PROBLEM TO BE SOLVED: To automatically switch an "all-wheel drive system" and a "front wheel drive system" when mowing a lawn while descending a slope, in a riding-type lawn mower which is driven by a hydraulic motor individually installed at each wheel by using a single hydraulic pump.SOLUTION: In this lawn mower of a riding type for driving all the wheels hydraulically and driving each hydraulic motor MF, MB provided at each wheel WF, WB individually by one hydraulic pump A incorporated in a position for forming one driving loop circuit in a main pipe passage L0, a check valve V1, an automatic selector valve V2, and a relief valve V3 are incorporated in a hydraulic circuit C1 to switch the "all-wheel drive system" and the "front-wheel drive system" alternately by using a time immediately before the ground contact pressure of the rear wheel WB is reduced and the slip of the rear wheel WB is generated and a time when the slip of the rear wheel WB is not generated due to an increase of the ground contact pressure of the rear wheel WB as a reference.COPYRIGHT: (C)2008,JPO&INPIT【課題】1個の油圧ポンプにより全車輪に個別に設けられた各油圧モータを駆動する形式の乗用芝刈機において、傾斜地を下って芝刈作業を行う際に、「全車輪駆動方式」と「前輪駆動方式」との切り替えを自動的に行うことである。【解決手段】主管路L0 における1つの駆動ループ回路が形成可能な位置に組み込まれた一個の油圧ポンプAにより、各車輪WF,WBに設けられた各油圧モータMF,MBをそれぞれ個別に駆動する形式の全車輪油圧駆動乗用芝刈機において、後輪WBの接地圧が小さくなって後輪WBに滑りが発生する直前の時点、及び後輪WBの接地圧の増大により前記滑りの発生がなくなった時点を基準にして、「全車輪駆動方式」と「前輪駆動方式」とを相互に切替えられるように、油圧回路C1 内にチェックバルブV1 と自動切替バルブV2 とリリーフバルブV3 とを組み込む。【選択図】 図8-B
