Patent ID: 7571786

Claim:
A wheeled working machine comprising: a lower traveling body having front wheels and rear wheels; an upper rotating body rotatably mounted on the lower traveling body; a cab arranged on one of the left and right sides of the upper rotating body for serving as an operating room during working as well as a driving room during traveling; and a steering apparatus, the steering apparatus comprising: mechanical steering means for steering the front wheels in accordance with the operation of a steering handle arranged within the cab by connecting the steering handle to a link member for changing the direction of the front wheels so as to transmit a steering force; full-hydraulic steering means, having steering cylinders for respectively changing the directions of the front wheels and the rear wheels, for steering at least one of the front wheels and the rear wheels by introducing hydraulic oil to predetermined steering cylinders in accordance with the operation of the steering handle and a selected mode relating to the steering of the front wheels and the rear wheels; and switching means for switching one of the mechanical steering means and the full-hydraulic steering means so as to be selectively actuated, wherein the mechanical steering means is constructed to assist the steering of the front wheels by introducing hydraulic oil to the steering cylinders for the front wheels of the full-hydraulic steering means in accordance with the rotational operation of the steering handle, and wherein the mechanical steering means includes a supply line for introducing hydraulic oil to the steering cylinders for the front wheels from a hydraulic source, a control valve arranged on the supply line for changing a communication state of the supply line in accordance with the rotational operation of the steering handle, and a double pilot check valve provided on the downstream side of the control valve for preventing pressurized oil from being returned to a tank circuit of the mechanical steering means when the full- hydraulic steering means is selected.