349F and 352F Excavators Hydraulic System Caterpillar

Illustration 1	g03409021
Pressure circuit for SWING RIGHT (partial schematic) (1) Passage (supply oil) (2) Makeup port (3) Check valve (4) Relief valve (5) Passage (6) Right swing motor (7) Motor rotary group (8) Passage (return oil) (9) Left swing motor (10) Check valve (11) Relief valve (12) Passage (13) Makeup line (14) Port (supply oil) (15) Port (return oil) (16) Check valve (17) Return passage (18) Swing control valve (19) Main control valve (20) Return line (21) Slow return check valve

When the swing joystick is moved to the NEUTRAL position during the swing right operation, swing control valve (18) shifts to the NEUTRAL position. Since the swing control valve is in the NEUTRAL position. The oil delivery through port (14) to motor rotary group (7) is now blocked at the swing control valve. The return oil from the motor rotary group through port (15) is also blocked at the swing control valve. The upper structure will attempt to continue to rotate after the swing joystick is returned to the NEUTRAL position. This action causes an internal leak of oil in the swing motor. As a result, a vacuum condition occurs at passage (1). In order to prevent this vacuum condition, makeup oil is delivered from the return hydraulic system to the swing motor.

Illustration 2	g02155891
(13) Makeup line (21) Slow return check valve

Illustration 3	g02155969
Side view of slow return check valve (17) Return passage (20) Return line (21) Slow return check valve

When all of the control valves for implements, swing and travel are in the NEUTRAL position, the oil delivery from the idler pump and the drive pump flows through return passage (17) to the hydraulic tank. Slow return check valve (21) is located between return passage (17) and the hydraulic tank. Slow return check valve (21) maintains the return oil pressure at 290 kPa (42 psi) in return passage (17). If a vacuum condition occurs at the swing motor during the stop of a swing operation, the slow return check valve causes return oil from return passage (17) to flow to motor rotary group (7) as makeup oil. The slow return check valve eliminates the vacuum condition in the swing motor due to internal leakage.

If the swing joystick is moved suddenly toward the NEUTRAL position from the FULL STROKE position, the swing control valve partially closes. Until the swing control valve reaches the NEUTRAL position, the return oil from the swing motor continues to flow through passage (8) and port (15) to return passage (17). The return oil pressure in passage (8) increases. The return oil pressure in passage (8) remains lower than the pressure setting of swing relief valve (11). Swing relief valve (11) remains in the CLOSED position.

A vacuum condition occurs at port (14) and passage (1) due to the insufficient oil delivery from the drive pump and due to the tendency of the motor rotary group to continue to rotate. Since relief valve (11) remains in the CLOSED position, makeup oil does not flow through relief valve (11), passage (12) and check valve (3) to passage (1) to motor rotary group (7).

Makeup oil is supplied to motor rotary group (7) from return passage (17). Return oil flows from return passage (17) through makeup line (13), port (2), passage (12), check valve (3), and passage (1) to motor rotary group (7). The vacuum condition in passage (1) is eliminated by the makeup oil from the return hydraulic system.

During a left swing operation, the return ports and the supply ports of the swing motor are reversed. Makeup oil flows through check valve (10) if a vacuum condition occurs in passage (8) during a swing left operation.