Abstract:
A tractor-mounted front end loader includes lift boom having a tool, disclosed as a bucket, pivotally mounted to one end of the boom for selected movement by one or more hydraulic adjustment cylinders. A safety valve is associated with one of the pressure chambers of each adjustment cylinder to prevent unintended lowering of the tool in the event a supply-return hose extending between a direction control valve and the adjustment cylinder should rupture. Also connected to this pressure chamber of each adjustment cylinder is an accumulator for providing a damping action. The connection between the pressure chamber and the accumulator is made by rigid tubing, and in one disclosed embodiment, includes a remotely operable on/off valve for selectively coupling the accumulator to the pressure chamber.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention concerns a hydraulic control arrangement for a mobile operating machine with at least one hydraulic cylinder, by means of which an operating tool can be moved, a directional control valve for the control of the path of the pressurized fluid between the pressure chamber of the cylinder, a source of pressurized fluid and a tank, a safety valve inserted between the directional control valve and the pressure chamber of the cylinder that can be opened for the repositioning of the cylinder, and with a pressure accumulator that can be connected with the pressure chamber of the cylinder.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lifting gear, for example, front loaders, are attached to vehicles and are used for lifting and transporting loads. They are provided with hydraulic cylinders for the lifting of the load, and as a rule, also for the pivoting of the load, which are supplied with pressurized hydraulic fluid from a carrier vehicle through removable, flexible hoses. In order to prevent the load from falling down in the case of a broken hose that leads to a fall-off of the pressure in the cylinder holding the load, so-called load retaining or safety valves are used. These valves block the connection between the cylinder and the hose as long as the operator does not initiate any movement of the lifting gear.  
           [0003]    Furthermore, it is possible to connect the hydraulic cylinders of the lifting gear with pressure accumulators acting as vibration dampers that are provided on the lifting or lowering side of a cylinder or on both sides (DE 197 34 658 A). Such pressure accumulators are connected at the connecting point on the vehicle side of the safety valves, since as a rule, they are located on the vehicle and are connected by a hose with the safety valve. In order to obtain damping even when the safety valve is turned off, the operator can manually deactivate the safety valve. The operator then assumes the responsibility of not performing any lifting operation. It is also conceivable that the safety valve can be activated as a function of the speed of the vehicle. If the vehicle is operated at a certain speed, a conclusion can be drawn that the vehicle is not operated in a lifting condition, but in a transport condition. Then the pressure accumulator is effective only during the transport operation.  
           [0004]    The problem underlying the invention is seen in the need to improve a load lifting arrangement with a safety valve in such a way that a damping of the cylinder is possible, even if the safety valve is turned off or that the safety valve remains effective even with a damped cylinder.  
         SUMMARY OF THE INVENTION  
         [0005]    According to the present invention, there is provided an improved hydraulic control system for a lifting device such as a loader.  
           [0006]    An object of the invention is to provide a hydraulic control system which provides damping against travel-generated oscillations of the lifting arms, no matter what the status is of one or more safety valves contained in the system.  
           [0007]    To accomplish the object, it is proposed that a pressure accumulator in the control system be connected on the side of the safety valve facing the lift cylinder. In this way, the result is that damping is achieved, even with an active safety valve, that is, a blocked safety valve. Thereby, damping of the operating tool is possible at all times, without the safety valve being manually deactivated or without it being necessary that the operation be conducted at a minimum speed.  
           [0008]    The pressure accumulator may be connected with the pressure chamber of the cylinder directly or through a valve that can be blocked, as a rule, one that can be remotely controlled. The valve makes it possible to turn off the pressure accumulator in case that an operator wants to eliminate the effect of the pressure accumulator in order to perform a more exact positioning of the operating tool.  
           [0009]    The pressure accumulator is appropriately coupled mechanically and rigidly to the cylinder and/or the safety valve so that it moves with the cylinder and the safety valve and no hoses are required that might be in danger of rupture. The pressure accumulator is preferably connected with the pressure chamber of the cylinder exclusively by means of mechanically rigid connecting elements, such as tubes and the like. Thereby, the risk of hose rupture is avoided that would result in a sudden dropping of a load.  
           [0010]    In particular, the line leading to the pressure accumulator may be connected with a second connection to the pressure chamber of the cylinder, whose first connection is connected to the safety valve, but the use of a T-fitting in the line between the safety valve and the pressure chamber is conceivable for the connection of the pressure accumulator or the use of an additional connection of the safety valve or the safety valve block. In another embodiment, the pressure accumulator is integrated into the housing of the cylinder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The drawings show four embodiments of the invention that shall be described in greater detail in the following.  
         [0012]    [0012]FIG. 1 is a schematic left side view of a lifting gear with an operating tool.  
         [0013]    [0013]FIG. 2 is a schematic representation of a hydraulic circuit constructed in accordance with a first embodiment of the present invention in which a pressure accumulator is connected with a safety valve and two cylinders.  
         [0014]    [0014]FIG. 3 is a schematic representation of a hydraulic circuit constructed in accordance with a second embodiment of the present invention in which a pressure accumulator is connected with a safety valve and a cylinder.  
         [0015]    [0015]FIG. 4 is a schematic representation of a third embodiment of the present invention in which a pressure accumulator is directly connected with a safety valve.  
         [0016]    [0016]FIG. 5 is a schematic representation of a fourth embodiment of the present invention in which a pressure accumulator that can be blocked, is connected with a safety valve and a cylinder. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    The hydraulic control arrangement, according to the invention, is explained on the basis of the example of an agricultural tractor with a front loader. It can nevertheless be applied to any desired operating machine with movable operating tools, such as wheel loaders, telescoping loaders, dredges, and the like. A lifting gear  10 , shown in FIG. 1, is attached to the front side of an agricultural tractor, that is merely indicated, and includes a mast  12  that is illustrated and that engages a lifting boom  16  in a bearing  14 . The lifting boom is configured with two arms and therefore is reinforced by means of a transverse carrier  18 . An operating tool  20 , for example, an earth shovel, a manure fork, a bale fork or a palette fork, is connected to the front end of the lifting boom  16  by means of a positioning arrangement  30  and a tool holder  32  so as to be freely movable and interchangable. In order to lift the operating tool  20 , the lifting boom  16  is pivoted about the bearing  14  by means of hydraulic cylinders  26 . To reposition the inclination of the operating tool  20 , a further hydraulic cylinder  28  engages the repositioning arrangement  30  with one end and the lifting boom  16  with its other end. Further hydraulic cylinders  26  and  28  are respectively arranged on the opposite side of the lifting boom  16  from the side shown. The repositioning arrangement  30  operates on the tool holder  32 , to which the operating tool  20  is connected directly, and is composed generally of first and second pivoting arms  34  and  36 , respectively, that are connected to each other in a joint  44 , with one end of the cylinder  28  being coupled at the joint  44 . The tool holder  32  is supported in a bearing  38  on the lifting boom  16 , and the first pivot arm  34  is supported in a bearing  40  on the lifting boom  16 , in both cases free to pivot.  
         [0018]    The second pivot arm  36  is connected in a bearing  46  with the tool holder  32 , free to pivot. The number call-out  48  denotes a locking bar that retains the operating tool  20  in a detent position. In the upper section of the tool holder  32 , a hook  50  and a pin  52  are provided that bring the operating tool  20  into engagement with the lifting gear  10 .  
         [0019]    [0019]FIG. 2 shows schematically the hydraulic circuit of a hydraulic control arrangement according to the invention for the operating machine of FIG. 1. On the agricultural tractor, a pump  54  is arranged as a source of pressurized fluid and a tank  56  is arranged to receive return fluid. The pump  54  and tank  56  are both connected over two directional control valves  58  and  60 , that can be actuated by the operator from the cab of the agricultural tractor, so as to selectively couple ports of a coupling  62  either to pressurized fluid or to the tank  56 . The hydraulic connections of the lifting gear  10  are releasably connected to the coupling  62 . The single-acting or double-acting cylinders  26  are connected over the coupling  62  directly with the directional control valve  60  so that the operating tool  20  can be raised or lowered by actuation of the directional control valve  60  in a manner known in itself.  
         [0020]    On the other hand, the further cylinders  28  are double-acting. Their piston end pressure chamber  64  is supplied with pressure in order to pivot the operating tool  20  upward. Analogously, its rod end pressure chamber  66  is supplied with pressure when the operating tool  20  is to be pivoted downward. In order to prevent an unintended lowering of the operating tool  20 , due to a rupture in a flexible hose coupled between the coupling  62  and a safety valve block  68 , which is connected to the piston end pressure chamber  64 , the safety valve block  68  is provided with first and second safety valves  70  and  72 , respectively. The safety valve block  68  is provided with two connections on the side of the cylinders to each of which a piston end pressure chamber  64  of the cylinders  28  is connected. The safety valves  70  and  72  are provided for each of the two possible directions of flow of the pressurized fluid to and from the chambers  64  and  66 . The first safety valve  70  contains a check valve that opens when the pressure in the piston end pressure chamber  64  is lower than the pressure in the associated connection of the coupling  62 . It opens automatically when the operator increases the pressure in the piston end pressure chamber  64  by actuating the directional control valve  58 . With the use of an electromagnetic remote control, there is the possibility of switching the first safety valve  70  between the check valve and a passage so that the safety valve  70  can be switched to a constant passage flow. The second safety valve  72  also contains a check valve that corresponds to that in the safety valve  70 . Under the control of the pressure in the pressure chambers  64  and  66 , the second safety valve  72  can switch between the check valve and a passage. Thereby, the passage is opened automatically when the pressure in the rod end pressure chamber  66  is greater than the pressure in the piston end pressure chamber  64 , that is, the cylinder  28  should retract so that the pressurized fluid can drain out of the piston end pressure chamber  64  into the tank  56 . As a result, the safety valves  70  and  72  are open when the operator initiates a movement of the cylinder  28  over the directional control valve  58 , and otherwise are closed.  
         [0021]    The piston end pressure chamber  64  of the cylinder  28 , shown at the right, is provided with first and second connections  76  and  78 , respectively. The first connection  76  is connected with the safety valve block  68 . The second connection  78  is connected to a pressure accumulator  74  over rigid connecting elements. The pressure accumulator  74  is fastened mechanically to the lifting boom  16  or to the cylinder  28 . The pressure accumulator  74  provides permanent damping for the cylinder  28  shown at the right, and over the first connection  76  and the rigid connection  90  also, for the cylinder  28  shown at the left, even with closed safety valves  70  and  72 . The pressure accumulator  74  could also be arranged in the housing of the cylinder  28  near the bottom side.  
         [0022]    A second pressure accumulator  80  is connected with the rod end pressure chamber  66  of the cylinders  28 , in order to provide further damping of each cylinder  28 . Since the rod end pressure chambers  66  are not critical from a safety standpoint as a pressure drop in them does not lead to a lowering of the operating tool  20 , safety valves associated with them can be omitted. The second pressure accumulator  80  may be fastened to the lifting boom  16  or to the frame of the agricultural tractor, where flexible hoses can be used.  
         [0023]    [0023]FIG. 3 schematically shows a second embodiment of a hydraulic circuit of a hydraulic control arrangement. Elements that conform to those of the first embodiment are identified with the same number call-outs. In contrast to the embodiment according to FIG. 2, each of the cylinders  28  is associated with a safety valve block  68 . Therefore, each cylinder  28  is also equipped with a second connection  78  and a pressure accumulator  74  and  74 ′, respectively, connected to them. However, for the rod end side of the cylinders  28 , only a common pressure accumulator  80  is provided, as in the embodiment according to FIG. 2.  
         [0024]    The third embodiment of a hydraulic control arrangement, according to the invention shown in FIG. 4, differs from that shown in FIG. 3 by the fact that the cylinders  28  are equipped with only a single connection  76 , and that the pressure accumulators  74  and  74 ′ are each connected with a connection  82  of the associated safety valve block  68 . The connection  82  is connected within the safety valve block  68  with the connection  76 . Thereby, the second connection  78  of the cylinders  28  can be omitted.  
         [0025]    [0025]FIG. 5 schematically shows a fourth embodiment of a hydraulic control arrangement according to the invention. It corresponds generally to the embodiment according to FIG. 3; however, the pressure accumulators  74 ,  74 ′ and  80  can be disabled by means of respective valve arrangements  84 . The valve arrangements  84  include check valves  86  that open when the pressure in the associated pressure accumulator  74 ,  74 ′, and  80  is lower than the pressure in the associated cylinder pressure chamber. Thereby, undesirable movements of the cylinders  28  can be avoided when the associated pressure accumulator is turned on. Furthermore, each valve arrangement  84  includes a directional control valve  88  that can be remotely, electromagnetically controlled and that can be moved between a through passage position, in which the pressure accumulators  74 ,  74 ′,  80  are connected with the associated cylinder pressure chambers, and a blocking position, in which the pressure accumulators are separated from the cylinder pressure chambers. The valve arrangements  84  make it possible to block the pressure accumulators in case the operator desires to reposition the operating tool  20  with increased precision.  
         [0026]    It should be noted that in place of, or in addition to, the cylinders  28 , the cylinder or the cylinders  26  could also be spring supported by pressure accumulators in the manner shown.  
         [0027]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.