Wednesday, December 5, 2012

770G and 772G Off-Highway Truck; Hydraulic Fan Drive System

Fan drive hydraulic system
(1) Hydraulic tank
(2) Case drain oil filter
(3) Fan drive pump
(4) Speed sensor
(5) Fan drive motor
(6) Cooler bypass valve(7) Oil filter fan

Left-hand side view of fan drive hydraulic system
(1) Hydraulic tank
(2) Case drain oil filter
(3) Fan drive pump
(4) Speed sensor
(5) Fan drive motor(7) Fan oil filter

Front view of cooling arrangement(6) Cooler bypass valve

Oil flows from the fan drive pump (3) to the fan drive motor (5). Oil flows from the fan drive motor (5) through the cooler bypass valve (6). The oil flows from the cooler bypass valve (6) through the fan oil filter (7) and the oil cooler to the hydraulic tank (1) .

The fan drive motor (5) is a fixed displacement motor. The fan speed is determined by the amount of flow from the fan drive pump (3). The fan drive pump (3) is a variable displacement piston pump that is controlled by the Brake/Cooling ECM.

Case drain oil flows from the fan drive motor (5) and the fan drive pump (3) through a case drain oil filter (2) to the hydraulic tank (1) .

If the supply oil to the fan stops suddenly, the fan and the motor may continue to rotate because of the mass of the fan. A valve assembly allows the oil in the circuit to flow from the return side to the supply side preventing a vacuum in the lines on the supply side.

Fan speed is controlled by the Brake/Cooling ECM. Fan speed will vary depending on many inputs.

Table 1
Standard Fan Speed Control Limits (RPM) 43" Fan
Engine Speed	Fan Speed Limit
1250 or Less	357
1300	378
1450	414
1500	429
1600	457
1700	486
1750	500
1800	500
1841	500
1900	500
1910	500
1950	500