Hydraulic height adjustment device for a motor vehicle and motor vehicle

Abstract

This utility model relates to a hydraulic height adjustment device for a motor vehicle and the motor vehicle itself. The hydraulic height adjustment device has at least one hydraulic pump unit connected via an inlet branch to at least two actuators for adjusting the vehicle level. The pump unit pumps hydraulic fluid from at least one tank unit to at least one of the at least two actuators. At least one switching valve is connected upstream of each actuator to allow or prevent the inflow or outflow of hydraulic fluid. The at least two actuators are connected to at least one tank unit via a return branch. A pilot valve pressure-controlled return valve is provided, configured to keep the return branch open when the pressure is below a defined pilot valve pressure and to keep the return branch closed when the pressure is below or above the pilot valve pressure. A pilot valve pressure line is provided between the pump unit and the return valve, through which pilot valve pressure can be provided.

Description

Technical Field

[0001] This article describes a hydraulic height adjustment device for a motor vehicle and a motor vehicle. Background Technology

[0002] The hydraulic height adjustment device for motor vehicles of the type described at the beginning, as well as the motor vehicle itself, are known in the prior art.

[0003] Known pneumatic height adjustment devices have an electronically controlled return valve in the return branch. During lifting or inflation, an electrically operated valve assigned to a damping strut is opened, while the electronically controlled return valve is closed. To vent air or pressure from the air springs, the electronic return valve and the electrically operated valve assigned to the corresponding air spring are opened until the desired height level is reached. This principle also applies to hydraulic solutions.

[0004] Electronic switching valves are relatively expensive. The price increases particularly with valve size, which is primarily determined by the maximum flow rate. For a pure rear axle solution, the valve must therefore be designed large enough to handle the flow rates of both air springs simultaneously; while if both the front and rear axles are equipped with air springs, twice the size of the return piping and a correspondingly larger return valve are required to achieve the same descent speed.

[0005] KR 10 2024 009 3011 A proposes that the flow to the hydraulic actuators should not be switched on or off at all, so that all actuators are pressurized at the same time. Instead, the switching valve should be placed on the pressure relief side.

[0006] The drawback here is that, with pressurized inlet flow, all actuators must always be raised evenly until the last actuator reaches the target height. Subsequently, the other actuators, now over-leveled due to the principle, are then lowered to the target level. While this principle saves a switching valve, in solutions with four actuators or actuators on both the front and rear axles, the following situation frequently occurs: three actuators must be overfilled with fluid until the last one reaches the target level. Afterward, the first three actuators must be emptied, reducing comfort, slowing down the adjustment to the desired level, and unnecessarily wasting energy. Furthermore, due to the different inlet pipe lengths to the front and rear axle actuators, pressure losses in the pipes must be compensated; otherwise, due to the principle, actuators with lower pressure losses will always be overfilled first before the others move. Utility Model Content

[0007] Therefore, the objective of this invention is to improve the hydraulic height adjustment device and the vehicle of the type described at the beginning, so that an electronic switching valve can be omitted, while still serving both pure rear axle systems and multi-axle systems.

[0008] This task is accomplished using the hydraulic height adjustment device of the motor vehicle and the motor vehicle itself.

[0009] This document describes a hydraulic height adjustment device for a motor vehicle, comprising at least one hydraulic pump assembly connected via an inlet branch to at least two actuators for adjusting the vehicle level. The pump assembly pumps hydraulic fluid from at least one tank assembly to at least one of the at least two actuators. At least one switching valve is connected upstream of each actuator to allow or prevent the inflow or outflow of hydraulic fluid. The at least two actuators are connected to the at least one tank assembly via a return branch. A pilot valve pressure-controlled return valve is provided, configured to keep the return branch open below a defined pilot valve pressure and closed at or above the pilot valve pressure. A pilot valve pressure line is provided between the pump assembly and the return valve, through which pilot valve pressure is provided.

[0010] The pilot valve pressure-controlled reflux valve is used to open or close the reflux branch as needed. To open or close, the pressure of the pilot valve pressure line is used, wherein the reflux valve opens when the pilot valve pressure is lower than the switching pilot valve pressure and closes when the switching pilot valve pressure is at or above that pressure.

[0011] By using a pilot-operated pressure-controlled return valve, the use of another expensive electronic switching valve can be avoided, especially the kind of electronic switching valve that must have a large hydraulic fluid throughput, such as the kind required in multi-bridge solutions.

[0012] In this way, the hydraulic height adjustment device can be equipped on motor vehicles with one hydraulically adjustable rear axle or multiple hydraulically adjustable axles. While some components may need adjustment depending on the number of adjustable axles, the size design of the pilot-operated pressure-controlled return valve is more economical than that of a corresponding electronically controlled valve. This is particularly applicable because, for a pilot-operated pressure-controlled valve, the required switching force is not necessarily related to the cross-section to be switched.

[0013] The corresponding pilot valve pressure can be provided by a pump unit or by a separate pressure generating device specifically designed for operating the pilot valve pressure-controlled reflux valve, or it can be used for other purposes.

[0014] The oil tank device can be a single oil tank, multiple oil tanks, or one or more oil tanks with optional auxiliary oil tanks (such as compensation containers). The compensation container can compensate for fluctuations in the volume of hydraulic fluid, such as fluctuations caused by temperature changes or vehicle operation.

[0015] The pump assembly may have a motor and a hydraulic pump. The pump assembly is used to deliver hydraulic fluid to the at least two actuators at a sufficiently high pressure.

[0016] In a first further embodiment, a pressure damper is provided to reduce pressure peaks when pressure changes occur in the inlet branch of the hydraulic system.

[0017] In another further embodiment, a pre-pressure valve is connected in the upstream inlet branch of the at least two actuators, which releases the fluid flow to the at least two actuators only when a minimum pressure is exceeded, wherein the pilot valve pressure line is arranged between the pump unit and the pre-pressure valve on one side and the return valve on the other side.

[0018] In this way, pressure can be established in the hydraulic fluid between the pump unit and the pre-pressure valve.

[0019] The preload valve can be a spring-loaded valve, where various spring types can be considered, such as helical springs or disc springs, which can be appropriately arranged tension or compression springs. In some embodiments, the spring force can be adjusted, for example, by adjusting the spring preload.

[0020] In another further embodiment, a bypass pipe is provided for reducing the pressure in the pilot valve pressure line after the at least one hydraulic pump device is shut down.

[0021] This is used to reduce the line pressure between the pump unit and the pilot-valve pressure-controlled return valve over time, thereby causing the pilot-valve pressure-controlled return valve to reopen.

[0022] In another further embodiment, the bypass pipe is formed in the return valve, establishing a fluid connection between the pilot valve pressure line and the return branch.

[0023] In this way, a separate bypass pipe can be eliminated.

[0024] In another further embodiment, the bypass pipe is configured as a groove or hole in the valve body.

[0025] This saves on additional parts and / or manufacturing steps.

[0026] In some embodiments, the bypass pipe may also be formed in the valve body or valve seat, etc.

[0027] In another further embodiment, the bypass pipe establishes a fluid connection between the inlet branch between the pump unit and the pre-pressure valve and the return branch between the return valve and the tank unit.

[0028] In another further embodiment, the reflux valve has a spring-loaded valve body that is held in the open position by the spring, wherein the valve body is brought to the closed position by switching pressure.

[0029] At this point, various types of springs can be considered, such as helical springs, which can be appropriately arranged tension or compression springs. In some embodiments, the spring force can be adjusted, for example, by adjusting the spring preload.

[0030] In another further embodiment, the valve body is a valve ball, a valve push rod, or a valve cylinder.

[0031] In another further embodiment, an overpressure line is provided between the pump unit and the return branch, which is locked by an overpressure valve during normal operation.

[0032] In another further embodiment, each of the at least two actuators includes at least one fluid chamber, which is at least partially defined by at least one elastomeric telescopic shell with rolling pleats, wherein the hydraulic system is configured such that when one or more of the at least two actuators are retracted, the entire flow of hydraulic fluid flows together into the return branch through the one return valve.

[0033] Compared to other systems, telescopic shell systems tend to require high volumetric flow rates at low pressures, while in piston-based structures, for example, the pressure tends to be higher, but the volumetric flow rate is smaller. In this context, valves with pilot-operated pressure control are particularly advantageous because the volumetric flow rate to be switched and the required switching force are not necessarily coupled. This is because the switching force is typically not in the same direction as the closing force, but rather transverse to it, and therefore independent of the closing force. However, in electronically switched valves, the closing force is typically arranged in the same direction as the switching force, thus a large cross-section to be switched results in a large switching force, and therefore requires large, expensive components, such as large copper coils.

[0034] The first independent topic concerns a motor vehicle with a hydraulic height adjustment device of the type previously described. Attached Figure Description

[0035] Other features, details, and advantages of this invention will become apparent from the wording of the claims and the following description of embodiments based on the accompanying drawings. The drawings schematically illustrate: Figure 1 A top view of a motor vehicle equipped with a hydraulic height adjustment device; Figure 2Switch diagram for a hydraulic height adjustment device used in a single-axle system; Figure 3 From Figure 1 A diagram of the switch for a hydraulic height adjustment device used in a dual-bridge system; Figure 4 The pilot valve pressure-controlled return valve in the first embodiment; and Figure 5 The second embodiment is a pilot valve pressure-controlled reflux valve.

[0036] List of reference numerals 2 Motor vehicles 4 Rear Axle 6. Front axle 8.1 First Chassis Assembly 8.2 Second Chassis Assembly 8.3 Third Chassis Assembly 8.4 Fourth Chassis Unit 10.1 First Wheel 10.2 Second Wheel 10.3 Third wheel 10.4 Fourth Wheel 12 First hydraulic height adjustment device 12' Second hydraulic height adjustment device 14 Control device 16.1 First Supply Pipeline 16.2 Second Supply Line 16.1 Third Supply Line 16.1 Fourth Supply Line 18.1 First Actuator 18.2 Second Actuator 18.3 Third Actuator 18.4 Fourth Actuator 20.1 First Fluid Chamber 20.2 Second Fluid Chamber 20.3 Third Fluid Chamber 20.4 Fourth Fluid Chamber 22.1 First Solenoid Valve 22.2 Second Solenoid Valve 22.3 Third Solenoid Valve 22.4 Fourth Solenoid Valve 24.1 First Horizontal Sensor 24.2 Second Horizontal Sensor 24.3 Third-level sensor 24.4 Fourth Level Sensor 26 First Distributor 26.1 Second Distributor 30 Inflow Branches 32 Pump Unit 32.1 Motor 32.2 Pump 34 fuel tank 36 Hydraulic fluid 38 Pre-pressure valve 40 Return Branches 42 Pilot valve pressure control of the first return valve 42' Pilot valve pressure controlled second return valve 44 Pilot valve pressure line 46 Bypass pipe 48 Overpressure piping 50 Overpressure Valve 52 Pressure Buffer 58 First shell 58' Second shell 60 Valve push rod 62 Valve body 63 Valve seat 64 First Spring 64' Second Spring 70 Valve Ball 72 stops pP pilot valve pressure pS switch pilot valve pressure Detailed Implementation

[0037] In the embodiments described below, for better readability, the same or equivalent parts or elements are referred to by the same reference numerals.

[0038] Figure 1 A top view of a motor vehicle 2 is shown.

[0039] The motor vehicle 2 has a rear axle 4 and a front axle 6. The motor vehicle 2 has a chassis with a first chassis assembly 8.1, a second chassis assembly 8.2, a third chassis assembly 8.3, and a fourth chassis assembly 8.4, wherein the first chassis assembly 8.1 is used for the left rear wheel 10.1, the second chassis assembly 8.2 is used for the right rear wheel 10.2, the third chassis assembly 8.3 is used for the left front wheel 10.3, and the fourth chassis assembly 8.4 is used for the right front wheel 10.4. Each chassis assembly can be height-adjusted by means of a first hydraulic height adjustment device 12.

[0040] The first hydraulic height adjustment device 12 has a control device 14, which has both control logic and necessary fluid delivery components. In different embodiments, the control device 14 may be arranged in one or more housings. The control device 14 is connected to first actuators 18.1, second actuators 18.2, third actuators 18.3, and fourth actuators 18.4 via first supply lines 16.1, second supply lines 16.2, third supply lines 16.3, and fourth supply lines 16.4. These actuators are respectively arranged on or incorporated into the first chassis assembly 8.1, second chassis assembly 8.2, third chassis assembly 8.3, and fourth chassis assembly 8.4.

[0041] The components of the first hydraulic height adjustment device 12, and in particular the components of the control device 14, will be described in more detail in the following description of the drawings. By means of the first hydraulic height adjustment device 12, the vehicle level of the motor vehicle 2 can be raised or lowered individually on each of the individual first wheels 10.1 to fourth wheels 10.4 to raise or lower the level of one or both of the rear axle 4 and the front axle 6, or to compensate on individual first wheels 10.1 to fourth wheels 10.4 when needed, for example, in cases of different loads due to loading or passenger distribution.

[0042] Figure 2 A switch diagram of a second hydraulic height adjustment device 12' for a single-axle system (typically the rear axle 4 of a motor vehicle) is shown.

[0043] The first actuator 18.1 and the second actuator 18.2 each have a first fluid chamber 20.1 and a second fluid chamber 20.2, which can be filled with hydraulic fluid and discharged from it. When filled with hydraulic fluid, the volume of the first fluid chamber 20.1 and the second fluid chamber 20.2 increases accordingly, thereby lifting the corresponding first chassis device 8.1 and second chassis device 8.2.

[0044] To control the filling of the first fluid chamber 20.1 and the second fluid chamber 20.2, a first solenoid valve 22.1 and a second solenoid valve 22.2 are respectively provided, which open or close based on data from their respective first level sensors 24.1 and 24.2. The respective first level sensors 24.1 and 24.2 send their data to a central controller (not shown), which determines whether the first wheel 10.1 and the second wheel 10.2 of each corresponding piece of equipment should be raised or lowered. This can be automatically adjusted and / or influenced by user commands, among other things.

[0045] The first actuator 18.1 and the second actuator 18.2 are in fluid communication with an inlet branch 30 via pipes 16.1 and 16.2 through a first distributor 26. The inlet branch can pump hydraulic fluid 36 from a tank 34 toward the first actuator 18.1 and the second actuator 18.2 by means of a pump device 32.

[0046] The pump unit 32 has a motor 32.1 and a pump 32.2, which can generate hydraulic pressure with hydraulic fluid 36, so that the corresponding first fluid chamber 20.1 and second fluid chamber 20.2 can be filled when the level of the bridge 4 needs to be raised.

[0047] A pre-pressure valve 38 is also arranged in the inlet branch 30. The pre-pressure valve 38 keeps the inlet branch 30 closed until a certain minimum pressure or switching pressure is reached, at which point the pre-pressure valve 38 opens. Below this minimum pressure, it is impossible to deliver hydraulic fluid 36 to the first actuator 18.1, the second actuator 18.2, or the first fluid chamber 20.1 and the second fluid chamber 20.2.

[0048] The first actuator 18.1 and the second actuator 18.2 are also connected to a return branch 40 via pipelines 16.1 and 16.2, through which hydraulic fluid 36 can be returned to the oil tank 34.

[0049] A pilot-valve pressure-controlled first return valve 42 is arranged in the return branch 40, which will be described in more detail below. If the level of the corresponding height-adjustable rear axle 4 or one of its first wheels 10.1 and second wheels 10.2 is to be lowered, the corresponding first solenoid valve 22.1 and / or second solenoid valve 22.2 are opened. With the pilot-valve pressure-controlled first return valve 42 open, hydraulic fluid 36 is forced into the oil tank 34 through the return branch 40 by the gravity of the motor vehicle.

[0050] The fuel tank 34 can be a single fuel tank or a multi-part fuel tank with a compensation container.

[0051] The inlet branch 30 and the return branch 40 transition to each other at the first distributor 26 or the second distributor 26.1. In some embodiments, the first distributor 26 and the second distributor 26.1 may be combined into a single component, or as shown here, at the two distribution points, the pipe section between the intersection points may be used as either inlet or return flow depending on the operating mode.

[0052] In the inlet branch 30, a branch is provided between the pump unit 32 and the pre-pressure valve 38, from which a pilot valve pressure line 44 leads to a pilot valve pressure-controlled first return valve 42. A pilot valve pressure pP is provided through the pilot valve pressure line 44. This pilot valve pressure-controlled first return valve 42 is configured such that it is either locked or opened according to the corresponding pilot valve pressure pP. At this time, the pilot valve pressure-controlled first return valve 42 is configured such that when the pressure is lower than the switching pilot valve pressure pS (see...) Figure 5 When the pressure of the pilot valve is pS or above, the first return valve 42 controlled by the pilot valve pressure switches to the locked state through the pilot valve pressure line 44, thereby closing the return line 40 and preventing the hydraulic fluid 36 from flowing into the oil tank 34.

[0053] The switching pressure of the pre-pressure valve 38 is higher than the switching pilot valve pressure pS, so that the pre-pressure valve 38 only opens when the pressure is high enough to keep the first return valve 42 closed under pilot valve pressure control.

[0054] Since the inlet branch 30 is locked by the pre-pressure valve 38 before sufficient pressure is generated, the pump assembly 32 can first establish and provide at least the pilot valve pressure pS, thereby initially closing the pilot valve pressure-controlled first return valve 42. Upon further pressure increase, the pre-pressure valve 38 opens, and hydraulic fluid 36 can be delivered through the inlet branch 30 to the first actuator 18.1 and / or the second actuator 18.2. In this state, the pressure at the pilot valve pressure-controlled first return valve 42 remains higher than the pilot valve pressure pS. Therefore, the pilot valve pressure-controlled first return valve 42 remains closed, preventing significant pressure loss when the first fluid chamber 20.1 and the second fluid chamber 20.2 are filled with hydraulic fluid 36.

[0055] The pump unit 32 remains active until the corresponding chassis level is reached at the first wheel 10.1 and the second wheel 10.2. Once this state is reached, the first solenoid valve 22.1 and the second solenoid valve 22.2 close, and the volumes of the corresponding first fluid chamber 20.1 and second fluid chamber 20.2 are determined.

[0056] To balance the overpressure established in the inlet branch 30 due to the pumping of hydraulic fluid 36 after operating the pump unit 32, a bypass pipe 46 is provided through which the corresponding overpressure between the pump unit 32 and the first actuator 18.1 and the second actuator 18.2 can be discharged. Then, the pilot valve pressure pP drops below the switching pilot valve pressure pS, and the first return valve 42 controlled by the pilot valve pressure enters the open state.

[0057] To lower the level of bridge 4, the first solenoid valve 22.1 and the second solenoid valve 22.2 are opened, allowing hydraulic fluid 36 to flow from their respective first fluid chamber 20.1 and second fluid chamber 20.2 and be guided into the oil tank 34. Once the desired level is reached, the respective first solenoid valve 22.1 and second solenoid valve 22.2 are closed. This can be controlled individually for each first wheel 10.1 and second wheel 10.2 via the first solenoid valve 22.1 and second solenoid valve 22.2, just as when raising the level. If the level of one first wheel 10.1 or second wheel 10.2 needs to be raised or lowered more or less, the corresponding solenoid valve remains open for a correspondingly longer or shorter time.

[0058] To prevent damage, an overpressure protection device is provided between the inlet branch 30 and the return branch 40, which takes the form of an overpressure line 48 and an overpressure valve 50. The overpressure valve 50 opens when the pressure exceeds the normal operating parameters of the first hydraulic height adjustment device 12 but remains below the possible burst limit of the corresponding line, valve, and actuator. This is particularly important for actuators with an elastomer telescopic housing and at least one rolling pleat, as they have relatively low burst pressures. This prevents damage to the corresponding first hydraulic height adjustment device 12.

[0059] Figure 3 Showing from Figure 1 A diagram showing the switch for a hydraulic height adjustment device used in a dual-axle system. To avoid duplication, refer to... Figure 2 The corresponding description of the second hydraulic height adjustment device 12' in the text.

[0060] The third actuator 18.3 and the fourth actuator 18.4 on the front axle 6 have the same construction as the first actuator 18.1 and the second actuator 18.2 on the rear axle 4, therefore detailed descriptions are omitted, and reference is made to... Figure 2 The corresponding principle is described in the text. However, the dimensions of the first actuator 18.1, the second actuator 18.2, the third actuator 18.3, and the fourth actuator 18.4 may differ in practice to accommodate different lifting and load requirements, such as in commercial vehicles with high payloads on one of the rear axle 4 or the front axle 6.

[0061] and Figure 2 Unlike other places, this place has only one intersection point where the inflow branch 30 and the return branch 40 transition to each other.

[0062] By selectively locking or opening the respective first solenoid valves 22.1 to the fourth solenoid valves 22.4, the level of each individual first wheel 10.1 to the fourth wheel 10.4 can be changed, for example, raised or lowered.

[0063] In addition, a pressure damper 52 is provided to prevent pressure spikes in the hydraulic system during switching. Such pressure spikes may occur, for example, when the pre-pressure valve 38 or the pilot valve pressure-controlled first return valve 42 switches on or off. Figure 2 A corresponding pressure buffer can also be installed in the second hydraulic height adjustment device 12'.

[0064] In the first hydraulic height adjustment device 12, no separate overpressure protection device is shown. However, in different implementation variations, it can be added accordingly or designed in various other forms, such as on the first return valve 42 controlled by the pump 32.2 or the pilot valve pressure.

[0065] Figure 4 A first reflux valve 42 with pilot valve pressure control in the first embodiment is shown.

[0066] The pilot valve pressure-controlled first return valve 42 has a first housing 58 in which a valve push rod 60 is axially movable. The valve push rod 60 moves a valve body 62 connected thereto, which, together with a valve seat 63, enables the opening or closing of the return branch 40.

[0067] The valve push rod 60 is preloaded by a tension spring 64 in the opposite direction to the closing direction, so that when there is no pilot valve pressure pP, the corresponding pilot valve pressure-controlled first return valve 42 is open. Only when the pilot valve pressure pP reaches the switching pilot valve pressure pS is the tension spring 64 fully stretched, and the valve body 62 is sealed in the valve seat 63, thereby closing the return branch 40.

[0068] In an alternative embodiment, a compression spring can be arranged instead of the tension first spring 64, which acts in the opposite direction and preloads the valve push rod 60 and the corresponding valve body 62 to the open position.

[0069] Figure 5 A second reflux valve 42' with pilot valve pressure control is shown in the second embodiment.

[0070] The pilot valve pressure controls the second return valve 42'. Figure 4 The first return valve 42, controlled by the pilot valve pressure, has a higher degree of integration. Therefore, the inlet branch 30 and the return branch 40 each pass through the corresponding second housing 58' of the second return valve 42', which is controlled by the pilot valve pressure. The pilot valve pressure line 44 is also integrated into the second housing 58'. This second housing 58' may be cast.

[0071] The valve body uses a valve ball 70, which is pressed against a stop 72 by a compression second spring 64' at a low pilot valve pressure pP. Only when the pilot valve pressure pP is greater than the switching pilot valve pressure pS is the second spring 64' fully compressed, causing the return branch 40 to be locked by the valve ball 70. When the pilot valve pressure pP drops again, the valve ball 70 releases the return branch 40 again.

[0072] In this embodiment, a bypass pipe can also be integrated, for example in the form of a groove, which can be formed separately or between the pilot valve pressure line 44 and the return branch 40.

[0073] Alternatively, in this implementation method and Figure 4 In the embodiments described herein, a groove may be formed in the respective valve body 62; valve ball 70 to prevent a perfect seal from being achieved on the respective valve seat 63.

[0074] This utility model is not limited to one of the aforementioned embodiments, but can be modified in many ways.

[0075] All features and advantages arising from the specification and drawings, including structural details, spatial arrangements and method steps, may exist individually or in various combinations and are essential to this invention.

Claims

1. A hydraulic height adjustment device for a motor vehicle, characterized in that, The hydraulic height adjustment device includes at least one hydraulic pump unit, which is connected via an inlet branch to at least two actuators for adjusting the vehicle level of the motor vehicle. The at least one hydraulic pump unit can pump hydraulic fluid from at least one tank unit to at least one of the at least two actuators. At least one switching valve is connected upstream of each of the at least two actuators to allow or prevent the inflow or outflow of hydraulic fluid. The at least two actuators are connected to the at least one tank unit via a return branch. A pilot valve pressure-controlled return valve is provided, configured to keep the return branch open when the pressure is below a defined pilot valve pressure, and to keep the return branch closed when the pilot valve pressure is present or above the pilot valve pressure. A pilot valve pressure line is provided between the at least one hydraulic pump unit and the return valve, through which pilot valve pressure can be provided.

2. The hydraulic height adjustment device for a motor vehicle according to claim 1, characterized in that, A pressure damper is provided to reduce the pressure peak when the pressure changes in the inlet branch of the hydraulic system.

3. The hydraulic height adjustment device for a motor vehicle according to claim 1 or 2, characterized in that, A pre-pressure valve is connected upstream of the at least two actuators in the inlet branch. The pre-pressure valve releases fluid flow to the at least two actuators only when the minimum pressure is exceeded. The pilot valve pressure line is arranged between the at least one hydraulic pump unit and the pre-pressure valve on one side and the return valve on the other side.

4. The hydraulic height adjustment device for a motor vehicle according to claim 3, characterized in that, A bypass pipe is provided for reducing pressure after the at least one hydraulic pump unit is shut down.

5. The hydraulic height adjustment device for a motor vehicle according to claim 4, characterized in that, The bypass pipe is formed in the return valve, and establishes a fluid connection between the pilot valve pressure line and the return branch.

6. The hydraulic height adjustment device for a motor vehicle according to claim 5, characterized in that, The bypass pipe is configured as a groove or hole in the valve body.

7. The hydraulic height adjustment device for a motor vehicle according to any one of claims 4 to 6, characterized in that, The bypass pipe establishes a fluid connection between the inlet branch between the at least one hydraulic pump unit and the pre-pressure valve on one hand, and the return branch between the return valve and the tank unit on the other hand.

8. The hydraulic height adjustment device for a motor vehicle according to claim 1, wherein, The reflux valve has a spring-loaded valve body that is held in the open position by the spring, wherein the valve body is brought to the closed position by switching pressure.

9. The hydraulic height adjustment device for a motor vehicle according to claim 8, characterized in that, The valve body is a valve ball, valve push rod, or valve cylinder.

10. The hydraulic height adjustment device for a motor vehicle according to claim 1, characterized in that, An overpressure line is provided between the at least one hydraulic pump unit and the return branch, which is locked by an overpressure valve during normal operation.

11. The hydraulic height adjustment device for a motor vehicle according to claim 2, characterized in that, Each of the at least two actuators includes at least one fluid chamber, the at least one fluid chamber being at least partially defined by at least one elastomeric telescopic shell with rolling folds, wherein the hydraulic system is configured such that when one or more of the at least two actuators are retracted, the entire flow of hydraulic fluid flows together into the return branch through the return valve.

12. A motor vehicle, characterized in that, The motor vehicle has a hydraulic height adjustment device according to any one of claims 1 to 11.

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