A relief valve is a device used to limit pressure in one or more locations in a hydraulic circuit. A relief valve uses a spool or poppet engaged to the closed position by a spring. A spool is a cylindrical piece of machined steel that slides within a machined body. A poppet is a flat piece of machined steel attached to a stem, and the face of the poppet rests against a seat to provide superior sealing. A spool often provides better metering characteristics, but has significantly more leakage than a poppet design.
When pressure rises in the portion of the circuit where the relief valve is installed, force acts upon the spool end or poppet face; the force applied by the spring opposes the force on the spool end or poppet face to keep the valve closed. Valve spring compression force is often variable — compression height can be reduced by an adjustable screw — although effective range is limited (for example, a spring might be effective between 1,000 and 5,000 psi or 100 and 1,200 psi, but rarely between 100 and 5,000 psi).
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