352F Mobile Hydraulic Power Unit (MHPU) Machine System Main Hydraulic System Caterpillar

Main Hydraulic Schematic

Illustration 1	g06066194
(7) Left swing motor (8) Right swing motor (12) Pilot control valve (travel) (13) Oil cooler (14) Bypass check valve (15) Slow return check valve (16) Stick drift reduction valve (17) Main control valve (18) Boom drift reduction valve (19) Pilot control valve (swing and stick) (20) Pilot control valve (boom and bucket) (21) Main relief valve (22) Pressure sensor (idler pump) (23) Pressure sensor (drive pump) (24) Pressure sensor (boom down control) (25) Implement pressure switch (26) Pressure sensor (swing control) (27) Reducing valve (boom priority mode) (28) Travel speed solenoid valve (29) Solenoid valve (swing parking brake) (30) Valve (31) Solenoid valve (hydraulic activation) (32) Accumulator (33) Pilot oil manifold (34) PRV for boom regeneration valve (35) Hydraulic tank (36) Pilot relief valve (37) Pilot filter (38) Drive pump (39) Pilot pump (40) Case drain filter (swing motors and travel motors) (41) Return filters (42) Case drain filter (pump) (43) Idler pump (44) Idler pump regulator (45) Proportional reducing valve (power shift pressure) (46) Drive pump regulator (47) Negative flow control valve

Hydraulic Schematic with Heavy Lift

Illustration 2	g03408590
Heavy lift (partial schematic) (17) Main control valve (21) Main relief valve (33) Pilot oil manifold (34) PRV for boom regeneration valve (36) Pilot relief valve (39) Pilot pump (48) Heavy lift solenoid valve

Hydraulic Schematic with Fine Swing

Illustration 3	g03408633
(7) Left swing motor (8) Right swing motor (17) Main control valve (35) Hydraulic tank (43) Idler pump (49) Fine swing solenoid valve

Hydraulic Pump Flow and Pressure Control System

Illustration 4	g02132554
Pump compartment (38) Drive pump (43) Idler pump (45) Proportional reducing valve (power shift pressure) (50) Delivery line (drive pump) (51) Delivery line (idler pump)

This machine is driven and controlled by the following systems.

• The main hydraulic system controls the cylinders, the travel motors, and the swing motors.

• The pilot hydraulic system supplies oil to the main pumps, the main control valve, the swing brakes, and the travel motors.

• The electronic control system controls the outputs of the engine and pump.

The main hydraulic system delivers oil flow from idler pump (43) and drive pump (38) to control the following components: bucket cylinder, stick cylinder, boom cylinders, right travel motor, left travel motor, left swing motor (7) and right swing motor (8).

Illustration 5	g02139255
Main control valve (21) Main relief valve (52) Right control valve body (53) Left control valve body

Idler pump (43) and drive pump (38) are axial piston pumps. The performance of both pumps is equal.

Drive pump (38) is directly connected to the engine by a flexible coupling. The drive pump delivers oil to the right control valve body (52) of the main control valve. Idler pump (43) is mechanically connected to the drive pump through gears. The idler pump delivers oil to the left control valve body (53) of the main control valve. Pilot pump (39) is a gear pump that supplies oil to the pilot hydraulic system. Pilot pump (39) is coupled directly to idler pump (43). All engine output is used to drive these three pumps.

As the load pressure increases during working conditions, the main pumps increase the delivery pressure and the pumps decrease the flow rate. The hydraulic horsepower remains constant even though the delivery pressure and the flow rates change. The hydraulic horsepower is approximately identical to the engine horsepower.

When no work is being performed, pump oil flows through main control valve (17) and into hydraulic tank (35). The main control valve sends a negative flow control signal to each main pump regulator to destroke the pump to the minimum output flow.

If an operation is being performed, main control valve (17) directs pump oil to the respective cylinders (boom, bucket, and stick) and/or motors (swing and travel). Main control valve (17) contains numerous valve stems, passages, check valves, and orifices to carry out a single operation or a combined operation. The working pressure of the main hydraulic system is regulated by main relief valve (21).

Illustration 6	g02122261
Cab (54) Joystick (stick and swing) (55) Joystick (boom and bucket) (56) Monitor panel (57) Left travel lever/pedal (58) Right travel lever/pedal (59) Engine speed dial

The pilot hydraulic system receives oil flow from pilot pump (39). The pilot hydraulic system controls the following functions.

1. The pilot hydraulic system controls the operation of the implement control valves.

   Pilot oil flows from pilot pump (39) through pilot oil manifold (33). The pilot oil then flows to the pilot control valves for machine operation (implement operations, swing operations, and travel operation). These pilot control valves are activated by the joysticks and the travel levers/pedals.

   When joystick (54), joystick (55), left travel lever/pedal (57) and/or right travel lever/pedal (58) are moved from the NEUTRAL position, the pilot oil flows through the pilot control valves to the corresponding spools at the main control valve (17).

   The pilot pressure oil at that end of the valve spool forces the valve spool to shift. The pilot oil on the other end of the valve spool drains to the hydraulic tank. When the valve spool shifts, oil is then delivered from idler pump (43) or drive pump (38) to the cylinders and motors.

   Thus, pilot oil drives each system of the main control valve.

2. The pilot hydraulic system controls the output flow of the main pumps.

   During machine operation, pilot pressure is sent to the main pump regulators as a signal pressure. This signal pressure is called power shift pressure. The electronic control system receives input signals from various components on the machine. The machine ECM processes the input signals. The electronic control system then sends an electrical signal to proportional reducing valve (45) at the idler pump regulator to regulate the power shift pressure. The power shift pressure controls the output flow of idler pump (43) and drive pump (38). Power shift pressure adjusts the output flow of the main pumps in accordance with the engine speed. For more information concerning power shift pressure, refer to Systems Operation, "Pilot Hydraulic System".

3. The pilot hydraulic system generates signal pressure to perform the following operations.

   1. Pilot signal pressure activates the Automatic Engine Speed Control (AEC) system. This causes functions to automatically reduce the engine speed when no hydraulic operation is called for.

   2. Pilot signal pressure releases the swing parking brake.

   3. Pilot signal pressure will automatically change the travel speed to either HIGH or LOW in accordance with the hydraulic system load.

   4. Pilot signal pressure operates the straight travel control valve. This maintains straight travel during the operation of an implement.

   5. Pilot signal pressure controls the operation of the valves that can be used during a loading operation or a trenching operation.

For more information concerning the pilot hydraulic system, refer to Systems Operation, "Pilot Hydraulic System".