Abstract:
In a stacker control of a stacker (fork lift) for high storage racks comprising a swing-shift fork (G) which is pivotable about a vertical axis ( 7 ) and horizontally movable in computer-assisted fashion by hydraulically controlled hydraulic motors (H 1 , H 2 ) via a directional control valve (W 1 , W 2 ) which is connected to a joint source of pressure ( 15 , P), a three-way pressure balance (D) is provided between the source of pressure ( 15 ) and the directional control valves (W 1 , W 2 ) for giving priority to one of said hydraulic motors (H 1 , H 2 ). the three-way pressure balance (D) being controlled by control pressures tapped upstream and downstream of the directional control valve, at least the directional control valve of the preferential hydraulic motor having an open zero position, and the three-way pressure balance (D) forming part of a neutral circulation circuit (U) led via the control piping of the further hydraulic motor from the source of pressure to tank.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a stacker control. 
     Shelf stackers (special sorts of fork lift trucks, the fork of which can be lifted and lowered vertically but also have additional fork functions such as additional lift, thrust, turning the fork), in particular stackers for high storage racks, comprising swing-shift forks require a high degree of safety during control of the fork if they operate within a laterally defined aisle area, e.g. between rows of shelves. For reasons of space the aisle area is often so limited that uncontrolled turn and thrust movements of the fork may easily lead to collisions. Therefore, for such complicated maneuvering operations, shelf stackers are often operable by means of a computer which guides the stacker control by simultaneously swinging and shifting the fork, so that the fork does not collide with the rows of shelves. To this end, the computer must not only be informed in a precise manner about the actual position of the fork, but the stacker control must be precisely guidable by the computer, i.e. exactly in proportion with the control signals. A further problem encountered in the case of a precise proportional control of the swing-shift fork arises from the fact that hydraulic motors which carry out swinging and shifting operations, respectively, optionally at the same time, require clearly distinct flow rates, e.g. in a ratio of 1:3 to 1:6, and the control of the hyraulic motor with the smaller rate must also be reliably proportional in cases where the hydraulic motor requiring the larger amount is moved at the same time. 
     It is known in practice that in the stacker control of a shelf stacker a three-way flow regulator (3-way control valve) and a proportional directional control valve with upstream supply regulator are assigned to each hydraulic motor to achieve precise proportionality during control, as is e.g. required for maneuvering operations within narrow aisle areas. Such a stacker control is not only very troublesome and expensive, but requires a lot of space, which is not readily available in the case of shelf stackers, because of the complicated hydraulic components. Therefore, such a stacker control is also very troublesome for the reason that a specific neutral circulation switching function of the hydraulic medium, which is delivered by the source of pressure, is required, for instance in case of pilot pressure tapping (PSL load sensing principle), in the directional control valves. 
     Furthermore, it is known in practice that the stacker control of such shelf stackers is equipped with a plurality of pressure balances, directional control valves and lowering brake valves (drop-rate braking valves) to save space and costs. However, it is thereby not possible to achieve a sufficiently proportional control, as is e.g. needed for maneuvering operations within limited aisle areas. Such shelf stackers first must move out of the aisle area for the swing-shift adjustment of the fork, and must again move into the aisle with the adjusted fork, or the aisle areas between rows of shelves must be made correspondingly wide in relation with the width of the fork. 
     In control devices which are known from WO86/06142, U.S. Pat. No. 3,911,942, U.S. Pat. No. 3,987,623 and DE-A-1 95 49 150, control priority is given to a selected group of consumers over other consumers. The control devices contain three-way pressure balances or multi-way valves which serve flow control purposes. 
     Further prior art is found in U.S. Pat. No. 4,517,800, U.S. Pat. No. 4,733,533, DE-A-301 61 57 and U.S. Pat. No. 4,543,031. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a stacker control of the aforementioned type which is inexpensive, space-saving and comfortable and which permits a safe maneuvering of the swing-shift fork within narrow aisle areas, possibly in computer-assisted fashion. 
     This object is achieved with the following features of claim  1 . 
     With the help of the three-way pressure balance, priority is always given in a constructionally simple and inexpensive manner to the hydraulic motor in which under simultaneous control of a further hydraulic motor the risk of a no longer exactly proportional control is given, i.e. normally to the hydraulic motor having the lower flow rate. In a shelf stacker or stacker for high storage racks, this is normally the hydraulic motor which is in charge to turn the fork about the vertical axis. Thanks to the action of the three-way pressure balance, the further hydraulic motor(s) only receive(s) the excess amount so that at least the preferential hydraulic motor can be controlled in a precisely proportional manner under all operating conditions. The installation space for the stacker control and also the equipment efforts are reduced because a few compact components are sufficient and because the three-way pressure balance permits a very simple neutral circulation switching through which the hydraulic medium delivered by the source of pressure, which is normally operated electrically, passes at a small throttling resistance and small counterpressure into the return means. The proportional directional control valve of the preferred hydraulic motor relieves the two work lines to the return means in its zero position so that a strong control pressure drop which can be used for the neutral circulation switching is given. The further hydraulic motor is also controllable in a precisely proportional manner via the three-way flow regulator, with a three-way flow regulator, the three-way pressure balance and the further directional control valves cooperating for the establishment of the neutral circulation circuit. 
     According to another feature of the invention the swing-type hydraulic motor for the fork turn function is the preferential hydraulic motor which requires a smaller flow rate than the further hydraulic motors and in which a precise proportional control is important for difficult maneuvers of the swing-shift fork. 
     According to another feature the precise proportional control is possible with the help of the proportional directional control valve which simultaneously contains the regulating aperture for the three-way pressure balance. 
     According to another feature there is provided a simple load holding device for hydraulically blocking each hydraulic motor and for holding a load without any leakage. The load holding device with the load holding valves which can alternately be pilot-operated from the work lines permits a sensitive motional control under load and helps, above all, to simplify the neutral circulation switching because in a load holding situation both work lines are relieved downstream of the directional control valve, and a pilot pressure for the three-way pressure balance is automatically present which switches the three-way pressure balance into the neutral circulation position. Furthermore, the load holding device permits the use of a short directional control valve which is open in the zero position and which, therefore, does not require separate flow paths for the neutral circulation. 
     According to another feature the pilot pressure for the three-way pressure balance and the neutral circulation switching is tapped in a constructionally simple manner without the directional control valve, which is thus compact, requiring any troublesome constructional measures. 
     According to another feature, the three-way pressure balance gives supply priority to the selected hydraulic motor over the further hydraulic motor(s) irrespective of whether and how said motors are acted upon. 
     According to another feature, the three-way pressure balance is of a particularly simple construction. 
     According to another feature, if no further hydraulic motor is acted upon, the pressurized fluid delivered by the source of pressure is guided in the neutral circulation circuit with the throttling loss into the return means via the directional control valve of the further hydraulic motor or said directional control valve and the three-way flow regulator arranged upstream of said valve. 
     According to another feature the further hydraulic motor is controlled either only in its direction at a speed depending on the amount left by the preferential hydraulic motor, or optionally via a delivery control of the source of pressure (speed-controlled pump). 
     According to another feature the swing-shift fork can be guided by the computer when complicated movement maneuvers are to be controlled, with the displacement and/or position detectors providing the actual parameters for the computer. 
     Embodiments of the subject matter of the invention will now be explained with reference to the drawing, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a shelf stacker; 
     FIG. 2 is a schematic top view on a shelf stacker during work within a restricted aisle area, 
     FIG. 3 is a block diagram of a first embodiment of a stacker control; 
     FIG. 4 is a block diagram of a further embodiment of a stacker control; and 
     FIG. 5 is a schematic longitudinal section through a detail. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A shelf stacker (fork lift truck) as shown in FIGS. 1 and 2, in particular a stacker F for high storage racks  10 ,  11  comprises a travel body  1  with actuating elements  2  of an actuating system  3  for a fork G and optionally, for a steering system (not shown in more detail), a travel drive, a lifting/lowering function and, if necessary, a tilting function. The actuating device  3  is connected to a stacker control S and, optionally, to a computer C. The fork G is horizontally movable in guide means  4  in a direction transverse to the longitudinal direction of the stacker F (horizontal thrust function) and is, for instance, pivotably mounted on a mast  6  (fork turn function) defining a vertical axis  7  (swing-shift fork). The mast  6  is held in a support construction  5 . The shift movement of the fork G is controlled with a hydraulic motor H 1 , whereas the swing movement of the fork G about the vertical axis  7  (turning range at least 180°) is controlled with at least one hydraulic motor H 2 . The computer C is linked or connectable to the stacker control S. Furthermore, there may be provided position or displacement detectors  8 ,  9  for the instantaneous position of the fork G, the signals of said detectors being supplied to the computer C. 
     FIG. 2 shows how the shelf stacker F operates within a laterally defined aisle area  12  between rows of shelves  10 ,  11 , with the aisle width being only slightly wider than the width of fork G. With simultaneous swing and shift movements of the fork G, said fork is pivoted within the aisle width from the basic position shown in unbroken lines, in which the fork arms are oriented in the direction of travel, by about 90° to one or the other side until the fork arms are oriented towards the rows of shelves. Such a maneuver may e.g. be assisted by the computer C. The movements of the hydraulic motors H 1 , H 2  are superimposed on one another in such fashion that the fork G does not collide with the rows of shelves  10 ,  11 . 
     According to FIGS. 3 and 4, the hydraulic motor HI for the shift movement is a rotatable hydraulic motor (or a pair of hydraulic cylinders, not shown), whilst the hydraulic motor H 2  for the swing movement of the fork G consists of two unilaterally actuable cylinders  13 ,  14  which pivot the fork G back and forth about the vertical axis  7 . Both cylinders  13 ,  14  are e.g. only acted upon at their piston rod sides whilst their piston sides are short-circuited. 
     The stacker control S in FIG. 3 is supplied with pressurized fluid from a source of pressure  15 , e.g. an electrically driven hydraulic pump. A pump line P which is connected to a return means R via a pressure relief valve  18  leads to the inlet port of a three-way pressure balance D. A pressure port  22  of the three-way pressure balance D is connected via a pressure line  16  to the pressure inlet of a proportional directional control valve W 1  which in both displacement directions can be adjusted by means of proportional magnets  33 ,  34  from a zero position which is centered by springs (4/3 directional control valve with open zero position). Two work lines A 1 , B 1  lead from the proportional directional control valve W 1  to the two cylinders  13 ,  14  of the hydraulic motor H 2 . A load holding valve assembly L is arranged in the work lines A 1 , B 2  with two load holding valves  35  which are each pilot-operated against a spring  36 , and with check valves  38  bypassing said valves. Each load holding valve  35  which is arranged in a work line A 1  or B 1  can be pilot-operated via a control line  37  from the respectively other work line B 1 , A 1  in response to pressure. 
     A transverse line  31  connects the two work lines A 1  and B 1  between the proportional directional control valve W 1  and the load holding device L. A changing valve  30  is provided in the transverse line  31  with a port  32  passing the respectively higher pressure in a work line. A control line  27  extends from port  32  via a throttle  28  to a control side of the three-way pressure balance D. A control line  25  which extends via a throttle  26  to the opposite control side of the three-way pressure balance D is branched off between the three-way pressure balance D and the proportional directional control valve W 1 . The three-way pressure balance contains a control piston  21  which is acted upon by means of the control pressure prevailing in the control line  25  in the closing direction of a connection between the pump line P and the pressure line  16 . By contrast, in the opposite direction, i.e. in the opening direction of said connection, the control piston  21  is acted upon by the control pressure prevailing in the control line  27  and by a control spring  24 . 
     A further pressure line  17  which leads to at least one further hydraulic motor, i.e. the shift type hydraulic motor H 1 , is connected to a second pressure port  23  of the three-way pressure balance D. In FIG. 3, the pressure line  17  has arranged therein a three-way flow regulator  19  which is adjustable by means of a proportional magnet  20  and possesses a connection to the return means R. Downstream of the three-way flow regulator  19 , the pressure line  17  is connected to a port  39  of a magnetically operated directional control valve W 2  (4/3 directional control valve with open zero position). Two work lines A 2 , B 2  lead from the directional control valve W 2  to the hydraulic motor H 1 . A further load holding device L is provided between the directional control valve W 2  and the hydraulic motor H 1 , in accordance with the load holding device L of the hydraulic motor H 2 . Open zero position means with respect to the directional control valve W 1 , W 2  that, as outlined in W 2 , both work lines are connected via a transverse channel  40  to the return means R in a central position which is centered by springs, while the pressure port  39  is shut off. The directional control valve W 2  could also be operated by proportional magnets and work in a precisely proportional manner. 
     In contrast to FIG. 3, the pressure line  17  in the stacker control S of FIG. 4 is directly guided to the directional control valve W 2  (4/3 directional control valve with open zero position), in which in the central position the transverse channel  40  connects both work lines A 2 , B 2  to the return means, and the pressure line  17  is also connected to the return means via its pressure port  39 ′ and the transverse channel  40 . 
     In both embodiments of the stacker control S of FIGS. 3 and 4, there is provided a neutral circulation circuit U via which the hydraulic means delivered by the source of pressure  15  is drained to the return means in case none of the hydraulic motors H 1 , H 2  is moved and a respective load is held. Part of the neutral circulation circuit U is the three-way pressure balance D which in the closed position of the connection from the pump line P to the pressure line  16  leads the hydraulic medium into the pressure line P from which in FIG. 3 it passes via the three-way flow regulator  19  to the return means, whereas it passes via the transverse channel  40  in the stacker control shown in FIG.  4 . 
     In the stacker control S in FIG. 4, a specific control of the shift-type hydraulic motor could e.g. be carried out by means of a speed-controllable hydraulic pump. 
     FIG. 5 shows a simple embodiment of the three-way pressure balance D of the preceding figures in longitudinal section. In a housing  41  comprising a housing hole  42  two pressure ports  16 ,  17  are provided with grooves  43 ,  44  which define control edges. In the housing hole  42 , the control piston K,  21  is sealingly movable in response to the control pressures acting in both axial directions, from the control lines  25  and  27 , respectively, with the force of the expediently adjustable control spring  24  being added to the control pressure of the control line  27 . The control spring is made very weak and, substantially, is only sufficient for holding the control piston K in the open position of the connection from the pressure port P to the pressure port  16  when the system is without any pressure. The force of the control spring  24  is substantially the relevant throttling factor which in the case of switching to the pressureless neutral circulation must be overcome by the source of pressure. 
     In the control piston K, a flow channel is shaped in the form of a groove  45  which is defined at both sides by control edges  46  and  47 . The control edges  46  and  47  alternately cooperate with the grooves  43 ,  44  or the control edges thereof in the manner of apertures to throttle the pressurized fluid accordingly and to pass it either to the pressure port  16  or to the pressure port  17 , and to keep constant the pressure difference set in the directional control valve W 1  by the electric current, through corresponding control movements. It is evident that the control pressure acting on the control piston at the lift side could also be tapped from the control port  16  via a throttle hole directly in the housing  41 .