| 摘要 |
PURPOSE:To improve the efficiency of a concrete pump by changing a set pressure in accordance with the gear position of a transmission unit so that the discharge oil volume of a hydraulic pump is changed. CONSTITUTION:A switch 29 is turned on when a transmission unit is set in a third gear position, meanwhile a switch 30 is turned on when the transmission unit is set in a fourth gear position. A solenoid 1 is energized and as well as a solenoid 2 or a solenoid 3 is energized by a diode 37 through a normal/reverse switch 36, when the switch 29 is turned on. However, the solenoid 1 is not energized when the switch 30 is turned on. A set pressure changing valve 25 is actuated when the solenoid 1 is energized so that a circuit for a low-pressure relief valve 27 is shut-off for operating a high-pressure relief valve 24 alone. With this arrangement, a concrete pump/motor 10 is operated under high pressure for pumping concrete to a high level position, etc. On the contrary, hydraulic pressure is set at a low value when the transmission unit is set in the fourth gear position so that the pump is operated at a high speed, thereby the operational efficiency of the pump is improved. |
