Control device for hydraulic operating cylinders of a combined lifting platform and a closing wall of a vehicle
FIELD OF THE INVENTION 
     The present invention relates to a control device for hydraulic operating cylinders of a platform to be used with vehicles which can be raised and lowered, to place loads on the platform and, by then rotating the platform about an axis close to one end of the platform, forming a closing wall, and more particularly to such a device which operates reliably even if the load on the platform is located off center, or is excessive. 
     BACKGROUND 
     It is known to provide a motor-driven hydraulic fluid pump which can be made to communicate with the fluid supply and, via pump branch lines beginning at a pump line, with a chamber of at least one lifting cylinder and of at least one closing cylinder, which rotates the lifting platform and closing wall combination, respectively, via associated control valves. A pressure transformer and piston cylinder unit is connected across the pump branch lines. 
     In previously known control devices of this type, a problem has arisen in practice that, if the load is not positioned on the combination lifting platform and closing wall in accordance with regulations, or directions of use, for example being off-center or an overload, tilting torques may arise which translate into increased tensile and compressive forces in the lifting arms and in the closing cylinders of the combination platform and closing wall. For example, the allowable forces may be exceeded by up to a factor of 5, which can damage or destroy the lifting arms and/or the closing cylinders. 
     THE INVENTION 
     It is an object of the invention to provide a device which, in case of unbalanced placement of loads, averts the dangers discussed above. 
     Briefly, a pressure-responsive element, such as an electrical switch, is provided which shuts down the pump drive motor in case an overload is sensed. The pressure-responsive device is coupled to the smaller one of the cylinder-piston chambers of the pressure transducer, which provides for a simple connection. Alternatively, the pressure sensing device may be a hydraulic pressure sensor, sensing the pressure in the smaller one of the piston-cylinder chambers of the pressure transducer. The action of this pressure sensor is such that the branch lines can be blocked off, or the pump line, providing a main hydraulic supply, is hydraulically coupled to a return line to the fluid supply; optionally, the pump motor can also be shut down, under hydraulic over-pressure. 
     As a result, in a structurally simple manner, the pump motor is shut down and/or the hydraulic fluid feed line is blocked before an overload or unbalance may cause damage, for example to the components to the lifting platform and closing wall such as the lifting arms and closing cylinders. 
     In accordance with a preferred feature of the invention, a counter mechanism for counting the number of times that pressure is exceeded can be activated by the pressure sensor, either by the electric pressure switch, or by the adjustable hydraulic pressure sensor which, in a simple form, is an adjustable hydraulic valve. 
    
    
     DRAWINGS 
     The invention is described in further detail below in terms of exemplary embodiments, referring to the drawings, in which: 
     FIG. 1 shows a hydraulic circuit diagram of a control device according to the invention; and 
     FIG. 2 is a circuit diagram of an alternative embodiment. 
    
    
     DETAILED DESCRIPTION 
     According to FIG. 1, a hydraulic fluid pump 1 is driven by an electric motor 2 and communicates via a line 3 with a hydraulic fluid supply 4. The pump 1 also communicates to a hydraulic system via a pump line 5, which includes a check valve 6, with a fluid return line 7 and with a line 8 that branches into branch lines 8&#39;, 8&#34;. The left-hand branch 8&#39; in FIG. 1, via a controllable lifting valve 9 and two controllable lifting cylinder valves 10, feeds two lifting cylinders 11 of the lifting platform and closing wall, which is not shown. 
     Analogously, the right-hand branch line 8&#34; in FIG. 1, via a controllable closing valve 12 and two controllable closing cylinder valves 13, feeds two closing cylinders 14 to rotate the lifting platform and closing wall. Between the valve 9 and 10 on one side and the valve 12 and 13 on the other, a pressure transformer and piston-cylinder unit 15, the larger cylinder-piston end 15a of which is oriented toward the lifting cylinder, is connected to the branch lines 8&#39;, 8&#34;. 
     The typical operation of such a combination loading and lifting platform and closing wall is this: 
     Let it be assumed that hydraulic pressure is present, namely that motor 2 operates pump 1. The cylinders 11 control down-up, that is, dropping and lifting movement of the combination platform-wall. The cylinders 14 control rotation of this combination loading and lifting platform and closing wall between horizontal and vertical orientation, pivoting about an axis close to the vehicle. When the platform is in essentially horizontal position, a load can be placed thereon and the load lifted. When the load then is even with the vehicle loading bay, the platform is rotated so that it will be in a vertical position, thus forming a closing wall for the vehicle. In addition, rotation about that axis of the platform is possible to permit the platform to tip somewhat downwardly at the end removed from the rotation axis, that is downwardly from a horizontal position, to permit rolling a load on the platform. The platform can then be rotated again into a horizontal position for lifting. This lifting movement is controlled by the lifting cylinder 11. All rotary movements, that is from a horizontal to a slightly downwardly tipped position for loading, and later from the horizontal to a vertical position to form a closing wall, is controlled by the closing cylinders 14. These operations are well known and standard in this field. 
     The present invention addresses itself to the problem which arises when the load is placed on the platform, when the platform is tilted downwardly for loading, and it is then attempted to tilt the platform back into a horizontal position so that, subsequently, can be raised, and to prevent damage if the load is excessive or placed in an unbalanced condition on the platform. 
     In accordance with a feature of the invention, a pressure sensor is coupled to the smaller cylinder-piston chamber 15b of the pressure transformer and piston cylinder unit 15. As shown in FIG. 1, this pressure sensor is an electrical switch 16. Switch 16 is electrically connected to a counting mechanism 17, and to an interrupter switch 18 which controls electric energy for the electrical pump motor, as shown schematically by connection line 18&#39; 
     The pressure switch 16, for example, can be set to 220 bar if the left-hand side, or larger side 15a of the pressure transformer 15 is set to 180 bar. 
     A counting mechanism 17 is provided, sealed inside the control system of the loading and lifting platform and closing wall combination. Each time an overload is sensed, it switches onward by one digit in order to record the number of times the load is exceeded over a relatively long period of time. 
     According to the embodiment shown in FIG. 2, the pressure switch 16 of FIG. 1 is replaced by an adjustable hydraulic switch-over valve or cut-off valve 19. The cut-off valve 19 is connected to the smaller chamber 15b of the pressure transformer and piston cylinder unit 15. This valve 19 is incorporated into the branching point or junction of the branch lines 8&#39;, 8&#34; and blocks them off if a predetermined pressure, for instance 220 bar, is exceeded. Optionally, the electrical pump motor feed line can also be interrupted via this hydraulic valve by a suitable electrical connection. 
     Instead of the hydraulic valve 19 shown, a piston slider or the like could be provided. A counter mechanism can also be activated via the hydraulic valve, analogously to the counting mechanism 17. 
     Optionally, and referring to FIG. 2, the hydraulic valve 19 may be so constructed that, in the event of an overload, the pump line 8 is placed into bypass communication with the fluid return line 7, so that the pump 1 idles. 
     The invention is not limited to the examples shown. On the contrary, they can be modified in various ways within the scope of the general concept of the invention.