Equalized hydraulic clamp force control

Abstract

Hydraulic valve circuitry adapted for hydraulic connection to a lift truck having a mast and at least one lift cylinder for alternately raising and lowering the mast. The hydraulic assembly is also adapted for hydraulic connection to an attachment to the lift truck, the attachment having opposed clamps actuated by respective clamp cylinders. The hydraulic assembly preferably comprises a valve circuit capable of causing the at least one lift cylinder to be an accumulator.

I claim: 
     
       1. A hydraulic assembly adapted for hydraulic connection to a lift truck having a mast and at least one lift cylinder for alternately raising and lowering the mast, the hydraulic assembly also adapted for hydraulic connection to an attachment to the lift truck, the attachment having opposed clamps actuated by respective clamp cylinders, the hydraulic assembly comprising a valve circuit capable of causing the at least one lift cylinder to be an accumulator, where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to at least one of a sensed change in load weight and a sensed change in gripping force by the respective clamp cylinders. 
     
     
       2. The hydraulic assembly of  claim 1  adapted for hydraulic connection to a lift truck having a first lift cylinder for moving the mast within a main lift stage, and a second lift cylinder for moving the mast within a second lift stage, and the valve circuit is capable of causing each of the first cylinder and second cylinder to be a said accumulator. 
     
     
       3. The hydraulic assembly of  claim 1  where the valve circuit causes the at least one lift cylinder to be an accumulator without concurrent manual actuation of pressure supply to either the at least one cylinder or the respective clamp cylinders. 
     
     
       4. The hydraulic assembly of  claim 1  where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to a sensed change in load weight. 
     
     
       5. The hydraulic assembly of  claim 1  where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to a sensed change in gripping force by the respective claim cylinders. 
     
     
       6. The hydraulic assembly of  claim 1  where the valve circuit comprises a check valve. 
     
     
       7. The hydraulic assembly of  claim 1  where the valve circuit comprises a one-way valve. 
     
     
       8. The hydraulic assembly of  claim 1  where the lift truck supplies pressure to the at least one lift cylinder through a first hydraulic line, and the attachment supplies pressure to the respective clamp cylinders through a second hydraulic line, and the valve circuit interconnects the second hydraulic line to the first hydraulic line at a location between the at least one lift cylinder and the lift truck. 
     
     
       9. The hydraulic assembly of  claim 1  where the valve circuit comprises a first portion connectable to the lift truck and a second portion connectable to the attachment. 
     
     
       10. The hydraulic assembly of  claim 9  where the lift truck supplies pressure to the at least one lift cylinder through a first hydraulic line in the first portion, and the attachment supplies pressure to the respective clamp cylinders through a second hydraulic line in the second portion, and the hydraulic circuit interconnects the second hydraulic line to the first hydraulic line at a location between the at least one lift cylinder and the lift truck. 
     
     
       11. A hydraulic assembly adapted for hydraulic connection to a lift truck having a mast and at least one lift cylinder for alternately raising and lowering the mast, the hydraulic assembly also adapted for hydraulic connection to an attachment to the lift truck, the attachment having opposed clamps actuated by respective clamp cylinders, the hydraulic assembly comprising a valve circuit capable of causing the at least one lift cylinder to be an accumulator, and where the lift truck supplies pressure to the at least one lift cylinder through a first hydraulic line, and the attachment supplies pressure to the respective clamp cylinders through a second hydraulic line, and the valve circuit interconnects the second hydraulic line to the first hydraulic line at a location between the at least one lift cylinder and the lift truck. 
     
     
       12. The hydraulic assembly of  claim 11  adapted for hydraulic connection to a lift truck having a first lift cylinder for moving the mast within a main lift stage, and a second lift cylinder for moving the mast within a second lift stage, and the valve circuit is capable of causing each of the first cylinder and second cylinder to be a said accumulator. 
     
     
       13. The hydraulic assembly of  claim 11  where the valve circuit causes the at least one lift cylinder to be an accumulator without concurrent manual actuation of pressure supply to either the at least one cylinder or the respective clamp cylinders. 
     
     
       14. The hydraulic assembly of  claim 11  where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to a sensed change in load weight. 
     
     
       15. The hydraulic assembly of  claim 11  where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to a sensed change in gripping force by the respective claim cylinders. 
     
     
       16. The hydraulic assembly of  claim 11  where the valve circuit comprises a check valve. 
     
     
       17. The hydraulic assembly of  claim 11  where the valve circuit comprises a one-way valve. 
     
     
       18. The hydraulic assembly of  claim 1  where the valve circuit comprises a first portion connectable to the lift truck and a second portion connectable to the attachment. 
     
     
       19. An attachment for a lift truck having a mast and at least one lift cylinder for alternately raising and lowering the mast, the attachment having respective clamp cylinders for clamping a load using hydraulic pressure, and the attachment comprising a hydraulic assembly adapted for hydraulic connection to the lift truck, the hydraulic assembly comprising a valve circuit capable of causing the at least one lift cylinder to be an accumulator, where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to at least one of a sensed change in load weight and a sensed change in gripping force by the respective clamp cylinders. 
     
     
       20. An elongate, longitudinally extensible lifting device having at least one lift cylinder for alternately raising and lowering a mast of a lift truck in a main lift stage and a free lift stage, the lifting device comprising a hydraulic assembly adapted for hydraulic connection to an attachment of the lift truck having opposed clamps actuated by respective clamp cylinders, the hydraulic assembly comprising a valve circuit capable of causing the at least one lift cylinder to be an accumulator, where the at least one lift cylinder, when operating as an accumulator, supplies fluid to the respective clamp cylinders automatically in response to at least one of a sensed change in load weight and a sensed change in gripping force by the respective clamp cylinders.