Hydraulic assembly with transfer port and lifting device

Abstract

The invention relates to a hydraulic assembly (1) comprising a cylinder (3) and a piston (2), guided in the cylinder (3), which can be adjusted between a working position and an end position, wherein the piston (2) sealingly divides a cylinder chamber into a first sub-chamber (5) and a second sub-chamber (6) in the working position, and wherein the first sub-chamber (5) and the second sub-chamber (6) are fluidically connected to one another by means of a transfer port (13) in the end position of the piston (3). A flow path (14) is formed in the piston (2), running between an end side of the piston (2) and an outer peripheral side of the piston (2) and wherein the flow path (14) at least partially forms the transfer port (13) in the end position of the piston.

Description

[0001] Hydraulic reservoir with overflow channel and lifting device

[0002] Description

[0003] The invention relates to a hydraulic unit according to the preamble of claim 1 and a lifting device according to the preamble of claim 14.

[0004] A hydraulic power unit of the type mentioned above can be used to move heavy loads. For this purpose, the hydraulic power unit typically has a cylinder within which a piston is adjustable. In an operating position, the piston seals and divides a cylinder chamber into two subchambers. If hydraulic fluid is pumped into one of the subchambers via an inlet, the piston is hydraulically adjusted. A piston rod connected to the piston allows the piston's adjustment movement to be mechanically transmitted to the load to be moved.

[0005] It is common practice to operate several hydraulic power units simultaneously, and synchronization between them typically must be ensured. This means that the pistons of the hydraulic power units can be moved to identical positions, especially their end positions, as needed. This is intended to prevent a load lifted via multiple force application points from becoming unstable.

[0006] When operating multiple hydraulic units, it is also known to operate multiple hydraulic units in pairs in separate hydraulic circuits to achieve a high level of reliability. Two pairs of hydraulic units can be provided, wherein the pistons of a first pair of hydraulic units and the pistons of a second pair of hydraulic units are mechanically coupled to one another. Furthermore, each of the hydraulic units of the first pair is hydraulically coupled to a hydraulic unit of the second pair. In the event of a malfunction in one of the hydraulic circuits, a lifted load can be held without sinking by the system pressure of the other hydraulic circuit and via the mechanical coupling of the pistons. A hydraulic circuit diagram of a lifting platform with two hydraulic units mechanically coupled in pairs and cross-connected is shown, for example, in DE 10 2014 113 301 A1.

[0007] However, wear on the seals or other components of the hydraulic units can lead to hydraulic fluid leakage from one of the hydraulic units. Furthermore, the hydraulic units can expand differently due to temperature differences. This can lead to variations in the amount of hydraulic fluid required to move the pistons of several hydraulic units to their respective end positions in synchronization.

[0008] In the case of the arrangement described above with two pairs of hydraulic units, which achieves a high degree of reliability, pressure differences can also arise between the hydraulically coupled hydraulic units. To avoid mechanical stress between the mechanically coupled hydraulic units, it is desirable to be able to perform pressure equalization.

[0009] One possibility for achieving the desired synchronization and pressure equalization between several hydraulic units is to connect a first hydraulic unit in a manner known per se to a hydraulic fluid source via which the required amount of hydraulic fluid can always be provided in order to move its piston to its end position. The first hydraulic unit is designed in such a way that its piston, in its end position, opens up an overflow channel via which the first and second sub-chambers of the first hydraulic unit are fluidically connected to one another. By means of such an overflow channel, hydraulic fluid can thus flow from the first sub-chamber into the second sub-chamber of the first hydraulic unit and can then be pumped from the second sub-chamber into a second hydraulic unit via an overflow line in order to also move its piston to its end position.In this way, synchronization between two hydraulic units can be achieved with a small number of hydraulic components and, in particular, with only one hydraulic fluid source. Furthermore, pressure equalization between the first and second hydraulic units can take place by means of the overflow channel. One design of the overflow channel is known from EP 2 428 482 A1. For this purpose, the inside of the cylinder has a recess which is formed on the end face of the inside of a cylinder. When the piston is in a working position, the recess lies entirely within a partial chamber of the cylinder. When the piston reaches its end position in the region of the recess, a fluid-conducting connection is created on the inside of the cylinder between the two partial chambers of the cylinder of the hydraulic unit.

[0010] A disadvantage of the previously known design of the overflow channel is that, in the piston's end position, the hydraulic fluid must flow past the entire piston height on the outside of the piston to flow from one subchamber to the other. This places particular stress on the sealing elements, which may be located on the outside of the piston. This results in significant wear and, consequently, high maintenance and repair requirements for the entire hydraulic unit.

[0011] The invention is therefore based on the object of proposing a hydraulic unit with an overflow function which is associated with a lower maintenance requirement.

[0012] The object is achieved by the features of the subject matter of claim 1. The object is further achieved by a lifting device according to claim 14. Advantageous embodiments are the subject matter of the respective dependent subclaims.

[0013] The hydraulic unit according to the invention comprises, in a conventional manner, a cylinder and a piston guided within the cylinder. The piston is adjustable between a working position and an end position. In the working position, the piston sealingly divides a cylinder chamber into a first subchamber and a second subchamber. In the end position of the piston, the first and second subchambers are fluidly connected to each other via an overflow channel.

[0014] It is essential for the invention that a flow path is formed in the piston, which runs between an end face and an outer peripheral side of the piston and that the overflow channel is formed at least partially by the flow path in the end position of the piston.

[0015] The hydraulic unit according to the invention offers advantages in terms of its service life and the required maintenance measures. This is because the inventive design of the flow path allows a hydraulic fluid flow to be directed between two subchambers of the hydraulic unit in the manner of a bypass line, thereby avoiding damaging contact with functionally critical components on the outer peripheral side of the piston. For example, at least one piston seal or guide band is arranged, relative to the longitudinal axis of the piston, between a front-end opening of the flow path and an outer peripheral opening of the flow path on the outer peripheral side.

[0016] According to the invention, the flow path is formed within the piston and, at least in sections, completely enclosed by the piston. Furthermore, the invention is not limited to a specific course of the flow path or to a cross-sectional geometry. In a simple embodiment, the flow path can run in the form of a straight bore from the end face of the piston to its outer peripheral side. Alternatively, the flow path in the piston can comprise two blind bores that intersect in the regions of their bore bases and are thus connected to one another in a fluid-conducting manner. One of the blind bores is arranged on the end face and the other of the blind bores is arranged on the outer peripheral side of the piston.

[0017] In a simple embodiment, the piston can be guided in a sealing manner within the cylinder in a working position, as is known per se. In this case, one end of the flow path on the end face of the piston opens into one of the two subchambers of the cylinder. The other end of the flow path on the outer peripheral side of the piston points towards the inner cylinder wall, on which the piston is guided in a sealing manner. If the piston is subjected to hydraulic pressure in this working position on the end face on which the flow path is formed, the piston is displaced in a known manner to its end position. Upon reaching the end position, the flow path on the outer peripheral side of the piston can open directly into the second subchamber. It is therefore within the scope of the invention that the overflow channel in the end position of the piston is formed exclusively by the flow path.In another embodiment, however, a further fluid path is provided, which also represents a component of the overflow channel and by means of which the fluid-conducting connection between the two sub-chambers is created.

[0018] For the purposes of the invention, the end position of the piston is not limited to a single position of the piston within the cylinder, but also includes a range in which the piston approaches this position. It is therefore within the scope of the invention that the overflow channel is not only opened when the piston reaches its end position, but already when the piston is in an end position range. Furthermore, the invention is not limited to how many end positions the piston can assume within the hydraulic unit. Rather, the end position of the piston can be defined by any desired mechanical stop within the cylinder, for example a cylinder cover, a cylinder base or a stop element protruding radially into the cylinder interior.

[0019] The invention is also not limited to the design of the hydraulic unit, its cylinder, or the piston. It is therefore within the scope of the invention for the hydraulic unit to be designed as a so-called double-action cylinder or a so-called differential cylinder. The piston preferably has a cylindrical basic shape. Furthermore, the piston can have a circumferential profile on its outer circumference, which can serve for the positive arrangement of sealing elements.

[0020] In an advantageous further development, the flow path has a flow resistance in order to reduce a pressure of a hydraulic fluid flowing in the flow path.

[0021] By means of the refinement described above, it is possible to provide sealing elements on the outer circumferential side of the piston, past which the overflowing hydraulic fluid does not bypass via the flow path, but rather comes into contact. Investigations by the applicant have shown that the damaging effect of the overflowing hydraulic fluid can be reduced or completely avoided if the hydraulic pressure of the hydraulic fluid is reduced beforehand. In this context, the flow path within the piston can be used not only to design the overflow channel, but also simultaneously to arrange the flow resistance, which serves to reduce the pressure. Reducing the pressure of the hydraulic fluid includes, in particular, reducing its static and / or dynamic pressure.

[0022] In an advantageous development, the piston has at least one sealing element on the outer peripheral side, which is arranged axially offset from the opening of the flow path on the outer peripheral side of the piston and which, in the end position of the piston, partially delimits the overflow channel. The sealing element is arranged downstream of the flow resistance with respect to a flow direction of the hydraulic fluid flowing in the overflow channel.

[0023] Investigations by the applicant have shown that the design of the flow path with the flow resistance is particularly advantageous when the overflowing hydraulic fluid flows past the sealing element, which is arranged on the outer circumferential side of the piston, and the pressure of the hydraulic fluid is reduced 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 unit. In particular, the sealing element and the overflowing hydraulic fluid are in direct contact in the end position of the piston. In particular, the sealing element is designed to divide the cylinder chamber into the first subchamber and the second subchamber in a sealing manner in the working position of the piston.

[0024] The flow resistance can be adjusted, for example, depending on a geometric feature of the flow path and / or a surface property of the flow path. Adjustable geometric features include, for example, the length of the flow path through which fluid flows and / or a curvature of the flow path and / or a macroscopic profile. Adjustable surface properties include, for example, a microscopic profile. The flow resistance can be designed either as an integral component of the flow path or as a differential component that can be interchangeably arranged within the flow resistance.

[0025] In a preferred embodiment of the hydraulic unit, the flow resistance is an orifice or a throttle. Using the orifice, it is possible to reduce the pressure of the flowing hydraulic fluid and set a constant volume flow of the hydraulic fluid independent of the system pressure. While the pressure of the hydraulic fluid can also be reduced using a throttle, the adjustable volume flow is typically linearly dependent on the system pressure. Accordingly, using an orifice, at least the dynamic pressure of the hydraulic flow can be reduced to a constant value, whereas using an orifice, it can be adjusted depending on the system pressure.Despite their different modes of operation, both the orifice and the throttle advantageously serve to reduce the pressure of the hydraulic fluid in such a way that, in particular, the sealing elements arranged on the outer circumference of the piston are protected in the event of contact with the overflowing hydraulic fluid.

[0026] The orifice and throttle can each be a standardized hydraulic component whose pressure-reducing effect can be determined using common design principles. It is also within the scope of an advantageous development for the orifice and throttle to be arranged in the flow path as an integral component or as a differentially designed component. The advantage achieved by the integral design is the reduction of downstream assembly steps in which the orifice or throttle must be joined to the piston. Alternatively, the orifice and throttle can be mounted in the piston as inserts.

[0027] In a preferred embodiment of the hydraulic unit, the flow path opens into an annular channel on the outer peripheral surface of the piston.

[0028] The design of the annular channel allows the hydraulic fluid emerging from the flow path to be partially or completely distributed over the piston's bypass. This is particularly advantageous because the hydraulic fluid can overflow regardless of the piston's rotational position. This is particularly difficult when the flow path, in its final position, must flow into a fluid path on the outer circumferential side of the piston to form the overflow channel. The annular channel thus allows the piston to be mounted in any rotational position relative to the cylinder's longitudinal axis.

[0029] In a preferred embodiment of the hydraulic unit, a recess is formed on the inner cylinder wall as a longitudinal groove, into which the flow path formed in the piston opens in the end position, thus forming part of the overflow channel in the end position of the piston. The recess can extend, at least in sections, parallel to a longitudinal axis of the cylinder. However, it is also within the scope of the preferred embodiment for the recess to extend at an angle and / or with a curved profile relative to the longitudinal axis of the cylinder.

[0030] The position and length of the recess can be used to easily determine the spatial area in which overflow occurs. The longer the groove length, the further the longitudinal groove extends into the cylinder interior and the sooner the hydraulic fluid overflow occurs when the piston approaches its end position. In addition, the damping properties of the hydraulic unit can be positively influenced when reaching the end position, as a jerky stop of the piston due to a mechanical stop in its end position can be avoided.

[0031] In an advantageous further development, the sealing element is located on the outer circumferential side of the piston in the end position of the piston along the longitudinal axis of the cylinder in the region of the recess. An overflowing hydraulic fluid can pass through the piston and out of the flow path formed therein and, in particular, come into contact with the sealing element at a reduced pressure without damaging it. 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 a section of the overflow channel. In a preferred embodiment, two or more longitudinal grooves are distributed over the circumference of the cylinder interior and, in the end position of the piston, each partially form the overflow channel.The distribution of the longitudinal grooves creates a multitude of possible overflow positions in which the outlet area of ​​the flow path can correspond with one of the longitudinal grooves when the piston rotates. This is particularly advantageous when the piston is designed without an annular channel. If an annular channel is provided, simultaneous overflow over the plurality of recesses is possible, allowing the hydraulic fluid to be evenly distributed over the outer circumference of the piston and enter the numerous recesses.

[0032] In a simple embodiment, the recess on one end face of the cylinder opens directly into a subchamber of the cylinder. In this embodiment, the cylinder and piston can be joined together in such a way that the flow path of the piston, in its end position, opens directly into the recess or indirectly via the annular channel. In this embodiment, the overflow channel comprises the flow path in the piston, optionally the annular channel, and the recess through which the overflowing hydraulic fluid can reach the second subchamber.

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

[0034] In a simple embodiment of the further development described above, the cylinder and the piston can be joined together in such a way that the flow path of the piston, in its end position, opens directly or indirectly into the side channel via the annular channel. The hydraulic fluid can escape from the overflow channel via the side channel and from there reach the second sub-chamber. For this purpose, a hydraulic line can be provided in a simple manner, which connects the sub-chambers to one another. In this embodiment, the overflow channel comprises the flow path in the piston, optionally the annular channel, and the side channel, via which the overflowing hydraulic fluid can reach the second sub-chamber. In an advantageous further development, the overflow channel also comprises the recess which opens into the side channel. The overflow channel therefore comprises the flow path in the piston, optionally the annular channel, the recess and the side channel.In this embodiment, the recess preferably has a circumferential edge formed on the inside of the cylinder. The recess is formed at a distance from a front end of the cylinder.

[0035] In an advantageous development, the sealing element is arranged on the outer circumferential side of the piston in the end position of the piston along the longitudinal axis of the cylinder between the side channel and the opening of the flow path on the outer circumferential side of the piston, in particular in the region of the annular channel. This makes it possible, in a structurally simple manner, to ensure that the sealing element of the piston is the only sealing component that comes into contact with the overflowing hydraulic fluid. In particular, it can be avoided that another component, for example a guide band arranged on the outer circumferential side of the piston, comes into contact with the overflowing hydraulic fluid. Despite a reduced pressure of the hydraulic fluid, wear on the components of the hydraulic unit can be prevented in this way.

[0036] In an advantageous development, a gap channel is formed between the outer peripheral side of the cylinder and a cover element that closes the cylinder interior at the front, with the side channel opening into the gap channel. Preferably, the gap channel is formed substantially coaxially with the longitudinal axis of the cylinder.

[0037] According to the embodiment described above, the cover element surrounds the cylinder on the outside and, at least in sections, has an oversize on the inside of the cover compared to the outside of the cylinder in order to form the gap channel. The cover element can define at least one mechanical stop which limits the adjustment range of the piston within the cylinder and defines the end position. The cover element is preferably designed such that, in the end position of the piston, the partial space into which the hydraulic fluid is intended to overflow is enclosed between the cover element and an end face of the piston. Furthermore, the cover element can have an overflow connection via which the overflowing hydraulic fluid can be guided into another hydraulic unit.

[0038] In a preferred embodiment, a first guide band and a second guide band are arranged on the outer circumferential surface of the piston and serve to guide the piston on the inner cylinder wall. The guide bands improve the support of the piston in its end position, since the overflow can at least temporarily create asymmetrical force relationships, whereby the associated forces can be supported by the guide bands on the inner cylinder wall. This leads to a lower mechanical load on sealing elements, which can also be arranged on the outer side of the piston. Preferably, at least one of the guide bands is arranged between the opening of the flow path on the end face of the piston and the opening on the outer circumferential side of the piston.

[0039] As explained above, the object underlying the invention is also achieved by the lifting device according to the invention.

[0040] In the lifting device according to the invention, in particular a lifting platform for motor vehicles, with at least a first and a second hydraulic hydraulic unit operating as a lifting drive for lifting the lifting device, each of the hydraulic units has an inlet for supplying and an overflow for discharging hydraulic fluid when the lifting device is raised.

[0041] The first hydraulic unit is designed as a command unit, with its overflow fluidly connected to the inlet of the second hydraulic unit, which is controlled as a slave unit. Each hydraulic unit has a cylinder and a piston sealingly guided within the respective cylinder, which divides the corresponding cylinder interior into a first and a second sub-chamber.

[0042] At least one of the hydraulic units of the lifting device has an overflow channel which is designed such that, at least in the end position of the associated piston when the lifting device is maximally raised or maximally lowered, the first and second subchambers of this unit are fluidly connected to one another via the overflow channel.

[0043] In the end position of the piston, the overflow channel corresponds to a flow path running from one end face of the piston to its outer circumferential surface.

[0044] By designing a lifting device of this type, the disadvantages of overflow in existing lifting devices can be avoided by reducing the stress on the components used, especially seals. This leads to a longer service life of the components in question and reduced maintenance frequency. Furthermore, safety when working with such lifting devices is increased by ensuring overflow occurs with high reliability, as the components of the hydraulic unit are protected.

[0045] Preferably, the lifting device has at least one hydraulic unit according to the invention or an advantageous development thereof.

[0046] Further advantages and embodiments can be found in the following description of embodiments based on the figures.

[0047] It shows:

[0048] Figure 1 is a sectional view of a hydraulic unit in working position;

[0049] Figure 2 is a sectional view of the hydraulic unit according to Figure 1 in the final position;

[0050] Figure 3 shows an embodiment of a lifting device with two hydraulic units according to Figures 1 and 2, one of which is designed as a command unit and the other as a follower unit; Figure 4 shows a schematic representation of the hydraulic system of the lifting device according to Figure 3.

[0051] Figures 1 and 2 show a hydraulic unit 1, the piston 2 of which is in a working position according to Figure 1 and in the region of an end position according to Figure 2.

[0052] The hydraulic unit 1 comprises a cylinder 3, within which the piston 2 is guided along a cylinder axis 4. When the piston 2 is in its working position (see Figure 1), it divides a cylinder chamber into a first sub-chamber 5 and a second sub-chamber 6 in a sealing manner. When the first sub-chamber 5 is filled with a hydraulic fluid, the piston 2 is hydraulically adjusted along the cylinder axis 4. The volume of the first sub-chamber 5 increases, while the volume of the second sub-chamber 6 is reduced. A cylinder cover 7, which delimits the second sub-chamber 6, has an overflow connection 8, through which the hydraulic fluid in the second sub-chamber 6 can escape. The piston 2 has two circumferential guide bands 9, 10 and two circumferential seals 11, 12 on its outer circumference.The guide bands 9, 10 serve to guide the piston 2 on the inner cylinder wall, while the seals 11, 12 serve to seal the partial chambers 5 and 6 from each other in the working position of the piston 2.

[0053] In the embodiment shown here, the hydraulic unit 1, together with another hydraulic unit (see Figures 3 and 4), serves to raise a vehicle, hold it in a raised position, and lower it again if necessary. It is desirable to be able to reliably move the pistons of both hydraulic units within the respective cylinders to their end positions so that the raised load does not become unstable. However, due to aging, fluctuating ambient conditions, and unavoidable leakage at the hydraulic seals, the amount of hydraulic fluid required to move the pistons of both hydraulic units to their respective end positions can vary.In order to achieve this with minimal design effort, the hydraulic unit shown in Figure 1 is connected to the other hydraulic unit via the overflow port 8 and is designed such that when the end position of the piston 2 is reached, hydraulic fluid can flow via an overflow channel 13 (only shown in Figure 2) first from the first sub-chamber 5 into the second sub-chamber 6 and from there via the overflow port 8 into the other hydraulic unit. This allows an unexpectedly required additional quantity of hydraulic fluid to be pumped via the first hydraulic fluid 1 into the other hydraulic fluid (see Figures 3 and 4). The design implementation of this function is explained in detail below.

[0054] As shown in Figure 1, the piston 2 has a flow channel 14 which runs from one end face of the piston to an outer peripheral side of the piston 2. In the exemplary embodiment shown here, the flow channel 14 is essentially formed from two blind bores which intersect in the region of their respective bore bases and thus form a fluid-conducting connection between their respective bore openings. In the working position of the piston 2 shown in Figure 1, the flow channel 14 is sealed off on the outer peripheral side of the piston 2 by the inner cylinder wall. By applying pressure to the piston on one side, it can be brought into its end position in the manner described above.

[0055] In the end position shown in Figure 2, the piston 2 is in mechanical contact with the cylinder cover 7. This exposes the above-mentioned overflow channel 13, which allows hydraulic fluid to flow between the first subchamber 5 and the second subchamber 6 in the end position of the piston 2. For this purpose, the overflow channel 13 in the end position of the piston 2 comprises a flow path 14, an annular channel 15, a plurality of recesses 16, a plurality of side channels 17, and a gap channel 18.

[0056] The design of the flow channel 14, which runs between the end face and the outer circumferential side of the piston 2, has the advantage that the guide band 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 comparison to a previously known overflow channel, in which the hydraulic fluid can only flow from the first subchamber 5 into the second subchamber 6 on the outer circumferential side of the piston 2, the guide band 10 is not stressed by the overflowing hydraulic fluid 19 and is thus protected. The annular channel 15 is designed as a circumferential groove that runs on the outer circumference of the piston 2 and into which the hydraulic fluid can already reach in the working position of the piston 2.An advantage associated with the annular channel 15 is that the hydraulic fluid contained in the annular channel 15, due to its hydraulic pressure, already has a centering effect on the piston 2 in the working position of the piston 2. This allows the components 9, 10, 11, 12 arranged on the outer circumference to be evenly loaded, thereby reducing their wear. Furthermore, the hydraulic fluid can be distributed via the annular channel 15 over the outer circumference of the piston 2 and thus enter the second subchamber 6 simultaneously via several recesses.

[0057] In the end position, the annular channel 15 opens into a plurality of recesses 16, which are designed as longitudinal grooves and each run essentially parallel to the cylinder axis 4. While the guide band 10 does not come into direct contact with the overflowing hydraulic fluid 19 by means of the flow path 14 described here, which resembles a bypass line, such contact between the hydraulic fluid and the seal 12 is not prevented. To nevertheless prevent damage to the seal 12, a flow resistance 20 is arranged in the flow path 14, which serves to reduce the hydraulic pressure of the overflowing hydraulic fluid in the overflow channel 13. Such a reduction in pressure has a positive effect on the wear phenomena on the seal 12.In the embodiment shown here, the flow resistance 19 is an orifice plate that serves to reduce the static pressure of the hydraulic fluid 19 located in the overflow channel 13. Alternatively, the flow resistance 20 can be a throttle.

[0058] The recess 16 is designed as a longitudinal groove extending substantially parallel to the cylinder axis 4. The length and position of the recess are selected such that an overflow of hydraulic fluid from the first subchamber 5 into the second subchamber 6 can occur even when the piston 2 is approaching its end position but has not yet fully reached it. Such an early overflow of hydraulic fluid makes it possible to brake the piston 2 before reaching its end position, thus preventing a sudden stop in the movement of the lifted load.

[0059] The design of the side channel 17 shown here makes it possible to make the recess 16 comparatively short and thus to reduce the notch effects occurring on the cylinder wall.

[0060] The cylinder cover 7 has an oversize relative to the cylinder outer wall on a side facing the cylinder 3. Thus, a region is enclosed between the cylinder cover 7 and the cylinder outer wall, extending in a gap-like manner and parallel to the cylinder axis 4. This region is referred to herein as the gap channel 18 and serves to fluidically connect the side channel 17 to the second subchamber 6. This eliminates the need for additional hydraulic lines to fluidically connect the side channel 17 to the second subchamber 6.

[0061] In a manner not shown here, an alternative embodiment of the hydraulic cylinder 1 can be designed such that the overflow channel 13 is formed solely by the flow path 14. In this case, the opening of the flow path 14 located on the outer circumferential side of the piston 2 can open directly into the second partial chamber 6 in the end position of the piston 2 in the region of the front cylinder edge. A further alternative embodiment can comprise an overflow channel 13 which is designed without an annular channel 15 and wherein the flow path 14 opens directly into a recess 16 or a side channel 17 in its end position. In a further alternative embodiment, a side channel 17 and a gap channel 18 can be completely dispensed with by selecting the position and length of the recess 16 such that said recess 16 opens directly into the second partial chamber 6.

[0062] Figure 3 shows a lifting device designed as a column lift 21. It comprises two lifting columns 22 and 23, which extend essentially vertically and on each of which a support scissor arm 24 and 25 are arranged. The support scissor arms 24 and 25 are vertically movable and each have supports 24a, 24b and 25a, 25b, respectively, which serve to accommodate a vehicle that is to be lifted by means of the column lift 21 and held in the raised position.

[0063] In a manner not shown in detail here, the lifting columns 22 and 23 each comprise two hydraulic units, each designed corresponding to the hydraulic unit 1 shown in Figures 1 and 2. A first pair of hydraulic units is arranged in the lifting column 22 and a second pair is arranged in the lifting column 23. The pistons of the hydraulic units of the first pair, which is arranged in the lifting column 22, are mechanically coupled to one another. The pistons of the hydraulic units of the second pair, which is arranged in the lifting column 23, are also mechanically coupled to one another. In addition, a hydraulic unit of the first pair in the lifting column 22 is hydraulically coupled to a hydraulic unit of the second pair in the lifting column 23, i.e. connected in series in the form of a command-sequence arrangement as described below.

[0064] The hydraulically coupled hydraulic units are each operated in a common hydraulic circuit, which is shown in detail in Figure 4. The combination of mechanical and hydraulic coupling between the hydraulic units of the lifting columns 22, 23 described above allows for a high degree of reliability. This is because, if the system pressure of one hydraulic circuit drops due to an unexpected failure, the lifted load is maintained via the system pressure of the other hydraulic circuit and the mechanical coupling of the hydraulic units in the lifting columns 22, 23.

[0065] Figure 4 shows a schematic representation of the hydraulic circuit of the lifting platform 21 according to Figure 3 and between two hydraulic units K and F, which are arranged in the lifting columns 22 and 23, respectively. Here, a hydraulic unit of the lifting column 22 acts as a so-called command unit, and a hydraulic unit of the lifting column 23 acts as a so-called follower unit.

[0066] To operate the lifting platform, hydraulic fluid is pumped from a tank 26 by a pump 27 through a suction filter 28 via a first supply line 29 to the command unit K. This causes the piston 30 of the command unit K to move vertically. The hydraulic fluid displaced by the piston 30 is fed to the follower unit F via the overflow line 31. This causes the piston 32 to also perform a lifting movement, displacing the hydraulic fluid contained in the follower unit F, which is fed to the tank 26 via a second overflow line 33.

[0067] If the piston 30 of the command unit K reaches its end position before the piston 32 of the follower unit F reaches its end position, the design of the command unit K in the form of the hydraulic unit 1 according to Figures 1 and 2 allows hydraulic fluid to overflow via the overflow line 31. This makes it possible to design the hydraulic system shown in Figure 4 simply by reducing the number of required components. In particular, only one pump device is required to actuate both the command unit K and the follower unit F. In addition, pressure equalization can take place between the hydraulic units K and F.

[0068] To move the pistons 30, 32 in the reverse direction and lower the raised support scissors, a 2 / 2-way valve 34 can be actuated. The hydraulic fluid contained in the command unit K or the follower unit F is displaced by the weight of the raised vehicle and fed to the tank 26 via a return line 35. The speed of the lowering movement is controlled by a lowering brake 37. For safety reasons, the supply line 29 and the return line 35 are connected to a pressure relief valve 38 via another line.

Claims

Claims 1. Hydraulic unit (1) with a cylinder (3) and a piston (2) guided in the cylinder (3), which is adjustable between a working position and an end position, wherein the piston (2) in the working position sealingly divides a cylinder chamber into a first partial chamber (5) and a second partial chamber (6), and wherein in the end position of the piston (3) the first partial chamber (5) and the second partial chamber (6) are fluidly connected to one another by means of an overflow channel (13), characterized in that a flow path (14) is formed in the piston (2), which runs between an end face of the piston (2) and an outer peripheral side of the piston (2), and wherein the flow path (14) in the end position of the piston at least partially forms the overflow channel (13).

2. Hydraulic unit (1) according to claim 1, wherein the flow path (14) has a flow resistance (20) which is designed to reduce a pressure of a hydraulic fluid (19) flowing in the flow path (14).

3. Hydraulic unit (1) according to claim 2, wherein the piston (2) has at least one sealing element (12) on the outer peripheral side, which is arranged axially offset from the opening of the flow path on the outer peripheral side of the piston and which, in the end position of the piston (3), partially delimits the overflow channel (13) and is arranged downstream of the flow resistance (20) with respect to a flow direction of the hydraulic fluid (19) flowing in the overflow channel.

4. Hydraulic unit (1) according to claim 2 or 3, wherein the flow resistance (20) is an orifice or a throttle.

5. Hydraulic unit (1) according to one of the preceding claims, in which the flow path (14) opens on the outer peripheral surface of the piston (2) into an annular channel (15) of the piston (2), which runs substantially coaxially to a longitudinal axis of the piston (2).

6. Hydraulic unit (1) according to one of the preceding claims, wherein the overflow channel (20) in the end position of the piston (2) comprises at least one recess (16) on the cylinder inner wall, which runs at least in sections parallel to a longitudinal axis (4) of the cylinder.

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

8. Hydraulic unit (11) at least according to claim 7, wherein the sealing element (12) and the recess (16) form a section of the overflow channel (20) in the end position of the piston (2).

9. Hydraulic unit (1) at least according to claim 6, in which two or more recesses (16) are arranged distributed over an inner circumference of the cylinder (3).

10. Hydraulic unit (1) according to one of the preceding claims, wherein the overflow channel (13) in the end position of the piston comprises a side channel (17) which runs radially through a cylinder wall.

11. Hydraulic unit (1) at least according to claims 6 and 10, in which the recess (16) opens into the side channel (17).

12. Hydraulic unit (1) at least according to claims 3 and 11, in which the sealing element (12) in the end position of the piston (2) along the longitudinal axis (4) of the cylinder is arranged between the side channel (17) and an opening of the flow path (14) on the outer peripheral side of the piston (2), in particular in the region of the annular channel (15). Hydraulic unit (1) according to claim 11, wherein the overflow channel (13) in the end position of the piston (2) comprises a gap channel (18) which extends between an outer wall of the cylinder and a cylinder cover (7) encompassing the outer wall, and wherein the side channel (17) opens into the gap channel (18). A lifting device (21), in particular a lifting platform for motor vehicles, comprising at least a first and a second hydraulic unit (1) operating as a lifting drive for raising the lifting device (21), wherein each of the hydraulic units (1) has an inlet for supplying and an overflow for discharging hydraulic fluid when the lifting device (21) is raised, and the first hydraulic unit (1) is designed as a command unit (K).in that its overflow is fluidically connected to the inlet of the second hydraulic unit (1) controlled as a follower unit (F), wherein each of the hydraulic units (1) has a cylinder (3) and a piston (2) which is sealingly guided in the cylinder (3) and divides a cylinder interior into a first (5) and a second partial chamber (6), and wherein at least one of the hydraulic units has an overflow channel (13) which is arranged and designed such that only in the region of an end position of the associated piston (2) when the lifting device (21) is maximally raised or maximally lowered are the first (5) and the second partial chamber (6) of this hydraulic unit fluidically connected to one another via the overflow channel (13), characterized in that a flow path (14) is formed in the piston (2),which runs between an end face of the piston (2) and an outer peripheral side of the piston (2) and wherein the flow path (14) in the end position of the piston (2) at least partially forms the overflow channel (13).

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