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


Piston Motor (Wheel Propel)
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Illustration 1g01416295
Location is Under the Machine Behind Hydraulic Tank
(1) Axle hydraulic motor
(2) Flushing valve
(3) Shift valve line

The axle motor is located under the machine on the front side of the axle. This motor is a piston motor with two speeds. The axle motor drives the axle through a planetary gear reducer. The planetary gear reducer can be equipped with an optional parking brake.

When the machine is in the high speed range, the shift solenoid is energized, and charge oil enters the axle motor through shift valve line (3). In the high speed range, the propulsion motor operates at the minimum angle.

Flushing valve block (2) is identical to the flushing valve block which is used on the drum motor.

The case drain line (axle motor) directs oil from the case to the return manifold. The axle motor is not shown.



Illustration 2g01416297
Cross Section of Axle Motor
(4) Piston
(5) Barrel assembly
(6) Minimum displacement limiter
(7) Flushing block
(8) Rod
(9) Control spool
(10) Actuator piston
(11) Maximum displacement limiter
(12) Control plate
(13) Flushing relief valve
(14) Flushing spool
(15) Check valve
(16) Control spool
(17) Shift port

The axle motor has a bent axis. The speed of this type of motor shifts when the angle of the motor is changed. The motor can be operated against either the minimum displacement limiter or the maximum displacement limiter. The minimum displacement limiter or the maximum displacement limiter can be adjusted in order to control the minimum and maximum motor speeds.

When the supply pump is generating flow, supply oil enters the motor. Supply oil is directed to the inlet port of the control plate. The control plate directs oil into the piston chamber in the barrel assembly. This pressure forces the pistons which are aligned with the inlet port to move out of the cylinder block.

As the pistons are forced out of the barrel assembly, the barrel assembly and pistons rotate. Since the pistons are connected to the output shaft, the output shaft also rotates.

As the barrel assembly rotates, pistons align with the outlet port in the control plate. The rotation of the barrel assembly forces oil out of the piston chambers and into the low pressure side of the hydrostatic loop. The low pressure oil then returns to the inlet side of the hydraulic pump, which completes the hydrostatic circuit.

With the hydraulic system in the LOW SPEED condition, the shift port is connected to the tank. The springs of the control spool move the spool. In this condition, the control spool directs oil from the high pressure circuit to the top of the actuator piston. This action shifts the actuator piston and this action shifts the control plate down. The bottom of the actuator piston is vented to the motor case.

Note: In order to shift the motor to the HIGH SPEED condition, charge oil is routed to the shift port. Charge oil then shifts the control spool against the spring force. With the control spool shift, the top of the actuator piston will be open to the motor case, and oil from the high pressure circuit will be directed to the bottom of the actuator piston. The piston and the control plate will shift up.

When the supply pump produces oil flow, the oil flushes through the circuit. Oil in the high pressure circuit causes the flushing spool to shift. Oil in the low pressure circuit now flows to the relief valve.

The flushing relief valve opens at 1800 kPa (261 psi). This pressure is lower than the setting of the charge relief valve in the supply pump. As a result, during normal operating conditions the flushing relief valve opens when the supply pump is producing oil flow. This results in oil being flushed from the hydrostatic circuit. An orifice in the motor controls the flow rate. The oil is flushed through the motor case.

Note: The orifice in the motor only allows a small amount of oil from the low pressure loop to flow into the motor case compared to the total amount of charge flow that is available. Therefore, even though the flushing relief valve opens at 1800 kPa (261 psi), because of the orifice, charge pressure only drops slightly when the machine is moving.

The main purpose of the flushing relief valve is to maintain sufficient pressure in order to release the parking brakes if charge flow decreases in the system. Charge flow can decrease for several reasons. This includes excessive internal leakage in the hydraulic system. Without a flushing relief valve, the orifice in the flushing system could allow the pressure in the charge system to fall below the level that is necessary in order to release the parking brakes. However, once the charge pressure reaches 1800 kPa (261 psi) on this machine, the flushing relief valve closes. This action blocks the oil flow through the orifice. In this case, flushing flow will stop. The system may be capable of creating enough pressure in order to unlock the parking brake so the machine can be loaded onto a truck.

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