Hydraulic device with overflow channel and lifting equipment

Abstract

The invention relates to a hydraulic device (1) having a cylinder (3) and a piston (2) guided in the cylinder (3), which piston (2) can be displaced between a working position and an end position, the piston (2) in the working position sealingly dividing the cylinder chamber into a first subspace (5) and a second subspace (6), and the piston (2) in the end position of the piston (3) in which the piston (2) is arranged in the first subspace (5) and the second subspace (6). The first subspace (5) and the second subspace (6) are fluidically connected to each other via an overflow channel (13). According to the invention, a flow path (14) is formed in the piston (2), said flow path (14) extending between the end side of the piston (2) and the outer circumferential side of the piston (2), and in the end position of the piston, the overflow channel (13) is at least partially formed by the flow path (14).

Description

[0001] The invention relates to a hydraulic device according to the preamble of claim 1 and to a lifting device according to the preamble of claim 14 .

[0002] Hydraulic devices of the type described above can be used to move heavy objects. For this purpose, the hydraulic device usually has a cylinder, inside which a piston is movable. In the working position, the piston seals the cylinder chamber into two subchambers. If hydraulic fluid is supplied via an inlet into one of the subchambers, the piston is hydraulically adjusted. Via a piston rod connected to the piston, the adjustment movement of the piston can be mechanically transmitted to the load to be moved.

[0003] It is common to operate several hydraulic devices simultaneously, and usually synchronization must be ensured between these hydraulic devices. This means that the pistons of the hydraulic devices can be brought to the same position, in particular their end position, when necessary. This should prevent the load being lifted at several points of force application from becoming unstable.

[0004] It is also known that when operating a plurality of hydraulic devices, the plurality of hydraulic devices are operated in pairs in separate hydraulic circuits in order to achieve a high level of reliability. Here, two pairs of hydraulic devices can be provided, the pistons of a first pair of hydraulic devices and the pistons of a second pair of hydraulic devices being mechanically coupled to each other. In addition, each hydraulic device in the first pair of hydraulic devices is hydraulically coupled to one hydraulic device in the second pair of hydraulic devices. In the event of a functional failure in one of the hydraulic circuits, the lifted load can be maintained by the system pressure of the corresponding other hydraulic circuit and by the mechanical coupling of the pistons, without the load falling. For example, DE 10 2014 113 301 A1 shows a hydraulic circuit diagram of a lifting platform having two pairs of hydraulic devices that are mechanically coupled and connected via a cross pipe.

[0005] However, wear phenomena occurring on seals or other components of the hydraulic devices can lead to hydraulic fluid escaping from one of the hydraulic devices due to leakage. In addition, the hydraulic devices can expand to different degrees due to temperature differences. This can lead to a change in the amount of hydraulic fluid required to bring the pistons of several hydraulic devices synchronously to their respective end positions.

[0006] Furthermore, in the above-described arrangement with two pairs of hydraulic devices for achieving a high level of reliability, pressure differences may occur between the hydraulically coupled hydraulic devices. In order to avoid mechanical tension between the mechanically coupled hydraulic devices, it is desirable to implement pressure compensation.

[0007] A method for achieving the required synchronization and pressure balance between a plurality of hydraulic devices consists in connecting a first hydraulic device in a manner known per se to a hydraulic fluid source, via which the necessary amount of hydraulic fluid can always be provided in order to be able to bring the piston of the first hydraulic device to its end position. Here, the first hydraulic device is designed so that the piston of the first hydraulic device releases an overflow channel in the end position of the piston, via which two subspaces of the first hydraulic device are fluidically connected to each other. That is, via this overflow channel, hydraulic fluid can enter the second subspace from the first subspace of the first hydraulic device, and then the hydraulic fluid can be conveyed from the second subspace to the second hydraulic device via the overflow line, in order to also bring the piston of the second hydraulic device to its end position. In this way, synchronization can be achieved between two hydraulic devices having a small number of hydraulic components and, in particular, only one hydraulic fluid source. In addition, pressure balance can be achieved between the first and second hydraulic devices via the overflow channel.

[0008] An embodiment of an overflow channel is known from EP 2 428 482 A1. For this purpose, the inner side of the cylinder has a recess which is formed on the end side of the inner side of the cylinder. If the piston is in the working position, the recess is completely located in one subspace of the cylinder. If the piston reaches its end position in the region of the recess, a fluid-conducting connection is established inside the cylinder between the two subspaces of the cylinder of the hydraulic device.

[0009] A disadvantage of this previously known embodiment of the overflow channel is that in the end position of the piston, the hydraulic fluid must flow over the entire piston height past the outside of the piston in order to pass from one subspace of the cylinder into the other subspace. In this case, in particular sealing elements which may be arranged on the outside of the piston are subject to high loads. This is accompanied by high wear and thus places high demands on maintenance and repair of the entire hydraulic device.

[0010] It is therefore an object of the present invention to provide a hydraulic device with an overflow function which requires less maintenance.

[0011] This object is achieved by the features of the subject matter of claim 1. This object is further achieved by a lifting device according to claim 14. Advantageous embodiments are the subject matter of the respective dependent claims.

[0012] The hydraulic device according to the invention comprises, in a manner known per se, a cylinder and a piston guided in the cylinder. The piston can be moved between a working position and an end position. In the working position, the piston seals the cylinder chamber into a first subspace and a second subspace. In the end position of the piston, the first subspace and the second subspace are connected to one another in a fluid-conducting manner via an overflow channel.

[0013] It is essential for the invention that a flow path is formed in the piston, which flow path extends between an end side and a circumferential side of the piston and that in the end position of the piston the transfer channel is at least partially formed by this flow path.

[0014] The hydraulic device according to the invention has advantages in terms of its service life and the required maintenance measures. This is because such an embodiment of the flow path according to the invention allows the hydraulic fluid flow to be guided in the form of a bypass line between two sub-spaces of the hydraulic device, while damaging contact with functionally critical components on the outer circumference of the piston can be avoided. For example, at least one piston seal or guide ring is arranged on the outer circumference between the opening on the end side of the flow path and the opening on the outer circumference of the flow path relative to the longitudinal axis of the piston.

[0015] According to the invention, the flow path is formed in the interior of the piston and is at least partially completely surrounded by the piston. In addition, the invention is not limited to a specific direction or cross-sectional geometry of the flow path. In a simple embodiment, the flow path can extend from the end side of the piston to the peripheral side of the piston in the configuration of a hole extending in a straight line. Alternatively, the flow path in the piston can include two blind holes, which intersect in the region of their hole bottoms and are thus connected to each other in a fluid-conducting manner. Here, one of the blind holes is arranged on the end side, and the corresponding other blind hole is arranged on the peripheral side of the piston.

[0016] In a simple embodiment, as is known per se, the piston can be guided in a sealed manner inside the cylinder in the working position. Here, one end of the flow path opens into one of the two subspaces of the cylinder on the end side of the piston. The other end of the flow path points to the inner wall of the cylinder on the outer peripheral side of the piston, on which the piston is guided in a sealed manner. If the piston is subjected to hydraulic pressure on its end side formed with the flow path in this working position, the piston is displaced in a manner known per se until its end position. When the end position is reached, the flow path directly opens into the second subspace on the outer peripheral side of the piston. Therefore, it is within the scope of the invention that, in the end position of the piston, the overflow channel is formed only by the flow path. However, in another embodiment, a further fluid path is provided, which also forms a component of the overflow channel and forms a fluid-conducting connection between the two subspaces via the fluid path.

[0017] For the purposes of the present invention, the end position of the piston is not limited to a single position of the piston inside the cylinder, but also includes the region of the piston close to this position. It is therefore within the scope of the present invention that the overflow channel is not only released when the piston reaches its end position, but also when the piston is in the region of the end position. Furthermore, the present invention does not impose any restrictions on how many end positions the piston can occupy inside the hydraulic device. Instead, the end position of the piston can be defined by any mechanical stop inside the cylinder, such as, for example, a cylinder cover, a cylinder bottom or a stop element which, for example, projects radially into the cylinder cavity.

[0018] The invention is also not limited to the structural form of the hydraulic device, its cylinder or piston. It is therefore within the scope of the invention that the hydraulic device is designed as a so-called synchronous cylinder or as a so-called differential cylinder. The piston preferably has a cylindrical basic shape. In addition, the piston can have a circumferential profile on its outer circumference, and the profile can be used for the arrangement of sealing elements.

[0019] In an advantageous development, the flow path has a flow resistance in order to reduce the pressure of the hydraulic fluid flowing in the flow path.

[0020] By means of the above-described improvement, a sealing element can be arranged on the outer peripheral side of the piston, and the overflowing hydraulic fluid is not guided around the sealing element via the flow path, but comes into contact with the sealing element. The applicant's research work has shown that here, when the hydraulic pressure of the hydraulic fluid is reduced beforehand, the damaging effects of the overflowing hydraulic fluid can be reduced or completely avoided. In this case, the flow path inside the piston can not only be used to realize an embodiment of the overflow channel, but can also be used to set the flow resistance for reducing the pressure. The pressure reduction of the hydraulic fluid specifically includes reducing its static and / or its dynamic pressure.

[0021] In an advantageous development, the piston has at least one sealing element on the outer circumference, which is arranged axially offset relative to the opening of the flow path on the outer circumference of the piston and partially defines the overflow channel in the end position of the piston. In this case, the sealing element is arranged downstream of the flow resistance in the flow direction of the hydraulic fluid flowing in the overflow channel.

[0022] The applicant's research work has shown that embodiments of the flow path with a flow resistance are particularly advantageous when the overflowing hydraulic fluid flows through a sealing element arranged on the outer circumference of the piston and reduces the pressure of the hydraulic fluid by means of the flow resistance. This has a particularly positive effect on the service life of the sealing element and the operational reliability of the entire hydraulic device. In particular, in the end position of the piston, the sealing element is in direct contact with the overflowing hydraulic fluid. In particular, the sealing element is designed to seal the cylinder chamber into the first subspace and the second subspace in the working position of the piston.

[0023] The flow resistance can be adjusted, for example, based on the geometrical features of the flow path and / or the surface characteristics of the flow path. Adjustable geometrical features include, for example, the length of the flow path through which the flow is passed and / or the curvature and / or the macroscopic profile of the flow path. Adjustable surface characteristics include, for example, the microscopic profile. The flow resistance can be configured as an integral component of the flow path or as a separately / differentiatedly designed component that can be arranged interchangeably within the flow resistance.

[0024] In a preferred embodiment of the hydraulic device, the flow resistance is an orifice plate or a throttle valve. The pressure of the flowing hydraulic fluid can be reduced by means of the orifice plate and a constant volume flow of the hydraulic fluid can be set independently of the system pressure. The pressure of the hydraulic fluid can also be reduced by means of a throttle valve, but the adjustable volume flow is usually linearly related to the system pressure. Thus, the dynamic pressure of the hydraulic flow can be reduced to a constant value at least by means of the orifice plate, while the dynamic pressure of the hydraulic flow can also be adjusted by means of the orifice plate as a function of the system pressure. Although the orifice plate and the throttle valve have different functional principles, both can be used to reduce the pressure of the hydraulic fluid in an advantageous manner, so that in particular the sealing elements arranged on the outer periphery of the piston are protected when they come into contact with the overflowing hydraulic fluid.

[0025] The orifice plate and the throttle valve can be standardized hydraulic components, respectively, whose pressure reduction effect can be determined according to common design rules. In this case, it is also within the scope of this advantageous improvement that the orifice plate and the throttle valve can each be arranged in the flow path as an integral component or as a separately / differentiated designed component. The advantage that can be achieved by the integral design is that subsequent installation steps are reduced, in which the orifice plate or the throttle valve must be assembled with the piston. Alternatively, the orifice plate and the throttle valve can be installed in the piston in the form of an insert.

[0026] In a preferred embodiment of the hydraulic device, the flow path leads to an annular channel on the outer peripheral surface of the piston.

[0027] The embodiment of the annular channel allows the hydraulic fluid flowing out of the flow path to be distributed partially or completely around the piston. This is particularly advantageous because an overflow of the hydraulic fluid can take place independently of the rotational position of the piston. This is particularly difficult if, in the end position, the flow path has to open into a fluid path on the peripheral side of the piston to form the overflow channel. The annular channel thus makes it possible to mount the piston in any rotational position relative to the longitudinal axis of the cylinder.

[0028] In a preferred embodiment of the hydraulic device, a recess is formed as a longitudinal groove on the inner wall of the cylinder, into which the flow path formed in the piston opens in the end position, so that in the end position of the piston, the recess forms part of the overflow channel. The recess can extend at least partially parallel to the longitudinal axis of the cylinder. However, it is also within the scope of this preferred embodiment that the recess extends at an angle to the longitudinal axis of the cylinder and / or extends in a curved shape.

[0029] The position and length of the recess can be used to determine in a structurally simple manner how large the spatial range is to be in which overflow occurs. The longer the groove length, the deeper the longitudinal groove reaches into the cylinder interior and the earlier the overflow of hydraulic fluid occurs when the piston approaches its end position. In addition, the damping properties of the hydraulic device are also positively influenced when the end position is reached, since a sudden stop of the piston in its end position due to a mechanical stop can be avoided.

[0030] In an advantageous development, in the end position of the piston, the sealing element on the outer circumference of the piston is arranged in the region of the recess along the longitudinal axis of the cylinder. Overflowing hydraulic fluid can pass through the piston and out of the flow path formed in the piston and come into contact with the sealing element, in particular with reduced pressure, without damaging the sealing element. The sealing element is preferably arranged in the region of the recess such that in the end position of the piston, the sealing element together with the recess forms part of the overflow channel.

[0031] In a preferred embodiment, two or more longitudinal grooves are arranged on the inner circumference of the cylinder, and each of the longitudinal grooves partially forms the overflow channel at the end position of the piston. This distribution of the longitudinal grooves forms a plurality of possible overflow positions, and when the piston rotates, the outlet area of ​​the flow path can correspond to one of the longitudinal grooves. This is particularly advantageous if the piston is designed without an annular channel. If an annular channel is provided, overflow can be carried out simultaneously through the plurality of recesses, in which case the hydraulic fluid can be evenly distributed on the outer circumference of the piston and can enter the plurality of recesses.

[0032] In a simple embodiment, the recess at one end side of the cylinder opens directly into a subspace of the cylinder. In this embodiment, the cylinder and the piston can be assembled to each other in such a way that the flow path of the piston opens into the recess directly or indirectly via an annular channel at the end position of the piston. In this embodiment, the overflow channel includes the flow path in the piston, if necessary, the annular channel and the recess, through which the overflowing hydraulic fluid can enter the second subspace.

[0033] In an advantageous development of the hydraulic device, the overflow channel comprises, in the end position of the piston, a side channel which extends radially through the cylinder wall.

[0034] In a simple embodiment of the above-mentioned improvement, the cylinder and the piston can be assembled to each other in such a way that the flow path of the piston directly or indirectly via the annular channel leads to the lateral channel at the end position of the piston. The hydraulic fluid can flow out of the overflow channel via the lateral channel and enter the second subspace. For this purpose, a hydraulic line connecting the subspaces to each other can be provided in a simple manner. In this embodiment, the overflow channel includes the flow path in the piston and, if necessary, the annular channel and the lateral channel, and the overflowing hydraulic fluid can enter the second subspace via the lateral channel.

[0035] In an advantageous development, the overflow channel further comprises the recessed portion opening into the lateral channel. Thus, the overflow channel comprises the flow path in the piston and, if necessary, the annular channel, the recessed portion and the lateral channel. In this embodiment, the recessed portion preferably has a circumferential edge, which is formed on the inner side of the cylinder. The recessed portion is spaced a distance from one end of the cylinder.

[0036] In an advantageous development, in the end position of the piston, the sealing element on the outer circumference of the piston is arranged along the longitudinal axis of the cylinder between the lateral channel and the opening of the flow path on the outer circumference of the piston, in particular in the region of the annular channel. This ensures in a structurally simple manner that the sealing element of the piston is the only component that comes into contact with the overflowing hydraulic fluid and has a sealing effect. In particular, it can be prevented that other components, such as a guide ring arranged on the outer circumference of the piston, come into contact with the overflowing hydraulic fluid. In this way, wear of the components of the hydraulic device can be prevented despite low pressures of the hydraulic fluid.

[0037] In an advantageous development, a gap channel is formed between the outer circumference of the cylinder and a cover element, which closes the interior of the cylinder at the end, into which the lateral channel opens. The gap channel is preferably formed substantially coaxially with respect to the longitudinal axis of the cylinder.

[0038] According to the previously described embodiment, the cover element surrounds the cylinder on the outside and the inside of the cover is at least partially overdimensioned relative to the outside of the cylinder. In order to form the gap channel. The cover element can define at least one mechanical stop, which is used to limit the adjustment range of the piston inside the cylinder and to define the end position. The cover element is preferably designed here so that in the end position of the piston, a subspace is enclosed between the cover element and one end side of the piston, into which the hydraulic fluid should overflow. In addition, the cover element has an overflow interface, through which the overflowing hydraulic fluid can be introduced into another hydraulic device.

[0039] In a preferred embodiment, a first guide ring and a second guide ring are arranged on the outer circumference of the piston, and the first guide ring and the second guide ring are used to guide the piston on the inner wall of the cylinder. The guide rings improve the support of the piston in its end position, because overflow may at least temporarily cause asymmetric force states, and the resulting forces can be supported on the inner wall of the cylinder via the guide rings. This subjects the sealing element to less mechanical load, and the sealing element can also be arranged on the outside of the piston. Preferably, at least one of the guide rings is arranged between the opening of the flow path on the end side of the piston and the opening on the outer circumference of the piston.

[0040] As explained above, the object of the invention can also be achieved by a lifting device according to the invention.

[0041] The lifting device according to the invention, in particular a lifting device for a lifting platform of a motor vehicle, has at least a first hydraulic device and a second hydraulic device as lifting drives for lifting the lifting device, each of the hydraulic devices having an inlet for supplying hydraulic fluid when the lifting device is raised and an overflow for discharging the hydraulic fluid.

[0042] The first hydraulic device is designed as a master device, whose overflow port is fluidically connected to the inlet of the second hydraulic device, which is actuated as a slave device. Each of the hydraulic devices has a cylinder and a piston, which is guided in a sealed manner in the respective cylinder and which divides the respective cylinder chamber into a first subspace and a second subspace.

[0043] At least one of the hydraulic devices of the lifting device has an overflow channel which is designed such that, when the lifting device is maximally raised or maximally lowered, a first subchamber and a second subchamber of the hydraulic device are connected to one another in a fluid-conducting manner via the overflow channel at least in the end position of the associated piston.

[0044] In the end position of the piston, the overflow channel corresponds to a flow path extending from one end side of the piston to the outer peripheral surface thereof.

[0045] This design of the lifting device makes it possible to avoid the disadvantages that occur in prior art lifting devices when overflow occurs by reducing the load on the components used, in particular the seals. This results in a longer service life for the components and a reduced maintenance frequency. In addition, overflow occurs with high reliability, since the components of the hydraulic device are protected, thereby increasing the safety of using the lifting device.

[0046] The lifting device preferably has at least one hydraulic device according to the invention or a preferred embodiment of the hydraulic device.

[0047] Further advantages and exemplary embodiments can be gathered from the following description of exemplary embodiments with reference to the drawings.

[0048] in:

[0049] Figure 1 a sectional view showing the hydraulic device in a working position;

[0050] Figure 2 Show according to Figure 1 A sectional view of the hydraulic device in the end position;

[0051] Figure 3 It is shown that there are two Figure 1 and 2 An embodiment of a lifting device of a hydraulic device, in which one hydraulic device is configured as a master device and the other hydraulic device is configured as a slave device;

[0052] Figure 4 Show according to Figure 3 Schematic diagram of the hydraulic system of the lifting equipment.

[0053] Figure 1 and Figure 2 A hydraulic device 1 is shown, the piston 2 of the hydraulic device 1 is Figure 1 is in working position and Figure 2 In the end position area.

[0054] The hydraulic device 1 comprises a cylinder 3, inside which a piston 2 is guided along a cylinder axis 4. When the piston 2 is in its working position (see Figure 1), the piston 2 seals the cylinder chamber into a first subspace 5 and a second subspace 6. When the first subspace 5 is filled with hydraulic fluid, the piston 2 moves along the cylinder axis 4 under the action of the hydraulic pressure. At this time, the volume of the first subspace 5 increases, while the volume of the second subspace 6 decreases. The cylinder head 7 that defines the second subspace 6 has an overflow interface 8, and the hydraulic fluid in the second subspace 6 can flow out through the overflow interface 8. The outer peripheral side of the piston 2 has two surrounding guide rings 9, 10 and two surrounding seals 11, 12. The guide rings 9, 10 are used to guide the piston 2 on the inner wall of the cylinder, and the seals 11, 12 are used to seal the subspaces 5 and 6 from each other in the working position of the piston 2.

[0055] In the embodiment shown here, the hydraulic device 1 is combined with another hydraulic device (see Figure 3 and 4 ) are used together to lift the vehicle, to keep it in the raised position and, if necessary, to lower it again. It is desirable to be able to reliably bring the pistons of the two hydraulic units into their end position within the respective cylinders so that the lifted load does not end up in an unstable tilted position. However, due to aging phenomena, fluctuating ambient conditions and unavoidable leaks at the hydraulic seals, the amount of hydraulic fluid required to be able to bring the pistons of the two hydraulic units into their respective end position can vary. In order to achieve this goal with a minimum of design effort, Figure 1 The hydraulic device shown in FIG. 1 is connected to another hydraulic device via an overflow port 8 and is designed so that when the piston 2 reaches the end position, the hydraulic fluid can firstly flow through the overflow channel 13 (only in FIG. 2 ). Figure 2 ) from the first sub-space 5 into the second sub-space 6, and then from here via the overflow interface 8 into another hydraulic device. Thus, the first hydraulic fluid 1 can be fed to another hydraulic fluid (see Figure 3 and Figure 4 ) in the supplementary delivery of the extra hydraulic fluid required beyond the expectation. The implementation of this functional structure is explained in detail below.

[0056] like Figure 1 As shown in FIG. 1 , the piston 2 has a flow channel 14 which extends from one end side of the piston to the outer peripheral side of the piston 2. In the embodiment shown here, the flow channel 14 is essentially formed by two blind holes which intersect in the region of their respective hole bottoms and thus form a fluid-conducting connection between their respective orifices. Figure 1 In the working position of the piston 2 shown in the figure, the flow channel 14 is sealed by the cylinder inner wall on the outer peripheral side of the piston 2. By loading pressure on one side of the piston, the piston can be brought to its end position in the manner described above.

[0057] exist Figure 2In the end position shown in FIG. 1 , the piston 2 and the cylinder cover 7 are in a mechanical stop position. At this time, the above-mentioned overflow channel 13 is exposed, through which the hydraulic fluid can overflow between the first subspace 5 and the second subspace 6 in the end position of the piston 2. To this end, in the end position of the piston 2, the overflow channel 13 includes a flow path 14, an annular channel 15, a plurality of recesses 16, a plurality of lateral channels 17 and a gap channel 18.

[0058] This design of the flow channel 14 extending between the end side and the outer circumference of the piston 2 has the advantage that the guide ring 10 in the end position of the piston 2 does not come into contact with the overflowing hydraulic fluid 19 which is guided through the piston 2. In contrast to previously known overflow channels, in which hydraulic fluid can only overflow from the first subspace 5 into the second subspace 6 at the outer circumference of the piston 2, the guide ring 10 is not loaded by the overflowing hydraulic fluid 19 and is thus protected.

[0059] The annular channel 15 is designed as a circumferential groove extending on the outer circumference of the piston 2 and into which the hydraulic fluid can already enter in the working position of the piston 2. The annular channel 15 has the advantage that in the working position of the piston 2, the hydraulic fluid located in the annular channel 15 already has a centering effect on the piston 2 due to its hydraulic pressure. As a result, the components 9, 10, 11, 12 arranged on the outer circumference are loaded uniformly, thereby reducing the wear of these components. In addition, the hydraulic fluid can be distributed on the outer circumference of the piston 2 via the annular channel 15 and can thus enter the second subspace 6 simultaneously via a plurality of recesses.

[0060] In the end position, the annular channel 15 opens into a plurality of recesses 16, which are designed as longitudinal grooves and each of which extends substantially parallel to the cylinder axis 4. Although the guide ring 10 does not come into direct contact with the overflowing hydraulic fluid 19 via the flow path 14 in the form of a bypass line described here, such contact between the hydraulic fluid and the seal 12 is not prevented. In order to avoid damage to the seal 12, a flow resistance 20 is provided in the flow path 14, which serves to reduce the hydraulic pressure of the hydraulic fluid overflowing in the overflow channel 13. This pressure reduction has a favorable effect on the wear phenomena on the seal 12. In the embodiment shown here, the flow resistance 19 is an orifice plate, which is used to reduce the static pressure of the hydraulic fluid 19 in the overflow channel 13. Alternatively, the flow resistance 20 can also be a throttle valve.

[0061] The recess 16 is designed as a longitudinal groove which extends substantially parallel to the cylinder axis 4. The length and position of the recess are selected here so that when the piston 2 approaches its end position and has not yet completely reached the end position, the hydraulic fluid can already overflow from the first subspace 5 into the second subspace 6. By this early overflow of the hydraulic fluid, the piston 2 can be braked before reaching its end position, whereby sudden stops in the movement of the lifted load can be avoided.

[0062] By means of the embodiment of the lateral channel 17 shown here, it is possible to form the recess 16 relatively short and thus reduce the stress concentration effects occurring on the cylinder wall.

[0063] The cylinder cover 7 has an oversize relative to the cylinder outer wall on the side facing the cylinder 3. In this way, a region is enclosed between the cylinder cover 7 and the cylinder outer wall, which extends in a gap-like manner and parallel to the cylinder axis 4. This region is referred to as the gap channel 18 in the present case and serves to connect the lateral channel 17 to the second subspace 6 in a fluid-conducting manner. In this way, additional hydraulic lines can be omitted in order to connect the lateral channel 17 to the second subspace 6 in a fluid-conducting manner.

[0064] In a manner not shown here, an alternative embodiment of the hydraulic cylinder 1 can be designed so that the overflow channel 13 is formed only by the flow path 14. In this case, in the end position of the piston 2, the opening of the flow path 14 on the outer peripheral side of the piston 2 can directly open into the second subspace 6 in the area of ​​the edge of the cylinder end side. Another alternative embodiment can include an overflow channel 13, which is designed without an annular channel 15 and the flow path 14 opens directly into the recess 16 or the lateral channel 17 in its end position. In another alternative embodiment, the lateral channel 17 and the gap channel 18 can be completely omitted by selecting the position and length of the recess 16 so that the recess 16 opens directly into the second subspace 6.

[0065] Figure 3 A lifting device is shown which is configured as a column lift 21. The lifting device comprises two lifting columns 22 and 23 extending substantially vertically, and a carrying fork 24 or 25 is arranged on each of the lifting columns 22 and 23. The carrying forks 24, 25 are vertically movable and have seats 24a, 24b or 25a, 25b, respectively, for receiving a vehicle which is lifted by the column lift 21 and held in the lifted position.

[0066] In a manner not shown in detail here, the lifting columns 22 and 23 each include two hydraulic devices, which are each configured to Figure 1 , 22. The first pair of hydraulic devices is arranged in the lifting column 22 and the second pair of hydraulic devices is arranged in the lifting column 23. The pistons of the first pair of hydraulic devices arranged in the lifting column 22 are mechanically coupled to each other. The pistons of the second pair of hydraulic devices arranged in the lifting column 23 are likewise mechanically coupled to each other. Furthermore, one of the first pair of hydraulic devices in the lifting column 22 is hydraulically coupled to one of the second pair of hydraulic devices in the lifting column 23, that is, is arranged in series in the form of a master-slave arrangement as explained below.

[0067] Hydraulic devices that are hydraulically coupled to each other operate according to a common hydraulic circuit, see Figure 4 By means of the combination of mechanical and hydraulic coupling between the hydraulic devices of the lifting columns 22, 23, good reliability can be achieved. This is because, if the system pressure of one hydraulic circuit drops due to an unexpected fault, the lifted load continues to be maintained by the system pressure of the corresponding other hydraulic circuit and by the mechanical coupling of the hydraulic devices in the lifting columns 22, 23.

[0068] Figure 4 Shown according to Figure 3 Schematic diagram of the hydraulic line arrangement between the lifting platform 21 and the two hydraulic devices K and F respectively arranged in the lifting columns 22 or 23. Here, the hydraulic device of the lifting column 22 serves as a so-called master device, and the hydraulic device of the lifting column 23 serves as a so-called slave device.

[0069] In order to operate the lifting platform, hydraulic fluid is conveyed from the tank 26 via the suction filter 28 to the master device K via the first supply line 29 by means of the pump 27. This causes the piston 30 of the master device K to perform a lifting movement in the vertical direction. The hydraulic fluid discharged by the piston 30 is supplied to the slave device F via the overflow line 31. This causes the piston 32 to also perform a lifting movement and in this case discharges the hydraulic fluid contained in the slave device F, which is supplied to the tank 26 via the second overflow line 33.

[0070] If the piston 30 of the master device K reaches its end position before the piston 32 of the slave device F reaches its end position, then according to Figure 1 and Figure 2 The design of the master control device K in the form of a hydraulic device 1 allows the hydraulic fluid to overflow through the overflow line 31. This can reduce the number of necessary components. Figure 4 The hydraulic system shown in FIG. 1 is designed to be relatively simple. In particular, only one pump device is required to actuate the master device K and the slave device F. In addition, pressure balance can be achieved between the hydraulic devices K and F.

[0071] In order to move the pistons 30, 32 again in the opposite direction and lower the raised carrying fork, the 2 / 2-way valve 34 can be actuated. The hydraulic fluid contained in the master K or in the slave F is now displaced by the weight of the lifted vehicle and supplied to the tank 26 via the return line 35. The speed of the lowering movement can be controlled by a lowering brake 37. For safety reasons, the supply line 29 and the return line 35 are connected to a pressure-limiting valve 38 via a further line.

Claims

1. A hydraulic device (1), comprising: A cylinder (3) and a piston (2) guided in the cylinder (3), the piston (2) being movable between a working position and an end position, wherein: The piston (2) in the working position seals the cylinder chamber into a first subspace (5) and a second subspace (6), and wherein: In the end position of the piston (3), the first subspace (5) and the second subspace (6) are connected to one another in a fluid-conducting manner via an overflow channel (13). It is characterized in that A flow path (14) is formed in the piston (2), the flow path (14) extending between the end side of the piston (2) and the outer peripheral side of the piston (2), and wherein: In the end position of the piston, the overflow channel (13) is at least partially formed by the flow path (14).

2. The hydraulic device (1) according to claim 1, wherein: The flow path (14) has a flow resistance (20) configured to reduce the pressure of a hydraulic fluid (19) flowing in the flow path (14).

3. The hydraulic device (1) according to claim 2, wherein: The piston (2) has at least one sealing element (12) on its outer peripheral side, the sealing element (12) being arranged axially offset relative to an opening of the flow path on the outer peripheral side of the piston, and the sealing element partially defines the overflow channel (13) at the end position of the piston (3) and is arranged downstream of the flow resistance (20) relative to the flow direction of the hydraulic fluid (19) flowing in the overflow channel.

4. The hydraulic device (1) according to claim 2 or 3, wherein: The flow resistance (20) is an orifice plate or a throttle valve.

5. The hydraulic device (1) according to any one of the preceding claims, wherein: The flow path (14) on the outer circumferential surface of the piston (2) opens into an annular channel (15) of the piston (2), and the annular channel extends substantially coaxially with the longitudinal axis of the piston (2).

6. The hydraulic device (1) according to any one of the preceding claims, wherein: In the end position of the piston (2), the overflow channel (20) comprises at least one recess (16) in the inner wall of the cylinder, the recess (16) extending at least partially parallel to the longitudinal axis (4) of the cylinder.

7. A hydraulic device (1) according to at least claims 3 and 6, wherein: In the end position of the piston (2), the sealing element (12) is arranged along the longitudinal axis (4) of the cylinder in the region of the recess (16).

8. A hydraulic device (11) at least according to claim 7, wherein: The sealing element (12) and the recess (16) form a part of the overflow channel (20) in the end position of the piston (2).

9. A hydraulic device (1) at least according to claim 6, wherein: Two or more recessed portions (16) are distributed on the inner circumference of the cylinder (3).

10. The hydraulic device (1) according to any one of the preceding claims, wherein: In the end position of the piston, the overflow channel (13) comprises a lateral channel (17) which extends radially through the cylinder wall.

11. A hydraulic device (1) according to at least claims 6 and 10, wherein: The recess (16) opens into the lateral channel (17).

12. A hydraulic device (1) according to at least claims 3 and 11, wherein: In the end position of the piston (2), the sealing element (12) is arranged along the longitudinal axis (4) of the cylinder between the lateral channel (17) and the opening of the flow path (14) on the outer peripheral side of the piston (2), in particular in the area of ​​the annular channel (15).

13. The hydraulic device (1) according to claim 11, wherein: In the end position of the piston (2), the overflow channel (13) comprises a gap channel (18) which extends between the outer wall of the cylinder and a cylinder cover (7) surrounding the outer wall, and into which the lateral channel (17) opens.

14. A lifting device, in particular a lifting device for a lifting platform of a motor vehicle, comprising at least a first hydraulic device (1) and a second hydraulic device (1) as lifting drives for lifting the lifting device (21), each of the hydraulic devices (1) having an inlet for supplying hydraulic fluid when the lifting device (21) is lifted and an overflow for discharging the hydraulic fluid, and the first hydraulic device (1) is designed as a master device (K), the overflow of which is connected in a fluid-conducting manner to the inlet of the second hydraulic device which is actuated as a slave device (F), in, Each of the hydraulic devices (1) comprises a cylinder (3) and a piston (2) guided in a sealed manner in the cylinder (3), the piston (2) dividing the cylinder chamber into a first subspace (5) and a second subspace (6), and at least one of the hydraulic devices comprises an overflow channel (13), the overflow channel (13) being arranged and configured such that the first subspace (5) and the second subspace (6) of the hydraulic device are connected to one another in a fluid-conducting manner via the overflow channel (13) only in the region of the end position of the associated piston (2) when the lifting device (21) is maximally raised or maximally lowered, It is characterized in that A flow path (14) is formed in the piston (2), and the flow path (14) is formed in the piston (2). The piston (2) extends between the end side and the outer peripheral side of the piston (2), and at the end position of the piston (2), The overflow channel (13) is at least partially formed by the flow path (14).

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