TUBE A 4H8624 - Caterpillar

Illustration 4 g00270338
Metering Position (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 maintains equal pressure on each end of spool (12) .The force of spring (14) causes pump pressure to be 1400 100 kPa (200 15 psi) greater than signal pressure. This difference is called margin pressure.With the spool in position A, pump pressure is greater than the combined force of spring (14) and of the signal oil pressure in cavity (18). This forces spool (12) to move upward.When spool (12) moves upward, pressure oil flows past the spool. The oil then flows through passage (19). The oil flows past compensator spool (13) and into passage (20). Oil flows through passage (20) to actuator piston (10) .The area of actuator piston (10) is larger than the area of bias piston (4). The force of the oil that is acting against piston (10) overcomes the combined force of spring (3) and of the oil that is acting against piston (4). Piston (10) moves to the right. This causes swashplate (2) to move toward the minimum angle. As the angle of swashplate (2) decreases, pump output also decreases. When the pump pressure is low enough, the combined force of the signal oil pressure in cavity (18) and of spring (14) causes spool (12) to move downward into position B.When spool (12) moves down, the flow of pump output oil into passage (19) is blocked. Oil in passage (20) flows from actuator piston (10) past spool (13). The oil then returns to the case drain.Destroking
Illustration 5 g00632991
Pump and Compensator Operation (2) Swashplate. (4) Bias piston. (6) Bias spring. (10) Actuator piston. (12) Flow compensator spool. (13) Pressure compensator spool. (14) Spring. (15) Spring. (16) Line to signal and pressure reducing valve. (17) Signal line from resolver. (18) Cavity. (19) Passage. (20) Passage. (21) Passage. (22) Passage. (23) Passage. (24) Case drain. (A) Signal pressure oil. (B) System pressure oil. (C) Return oil.Destroking of the pump occurs because of decreased flow demand. The decreased flow demand is met by decreased pump output. The decreased flow demand causes a signal pressure in line (17). This signal pressure combines with the force of spring (14) in cavity (18). This combination of signal pressure and of spring force is less than the pump pressure in passage (21). This causes spool (12) to move upward. The oil behind actuator piston (10) cannot flow through passage (22) to case drain (24). Pump oil now flows through passage (21). The oil then flows past spool (12), through passage (20) and into actuator piston (10). The pump pressure behind actuator piston (10) is now greater than the combined force of bias piston (4) and of bias spring (6). The angle of the swashplate (2) decreases. This decreases the pump output and the system pressure.When the lower flow requirements are met, the flow compensator spool (12) moves downward to the metering position. Swashplate (2)