422E, 428E, 432E, 434E, 442E and 444E Backhoe Loaders Hydraulic and Steering System Caterpillar

Illustration 1	g01277795
(1) Pump control valve (2) Piston (3) Bias spring (4) Actuator Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (20) Spring (21) Spring

Illustration 2	g01278842
(1) Pump control valve (12) Pressure compensator spool (13) Flow compensator spool (20) Spring (21) Spring

The piston pump for the hydraulic system is controlled by the load signal from the work tools. The piston pump senses both pressure and flow needs. When none of the hydraulic circuits are being used, the pump is at low pressure standby.

If one or more circuits are being used, the resolver network compares the control valve work port pressures. The single highest pressure felt flows to flow compensator spool (13). The flow compensator (13) keeps the pump output at a level that is needed in order to fulfill the flow and pressure requirements of the system.

The pump outlet pressure is maintained about 2200 kPa (320 psi) above the needed pressure of the work port by flow compensator spool (13) .

The system pressure will be greater than the requirements for the highest work port pressure unless the pump is at full stroke. Margin pressure is the difference between the signal pressure and the system pressure.

The pressure compensator valve prevents a high pressure spike at the pump. The hydraulic system pressure is limited by the signal relief valve in the control valves. This protects the hydraulic system from damaging high pressures.

When the work port pressure rises above the pressure in Table 2, pressure compensator spool (12) overrides flow compensator spool (13). This causes the pump to destroke.

The pump has one piston (2) and a bias spring (3). The bias spring (3) causes swashplate (7) to upstroke. The pressure in the piston (2) causes the pump to reduce flow.

The pressure in the piston (2) is controlled by the flow compensator (13) or the pressure compensator (12) .

The flow compensator (13) regulates the pressure in the actuator piston (4) by one of two ways. The flow compensator (13) will communicate with the tank when the setting for the margin pressure is not met. The flow compensator (13) will communicate the pump discharge pressure to the piston (2) when the setting for the margin pressure is met.

Low Pressure Standby

Illustration 3	g01298461
(1) Pump control valve (2) Piston (3) Bias spring (4) Actuator Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (14) Passage (15) Passage (16) Passage (17) Passage (18) Cavity (19) Signal line from the control valve (20) Spring (21) Spring

When you start the machine with the work tool controls in the HOLD position and the steering is inactive, the pump is at low pressure standby.

There are no flow or pressure demands from the implements on the pump but there is signal pressure in line (19) due to the orifice in the priority spool (dynamic signal orifice). This maintains a charge in the signal line from the metering pump. This charge causes quick steering response.

Before the engine is started, bias spring (3) holds swashplate (7) at a maximum angle. As the pump begins to turn, this causes oil to flow. The flow of oil builds up pressure in the system because of the closed centers of the implement valves.

The pressure in passage (15) is felt at the bottom of the spool for flow compensator (13) and the spool for pressure compensator (12). As this pressure increases, the pressure pushes the spool for the flow compensator against spring (20). Spool (13) moves up. The movement of the spool opens passage (15) to passage (16) in order to allow the pressure oil to flow to piston (4) .

The oil acts against the piston (2). The oil pressure overcomes the force of bias spring (3) and supply pressure. This causes the piston to move to the right. The piston moves the swashplate (7) toward the minimum angle. The actuator piston (4) continues to move to the right. The actuator piston (4) allows oil to drain into the case.

The draining oil limits the maximum travel of the piston (2) to the right. The pump will produce enough flow in order to account for the system leakage. The pump will also produce enough flow in order to account for the leakage to the pump case.

Upstroking

Illustration 4	g01298465
(1) Pump control valve (2) Piston (3) Bias spring (4) Actuator Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (14) Passage (15) Passage (16) Passage (17) Passage (18) Cavity (19) Signal line from the control valve (20) Spring (21) Spring

When the pump increases the output due to the demand for the increased flow of oil, the pump is upstroking. Because of the increased flow demand, the signal pressure combines with the force of the spring (20) in the cavity (18) .

This combination of signal pressure and spring force is greater than the pump discharge pressure. This combination of signal pressure and spring force causes the spool (13) to move down. As the spool moves down, the spool blocks the flow of the supply oil to piston (4). Oil in the passage (14) for piston (2) can now drain to passage (14) .

The swashplate (7) upstrokes because of the force from the bias spring (3) .

This increases the pump flow. As the flow requirements are satisfied the pump output pressure increases. The pressure in passage (14) moves spool (12) up to the metering position. This means that the setting for the margin pressure of the flow compensator has been met.

Metering

Illustration 5	g01298468
(1) Pump control valve (2) Piston (3) Bias spring (4) Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (14) Passage (15) Passage (16) Passage (17) Passage (18) Cavity (19) Signal line from the control valve (20) Spring (21) Spring

In the metering position, the signal oil pressure in line (19) and the force of spring (20) equals the pump supply pressure below spool (12) .

The pump will maintain the flow that is required in order to maintain system pressure. The pump will stay in the metering position until the flow requirements change.

Destroking

Illustration 6	g01298469
(1) Pump control valve (2) Piston (3) Bias spring (4) Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (14) Passage (15) Passage (16) Passage (17) Passage (18) Cavity (19) Signal line from the control valve (20) Spring (21) Spring

When the pump decreases the output due to the decreasing demand for the flow of oil, the pump is destroking. The decreasing demand for the flow of oil causes the pump discharge pressure to increase. The discharge pressure is higher than the signal pressure and the force of the spring (20) in the flow compensator. The spool (13) is pushed up.

Pump oil now flows from passage (15) past spool (12). The oil flows through passage (16) around pressure compensator spool (12) to passage (14). then, the oil flows into piston (2). Pump pressure behind piston (2) is now greater than the bias piston (3). The angle of swashplate (7) decreases. This decreases pump flow.

Once the lower flow requirements are met, flow compensator spool (13) moves down to the metering position. Swashplate (7) will maintain an angle that is sufficient to provide the lower required flow. If all implements are returned to hold, the pump goes to the low pressure standby.

High Pressure Spike

Illustration 7	g01298473
(1) Pump control valve (2) Piston (3) Bias spring (4) Piston (5) Slipper (6) Input shaft (7) Swashplate (8) Slipper Retainer (9) Cylinder block (10) Cylinder block spring (11) Valve plate (12) Pressure compensator spool (13) Flow compensator spool (14) Passage (15) Passage (16) Passage (17) Passage (18) Cavity (19) Signal line from the control valve (20) Spring (21) Spring

When the control valve is rapidly centered or when the cylinders are suddenly stopped, there can be high pressure in the pump discharge line. The signal oil pressure in line (19) and cavity (18) becomes equal to the pump output pressure. Spring (20) keeps spool (12) shifted downward.

When the pressure in passage (15) rises above the pressure in Table 2, the upward force on the spool (13) will overcome the force of spring (21). This will cause spool (13) to move up. Supply oil flows through passage (14) to piston (2).

Oil pressure that is felt on the piston (4) will destroke the pump.