PUMP SOLENOID TORQUE PARAMETERS
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The torque limiter monitors the pump swashplate angle and the pump discharge pressure. The torque limiter pressure setting varies with the swashplate angle: the greater the swashplate angle, the lower the pressure setting on the torque limiter spring due to the mechanical linkage between the swashplate and the torque limiter. As the swashplate angle decreases, the
mechanical linkage increases the force on the torque limiter springs, therefore, increasing the pressure setting of the torque limiter.
When a higher flow rate from the pump is required, the flow compensator receives a signal from the control valve to move the pump to maximum displacement. With the increased pressure in the passage between the torque control spool and the torque limiter, the torque limiter opens and allows oil to drain to the tank.
As the oil drains to tank at the torque limiter, a pressure reduction occurs at the top of the torque control spool, which allows oil to start flowing through the axial drilled passage in the torque control spool. Due to the reduced pressure at the top of the torque control spool and increased pump discharge pressure at the bottom of the torque control spool, the torque control spool shifts up. When the torque control spool shifts up, oil is metered across the flow
compensator spool to the large actuator piston which moves the swashplate toward minimum angle.
STEERING AND IMPLEMENT PUMP
TORQUE LIMITER OPEN
Pump
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When the appropriate engine speed for a loading or backhoe operation is obtained, the torque control solenoid is ENERGIZED. With the torque control solenoid energized, the variable displacement pump flow increases.
Pump discharge oil flows through the solenoid and a large and small orifice. The combined discharged pump flow increases the pressure on the top of the torque control spool. This increased pressure at the top of the torque control spool keeps the torque control spool seated.
At the same time a higher work port pressure occurs which generates a higher signal pressure to keep the flow compensator in the down position. The combination of the two spools assists in keeping the swashplate in a partially upstroked position to meet the higher operating flow requirements and system pressure.
Due to the mechanical linkage working between the swashplate of the pump and the torque limiter, the torque limiter spring force increases. With the increased spring force the pump will destroke at a higher pressure.
In summary, full pump supply pressure is always available, however, flow is regulated to maintain maximum input torque from the engine.
STEERING AND IMPLEMENT PUMP
TORQUE CONTROL SOLENOID ON
Pump
Due to the torque control solenoid, the torque limiter has two different torque curves: one for the low torque mode, and one for the high torque mode.
When the hydraulic demand falls below the conditions stated above, the torque limiter closes.
The flow compensator now has control over the pump swashplate angle to regulate pump flow and meet the required system pressure and flow demands.
Mechanical Loader Valve Group
The 420E/430E machines can be equipped with a mechanically controlled loader valve (1) or a pilot controlled loader valve (not shown). The loader valve is accessed from below the floor plate. The loader control valve group contains an inlet manifold (2), lift control valve (3), tilt control valve (4), and auxiliary control valve (5, if equipped).
NOTE: 420E/430E machines with a mechanically controlled loader valve or a pilot controlled loader valve use the same inlet manifold.
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3 2 5 4
The loader joystick (1) controls the position of the lift and tilt valve spools in the loader control valve. The auxiliary control lever (2) controls the position of the auxiliary valve spool.
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1 2
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This illustration shows the mechanically controlled loader valve on the 420E/430E machines.
The loader valve is closed-center. The loader valve includes the lift and tilt valves in the standard loader valve group. The tilt valve contains two line relief valves. The auxiliary valve (if equipped) contains one line relief valve.
The loader valves in the 420E/430E are a PPPC type, but there are no resolvers in the signal network. The pressure compensator creates the load signal pressure by diverting part of the supply oil going to the cylinder into the signal line. The highest load signal is sensed at all compensator valves and at the pump control valve.
The pressure compensator valve maintains a controlled pressure differential across the spool to control flow to the circuit. If more than one circuit is used at a time, the circuit with the highest workport pressure is used to regulate the flow through each control valve.
The "E" Series Backhoe Loader hydraulic system is similar to the PPPC hydraulic system used on other Caterpillar products. The system shown incorporates the flow control, signal
duplication, and resolver into one component called a pressure compensator valve.
420E / 430E HYDRAULIC SYSTEM
MECHANICALLY CONTROLLED LOADER VALVE HOLD
Ride Control Valve
Priority
The pressure compensator valve acts as a resolver to compare the different circuit workport pressures and to send a signal pressure equal to the highest circuit pressure back to the pump.
The priority valve in the 420E/430E loader valve is a three-section valve that operates similarly to the two-section priority valve in the 416E/422E/428E loader valve previously described.
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This illustration shows the loader inlet manifold in the 420E/430E/432E/434E/442E and 444E machines.
NOTE: The 420E/430E machines with a mechanically controlled loader valve and the 420E/430E/432E/434E/442E and 444E machines with a pilot controlled loader valve use the same inlet manifold.
Oil flows from the pump to the priority valve. Before the engine is started, the spring holds the priority spool to the left. When the engine is started, oil flow from the pump is directed to the priority valve. With the spool to the right, the pump supply oil is directed to the HMU. As the HMU passage fills, oil also enters the axial-drilled hole in the priority spool and flows to the right end of the spool.
As pressure builds on the right end, the spool starts to move to the left, against spring force.
Oil continues to flow to the HMU and restricted oil flows to the loader valves. As the priority valve spool continues to move to the left, oil is restricted by the priority spool from entering the HMU supply passage and all oil flows to the loader implement valves.
When the priority spool is moved completely to the left, oil flow to the HMU supply passage is blocked.