In an excavator’s hydraulic system, relief valves are critical control components. Their main function is to control and limit the system pressure to prevent overload. Depending on their installation position and function, the common types of relief valves and their roles are as follows:

1. Main Relief Valve

• Location: Usually installed at the main control valve assembly or at the pump outlet.

• Function:

  • Controls the maximum working pressure of the system;

  • Prevents damage to hydraulic components caused by overpressure;

  • Releases excess pressure to maintain system stability when operations are suddenly stopped.

2. Secondary or Safety Relief Valve

• Location: Installed in individual actuator circuits, such as boom, arm, bucket, swing, and travel circuits.

• Function:

  • Sets independent pressure limits for each hydraulic circuit;

  • Acts as a backup protection if the main valve sticks or the main relief valve fails;

  • Prevents local overpressure that could rupture cylinders or pipelines.

3. Pilot Relief Valve

• Location: In the pilot system (e.g., control lever and pilot control circuit).

• Function:

  • Controls pilot pressure (usually lower, around 3–5 MPa);

  • Ensures stable operation of the pilot system;

  • Protects solenoid valves and the pilot pump;

  • Supplies stable control pressure to the main or proportional valves.

4. Travel Relief Valve

• Location: In the travel motor circuit.

• Function:

  • Prevents abnormal pressure rise when climbing or during sudden stops;

  • Protects the travel motor and drive mechanism from damage.

5. Swing Relief Valve

• Location: In the swing mechanism hydraulic circuit.

• Function:

  • Releases excess pressure during sudden stops or fast swings;

  • Prevents damage to the swing motor or swing gear caused by impact loads.

6. Load-holding Relief Valve / Pressure-holding Relief Valve

• Function:

  • Prevents unintended movement of the cylinder or motor caused by gravity or external force;

  • Maintains stable pressure in a specific circuit to avoid sudden drops or bouncing of the equipment.

Summary Table: