Hydraulic unit having an overflow passage and a lifting device

Abstract

The invention relates to a hydraulic unit (1) having a cylinder (3) and a piston (2) guided in the cylinder (3) and adjustable between an operating position and an end position, in which the piston (2) sealingly divides the cylinder chamber into a first partial chamber (5) and a second partial chamber (6), in which, 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 an overflow channel (13). A flow channel (14) is formed in the piston (2) extending between the front of the piston (2) and its outer periphery, the flow channel (14) at least partially forming the overflow channel (13) in the end position of the piston.

Description

[Technical Field]

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

[0002] Hydraulic units of the type mentioned at the outset can be used to move heavy loads. For this purpose, they generally have a cylinder in which a piston is displaceably guided. In the working position, the piston sealably divides the cylinder chamber into two partial chambers. When hydraulic fluid is supplied to one of the partial chambers via an inlet, the piston is displaced hydraulically. A piston rod connected to the piston makes it possible to mechanically transmit the displacement movement of the piston to the load to be moved.

[0003] It is common for several hydraulic units to operate simultaneously, and in this case it is generally necessary to ensure synchronization between them, which means that the pistons of the hydraulic units can be moved to the same position, especially to an end position, as needed, in order to prevent unstable tilting of the lifted load via several points of application of force.

[0004] It is also known to operate multiple hydraulic units in pairs in separate hydraulic circuits to achieve high reliability. In this case, two pairs of hydraulic units can be provided, with the pistons of the first pair of hydraulic units mechanically connected to the pistons of the second pair of hydraulic units. Each hydraulic unit of the first pair is hydraulically connected to a respective one of the second pair of hydraulic units. In the event of a loss of function in one hydraulic circuit, the lifted load can be maintained without loss through the system pressure of the other hydraulic circuit and the mechanical connection of the pistons. A hydraulic circuit diagram for a car lift with two cross-connected hydraulic units mechanically connected in pairs is shown, for example, in German Patent Application No. 102014113301.

[0005] However, signs of wear in seals or other components of the hydraulic units can lead to leaks, resulting in hydraulic fluid leaking from one hydraulic unit. Additionally, the hydraulic units can expand differently as a result of temperature differences, which can lead to different amounts of hydraulic fluid being required to synchronously move the pistons of the hydraulic units to their respective end positions.

[0006] In the case of the above-mentioned configuration having two pairs of hydraulic units, which can achieve high reliability, a pressure difference may occur between the hydraulically connected hydraulic units, and it is desirable to be able to perform pressure equalization to avoid mechanical tension between the mechanically connected hydraulic units.

[0007] One way to achieve the desired synchronization and pressure equalization between multiple hydraulic units is to connect a first hydraulic unit to a hydraulic fluid source in a known manner, and always supply the necessary amount of hydraulic fluid via the hydraulic fluid source so that the piston of the hydraulic unit can be moved to its end position. The first hydraulic unit is configured so that the piston opens an overflow passage in its end position, and the first and second partial chambers of the first hydraulic unit are fluidically connected to each other via the overflow passage. This overflow passage allows hydraulic fluid to flow from the first partial chamber of the first hydraulic unit to the second partial chamber, and then from the second partial chamber via the overflow pipe to the second hydraulic unit, thereby moving the piston of the second hydraulic unit to its end position as well. In this way, synchronization between the two hydraulic units can be achieved with a small number of hydraulic components, in particular with only one hydraulic fluid source. Additionally, pressure equalization between the first and second hydraulic units can be achieved via the overflow passage.

[0008] The design of the overflow channel is known from EP 2 428 482 A1. For this purpose, the inside of the cylinder has a recess formed directly in front of it. When the piston is in its working position, the recess is located completely within one of the cylinder's partial chambers. When the piston reaches its end position in the area of ​​the recess, a fluid-conducting connection is formed inside the cylinder between the two partial chambers of the cylinder of the hydraulic unit.

[0009] A disadvantage of the known overflow channel designs is that to reach one partial chamber of the cylinder, the hydraulic fluid must flow past the outside of the piston over the entire height of the piston in its end position, which places particularly strong stresses on sealing elements that may be arranged on the outside of the piston, which leads to high wear and therefore to high maintenance and repair needs for the entire hydraulic unit. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] German Patent Application Publication No. 102014113301 [Patent Document 2] European Patent Application Publication No. 2428482 Summary of the Invention [Problem to be solved by the invention]

[0011] The invention is therefore based on the task of proposing a hydraulic unit with an overflow function which requires less maintenance. [Means for solving the problem]

[0012] This problem is solved by the features of claim 1. Furthermore, this problem is solved by a lifting device according to claim 14. Advantageous embodiments are the subject of the respective dependent claims.

[0013] The hydraulic unit according to the invention has a cylinder and a piston guided in the cylinder in a known manner. The piston is displaceable between an operating position and an end position. In the operating position, the piston sealably divides the cylinder chamber into a first partial chamber and a second partial chamber. In the end position of the piston, the first and second partial chambers are fluidly connected to one another by an overflow channel.

[0014] Important to the invention is that the piston is formed with a passage extending between the front and outer periphery of the piston, and the overflow path is at least partially formed by the passage in the end position of the piston.

[0015] The hydraulic unit according to the invention has advantages with regard to the service life of the hydraulic unit and the necessary maintenance measures, since the configuration of the flow passage according to the invention makes it possible to direct the flow of hydraulic fluid between the two partial chambers of the hydraulic unit, such as a bypass pipe, and to avoid damaging contact with functionally important parts at the outer periphery of the piston, for example by means of at least one piston seal or guide band arranged between the front opening of the flow passage relative to the longitudinal axis of the piston and the outer periphery opening of the flow passage.

[0016] According to the invention, the flow passage is formed in the piston and is completely surrounded by the piston at least in part. Furthermore, the invention is not limited to a specific path or cross-sectional shape of the flow passage. In a simple embodiment, the flow passage can extend in the form of a hole extending linearly from the front face to the outer periphery of the piston. Alternatively, the flow passage in the piston can comprise two blind holes that are fluidly connected to each other by intersecting in the region of the hole bottom. One of the blind holes is located at the front face and the other blind hole is located on the outer periphery of the piston.

[0017] In a simplified embodiment, the piston may be guided sealingly in the cylinder in the working position, as is known. In this case, one end of the flow passage leads directly to one of the two partial chambers of the cylinder at the front of the cylinder. The other end of the flow passage on the outer periphery of the piston faces the inner wall of the cylinder, against which the piston is guided sealingly. When hydraulic pressure is applied directly to the piston in the working position, the piston is displaced in a known manner to its end position, directly in front of the flow passage. Upon reaching this end position, the flow passage on the outer periphery of the piston can directly connect to the second partial chamber. It is therefore within the scope of the present invention that the overflow passage is formed only by the flow passage in the end position of the piston. However, in another embodiment, a further flow passage is provided, which also forms part of the overflow passage, and which forms a fluid-conducting connection between the two partial chambers.

[0018] According to the spirit of the present invention, the end position of the piston is not limited to a single position of the piston in the cylinder, but also includes the region where the piston is close to the position. Therefore, it is within the scope of the present invention that the overflow passage is not only opened when the piston reaches the end position, but is already opened when the piston is in the end position region. Furthermore, the present invention is not limited by how many end positions the piston can assume in the hydraulic unit. Rather, the end position of the piston can be defined by any mechanical stop within the cylinder, such as a cylinder cover, a cylinder bottom, or a stop element that protrudes, for example, radially into the interior space of the cylinder.

[0019] The present invention is not limited to the structure of the hydraulic unit, its cylinders or pistons. It is therefore within the scope of the present invention for the hydraulic unit to be configured as a so-called synchronous cylinder or a so-called differential cylinder. The pistons preferably have a cylindrical basic shape. Furthermore, in this case, the pistons may have a circumferential profile on their outer periphery, which may be used to position the sealing elements in a complementary engagement.

[0020] In an advantageous development, the flow passage comprises a flow resistor in order to reduce the pressure of the hydraulic fluid flowing in the flow passage.

[0021] The aforementioned development allows the piston to be provided with a sealing element on its outer periphery, where the overflowing hydraulic fluid comes into contact with the flow passage without passing through the flow passage. The applicant's investigations have shown that the harmful effects of the overflowing hydraulic fluid can be reduced or completely avoided by reducing the hydraulic pressure of the hydraulic fluid in advance. In this connection, the flow passage in the piston can be used not only to form an overflow path, but also to arrange a flow resistor, which simultaneously serves to reduce the pressure. The reduction in the pressure of the hydraulic fluid includes, in particular, a reduction in the static and / or dynamic pressure.

[0022] In an advantageous development, the piston has at least one sealing element on its outer periphery, which is arranged axially offset with respect to the opening of the flow passage on the outer periphery of the piston and which partially limits the overflow channel in the end position of the piston, the sealing element being arranged downstream of the flow resistor with respect to the flow direction of the hydraulic fluid through the overflow channel.

[0023] The applicant's investigations have shown that a flow path configuration with a flow resistor is particularly advantageous when the overflowing hydraulic fluid flows through a sealing element arranged on the outer periphery of the piston and the pressure of the hydraulic fluid is reduced by the flow resistor. 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 come into direct contact in the end position of the piston. In particular, the sealing element is configured to divide the cylinder chamber into a first partial chamber and a second partial chamber in the operating position of the piston for sealing purposes.

[0024] The flow resistor can be adjusted, for example, depending on the geometric features of the flow path and / or the surface properties of the flow path. Adjustable geometric features include, for example, the length through which the flow passes and / or the curvature and / or macroscopic profile of the flow path. Adjustable surface properties include, for example, the microscopic profile. The flow resistor can be formed as an integral part of the flow path or as a separately configured component that can be interchangeably positioned within the flow resistor.

[0025] In a preferred embodiment of the hydraulic unit, the flow resistor is an orifice or a throttle. An orifice can be used to reduce the pressure of the flowing hydraulic fluid, thereby adjusting a constant volumetric flow of hydraulic fluid independent of the system pressure. A throttle can also be used to reduce the pressure of the hydraulic fluid, but the adjustable volumetric flow generally has a linear dependence on the system pressure. Thus, an orifice can at least reduce the hydraulic pressure of the hydraulic flow to a constant value, whereas a throttle can adjust the hydraulic pressure depending on the system pressure. Although the operating principles of orifices and throttles are different, throttles as well as orifices are advantageously used to reduce the pressure of the hydraulic fluid, in particular so that sealing elements arranged on the outer periphery of the piston are protected in the event of contact with overflowing hydraulic fluid.

[0026] The orifice and throttle may be standardized hydraulic components whose respective pressure reducing effects can be determined by known design rules. It is also within the scope of this preferred development that the orifice and throttle are each arranged in the flow passage as an integrated part or as a separately formed component. The advantage of an integrated configuration is that it reduces the subsequent assembly steps that are required to connect the orifice or throttle to the piston. Alternatively, the orifice and throttle can be assembled to the piston in the form of an insert.

[0027] In a preferred embodiment of the hydraulic unit, the flow passage extends into the annulus at the outer periphery of the piston.

[0028] The annular passage configuration allows the hydraulic fluid exiting the flow passage to be distributed partially or completely around the circumference of the piston. This is particularly advantageous because hydraulic fluid overflow can occur regardless of the rotational position of the piston. This becomes particularly difficult when the flow passage must connect to a fluid passage at the end position of the piston's outer periphery to form an overflow path. The annular passage thus allows the piston to be assembled in any rotational position relative to the longitudinal axis of the cylinder.

[0029] In a preferred embodiment of the hydraulic unit, a recess is formed as a longitudinal groove in the inner wall of the cylinder, into which a passage formed in the piston in the end position connects, thus forming part of an overflow path in the end position of the piston. The recess may extend at least partly parallel to the longitudinal axis of the cylinder. However, it is also within the scope of this preferred embodiment for the recess to extend at an angle to the longitudinal axis of the cylinder and / or in a curved path.

[0030] The position and length of the recess can be used to determine in a structurally simple way how large the spatial area in which overflow occurs should be. The longer the groove, the more the longitudinal groove penetrates into the internal space of the cylinder and the sooner the hydraulic fluid overflows when the piston approaches the end position. Furthermore, the mechanical stop of the piston at the end position can be avoided, which can have a positive effect on the damping characteristics of the hydraulic unit when the end position is reached, since it is possible to avoid sudden stopping of the piston.

[0031] In a preferred development, the sealing element is arranged along the longitudinal axis of the cylinder in the region of the recess on the outer circumferential surface of the piston in its end position. The overflowing hydraulic fluid passes through the piston and exits a channel formed in the piston, and can come into contact with the sealing element without damaging it, particularly under reduced pressure. The sealing element is preferably arranged in the region of the recess so that, together with the recess, it forms part of the overflow path in the end position of the piston.

[0032] In a preferred embodiment, two or more longitudinal grooves are distributed around the inner circumference of the cylinder, each forming a partial overflow channel in the end position of the piston. The distribution of the longitudinal grooves results in multiple possible overflow positions, and the outlet area of ​​the flow path can correspond to 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, the hydraulic fluid can overflow simultaneously through multiple recesses, and the hydraulic fluid can enter the multiple recesses in an evenly distributed manner over the outer circumference of the piston.

[0033] In a simplified embodiment, the recess leads directly to one partial chamber of the cylinder directly in front of the cylinder. In this embodiment, the cylinder and the piston can be connected to each other in such a way that the flow passage of the piston leads to the recess in the end position either directly or indirectly via the annular passage. In this embodiment, the overflow passage comprises a flow passage in the piston, possibly an annular passage, as well as a recess, via which the overflowing hydraulic fluid can reach the second partial chamber.

[0034] In an advantageous development of the hydraulic unit, the overflow channel comprises a side channel which passes radially through the cylinder wall in the end position of the piston.

[0035] In a simplified embodiment of the aforementioned development, the cylinder and the piston can be connected to one another in such a way that the flow passage in the piston, in its end position, leads to a bypass passage either directly or indirectly via the annular passage. Via the bypass passage, hydraulic fluid can flow out of the overflow passage and reach the second partial chamber from there. For this purpose, hydraulic lines connecting the partial chambers to one another can be provided in a simple manner. In this embodiment, the overflow passage comprises a flow passage in the piston, possibly the annular passage, as well as a bypass passage, via which the overflowing hydraulic fluid can reach the second partial chamber.

[0036] In an advantageous development, the overflow passage also includes a recess leading to the bypass passage. The overflow passage thus includes a flow path in the piston, possibly an annular passage, a recess, and a bypass passage. In this embodiment, the recess preferably has a circumferential edge formed on the inside of the cylinder. The recess is spaced apart from the front end of the cylinder.

[0037] In an advantageous development, the sealing element is arranged on the outer circumferential surface of the piston along the longitudinal axis of the cylinder in the piston's end position between the side passage and the opening of the flow passage on the outer circumferential surface of the piston, particularly in the region of the annular passage. This ensures in a structurally simple manner that the sealing element on the piston is the only sealing component that comes into contact with the overflowing hydraulic fluid. In particular, it is possible to prevent other components, such as guide bands arranged on the outer circumferential surface of the piston, from coming into contact with the overflowing hydraulic fluid. This prevents the components in the hydraulic unit from showing signs of wear even when the hydraulic fluid is decompressed. In a preferred development, a clearance passage is formed between the outer circumferential surface of the cylinder and a cover element that closes the interior space of the cylinder at the front side, and the side passage leads to the clearance passage. The clearance passage is preferably formed essentially coaxially with the longitudinal axis of the cylinder.

[0038] According to the aforementioned embodiment, the cover element surrounds the outside of the cylinder and is at least partially oversized compared to the outside of the cylinder to form a clearance channel inside the cover. The cover element may define at least one mechanical stop that limits the displacement range of the piston in the cylinder and defines the end positions. The cover element is preferably configured so that a partial chamber into which hydraulic fluid should overflow is included between the cover element and directly in front of the piston in the end positions of the piston. Furthermore, the cover element may have an overflow connection through which overflowing hydraulic fluid can be guided to another hydraulic unit.

[0039] In a preferred embodiment, a first guide band and a second guide band are arranged on the outer periphery of the piston and are used to guide the piston on the inner wall of the cylinder. The guide bands improve support of the piston in its end position because the forces associated with the at least temporarily asymmetric strength ratio caused by the overflow can be supported by the guide bands on the inner wall of the cylinder. This reduces the mechanical load on a sealing element that can also be arranged on the outer periphery of the piston. It is preferred that at least one of the guide bands is arranged between the opening of the flow passage directly in front of the piston and the opening on the outer periphery of the piston.

[0040] As mentioned above, the problem on which the invention is based is also solved by the lifting device according to the invention.

[0041] The lifting device according to the present invention, which is in particular an automobile lift for vehicles, has at least first and second hydraulic units which act as lifting drives for lifting the lifting device, each having an inlet for supplying hydraulic fluid when lifting the lifting device and an overflow for discharging hydraulic fluid.

[0042] The first hydraulic unit is formed as a master unit by having its overflow connected fluidly to the inlet of a second hydraulic unit, which acts as a slave unit, and each hydraulic unit has a cylinder and a piston sealingly guided in the respective cylinder, which divides the interior space of the corresponding cylinder into first and second partial chambers.

[0043] At least one of the hydraulic units of the lifting device has an overflow passage, which is configured in such a way that, at least in the end position of the corresponding piston, the first and second partial chambers of the unit are fluidly connected to each other via the overflow passage when the lifting device is maximally raised or maximally lowered.

[0044] The overflow passage corresponds to a passage extending from directly in front of the piston to the outer periphery of the piston in its end position.

[0045] Such a design of the lifting device makes it possible to avoid the drawbacks of overflows that are present in existing lifting devices by reducing the strain on the components used, especially the seals, which increases the service life of the aforementioned components and reduces the frequency of their maintenance. Furthermore, the components of the hydraulic unit are protected, and the overflow is carried out reliably, which increases the safety of working with such a lifting device.

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

[0047] Further advantages and embodiments can be found in the following description of example embodiments with reference to the figures. [Brief explanation of the drawings]

[0048] [Figure 1] FIG. 3 is a cross-sectional view of the hydraulic unit in the working position. [Figure 2] 2 is a cross-sectional view of the hydraulic unit according to FIG. 1 in an end position; [Figure 3] 3 shows an embodiment of a lifting device with two hydraulic units according to FIGS. 1 and 2, one hydraulic unit being configured as a master unit and the other as a slave unit; [Figure 4] Schematic diagram of the hydraulic system of the lifting device according to Figure 3. DETAILED DESCRIPTION OF THE INVENTION

[0049] 1 and 2 a hydraulic unit 1 is illustrated, the piston 2 of which is in the region of the working position according to FIG. 1 and the end position according to FIG.

[0050] The hydraulic unit 1 comprises a cylinder 3 in which a piston 2 is guided along a cylinder axis 4. When the piston 2 is in its working position (see FIG. 1 ), it sealingly divides the cylinder chamber into a first partial chamber 5 and a second partial chamber 6. When the first partial chamber 5 is filled with hydraulic fluid, the piston 2 is hydraulically displaced along the cylinder axis 4. In this case, the volume of the first partial chamber 5 increases, while the volume of the second partial chamber 6 decreases. A cylinder cover 7, which defines the second partial chamber 6, has an overflow connection 8 through which the hydraulic fluid in the second partial chamber 6 can escape. The piston 2 has two encircling guide bands 9, 10 and two encircling seals 11, 12 on its outer periphery. The guide bands 9, 10 serve to guide the piston 2 along the inner wall of the cylinder, while the seals 11, 12 sealingly separate the partial chambers 5 and 6 from each other when the piston 2 is in its working position.

[0051] In the illustrated embodiment, hydraulic unit 1, together with another hydraulic unit (see FIGS. 3 and 4), is used to lift a vehicle, hold it in the lifted position, and lower it again if necessary. It is desirable to ensure that the pistons of both hydraulic units are reliably moved to their end positions in their respective cylinders to prevent the lifted load from becoming unstable. However, signs of deterioration, changing environmental conditions, and inevitable leaks in hydraulic seals can cause variations in the amount of hydraulic fluid required to move the pistons of both hydraulic units to their end positions. To achieve this with minimal structural effort, the hydraulic unit shown in FIG. 1 is connected to the other hydraulic unit via an overflow connection 8. When piston 2 reaches its end position, hydraulic fluid can flow via overflow path 13 (shown only in FIG. 2) first from first partial chamber 5 to second partial chamber 6, and from the second chamber to the other hydraulic unit via overflow connection 8. This allows additional hydraulic fluid to be supplied from first hydraulic unit 1 to the other hydraulic unit (see FIGS. 3 and 4) if it is unexpectedly needed. The structural implementation of this functionality is detailed below.

[0052] As shown in Fig. 1, the piston 2 has a flow passage 14, which extends from the front of the piston to the outer periphery of the piston 2. In the illustrated embodiment, the flow passage 14 is essentially formed by two blind holes that intersect in the region of their bottoms to form a fluid-conducting connection between the openings of the holes. In the working position of the piston 2 shown in Fig. 1, the flow passage 14 is sealingly closed by the inner wall of the cylinder at the outer periphery of the piston 2. By applying pressure to the piston from one side, it is possible to move the piston to its end position in the manner described above.

[0053] 2, the piston 2 is mechanically stopped by the cylinder cover 7. In this case, the aforementioned overflow path 13 is open, so that an overflow of hydraulic fluid is possible between the first partial chamber 5 and the second partial chamber 6 in the end position of the piston 2. For this purpose, the overflow path 13 comprises a flow path 14, an annular path 15, a number of recesses 16, a number of side paths 17 and a gap path 18 in the end position of the piston 2.

[0054] The design of the flow passage 14 extending between the front and outer periphery of the piston 2 has the advantage that the guide band 10 is not in contact with the overflowing hydraulic fluid 19 that is conducted through the piston 2 in its end positions. Compared to known overflow paths in which the hydraulic fluid can overflow exclusively at the outer periphery of the piston 2 from the first partial chamber 5 into the second partial chamber 6, the guide band 10 is protected as it is not subjected to stress by the overflowing hydraulic fluid 19.

[0055] The annular passage 15 is configured as a circumferential groove that extends around the outer periphery of the piston 2 so that the hydraulic fluid can reach it even when the piston 2 is in its working position. The advantage of the annular passage 15 is that the hydraulic fluid in the annular passage 15 exerts a centering effect on the piston 2 by its hydraulic pressure even when the piston 2 is in its working position. This allows the components 9, 10, 11, 12 arranged on the outer periphery to be loaded evenly, thereby reducing wear on the components. Furthermore, the hydraulic fluid can be distributed over the outer periphery of the piston 2 via the annular passage 15 and enter the second partial chamber 6 simultaneously via several recesses.

[0056] In the end position, the annular passage 15 is connected to a number of recesses 16, which are formed as longitudinal grooves and each extend essentially parallel to the cylinder axis 4. The described flow passage 14 prevents the guide band 10 from coming into direct contact with the overflowing hydraulic fluid 19 in the manner of a bypass, while this type of contact between the hydraulic fluid and the seal 12 is not prevented. Nevertheless, to avoid damage to the seal 12, a flow resistor 20 is arranged in the flow passage 14, which serves to reduce the hydraulic pressure of the overflowing hydraulic fluid in the overflow passage 13. Such a pressure reduction has a positive effect on the signs of wear in the seal 12. In the illustrated embodiment, the flow resistor 20 is an orifice, which serves to reduce the static pressure of the hydraulic fluid 19 in the overflow passage 13. Alternatively, the flow resistor 20 can be a throttle.

[0057] The recess 16 is formed as a longitudinal groove extending essentially parallel to the cylinder axis 4. The length and position of the recess are selected so that an overflow of hydraulic fluid can occur from the first partial chamber 5 to the second partial chamber 6 already when the piston 2 is approaching but not yet completely reaching its end position. Such an early overflow of hydraulic fluid makes it possible to brake the piston 2 before it reaches its end position, thereby avoiding a sudden stop in the movement of the lifted load.

[0058] The structure of the bypass 17 shown here makes it possible to reduce the influence of the notch effect occurring in the cylinder wall by making the recess 16 relatively short.

[0059] The cylinder cover 7 has an oversized dimension relative to the outer wall of the cylinder at its face facing the cylinder 3. This surrounds a gap-like area between the cylinder cover 7 and the outer wall of the cylinder, which area extends parallel to the cylinder axis 4. This area is referred to in the present text as a gap channel 18 and is used to fluidically connect the bypass channel 17 to the second partial chamber 6. This makes it possible to dispense with an additional hydraulic line for fluidically connecting the bypass channel 17 to the second partial chamber 6.

[0060] In a manner not shown here, it is also possible to configure a further embodiment of the hydraulic unit 1 in such a way that the overflow channel 13 is formed exclusively by the flow channel 14. In this case, the opening of the flow channel 14 on the outer periphery of the piston 2 can lead directly to the second partial chamber 6 in the end position of the piston 2 in the region of the cylinder edge directly in front of it. Another further embodiment can comprise an overflow channel 13 formed without an annular channel 15, in which case the flow channel 14 leads directly to one recess 16 or one bypass channel 17 in the end position. In yet another further embodiment, it is possible to completely omit the bypass channel 17 and the gap channel 18 by selecting the position and length of the aforementioned recess 16 so that it leads directly to the second partial chamber 6.

[0061] 3 shows a lifting device configured as a lift pillar / car lift 21. The lifting device comprises two lift pillars 22 and 23, which extend essentially vertically and to which one shank 24 or 25 is respectively arranged. The shanks 24, 25 are vertically movable and have supports 24a, 24b or 25a, 25b, respectively, which are used to accommodate a vehicle, lift it by means of the lift pillar / car lift 21 and hold it in the lifted position.

[0062] In a manner not shown in detail here, the lift pillars 22 and 23 each comprise two hydraulic units, each configured corresponding to the hydraulic unit 1 shown in FIGS. 1 and 2. A first pair of hydraulic units is arranged on the lift pillar 22, and a second pair on the lift pillar 23. The pistons of the first pair of hydraulic units arranged on the lift pillar 22 are mechanically connected to one another. The pistons of the second pair of hydraulic units arranged on the lift pillar 23 are likewise mechanically connected to one another. Furthermore, one hydraulic unit of the first pair on the lift pillar 22 is connected in series with one hydraulic unit of the second pair on the lift pillar 23 in a command-slave arrangement, as will be explained below.

[0063] The hydraulic units, which are hydraulically connected to one another, are each driven by a common hydraulic circuit, which is shown in detail in Figure 4. The aforementioned combination of mechanical and hydraulic connections between the hydraulic systems of the lifting pillars 22, 23 makes it possible to obtain good reliability, since even if the system pressure of one hydraulic circuit drops as a result of an unexpected fault, it is still possible to maintain the lifted load via the system pressure of the respective other hydraulic circuit and the mechanical connection of the hydraulic systems of the lifting pillars 22, 23.

[0064] Figure 4 shows a schematic hydraulic diagram of the car lift 21 according to Figure 3 and the hydraulic diagram between two hydraulic units K and F arranged on the lift pillars 22, 23. Here, one hydraulic unit on the lift pillar 22 acts as a so-called master unit and one hydraulic unit on the lift pillar 23 acts as a so-called slave unit.

[0065] To operate the car lift, hydraulic fluid is supplied from the tank 26 via a pump 27 through a suction filter 28 to the master unit K via a first supply pipe 29. This causes the piston 30 of the master unit K to move up vertically. The hydraulic fluid moved by the piston 30 is supplied to the slave unit F via an overflow pipe 31. This also causes the piston 32 to move up and down, and at that time, the hydraulic fluid contained in the slave unit F is moved and the hydraulic fluid is supplied to the tank 26 via a second overflow pipe 33.

[0066] As long as the piston 30 of the master unit K reaches its end position before the piston 32 of the slave unit F reaches its end position, the construction of the master unit K in the form of the hydraulic unit 1 according to FIGS. 1 and 2 allows for an overflow of hydraulic fluid via the overflow pipe 31. This allows for a simplified construction of the hydraulic system shown in FIG. 4 by reducing the number of components required. In particular, only one pumping device is required to operate not only the master unit K but also the slave unit F. Furthermore, pressure equalization can be performed between the hydraulic units K and F.

[0067] To move the pistons 30, 32 again in the opposite direction and lower the lifted shank, the 2 / 2-way valve 34 can be actuated. In this case, the hydraulic fluid contained in the master unit K or the slave unit F is displaced by the weight of the lifted vehicle and supplied to the tank 26 via a return line 35. The speed of the lowering movement can be controlled via 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 a separate line.

Claims

1. It comprises a cylinder (3) and a piston (2) guided in the cylinder (3) and adjustable between an operating position and an end position, the piston (2) in the working position sealingly divides the cylinder chamber into a first partial chamber (5) and a second partial chamber (6); In the hydraulic unit (1), the first partial chamber (5) and the second partial chamber (6) are fluidically connected to one another by an overflow channel (13) in the end position of the piston (3), A flow path (14) is formed in the piston (2) and extends between the front of the piston (2) and the outer circumferential surface of the piston (2); The hydraulic unit (1) is characterized in that the flow passage (14) at least partially forms the overflow passage (13) in the end position of the piston (2).

2. 2. The hydraulic unit (1) according to claim 1, characterized in that the flow path (14) has a flow resistor (20) formed to reduce the pressure of the hydraulic fluid (19) flowing through the flow path (14).

3. the piston (2) has at least one sealing element (12) on the outer peripheral surface of the piston (2), the sealing element being axially offset from the opening of the flow passage on the outer peripheral surface of the piston; 3. A hydraulic unit (1) according to claim 2, characterized in that in the end position of the piston (2), it partially limits the overflow path (13) and is arranged downstream of the flow resistor (20) with respect to the flow direction of the hydraulic fluid (19) flowing through the overflow path.

4. A hydraulic unit (1) according to claim 2 or 3, characterized in that the flow resistor (20) is an orifice or a throttle.

5. 5. The hydraulic unit (1) according to claim 1, wherein the flow path (14) is connected to an annular passage (15) of the piston (2) at the outer circumferential surface of the piston (2), the annular passage (15) extending substantially coaxially with respect to the longitudinal axis of the piston (2).

6. 6. A hydraulic unit (1) according to claim 1, characterized in that the overflow channel (13) comprises at least one recess (16) in the inner wall of the cylinder in the end position of the piston (2), the recess extending at least partly parallel to the longitudinal axis (4) of the cylinder.

7. 7. A hydraulic unit (1) according to at least claim 3 or 6, characterized in that the sealing element (12) is arranged in the region of the recess (16) along the longitudinal axis (4) of the cylinder in the end position of the piston (2).

8. 8. Hydraulic unit (1) according to at least claim 7, characterized in that the sealing element (12) and the recess (16) form part of the overflow channel (20) in the end position of the piston (2).

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

10. 10. A hydraulic unit (1) according to any one of claims 1 to 9, characterized in that the overflow passage (13) comprises a side passage (17) extending radially through the cylinder wall in the end position of the piston.

11. A hydraulic unit (1) according to at least claims 6 and 10, characterized in that the recess (16) leads to the bypass (17).

12. 12. A hydraulic unit (1) according to at least claim 3 or 11, characterized in that the sealing element (12) is arranged in the end position of the piston (2) along the longitudinal axis (4) of the cylinder between the side passage (17) and the opening of the flow passage (14) on the outer circumferential surface of the piston (2), in particular in the region of the annular passage (15).

13. 12. The hydraulic unit (1) according to claim 11, characterized in that the overflow passage (13) comprises a gap passage (18) extending between the outer wall of the cylinder and a cover element (7) covering the outer wall in the end position of the piston (2), and the side passage (17) leads to the gap passage (18).

14. A lifting device (21), in particular a car lift for vehicles, comprising: at least first and second hydraulic units (1) operating as at least first and second lifting drives for lifting the lifting device (21); Each of the hydraulic units (1) has an inlet for supplying hydraulic fluid when lifting the lifting device (21) and an overflow for discharging the hydraulic fluid; the first hydraulic unit (1) is formed as a master unit (K) by having its overflow connected in fluid communication with the inlet of the second hydraulic unit (1) acting as a slave unit (F); Each of the hydraulic units (1) has a cylinder (3) and a piston (2) sealingly guided in the cylinder (3), the piston (2) fluidly separating the interior space of the cylinder (3) into a first partial chamber (5) and a second partial chamber (6); At least one of the hydraulic units has an overflow channel (13), which is arranged so that the first partial chamber (5) and the second partial chamber (6) of the hydraulic unit are fluidically connected to one another via the overflow channel (13) only in the region of an end position of the corresponding piston (2) when the lifting device (21) is at its maximum raised or lowered position, A flow path (14) is formed in the piston (2), the flow path extending between a front face of the piston (2) and an outer peripheral surface of the piston (2), and the flow path (14) at least partially forms the overflow path (13) at the end position of the piston (2).

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