Abstract:
A control device for a working device ( 12 ) connected to a hydraulic circuit ( 10 ), especially for a load fork ( 16 ) which can be raised and lowered with a working cylinder ( 14 ) in a fork-lift truck. A valve control unit moves into an adjustment position when the load fork ( 16 ) is lowered even when carrying a load ( 18 ). A predefinable maximum volume flow is maintained in the hydraulic circuit ( 10 ). By providing the valve control unit with a pressure regulator ( 26 ) limiting a pilot control pressure for the control valve ( 28 ), once a predefinable regulating pressure differential is reached in the hydraulic circuit, the control valve moves into a regulating position to maintain the maximum volume flow. The known flow control valve is replaced in the main flow by a pressure regulator ( 26 ) disposed in the pilot control circuit ( 40 ), and only needs to be dimensioned for low volume flows. As a result, the control system can be constructed using simple, low-cost components which operate in a functionally reliable manner and enable stable control behavior.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a control device for a working device connected to a hydraulic circuit, particularly for a load fork of a forklift which may be raised and lowered by an operating cylinder. In the control device, a valve control unit which assumes a normal position when the forklift is lowered, even when under a load. A predetermined maximum volume flow is maintained in the hydraulic circuit in that position. 
   BACKGROUND OF THE INVENTION 
   Seat valves with a constant opening pattern often participate in proportional load lowering of load forks of forklifts. The seat valves involved may be actuated directly or under pilot control. Use of seat valves is necessary, since forklift manufacturers require a so-called “lift-dense” load retention because only very slight leakage is permissible. Preventing leaks in the hydraulic circuit ensures that the load fork cannot be lowered independently, with or without load, something which would present a safety problem. The function of restricting the maximum volume flow independently of the load is assumed in the conventional solutions by a constant-volume flow regulator connected in series to the other components of the hydraulic circuit. 
   The control devices used for this purpose presently are disadvantageous. During lowering of the load fork when empty, only the dead weight of the moving parts, especially that of the load fork, is available on the lift mast to send the hydraulic fluid from the operating cylinder (plunger cylinder) back to the tank as a component of the hydraulic circuit. The pressure on the cylinder may drop to values&lt;8 bar in certain lift mast configurations. 
   The greater the number of components through which the flow must progress during lowering, the lower is the volume flow. The conventional constant-volume flow regulator presents a particularly great obstacle, since the aperture selected for the regulator must be so small that a control pressure difference of at least 7 bar may be established. Lower control pressures would result in unstable behavior in the hydraulic circuit and may also be unacceptable for safety reasons. Another problem is presented by the falling characteristic required by the customer for the volume flow regulator. Consequently, for considerations of safety and practicability, it must be ensured that the lowering speed will decrease with increases in the load. While this is effected by the constant-volume flow control unit in the conventional control devices, in theory this arrangement as well results in increased instability within the hydraulic circuit. 
   DE 37 08 143 C1 discloses a safety circuit for the control circuit of a power lift. In the circuit, the position of the hydraulic jack is assigned a nominal value as a function of the pressure differential between adjacent control lines. The pressure in a control line is affected by a load receiving element, and the pressure level in the power lines is affected by a nominal value transmitter. A control circuit is provided by which the drive of the working chamber of the power lift is disconnected from the control valve when the pressure medium supply circuit is disconnected. This switching state is maintained after starting of the pressure medium supply circuit until the desired value set at the nominal value transmitter is brought to the actual position value. This arrangement ensures reliable prevention of unintentional raising or lowering of an operating device mounted on a tractor after the tractor has been started, because an adjustment of the nominal value transmitter has inadvertently been made with the pressure medium supply circuit disconnected. Damage and accidents are thereby excluded. In the case of the conventional solution, however, the lowering behavior of the operating device is as unfavorable as in the case of the technical solutions in this field as described. 
   SUMMARY OF THE INVENTION 
   Objects of the present invention are to provide an optimal control devices achieving improved lowering behavior for an operating device of a machine, especially for a load fork of a forklift, while maintaining the desired stability criteria for the hydraulic circuit. 
   These objects are attained by a control device where a valve control unit is provided with a pressure regulator. When a predetermined control pressure differential in the hydraulic circuit limits the pilot control pressure for a control valve so that this control valve assumes a normal position for maintaining the maximum volume flow, the conventional flow control valve in the main flow is replaced by a pressure regulator in the pilot control circuit which needs to be configured only for low volume flows. Consequently, the control system may be produced with simple and cost-effective components which are also dependable in operation and permit a stable control behavior. In addition, by the present invention, the volume flow during lowering of the operating device, such as one in the form of a load fork, may be appreciably increased even when the device carries no load, so that no obstacles may arise in operation during lowering. Reduction of the lowering volume flow at the same pilot control pressure as that of the load increases is also achieved. This arrangement results in a characteristic curve of the system such that the operator of the forklift requires more adjusting distance to reach the same lowering speed for the load fork as the load increases. The degressive control behavior involved on the part of the operator is desirable for safety reasons and is achieved immediately with the control device of the present invention. 
   The control device of the present invention uses a pilot-controlled seat valve controlled by a proportional pressure reducer as the control aperture for a maximum volume flow restriction integrated into the system. Preferably, a measuring nozzle screw-mounted in the control unit of the operating device may be used as orifice gauge which must be designed for a control pressure differential of only 3 to 4 bar. The pressure differential of the measuring nozzle is monitored by the pressure regulator in the pilot control circuit of the seat valve. When the pilot differential pressure is reached, the pressure regulator opens, thereby limiting the maximum pilot control pressure of the seat valve. The respective lift of the seat valve then assumes a normal position for maintaining the maximum volume flow. 
   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 preferred embodiments of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram illustrating the circuitry of the control device according to a first embodiment of the present invention; 
       FIG. 2  is a graph illustrating the pattern of the respective lowering volume flow V s  at constant pilot control pressure p s (isobars); and 
       FIG. 3  is a schematic circuit diagram illustrating the circuitry of the control device shown according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A control device according to an embodiment of the present invention has a closed hydraulic circuit  10  with a hydraulic pump P for fluid and pressure supply and a tank T for fluid storage. The hydraulic circuit  10  is connected to an operating device, designated as a whole as  12 , which has a conventional operating cylinder  14  for raising and lowering a load fork  16  of a forklift of conventional design (not shown). The load fork  16  may be provided on its upper side with a load  18 . The directions of lifting and lowering the load fork  16  are indicated in  FIG. 1  by arrows  20 . If the lift valve  22  is selected, it is connected for the hydraulic medium to be fed into the operating cylinder  14  under pressure. The return valve  24  is open, and thus, the load fork  16  is lifted together with its load  18 . 
   The control device is provided with a valve control unit, which assumes a normal position when the load fork  16  carrying the load  18 . That normal position maintains a predetermined maximum volume flow in the hydraulic circuit  10 . The valve control unit is provided for this purpose with a pressure regulator  26  which, when a predetermined control differential is reached in the hydraulic circuit  10 , limits the pilot control pressure for a control valve  28  so that this valve assumes a normal position for maintaining the maximum volume flow. The pressure regulator  26  has its two control connections  30  and  32  upstream and downstream from an orifice gauge or choke  34 , and is connected to the hydraulic circuit  10 . The orifice gauge  34  delivers the control pressure differential for operation of the pressure regulator  26 . The orifice gauge  34  is inserted into the hydraulic circuit  10  between the operating device  12  and the control valve  28 . 
   The control valve  28  may be operated by a proportional pressure reducer  36 . The outlet  38  of the proportional pressure reducer is connected to the pilot control circuit  40  of the control valve  28 . The purpose of the pressure regulator  26  is to limit the pilot control pressure when the normal pressure differential is reached at the orifice gauge  34 . In addition, the input  42  of the proportional pressure reducer  36  is connected at a connection point  44  in the hydraulic circuit  10 . The connection point  44  opens into hydraulic circuit  10  between the orifice gauge  34  and the operating device  12 . The hydraulic pump P with operating valve  22  may also open into this connection point  44  to the hydraulic circuit  10  or, as illustrated in  FIG. 1 , at an upstream point between the connection point  44  and the operating device  12 . The output  38  of the proportional pressure reducer  36 , as shown in  FIG. 1 , has another aperture  46 . The characteristic of the proportional pressure reducer  36  may be adjusted. The reducer, in its switched position shown in  FIG. 1 , establishes a fluid conducting connection between the output  38  and a tank connection  48 . In its other switched position, the reducer establishes a fluid conducting connection between input  42  and output  38 . In addition, in its switched position shown in  FIG. 1  the pressure regulator  26  assumes a blocking position in which the pilot control circuit  40  is blocked from another tank connection  50 . In the other switched position of the pressure regulator  26 , a fluid conducting connection is established between the pilot control circuit  40  and the other tank connection  50 . In addition, an emergency drain function of the forklift is effected by a conventional blocking unit  52 . 
   The pilot control valve  28  is represented by a pilot-controlled seat valve  54  with an aperture cross-section continuously variable as a function of the pilot control pressure. This valve is illustrated in  FIG. 1  both by a switching symbol and by its actual internal structure. The pilot control is effected by way of a force transmitter  56  with a transmitting piston  58 . 
   This force transmitter  56  is necessary to achieve full opening of the control valve  28  when the lift is lowered without a load (load pressures possibly&lt;8 bar) for the lowest possible flow resistance. This arrangement satisfies the requirement for the load lowering function must be performed without the pump running, that is, the primary pressure supply of the proportional pressure reducer  36  can be provided only by tapping the load pressure. Consequently, the pilot control pressure may never be higher than the current lift pressure in lowering without a load. The piston side of the force transmitter  58  is connected to the pilot control circuit  40  so that fluid is conducted. The force transmitter rod side is connected to the tank so that fluid is conducted. In addition, the force transmitting piston  58  is connected by its control rod in operating connection to the other switching elements of the seat valve. Also acting on these other switching elements, in the direction opposite that of the force transmitting piston  58 , is a pressure spring  60  whose pretensioning is set by means of the plug screw  62 . The configuration for pilot-controllable seat valve  54  is conventional, and will not be described in detail at this point. In the switched position shown in the  FIG. 1  the pilot-controllable seat valve  54  seals the operating device  12  off free of leakage from the tank connection T, in the manner of a seat valve. In the other switched position, the control aperture  64  with an aperture value continuously variable as a function of the pilot control pressure establishes the respective connection with a proportionally variable characteristic throttle curve. 
   The pressure regulator  26  in the pilot control circuit  40  is designed for low volume flows, such as ones of an order of magnitude of approximately 1 l/min. In addition, the pressure drop at the orifice gauge  34 , especially one in the form of a measuring nozzle, is monitored by the pressure regulator  26 . Pressure regulator  26  effects connection of the pilot control line of the seat valve  54  to the tank connection when the pilot pressure differential is reached, and thus prevents further increase in the lowering volume flow. The maximum lowering volume flow may be varied by changing the orifice gauge diameter. In addition, the maximum aperture cross-section selected for the control aperture  64  of the control valve  28  is very large in relation to the pilot control pressure limit of the pressure regulator  26 . In particular, the aperture cross-section for the control aperture  64  of the control valve  28  is configured so that, with the pilot control pressure in the pilot control circuit  40  remaining the same, the lowering volume flow decreases with an increase in the load  18  on the operating device  12 . In addition, both the output  66  of the pressure regulator  26  and the output  38  referred to above of the proportional pressure reducer  36  are introduced into the pilot control circuit  40  of the control valve  28 . In addition, the input  42  of the proportional pressure reducer  36  and the control connection  30  are interconnected so as to conduct fluid as triggering input for the pressure regulator  26 , by way of a branch-off point  68  in the hydraulic circuit  10 . 
   The control device of the present invention uses the pilot-controlled seat valve  54  with control aperture  64  controlled by the proportional pressure reducer  36  for the maximum volume flow restriction required in the hydraulic circuit  10 . The orifice gauge  34  in the form of a measuring nozzle is designed for a control differential pressure of only 3 to 4 bar at rated volume flow. The control differential pressure on the orifice gauge  34  is monitored by the pressure regulator  26  in the pilot control circuit  40  of the seat valve  54 . When the predetermined control differential pressure is reached, the pressure regulator  26  opens into its switched position permitting flow of fluid, and thereby, limits the maximum pilot control pressure of the seat valve  54 . The lift of the seat valve assumes a normal position for the purpose of maintaining the maximum volume flow. The respective layout permits increase in the volume flow during lowering without a load, so that obstacles to operation may not arise during lowering of the load fork  16 . As is illustrated in  FIG. 1 , during the respective lowering movement the hydraulic pump P is separated from the hydraulic circuit  10  by the operating valve  22  being switched to the blocking position. 
   The maximum aperture cross-section selected for the seat valve with its control aperture  64  may be very large, since adjustment of the piston position is made by the pilot control pressure restriction of the pressure regulator  26 . This arrangement also results in an additional increase in the lowering speed. 
   In addition, the geometry of the seat valve  54  is configured so that with increased load pressure, the forces of flow require considerable increase in the pilot control pressure to achieve the same lift. The result is that the lowering volume flow drops with increase in the load under the same pilot control pressure. As a result, a system characteristic may be achieved such that the operator of the forklift requires greater adjusting distance for reaching the same lowering speed as the load increases. This respective degressive behavior favors good operation and also allows for safety aspects. The lowering volume flows V s  at constant pilot control pressure p s (isobars) are shown in  FIG. 2 . The line at the top  FIG. 2 , indicated by an arrow  70 , reflects the maximum restriction by the pressure regulator  26 . 
   Along with its original function of seat valve open at all times, the control aperture  64  of the control valve  28  is consequently assigned the additional function of control aperture of a constant flow control valve made up of orifice gauge  34 , pressure regulator  26 , and control aperture  64  for restriction of the maximum lowering speed. This arrangement results in a reduction of the functional surfaces present in the main volume flow of the lowering function and accordingly an increase in the lower speed without a load. 
   In contrast with the control differentials of &gt;7 bar customarily assigned to constant-flow controllers, the constant-flow controller made up of orifice gauge  34 , pressure regulator  26 , and control aperture  64  with a control pressure differential of only 3 to 4 bar can ensure stable lowering. This arrangement leads to a greater measuring aperture diameter, something which also results in an increase in the lowering speed without load. 
   The control valve  28  is triggered by a force transmitter made up essentially of valve element  56  and piston  58 . To permit load lowering even with the hydraulic pump switched off (conservation of energy especially in the case of battery operated devices), the primary connection of the proportional pressure reducer  36  is supplied by the load pressure of the lift function. The force transmitter  56  eliminates the problem that only an extremely low pilot control pressure (&lt;7 bar depending on the lift mast design) is available for opening the control valve  28  in lowering with no load. Only use of the force transmitter  56  makes possible full opening of the control valve under all circumstances, and accordingly also extremely low flow resistance in lowering with no load. 
   A second embodiment of the control device is illustrated in  FIG. 3 . The same components in  FIG. 1  are identified by the same reference numbers, while the statements made in the foregoing also apply to the embodiment shown in  FIG. 3 . In addition, the embodiment shown in  FIG. 3  is explained only to the extent that it differs significantly from the embodiment shown in  FIG. 1 . 
   In the embodiment shown in  FIG. 3 , the orifice gauge  34  is connected to a tank connection line, between the connection line T at the control valve  28  and the tank T. In addition, the first control connection  30  is mounted continuously between pressure regulator  26  and tank connection T of the control valve  28 . As in the embodiment illustrated in  FIG. 1 , the second control connection  32  at one end is connected to the pressure regulator  26 , and extends, as viewed in the direction of flow of fluid downstream from the orifice gauge  34 , into the connecting line to the tank T at its other end. This configuration presents the advantage that all connections of the pressure regulator  26  extend through the valve seat  64 , disconnected from the consuming device  12 . Only the proportional pressure reducer valve  36  remains as a potential leakage point. However, it may be dimensioned in such a way that the leakage from this valve  36  is far below the lift density required. The configuration shown in  FIG. 3  consequently allows almost leak-proof operation, and yet, very good control behavior for the device as a whole. 
   The device of the present invention need not be restricted to forklifts, but may also be applied to comparable problems. In addition, hydraulics may also be construed to include the use of pneumatic means. 
   The largely viscosity independent orifice gauge  34  may also be replaced by a corresponding orifice or a throttle with adjustable cross-section which provides the control pressure differential required for the pressure regulator  26 . 
   The control device of the present invention represents a highly cost-effective, reliably operating system by means of which specific lowering movements may be executed by operating devices, and the triggering behavior may be regarded as stable. In addition, it is possible to permit a slower lowering speed with increasing load during operation of the control device. This capability complies with more rigid safety requirements. 
   While various embodiments have 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.