Abstract:
A filter device includes at least one filter unit ( 10 ), connectable to a hydraulic circuit ( 14 ), with at least one supply line ( 16 ), one discharge line ( 18 ), and at least one cooling unit ( 22 ). The cooling unit is connected to a secondary branch line ( 20 ), is connected to the hydraulic unit ( 14 ) parallel to the supply line ( 16 ), and is supplied with a hydraulic medium by a switching device ( 24 ) once a predetermined temperature threshold value of the hydraulic medium to be cooled is attained. A permanent supply ( 36 ) for the cooling unit ( 22 ) is provided in the direction of flow of the hydraulic medium in the supply line ( 18 ) before the switching device ( 24 ). This permanent supply ( 36 ), connected in parallel to the switching device ( 24 ), flows into the secondary branch line ( 20 ) to control the volume flow in reverse flow according to the fluid temperature. A partial flow of the hydraulic medium in the supply line directly reaches the cooling unit at low or reduced temperatures, at which the switching device has not yet been actuated to ensure that the cooling unit is permanently supplied with the hydraulic medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a filter device with at least one filter unit which can be connected to a hydraulic circuit, with at least one supply line and one drain line. At least one cooling unit is connected in a secondary branch line and is connected parallel to the supply line into the hydraulic circuit. At a definable temperature threshold of the hydraulic medium to be cooled is supplied to the cooling unit by a changeover means. 
     BACKGROUND OF THE INVENTION 
     In known approaches, as are freely available on the market, filter devices are used in all types of hydraulic systems, especially in construction machines. Filter units, called return line intake filters, are in the return lines of the respective hydraulic system discharge. The return lines are combined into collectors which ensure central inflow of the fouled fluid or hydraulic medium to the respective return line intake filter unit. After filtration, the cleaned fluid is relayed to an intake connection to supply the hydraulic system with the cleaned hydraulic medium. 
     To prevent overheating of the hydraulic system, a cooling unit can be connected upstream to the supply line into which the return lines discharge before admission into the actual filter circuit in the secondary branch line. A conventional thermostat controller can turn on or off the cooling unit depending on the operating temperature of the hydraulic medium. Especially at low temperatures of the hydraulic medium, the changeover means, with the thermostat controller and/or the components of the cooling circuit itself can produce a banking-up pressure which leads to pressure peaks in the upstream units. For example, in a hydraulic motor and a hydraulic pump pressure peaks can damage the sealing elements. Furthermore, the fluid-carrying components in the known approach are connected to one another by the indicated collectors. Lines producing the pertinent fluid-carrying connections, can take place by conventional tubing technology using weld connections. Therefore, the known filter devices are structurally large and expensive to produce. 
     DE-A-34 02 884 A1 discloses a filter device with a thermostat installed in the system for regulating the temperature of the working liquid and opening or closing automatically the branch to the cooler depending on the temperature of the reversible working liquid in the system. The heated reversible working liquid flows directly out of the thermostat upstream of the filter into the intake branch of the filling liquid pump, i.e., bypassing the cooler and the tank. The thermostat closes the throughflow of working liquid into the part of the circuit which contains the cooler and tank when the temperature of the working liquid is low. When the temperature of the working liquid reaches the value at which the thermostat opens, part of the working liquid flows through the connecting branch into the cooler and on into the tank. The thermostat opens entirely when the temperature of the working liquid in the tank reaches the optimum value. All the working liquid which returns from the closed hydraulic circuit continues to flow only by way of the cooler into the tank. In this known approach, the thermostat can produce a banking-up pressure which leads to damaging pressure peaks in the upstream units especially at low temperatures of the hydraulic medium within the hydraulic circuit. 
     U.S. Pat. No. 3,990,424 discloses a generic filter device with at least one filter unit connected to a hydraulic circuit with at least one supply and drain line. At least one cooling unit is connected in a secondary branch line and is connected parallel to the supply line into the hydraulic circuit. By a changeover means, at a definable temperature threshold value of the hydraulic medium to be cooled, the medium is supplied to the circuit. In the flow direction of the hydraulic medium in the supply line upstream of the changeover means, a permanent supply for the cooling unit is provided. This permanent supply is connected parallel to the changeover means discharging into the secondary supply line. In this known filter device used especially in jet-propelled racing boats, the permanent supply is induced by the valve chamber to which the cooling unit is permanently connected to carry fluid. Depending on the respective temperature threshold value and therefore depending on the position of the changeover means, at low temperatures of the hydraulic medium, the medium is supplied directly to the filter unit. At high temperatures of the hydraulic medium, a detour takes place by the cooling unit before the medium which has then been cooled is supplied to the filter unit. As a result of the direct connection of the cooling unit to the valve chamber in the system, a damaging banking-up pressure can build up which leads to pressure peaks in the upstream units and in the cooling unit itself. In addition, the cooling unit can be exactly matched as a function block to the intended valve chamber using connection technology This arrangement causes production of the known design to become expensive. 
     SUMMARY OF THE INVENTION 
     Objects of the present invention are provided to improve filter devices such that the damaging banking-up pressure in the system is avoided while reliable operation is achieved. In addition, objects of the present invention are to provide a filter device having a small structure and being economical both in production and in operation. 
     The pertinent objects are basically achieved by a filter device which controls the volumetric flow in the return line depending on the fluid temperature such that even at low temperatures at which the changeover device has not yet been actuated, a partial flow of the hydraulic medium in the supply line travels directly to the cooling unit. The cooling unit is then permanently supplied with the hydraulic medium. Due a bypass circuit, the banking-up pressure is on the cooling unit independently of the actuation situation of the changeover device, by which the cooling unit can be vented at any time if necessary. 
     The permanent supply can be easily combined structurally with the changeover means, especially in one component so that little construction space is required. The filter device according to the present invention can be produced and operated easily and economically. 
     In one preferred embodiment of the filter device of the present invention, the changeover device has an oil temperature controller. The oil temperature controller has an operating characteristic such that, at low temperatures of the hydraulic medium, it clears a path for it to the filter unit. When the temperature threshold is reached, the oil temperature controller supplies the heated hydraulic medium to the cooling unit, which relays the cooled fluid to the filter unit by the secondary branch line. In this way the cooling unit can be triggered with a high degree of precision for a cooling process which operates in a defined manner by means of the oil temperature controller. The oil temperature controller can be a commercial component, known for example from the cooling circuit of internal combustion engines. 
     By way of a choke, venting of the downstream cooling unit is possible in any operating situation of the filter device. In the known filter devices, the venting is often only possible in the “high temperature” operating position or in an emergency operating position which, however, is to some extent only optionally available in the known devices. 
     In another preferred embodiment of the filter device of the present invention, viewed in the flow direction of the hydraulic medium, upstream of the changeover device, the return lines of a hydraulic motor and a hydraulic pump, particularly are combined into a hydrostatic transmission, discharge into the supply line. In the flow direction of the hydraulic medium downstream of the changeover device, the return line of the working hydraulics likewise discharges into the supply line. In this way, a compact structure for the filter device can be achieved. Also, the conventional connecting lines and collector means which are to be welded together can be omitted. Therefore, all connection possibilities for all existing modules are present combined into one unit. This combination leads to a filter device with a small structure. Preferably, at least the changeover device and the permanent supply are a component of a filter head be connectable to a housing part which holds the filter unit. In this way, the filter device can be included in the common filter housing of the filter unit and trigger unit. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a schematic circuit diagram for a known filter device, as is freely available on the market; 
         FIG. 2  is a schematic circuit diagram of the filter device according to an embodiment of the present invention, with its essential components; 
         FIG. 3  is a perspective view of the filter device with its connections according to an embodiment of the present invention; 
         FIG. 4  is a top plan view in section through the filter housing of the filter device of  FIG. 3 , with the cooling unit connected; and 
         FIGS. 5 and 6  are side elevational views of filter housing of  FIG. 4  in the positions of the oil temperature controller at low and high temperatures, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The known or conventional filter device shown in  FIG. 1  has a filter unit  10  of conventional design. The filter unit  10  is a component of a return line intake filter  12 ; its important components being encircled in  FIG. 1  with a broken-line frame. The return line intake filter  12  is provided with a fouling display VA which delivers an optical or other signal indication with respect to the state of fouling of the actual filter unit  10 . A spring-loaded check valve V 2 , in the manner of a bypass valve, is connected to the bypass line of the filter unit  10 . If the filter unit  10  is clogged by fouling, the pretensioned bypass valve V 2  opens to the tank T. In this way, dangerous pressure peaks cannot build up in the system. In the flow direction downstream of the filter unit  10 , the intake connection B is provided which, in conjunction with the spring-loaded check valve V 1 , produces a pretensioning pressure which ensures that the oil column in the hydraulic circuit  14  cannot separate. The possible closing pressure of the bypass valve V 2  is chosen to be greater than that of the check valve V 1  to prevent the fluid, which is be filtered and which is supplied by way of the connection A, from being drained unintentionally in the direction of the tank unfiltered into the latter with bypassing of the filter unit  10 . 
     The hydraulic circuit  14  has a supply line  16  and a drain line  18 , with the drain line being connected to the connection A of the return line intake filter  12 . The cooling unit  22  is connected in the secondary branch line  20  connected parallel to the supply line  16  in the hydraulic circuit  14 . Viewed in the direction of  FIG. 1 , the secondary branch line  20  has one free end discharging into the changeover means  24  and has its other free end discharging into a collector  26  into which the supply line  16  and the drain line  18  discharge. At a definable temperature threshold value of the hydraulic medium to be cooled, the changeover means  24  is actuated and supplies the hydraulic medium which is to be cooled to the cooling unit  22 . The cooled hydraulic medium is then relayed by the secondary branch line  20  to the collector  26  and from there conveyed by the drain line  18  into the return line intake filter  12 . 
     After cleaning the fouled medium by the filter unit  10 , the cleaned fluid or hydraulic medium is returned by the intake connection B to the hydraulic circuit  14  in which, for example, a hydraulic pump of a machine or machine tool is connected. Next to the collector  26 , there is another or second collector  28  into which the return line  30  of the hydraulic motor (not shown) and the return line  32  of a hydraulic pump (not shown) discharge. The hydraulic motor and the hydraulic pump are components of a hydrostatic transmission for which the illustrated filter device is used. Since the arrangements in this regard, especially the hydrostatic transmission, are conventional or prior art, they are not described in detailed. Moreover the return line  34  of the actual working hydraulics (not shown), which has for example a conventional working cylinder for an excavator or the like, discharges into the collector  26 . 
     Especially at low temperatures for the hydraulic medium, which arise for example when the hydraulic system is started up after having been idle for a long time, the components, especially return line intake filter  12 , in the supply line  16  produce a resistance in the form of an increased banking-up pressure. In upstream units in the form of a hydraulic motor and hydraulic pump, this banking-up pressure can lead to damage of their sealing elements, causing failure of the entire hydraulic system. To prevent this failure, in the illustrated embodiment of the filter device of the present invention shown in  FIG. 2 , in the flow direction of the hydraulic medium in the supply line  16  upstream of the changeover means  24  and independently of its position, a permanent supply  36  is provided for the cooling unit  22 . This permanent supply  36  is connected in parallel to the changeover means  24  discharging into the secondary branch line  20  with the cooling unit  22 . The permanent supply  36  is made essentially in the manner of another inflow line or inflow channel. 
     The components according to the filter device as of the present invention shown in  FIG. 2  are, to the extent they correspond to the known filter device shown in  FIG. 1 , labelled with the same reference numbers. The statements above in this regard also apply to the present invention shown in  FIG. 2 . Another difference is that the known collectors  26  and  28  in the embodiment of the present invention are completely omitted. Rather a direct connection technique of the components among one another with their supply lines is achieved, as detailed below. 
     The present invention also uses for the changeover means  24  a conventional oil temperature controller as is known for example in heating engineering. Its operating characteristic, at low temperatures of the hydraulic medium, clears the path for the medium to the filter unit  10  of the return line intake filter  12 . When the temperature threshold is reached, it supplies the heated hydraulic medium to the cooling unit  22  which relays the cooled fluid to the filter unit  10  by the secondary branch line  20 . The permanent supply  36 , in the form of a corresponding connecting line or a connecting channel, is provided with a choke or a diaphragm  38  such that, especially in the operating “low temperature” situation for the hydraulic medium, a damaging banking-up pressure in the supply line  16  is avoided. Independently of the choke  38 , in any case the line cross sections for the permanent supply  36  and the secondary branch line  20  are increased to avoid resistances in the hydraulic circuit  14 , with the banking-up pressure in the return lines  30 ,  32  being significantly increased. The permanent supply  36  in this regard with the choke  38  also allows venting relative to the cooling unit  22  in any operating situation of the filter device. There can also be a venting capability which can be actuated by hand directly on the changeover means  24 . 
     As shown in  FIG. 3 , in the preferred embodiment of the filter device of the present invention, all its components are combined into one unit. At least the changeover means  24  and the permanent supply  36  are components of a termination, cover or head part  40  made flange-like and joinable to the pot-like lower housing part  42  holding the filter unit  10 . In the area of the upper cover part  40 , connection possibilities for the working hydraulics A, the intake connection B and for the hydraulic motor or the hydraulic pump with respect to their return lines  30  and  32  can be provided. In addition to these connecting points, there are essentially in one plane the connecting points  44  and  46  for the connection of the secondary branch line  20  leading to the cooling unit  22  and originating from the cooling unit  22 , respectively. There is furthermore the tank connection T on the bottom of the pot-like housing part  42 . 
       FIGS. 4–6  impart an improved structure and operation of the changeover means  24  in conjunction with the structure and the action of the permanent supply  36 . The changeover means  24  in the form of the oil temperature controller has an expansion element  50  on which an energy storage device in the form of a compression spring  48  acts. Expansion elements  50  in this respect are relatively well known from motor vehicle engineering. The expansion element  50  triggers a sleeve-shaped pilot valve  52  guided in the valve space  54  to move lengthwise against the action of the compression spring  48 . As  FIG. 4  shows, the return lines  30  and  32  from the hydraulic motor and hydraulic pump discharge into the valve space  54 . The supply line  16 , leading to the filter unit  10  and passing at least partially as part of the valve space  54  into it, discharges into the valve space  54 . 
     In order to increase the free cross-sectional area of the drain line  18 , it is divided into two component branches  18   a,b , viewed in the direction of  FIG. 4 . The pilot valve actuating sleeve  52  in the controller position “low temperature” for the hydraulic medium covers the upper drain line branch  18   a  except for a fluid-carrying passage  56 . Viewed in the lengthwise direction of the actuating sleeve  52 , passage  56  passes through the sleeve wall part  58  in the middle and produces the fluid-carrying connection between branch  18   a  and the interior of the valve space  54 . The secondary branch of the drain line  18   b , which branch is the bottom one viewed in the direction of  FIG. 4 , is conversely kept entirely free from the wall part  58  of the pilot valve  52 . The lower boundary of the wall part  58  ends above the lower drain line  18   b . Between the two drain line branches  18   a,b , viewed in the direction of  FIG. 4  from the right side, the connecting point  44  leads by the secondary branch line  20  to the cooling unit  22 , and meshes with the valve space  54 . In the controller position “low temperature”, the pertinent connecting point is closed by the wall part  58  of the pilot valve  52 , except for the choke  38  of the permanent supply  36  which establishes a permanent fluid-carrying connection between the connecting point  44  and the supply line  16 . The choke  38  is formed by a fluid-carrying passage between the annulus as part of the valve housing, and discharges into the connecting point  44  on one side and the outside of the annular wall part  58  on the other side. The connecting point  46 , to which part of the secondary branch line  20  is connected and which originates from the cooling unit  22 , discharges directly into the fluid-carrying filter space of the filter unit  10 . The lengthwise alignment of the valve space  54  extends crosswise to the lengthwise axis of the housing part  42  with the filter unit  10 . The possible direction of flow through the cooling unit  22  via the secondary branch line  20  is shown in  FIGS. 1 ,  2  and  4  with arrows. 
     The controller positions are detailed below using  FIGS. 5 and 6 .  FIGS. 5 and 6  are views in planes extending 90° to the view of  FIG. 4 . In the controller position of “low temperature”, the expansion element  50  is in its retracted position, with the compression spring  48  holding the sleeve-shaped pilot valve  52  in its end position, the right position viewed in the direction of  FIG. 5 . In this position, the top of the pilot valve  52  adjoins the right boundary wall of the valve space  54 .  FIG. 5  corresponds to the position of the controller in  FIG. 4 . The cold hydraulic medium supplied through the inflow lines  16 ,  30  and  32  to the valve space  54  is relayed via the valve space  54  or the passage  56  directly to the drain line  18  with its branches  18   a ,  18   b  so that the cold hydraulic medium can be supplied directly to the filter space with the filter unit  10 , bypassing the cooling unit  22 . The cooling unit  22  is supplied permanently by way of the choke  38  with hydraulic medium of low temperature. Accordingly, a damaging banking-up pressure in the return lines  30  and  32  cannot occur. 
     For the controller position of “high temperature”, as shown in  FIG. 6 , the expansion element  50  is extended and the sleeve-shaped pilot valve  52  assumes it left-most position against the force of the compression spring  48  viewed in the direction at  FIG. 6 . In this position, the free bottom edge of the pilot valve  52  adjoins the left boundary wall of the valve space  54  as the stop part. The middle passage point  56  in the wall part  58  is then at the same height as the annular channel of the connecting point  44  which forms the choke  48 . The hydraulic medium to be cooled travels solely via the connecting point  44  and the secondary branch line  20  to the cooling unit  22 . After passing through the cooling unit  22 , the cooled hydraulic medium travels via the connecting point  46  into the filter space with the filter unit  10 . The remaining drain line  18 , with its branches a and b, is then closed essentially fluid-tight by the wall part  58  of the pilot valve  52 . Therefore in the controller position of “high temperature”, hot hydraulic medium does not travel directly via the drain line  18  to the filter unit  10 . With respect to the passage point  56  which is made large in cross section, in the controller “high temperature” position, essentially hot hydraulic medium no longer flows through the choke  38 . The choke function is essentially canceled by clearing its choked cross section, and drainage proceeds on a priority basis by way of the passage  56  in the wall part  58  of the pilot valve  52 . 
     Because an overflow channel  18   b , parallel to the main channel  18   a , is in the flow direction of the hydraulic medium in the supply line upstream of the changeover means, overflow channel  18   b  enlarges the flow cross section at low temperatures. A damaging banking-up pressure regardless of the actuation situation of the changeover means is thereby avoided. This overflow channel can be easily integrated structurally in the filter head, especially by combining the components of the filter and of the changeover means. Less installation space is required. The filter device of the present invention can accordingly be produced easily and economically using casting technology, as well as be easily and economically operated. 
     In the embodiment of a filter device of the present invention shown in  FIG. 2 , the two channels  18   a  and  18   b  shown in  FIG. 4  are attached in the drain line  18  and lead to the filter unit  10 . Such arrangement reduces the hydraulic resistance for the operating state “low temperatures”. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.