797B Off-Highway Truck Fan Drive Hydraulic System: Piston Pump (Fan Drive)

Location of fan drive pump (1) Fan drive pump (2) High pressure cutoff valve (3) Signal hose for the reducing valve (4) Displacement solenoid (5) Pump for the steering system (6) Current adjustment screw (7) Stop screw (minimum angle)(8) Pressure and flow compensator valve

Fan drive pump (1) is part of a double piston pump group. The steering piston pump is the other part of the pump group. Both pumps are variable displacement pumps. A charging pump is located between the fan drive pump and the steering pump. The charging pump is used to keep both pumps supplied with oil. Fan drive pump (1) is located at the front bottom section of the pump drive. The pump drive is located on the right hand frame rail near the rear of the flywheel housing. The Brake/Cooling ECM controls the flow of oil from the fan drive pump by energizing displacement solenoid (4) .
The Brake/Cooling ECM will analyze the sensor inputs for the temperatures, the status of the brakes and the ground speed. The Brake/Cooling ECM will send 0 to 680 milliampere to the solenoid. At 0 milliampere, the pump is at the maximum displacement and the fan speed is at maximum speed. At 680 milliampere, the pump is at the minimum displacement and the fan speed is at minimum speed.
Displacement solenoid (4) moves a spool in pressure and flow compensator valve (8) in order to control the flow of pump output pressure to the actuator piston (minimum angle). The actuator piston (minimum angle) will move the swashplate to the position of minimum flow. Current adjustment screw (6) controls the minimum current that is required to start destroking the pump.
Note: Never adjust the current adjustment screw in chassis. This adjustment should be performed on a hydraulic test stand.
High pressure cutoff valve (2) controls the maximum pressure in the fan drive system. The high pressure cutoff valve (2) controls the system pressure by controlling the flow of pump output pressure to the actuator piston (minimum angle). When the system pressure is at the maximum, high pressure cutoff valve (2) will send oil to the actuator piston (minimum angle). The actuator piston (minimum angle) will move the swashplate to the position of minimum flow.
Stop screw (minimum angle) (7) is located near pressure and flow compensator valve (8). The stop screw (maximum angle) is located on the other side of the pump.
At zero pressure, the spring on the actuator piston will hold the pump at the maximum angle. A reducing valve provides a signal pressure through hose (3) and a shuttle valve to the fan drive pump (1) and the brake cooling drive pump. The fan drive pump (1) needs the signal pressure so that displacement solenoid (4) can position fan drive pump (1) at the minimum angle at start-up and during cold temperatures. Without having the signal pressure, the pump could not stay at the minimum angle in order to provide zero fan speed at start-up and during cold temperatures.

Sectional view of the fan drive pump (4) Displacement solenoid (7) Stop screw (minimum angle) (8) Pressure and flow compensator valve (9) Swashplate (10) Actuator piston (maximum angle) (11) Piston (12) Drive shaft (13) Stop screw (maximum angle) (14) Rotary group (15) Actuator piston (minimum angle)(16) Impeller (charge pump)

Oil flows from the pump through a makeup valve to the fan motor. Fan speed is controlled by controlling the flow from the pump to the fan motor.
Oil from the hydraulic tank enters the pump in the port below impeller (16). The charge pump keeps the fan drive pump full of oil.
The large spring around actuator piston (10) holds swashplate (9) at the maximum angle. Pump output pressure is always present on the right side of actuator piston (10). Pump output pressure also helps to hold swashplate (9) at the maximum angle. When swashplate (9) is at the maximum angle, the pump output is at the maximum flow. The fan speed is at maximum speed. This is the position of the pump when displacement solenoid (4) receives 0 milliampere from the Brake/Cooling ECM. When displacement solenoid (4) is receiving 0 to 680 milliampere from the Brake/Cooling ECM, displacement solenoid (4) will move a spool in pressure and flow compensator valve (8). The spool allows pump output pressure to flow to actuator piston (15) .
Actuator piston (15) has a larger diameter than actuator piston (10). Actuator piston (15) moves swashplate (9) toward the position of minimum flow. The swashplate angle, pump flow and fan speed will modulate with the amount of current at displacement solenoid (4). When swashplate (9) is at the minimum angle, the pump output is at the minimum flow and the fan speed is at minimum speed. This is the position of the pump when the displacement solenoid receives 680 milliampere from the Brake/Cooling ECM.
Before swashplate (9) contacts stop screw (7), actuator piston (15) will open a small drain port to the tank. This will stop the movement of swashplate (9). Draining the oil from the actuator piston will prevent swashplate (9) from contacting stop screw (7) repeatedly. Repeated contact between the swashplate and the stop screw can be noisy and the contact may cause damage to the pump.

(17) Oil filter (case drain)

Oil that leaks past the pistons into the pump housing provides lubrication for the rotating components. This oil leakage is referred to as case drain oil. Case drain oil flows through the case drain port and oil filter (17) to the hydraulic tank.

(1) Fan drive pump (2) High pressure cutoff valve (4) Displacement solenoid (6) Current adjustment screw (10) Actuator piston (maximum angle) (15) Actuator piston (minimum angle) (16) Impeller (charging pump) (18) Shuttle valve(19) Case drain

Charge pump (16) pulls oil from the hydraulic tank which keeps the fan drive pump full of oil. Oil flows from the pump to high pressure cutoff valve (2), displacement valve (4) and actuator piston (10) .
The pump output oil and the spring around actuator piston (10) holds swashplate (9) at the maximum angle. This is the position of the pump when displacement solenoid (4) receives 0 milliampere from the Brake/Cooling ECM and the pump output pressure is low.
When displacement solenoid (4) is receiving 0 to 680 milliampere from the Brake/Cooling ECM, displacement solenoid (4) moves the valve spool. The spool allows pump output pressure to flow to actuator piston (15). Actuator piston (15) has a larger diameter than actuator piston (10). Actuator piston (15) moves swashplate (9) toward the position of minimum flow.
Current adjustment screw (6) controls the spring pressure in displacement valve (4). The current adjustment screw (6) changes the minimum current that is required to start destroking the pump.
High pressure cutoff valve (2) controls the maximum pressure in the fan drive system. High pressure cutoff valve (2) controls the flow of pump output pressure to actuator piston (16). When system pressure is at the maximum, the high pressure cutoff valve sends oil to actuator piston (15). Actuator piston (15) moves swashplate (9) to the position of minimum flow.
The oil that leaks past pistons (11) into the pump housing provides lubrication for the rotating components. This oil leakage is referred to as case drain oil (19). Case drain oil (19) flows through the case drain port and a case drain oil filter into the hydraulic tank for the steering/fan drive.

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797B Off-Highway Truck Fan Drive Hydraulic System: Piston Pump (Fan Drive) 2013-01-25T19:40:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown