349D2 MHPU Mobile Hydraulic Power Unit Hydraulic System Oil Makeup (Swing System) Caterpillar

Illustration 1	g06016605
Pressure circuit for SWING RIGHT (partial schematic) (1) Passage (return oil) (2) Swing motor (3) Passage (4) Check valve (5) Motor rotary group (6) Passage (7) Check valve (8) Passage (supply oil) (9) Relief valve (10) Makeup port (11) Relief valve (12) Makeup line (13) Port (return oil) (14) Port (supply oil) (15) Main control valve (16) Return line (17) Check valve (18) Slow return check valve (19) Return line

When the swing joystick is moved to the NEUTRAL position during the swing right operation, the swing control valve 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 (5) is blocked at the swing control valve. The return oil from the motor rotary group through port (13) 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 causes an internal leak of oil in the swing motor. As a result, a vacuum condition occurs at passage (8). To prevent this vacuum condition, makeup oil is delivered from the return hydraulic system to the swing motor.

Illustration 2	g06016656
(12) Makeup line (18) Slow return check valve (19) Return line

Illustration 3	g06016662
Slow return check valve (12) Makeup line (16) Return line from main control valve (18) Slow return check valve (19) Return line

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 line (16) to the hydraulic tank. Slow return check valve (18) is located between return line (16) and the hydraulic tank. Slow return check valve (18) maintains the return oil pressure at 290 kPa (42 psi) in return line (16). 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 line (16) to flow to motor rotary group (5) 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 (1) and port (13) to return line (16). The return oil pressure in passage (1) increases but the return oil pressure in passage (1) 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 (8) 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 (3) and check valve (7) to passage (8) to motor rotary group (5).

Makeup oil is supplied to motor rotary group (5) from return line (16). Return oil flows from return line (16) through makeup line (12), port (10), passage (3), check valve (7) and passage (8) to motor rotary group (5). The vacuum condition in passage (8) 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 (4) if a vacuum condition occurs in passage (1) during a swing left operation.