The boom lowering control valve has two functions:
- A check valve prevents the boom from falling if a hydraulic supply line or a supply tube to the boom cylinder fails.
- A manual valve allows the operator to manually lower the boom when the engine is shut down.
Boom Raise Operation
 | |
| Illustration 1 | g02164998 |
Partial Hydraulic Schematic (1) Boom cylinder (2) Boom lowering control valve (3) Supply line to rod end of boom cylinder (4) Line (5) Proportional reducing valve (6) Pilot supply line (7) Manual boom lowering valve (8) Drain line (9) Supply line (pilot oil) (10) Supply line to head end of boom cylinder (11) Boom control valve (12) Drain line (13) Passage (pump delivery flow) (14) Passage (pump delivery flow) (15) Pilot pump (16) Rear pump (17) Front pump (18) Passage (oil flow to boom lowering control valve) (19) Return passage (20) Return line (21) Hydraulic tank | |
 | |
| Illustration 2 | g02165653 |
Boom lowering control valve (6) Pilot supply line (10) Supply line to head end of boom cylinder (12) Drain line (22) Passage (pilot oil) (23) Check valve (24) Supply line to head end of boom cylinder (25) Spool valve (26) Drain port (27) Drain passage (28) Relief valve (head end) | |
 | |
| Illustration 3 | g02166513 |
Boom Cylinders (1) Boom cylinder (2) Boom lowering control valve | |
When the boom joystick is operated, an electrical signal from the joystick is sent to the machine ECM. The machine ECM energizes the proportional solenoid valve for boom cylinder head end.
When the proportional solenoid valve for the boom cylinder head end is energized, the spool in boom control valve (11) shifts UP. The oil delivery from the main pumps flows from passage (13) through control valve (11), passage (14) and passage (18). The oil delivery from the main pumps in passage (18) now flows through line (10) to boom lowering control valves (2). The oil flows from boom lowering control valves (2) through check valve (23) to the head end of boom cylinders (1) .
The oil from the rod end of boom cylinders (1) flows through line (3), boom control valve (11), and passage (19) to return line (20). The return oil now flows back to hydraulic tank (21) .
If a line breaks, the boom will not fall because check valve (23) prevents the oil in the head end of the boom cylinders from flowing out of the boom cylinders.
Boom Lower Operation
 | |
| Illustration 4 | g02167135 |
Boom lowering control valve (6) Pilot supply line (10) Supply line to head end of boom cylinder (12) Drain line (22) Passage (pilot oil) (23) Check valve (24) Supply line to head end of boom cylinder (25) Spool valve (26) Drain port (27) Drain passage (28) Relief valve (head end) | |
When the boom joystick is operated in order to perform a BOOM LOWER operation, an electrical signal is sent from the boom joystick to the machine ECM. The machine ECM energizes the proportional solenoid valve for the boom cylinder rod end. The machine ECM also energizes proportional reducing valve (5) for boom lowering control valves (2) .
The oil in the head end of the boom cylinders (1) flows into boom lowering control valves (2). Pilot oil flows from solenoid valve (5) through line (6). The pilot oil shifts valve (25). Return oil from the head end of the boom cylinders now flows through valve (25) .
The oil from the head end of the boom cylinders flows through spool valve (25) and through line (10) to boom control valve (11). The return oil then flows through passage (19) to return line (20) and returns to hydraulic tank (21) .
Note: Solenoid valve (5) of the boom lowering control valves receives an electrical signal from the machine ECM for a BOOM LOWER operation only.
Manual Boom Lowering Operation
 | |
| Illustration 5 | g02166841 |
Emergency valve group (29) Port (30) Seat (31) Locknut (32) Valve (33) Ball (34) Port | |
In case the boom cannot be lowered because of engine problems or hydraulic system problems, the boom can be manually lowered by opening manual boom lowering valve (7) gradually.
As seat (30) is turned counterclockwise, ball (33) will shift. As ball (33) is shifted, oil is allowed to flow through port (29) and valve (32) to port (34). Oil from the head end of the boom cylinder flows through port (34) to the hydraulic tank. This function 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".