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| Illustration 1 | g00425496 |
Priority Valve | |
The 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.
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| Illustration 2 | g00356637 |
The cross section of the priority valve The 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.
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| Illustration 3 | g00453673 |
The cross section of the priority valve The 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), while the oil is restricted to the brake system (9) .
The force of the spring (11) moves the spool (6) to the right. The increase of the pressure in chamber (10) moves the spool (6) to the right. This is shown in Illustration 3. Supply oil (AA) from port (5) flows to port (8) to the HMU. The flow of oil (BB) to the brake boost circuit is blocked from supply oil (AA). Oil to the HMU flows through the HMU to the steering cylinders. Return oil from the HMU flows to the brake system.
The signal oil from port (13) flows through passage (2), into chamber (4) and to the limiter valve for the signal pressure (3). When the signal pressure in chamber (13) reaches 21360 + 690 - 0 kPa (3100 + 100 - 0 psi), the limiter valve moves against the force of the spring. The limiter valve shifts to the left. The signal oil exits the priority valve through port (1). The signal oil goes to the hydraulic tank.