TRANSMISSION AR 2378186 - Caterpillar

System Operation
Illustration 1. Fan Drive System. (A) Oil Cooler with Bypass. (B) Steering Valve. (C) Variable Displacement Pump. (D) Case Drain Filter with Bypass. (E) Fixed Displacement Motor with Make-Up Valve. (F) To Tank.The fan drive system receives its oil from the steering circuit. Part of the steering return oil goes directly to tank through an orfice with the remaining oil supplied to the hydraulic oil cooler and then to the fan drive circuit. A variable displacement pump drives the fixed displacement fan motor. The pump and motor case drain oil is filtered before retuning to tank. The fan motor has a make-up valve to prevent motor cavitation during engine deacceleration. Oil from the fan motor returns to tank.
Illustration 2. Fan Drive Pump.
Illustration 3. Compensator Valve.The fan drive pump is an axial piston pump. Pump output is controlled by a compensator valve group. The compensator group senses both pressure and flow requirements for the system. The pump provides the high pressure oil for the fan drive system. The pump is mounted to the front housing of the engine.
With the engine running, drive shaft (1) turns. The drive shaft causes cylinder barrel (8) to also turn. Nine pistons (7) are held against swashplate (3) by retaining plate (6). The swashplate does not rotate.
At maximum swashplate angle, some of pistons (7) are pulled out of cylinder barrel (8). At the same time some of them are pushed into the cylinder barrel. The rotation of the cylinder barrel causes the pistons to move in and out of the cylinder barrel.
As a piston moves out of its barrel it draws oil into the pump. As a piston moves back into its barrel it forces the oil back out of the pump.
The angle of swashplate (3) determines how much oil is drawn into and forced out of each piston barrel per drive shaft revolution. The more the swashplate moves towards its maximum angle, the greater the amount of oil drawn into and discharged from the pump.
Rod (11) determines the maximum displacement of the pump.
The compensator valve group keeps pump pressure and flow at a level needed to regulate maximum fan rpm. The compensator group contains a control piston which works to continually adjust the angle of the pump swashplate.
Stroking piston (5) causes swashplate (3) to destroke the pump by moving the swashplate to its minimum angle. This decreases pump output.
Spring (4) causes the swashplate to upstroke the pump. Oil flows from the yoke stroking piston to case drain. Spring (4) works against stroking piston (5) to move the swashplate and changes pump displacement by regulating the amount of swashplate angle.
Pump outlet pressure is maintained at 8680 kPa (1260 psi) by the pressure compensator spool.
The pump supplies the fan drive motor with enough oil flow at 1720 engine rpm to drive the fan at full speed (685 rpm). At speeds above 1720 rpm, the pressure compensator valve on the pump, decreases the pump displacement to maintain fan speed at full speed (685 rpm).
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