7
Section IV
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Operation
Figure 2 illustrates the basic operation of the various types of
valves. The point at which the valve begins to function is
determined by the position of the adjusting screw (which
varies the force exerted by the spring on the spool) referred
to as the pressure setting of the valve. Clockwise rotation of
the screw increases pressure. Counterclockwise rotation
decreases pressure.
The effective areas of the spool exposed to hydraulic pres-
sure in primary chamber A being equal, the spool is hydrauli-
cally balanced and held in a normally closed position by the
force of the spring. When pressure is primary chamber A -
which is effective on the piston through passages C - ex-
ceeds the adjusted pressure setting, the piston is forced up-
ward moving the spool accordingly.
The spool moving upward opens secondary chamber B per-
mitting pump delivery to flow into the secondary circuit, while
maintaining a minimum pressure in primary chamber A equal
to the pressure setting of the valve.
When the primary pressure decreases, compression of the
spring overcomes the hydraulic force effective on the piston
and spool closes blocking flow to chamber B.
Fluid leakage trapped between the piston and the spool es-
capes through the center hole of the spool, into spring cham-
ber D, through drain passage E to chamber B and tank, or
externally to tank depending on the position of the top cover.
Valves with pressure ratings identified as A, B, D or F,
require the pilot piston in the bottom cover.
Valves of the Z (or low pressure rating) do not use the pilot
piston, but use a spool without the center drain hole, in order
to prevent the pilot fluid from escaping through the drain
passage.
In addition to the basic units, some models may include spe-
cial features for specific applications.
Model code breakdown on page 4, describes in detail the
valve types, pressure ranges and special applications avail-
able.
Figure 3 schematically illustrates two special feature valves.
The normally open spool is identified by the letter O in the
model number and the auxiliary remote control feature is
identified by the letter P in the model number.
