BOOM CYLINDER DRIVER FOR OIL-PRESSURE SHOVEL

PURPOSE:To save energy by reducing the amount of oil to be supplied by a method in which a connection switch valve to switch the oil path of each boom cylinder to a parallel connection and a serial connection is provided, and at the time of boom lowering operation, each cylinder is switched to serial connection. CONSTITUTION:A connection switch valve 208 for boom cylinder is switched to serial connection, and an oil path switch control valve 201 for boom cylinder is operated to the direction of X. Hereupon, working oil of an oil-pressure pump 300 flows through a control valve 201 into the rod side pressure chamber of the boom cylinder to push out oil as many as two-fold from the head side pressure chamber. The oil so pushed out flows by 1/2 each through a switch valve 208 into cylinders 11a and 13a, and oil as many as two-fold is returned through the control valve 201 from the head side pressure chamber to an oil tank 301. Three cylinders 11a, 11b, and 11c are integrally contracted and the lowering operation is made by 1/3 necessary horsepower. The switch valve can thus be miniaturized.