TM 5-3805-262-343-22. CONTROL VALVE ASSEMBLY MAINTENANCE (CONT)THEORY OF OPERATIOIN (CONT)Main Relief Valve. The main relief valve is apilot operated relief valve. The plunger con-trols pressure at which the main relief valveopens.When a piston rod in a cylinder stops moving forany reason with associated spool in a power po-sition, the main relief valve opens to protectthe hydraulic system. While the main reliefvalve is open, oil flow from hydraulic pump goesthrough the drain sleeve to the return passageand to the hydraulic reservoir.1. When pressure in open center passage is atpressure setting of main relief valve, plungeris pushed off plunger seat.2. Oil flows through plunger seat to return pas-sages and to hydraulic reservoir. This flow ofoil causes pressure inside the poppet to be lessthan pressure on the outside of the poppet. Thehigher pressure on the outside of the poppetpushes poppet off drain sleeve.3. Oil flows through drain sleeve to re-turn passages and to hydraulic reservoir.4. Main relief valve will stay open untilcylinder piston rod moves or associatedspool is returned to neutral.3-646
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