CP-563E, CP-573E, CS-563E, CS-573E and CS-583E Vibratory Soil Compactors Propel System Piston Motor (Drum Propel) Caterpillar


Piston Motor (Drum Propel)
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1.1. Location and Function
2.1. Components of the Drum Propel Motor
3.1. Oil Flow
4.2. Propel Lever in FORWARD Position and Propel Range Switch in LOW SPEED Position

Location and Function




Illustration 1g00943182

The drum propel motor is bolted to the final drive planetary, as shown. The final drive planetary is bolted to the drum. The drum propel motor receives oil from the drum propel pump. Power is transmitted from the motor to the drum.

Components of the Drum Propel Motor




Illustration 2g00943134

Drum Drive Motor

(1) Drive shaft

(2) Motor case

(3) Retainer

(4) Pistons

(5) Spring

(6) Barrel

(7) Adjustment screw (minimum displacement)

(8) Head

(9) Control spool

(10) Flushing relief valve

(11) Swivel pin

(12) Control piston

(13) Adjustment screw (maximum displacement)

(14) Control plate

(15) Control slot

(16) Control slot

(17) Pivot pin

The drum drive motor is a variable displacement axial piston motor with a bent axis.

The drum drive motor in LOW speed has the following characteristics:

  • High torque

  • Large displacement

The drum drive motor in HIGH speed has the following characteristics:

  • Low torque

  • Small displacement

In LOW speed, the motor has a displacement of 55 cc/rev (3.35 cu in/rev). In HIGH speed, the motor has a displacement of 28 cc/rev (1.71 cu in/rev). When the machine is shifted from LOW to HIGH, the displacement and the torque of the drum drive motor are decreased by one half. At the same time, the rotating speed is doubled. The drum drive motor can operate in either direction.

The following four lines are connected to the motor:

  • Two high pressure loop lines

  • One displacement selector pilot line

  • One case drain line

Oil is supplied to the drum drive motor by the drum propel pump. The direction of the motor rotation is controlled by the oil flow direction from the propel pump. The displacement and the speed are controlled by the shift valve.

The following components of the drum drive motor rotate:

  • Drive shaft (1)

  • Retainer (3)

  • Pistons (4)

  • Barrel (6)

The following components of the drum drive motor do not rotate:

  • Motor case (2)

  • Head (8)

  • Control plate (14)

Oil Flow

Propel Lever in FORWARD Position and Propel Range Switch in LOW SPEED Position




Illustration 3g00943263

Drum Propel Motor

(1) Drive shaft

(3) Retainer

(4) Pistons

(5) Spring

(6) Barrel

(7) Adjustment screw (minimum displacement)

(9) Control spool

(10) Flushing relief valve

(11) Swivel pin

(12) Control piston

(14) Control plate

(15) Control slot

(17) Pivot pin

(18) Rotating group

(19) Flushing shuttle spool

(20) Flushing block

(21) Check valves

(22) Oil port (shift valve)

(AA) System pressure

(AB) Sump

(AC) Activated components

(AD) First pressure reduction




Illustration 4g00943366

Hydraulic Schematic

(9) Control spool

(10) Flushing relief valve

(12) Control piston

(18) Rotating group

(19) Flushing shuttle spool

(21) Check valves

(23) Line from shift valve

(24) Line from drum propel pump

(25) Line from drum propel pump

(26) Line to thermal bypass and return manifold

(27) Line to drum propel pump

(28) Drum brake (piston)

(AA) System pressure

(AB) Sump

(AC) Activated components

(AD) First pressure reduction

(AE) Charge pressure

The drum propel motor will turn when a pressure differential exists between the forward and reverse ports. When the propel lever is in the FORWARD position, the directional control valve in the drum propel pump will control the amount of output oil by moving the swashplate in the drum propel pump. The output oil flow from the drum propel pump is directly proportional to the mechanical input from the propel lever to the directional control valve.

When the propel range switch is in the LOW SPEED position, charge oil (19) (shift port) is vented to the hydraulic tank. The spring in control spool (18) moves the control spool to the right. In this condition, the control spool allows the system pressure from the drum propel pump to flow through the control spool (18) to the top of control piston (11). Control piston (11) will shift control plate (13) downward. The bottom of the servo piston is vented to the motor case drain.

Note: To shift the drum propel motor to HIGH SPEED, the propel range switch should be in the HIGH SPEED position. Charge pressure oil that is coming from the line from shift valve (27) will shift control spool (18). This will allow system pressure oil to enter control piston (11). As a result, the angle of the swashplate in rotating group (17) will become larger. This will cause the speed of the drum propel motor to increase to HIGH SPEED.

When the parking brake switch is in the ON position, the solenoid on the drum interlock valve will be de-energized. The flow of oil in line from drum propel pump (24) will be blocked. As a result, the drum brake will be applied.

Reference: See Specifications, Systems Operation/Testing and Adjusting, "Propel System" for additional information.

Spring (5) pushes barrel (6) against control plate (14). This makes a high pressure seal between the barrel and the control plate. A seal is also made between the control plate and the head.

When system pressure oil flows through the closed loop line, the oil also flows to control slot (15). Oil in the control slot goes into the cylinders of barrel (6) that are over the control slot.

The spherical piston heads are held in the sockets in drive shaft (1) by retainer (3). Seven pistons (4) are held by barrel (6). The barrel rotates around pivot pin (17) which is at an angle to the axis of the drive shaft (1). The design of the bent axis between the barrel and the shaft causes each piston to move. The pistons, the barrel, and the drive shaft rotate as system pressure oil enters the cylinders.

The pistons are fully retracted when the pistons are in the top center position. The cylinder overlaps control slot (15) on the low pressure side of the loop at this point. When the piston starts to move down, the oil is pushed out of the cylinder. The oil also moves out of the control slot and the other closed loop port for the system pressure. The oil then exits to the low pressure side of the closed loop circuit.

The drum drive motor is lubricated by oil leakage from the pistons and the barrel.

The drum drive motor operates at either a large displacement or at a small displacement. When the propel motor operates at a large displacement, the barrel and the shaft are at the maximum angle. At this point, the barrel and the shaft are against the adjustment screw (13) (maximum displacement). When the propel motor operates at a small displacement, the barrel and the control plate are at the minimum angle against adjustment screw (7) (minimum displacement).

Information System:

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