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| Illustration 1 | g00886859 |
Components of the Front Propel Pump (1) Valve Plate (2) Inlet/Outlet Port (3) Barrel Assembly (4) Servo Piston (5) Piston (6) Electrical Displacement Control (EDC) (7) Shaft (8) Charge Pump (9) Inlet/Outlet Port (10) Spring (11) Swashplate (12) Housing | |
When the engine is running, the following components are rotating: shaft (7), barrel assembly (3) and charge pump (8). There are nine pistons (5) in barrel assembly (3). Valve plate (1) and swashplate (11) are fastened to housing (12). Valve plate (1) and swashplate (11) do not rotate. Spring (10) keeps a force on barrel assembly (3) in order to make a high pressure seal between barrel assembly (3) and valve plate (1). When barrel assembly (3) is rotating, pistons (5) follow the angle of the swashplate. If the angle of the swashplate is at zero, the pistons do not move in and out of the barrel. Also, there is no oil flow. Charge oil from charge pump (8) maintains oil pressure in the propel pump. The oil pressure in the propel pump performs the following functions: keeping barrel assembly (3) full of oil, lubricating the components of the pump and recovering the internal loss of oil due to leakage.
The position of swashplate (11) is controlled by electrical displacement control (EDC) (6) and by servo piston (4). When EDC (6) receives an electrical signal from the rotary switch on the propel lever, the electrical signal causes a valve inside EDC (6) to shift. The servo valve routes control oil in order to activate servo piston (4). The servo piston controls the direction of swashplate (11). Also, the servo piston controls the amount of the angle of swashplate (11) .
Oil flows from the propel pump to the propel motor and back to the propel pump through inlet/outlet ports (2) and (9). The position of swashplate (11) determines the direction of flow. The position of swashplate (11) also determines the line that is the high pressure line.
Charge Relief Valve
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| Illustration 2 | g00886862 |
Front Propel Pump (8) Charge Pump (13) Charge Relief Valve (Neutral) | |
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| Illustration 3 | g00886863 |
Front Propel Pump (14) Charge Pump Inlet (15) Main Loop Ports | |
The front propel system contains two charge relief valves. Neutral charge relief valve (13) is located in the propel pump. The forward/reverse loop flushing charge relief valve is located in the front propel motor.
Charge oil enters the pump through charge pump inlet (14). Neutral charge relief valve (13) provides a path of relief through the pump case drain to the tank. As the pump is stroked, oil (high pressure) shifts a shuttle valve in the motor. This allows the low pressure side of the loop to act against the flushing relief valve.
Oil that is flushed from the motor flows through the pump case drain to the oil cooler. Oil from the oil cooler then flows to the tank. The process of flushing allows fresh charge oil to replenish the hydrostatic loop. This keeps the components cool and clean during the operation.
Charge Pump
Charge pump (8) is a gerotor type pump. The following functions are functions of charge pump (8) : drawing oil from the hydraulic tank through a filter, supply oil in order to replenish the main hydrostatic loop, supply charge oil in order to fill the main loop through the charge check function of the multifunction valves and supply oil to the electric displacement control (EDC).
Multifunction Valves
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| Illustration 4 | g00886864 |
Front Propel Pump (17) Multifunction Valves | |
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| Illustration 5 | g00886869 |
Multifunction Valves (18) Pressure Limiter Section (19) Main Relief Valve Section (20) Charge Check Valve Section (21) Adjustment Screw (22) Bypass Actuator Nut (23) Pressure Limiter Valve | |
The propel pump contains two multifunction valves (17). One valve is for the forward circuit and one valve is for the reverse circuit. Each valve performs the following functions in the circuit:
- pressure limiter valve
- bypass circuit
- charge check valve
- high pressure relief valve
High Pressure Function
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| Illustration 6 | g00886873 |
Multifunction Valve Cartridge (21) Adjustment Screw (23) Pressure Limiter Valve | |
The high pressure function of the multifunction valve operates in the high pressure side of the main circuit. Pressure limiter valve poppet (23) acts as a pilot valve for the main relief pressure. When the system pressure reaches the pressure limiter setting of 38600 kPa (5600 psi), poppet (23) is forced off the seat. High pressure oil flows to the control circuit in order to destroke the pump. When the pump is destroked, the output pressure of the pump is reduced. This function maintains system pressure. The function may occur when the machine is operating under heavy load conditions. The pressure limiter setting is adjusted with adjustment screw (21) .
Bypass Circuit Function
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| Illustration 7 | g00886874 |
Multifunction Valve Cartridge (21) Adjustment Screw (22) Bypass Actuator Nut (24) makeup valve (25) Pressure Limiter Valve Spring | |
Bypass actuator nut (22) is used when conditions require flow but there is no pressure in the propel system. Loosen bypass actuator nut (22) by three turns. The force of pressure limiter valve spring (25) is removed. When bypass actuator nut (22) is open, very low oil pressure lifts makeup valve (24) off the seat. Oil flows from the high pressure circuit to the low pressure circuit, as shown.
Adjustment screw (21) is a part of bypass actuator nut (22). When bypass actuator nut (22) is moved, the adjustment screw moves the same number of turns, so the main relief valve setting does not change. After the machine has been towed, tighten bypass actuator nut (22) by three turns. Tighten the nut to the torque specification. Adjustment to the setting of the main relief valve is not necessary for this function.
Makeup Check Valve
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| Illustration 8 | g00886875 |
Multifunction Valve Cartridge (24) Makeup Valve (26) Pressure Limiter Valve Spring (27) Charge Check Valve | |
The makeup valve function of the multifunction valve operates in the low pressure side of the main circuit.
The right side of makeup valve (24) and the charge check valve seat is open to the charge oil circuit. As leakage and loop flushing occurs in the closed circuit, the charge oil pressure and the low pressure side of the closed circuit vary. Charge pressure overcomes the force of pressure limiter valve spring (26) and the oil pressure of the low side of the closed circuit. The pressure moves makeup valve (24) to the left. Makeup valve (24) contacts a snap ring on check valve (27). Makeup valve (24) continues to move as charge check valve (27) raises off the seat. When check valve (27) is off the seat, charge oil flows across the seat in order to replenish the closed circuit.
High pressure oil from the propel pump flows through the opening. The oil acts on the inlet end of the high pressure valve. The oil acts on the opposite end of the charge check valve. As the circuit pressure increases, the charge oil pressure is overcome. Charge check valve (27) shifts to the right. Charge check valve (27) will also reseat. This stops the flow between the circuit and the charge circuit.
High Pressure Relief
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| Illustration 9 | g00887677 |
Multifunction Valve Cartridge (27) Charge Check Valve | |
The high pressure relief function of the multifunction valve operates in the high pressure side of the main circuit. This function protects the circuit from high pressure spikes. When the system pressure reaches the high pressure relief setting, charge check valve (27) is forced off the seat. This action dumps high pressure oil instantly to the low pressure side of the circuit, as shown. This condition may occur if the machine encounters a large external force. The high pressure relief is set at a pressure of 3445 kPa (500 psi) above the pressure limiter setting.
Electrical Displacement Control (EDC)
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| Illustration 10 | g00886876 |
Electrical Displacement Control (EDC) (1) Pressure Control Pilot Valve (PCP Valve) (2) Path to the Servo Piston (3) Path to the Servo Piston (4) Metering Spool (5) Control Linkage (6) Swashplate Feedback (7) Path from the Charge Pump | |
The electrical displacement control (EDC) is a two-stage electro-hydraulic control. The EDC uses swashplate feedback (6) and ported oil from charge pump (7) in order to set up a closed swashplate circuit. The EDC receives electrical input from the ECM that is mounted in the operator's console on PCP valve (1) .
Current from the ECM meters charge oil pressure in PCP valve (1) to metering spool (4). This causes metering spool (4) to shift. When the metering spool shifts, passages (2) and (3) open. The valves allow oil to pass from the EDC to the servo piston. The oil flows to the EDC and returns to the hydraulic tank. Oil that is acting upon the servo piston causes the piston to shift. The piston then rotates the swashplate.
One end of control linkage (5) is attached to the metering spool. The other end is attached to the swashplate. The control linkage provides mechanical feedback between the swashplate and metering spool (4). As the swashplate rotates, control linkage (5) moves with the swashplate. When control linkage (5) moves, force is created on metering spool (4) by way of a spring collar. The reaction force and the pressure from PCP valve (1) balance the swashplate at an angle. The pump stroke angle is proportional to the ECM input to the EDC.
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| Illustration 11 | g00887059 |
Pressure Control Pilot Valve (PCP Valve) (8) Pole Piece (9) Centering Springs (10) Armature (11) Pole Piece (12) Magnet (13) Magnet (14) Pivot Point (15) Flapper (16) Nozzle (17) Nozzle (18) Oil Supply Port (19) Orifice (20) Orifice (21) Control Port (22) Oil Return Port (23) Control Port | |
PCP valve (1) is the first stage of electrical displacement control (EDC). The PCP valve controls the speed and the direction of the travel of the machine.
The hydraulic portion of the PCP valve is a closed circuit that uses internal hydraulic feedback from the swashplate piston of the pump. The PCP electrical section receives a direct current input in the torque motor stage. The torque motor stage of the PCP consists of armature (10) that is mounted on pivot point (14) and suspended in a magnetic field air gap. Magnets (12) and (13) are permanent magnets of parallel polarity. Magnets (12) and (13) form a magnetic bridge.
When the EDC is a pull, the armature is centered in the air gap. The equal magnetic forces of the opposing magnets and centering springs (9) keep the armature in the centered position. While the armature is centered, flapper (15) is centered between nozzles (16) and (17). Orifices (19) and (20) are located upstream from the nozzles. A control port is located between each orifice and each nozzle.
In the null position, charge oil enters oil supply port (18) and passes through the two orifices. Because the flapper is centered between the two nozzles, there is no pressure difference. Equal amounts of oil flows through the nozzles and past the flapper. The oil then flows through oil return port (22). Pressures in control port (21) and in control port (23) are equal so that there is no forces on metering spool (4). Metering spool (4) is then centered and the swashplate returns the pump to zero flow.
As current is increased in one direction, the end of the armature becomes polarized. The armature then moves toward the opposing magnetic field. The amount of movement is dependent upon the amount of current that is introduced to pole pieces (8) and (11) .
As current is applied and the armature moves toward magnet (13), a torque shift occurs at pivot point (14). The torque shift causes the flapper to move closer to nozzle (16). As the flapper moves closer to nozzle (16), the pressure rises between nozzle (16) and orifice (20). The pressure increase causes the oil to flow through control port (21) to one side of metering spool (4). The feedback springs cause metering spool (4) to equalize.
Due to the input from the control current, the flapper closes off the nozzle. This sends all oil though the control port and pushes metering spool (4) to the full stroke. This causes a rapid stroke of the servo piston to the position that is commanded. As the oil pressure rises in the passageway between the nozzle and the orifice, the pressure moves the flapper toward the null position. When the torque outputs of the torque motor stage and the pressure feedback are equal, the pilot control system is equalized. When the pilot control system is equalized, the flapper is positioned so that the machine will maintain the speed setting that is desired. Metering spool (4) can be balanced in equilibrium with the forces of the reaction spring from the swashplate feedback.