VALVE A 0006133 - Caterpillar

Illustration 1 g01097564
(1) Spool (2) Port for pilot oil (3) Port for signal oil (4) Passage (5) Port to drain line (6) SpringUnder normal operating conditions, pilot oil from the rear piston pump enters port (2). The pilot oil forces spool (1) against spring (6). Since the force of spring (6) is greater than the pressure of the pilot oil from port (2), spool (1) does not move. Since spool (1) does not move, signal oil from port (3) is blocked from passage (4). As a result, the pressure of the signal oil does not change.
Illustration 2 g01097565
(1) Spool (2) Port for pilot oil (3) Port for signal oil (4) Passage (5) Port to drain line (6) SpringWhen the machine experiences an overspeed condition, the pilot oil from the rear piston pump increases in pressure. The pilot oil enters port (2) and the pilot oil forces spool (1) against spring (6). Since the pressure of the pilot oil is greater than the force of spring (6), spool (1) moves downward. As spool (1) moves downward, passage (4) is open to the signal oil in port (3). The signal oil flows from port (3) through passage (4) to port (5). The oil flows from port (5) through a return line back to the hydraulic tank.