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

Illustration 1	g01415997
Left Side of Machine (1) Drum brake line (2) Drum motor (3) Flushing valve (4) Case drain line (drum motor) (5) Shift valve line (6) Flushing relief valve

The drum motor is located on the left side of the drum. This motor is a piston motor with two speeds. The drum motor drives the drum through a planetary gear reducer. The planetary gear reducer contains an integral parking brake.

The parking brake is spring applied and hydraulically released. The brake inlet line (1) on the motor provides charge oil in order to release the parking brake.

The drum motor receives supply oil from the drum propulsion pump through the forward and reverse circuit lines. Also, the drum motor returns low pressure oil to the drum propulsion pump through the forward and reverse circuit lines.

When the machine is in the high speed range, the shift solenoid is energized. In this case, charge oil enters the propulsion motor through shift valve line (5). In the high speed range, the propulsion motor operates at the minimum angle.

The drum motor contains flushing valve (3). The flushing valve spool directs oil from the low pressure side of the drum propulsion loop to flushing relief valve (6). When the pressure in the low pressure side of the drum propulsion loop is greater than 1800 kPa (261 psi), the flushing relief valve opens. When the flushing relief valve is open, an orifice diverts 8.7 L/min (2.3 US gpm) of oil from the low pressure side of the drum propulsion loop into the drum motor case drain.

The drum motor case drain line (4) directs oil from the drum motor case to the return manifold.

Illustration 2	g01415999
Cross Section of Drum Motor (7) Piston (8) Barrel assembly (9) Minimum displacement limiter (10) Control spool (11) Flushing block (12) Rod (13) Actuator piston (14) Maximum displacement limiter (15) Control plate (16) Flushing spool (17) Flushing relief valve (18) Shift port (19) Check valve

The drum 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.

A pressure differential between the forward port and the reverse port of the motor causes the motor to rotate. When the supply pump is not generating flow, pressure in the forward circuit and pressure in the reverse circuit is equal to charge pressure. In this case, a pressure differential does not exist, and the motor will not turn.

When the motor is not operating, the springs of the flushing spool move the spool into the center position. Also, the flushing relief valve is closed. In this condition, oil is trapped between the inner lands of the shuttle spool and the relief valve, and no flushing flow occurs.

Illustration 3	g01416181
Cross Section of Drum Motor (7) Piston (8) Barrel assembly (9) Minimum displacement limiter (10) Control spool (11) Flushing block (12) Rod (13) Actuator piston (14) Maximum displacement limiter (15) Control plate (16) Flushing spool (17) Flushing relief valve (18) Shift port (19) Check valve

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 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 shifted, the top of the actuator piston will be open to the motor case. Also, oil from the high pressure circuit will be directed to the bottom of the actuator piston. The piston will shift up and the control plate will shift up.

When the supply pump produces flow, 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 will open 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 will open when the supply pump is producing flow. This will result 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 will only allow 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 slightly drops 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 when charge oil 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.