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| Illustration 1 | g01093822 |
The propel pump is located on the right side of the engine compartment. The engine compartment cover provides access to this component.
The propel pump is a variable displacement, axial piston pump. The pump is driven by the engine flywheel. The propel pump provides flow to the hydrostatic drive system.
Pump servo (1) controls the angle of the swashplate in the propel pump. The direction control valve controls the position of the pump servo. The swashplate in this pump is bidirectional.
The housing of the propel pump contains a charge pump. The charge pump supplies makeup oil to the hydrostatic drive system. The charge pump also supplies the oil that is used in order to release the parking brakes. Charge relief valve (5) limits the pressure in the charge circuit to 2480 ± 140 kPa (360 ± 20 psi). Pressure in the charge circuit can be measured at the charge pressure tap (2) .
The pump housing contains two combination valves. Forward combination valve (7) is located on the bottom face of the pump. Reverse combination valve (3) is located on the top face of the pump. The combination valves act as high pressure relief valves, makeup valves, and bypass valves. The high pressure relief function of the combination valves limits maximum system pressure in the forward circuit and the reverse circuit to 48020 ± 1730 kPa (6960 ± 250 psi).
Note: The high pressure relief function of the combination valve cannot divert full pump flow into the low pressure side of the circuit.
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| Illustration 2 | g01093835 |
A mechanical linkage connects the propel lever to direction control valve (9). The direction control valve controls the angle of the swashplate in the propel pump. The angle of the swashplate controls the speed and direction of the machine.
The direction control valve also operates the neutral start switch (8). The neutral start switch is closed when the direction control valve is in the NEUTRAL position. The neutral start switch is open when the direction control valve is in the FORWARD position. The neutral start switch is also open when the direction control valve is in the REVERSE position.
The neutral start switch ensures that the machine cannot be started when the propel lever is out of the NEUTRAL position. This switch also ensures that the parking brake cannot be released while the propel lever is out of the NEUTRAL position.
Interlock solenoid (10) prevents the operator from releasing the parking brake when the propel lever is out of neutral. When the interlock solenoid is energized, the pressure on each side of the pump servo is equalized. In this situation, the swashplate in the propel pump cannot be moved from the zero position.
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| Illustration 3 | g01093841 |
Pressure override relief valve (11) limits the maximum working pressure in the forward circuit and in the reverse circuit of the propel system to 45020 ± 1380 kPa (6525 ± 200 psi).
Note: The high pressure relief function of the combination valve cannot divert full pump flow into the low pressure side of the circuit. However, the combination valve reacts faster than the relief section of the pressure override relief valve. For this reason, the high pressure relief section of the combination valve removes pressure peaks in the system. The relief section of the pressure override relief valve serves as the main relief valve for the circuit.
Piston Pump in NEUTRAL
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| Illustration 4 | g01093842 |
Engine rotation turns the pump drive shaft. This action rotates the barrel assembly. The pistons in the propel pump rotate with the barrel assembly. The slipper pads allow the pistons to follow the angle of the swashplate. The propel pump only generates flow when the swashplate is not at the minimum angle.
When the propel lever is in the NEUTRAL position, both sides of the servo piston are open to the tank. The servo piston springs center the servo piston, and the swashplate remains at the minimum angle. With the swashplate at the minimum angle, the pistons do not move in and out of the barrel assembly as the pistons rotate. Therefore, the pump does not generate oil flow.
If the pressure in either closed circuit line falls below charge pressure, charge oil will act against the corresponding combination valve. Charge oil will unseat the makeup valve poppet. As the makeup valve poppet opens, charge oil enters the corresponding circuit. The charge circuit maintains pressure in the propel pump in order to keep the barrel assembly full of oil. Charge oil lubricates the pump components. Charge oil also replenishes oil that is lost to internal leakage.
If pressure in the charge circuit increases to 2480 ± 140 kPa (360 ± 20 psi), the oil pressure overcomes the spring force, and the charge relief valve opens. This action directs excess charge pump flow into the pump case.
Propel Pump in FORWARD
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| Illustration 5 | g01093846 |
When the propel control lever is moved into the FORWARD position, the direction control valve rotates. Charge oil now flows through an orifice, across the direction control valve spool, and into the chamber on the forward side of the servo piston. The direction control valve spool directs the oil in the chamber on the reverse side of the servo piston into the pump case.
Oil pressure in the forward chamber causes the servo piston to move. As the servo piston moves, the action of the control linkage causes the angle of the swashplate to change. The swashplate angle corresponds to the position of the propel lever.
As the input shaft rotates, the slipper pads follow the angle of the swashplate. This action causes the pistons to move in and out of the barrel assembly. As the pistons move out, oil in the reverse circuit is drawn into the piston chamber. As the pistons move in, oil in the piston chamber is forced out of the forward port.
Pressure in the forward circuit acts against the seat of the high pressure relief section of the forward combination valve. If an external force causes the pressure in the forward circuit to increase above 48020 ± 1730 kPa (6960 ± 250 psi), the oil that is acting against the valve seat compresses the lower spring. This action allows oil from the forward circuit to flow into the charge circuit.
Pressure in the reverse circuit acts in conjunction with the upper spring of the reverse combination valve. Charge oil acts on the lower makeup valve poppet of the reverse combination valve. Charge oil acts with opposite force to the pressure in the reverse circuit. If the pressure in the reverse circuit falls below charge pressure, the lower makeup valve poppet opens. This action allows charge oil to open the upper makeup valve poppet, and charge oil flows into the reverse circuit. When the pressure in the reverse circuit becomes greater than the charge pressure, the upper poppet and the lower poppet seat. Now, charge oil no longer flows into the reverse circuit.
Pressure in the forward circuit acts on one end of the shuttle valve in the pressure override relief valve. Pressure in the reverse circuit acts on the other end of the shuttle valve in the pressure override relief valve. Since pressure in the forward circuit is greater than pressure in the reverse circuit, the shuttle valve shifts. This shift allows pressure in the forward circuit to act on the piston. When the pressure in the forward circuit is greater than 45020 ± 1380 kPa (6525 ± 200 psi), the piston is unseated. The valve spool moves until the passage from the charge circuit is open to the case drain of the pump.
When the charge circuit is open to the case drain of the pump, the pressure in the charge circuit decreases. The spring in the reverse chamber of the servo piston moves the servo piston toward the forward side. This movement reduces the swashplate angle. The pressure in the forward circuit is reduced.