Equalized hydraulic clamp force control

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.

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.

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.