GEAR 4M7839 - Caterpillar

Illustration 3 g00270338
Metering Positions (A) Spool position at high pump pressure. (B) Spool position at low pump pressure.Slight upward movement and slight downward movement is called metering. Metering is done by equal forces on the spool (12) .With spool (12) in position A, pump pressure is greater than the combined force of spring (18) and the signal oil pressure in cavity (20). This forces spool (12) to move upward.When spool (12) moves upward, pressure oil flows past the spool. The oil then flows through passage (23). The oil flows past compensator spool (12) and into passage (22). Oil flows through passage (22) to actuator piston (10). This action causes the pump to destroke.The area of actuator piston (10) is larger than the area of bias piston (4). This difference causes the oil force acting against actuator piston (10) to overcome the combined spring (6) force and oil acting against bias piston (4). Actuator piston (10) moves to the left. This causes swashplate (2) to move toward the minimum angle.As the angle of swashplate (2) decreases, pump output also decreases. When pump pressure decreases enough, the combined force of signal pressure in cavity (20) and spring (18) causes spool (13) to move into B.When spool (13) moves down, the flow of pump output oil into passage (20) is blocked. Oil in passage (22) flows from actuator piston (10) past spool (12). The oil then returns to the case drain.Destroking
Illustration 4 g00297452
Pump and Compensator Operation (1) Drive shaft. (2) Swashplate. (4) Bias piston. (6) Bias spring. (10)