A hydraulic circuit of winches for a crane, comprising:
- a first hydraulic motor for driving a first winch;
- a second hydraulic motor for driving a second winch;
- first and second hydraulic pumps;
- first and second control valves for merging pressure oil discharged from said first and second hydraulic pumps and for supplying the pressure oil to said first and second hydraulic motors in accordance with operation of first and second operation levers, said first control valve being a circuit in which a first-gear spool for the first winch stroked by secondary pressure of a first remote control valve operated by said first operation lever and a second-gear spool for the second winch stroked by secondary pressure of a second remote control valve operated by said second operation lever are connected in series, said second control valve being a circuit in which a second-gear spool for the first winch stroked by the secondary pressure of said first remote control valve operated by said first operation lever and a first-gear spool for the second winch stroked by the secondary pressure of said second remote control valve operated by said second operation lever are connected in series;
- four second-gear blocking switching valves respectively provided in pilot pipe lines providing communication between said remote control valves and pilot operation portions of valve sections of said second-gear spools of said control valves, said second-gear blocking switching valves being switchable between transmission positions for transmitting the secondary pressure of said remote control valves to said pilot operation portions of said valve sections of said second-gear spools of said control valves and block positions for blocking the transmission of the secondary pressure and for providing communication between said pilot operation portions and a tank;
- a switching selection means for switching said second-gear blocking switching valves by remote control operation;
- four proportional pressure reduction valves changing secondary pressure to be outputted in accordance with the secondary pressure of said remote control valves;
- and
- four secondary pressure changing switching valves provided in pilot pipe lines providing communication between said remote control valves and pilot operation portions of valve sections of said first-gear spools of said control valves in correspondence with said proportional pressure reduction valves, respectively, said secondary pressure changing switching valves being linked with said switching selection means so as to be switched to first positions for transmitting the secondary pressure of said remote control valves to said pilot operation portions of said valve sections of said first-gear spools when said second-gear blocking switching valves are switched to the transmission positions by said switching selection means, and so as to be switched to second positions for transmitting the secondary pressure of said corresponding proportional pressure reduction valves to said pilot operation portions of said valve sections of said first-gear spools when said second-gear blocking switching valves are switched to the block positions by said switching selection means,
- wherein each of the proportional pressure reduction valves has such a characteristic of change in a secondary pressure that a secondary pressure of each of the proportional pressure reduction valves at stroking start points of said first-gear spools in a case where the second-gear blocking switching valves are switched to the block position so that a one-pump and one-motor independent circuit type is selected is the same as a secondary pressure of a corresponding remote control valve at stroking start points of said first gear spools in a case where the second-gear blocking switching valves are switched to the transmission position so that a two-pump merging series circuit type is selected.