Propel Pump
 | |
| Illustration 1 | g01229283 |
The Propel Pump is Located at the Rear of the Engine. (1) Charge relief valve (2) Forward multifunction valve (3) Reverse multifunction valve | |
The propel pump is mounted on the rear of the engine. The propel pump is a piston pump with a variable displacement. The propel pump contains an integral charge pump. The charge pump is a gerotor type pump.
The propel pump and the internal charge pump rotate when the engine is running. The angle of the swashplate in the propel pump determines the amount of flow that the pump generates. The charge pump is a positive displacement pump. The speed of the engine determines the amount of flow which the charge pump generates.
The charge pump provides makeup oil for the propel circuit. The charge pump also provides the oil which moves the servo piston in the pump and the oil which shifts the propel motors.
Oil from the charge pump flows to an externally mounted filter. Filtered oil then flows to the center manifold, to the flow divider valve, and to the propel pump.
Inside the propel pump, charge oil is sent to the following components:
- Charge relief valve (1)
- Forward multifunction valve (2)
- Reverse multifunction valve (3)
- Pump EDC
Charge Relief Valve
Charge relief valve (1) limits the pressure (absolute) in the charge circuit to 2410 ± 69 kPa (350 ± 10 psi).
Multifunction Valves
Forward multifunction valve (2) and reverse multifunction valve (3) act as the following:
- Charge system check valves
- High pressure main relief valves
- System pressure limiters
- Bypass valves
The pressure limiter function of the multifunction valves limits system pressure to 37700 ± 690 kPa (5470 ± 100 psi). The main relief function of the valves is set approximately 3450 kPa (500 psi) higher than the setting of the pressure limiter valve.
The system bypass function of the multifunction valves allows a disabled machine to be towed slowly for short distances. In order to prevent the propel circuit from building pressure, the middle nut on the multifunction valve must be loosened three turns.
Propel Pump Electronic Displacement Control (EDC)
 | |
| Illustration 2 | g01229290 |
The Propel Pump EDC is Located at the Rear of the Engine Compartment. (4) Propel pump EDC | |
Propel pump EDC (4) contains a pressure control pilot valve and a metering spool. The machine ECM uses the EDC in order to control the hydraulic output from the pump.
In order to control the EDC, the machine ECM analyzes input from the following:
- Parking brake switch
- Propel lever
- Engine speed signal from the engine ECM
The pressure control pilot valve converts the electrical signal from the machine ECM into a hydraulic signal. The hydraulic signal acts against the EDC metering spool. The position of the EDC metering spool determines the oil pressure in the pump servo piston chambers. The position of the servo piston controls the angle of the swashplate in the propel pump. System software prevents the forward commands and reverse commands from being sent to the propel pumps simultaneously.
Colored Cross Section (Oil Flow)
 | |
| Illustration 3 | g01229295 |
Note: The colors on the cross sections and on hydraulic schematics throughout this presentation signify various conditions within the system. The legend is as follows:
- Red (pump supply pressure, or pressure in the high pressure side of the loop)
- Red and white stripe (reduced pump supply pressure, or oil in the low pressure side of the propel loop)
- Red and white hatch (further reduced pump supply pressure oil)
- Orange (charge or pilot pressure)
- Orange and white stripe (reduced charge pressure)
- Orange and white hatch (further reduced charge pressure)
- Green (line is open to tank)
- Blue (blocked oil)
- Yellow (moving part, or active valve envelope)
The pump input shaft turns the rotating group and the charge pump when the engine is running. The rotating group contains a piston assembly and a cylinder barrel. Charge oil flows through internal passages to the charge relief valve. External lines carry charge oil to the charge filter and back to the propel pump. Filtered charge oil re-enters the propel pump. The filtered oil then flows to the multifunction valves and flows to the EDC.
EDC
The EDC electric coils are mounted in a permanent magnet frame. The armature is mounted on a pivot and suspended in the magnetic field.
Note: A flapper, which is part of the armature, is centered between the nozzles when there is no current supplied to the coils. The electric coils, the magnet, and the armature assembly make up the pressure control pilot valve. This valve is also referred to as a "torque motor".
When the machine ECM sends an output signal to the EDC, electric current flows through the coils. Then, the ends of the armature become magnetically biased as either north or south pole. This causes the armature flapper to move toward one of the nozzles, which causes a pressure increase at that nozzle. The increased nozzle pressure attempts to move the flapper back toward the center position. When the armature flapper force equals the nozzle pressure feedback force, the system reaches equilibrium.
When the system is in equilibrium, a pressure differential exists at both ends of the metering spool. The pressure differential causes the spool to move, which meters charge oil to the appropriate servo cylinder. The angle of the swashplate in the pump then tilts in the amount proportional to the strength of the EDC input signal.
Servo Piston and Swashplate
Metered charge oil from the EDC metering spool acts against one side of the servo piston. The opposite side of the servo piston is vented to the case drain. The pressure of the metered charge oil on the servo piston causes the piston to move. This movement tilts the swashplate, and the movement causes piston displacement in the rotating group. As the swashplate tilts, the rotation of the pump generates oil flow. Pump supply oil then flows through the propel motor. The oil which is flowing through the motor causes the output shaft to rotate.
Feedback Linkage
As the swashplate moves, the feedback linkage moves the barrel feedback in the opposite direction of the EDC metering spool's original motion. Once the angle of the swashplate, metering spool and feedback link have reached equilibrium, the swashplate remains in position proportional to the amount of metering spool movement. As pressure rises in the loop, the swashplate is moved toward neutral. This movement causes the feedback link and the barrel feedback to open the metering spool an additional amount. As the metering spool opens, additional charge oil is directed to the servo piston. This action holds the swashplate in the same position. The feedback linkage prevents the servo from tilting the swashplate too far.
Multifunction Valves
Pump supply oil acts against the bottom multifunction valve. In the high pressure side of the loop, the multifunction valve acts as a pressure limiter the multifunction valve acts as a pressure relief valve. When pump supply pressure reaches the pressure limiter setting, the pressure limiter opens. When the pressure limiter opens, pump supply oil is metered to the end of the servo piston which is open to the case drain. This causes the pump to destroke. This action relieves the excessive load on the propel system.
Note: The pressure limiter function is used because no heat is generated and no load is placed on the engine.
However, a sequenced pressure relief function limits the momentary pressure surges which normally occur with machine acceleration and with a change of direction. These pressure surges are purged into the charge circuit. The relief pressure setting is greater than the pressure limiter setting.
When leakage and flushing occur in the propel circuit, the pressure in the return circuit becomes less than charge pressure. The multifunction valve in the return side of the loop then allows charge oil to flow into the low pressure side of the propel circuit, which replenishes the oil which was lost.
Note: The servo pressure relief valves prevent excessive servo cylinder pressures by purging momentary pressure surges into the charge circuit. These valves are not adjustable.
Multifunction Valve (Low Pressure Side Function)
 | |
| Illustration 4 | g01229302 |
Cross Section of Multifunction Valve | |
In the low pressure side of the propel hydraulic circuit, the multifunction valve acts as a makeup valve.
Low Pressure Operation
The spring chamber of the multifunction valve (on the right side of the valve cartridge) is filled with return oil which comes from the low pressure side of the circuit. Charge oil acts against the effective area of the makeup valve. The cartridge's left spring chamber is filled with charge oil.
Flushing and internal leakage cause pressure in the return side of the closed circuit to decrease. This decrease causes charge pressure to move the makeup valve to the left. The makeup valve contacts the snap ring at the left end of the check valve. When circuit return pressure is low enough, the makeup valve continues to move left. The makeup valve eventually unseats the check valve. Charge oil then flows into the circuit return side and replenishes the circuit.
Multifunction Valve (High Pressure Side Function)
 | |
| Illustration 5 | g01229305 |
Cross Section of Multifunction Valve | |
In the high pressure side of the propel circuit, the multifunction valve acts as a relief valve.
High Pressure Operation
High pressure oil from this circuit does the following:
- Fills the spring chamber (on the right side of the valve cartridge)
- Flows through the cross-drilled holes which are adjacent to the pressure limiter spool
- Flows to the right end of the spool chamber through the clearance between the spool and the housing
When the pressure in the high side of the circuit becomes great enough, the force which acts against the right side of the spool will shift the spool to the left. In this situation, oil is directed to the tank side of the pump servo piston. The pump will then destroke.
Note: The pressure limiter can be adjusted when the small locknut is loosened and the limiter adjusting screw is turned.
If a pressure surge occurs, some oil is quickly transferred from the pressure side of the circuit into the charge circuit. At relief pressure, the pressure limiter spool has moved left, which causes a pressure drop in the right spring chamber of the cartridge. This higher pressure acts against the effective area on the right side of the check valve. The higher pressure is sufficient to overcome the combined force from the spring and the pressure on the left. The check valve moves left, which allows some closed circuit oil to enter the charge circuit. Therefore, the limiter and relief functions operate in sequence. Main relief pressure is controlled by the pressure limiter setting, and therefore is not directly adjustable. Main relief pressure will always be approximately 3450 kPa (500 psi) above the pressure limiter setting.