374D Excavator Hydraulic System Return Hydraulic System Caterpillar

Illustration 1	g02161418
Return system at NO-LOAD condition (1) Left travel motor (2) Line (case drain) (3) Right travel motor (4) Line (case drain) (5) Makeup line (6) Left swing motor (7) Return check valve (8) Swing control valve (9) Right swing motor (10) Line (11) Solenoid valve (neutral bypass) (12) Neutral bypass valve (13) Differential pressure relief valve (14) Passage (pump output flow) (15) Slow return check valve (16) Bypass tube (17) Passage (return oil flow from control valve spools) (18) Main control valve (19) Passage (return oil flow from control valve spools) (20) Passage (pump output flow) (21) Differential pressure relief valve (22) Bypass check valve (23) Passage (return oil from neutral bypass valve) (24) Return line (25) Return filter (26) Return line (27) Return filter (28) Case drain filter (29) Return line (30) Hydraulic oil cooler (31) Fan motor (32) Case drain filter (33) Hydraulic tank (34) Suction line (35) Pilot pump (36) Return line (37) Fan pump (38) Swing pump (39) Line (case drain) (40) Front pump (41) Rear pump (42) Line (case drain)

Illustration 2	g02161921
Return oil system components (oil flow for implement or travel) (15) Slow return check valve (16) Bypass tube (17) Passage (return oil flow from control valve spools) (19) Passage (return oil flow from control valve spools) (22) Bypass check valve (25) Return filter (26) Return line (27) Return filter (33) Hydraulic tank (34) Suction line

Return Oil Flow

At NO-LOAD condition, the oil delivery from the front pump (40), rear pump (41), swing pump (38), and fan pump (37) returns to the hydraulic tank in the following manner.

• The oil delivery from the front pump and the rear pump flows into passages (14) and (20) of main control valve (18). With no input to the machine ECM for implement movement from the joysticks, solenoid valve (11) for the neutral bypass is energized to shift neutral bypass valve (12). Oil then flows through neutral bypass valve (12) and passage (23). The return oil can then return to tank with the following two oil paths. Part of the return oil flows into bypass tube (16) to slow return check valve (15) and oil cooler (25). The return oil then flows through return line (26), return filters (25) and (27), and into hydraulic tank (33). The remainder of the return oil flows through bypass check valve (22), return line (24), return filters (25) and (27), and into hydraulic tank (33) .

• The oil delivery from swing pump (38) flows through swing control valve (8), through return check valve (7) and into return line (10). Part of the return oil flows into bypass tube (16) to slow return check valve (15) and oil cooler (25). The return oil then flows through return line (26), return filters (25) and (27), and into hydraulic tank (33). The remainder of the return oil flows through bypass check valve (22), return line (24), return filters (25) and (27), and into hydraulic tank (33)

At LOAD condition, return oil from the cylinders and motors returns to the hydraulic tank in the following manner.

• Return oil from the travel motors and the implement cylinders enters main control valve (18). Return oil from the travel control valve and the implement control valves in the main control valve flows into passages (17) and (19). The return oil can then return to tank with the following two oil paths. Part of the return oil flows into bypass tube (16) to slow return check valve (15) and oil cooler (25). The return oil then flows through return line (26), return filters (25) and (27), and into hydraulic tank (33). The remainder of the return oil flows through bypass check valve (22), return line (24), return filters (25) and (27), and into hydraulic tank (33) .

• Return oil from the swing motors flows through swing control valve (8) and return check valve (7). The return oil can then return to tank with the following two oil paths. Part of the return oil flows into bypass tube (16) to slow return check valve (15) and oil cooler (25). The return oil then flows through return line (26), return filters (25) and (27), and into hydraulic tank (33). The remainder of the return oil can flow through bypass check valve (22), return line (24), return filters (25) and (27), and into hydraulic tank (33) .

Regardless of the presence of load, when the return oil is at a low temperature, most of the return oil flows through bypass check valve (22) into hydraulic tank (33). As the oil temperature increases, the amount of return oil flow through bypass check valve (22) decreases. The amount of return oil that flows from slow return check valve (15) to hydraulic oil cooler (30) increases.

At UNLOAD condition, the oil delivery from front pump (40) and rear pump (41) returns to the hydraulic tank in the following manner.

• When the load is released, the oil delivery from the front pump and the rear pump flows into passage (14) and into passage (20) rapidly. When the oil pressure from the front pump and the rear pump is 2940 kPa (430 psi) higher than the true load signal pressure of the motors and implements, pump delivery pressure acts on differential pressure relief valve (13) and differential pressure relief valve (21). The differential pressure relief valves open. The oil delivery from the front pump and the rear pump flows into passage (17) and passage (19) into the hydraulic tank.

The case drain oil for the pumps and the motors flows in the following manner.

• Case drain oil from front pump (40), rear pump (41) and swing pump (38) flows through case drain lines (42) and (39) to case drain filter (32). The oil then flows to hydraulic tank (33) .

• The case drain oil from travel motors (1) and (3), swing motors (6) and (9), fan motor (31), and fan pump (37) flows into case drain filter (28). The oil then flows to hydraulic tank (33) .

Return check valve (7) restricts the return oil from the swing motors in order to prevent a vacuum condition in the motors. The pressure in return lines (10) and (16) increases due to bypass check valve (22) and slow return check valve (15). If a vacuum condition occurs in the swing motors, the oil in return lines (10) and (16) is delivered to the motors through makeup line (5) in order to release the vacuum.

The hydraulic oil cooling system and the engine cooling system for this machine type are different from the systems for other machine types. The cooling fan is not driven through the engine. The cooling fan is driven by an exclusively equipped fan motor (31). The oil delivery of fan pump (37) flows to the fan motor (31). The cooling fan blades are driven by the fan motor in order to cool hydraulic oil cooler (30), the radiator, and the condenser for the air conditioner. The rotational speed of fan motor (31) is limited by the flow of fan pump (37) .