3-22. CONTROL VALVE ASSEMBLY MAINTENANCE (CONT)
THEORY OF OPERATIOIN (CONT)
Main Relief Valve. The main relief valve is a
pilot operated relief valve. The plunger con-
trols pressure at which the main relief valve
When a piston rod in a cylinder stops moving for
any reason with associated spool in a power po-
sition, the main relief valve opens to protect
the hydraulic system. While the main relief
valve is open, oil flow from hydraulic pump goes
through the drain sleeve to the return passage
and to the hydraulic reservoir.
1. When pressure in open center passage is at
pressure setting of main relief valve, plunger
is pushed off plunger seat.
2. Oil flows through plunger seat to return pas-
sages and to hydraulic reservoir. This flow of
oil causes pressure inside the poppet to be less
than pressure on the outside of the poppet. The
higher pressure on the outside of the poppet
pushes 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 until
cylinder piston rod moves or associated
spool is returned to neutral.