PURPOSE: To surely prevent reverse rotation of a hydraulic motor at the time of releasing braking by providing brake control valves switched according to a pressure difference of a counterbalance valve in the midway of a pipe line branched from a main pipe line.
CONSTITUTION: Brake control valves 21A, B are always put in the braking position, a pressure difference is produced in front and rear of a counterbalance valve 6 in the main pipe lines 4A, B, and when the oil pressure source 2 side becomes higher pressure than the hydraulic motor 1 side, one of brake control valves 21A, B is switched to the non-braking position. The oil pressure is fed from the high-pressure side main pipe lines 4A, B to branch pipe lines 11A, B and supplied through a high pressure select valve 12 and a brake pipe line 13 into a brake device 10 to release braking of the hydraulic motor 1. For example, when a hydraulic shovel is forced to ascend a slope from the stop state in the midway of the slope, if a car body is about to move downward, a pressure difference is produced in the main pipe lines 4A, B, and the brake control valves 21A, B are switched from the non-braking position to the braking position, so that the car body is kept from moving downward.
KIMURA KENICHI
INO KAZUYUKI
KOBAYASHI TAKESHI
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