AIR CLEANER & MOUNTING GP 3843664 - Caterpillar

Illustration 2 g00821612
Upstroking means that the pump is increasing the displacement (output). Upstroking occurs when the signal oil pressure increases due to an increased load. The highest signal oil pressure from the control valves flows through line (18) and the oil fills cavity (17). The signal oil pressure plus the force of spring (1) now move margin spool (16) in the downward direction. The spool movement blocks the flow of pump oil to actuator piston (4). The spool movement also opens the passages to actuator piston (4) to passage (12) and through the case to case drain (10). The combined force of bias spring (9) and of the pump oil pressure that is behind bias piston (8) now moves swashplate (5) toward the maximum angle (upstroke).Pump output is increased by upstroking until the pressure in passage (13) moves margin spool (16) upward to the metering position. When the margin spool is first at the metering position, system pressure in passage (13) exceeds the force of the signal oil pressure in cavity (17) and of spring (1). The greater pressure moves the margin spool farther in the upward direction. This sends pump oil pressure through passage (14) to actuator piston (4) .Because the force of actuator piston (4) is greater than the force of bias piston (8) plus the force of bias spring (9), the force that is on swashplate (5) toward the minimum angle is greater than the force that is on swashplate (5) toward the maximum angle. The difference in force causes a decrease in the angle of the swashplate, the pump output, and the system pressure. When the system pressure decreases enough, the combined signal pressure and the force of the spring in cavity (17) cause the margin spool to move downward. The pressure that is behind actuator piston (4) flows to the case drain again. The bias piston and the bias spring increase the swashplate angle.The continuous oscillation (metering) of margin spool (16) maintains the pump pressure in passage (13) at 2100 kPa (305 psi) higher than the signal pressure that is in cavity (17), because the force of spring (1) is equal to 2100 kPa (305 psi). The difference is called margin pressure.Destroking
Illustration 3 g00821649
Destroking means that the pump is decreasing the displacement (output). Destroking occurs when signal oil pressure decreases from lower loads. Destroking also occurs when all of the control valves are in the HOLD position and signal oil pressure decreases to 0 kPa (0 psi). The lower signal oil pressure in cavity (17) causes the higher system pressure in passage (13) to move margin spool (16) upward. The spool movement allows system pressure to flow through passage (14) to actuator piston (4). The force of the system pressure that is behind actuator piston (4) is now higher than the force of bias spring (9) and the force of the system pressure that is behind bias piston (8). The angle of the swashplate decreases. Pump output decreases until the pump flow is not adequate