PROBLEM TO BE SOLVED: To reduce a size of a device including a hydraulic pump, a valve block and piping for a return oil passage.SOLUTION: A hydraulic drive unit of a working machine comprises: a static hydraulic stepless gear change device 9 a single acting cylinder 12 a hydraulic pump 13 and a control valve unit 3 having a directional control valve 4 for controlling the supply and discharge of a working fluid. The directional control valve 4 is constituted of a linear electromagnetic proportional control valve which can select an operation position to an ascending position U for opening a communication oil passage to a cylinder port C, a descending position D for opening the communication oil passage to a tank port T, and a neutral position N. The single acting cylinder 12 comprises a first oil chamber 12a to which a pressure oil supply/discharge passage 32 continuing to the cylinder port C is connected, and a second oil chamber 12b located at a side opposite to the first oil chamber 12a with a piston 12c sandwiched therebetween. A second supply/discharge passage 16 for supplying and discharging the working fluid which is connected to the second oil chamber 12b is connected to a communication oil passage 17 continuing to the tank port T in a valve block 30, and a drain oil passage 33 continuing to the tank pot T is connected to a charge oil passage 93 of the static hydraulic stepless gear change device 9.SELECTED DRAWING: Figure 2COPYRIGHT: (C)2018,JPO&INPIT【課題】油圧ポンプやバルブブロック、及び戻り油路用の配管類を含めての装置の小型化を可能にする。【解決手段】静油圧式無段変速装置9と、単動シリンダ12と、油圧ポンプ13と、作動油の給排を制御する方向制御弁4を設けた制御弁ユニット3を備え、方向制御弁4は、シリンダポートCへの通油路を開く上昇位置Uと、タンクポートTへの通油路を開く下降位置Dと、中立位置Nと、に操作位置を選択可能な直動形電磁比例制御弁で構成され、単動シリンダ12は、シリンダポートCに連なる圧油給排路32が接続された第一油室12aと、ピストン12cを挟んで第一油室12aとは反対側に位置する第二油室12bを備え、第二油室12bに接続された作動油給排用の第二給排路16が、バルブブロック30内でタンクポートTに連なる通油路17に接続され、タンクポートTに連なるドレン油路33が静油圧式無段変速装置9のチャージ油路93に接続されている。【選択図】図2
