Patent ID: 11905153
Assignee: JUNGHEINRICH AKTIENGESELLSCHAFT
Field: Handling (Mechanical engineering)
Classification: CPC B  F | IPC B

Claim 0:
1. An industrial truck comprising:
a lift mast comprising
at least one mast lift stage driven by at least one mast lift cylinder, and
a free lift stage having a load receiving means driven by at least one free lift cylinder such that the load receiving means is displaceable along the lift mast; and

a hydraulic system for supplying a hydraulic fluid to and for unloading the hydraulic fluid from the at least one mast lift cylinder and the at least one free lift cylinder;
wherein the hydraulic system comprises a control programmed to actuate the at least one mast lift cylinder and the at least one free lift cylinder in a load lowering operation such that the mast lift stage and the free lift stage are simultaneously retracted,
wherein the hydraulic system comprises separate hydraulic return lines for unloading the at least one mast lift cylinder and the at least one free lift cylinder in the load lowering operation,
wherein the hydraulic system comprises a first hydraulic return line that runs between the at least one free lift cylinder and a reservoir for the hydraulic fluid,
wherein the hydraulic system further comprises a second hydraulic return line that runs between the at least one mast lift cylinder and the reservoir,
wherein a first lowering valve is integrated in the first return line,
wherein a second lowering valve is integrated in the second return line,
wherein the control is programmed to open the first and the second lowering valve simultaneously at the start of the load lowering operation when the load receiving means is lowered and the mast lift stage and the free lift stage start simultaneous retraction,
wherein the lift truck further comprises a displacement sensor provided on the mast lift stage and on the free lift stage for determining a measured value of respective retraction speeds of the mast lift stage and the free lift stage, and
wherein the first and second lowering valves are proportional valves and the control is further programmed to control or regulate a first volumetric flow through the first lowering valve and a second volumetric flow through the second lowering valve such that, based on the measure value, the at least one mast lift stage and the load receiving means reach a lower end position at least approximately simultaneously in the end of the load lowering operation, when the load receiving means is completely lowered.