374D Excavator Hydraulic System Main Control Valve Caterpillar

Hydraulic Schematic for the Main Control Valve

Illustration 1	g03125000
(1) Proportional solenoid valve for right travel (reverse) (2) Proportional solenoid valve for left travel (reverse) (3) Proportional solenoid valve for stick cylinder head end (4) Stick drift reduction/regeneration valve (5) Solenoid valve (stick drift reduction valve) (6) Line relief valve (stick cylinder rod end) (7) Main control valve (8) Pilot manifold (9) Proportional solenoid valve for boom cylinder head end (10) Proportional solenoid valve for bucket cylinder head end (11) Proportional solenoid valve for attachment spool (12) Solenoid valve (neutral bypass) (13) Neutral flow control valve (14) Flow compensator valve (right travel) (15) Right travel control valve (16) Flow compensator valve (left travel) (17) Left travel control valve (18) Flow compensator valve (stick) (19) Stick control valve (20) Signal duplication valve (21) Boom control valve (22) Load check valve (23) Flow compensator valve (boom) (24) Bucket control valve (25) Flow compensator valve (bucket) (26) Attachment control valve (27) Flow compensator valve (attachment) (28) Differential relief valve (29) Return port (30) Line relief valve (stick cylinder head end) (31) Inlet port (rear pump) (32) Inlet port (front pump) (33) True load signal port (34) Duplicate load signal port (35) Signal relief valve (low pressure) (36) Signal relief valve (high pressure) (37) Solenoid valve (high/low relief) (38) Line relief valve (boom cylinder rod end) (39) Line relief valve (bucket cylinder rod end) (40) Line relief valve (bucket cylinder head end) (41) Differential relief valve (42) Return port (43) Proportional solenoid valve for right travel (forward) (44) Proportional solenoid valve for left travel (forward) (45) Proportional solenoid valve for stick cylinder rod end (46) Load signal reduction manifold (47) Load signal selector solenoid (48) Boom drift reduction/regeneration valve (49) Line relief valve (boom cylinder head end) (50) Solenoid valve (boom drift reduction valve) (51) Proportional solenoid valve for boom cylinder rod end (52) Proportional solenoid valve for bucket cylinder rod end (53) Proportional solenoid valve for attachment spool (54) Hydraulic tank (55) Pilot pump (56) Fan pump (57) Swing pump (58) Front pump (59) Rear pump

Introduction

The main control valve is located in the hydraulic circuit between the main pumps, and the cylinders and travel motors. The oil flow from front pump (58) , rear pump (59) , and pilot pump (55) is supplied to the main control valve.

The main control valve distributes the oil flow from the pumps to the cylinders and travel motors. When the operator moves a joystick or travel lever/pedal, an electrical signal is sent to the machine ECM. The machine ECM receives the input and sends an electrical signal to the proportional solenoid valves on the main control valve. The electrical signals that are sent to the proportional solenoid valves control pilot oil pressure at each control valve spool. Pilot oil pressure is used to shift the control valve spools. The direction and speed of the cylinders and the motors is proportional to the input of the operator controls. This control also limits the pressure of the supply oil.

Illustration 2	g02157623
Main control valve (top view) (1) Proportional solenoid valve for right travel (reverse) (2) Proportional solenoid valve for left travel (reverse) (3) Proportional solenoid valve for stick cylinder head end (4) Stick drift reduction/regeneration valve (5) Solenoid valve (stick drift reduction valve) (6) Line relief valve (stick cylinder rod end) (9) Proportional solenoid valve for boom cylinder head end (10) Proportional solenoid valve for bucket cylinder head end (11) Proportional solenoid valve for attachment spool (12) Solenoid valve (neutral bypass) (13) Neutral flow control valve (14) Flow compensator valve (right travel) (16) Flow compensator valve (left travel) (18) Flow compensator valve (stick) (20) Signal duplication valve (23) Flow compensator valve (boom) (25) Flow compensator valve (bucket) (27) Flow compensator valve (attachment) (28) Differential relief valve (29) Return port (30) Line relief valve (stick cylinder head end) (33) True load signal port (34) Duplicate load signal port (35) Signal relief valve (low pressure) (36) Signal relief valve (high pressure) (37) Solenoid valve (high/low relief) (38) Line relief valve (boom cylinder rod end) (39) Line relief valve (bucket cylinder rod end) (40) Line relief valve (bucket cylinder head end) (41) Differential relief valve (42) Return port (43) Proportional solenoid valve for right travel (forward) (44) Proportional solenoid valve for left travel (forward) (45) Proportional solenoid valve for stick cylinder rod end (48) Boom drift reduction/regeneration valve (49) Line relief valve (boom cylinder head end) (50) Solenoid valve (boom drift reduction valve) (51) Proportional solenoid valve for boom cylinder rod end (52) Proportional solenoid valve for bucket cylinder rod end (53) Proportional solenoid valve for attachment spool (60) Main control valve body (boom side) (61) Center manifold (62) Main control valve body (stick side)

Illustration 3	g02157626
Main control valve (60) Main control valve body (boom side) (61) Center manifold (62) Main control valve body (stick side)

The main control valve consists of body (60) (boom side), body (62) (stick side), and center manifold (61) .

1. Boom control valve (21) , bucket control valve (24) , and attachment control valve (26) are arranged in parallel in the boom side of body (60) . The pump oil is delivered through return port (42) to hydraulic tank (54) . In addition, the following components are located in the boom side of the body:

   1. Differential relief valve (41) , line relief valve (38) , line relief valve (49) , line relief valve (39) , and line relief valve (40) limit circuit pressure.

   2. Flow compensator valves (23) , (25) , and (27) are part of the load sensing hydraulic system.

   3. Boom drift reduction valve (48) prevents boom drift when the joystick for the boom is in the NEUTRAL position. Line relief valve (49) for the boom cylinder head end is mounted on the boom drift reduction valve.

   4. A proportional solenoid valve is located at the end of each stem in the boom side of the main control valve body. Pilot oil pressure flows to each proportional solenoid valve in order to control the stem movement in the main control valve. Pilot oil pressure is controlled at the main control valve by the following proportional solenoid valves: proportional solenoid valve (11) for the attachment spool, proportional solenoid valve (10) for the bucket cylinder rod end, proportional solenoid valve (9) for the boom cylinder rod end, proportional solenoid valve (53) for the attachment spool, proportional solenoid valve (52) for the bucket cylinder head end and proportional solenoid valve (51) for the boom cylinder head end.

   5. When the joysticks are in the NEUTRAL position, the machine ECM energizes solenoid valve (12) . Pilot oil flows to neutral flow control valve (13) . Neutral flow control valve (13) shifts. Main system oil in the main control valve flows to hydraulic tank (54) .

   6. Boom control valve (21) , bucket control valve (24) , and attachment control valve (26) are equipped with load check valves (22) . The load check valves prevent oil in the high-pressure circuit from flowing into the low-pressure circuit when two circuits are operated simultaneously. The load check valves are part of flow compensator valves (27) , (25) , and (23) .

2. Stick control valve (19) , left travel control valve (17) , and right travel control valve (15) are located in the stick side of body (62) . The pump oil is delivered through return port (29) to hydraulic tank (54) . In addition, the following components are located in the stick side of the body:

   1. Differential relief valve (28) , line relief valve (30) , and line relief valve (6) limit the circuit pressure due to external forces.

   2. Flow compensator valve (18) (stick), (16) (left travel), and (14) (right travel) are part of the load sensing hydraulic system.

      Note: Left travel control valve (17) and right travel control valve (15) do not have load check valves.

   3. Stick drift reduction valve (4) prevents stick drift when the joystick for the stick is in the NEUTRAL position. Line relief valve (6) for the stick cylinder rod end is mounted on the stick drift reduction valve.

   4. A proportional solenoid valve is located at the end of each stem in the stick side of the main control valve body. Pilot oil pressure flows to each proportional solenoid valve in order to control the stem movement in the main control valve. Pilot oil pressure is controlled at the main control valve by the following proportional solenoid valves: proportional solenoid valve (3) for the stick cylinder head end, proportional solenoid valve (2) for the left travel (reverse), proportional solenoid valve (1) for the right travel (reverse), proportional solenoid valve (45) for the stick cylinder rod end, proportional solenoid valve (43) for the left travel (forward) and proportional solenoid valve (44) for the right travel (forward).

   5. Stick control valve (19) is equipped with a load check valve that is like valve (22) . The load check valve prevents oil in the high-pressure circuit from flowing into the low-pressure circuit when two circuits are operated simultaneously. The load check valve is part of flow compensator valve (18) (stick).

3. The front pump delivers oil to center manifold (61) through inlet port (32) . The rear pump delivers oil to center manifold (61) through inlet port (31) . Front pump oil and rear pump oil are combined in the center manifold. In addition, the following components are located in the center manifold:

   1. Signal duplication valve (20) is part of the load sensing hydraulic system.

   2. Circuit pressure is controlled by the following components: signal relief valve (35) (low pressure), signal relief valve (36) (high pressure) and solenoid valve (37) (high/low relief).

Main Control Valve Operation in NEUTRAL Position

Illustration 4	g02157628
Main control valves (NEUTRAL position) (12) Solenoid valve (neutral bypass) (13) Neutral flow control valve (15) Right travel control valve (17) Left travel control valve (19) Stick control valve (21) Boom control valve (24) Bucket control valve (26) Attachment control valve (31) Inlet port (rear pump) (32) Inlet port (front pump) (42) Return port (58) Front pump (59) Rear pump (60) Main control valve body (boom side) (61) Center manifold (62) Main control valve body (stick side) (63) Passage (64) Lands (65) Throttling slots (66) Throttling slots (67) Lands (68) Passage

In this description, the joysticks and the travel joysticks are in the NEUTRAL position. The boom cylinders, the stick cylinder, the bucket cylinder, and the travel motors are in a no-load condition.

Front pump oil flows through inlet port (32) of center manifold (61) . Rear pump oil flows through inlet port (31) of the center manifold. After the pump oil enters the main control valve, the pump oil is combined in the center manifold.

Valve (15) , valve (17) , valve (19) , valve (21) , valve (24) , and valve (26) are maintained in the NEUTRAL position. Pump oil does not flow to the cylinders or motors.

When the main control valves are in the NEUTRAL position, the machine ECM sends an electrical signal to neutral bypass solenoid valve (12) . When solenoid valve (12) is energized, pilot oil flows to neutral flow control valve (13) . Pilot oil shifts neutral flow control valve (13) . Pump oil flows through neutral flow control (13) valve to the hydraulic tank.

Neutral flow control valve (13) is open only when the main control valves are in the NEUTRAL position.

When the joysticks and/or the travel levers/pedals are operated, an electrical signal is sent to the machine ECM. The machine ECM de-energizes the solenoid valve for the neutral flow control valve, and the neutral flow control valve closes. Pump oil is blocked at the neutral flow control valve, and the pump oil flows to the stems in the main control valve body.

When the joysticks and/or the travel levers/pedals are returned to the NEUTRAL position, the control valves return to the NEUTRAL position. The machine ECM energizes the solenoid valve for the neutral flow control valve, and the neutral flow control valve opens. Most of the oil from the pumps flow through passage (63) , neutral flow control valve (13) , and return port (42) . The oil then flows to the hydraulic tank.

When the control valves are in the NEUTRAL position, the machine is under a no-load condition. The main pumps provide a small amount of oil delivery to passages (63) and (68) . Since neutral flow control valve (13) is open, the pump oil passes through the neutral flow control valve. The delivery pressure of this oil is maintained at 1960 kPa (285 psi) by the differential relief valves.

Each control valve is like the bucket in construction. The bucket control valve is shown as an example.

Illustration 5	g02157633
Bucket control valve in NEUTRAL position (sectional view) (63) Passage (64) Lands (65) Throttling slots (66) Throttling slots (67) Lands

When the bucket control valve is in the NEUTRAL position, the passages to lands (64) and (67) , and throttling slots (65) and (66) are closed. Pump oil in passage (63) does not flow to the bucket cylinder.

Signal Duplication Valve

The signal duplication valve is located in the center manifold of the main control valve. Signal duplication valve (20) is used to provide a smooth duplicate load signal pressure to the pump regulators during implement operation. Pressure spikes in the pump delivery pressure are undesirable during implement operation.

The duplicate load signal pressure is routed to the flow compensator valves and to the differential relief valves.

ReferenceFor more information about signal duplication valve (20) , refer to Systems Operation, "Signal Duplication Valve".

Individual Valve Operation

Illustration 6	g03125141
Circuit diagram for individual control valve operation (BUCKET CLOSE) (1) Bucket cylinder (2) Passage (load pressure) (3) Signal duplication valve (4) Resolver valve (5) Resolver valve (6) Passage (pump flow) (7) Resolver valve (9) Passage (pump flow to flow compensator valve) (10) Passage (pump flow to spool) (12) Flow compensator valve (14) Passage (load pressure) (15) Passage (pump flow from flow compensator valve) (16) Bucket control valve (17) Passage (pump flow to control valve) (19) Line (duplicate load signal pressure) (20) Passage (return pilot oil) (21) Passage (pump flow to bucket cylinder) (22) Proportional solenoid valve for bucket cylinder head end (23) Pilot manifold (24) Pilot pump (25) Front pump (26) Rear pump (33) Line (load signal pressure to pumps) (34) Load signal selector valve (35) Line (36) Load signal reduction manifold (PL) True load pressure

Illustration 7	g03184476
Bucket control valve (BUCKET CLOSE position) (5) Resolver valve (10) Passage (12) Flow compensator valve (14) Passage (15) Bridge passage (20) Passage (21) Passage (22) Proportional solenoid valve for bucket cylinder head end (29) Spool (30) Spring chamber (31) Passage (32) Spool (PL) Load pressure

Front pump oil and rear pump oil enters the main control valve. The front pump oil and rear pump oil is combined in main control valve.

Note: The load signal pressure controls the output of both main pumps through one line. Only one line is necessary for pump control because the output is combined in the main control valve.

Main pump oil flows through passages (17) , (6) , and (10) to bucket control valve (16) .

When the joystick for the bucket is moved to the BUCKET CLOSE position, an electrical signal is sent to the machine ECM. The machine ECM sends an electrical signal to proportional solenoid valve (22) . Spool (29) in the proportional solenoid valve shifts. The pilot oil pressure in spring chamber (30) flows through passage (20) to the hydraulic tank. The pilot oil pressure that acts on the left end of spool (29) decreases. The pilot oil pressure in the right end of spool (29) is higher than the pilot oil pressure in the left end of the spool. Spool (29) shifts to the left.

Main pump oil flows through passage (9) , flow compensator valve (12) , passage (14) , and passage (15) . The oil then flows to the head end of the bucket cylinder (1) . The oil in the rod end of the bucket cylinder returns to the main control valve and is then directed through passages to the hydraulic tank. The bucket cylinder extends.

The true load signal pressure (PL) for the bucket cylinder is generated in passage (14) and (15) . True load signal pressure in passage (14) flows through resolver valve (5) . From resolver valve (5) , the true load signal pressure flows to resolver valve (7) , passage (2) , and resolver valve (4) . The true load signal pressure then flows to signal duplication valve (3) and passage (19) .

Because an implement operation is being performed, the machine ECM energizes load signal selector solenoid valve (34) . The spool of load signal selector solenoid valve (34) shifts. The flow of true load signal pressure is blocked at load signal selector solenoid valve (34) . Duplicate load signal pressure flows through line (19) , solenoid valve (34) , line (35) , load signal reduction valve (36) , and line (33) to the pump regulators. The duplicate load signal pressure regulates the output of the main pumps.

The output of front pump (25) and rear pump (26) combine in the main control valve, so duplicate load signal pressure controls both pumps.

The oil flows in the same manner as bucket control valve (16) for all of the control valves in the main control valve.

Combined Control Valve Operation

Illustration 8	g03125342
Circuit diagram for combined control valve operation (BOOM RAISE and STICK OUT operation) (1) Stick cylinder (2) Boom cylinder (3) Proportional solenoid valve for stick cylinder rod end (5) Signal duplication valve (6) Return passage (7) Flow compensator valve (boom) (8) Boom control valve (9) Flow compensator valve (stick) (10) Stick control valve (11) Bridge passage (12) Bridge passage (14) Passage (return pilot oil) (15) Proportional solenoid valve for boom cylinder head end (16) Pilot pump (17) Front pump (18) Rear pump (19) Resolver valve (20) Line (load signal pressure to pumps) (21) Boom drift reduction valve (22) Stick drift reduction valve (23) Load signal selector valve (24) Load signal reduction manifold (PL) Load pressure (the higher pressure of boom cylinder or stick cylinder)

An example of a combined function is described with BOOM RAISE and STICK OUT. The joystick for the boom is moved to the BOOM RAISE position, and the joystick for the stick is moved to the STICK OUT position. Each joystick sends an electrical signal input to the machine ECM. The machine ECM then sends an electrical signal to proportional solenoid valve (3) and (15) .

The pilot oil pressure at proportional solenoid valve (15) flows through return passage (14) to the hydraulic tank. The pilot oil pressure that acts on the bottom end of the spool in boom control valve (8) decreases. The spool in the boom control valve shifts in order to perform the BOOM RAISE operation.

The pilot oil pressure at proportional solenoid valve (3) flows through return passage (6) to the hydraulic tank. Pilot pressure that acts on the top end of the spool in stick control valve (10) decreases. The spool in the stick control valve shifts in order to perform the STICK OUT operation.

Front pump oil and rear pump oil enters the main control valve. The front pump oil and the rear pump oil are combined. Pump oil then flows through boom control valve (8) to flow compensator valve (7) . Pump oil then flows through valve (8) to boom drift reduction valve (21) and out to the head end of boom cylinders (2) . Pump oil also flows through stick control valve (10) to flow compensator valve (9) . Pump oil then flows through valve (10) to stick drift reduction valve (22) and out to the rod end of stick cylinder (1) .

The true load pressure (PL) from the boom cylinders in bridge passage (11) is transmitted to resolver valve (19) . The true load pressure (PL) from the stick cylinder in bridge passage (12) is also transmitted to resolver valve (19) .

The basic use of the load sensing regulation is described as follows:

1. The true load pressure is not used for pump regulation during implement operations. True load pressure is an input for the operation of the signal duplication valve.

2. The duplicate load pressure is sent to the pump regulators to control the output of the pumps during implement operations.

For implement-only functions, load signal selector valve (23) is energized by the machine ECM. The true load signal is not used for pump regulation. The duplicate load signal flows through valve (23) to load signal reduction manifold (24) . The duplicate load signal flows through manifold (24) to the pump regulators. The output of the pumps is controlled by the pump regulators by using the duplicate load signal.

Note: True load signal is used for pump regulation during a travel/travel with implement operation.

For true load signal, only the highest pressure flows through resolver valve (19) to signal duplication valve (5) . The duplicate load signal pressure is more closely matched to the true load signal pressure. The duplicate load signal pressure controls the output flow of the pumps.