The boom lowering control valves are located on the head end of the boom cylinders.
The boom lowering control valve has two functions:
- The boom lowering control valve prevents the boom from falling if a hydraulic supply line fails or if a supply tube to a boom cylinder fails.
- The boom lowering control valve allows the operator to manually lower the boom when the engine is shut down.
ReferenceFor information on lowering the boom manually, see Operation and Maintenance Manual, "Equipment Lowering with Engine Stopped".
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| Illustration 1 | g02708420 |
Partial schematic (1) Boom cylinders (2) Boom lowering control valves (3) Main control valve (4) Manual boom lower (5) Pilot control valve | |
BOOM RAISE Operation
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| Illustration 2 | g02157425 |
Boom lowering control valve (1) Spool (2) Orifice (3) Passage (4) Port (5) Port (pilot) (6) Port (boom cylinder head end) (7) Passage (8) Check valve (9) Spring (10) Port (11) Line relief valve (12) Tank port (13) Drain port | |
When the operator moves the joystick to the BOOM RAISE position, oil from the main control valve flows through port (4) in the boom lowering control valve. As the oil pressure increases, check valve (8) shifts against the force of spring (9). This action allows oil to flow through port (10), passage (7), and port (6) to the head end of the boom cylinders. The rod is extended and the boom raises.
BOOM LOWER Operation
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| Illustration 3 | g02157427 |
Boom lowering control valve (1) Spool (2) Orifice (3) Passage (4) Port (5) Port (pilot) (6) Port (boom cylinder head end) (7) Passage (8) Check valve (9) Spring (10) Port (11) Line relief valve (12) Tank port (13) Drain port | |
When the operator moves the joystick to the BOOM LOWER position, pilot oil flows from the pilot control valve to port (5) of the boom lowering control valve. The pilot pressure moves spool (1) to the left. Orifice (2) opens allowing oil to flow to passage (3) .
As the joystick is moved further to the BOOM LOWER position, the pilot pressure in port (5) increases. Spool (1) shifts further to the left which allows more oil from passage (7) to flow to passage (3). Oil from the head end of the boom cylinder now flows through port (6), to port (4), and then to the main control valve. The boom cylinder rod retracts and the boom lowers.
Manual Boom Lower
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| Illustration 4 | g02076115 |
Emergency valve group (14) Port (15) Seat (16) Locknut (17) Valve (18) Ball (19) Port | |
As seat (15) is turned counterclockwise, ball (18) will shift. As ball (18) is shifted, oil is allowed to flow through port (14) and valve (17) to port (19). Oil from the head end of the boom cylinder flows through port (19) to the hydraulic tank. This action allows the boom to lower.
ReferenceFor information on the location of the manual lower valve, see Operation and Maintenance Manual, "Equipment Lowering with Engine Stopped".