Filter device for a hydraulic system

Abstract

Filter device for a hydraulic system, in particular on a forklift truck, comprising a filter element (20) with a filter body (22) and a valve element (30), wherein the valve element (30) opens a first bypass channel (A) bypassing the filter body (22) for fluid flowing through the filter device (10) below a predetermined first system pressure and opens a second bypass channel (B) bypassing the filter body (22) for fluid flowing through the filter device (10) above a predetermined second system pressure, wherein the second system pressure is greater than the first system pressure.

Description

[0001] The invention relates to a filter device for a hydraulic system, which can be used in particular in an industrial truck.

[0002] Industrial trucks usually have a hydraulic system which controls the load-bearing parts of the industrial truck. The hydraulic system can be used, for example, to control a mast with a lifting mast and a load section connected to the mast, for example to lift or lower loads. For this purpose, the hydraulic system usually comprises several hydraulic cylinders acting on the mast, to which hydraulic fluid is supplied from a tank by a hydraulic unit. The hydraulic fluid is distributed to or released from the cylinders via a valve arrangement. By actuating the hydraulic cylinders, a load on the load section of the mast can be moved, for example lifted via a lifting cylinder. To lower the load, hydraulic fluid is drained from the lifting cylinder via the valve arrangement and returned to the hydraulic tank.

[0003] Furthermore, such a hydraulic system usually includes a filter with which impurities can be filtered out of the hydraulic fluid. These impurities can, for example, be particles caused by material abrasion or clumps in the hydraulic oil itself. The filter is usually a return line filter, i.e. it filters the fluid returned to the hydraulic tank. Conventional return line filters have the disadvantage that fluid flowing through the filter is exposed to high flow resistance while achieving the desired filtering effect. This means that returned fluid can only escape from the hydraulic cylinder slowly, which in turn results in slow load movement. This is particularly disadvantageous when lowering the load section or the mast without a load on it, since without a load it is usually only the dead weight of the mast or load section that displaces the hydraulic fluid from the hydraulic cylinder.This means that lowering without a load is particularly slow.

[0004] It is known to increase the lowering speed without load by bypassing the return filter when the pressure in the hydraulic system is low. EP 2 236 453 B1 describes a hydraulic lifting device with a hydraulic system that includes a return filter and a filter control valve upstream of the return filter. The filter control valve includes a valve member that connects a filter inlet optionally to a first filter outlet or a second filter outlet, or to both outlets. A spring biases the valve member into a first position in which the valve member directs fluid flowing into the filter control valve to the first valve outlet when the system pressure is low, thus bypassing the hydraulic filter via a bypass circuit.As system pressure increases, the valve element is displaced against the spring force, connecting the valve inlet to the second valve outlet, which is connected to the hydraulic filter. Such a complete bypass of the filter is complex from a control perspective and also requires a lot of space.

[0005] The invention is therefore based on the object of providing a filter device which enables the filtration of the hydraulic fluid to be switched off in several different pressure ranges.

[0006] The invention solves the problem by a filter device according to claim 1. Advantageous embodiments are the subject of the subclaims, the description and the figures.

[0007] The filter device according to the invention for a hydraulic system, in particular on an industrial truck, comprises a filter element with a filter body and a valve element, wherein the valve element opens a first bypass channel for fluid flowing through the filter device below a predetermined first system pressure, bypassing the filter body, and opens a second bypass channel for fluid flowing through the filter device above a predetermined second system pressure, bypassing the filter body, wherein the second system pressure is greater than the first system pressure.

[0008] The system pressure in the hydraulic system depends, among other things, on the permeability of the filter body and the mass of the load. If, for example, the hydraulic system comprises a lifting device with a lifting frame and / or load section operated by a lifting cylinder, a load on the load section exerts a force on the hydraulic fluid in the hydraulic system via the lifting cylinder. With a heavy load, for example, the system pressure is higher than with a light load. Especially when there is no load on the load section, the system pressure is usually low. In order to enable the load section of the lifting device to be lowered quickly even without a load, a first bypass channel is opened below the first system pressure, bypassing the filter body. This means that hydraulic fluid flowing through the filter device flows through the first bypass channel below the first system pressure.The first system pressure can therefore be defined so that the first bypass channel opens when no load is resting on the load part. Consequently, it can be specified that the first system pressure is not reached when only the dead weight of the load part and / or the mast is bearing down on the hydraulic system.

[0009] According to the invention, a second bypass channel is further provided, which is opened by the valve element when the system pressure is above a predetermined second system pressure. This means that the filter body of the filter element can also be bypassed in a second, higher pressure range. Bypassing the filter element enables a faster flow rate of the hydraulic fluid guided through the hydraulic system and thus a faster movement of the hydraulic cylinders connected to the hydraulic system. By bypassing the filter body even at high system pressures above the second system pressure, the hydraulic system and thus the industrial truck can be used reliably even when the filter is heavily clogged with contaminants. The predetermined second system pressure is therefore preferably selected such that the hydraulic system only reaches such a pressure value when the permeability of the filter body has (significantly) decreased.

[0010] The invention enables the filter body to be bypassed in two different, separate pressure ranges. In particular, the filter body can thus be bypassed not only at low system pressure, but also at high system pressure. It can be provided that even when the first and / or second bypass channel is open, fluid continues to flow through the filter body itself and is thus filtered. However, since the bypass channels have a significantly lower flow resistance, the majority of the fluid flow will still flow through them in this case.

[0011] According to a preferred embodiment, both bypass channels are closed above the first system pressure and below the second system pressure, so that fluid flowing through the filter device only flows through the filter body of the filter element. Thus, if the system assumes a pressure value between the first and second system pressures, both the first bypass channel and the second bypass channel are closed to enable complete filtration of the fluid. Within this normal pressure range, the filter device enables reliable filtration of contaminants from the fluid.

[0012] According to a further embodiment, the first bypass channel is closed when the second bypass channel is open. Accordingly, the second bypass channel is closed when the first bypass channel is open. This ensures that the fluid only ever flows through one of the bypass channels. As explained above, the bypass channels are controlled in a pressure-dependent manner. It can therefore be provided that below the first system pressure, the first bypass channel is open and the second bypass channel is closed, while above the second system pressure, the second bypass channel is open but the first bypass channel is closed. In particular, at a pressure value between the first system pressure and the second system pressure, both bypass channels can be closed, as mentioned above.

[0013] According to a preferred embodiment, the filter element has a passage channel extending along a longitudinal axis of the filter element and surrounded by the filter body, wherein the filter element has a fluid inlet at one longitudinal end and a fluid outlet at the other longitudinal end, wherein the fluid inlet and the fluid outlet are connected to one another via the passage channel, and wherein the valve element is arranged at the longitudinal end of the filter element at which the fluid outlet is located. The filter element can, for example, have a cylindrical shape, wherein the filter body of the filter element is in particular designed as a hollow cylinder. Preferably, the filter element has a circular cylindrical shape. The passage channel can extend along the (hollow) cylinder longitudinal axis through the filter body. The filter element can further comprise, for example, a filter housing which supports the filter body.The filter element may have two opposite longitudinal ends, wherein a fluid inlet is provided at one longitudinal end of the filter element through which fluid can penetrate into the filter element, move along the passage channel and exit from the filter element through a fluid outlet at the other longitudinal end of the filter element.

[0014] The valve element is arranged at the fluid outlet end of the filter element. This allows fluid supplied to the filter element by a hydraulic system to flow through the fluid inlet into the passage channel of the filter element. At a system pressure below the first predetermined system pressure, the first bypass channel is opened, allowing the fluid flowing through the filter element to exit the fluid outlet through the valve element. At least a small portion of this fluid flow can also penetrate into the filter body surrounding the passage channel in a direction substantially perpendicular to the longitudinal axis of the filter element and thus exit again in a filtered state on the other side of the filter body along the lateral surface of the cylindrical filter element.At a system pressure that lies between the first and the second system pressure value, both bypass channels of the valve element are preferably closed so that no liquid flowing into the passage channel can leave the filter element via the fluid outlet. Instead, all liquid flows into the filter body and leaves the filter body via the outer surface of the cylindrical filter element. Above the second system pressure value, the valve element opens the second bypass channel and thus allows the liquid flowing into the filter element via the fluid inlet to escape unfiltered from the fluid outlet after passing through the passage channel. However, here too, it cannot be ruled out that at least some of the liquid also penetrates to the outside via the filter body.In principle, the first bypass channel and the second bypass channel do not necessarily bypass the filter body completely; at least a residual flow can also occur through the filter body.

[0015] According to this embodiment, the valve element is therefore integrated directly into the filter element. With this design of the filter device, no external valve technology is necessary to enable the pressure-based bypass control according to the invention. Instead of bypassing the filter element through additional lines and switching valves, the fluid always flows through the filter element, but when the bypass is open, it flows through the passage channel past the filter body. In particular, this design of the filter device ensures that no or at least hardly any contaminants can be washed out of the filter body when the fluid flows through one of the bypass channels. This design requires particularly little installation space and is also easy to install. The entire filter device can be stored pre-assembled and integrated into an existing hydraulic system. This means that the filter device can be easily retrofitted.The valve element can also be designed for retrofitting. Thus, a filter element having a through-channel can be supplemented with the valve element according to the invention. For example, a possibly existing maximum pressure valve at the outlet end of the valve element can be replaced by the valve element according to the invention.

[0016] According to a preferred embodiment, the valve element comprises a valve seat and a two-part valve member, wherein the two-part valve member comprises a first valve body with a through-opening as part of the first bypass channel and a second valve body that is adjustable relative to the first valve body, wherein below the first system pressure the second valve body is spaced from the first valve body in order to open the first bypass channel extending through the through-opening, wherein above the first system pressure the second valve body bears against the first valve body in order to block the through-opening and thus close the first bypass channel. The valve element can therefore comprise a valve seat and a two-part valve member with a first and a second valve body. The second valve body can be adjusted relative to the first valve body and thus open or close a through-opening in the first valve body.The first valve body thus forms a seat for the second valve body. The adjustment of the second valve body relative to the first valve body is pressure-dependent. If the system pressure is below the first system pressure, the second valve body is spaced from the first valve body. For example, the second valve body can be preloaded into this open position of the first bypass channel. At low pressure, the first bypass channel is opened by spacing the second and first valve bodies. If the system pressure in the hydraulic system now rises above the first system pressure, the second valve body moves towards the first valve body and thus closes the through-opening of the first valve body. For this purpose, the second valve body can in particular engage in the through-opening of the first valve body.This movement of the second valve body relative to the first valve body preferably occurs passively, solely due to the system pressure in the filter device. This allows a pressure-controlled valve element to be provided in a structurally simple manner.

[0017] The valve seat of the valve element serves as the seat for the first valve body. This valve seat preferably comprises a seat opening as part of the second bypass channel, wherein below the second system pressure the first valve body rests against the valve seat in order to close the second bypass channel, wherein above the second system pressure the first valve body together with the second valve body is spaced from the seat opening in order to open the second bypass channel extending through the seat opening. The first bypass channel is therefore opened between the first valve body and the second valve body and the second bypass channel is opened between the first valve body and the valve seat. Below the second system pressure the first valve body rests against the valve seat and thus closes the second bypass channel.If the first system pressure is exceeded, the second valve body also rests against the first valve body, so that the first bypass channel is closed accordingly, as explained above. If the system pressure rises to a value above the second system pressure, the second bypass channel is opened by the first valve body lifting off the valve seat of the valve element. The second valve body is also lifted off the valve seat together with the first valve body, whereby the first bypass channel remains closed. Consequently, below the first system pressure only the first bypass channel is open, between the first and second system pressure both bypass channels are closed so that hydraulic fluid can only escape from the filter device via the filter body, and above the second system pressure the first bypass channel is closed, but the second bypass channel is open.In particular, in this embodiment the first bypass channel can extend through the seat opening and through the passage opening, while the second bypass channel only runs through the seat opening. The passage opening and the seat opening can be arranged in alignment one behind the other. This embodiment enables a particularly compact design of the valve element. In particular, with this design the valve body can move passively solely through the existing system pressure. Preferably the valve element with its valve seat and its two-part valve member is integrated into the filter element in the manner described above, i.e. arranged at the fluid outlet of the filter element. For example the second valve body can be adjusted relative to the first valve body and the first valve body relative to the valve seat along the longitudinal axis of the filter element.

[0018] According to a further embodiment, the valve element is connected to a filter cover of the filter element, wherein the filter cover forms the valve seat of the valve element. The valve element and filter element can therefore be connected to one another via the filter cover, wherein the filter cover fulfills two functions. Firstly, it holds the filter body in the filter element, for example via a filter housing, and secondly, it serves as the valve seat of the valve element, i.e., it has the seat opening. This design is particularly compact. The filter cover is preferably located at a longitudinal end of the filter element, at which the fluid outlet is located. The filter element therefore preferably has a passage channel extending along a longitudinal axis, through which the liquid can flow into the filter body and, if necessary, leave the filter element via the valve element, as explained above.

[0019] The valve element preferably further comprises a guide rod on which the valve seat and the two-part valve member are mounted. This guide rod can protrude through the through-opening of the first valve body as well as through the seat opening and optionally also through the second valve body. During the above-explained movements of the second valve body relative to the first valve body or of the first valve body together with the second valve body relative to the valve seat, the first valve body and in particular also the second valve body can move relative to the guide rod. However, it can also be provided that the second valve body is fixedly connected to the guide rod and is moved together with it relative to the first valve body. If the valve element is integrated into the filter element as described above, the guide rod can protrude into the through-channel, in particular along the longitudinal axis of the filter element.Thus, the second valve body can be adjusted relative to the first valve body, and the first valve body relative to the valve seat, along the longitudinal axis of the filter element. In particular, the second valve body can move into the passageway during the described switching processes. This, in turn, makes the filter device more compact.

[0020] According to a preferred embodiment, the first valve body is prestressed into a closed position relative to the valve seat and / or the second valve body is prestressed into an open position relative to the first valve body. The prestressing of the first valve body into the closed position relative to the valve seat can be achieved in particular by means of a first spring. The first spring can, for example, be arranged so as to surround the guide rod between the valve seat and the first valve body. Likewise, the prestressing of the second valve body into the open position relative to the first valve body can be achieved by means of a second spring. The second spring can, for example, be arranged so as to surround the guide rod between the first valve body and the second valve body.By prestressing the second valve body into an open position, the valve element releases the first bypass channel in the depressurized state, in particular therefore below the first predetermined system pressure. In this state, the second bypass channel is closed by the prestressing of the first valve body relative to the valve seat in a closed position. If the system pressure exceeds the first predetermined system pressure, the prestressing of the second valve body relative to the first valve body is overcome and the first bypass channel is thus closed. If the second system pressure is also exceeded, the prestressing of the first valve body relative to the valve seat is overcome and the second bypass channel is opened by the first valve body being lifted off the valve seat. The first bypass channel remains closed; in particular, the second valve body is lifted off the valve seat together with the first valve body.This preload against the guide rod enables, in particular, completely passive control of the valve element via the system pressure. The entire filter device can thus switch in the manner according to the invention without electrical control. By selecting suitable spring forces for the first spring and the second spring, the bypass channels can be adapted to the desired pressure ranges.

[0021] According to a further embodiment, the guide rod extends through the seat opening of the valve seat and the through-opening of the first valve body, wherein the valve seat and the first valve body are mounted on the guide rod via radial webs. The guide rod can preferably extend through the seat opening of the valve seat and the through-opening of the first valve body centrally, in particular run along a longitudinal axis of the filter element, as mentioned above. The valve seat can therefore have radial webs that extend from an inner edge of the seat opening into the seat opening and are in contact with the guide rod. Likewise, the first valve body can have radial webs that extend from an inner edge of the through-opening into the through-opening and are in contact with the guide rod. In this way, the first valve body and the valve seat can be mounted on the guide rod.If a filter cover provided on the filter element serves as a valve seat, as explained above, the guide rod itself can be mounted on the valve seat. In particular, the guide rod can then be firmly connected to the valve seat, allowing the valve bodies mounted on the guide rod to move along the guide rod relative to the valve seat. The radial webs projecting into the through-hole or seat opening are preferably dimensioned such that they only slightly impede the flow of fluid through the through-hole or seat opening.

[0022] According to a preferred embodiment, the valve element for opening the first and / or second bypass channel is controlled solely by the existing system pressure. As already mentioned, the valve element can therefore be controlled entirely passively. In particular, the first valve body as well as the second valve body can be moved mechanically solely on the basis of the existing system pressure such that the first and second bypass channels are opened and closed, respectively. However, it can also be provided that, for example, only the second valve body or only the first valve body is passively controlled by the system pressure, while the other valve body is actively controlled electrically.

[0023] According to a further embodiment, it can be provided that the valve element for opening the first and / or second bypass channel is electrically controllable, and the filter device comprises at least one pressure sensor for detecting the system pressure, wherein an active control of the valve element takes place as a function of a system pressure detected by the pressure sensor. It can therefore be provided that the system pressure present in the filter device is determined via a pressure sensor. The pressure sensor can, for example, be connected to a control device which opens or closes the first or second bypass channel as a function of the pressure values ​​determined by the pressure sensor. It can also be provided, for example, that the movement of the second valve body relative to the first valve body takes place passively, while the movement of the first valve body relative to the valve seat takes place actively.Thus, the first bypass channel can be closed or opened passively, and the second bypass channel can be closed or opened actively. Passive control is particularly simple, error-free, and easy to implement. Active control, on the other hand, has the advantage that the predetermined system pressures can be changed and adjusted retrospectively.

[0024] The invention further relates to a hydraulic tank for an industrial truck, comprising a tank shell and a filter device according to the invention, wherein the filter device has an inlet opening for supplying fluid to the filter device and an outlet opening for draining fluid from the filter device into the tank shell, wherein the filter element of the filter device is arranged in the flow path of the fluid between the inlet opening and the outlet opening. The filter device according to the invention can therefore be integrated into a hydraulic tank for an industrial truck. Returned hydraulic fluid can be supplied to the filter device via the inlet opening. Depending on the switching state of the valve element, this supplied hydraulic fluid flows either through the filter body of the filter element, through the first bypass channel, or through the second bypass channel. If the fluid flows through one of the bypass channels, it returns unfiltered to the tank shell.When the fluid flows through the filter element's filter body, it also enters the tank shell via the outer surface of the filter body, but in this case, it is filtered. From the tank shell, the hydraulic fluid can then be fed via the hydraulic system to the desired hydraulic cylinders.

[0025] The invention further achieves the object by an industrial truck, in particular a forklift truck, comprising a filter device according to the invention or a hydraulic tank according to the invention.

[0026] The invention is explained below with reference to the figures. They show: Fig. 1 the filter device according to the invention in a sectional view with the first bypass channel open, Fig. 2 the filter device according to the invention in a sectional view with closed bypass channels, Fig. 3 the filter device according to the invention in a sectional view with the second bypass channel open, Fig. 4 a hydraulic tank for an industrial truck in a side view, Fig. 5 the hydraulic tank Fig. 4 rotated by 90° in a sectional view, and Fig. 6 a hydraulic diagram of a hydraulic system of an industrial truck.

[0027] Unless otherwise stated, the same reference symbols refer to the same items.

[0028] The Fig. 1 to 3 show a filter device 10 according to the invention with a filter element 20 and a valve element 30. The filter element 20 has a circular cylindrical shape and comprises a hollow cylindrical filter body 22, which surrounds a passage channel 24 extending along the longitudinal axis L of the filter element. Furthermore, the filter element 20 has a fluid inlet 26 at one of its longitudinal ends, as shown in particular in Fig. 3 or in Fig. 5, and at its other longitudinal end via a fluid outlet 28. The fluid inlet 26 of the filter element 20 is in the Fig. 1 and Fig. 2, since the lower end of the filter device 10 is not shown there. The fluid inlet 26 and the fluid outlet 28 are connected to one another via the passage channel 24. At its outlet end, the filter element 20 also has a filter cover 29 with an opening for the fluid outlet 28. The valve element 30 is arranged on this filter cover 29. The valve element 30 is thus integrated into the filter element 20. The valve element can, in particular, be retrofittable. The fluid outlet 28 is formed by through-openings 42 and an annular gap-shaped opening 76, as explained further below.

[0029] The valve element 30 comprises a valve seat 32, which in the present case is formed by the filter cover 29 of the filter element 20, and a two-part valve member 34 consisting of a first valve body 36 and a second valve body 38, which are connected to one another via a guide rod 40. The first valve body 36 has a through-opening 42, which is divided into several opening sections by radial webs 44, which extend from an inner wall of the first valve body 36 into the through-opening 42 in the direction of the guide rod 40. The radial webs 44 are connected to a bearing ring 46 located centrally in the through-opening 42, via which bearing ring 46 the first filter body 36 is movably mounted on the guide rod 40. The valve seat 32 has a seat opening 48 which is divided into several sections by radial webs 50 which extend from an inner wall of the valve seat 32 into the seat opening 48.The radial webs 50 terminate centrally in the seat opening 48 into a ring element 52, via which the guide rod 40 is fixed to the valve seat 32 and thus to the filter cover 29. The through opening 42 and the seat opening 48 are arranged in alignment one behind the other. As shown particularly in FIG. Fig. 1, the through-opening 42 and the seat opening 48 correspond to one another in such a way that the first valve body 36 or the valve seat 32 does not cover the opening 48, 42 of the other part. The second valve body 38 has engagement elements 54 that are designed to correspond to the through-opening 42 and the seat opening 48. The second valve body 38 is movably mounted on the guide rod 40 via a bearing section 56.

[0030] The filter device 10 is part of a hydraulic system, which further comprises a tank shell 60 with an inlet nozzle 61 for filling the tank shell 60 with hydraulic fluid, a tank inlet 62 arranged within the inlet nozzle 61 for returning hydraulic fluid, and a tank outlet 64. Hydraulic fluid can be pumped from the tank shell 60 to one or more hydraulic cylinders (not shown) via the tank outlet 64 by means of a pump 66. The tank shell 60 has a receptacle 68 for the filter device 10, wherein the receptacle 68 can be closed with a screw cap 70 after the filter device 10 has been inserted. The cap 70 can be removed again accordingly in order to replace the filter device 10.The fluid inlet 26 of the filter device 10 is connected to a hose 74 via a hose connection having an inlet opening 72, via which hydraulic fluid returned from the hydraulic system through the tank inlet 62 can be supplied to the tank shell 60 of the filter device 10. The fluid outlet 28 is connected to or forms an outlet opening 73 of the filter device.

[0031] If hydraulic fluid is now returned from the hydraulic system via the inlet 62 back into the tank 60, it flows via the hose 74 and the fluid inlet 26 into the filter device 10. The fluid enters the passage channel 24 of the filter element 20 and flows in the direction of the fluid outlet 28 of the filter element 20. As long as the flow pressure does not exceed a first predetermined system pressure, the hydraulic fluid flows past the second valve body 38 through the seat opening 48 and the through opening 42 and thus reaches the tank shell 60 via the fluid outlet 28. The second valve body 38 is pretensioned into its open position via the guide rod 40, as shown in Fig. 1. For this purpose, a spring (not shown) can be arranged between the second valve body 38 and the first valve body 36. The liquid thus flows along the Fig. 1 through the first bypass channel A. The first bypass channel A thus runs past the second valve body 38 through the seat opening 48 of the valve seat 32 and the through opening 42 of the first valve body 36. The first predetermined system pressure is preferably selected such that a lifting cylinder controlled via the hydraulic circuit allows a rapid lowering of a load part connected to the lifting cylinder even without a load. This can be achieved, for example, by appropriately preloading the second valve body 38 into the open position. At this time, the first valve body 36 is in its closed position, into which it is preloaded. This preload can be provided, for example, by a spring (not shown).

[0032] If the pressure now exceeds the first predetermined system pressure, the fluid flow flowing through the passage channel 24 pushes the second valve body 38 along the guide rod 40 in the direction of the first valve body 36. The engagement elements 54 penetrate into the seat opening 48 and the through opening 44 and close both, as shown in Fig. 2. The second valve body 38 is thus moved into its closed position. If the system pressure is above the first but below the second system pressure, no bypass channel is opened and the hydraulic fluid flowing through the passage channel 24 instead leaves the filter device 10 via the filter body 22, according to the Fig. 2. In this case, the fluid is completely filtered. After passing through the filter body 22, the hydraulic fluid passes through a Fig. 5 illustrated housing 21 of the filter device 10 and enters the interior of the tank shell 60.

[0033] If the system pressure now increases further so that a second predetermined system pressure is exceeded, the flow pressure acting on the two-part valve member 34 becomes so great that the first valve body 36 together with the second valve body 38 is displaced in the flow direction along the longitudinal axis L on the guide rod 40. In this case, the force generated by the preload of the first valve body 36 in its closed position is overcome and thus the second bypass channel B is opened, as in Fig. 3. In this case, too, the second valve body 38 is pressed by the flow pressure toward the first valve body 36 and completely closes its through-opening 42 with its engagement elements. The first bypass channel A is therefore closed in this case. However, the seat opening 48 of the valve seat 32 has a larger opening cross-section than the through-opening 44, so that - as in Fig. 3 - the liquid flow at high pressure in the second valve body 38 can pass through the seat opening 48, according to the Fig. 2. Since the first valve body 36 is lifted from the valve seat 32 due to the high pressure, an annular gap-shaped opening 76 is formed through which the fluid flow can escape from the filter device 10 along an underside of the first filter body 36 and along an upper side of the valve seat 32 (or the filter cover 29). Movement of the second valve body 38 in the flow direction along the guide rod 40 is stopped above the second system pressure by the annular element 52 of the valve seat 32.

[0034] Due to the special design of the valve element 30, the filter device 10 according to the invention consequently enables bypassing of the filter body 22 at low as well as high pressure in the hydraulic system. At normal pressure, i.e. a pressure value between the first and second system pressures, hydraulic fluid flowing into the filter device flows exclusively through the filter body 22. At system pressures below the first system pressure and above the second system pressure, the hydraulic fluid flows essentially only through the first and second bypass channels, respectively, as explained above. Depending on the permeability of the filter and the system pressure, however, at least a small portion of the fluid flow can escape through the filter body to the outside even when the bypass channels are open.

[0035] The valve element described in this embodiment is passively controlled solely by the existing system pressure, as just explained. However, it is also conceivable for the valve element to be electrically controlled. For this purpose, the second valve body 38 can, for example, be firmly connected to the guide rod 40, wherein the guide rod can be moved into or out of the passage channel 24 together with the second valve body. In this case, the first valve body 36 can also continue to be passively controlled solely by the existing system pressure. In this case, the guide rod 40 is then not firmly connected to the ring element 52 of the valve seat 32 or the filter cover 29.

[0036] In Fig.Figure 6 shows a hydraulic diagram for an industrial truck with a filter device according to the invention. A lifting unit (not shown) of the industrial truck can be moved via cylinders 100 and 102, with cylinder 100 acting on a lifting mast of the industrial truck and cylinder 102 acting on a load part connected to the lifting mast, for example, a fork. Thus, hydraulic cylinder 100 controls the mast lift, while hydraulic cylinder 102 controls the free lift. Hydraulic fluid is pumped from the tank shell 60 via pump 66 through a check valve 104 into the mast lift cylinder 100 and / or the free lift cylinder 102. For this purpose, an electrically actuated two-way proportional valve 106 is connected upstream of the mast lift cylinder 100, which valve can be switched to a through position to allow an inflow and a return flow of hydraulic fluid to and from the mast lift cylinder 100, and can be switched to a blocking position.An electrically actuated two-way valve 108 is connected upstream of the free-lift cylinder 102, which in a through position allows an inflow to the free-lift cylinder 102 and a return flow from the free-lift cylinder 102 and in a blocking position blocks these inflows.

[0037] If fluid is now drained from one of the hydraulic cylinders 100, 102 through a corresponding passage position of the valves 106, 108, this fluid reaches the filter device 10. The filter device 10 directs the hydraulic fluid in the manner described above either through the filter body or through one of the two bypass channels back into the tank shell 60. In addition, a two-way two-way valve 112 and / or a pressure control valve 114 can be connected upstream of the filter device. List of reference symbols 10 Filter device 20 filter element 22 filter bodies 24 passage channel 26 Fluid inlet 28 Fluid outlet 29 Filter cover 30 valve element 32 valve seat 34 two-piece valve element 36 first valve body 38 second valve body 40 guide rod 42 passage opening 44 radial webs 46 Bearing ring 48 seat opening 50 radial webs 52 ring element 54 engagement elements 56 storage section 60 tank tray 61 inlet nozzle 62 Tank inlet 64 Tank outlet 66 Pump 68 recording 70 screw caps 72 Inlet opening 73 Outlet opening 74 hose 76 annular gap-shaped opening 100 mast lift cylinders 102 free lift cylinders 104 Check valve 106 Two-way proportional valve 108 Two-way valve 112 Two-way valve 114 Pressure control valve A first bypass channel B second bypass channel L Longitudinal axis

Claims

[1] Filter device for a hydraulic system, in particular on an industrial truck, comprising a filter element (20) with a filter body (22) and a valve element (30), wherein the valve element (30) opens a first bypass channel (A) for fluid flowing through the filter device (10) below a predetermined first system pressure, bypassing the filter body (22), and opens a second bypass channel (B) for fluid flowing through the filter device (10) above a predetermined second system pressure, bypassing the filter body (22), wherein the second system pressure is greater than the first system pressure. [2] Filter device according to claim 1, characterized by that above the first system pressure and below the second system pressure both bypass channels (A, B) are closed, so that fluid flowing through the filter device (10) flows only through the filter body (22) of the filter element (20). [3] Filter device according to claim 1 or 2, characterized by that the first bypass channel (A) is closed when the second bypass channel (B) is open. [4] Filter device according to one of the preceding claims, characterized by in that the filter element (20) has a passage channel (24) extending along a longitudinal axis (L) of the filter element (20) and surrounded by the filter body (22), wherein the filter element (20) has a fluid inlet (26) at one longitudinal end and a fluid outlet (28) at the other longitudinal end, wherein the fluid inlet (26) and the fluid outlet (28) are connected to one another via the passage channel (24), and wherein the valve element (30) is arranged at the longitudinal end of the filter element (20) at which the fluid outlet (28) is located. [5] Filter device according to one of the preceding claims, characterized byin that the valve element (20) comprises a valve seat (32) and a two-part valve member (34), wherein the two-part valve member (34) comprises a first valve body (36) with a through-opening (42) as part of the first bypass channel (A) and a second valve body (38) which is adjustable relative to the first valve body (36), wherein below the first system pressure the second valve body (38) is spaced from the first valve body (36) in order to open the first bypass channel (A) extending through the through-opening (24), wherein above the first system pressure the second valve body (38) bears against the first valve body (36) in order to block the through-opening (24) and thus close the first bypass channel (A). [6] Filter device according to claim 5, characterized byin that the valve seat (32) comprises a seat opening (48) as part of the second bypass channel (B), wherein below the second system pressure the first valve body (36) bears against the valve seat (32) in order to close the second bypass channel (B), wherein above the second system pressure the first valve body (36) together with the second valve body (38) is spaced from the seat opening (48) in order to open the second bypass channel (B) extending through the seat opening (48). [7] Filter device according to claim 5 or 6, characterized by that the valve element (30) is connected to a filter cover (29) of the filter element (20), wherein the filter cover (29) forms the valve seat (32) of the valve element (30). [8] Filter device according to one of claims 5 to 7, characterized by that the valve element (30) further comprises a guide rod (40) on which the valve seat (32) and the two-part valve member (34) are mounted. [9] Filter device according to claim 8, characterized by that the first valve body (36) is prestressed in a closed position relative to the valve seat (32), and / or that the second valve body (38) is prestressed in an open position relative to the first valve body (36). [10] Filter device according to claim 8 or 9, characterized by that the guide rod (40) extends through the seat opening (48) of the valve seat (32) and the through opening (42) of the first valve body (36), wherein the valve seat (32) and the first valve body (36) are mounted on the guide rod (40) via radial webs (44, 50). [11] Filter device according to one of the preceding claims, characterized by that the valve element (30) for opening the first and / or second bypass channel (A, B) is passively controlled solely by the existing system pressure. [12] Filter device according to one of claims 1 to 10, characterized bythat the valve element (30) for opening the first and / or second bypass channel (A, B) can be electrically controlled, and the filter device (10) comprises at least one pressure sensor for detecting the system pressure, wherein an active control of the valve element (30) takes place as a function of a system pressure detected by the pressure sensor. [13] Hydraulic tank for an industrial truck, comprising a tank shell (60) and a filter device (10) according to one of the preceding claims, wherein the filter device (10) has a connection (72) for supplying fluid to the filter device (10) and an outlet opening (73) for draining fluid from the filter device (10) into the tank shell (60), wherein the filter element (20) of the filter device (10) is arranged in the flow path of the fluid between the connection (72) and the outlet opening (73). [14] Industrial truck, in particular a forklift truck, comprising a filter device (10) according to one of claims 1 to 12 or comprising a hydraulic tank according to claim 13.

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