CP56, CS56, CP64, CS64, CP74 and CS74 Vibratory Compactors Propel System Piston Pump (Propel) Caterpillar


Piston Pump (Propel)
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Illustration 1g01415890
Located Under Operator Platform
(1) Axle interlock solenoid
(2) Reverse pressure tap (axle pump)
(3) Reverse balance line
(4) Forward balance line
(5) Reverse pressure tap (drum pump)
(6) Drum interlock solenoid
(7) Forward pressure tap (axle pump)
(8) Forward pressure tap (drum pump)
(9) Forward combination valve (drum pump)

The machine is equipped with two propulsion pumps. One pump is for the axle drive system, and one pump is for the drum drive system. The propulsion pumps are located under the operator platform. The axle pump is driven by the engine flywheel, and the drum pump is splined to the axle pump. The propulsion pumps provide flow to the closed circuit, hydrostatic drive system. Functionally, the axle and drum pumps are identical.

The forward circuits of the axle and drum propulsion systems are connected by forward balance line (4). The reverse circuits of the axle and drum propulsion systems are connected by reverse balance line (3). Each balance line contains an orifice that restricts oil flow between the axle circuit and the drum circuit. The orifices prevent all flow from the drum circuit to flow to the axle circuit if the tires begin to spin. The orifices also prevent all flow from the axle circuit to flow to the drum circuit if the drum begins to spin.

The balance lines allow oil to flow between the drum and axle propulsion circuits. This oil transfer modulates the pressure difference in the hydrostatic drive circuits of the drum and axle propulsion systems. Oil flow between the two systems is necessary to compensate for the following situations:

  • Differences in underfoot conditions

  • Differences in speeds between the drum and the wheels during turns

  • Differences in rolling radii between the drum and the tires

Each pump housing contains two combination valves. One combination valve is for the reverse drive circuit and one is for the forward drive circuit. Forward combination valves (9) (one is shown) are located on the right side of the pumps.

The combination valves act as high pressure relief valves, makeup valves, and towing valves. The high pressure relief function of the combination valves limits maximum system pressure in the forward circuit and the maximum system pressure in the reverse circuit to 46500 ± 1000 kPa (6750 ± 145 psi).

Note: The high pressure relief function of the combination valve is fast acting. The purpose of the high pressure relief valve is to relieve peak pressure in the closed circuit. The high pressure relief valve bypasses all pump flow until the slower acting POR valve has time to react.

The makeup section of the combination valves allows charge oil to flow into the low pressure circuit in order to replenish the oil lost to loop flushing and internal leakage.

Each combination valve is equipped with a towing plunger. When this plunger is screwed in, the high pressure relief valve in the circuit is opened. When the forward and reverse towing plungers are both screwed in, the forward circuit and the reverse circuit are connected. In this case, pressure cannot build in the system and the machine can be towed.

Reverse pressure taps (2) and (5) measure pressure in the reverse drive loops of the propulsion circuit. Forward pressure taps (7) and (8) measure pressure in the forward drive loop of the propulsion circuit.

Axle interlock solenoid (1) is controlled by brake relay "P1". Contact "1" of the solenoid is connected to contact "87A" of the relay. When the axle pump interlock solenoid is energized, charge oil is directed to the displacement control spool in the axle pump. The displacement control spool controls the flow of charge oil which acts against either side of the servo piston. The servo piston moves in the servo cylinder bore in order to control the position of the swashplate in the pump.

When the solenoid is de-energized, charge pressure cannot reach the control spool. In this case, the pump destrokes since charge pressure is not available to the servo cylinder bore.

Drum interlock solenoid (6) is controlled by brake relay "P1". Contact "1" of the solenoid is connected to contact "87A" of the relay. When the drum pump interlock solenoid is energized, charge oil is directed to the displacement control spool in the drum pump. The displacement control spool controls the flow of charge oil which acts against either side of the servo piston. The servo piston moves in the servo cylinder bore in order to control the position of the swashplate in the pump.

When the interlock solenoid on the drum pump is energized, charge oil is also directed to the brake release port in the drum drive planetary, and the parking brake is released.

When the interlock solenoid on the drum pump is de-energized, charge pressure cannot reach the control spool. In this case, the pump destrokes since charge pressure is not available to the servo cylinder bore. When the solenoid is de-energized, the brake release port in the drum drive planetary is vented to the hydraulic tank, and the parking brakes are engaged. The brakes are spring applied and hydraulically released.

Note: If the machine is equipped with axle brakes, the interlock solenoid of the drum pump also controls the axle brakes.

Note: The charge relief valves are located on the bottom of the pumps. Each pump has a charge relief valve.



Illustration 2g01415893
Located Under Operator Platform
(10) Displacement control spool (drum pump)
(11) Synchronization link
(12) Neutral start switch
(13) Reverse combination valve (drum pump)

The servo piston in each pump controls the angle of the swashplate in the pump. Displacement control lever (10) (drum pump shown) on each pump provides input to the displacement control spool in the pump and the pump servo. The displacement control lever also operates neutral start switch (12). Synchronization link (11) connects the displacement control levers on the two pumps. If the axle pump control lever is not in the center position, the neutral start switch is open, and the start relay will not energize.

Neutral start switch (12) is activated by the position of the displacement control lever. This switch makes sure that the parking brake remains engaged if the parking brake switch is moved from the ON position to the OFF position while the propulsion lever is out of the NEUTRAL position. The neutral start switch also prevents the starter from cranking the engine when the propulsion lever is out of the NEUTRAL position.

When the propulsion lever is in the NEUTRAL position, neutral start switch (12) is closed. When the propulsion lever is in the FORWARD position, the neutral switch is open. When the propulsion lever is in the REVERSE position, the neutral start switch is open.

Contact "1" of neutral start switch (12) receives power from contact "R" of the engine start switch when the switch is in the ON position or in the START position. When the neutral start switch is closed, power transfers to contact "86" of neutral start relay "P4".

Note: Reverse combination valves (13) (drum pump shown) are located on the left side of the pumps. The pressure override relief (POR) valves (not shown) are located on the lower left sides of the pumps. Each pump contains one pressure override relief valve. These valves destroke the pumps at a system pressure of 44000 ± 1000 kPa (6380 ± 145 psi).



Illustration 3g01415897
Cross Section of the Propulsion Pump
(14) Swashplate
(15) Drive shaft
(16) Servo piston
(17) Feedback lever
(18) Direction control valve
(19) POR valve
(20) Port plate
(21) Barrel assembly
(22) Piston feedback lever
(23) Slipper pad
(24) Shuttle valve
(25) Piston
(26) Spool
(27) Reverse port plate
(28) Orifice
(29) Reverse combination valve
(30) Charge relief valve
(31) Forward combination valve
(32) Forward port
(33) Feedback lever
(34) Direction control valve
(35) Neutral start switch
(36) Servo piston

Engine rotation turns the pump drive shaft. This action rotates the barrel assembly. The pistons in the propulsion pump rotate with the barrel assembly. The piston slipper pads allow the pistons to follow the angle of the swashplate. The propulsion pump only generates flow when the swashplate is not at the minimum angle.

When the propulsion lever is in the NEUTRAL position, both sides of the servo piston are open to the pump case. 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 during rotation. 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. This 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 propulsion pump in order to keep both sides of the hydrostatic loop 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 2600 ± 250 kPa (377 ± 36 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. Flow over the charge relief valve flushes the pump case in order to cool components in the system.



Illustration 4g01415913
Cross Section of the Propulsion Pump
(14) Swashplate
(15) Drive shaft
(16) Servo piston
(17) Feedback lever
(18) Direction control valve
(19) POR valve
(20) Port plate
(21) Barrel assembly
(22) Piston feedback lever
(23) Slipper pad
(24) Shuttle valve
(25) Piston
(26) Spool
(27) Reverse port plate
(28) Orifice
(29) Reverse combination valve
(30) Charge relief valve
(31) Forward combination valve
(32) Forward port
(33) Feedback lever
(34) Direction control valve
(35) Neutral start switch
(36) Servo piston

When the propulsion 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.

The pressure differential between the two sides of the servo piston causes the piston to move. This action tilts the swashplate, and the piston displacement in the rotating group changes. As the swashplate moves, the feedback linkage tends to move the direction control spool back to neutral through an internal feedback spring. This action prevents the servo piston from tilting the swashplate too far by blocking the charge oil supply, once the tilt angle is proportional to the input from the direction control 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.

As the load on the propulsion system increases, increased pressure in the system tends to move the swashplate toward the neutral position. This movement causes the feedback linkage to reposition the direction control spool in order to send additional charge oil into the servo piston. The increased pressure in the servo piston maintains the position of the swashplate in order to maintain pump supply flow.

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 46500 ± 1000 kPa (6750 ± 145 psi), the oil acting against the valve seat compresses the lower spring. This action allows oil from the forward circuit to flow into the charge circuit.

In the reverse circuit, low pressure oil acts against one side of the makeup valve poppet in conjunction with the small spring, while charge oil acts on the opposite side of the makeup valve poppet. Loop flushing causes the pressure in the reverse circuit to fall below charge pressure. In this case, the makeup valve opens, and charge oil flows into the reverse circuit.

Each combination valve is equipped with a towing bypass stud. When the locknut is loosened and the stud is tightened two full turns beyond the first contact with the cartridge assembly, the cartridge assembly is moved against the makeup spring. When this action is performed on both combination valves, oil is allowed to freely flow between the forward and reverse sides of the hydrostatic loop. Manipulation of this stud has no affect on the relief setting of the combination valves.

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 44000 ± 1000 kPa (6380 ± 145 psi), the piston is unseated. The valve spool moves until the passage from the high pressure side of the servo piston is open to the case drain of the pump.

An orifice in the system causes the pressure in the high pressure servo piston cavity to decrease. The springs in the servo then move the swashplate to a smaller angle, and the flow from the pump decreases. Decreased pump flow results in reduced pressure in the high pressure circuit. When the pressure drops below 44000 ± 1000 kPa (6380 ± 145 psi), the POR valve begins to close. The POR valve will reach equilibrium. At this point, the POR valve maintains the pressure in the high pressure circuit at 44000 ± 1000 kPa (6380 ± 145 psi) until the pressure that is required to propel the machine decreases.

The pressure override relief valve is set to a lower pressure than the high pressure relief valves in the combination valves. The lower setting allows the machine to work at high pressures with less heat generation. The pressure override relief valve also reduces the horsepower draw on the engine when the machine is being accelerated.

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