The basic principle.
A pressure-relief valve opens a route when the pressure acting on its mechanism overcomes the opposing force. It is commonly used to limit pressure by diverting flow to a lower-pressure return path.
It does not normally control cylinder speed in the way a metering valve does. Pressure behaviour depends on the valve design and flow through it; a nominal setting is not a perfectly constant pressure under every condition.
A load cannot move.
If pump flow enters a circuit that cannot accept it, pressure rises. A correctly designed relief path can divert flow rather than allow an uncontrolled pressure rise.
Follow the relationship.
- Pressure risesActs on the valve mechanism.
- Relief opensCreates a return path.
- Energy dissipatesFluid power becomes heat through the restriction.
Connect it to the lift.
Prolonged relief flow can heat the oil and waste power. A lift that fails to rise should not be “fixed” by increasing a relief setting: load, pump delivery, valve paths and mechanical conditions require investigation against the design.
A closer look
Direct-acting and pilot-operated relief valves have different response characteristics. Pressure-reducing valves perform a different function by controlling pressure in a downstream branch.
Check your understanding
Does a relief valve make unused hydraulic power disappear?
Reveal the explanation
No. Power dissipated through the relief path becomes heat, which the system must manage.
Further reading.
Optional reading to explore this lesson in more detail.
- Blain: service manual (PDF)
Technical example: locate relief valve S and the pump and tank connections in the EV100 cutaway, pages 4–5.