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| Illustration 1 | g01164513 |
Boom hydraulic system (1) Boom cylinders (2) Boom dampening system (3) Load control valves (4) Accumulator (5) Lowering control valve (if equipped) (A) Line from the control valve for RAISE (B) Line from the control valve for LOWER (C) Return line to tank | |
The boom hydraulic system consists of the following main components:
- Boom cylinder (1)
- Boom dampening system (2)
- Load control valves (3)
- Accumulator (4)
- Lowering control valve (5)
The boom hydraulic system controls the following functions of the boom:
- RAISE
- LOWER
Oil is supplied to the boom hydraulic system by the piston pump. For more information, refer to Systems Operation, "Piston Pump (Implement and Steering)". The flow of oil to the boom hydraulic system is controlled by the control valve. The control valve is sometimes referred to as the bank valve. The spool in the boom section of the control valve is operated by solenoids. The solenoids are activated by electronic signals that are sent from the joystick to the ECM. Operating the joystick in the longitudinal direction will shift the spool in the boom section of the control valve.
Control valve (implement)
The following text describes the operation of the implement control valve. The operation of the implement control valve sections for the boom cylinder, the telescoping cylinder, and the tilt cylinder is the same operation.
Valve for the Cylinder in the HOLD Position
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| Illustration 2 | g01009187 |
(1) Pilot oil passage (2) Passage to cylinder head end (3) Passage to cylinder rod end (4) Electronic Pilot Valve (implement) (5) Tank return passage (6) Solenoid (7) Proportional pilot reducing valve (8) Chamber (9) Tank return passage (10) Load sensing signal passage (11) Spring (12) Passage (13) Control valve spool (14) Load check valve (15) Passage (16) Oil passage (17) Spring (18) Chamber (19) Pressure compensator (20) Pump supply passage (21) Proportional pilot reducing valve (22) Chamber (23) Solenoid (24) Screen (25) Pilot oil passage | |
Solenoids (6) or (23) and proportional pilot reducing valves (7) or (21) are part of Electronic Pilot Valve (implement) (4). When the joystick control is in the HOLD position, neither of the solenoids (6) or (23) on proportional pilot reducing valves (7) or (21) are energized. Pilot oil in pilot oil passage (26) is blocked at the proportional reducing valves. Therefore, no pilot oil is present on the downstream side of the proportional pilot reducing valves. If pilot oil is not acting on control valve spool (13), springs (11) and (17) will keep the control valve spool in the center HOLD position.
Supply oil from the hydraulic pump enters the hydraulic control valve and flows through pump supply passage (20). Pump supply passage (20) is a common passage through all the sections of the Control Valve (implement). When the joystick control is in the HOLD position, and the main control valve spool is centered, supply oil in passage (20) is blocked by control valve spool (13) .
Load check valve (14) is seated. The load check valve prevents implement drift until the supply pressure builds enough pressure in order to move the load. Load check valve (14) and control valve spool (13) blocks the flow of oil in passages (2) and (3) from the cylinder.
When the flow of oil from the cylinder in passage (2) and in passage (3) is blocked at control valve spool (13), the implements are held in the HOLD position. Control valve spool (13) also blocks the flow of oil to pressure compensating valve (19). When there is no oil pressure against the pressure compensating valve the valve remains in the down position.
Each implement section contains pressure compensating valve (19). All pressure compensating valves are interconnected. The pressure compensating valves allow the oil flow to be proportionally divided between each of the operating circuits when the flow demands of the system exceed the total flow that is available from the pump. All of the hydraulic actuators will continue to function. However, the actuators will operate at a reduced speed.
Also, the pressure compensating valves can determine the function that has the highest demand in all of the valve sections. Load sensing signal pressure reacts to the highest demand and flows through load sensing passage (10) to the pump controller. The pump controller adjusts the pump output in order to maintain the required demand. Load sensing passage (10) is a common passage through all the sections of the Control Valve (implement). The pump uses the load sensing signal in order to determine the amount of flow that is required.
Note: Load sensing signal pressure is sometimes referred to as signal oil. The two terms have the same meaning throughout this story.
The pressure compensating valves maintain cylinder speeds which are proportional to the position of the joystick control. The cylinder speed will not increase if the engine speed is increased in order to engage an additional system. These characteristics aid smooth machine operation when more than one system is in operation.
The oil in passage (12) will drain to the hydraulic tank through tank return passage (9). Passage (9) is a common passage through all the sections of the Control Valve (implement).
Valve for the Cylinder in the EXTEND Position
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| Illustration 3 | g01009197 |
(1) Pilot oil passage (2) Passage to cylinder head end (3) Passage to cylinder rod end (4) Electronic Pilot Valve (implement) (5) Return to tank passage (6) Solenoid (7) Proportional pilot reducing valve (8) Chamber (9) Return to tank passage (10) Load sensing signal passage (11) Spring (12) Passage (13) Control valve spool (14) Load check valve (15) Passage (16) Oil passage (17) Spring (18) Chamber (19) Pressure compensator (20) Pump supply passage (21) Proportional pilot reducing valve (22) Chamber (23) Solenoid (24) Screen (25) Pilot oil passage | |
When the valve ECM receives a signal from the machine ECM, the signal is sent to boom extend solenoid (6). The boom extend solenoid is part of Electronic Pilot Valve (implement) (4). The solenoid is energized and proportional pilot reducing valve (7) is shifted to the right.
This allows pilot oil from the pressure reducing valve to flow through the proportional pilot reducing valve (7). The pressure reducing valve is located in the outlet manifold of the Control Valve (implement). The pilot oil flows from the pressure reducing valve into Electronic Pilot Valve (implement) (4) through pilot oil passage (25). Pilot oil passage (25) is a common passage through the Electronic Pilot Valve .
The pilot oil flows through screen (24) into pilot oil passage (1). The pilot oil then flows into chamber (8) in order to move the spool in the control valve downward. The amount of current that is applied to solenoid (6) determines the amount of movement of the spool. As the proportional pilot reducing valve spool moves to the right the pressure in pilot chamber (8) increases. The spool moves downward as the pressure in pilot chamber (8) increases. The pressure in chamber (8) determines the movement of control valve spool (13) .
Because boom retract solenoid (23) does not receive a signal from the machine ECM, proportional pilot reducing valve (21) does not move. A spring causes proportional pilot reducing valve (21) to remain shifted to the left. When proportional pilot reducing valve (21) is in this position any pilot oil in chamber (22) flows through the proportional pilot reducing valve into the return to tank passage (5). The return to tank passage (5) is a common passage through the Electronic Pilot Valve .
Supply oil from the hydraulic pump flows into the valve section for the cylinder through pump supply passage (20). Supply passage (20) is a common passage through the Control Valve (implement).
The force of the pilot oil in chamber (8) overcomes the tension of spring (17). This action causes control valve spool (13) to move downward. This allows supply oil from the hydraulic pump to flow past control valve spool (13) into chamber (18). The pressure in chamber (18) causes the spool in pressure compensator (19) to move up.
A portion of the pump supply oil flows through pressure compensator (19) into oil passage (16). The oil flows through passage (16) into load sensing oil passage (10). Load sensing oil passage (10) is a common passage through the Control Valve (implement). The load sensing oil is routed to the compensator valve that is mounted on the hydraulic pump. This causes the pump to upstroke.
Note: Load sensing signal pressure is sometimes referred to as signal oil. The two terms have the same meaning throughout this story.
Reference: See Systems Operation/Testing and Adjusting, "Piston Pump (Implement and Steering)" for more information about the load sensing signal circuit.
The majority of the oil that flows through pressure compensator (19) flows into oil passage (15). The oil then flows to load check valve (14). The increasing pressure opens load check valve (14). This allows the oil to flow past open control valve spool (13). The oil then flows through passage (2) and into the head end of the cylinder. The oil pressure that is acting against the head end of the cylinder forces the cylinder rod to move out of the cylinder. As the rod moves out of the cylinder, the rod causes the boom to extend.
The oil that is displaced from the rod end of the cylinder flows out of the cylinder into passage (3). The oil flows through passage (3) past open control valve spool (13). The oil then flows into passage (12) and into the return to tank passage (9). The return to tank passage (9) is a common passage through the Control Valve (implement). The oil then returns to the hydraulic tank.
Valve for the Cylinder in the RETRACT Position
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| Illustration 4 | g01009220 |
(1) Pilot oil passage (2) Passage to cylinder head end (3) Passage to cylinder rod end (4) Electronic Pilot Valve (implement) (5) Return to tank passage (6) Solenoid (7) Proportional pilot reducing valve (8) Chamber (9) Return to tank passage (10) Load sensing signal passage (11) Spring (12) Passage (13) Control valve spool (14) Load check valve (15) Passage (16) Oil passage (17) Spring (18) Chamber (19) Pressure compensator (20) Pump supply passage (21) Proportional pilot reducing valve (22) Chamber (23) Solenoid (24) Screen (25) Pilot oil passage | |
When the valve ECM receives a signal from the machine ECM, the signal is sent to boom retract solenoid (23). The boom retract solenoid is part of Electronic Pilot Valve (implement) (4). The solenoid is energized and proportional pilot reducing valve (21) is shifted to the right.
This allows pilot oil from the pressure reducing valve to flow through the proportional pilot reducing valve (21). The pressure reducing valve is located in the outlet manifold of the Control Valve (implement). The pilot oil flows from the pressure reducing valve into Electronic Pilot Valve (implement) (4) through pilot oil passage (25). Pilot oil passage (25) is a common passage through the Electronic Pilot Valve .
The pilot oil flows through screen (24) into pilot oil passage (1). The pilot oil flows into chamber (22) in order to move the spool in the control valve upward. The amount of current that is applied to solenoid (23) determines the amount of movement of the spool. As the proportional pilot reducing valve spool moves to the right the pressure in pilot chamber (22) increases. The spool moves downward as the pressure in pilot chamber (22) increases. The pressure in chamber (22) determines the movement of control valve spool (13) .
Because boom extend solenoid (6) does not receive a signal from the machine ECM, proportional pilot reducing valve (7) does not move. A spring causes proportional pilot reducing valve (7) to remain shifted to the left. When proportional pilot reducing valve (7) is in this position any pilot oil in chamber (8) flows through the proportional pilot reducing valve into the return to tank passage (5). The return to tank passage (5) is a common passage through the Electronic Pilot Valve .
Supply oil from the hydraulic pump flows through pump supply passage (20) and into the valve section for the cylinder. Supply passage (20) is a common passage through the Control Valve (implement).
The force of the pilot oil in chamber (22) overcomes the tension of spring (11). This action causes control valve spool (13) to move upward. This allows supply oil from the hydraulic pump to flow past control valve spool (13) into chamber (18). The pressure in chamber (18) causes the spool in pressure compensator (19) to move up.
A portion of the pump supply oil flows through pressure compensator (19) into oil passage (16). The oil flows through passage (16) into load sensing oil passage (10). Load sensing signal passage (10) is a common passage through the Control Valve (implement). The load sensing oil is routed to the compensator valve that is mounted on the hydraulic pump. This causes the pump to upstroke.
Note: Load sensing signal pressure is sometimes referred to as signal oil. The two terms have the same meaning throughout this story.
Reference: See Systems Operation/Testing and Adjusting, "Piston Pump (Implement and Steering)" for more information about the load sensing signal system.
The majority of the oil that flows through pressure compensator (19) flows into oil passage (15). The oil then flows to load check valve (14). The increasing pressure opens load check valve (14). The oil then flows through passage (3) and into the rod end of the cylinder. The oil pressure that is acting against the rod end of the cylinder forces the cylinder rod to move into the cylinder. As the rod moves into the cylinder, the rod causes the boom to retract.
The oil that is displaced from the head end of the cylinder flows out of the cylinder into passage (2). The oil flows through passage (2) past open control valve spool (13). The oil then flows into passage (12) into the return to tank passage (9). The return to tank passage (9) is a common passage through the Control Valve (implement). The oil then returns to the hydraulic tank.
Load Control Valves
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| Illustration 5 | g01167245 |
Load control valve in the LOWER position (6) Check valve (7) Lock valve (A) Line from the control valve for RAISE (B) Line from the control valve for LOWER (D) Pilot line from the boom dampening system | |
When the engine is OFF, the load control valves prevent the boom from lowering. Oil in the head end of the boom cylinder is trapped by the check valve (6) and the lock valve (7) .
Boom Dampening System
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| Illustration 6 | g01167477 |
(2) Boom Dampening System (4) Accumulator (B) Line from the control valve for LOWER (D) Pilot line to the load control valve | |
The boom dampening system (2) allows the lock valves (7) on the two boom cylinders to open at the same time when the boom is lowered. This provides greater stability for the boom. The accumulator (4) dampens any fluctuations in the supply to the lock valves.
Lowering Control Valve (If Equipped)
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| Illustration 7 | g01164556 |
(5) Lowering control valve (7) Lock valves (A) Line from the control valve for RAISE (B) Line from the control valve for LOWER (C) Return line to tank | |
The lock valves (7) can be manually shifted and the boom control valve can be manually shifted in order to open the head end of the boom cylinders and the rod end of the boom cylinders to tank. The weight of the boom should cause the boom to lower. The lowering control valve (5) provides a means of lowering the boom when the boom will not lower under gravity. For more information, refer to Operation and Maintenance Manual, "Equipment Lowering with Loss of Hydraulic Power".
Boom RAISE
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| Illustration 8 | g01167507 |
(1) Boom cylinders (6) Check valves (A) Line from the control valve for RAISE (B) Line from the control valve for LOWER (C) Return line to tank | |
When the boom control valve is in the RAISE position, high pressure oil is directed to line (A). The high pressure oil flows through check valves (6) to the head end of the boom cylinders (1). Return oil in the rod end of the boom cylinders flows to line (B). Line (B) is open to the tank.
Boom LOWER
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| Illustration 9 | g01167508 |
(1) Boom cylinders (2) Boom dampening system (7) Lock valves (A) Line from the control valve for RAISE (B) Line from the control valve for LOWER (C) Return line to tank | |
When the boom control valve is in the LOWER position, high pressure oil is directed to line (B). The high pressure oil flows to the boom dampening system (2) and to the rod end of the boom cylinders (1). Pilot oil from the boom dampening system shifts the lock valves. Return oil flows from the head end of the boom cylinders through the lock valves (7) and to line (A). Line (A) is open to the tank.