HYDRAULIC DRIVE UNIT

摘要

In order to release an excessive rod pressure in a first hydraulic cylinder and make effective use of the pressure oil in the rod-side chamber thereof at the time of operation when pressure oil is supplied to the bottom-side chamber of a second hydraulic cylinder, the hydraulic drive unit comprises a boom cylinder (6), an arm cylinder (7), a communication passage (40) establishing communication between the rod-side chamber (6b) of the boom cylinder (6) and the bottom-side chamber (7a) of the arm cylinder (7), and a switching valve (57) installed in the communication passage (40) for opening or closing the communication passage (40) according to the rod pressure in the boom cylinder (6), wherein an arm crowd single-operation is performed, in which when the reaction force of excavation causes the rod pressure in the boom cylinder (6) to take a predetermined high pressure value or above, a switching valve (57) switches from the cut-off position to the communication position, whereupon the pressure oil in the rod-side chamber (6b) of the boom cylinder (6) is supplied to the bottom-side chamber (7a) of the arm cylinder (7), and the boom cylinder (6) is automatically extended, thereby precluding the possibility of the vehicle body of the hydraulic shovel floating up.