Hydraulic System (General Information)

Closed-Center Hydraulic System

Machine is equipped with a combination of closed-center hydraulics and open center hydraulics.

Closed-center hydraulic system uses the variable flow pump block that is a pressure flow compensated pump (PFC pump).

Variable flow pump block has four main components:

• Variable flow piston pump
• Swash plate piston
• Pressure limit valve
• Load sense valve

Variable piston pump is an axial piston pump that varies the pressure and flow required to meet individual systems immediate oil pressure and flow requirements. Configurations of signal lines are used to relay pressure requirements to the variable flow pump block. The pump only produces enough oil pressure and flow required to meet system demands when they occur.

Variable flow pump block is shaft driven by the speed reduction gearcase that is mounted to the rear of the hydrostatic pump assembly. The variable piston pump turns whenever the engine is running.

The pressure flow compensated pump (PFC pump) system creates pressure and flow for the following hydraulic systems:

• Steering
• Reel Fore/Aft
• Lateral Tilt
• Grain Tank Door
• Unloading Auger Swing
• Header Lift
• Reel Lift
• Variable Speed Cylinder Drive
• Header Tilt (fore/aft)
• Hydraulic Service Brakes

NOTE: Service brakes, steering valve block, and electro-hydraulic valve block have flow priority over all other closed-center hydraulic circuits.

Variable Flow Pump Block Operation

Variable flow pump block is a pressure flow compensated (PFC) pump that operates in one of three modes:

1. Low pressure standby (neutral).
2. Increased flow (when demand for pressure and flow begins to ramp up).
3. High-pressure standby (system is at or near relief pressure).

When the drive shaft of the variable piston pump is rotating, the piston cylinder group, which is splined to the drive shaft, also turns. Variable piston pump contains nine pistons assemblies that ride against the machined surface of the swash plate.

At maximum swash plate tilt the piston shoes follow the inclined surface of the swash plate and begin moving the pistons in and out of the piston group bore. Half the piston assemblies are being pulled out of the piston group while the other half of the pistons are being pushed back into the piston group. As the pistons are pulled from the piston group, they draw oil into the piston group bores from the intake port. As the pistons pass over top dead-center, the pistons push oil out of the piston group bores to an outlet pressure port. Each of the nine pistons completes this cycle for each revolution of the pump shaft. This creates a continuous, even flow of oil from the variable flow pump block. The greater the swash plate tilt, the greater the piston stroke.

Engine Start-Up - Low Pressure Standby

When the engine is not running the spring on the swash plate piston holds the variable piston pump and swash plate at full stroke.

When the engine is started the variable piston pump begins to rotate, and momentarily tries to pump oil creating pressure. This pressure is directed to the load sense valve set and the pressure limit valve. Pressure builds shifting the load sense valve against its spring routing oil pressure to the swash plate piston extending the piston. The swash plate moves to a nearly zero degree angle, destroking the pump. In this condition, the variable piston pump creates enough oil flow to make up for the internal leakage within the system and maintains pressure.

Variable piston pump remains in this position until there is a requirement for oil. In low pressure standby mode the variable flow pump produces less heat and uses less horsepower than an open center system. Low pressure standby also makes starting the engine easier.

Increased Flow and Pressure

Activating a closed-center hydraulic valve creates a pressure drop in the variable piston pump outlet port. Pump outlet pressure is routed to the spool side of the load sense valve and pressure limit valve. At the same time the signal line sends working pressure to the spring side of the load sense valve and pressure limit valve in the variable flow pump. This creates a pressure differential between the pump side of the valves and the spring – signal line side. Signal line pressure and spring pressure shift the low pressure valve allowing oil to exit the swash plate piston, stroking the variable piston pump creating flow and pressure. Pressure in the swash plate piston is routed to the hydraulic reservoir. Stroking the variable flow pump to meet pressure and flow requirements to the point when the pump outlet pressure and signal line pressure begin to equalize. When flow and pressure requirements are met, the signal line relays working pressure to the spring side of the valves. Spring pressure and working pressure overwhelm the pump side pressure. The valve shifts and pressure routes to the swash plate piston destroking the variable flow pump.

Signal Line Function

Signal line pressure is created when a closed-center hydraulic functional valve is opened or the pressure compensation valve is activated.

Closed-center hydraulic signal lines originate in the:

• Primary control valve block
• Electro-hydraulic valve block (AutoTrac)
• Manual steering valve block
• Header height valve block
• Reel function valve block
• Chassis tilt valve block (If Equipped)

When activating a closed-center hydraulic functional valve, the signal line routes working pressure to the shuttle valve located in the primary control valve block.

NOTE: Shuttle valve gives priority to signal line pressure from electro-hydraulic valve block.

From the primary control steering valve block signal line pressure is routed to the variable flow pump.

High Pressure Standby

Whenever a cylinder reaches the end of the stroke or a system valve is held on, the closed-center hydraulic system goes to high-pressure standby until the load is overcome or the valve returned to neutral.

Pump outlet pressure is routed to the spool side of the load sense valve and pressure limit valve. Signal line pressure from the activated system is routed to the load sense valve. The combined signal line pressure with the valve spring tension keeps the path open to the pressure limit valve from the swash plate piston. When oil pressure is equal to the spring pressure of the pressure limit valve, the spool shifts sending flow to the swash plate piston. Oil flow extends the swash plate piston adjusting the swash plate to hold the system pressure no higher than 21800 ± 400 kPa (218 ± 4 bar) (3100 ± 50 psi). The pump remains at the high pressure standby mode until the load is overcome or the operator commands the valve returned to neutral.

Limited pressure is available to the signal line. The bleed-off orifice relieves pressure in the signal line circuit. Destroking the variable flow piston pump returns the system to low-pressure standby.