Electric Lowering Control Pump (If Equipped)
When there is a loss of hydraulic power, the load control valve on the telescoping cylinder locks oil in the cylinder in order to prevent the boom from retracting. Refer to Systems Operation, Testing and Adjusting, "Load Control Valves (Lock)" for information on the operation of the load control valve. When this happens the boom can be retracted by manually opening the lock valve on the telescoping cylinder and by manually opening the implement control valve for the telescoping cylinder. The oil is then free to flow from the head end of the cylinder and the cylinder will retract under the weight of the load. Refer to Operation and Maintenance Manual for more information.
Note: The boom on the TH580B can not be retracted or lowered by gravity.
When the angle of the boom is below approximately 20 degrees, the deflection of the second stage of the boom may prevent the boom from fully retracting. In this situation, the boom can be retracted provided that the machine is equipped with an electric lowering control pump.
The electric motor and the pump are directly connected by hydraulic hoses to the telescoping cylinder and to the load control valve for the telescoping cylinder. The pump provides the following functionality:
- Allow a means of retracting the boom when the boom will not retract under gravity.
Retracting the Boom
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| Illustration 1 | g01194990 |
Hydraulic Schematic for retracting the boom | |
Press toggle Switch (13) toward the rear of the machine. The motor (14) will activate. The motor turns pump (15) . The oil passes from pump (15) through line (11) past check valve (10A) . Since the solenoid for sequence valve (9) is not energized, the oil from the pump continues through the sequence valve and past check valve (6) . The pump pressure opens lock valve (1) . The increased oil pressure enters the rod end of the telescopic cylinder (2) . The boom begins to retract. The oil in the head end of the telescopic cylinder is then forced backward through lock valve (1) . The oil continues through line (19) to the telescopic control valve. The oil is then blocked at the control. Pilot oil pressure from line (11) to check valve (10B) holds the check valve open. Oil from line (17) passes through check valve (10B) and the oil returns to tank (18) . Continue to retract the boom until the boom is fully retracted.
Lowering the Boom
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| Illustration 2 | g01195008 |
Hydraulic Schematic for lowering the boom | |
Lowering the boom is similar to retracting the boom.
Press toggle Switch (13) toward the front of the machine. The motor (14) will activate. Simultaneously, the solenoid for sequence valve (9) energizes and the valve spool shifts to the right. This action allows the hydraulic oil to be directed to the boom cylinder. The motor turns pump (15) . The oil passes from pump (15) through line (11) past check valve (10A) . The oil from the pump continues through the sequence valve. The pump pressure opens lock valve (7) . The increased oil pressure enters the rod end of the boom cylinder (4) . The oil in the head end of the boom cylinder is forced backward through lock valve (7) . The boom begins to lower. The oil continues through line (22) to the boom cylinder control valve. The oil is then blocked at the control valve. Pilot oil pressure from line (11) to check valve (10B) holds the check valve open. Oil from line (21) passes through check valve (10C) . Then, the oil flows through line (17) past check valve (10B) and returns to tank (18) . Continue to lower the boom until the boom is fully lowered.