SEAL 1519573 - Caterpillar

Illustration 1 g00425496
Priority ValveThe priority valve is attached to the frame on the left side of the machine. The metering pump (HMU) is located under the floorplate of the cab.
Illustration 2 g00356637
The cross section of the priority valveThe service brakes are applied and the steering wheel is not being turned. (AA) Supply oil. (BB) Blocked oil. (CC) Signal oil. (DD) Hydraulic oil (tank pressure). (1) Outlet to the hydraulic oil tank. (2) Passage. (3) Limiter valve for the signal pressure. (4) Chamber. (5) Inlet port from the pump. (6) Priority valve spool. (7) Passage inside spool. (8) Port to steering system. (9) Port to brake system. (10) Chamber. (11) Spring. (12) Dynamic bleed orifice. (13) Load sensing port.There are five hydraulic lines that are connected to the priority valve. Oil flows from the pump into port (5). Oil flows to the HMU from port (8). Oil flows from port (9) to the brake valve. Signal oil from the HMU enters through port (13). Oil exits the priority valve to the hydraulic tank through port (1) .The steering system has priority over the brake system. When there is no pressure in the system, the spring (11) holds the priority valve spool (6) to the right. This restricts port (9) to the brake system and port (8) is opened to the steering system. When the steering wheel is not turned, the pressure increases in the HMU and in port (8). The oil pressure is felt in passage (7) that is in spool (6). The oil pressure is felt on the right side of spool (6). The pressure shifts spool (6) to the left. This is shown in Illustration 2. Supply oil (AA) from port (5) flows to the brake boost circuit through port (9) .Oil in passage (7) also flows to spring chamber (10), through dynamic bleed orifice (12) and exits through port (13). Hydraulic oil (CC) from port (13) flows through a signal line to the HMU and back to the tank.
Illustration 3 g00453673
The cross section of the priority valveThe service brakes are released and the steering wheel is being turned. (AA) Supply oil. (BB) Blocked oil. (CC) Signal oil. (DD) Hydraulic oil (tank pressure). (1) Outlet to the hydraulic oil tank. (2) Passage. (3) Limiter valve. (4) Chamber. (5) Inlet port from the pump. (6) Priority valve spool. (7) Passage inside spool. (8) Port to steering system. (9) Port to brake system. (10) Chamber. (11) Spring. (12) Dynamic bleed orifice. (13) Load sensing port.When the steering wheel is turned, the pressure decreases in the HMU supply line to port (8). The pressure also decreases in passage (7) and in the chamber on the right side of the spool (6). Signal oil (CC) from port (13) is connected to the HMU. The spring (11) and the pressure in the dynamic bleed orifice (12) in chamber (10) shift the priority valve spool (6) to the right. This allows the supply oil from port (5) to flow to the steering system (8),