined for the actuator (340) is exceeded and, as a result, enables the inflow of hydraulic fluid from the feed-in port3567via the feed-in port3553into a hollow chamber3565of the damping piston3561. The closing body3550and the pressure limiting spring3552of the pressure limiting valve355are located within the hollow chamber3565.

On the side facing away from the feed-in port3567, i.e., in the spring chamber of the damper spring3562, the hydraulic damper356or the damper housing3560is vented via a lateral drain hole3568to the transmission interior or to the tank (38). The hollow chamber3565of the damper piston3561also has a lateral drain hole3554. The damper piston3561itself is axially displaceably arranged, in a known way, in a bore of the housing3560of the hydraulic damper356. Since the pressure limiting spring3552is arranged, as viewed spatially, within the hollow chamber3565and the damper spring3562is arranged, as viewed spatially, above the hollow chamber3565of the damper piston3561, the damper spring3562and the pressure limiting spring3552are connected in series with respect to force.

In the upper part ofFIG.6, the damper piston3561is 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 valve355has also not yet been reached, by far. The spring characteristics of the damper spring3562preloading the damper piston3561are attuned to the pressure fluctuations and pressure peaks to be damped and are comparatively flat. The spring characteristics of the pressure limiting spring3552preloading the closing body3550, however, are attuned to the maximum pressure to be safeguarded and, therefore, are comparatively steep.

In the lower part ofFIG.6, the system pressure P_sys is at a level that is so high that the damper piston3561has compressed the damper spring3562to the fully compressed size, with the result that the hollow chamber3565of the damper piston3561is now fluidically connected via the drain hole3554of the hollow chamber3565to the drain hole3568of the damper housing3560and, as a result, is also vented to the transmission interior or to the tank (38). In addition, in the lower part ofFIG.6, the system pressure P_sys applied at the feed-in port3567has exceeded the permissible maximum pressure, and so the pressure limiting valve355is 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 port3567is limited to the permissible maximum pressure.

The third exemplary design represented inFIG.7is a technically simplified and installation space-saving variant of the pressure limiting valve355shown inFIG.6and is also integrated into the damper piston3561of the hydraulic damper356. The damper piston3561—similarly toFIG.6—is axially displaceably arranged in a bore of a housing3560of the hydraulic damper356, wherein system pressure P_sys is applicable or applied at the top-side end face of the damper piston3561via a feed-in port3567provided in the housing3560. 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 port3567, the damper piston3561forms a spring chamber for a damper spring3562, which preloads the damper piston3561with respect to the housing3560. The spring chamber is vented via a drain hole3568to the interior space or to the tank (38) of the transmission (3). As inFIG.6, the spring characteristics of the damper spring3562are attuned to the pressure fluctuations and pressure peaks to be damped.

In contrast toFIG.6, the pressure limiting valve355is now designed as a spring-loaded plate valve, which is arranged within a longitudinal bore3563of the damper piston3561. A pressure limiting spring3552provided for preloading the pressure limiting valve355is also arranged within the longitudinal bore3563and presses a closing body3550—which is now designed as a ring-shaped plate—of the pressure limiting valve355against a closing body seat3551—which is now flat—at the damper piston3561. The pressure limiting spring3552is supported via a disk3556at a securing or snap ring3558, which is placed into an annular groove3557of the longitudinal bore3563, at the damper piston3561, and so the closing body3550is axially mounted between the closing body seat3551and the annular groove3557. Viewed spatially, the pressure limiting spring3552is concentrically arranged within the damper spring3562, at least partially depending on the length of the guidance of the damper piston3561in the bore of the housing3560.

Hydraulic fluid is applied at the closing body3550of the pressure limiting valve355on the side opposite the pressure limiting spring3552via a feed-in port3553, which is provided in the damper piston3561and is constantly fluidically connected to the feed-in port3567provided in the housing3560of the hydraulic damper356. The spring characteristics of the pressure limiting spring3552are designed in such a way that the pressure limiting valve355opens as soon as the system pressure P_sys has exceeded a predefined maximum value. If the pressure limiting valve355is open, the longitudinal bore3563of the damper piston3561simultaneously functions as a drain hole for the excess hydraulic fluid resulting from overpressure, which is then discharged via the drain hole3568of the damper housing3560to the interior space or to the tank (38) of the transmission (3).

Contrary to the representation inFIG.7, it can also be provided that the plate-shaped closing body3550of the pressure-limiting valve355is laterally guided in the longitudinal bore3563of the damping piston3561. In this case, the closing body3550needs 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 port3553via the longitudinal bore3563to the drain hole3568when the pressure limiting valve355is open.

REFERENCE CHARACTERS

1motor vehicle2prime mover of the motor vehicle20crankshaft of the prime mover3transmission of the motor vehicle30starting component between the prime mover and the transmission31input shaft of the transmission32output shaft of the transmission33shift elements of the transmission34parking lock of the transmission340actuator of the parking lock341hydraulic piston of the actuator342interlocking device343electronic component of the interlocking device344pin of the interlocking device345engagement spring of the parking lock346pressure chamber of the actuator347pressure line to the pressure chamber35electrohydraulic control unit of the transmission350electromagnetically actuatable hydraulic valve of the electrohydraulic control unit for actuating a shift element351electromagnetically actuatable hydraulic valve of the electrohydraulic control unit for generating the system pressure352choke unit353orifice of the choke unit354non-return valve of the choke unit3540closing body of the non-return valve3541closing body seat of the non-return valve3542spring of the non-return valve355pressure limiting valve3550closing body of the pressure limiting valve3551closing body seat of the pressure limiting valve3552pressure limiting spring3553feed-in port of the pressure limiting valve3554drain hole of the pressure limiting valve3555air escape of the pressure limiting valve3556disk3557annular groove3558securing ring356hydraulic damper3560housing of the damper3561damper piston3562damper spring3563longitudinal bore in the damper piston3564hollow chamber in the damper piston3566drain hole of the hollow chamber3567feed-in port of the damper3568drain hole of the damper3569leading-edge dimension357hydraulic line36electronic control unit of the transmission37pump of the transmission38tank; oil sump4drive axle of the motor vehicle5operating unit for the transmissionA position of the hydraulic piston in the disengaged condition of the parking lockE position of the hydraulic piston in the engaged condition of the parking lockP_k clutch pressureP_p pump pressureP_sys system pressure