349E Excavator Hydraulic System Return Hydraulic System Caterpillar


Return Hydraulic System
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Illustration 1g02716204

(1) Travel motors

(2) Swing motors

(3) Hydraulic oil cooler

(4) Bypass check valve

(5) Slow return check valve

(6) Case drain line

(7) Main control valve

(8) Case drain line

(9) Makeup line (makeup oil to swing motor)

(10) Return Passage

(11) Negative flow control orifice

(12) Return passage

(13) Return Passage

(14) Center bypass passage

(15) Center bypass passage

(16) Return Passage

(17) Negative flow control orifice

(18) Return line

(19) Return line

(20) Case drain filter (swing motors and travel motors)

(21) Hydraulic tank

(22) Return filter

(23) Case drain filter (Pump)

(24) Case drain line

(25) Pilot pump

(26) Drive pump

(27) Idler pump

(28) Suction line

The oil delivery from drive pump (26) and idler pump (27) enters main control valve (7). The oil then flows through the return system in one of the following manners:

  • When all joysticks and/or travel levers/pedals are in the NEUTRAL position, oil flow at standby pressure from drive pump (26) flows through center bypass passage (15) and negative flow control orifice (17) to return passages (12) and (10). Oil flow at standby pressure from idler pump (27) flows through center bypass passage (14) and negative flow control orifice (11) to return passages (12) and (10) .

  • When any one of the joysticks and/or travel levers/pedals are shifted from the NEUTRAL position, center bypass passages (14) and (15) are blocked. The return oil from the cylinders and/or motors now flows through return passage (12) and return passage (13) to passage (10) .

The return oil flows from return passage (10) through slow return check valve (5). When the oil temperature is cold, most of the oil is returned through bypass check valve (4) to hydraulic tank (21). The remainder of the oil flows through oil cooler (3), return line (19), and return filters (22). Return filters (22) are located within hydraulic tank (21) .

When the oil temperature increases, the rate of oil flow through bypass check valve (4) decreases. This decrease in oil flow will cause the rate of oil flow through oil cooler (3) to increase.

ReferenceFor more information concerning the bypass check valve, refer to Systems Operation, "Bypass Valve (Return) (Bypass Check Valve)".

Case drain oil from pilot pump (25), drive pump (26), and idler pump (27), flows into case drain line (24). The case drain oil from the pumps flow through case drain filter (23) to hydraulic tank (21) .

Case drain oil from travel motors (1) and swing motors (2) flows into case drain lines (6) and (8). The case drain oil from the motors will combine and flow through the case drain line (18), through case drain filter (20), and to hydraulic tank (21) .

Makeup line (9) will route return oil to the inlet port of the swing motor if a vacuum condition occurs at the swing motor during swing stop.

For more information concerning the makeup operation of the return hydraulic system at the swing motor, refer to the following sections in this manual.

  • Systems Operation, "Check Valve (Return Makeup)"

  • Systems Operation, "Oil Makeup (Swing System)"

  • Systems Operation, "Relief Valve (Swing)"

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