Daewoo 225-9 boom holding valve is an important hydraulic valve installed between the boom control valve and the bottom of the boom cylinder. Its main function is to prevent the boom from falling naturally when the boom control lever returns to the middle position, so that the boom can be kept in place.

     Working principle

       - When the boom is lifted: the boom lever performs lifting action, the boom lifting PPC pressure is output to the boom control valve, causing the valve core to move to the left, and the main pump pressure oil passes through the boom control valve core to the boom holding valve oil port V. The pressure oil pushes the poppet upward, and the main pump pressure oil flows into the bottom end of the boom cylinder through the boom holding valve oil port Cy, and the boom is lifted.

      - The boom is in the neutral position: the boom lever returns to the neutral position, the boom control valve spool also returns to the neutral position, the poppet returns to the neutral position under the action of the spring, and the pressure oil at the bottom end of the boom cylinder is closed by the poppet, the boom cylinder does not drop, and the boom remains in place.

     - When the boom is lowered: the boom joystick makes a lowering action, the boom lowers the PPC pressure to the boom control valve, the valve core moves to the right, the main pump pressure oil passes through the boom control valve valve core to the boom cylinder head, flows into the boom holding valve oil port Pi, pushes the pilot slide valve to move to the left, and the pressure oil from the bottom of the boom cylinder flows to the oil tank after throttling through the small hole, generating a pressure difference at both ends of the throttle port, the poppet opens, the pressure oil flows from the Cy port to the V port, the oil flows back to the oil tank through the boom control valve, and the boom is lowered. If abnormal pressure is generated in the circuit at the bottom end of the boom cylinder, the safety valve will work.

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