| 摘要 |
PURPOSE:To lengthen the lift of an oil-pressure shovel without the burden of excessive load on brake and reduction gear by a method in which a parking brake is operated only when a switch valve for operating traveling motor is operated and the operation of the brake is released during the other operations. CONSTITUTION:When only switch valves 4 and 8 for operating traveling motor are operated, auxiliary switch valves 13 and 14 switched to sides except for neutral and opened, and discharged oil from an oil-pressure pump 15 returns through left and right-handed circuits to a tank 42, where a parking brake 43 is kept in the locked state without acting on the connecting valve 45. In case where any one of switch valves 5, 9, and 10 for operating arm cylinder 21, bucket cylinder 22, and boom cylinder 23 or a switch valve 6 for operating a slewing motor 32 and switch valves 13 and 14 are concurrently operated, the pressure oil of the pump 15 flows through each path to a shuttle valve 50, and the valve 45 is switched to release the brake 43.
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