374D Excavator Hydraulic System Main Control Valve - Load Signal Regulation Caterpillar

Introduction

The output flow of the main pumps is controlled by the true load signal or the duplicate load signal. The true load signal is used during travel or travel with implement functions. The duplicate load signal is used during implement only functions. When the joysticks or travel levers/pedals are moved to the FULL STROKE position, the load signal system functions in the following manner:

• During a single operation, one load signal pressure is sent to both pumps.

• During a combined control valve operation, the highest load signal pressure is sent to both pumps.

The displacement of the pumps is proportional to the amount of movement of the joysticks or the travel lever/pedals. When a joystick or a travel lever/pedal is moved to the NEUTRAL position, the pump swashplate is moved to the minimum displacement position.

Load Signal During Individual Cylinder Operation

Illustration 1	g03123456
Circuit diagram for load signal regulation during the operation of an individual cylinder (BUCKET CLOSE) (1) Main control valve (2) Resolver valve (3) Passage (load signal pressure to signal duplication valve) (4) Passage (true load signal pressure) (5) Passage (load signal pressure) (6) Passage (load signal pressure) (7) Passage (pump flow) (8) Signal duplication valve (9) Passage (10) Orifice (11) Passage (duplicate load signal) (12) Passage (pump flow) (13) Passage (pump flow) (14) Resolver valve (15) Passage (pump flow to the bucket control valve) (16) Bucket control valve (17) Load check valve (18) Flow compensator valve (bucket) (19) Bridge passage (bucket) (20) Resolver valve (21) Load signal reduction manifold (22) Line (load signal pressure) (23) Solenoid valve (load signal selector (24) Passage (head end of the bucket cylinder) (25) Line (load signal pressure to pump regulators) (26) Front pump (27) Rear pump (28) Bucket cylinder

When the joystick for the bucket is moved to the BUCKET CLOSE position, bucket control valve (16) shifts and pump oil flows in the following manner:

1. Oil from the front pump (26) and the rear pump (27) flows into main control valve (1) and into passage (12) . Pump oil then flows to passage (13) and to passage (15) . Pump oil then enters bucket control valve (16) and flows through load check valve (17) . Pump oil then flows through flow compensator valve (18) to bridge passage (19) and through bucket control valve (16) . The pump oil flows from bucket control valve (16) to passage (24) and then to the head end of bucket cylinder (28) . The bucket cylinder extends.

2. The true load pressure in bridge passage (19) flows to resolver valve (20) and to passage (5) . The true load pressure then flows through resolver valve (14) to passage (4) . The true load pressure then flows through resolver valve (2) to passage (6) . The true load pressure flows trough passage (3) to signal duplication valve (8) and load signal selector solenoid (23) .

   1. True load signal in passage (3) flows to signal duplication valve (8) to assist in creating the duplicate load signal pressure. The remainder of the true load signal oil flows to load signal selector solenoid (23) . During implement-only operations, load signal selector solenoid (23) is energized. The true load signal pressure is blocked from flowing to the pump regulators.

      Note: During a travel or travel with implement operation, load signal selector solenoid (23) is de-energized. True load signal pressure is sent to the pump regulators.

3. Pump oil enters passage (7) and flows into signal duplication valve (8) . The pump oil flows through signal duplication valve (8) and into passage (9) . Orifice (10) creates back pressure in passage (9) and causes the duplication valve to close partially. True load signal pressure in passage (3) counteracts the pressure in passage (9) . Signal duplication valve (8) maintains a lowered pressure after orifice (10) then in passage (9) . The lowered pressure after orifice (10) is the duplicate load signal pressure.

4. Duplicate load signal pressure flows into passage (11) . Passage (11) supplies the duplicate load signal pressure to the following components: the flow compensator valves for travel, stick, boom, bucket, and the attachment, the two pressure differential relief valves, the signal relief selector solenoid valve and the load signal selector solenoid valve. The function of the duplicate load signal pressure in these components is listed as follows:

   1. The flow compensator valves use the duplicate load signal pressure to reduce the amount of pump oil pressure to an implement that does not require high pressure or flow to operate.

   2. The pressure differential relief valves use the duplicate load signal pressure to maintain a margin of main pump pressure and the load signal circuit.

   3. The signal relief selector solenoid valve controls the flow of the duplicate load signal pressure to the signal relief valves. The signal relief valves limit the amount of pressure of the duplicate load signal pressure.

   4. Load signal selector solenoid valve (23) controls the flow of load signal pressure to the pump regulators. When load signal selector solenoid valve (23) is de-energized, true load signal pressure flows to the pump regulators. When load signal selector solenoid valve (23) is energized, duplicate load signal pressure flows to the pump regulators.

5. Because an implement-only operation is performed, load signal selector solenoid valve (23) is energized. Duplicate load signal pressure is used for pump regulation. The duplicate load signal flows through load signal selector solenoid valve (23) to line (22) . The duplicate load signal pressure then flows into load signal reduction manifold (21) and into line (25) . The duplicate load signal pressure then flows to the pump regulators. The pump regulators control the output of front pump (26) and rear pump (27) .

The oil from front pump (26) and the oil from rear pump (27) is combined at main control valve (1) . Therefore, the load signal pressure controls both pumps.

The oil flows in the same manner for all of the control valves in the main control valve.

Load Signal During the Slight Operation of an Individual Cylinder

Illustration 2	g03124616
Bucket control valve (BUCKET CLOSE) (4) Passage (true load signal pressure) (15) Passage ((pump flow to the bucket control valve)) (17) Load check valve (18) Flow compensator valve (bucket) (19) Bridge passage (bucket) (20) Resolver valve (24) Passage (head end of the bucket cylinder) (28) Bucket cylinder (29) Passage (duplicate load signal pressure) (30) Feeder passage (31) Passage (pump flow to feeder passage) (32) Spool

Combined flow from the front pump and the rear pump enters the bucket control valve. Spool (32) moves a slight amount to the left. Passage (31) opens a slight amount. Pump oil flows from passage (15) to feeder passage (30) . Pump oil then flows through load check valve (17) to bridge passage (19) . Pump oil then flows to passage (24) and to bucket cylinder (28) . Bucket cylinder (28) extends.

True load signal pressure is generated in bridge passage (19) . True load signal pressure in bridge passage (19) flows through resolver valve (20) and passage (4) to the signal duplication valve. The signal duplication valve creates the duplicate load signal pressure. The duplicate load signal pressure flows to passage (29) and to each flow compensator valve for the implements. The duplicate load signal pressure also flows to the front pump and the rear pump in order to regulate the output flow of the pumps.

The output flow of the pumps is regulated in order to maintain a pressure difference between the pump delivery pressure and the duplicate load signal pressure to 1960 kPa (285 psi). If the bucket joystick is moved closer to the NEUTRAL position, the size of the opening at passage ( ) decreases.

As the pump oil flow through passage (31) decreases, pressure in feeder passage (30) decreases. The true load signal pressure in bridge passage (19) , duplicate load signal pressure, and the force of the spring on flow compensator valve (18) pushes flow compensator valve (18) downward. Flow compensator (18) moving downward will further reduce the pump oil flow through load check valve (17) . The pressure decreases in bridge passage (19) . The reduced true load signal pressure in bridge passage (19) flows to the signal duplication valve. Reduced true load signal pressure also reduces the duplicate load signal pressure. The output flow of the pumps decreases further by duplicate load signal regulation.

When the output flow of the pumps is decreased, the amount of flow through compensator valve (18) is also decreased.

Load Signal During Combined Control Valve Operations

Illustration 3	g03124741
Circuit diagram for combined control valve operation (BOOM RAISE and STICK OUT) (1) Main control valve (2) Resolver valve (3) Passage (load signal pressure to signal duplication valve) (4) Passage (true load signal pressure) (6) Passage (load signal pressure) (7) Passage (pump flow) (8) Signal duplication valve (9) Passage (10) Orifice (11) Passage (duplicate load signal) (12) Passage (pump flow) (14) Resolver valve (21) Load signal reduction manifold (22) Line (load signal pressure) (23) Solenoid valve (load signal selector (25) Line (load signal pressure to pump regulators) (26) Front pump (27) Rear pump (33) Resolver valve (34) Flow compensator valve (stick) (35) Load check valve (36) Stick control valve (37) Bridge passage (stick) (38) Feeder passage (stick) (39) Passage (true load signal pressure) (40) Passage (duplicate load signal pressure) (41) Line (stick cylinder rod end) (42) Passage (duplicate load signal pressure) (43) Boom control valve (44) Load check valve (45) Feeder passage (boom) (46) Flow compensator valve (boom) (47) Bridge passage (boom) (48) Line (boom cylinder head end) (49) Stick cylinder (50) Boom cylinders

In the following explanation of oil flow, the joysticks are at the FULL STROKE position. The boom hydraulic circuit and the stick hydraulic circuit are operated for a combined operation. The load on the stick hydraulic circuit is greater than the load on the boom hydraulic circuit. The boom hydraulic circuit is activated first, and the stick hydraulic circuit is activated during boom movement.

The joystick for the boom is first moved to the FULL STROKE position in order to perform a BOOM RAISE operation. Oil from the front pump (26) and the rear pump (27) flows into main control valve (1) and into passage (12) . Pump oil then enters boom control valve (43) and flows through load check valve (44) . Pump oil then flows through flow compensator valve (46) to bridge passage (47) and through boom control valve (43) . The pump oil flows from boom control valve (43) to the boom drift reduction valve. Pump oil flows to line (48) and then to the head end of boom cylinders (50) . The boom cylinders extend.

The true load signal pressure from the boom cylinders in bridge passage (47) flows through resolver valve (14) and to passage (4) . The true load signal pressure then flows to resolver valve (2) , passage (6) , and passage (3) to signal duplication valve (8) .

Pump oil enters passage (7) and flows into signal duplication valve (8) . The pump oil flows through signal duplication valve (8) and into passage (9) . Orifice (10) creates back pressure in passage (9) and causes the duplication valve to close partially. True load signal pressure in passage (3) counteracts the pressure in passage (9) . Signal duplication valve (8) maintains a lowered pressure after orifice (10) then in passage (9) . The lowered pressure after orifice (10) is the duplicate load signal pressure.

The remainder of the true load signal oil flows to load signal selector solenoid (23) . During implement-only operations, the load signal selector solenoid is energized. The true load signal pressure is blocked from flowing to the pump regulators.

Because an implement-only operation is performed, load signal selector solenoid valve (23) is energized. Duplicate load signal pressure is used for pump regulation. The duplicate load signal flows through load signal selector solenoid valve (23) to line (22) . The duplicate load signal pressure then flows into load signal reduction manifold (21) and into line (25) . The duplicate load signal pressure then flows to the pump regulators as load signal pressure.

When the stick joystick is also moved to the FULL STROKE position during a boom operation, the load signal regulation of the pumps will change. The delivery pressure and the duplicate load signal pressure then operate in the following manner:

The joystick for the stick is first moved to the FULL STROKE position in order to perform a STICK OUT operation. Oil from the front pump (26) and the rear pump (27) flows into main control valve (1) and into passage (12) . Pump oil then flows to stick control valve (36) and flows through load check valve (35) . Pump oil then flows through flow compensator valve (34) to bridge passage (37) and through control valve (36) . The pump oil flows from control valve (36) to the stick drift reduction valve. Pump oil flows to line (41) and then to the rod end of stick cylinder (49) . The stick cylinder retracts.

The true load signal pressure in bridge passage (37) of stick cylinder (49) flows to resolver valve (33) . The true load signal flows from resolver valve (33) to passage (39) and then to resolver valve (2) . The stick circuit true load signal pressure is greater than the boom true load signal pressure. The stick true load signal pressure causes resolver valve (2) to shift to the right. The boom load signal pressure is now blocked from flowing to signal duplication valve (8) . The stick true load signal pressure flows to signal duplication valve (8) . True load signal pressure in passage (40) is used to duplicate the stick true load signal pressure at signal duplication valve (8) .

The true load signal pressure from the stick circuit is greater than the true load signal pressure from the boom circuit. The duplicate load signal pressure increases in passage (11) . The increased duplicate load signal pressure causes the main pump flow to increase.

Duplicate load signal pressure flows through passage (11) and passage (42) to boom flow compensator valve (46) . Duplicate load signal pressure forces flow compensator valve (46) to close partially. Flow compensator valve (46) restricts pump delivery flow through boom control valve (43) .

Since flow compensator valve (46) is partially closed, pump delivery pressure in the boom feeder passage increases. If the pressure in the boom feeder passage becomes higher than the stick feeder passage, pump output will increase. The pump output will increase in order to maintain the balance of the flow and pressure needed to perform the operations.

The duplicate load signal pressure also flows through passage (40) to the top of stick flow compensator valve (34) . Pump delivery pressure in passage (38) is higher than the duplicate load signal pressure. The duplicate load signal pressure does not close stick flow compensator valve (34) . Pump delivery pressure is not restricted from flowing into bridge passage (37) .

The duplicate load signal pressure also flows through passage (11) to load signal selector solenoid valve (23) . During an implement-only operation, the machine ECM energizes solenoid valve (23) . True load signal pressure is blocked and duplicate load signal pressure flows through solenoid valve (23) . Duplicate load signal pressure then flows through line (22) to load signal reduction manifold (21) . The duplicate load signal pressure then flows through line (25) to the pump regulators.

During combined control valve operations with the joysticks at the FULL STROKE position, the hydraulic circuit with the highest true load signal pressure will control the duplicate load signal pressure. When the true load signal pressure is high, the duplicate load signal pressure will be higher as well. When the true load signal pressure is low, the duplicate load signal pressure will be lower as well.

A slight amount of movement of the boom joystick and the stick joystick causes the duplicate load signal regulation to decrease the output flow of the pumps. As a result, the speed of both the stick cylinder and the boom cylinders is decreased.

If the joystick for the boom is at the FULL STROKE position and the joystick for the stick is moved slightly, the speed of the boom cylinders will be normal. The higher true load signal pressure from the boom circuit will cause the duplicate load signal pressure to be higher as well. The speed of the stick cylinder will be decreased. No change in pump output flow will occur due to the higher duplicate load signal system.