Patent ID: 7770697

Claim:
An energy recovering method in a hydraulic lift device of a battery operated industrial truck which is provided with a lift cylinder for lifting a lift means, an electric motor driven by a battery power source, a hydraulic pump for supplying pressurized working fluid to said lift cylinder to lift said lift means, and a control valve disposed between said lift cylinder and said hydraulic pump, whereby said electric motor is allowed to function as an electric generator driven by said hydraulic pump which is allowed to work as a hydraulic motor driven by pressurized working fluid returning from said lift cylinder to the hydraulic pump when a load on said lift means is lowered, and potential energy of the load is recovered as electric power to the battery, wherein lowering speed of said lift means is controlled by controlling the amount of opening of said control valve for controlling the flow rate of the pressurized working fluid returning to the hydraulic pump and rotation speed of said electric motor, and said control valve is opened to be larger than the amount of valve opening corresponding to rotation speed of the electric motor, in which rotation speed depends on the flow rate of the returning working fluid and therefore depends on the amount of the valve opening, so that lowering speed of said lift means is controlled by directly controlling rotation speed of the electric motor.