These operations are made with full return spring,
with centering spring and without spring. A full
return spring moves a valve’s stem to its position
at the other end of its travel with the removal of an
operating force that had moved it from this posi-
tion. A centering spring returns a valve’s stem to
its mid-position with the removal of an operating
force that had moved it from this position. Two-
way valves never have centering springs. Three
and four-way valves have centering springs when
the valve is to have the ”inching” feature, that is
the ability to admit or to exhaust quantities of fluid
from the delivery line and then close all ports to
retain the fluid in the delivery line without change.
The stem of the valve having an operator without
spring remains in the position in which an
operating force moves it to. The use of a “D”
PILOTAIR® Valve determines the kind of operator
it has and whether it has a full return spring, a
centering spring or no springs. Lever, button or
treadle operators without springs can be used
alone on a valve as they can move a stem in both
directions. Pedal, cam, pilot, cylinder, or solenoid
operators without springs or with centering
springs cannot be used alone, since they can only
push the stem in one direction, and the centering
spring can only return the stem to its mid-position.
To be used alone, pedal, cam, pilot cylinder and
solenoid operators must have full return springs.
However, these operators without springs or with
centering springs may be used when there is
another operator on the other end of the valve.
Normally a “D” PILOTAIR Valve has one operator,
though in some applications it has two-one on
each end of the valve. Two operators are used
when stem movement in each direction is to be
controlled separately. When used in pairs,
operators without springs or with centering
springs are used. Operators with full return
springs are never used in pairs.
“D” PILOTAIR Valves with either full return
springs or centering springs have “normal” stem
positions; positions to which the springs return
the stems when operating forces acting on the
valves are removed. In two and three-way valves
with full return springs, these positions are either
“normally open” or “normally closed” depending
upon which end of the valve and full return spring
is fastened.
The ends of the “D” PILOTAIR Valve are
identified as “A” and “B”. The “A” end of the
valve is the one with the nameplate. A full
return spring on this end of a two-way valve
or a three-way valve with open exhaust
makes it “normally closed”. A “normally
closed” valve has the passage between its
supply and delivery ports closed when no
operating force is acting on the valve. A
“normally open” valve has this passage open
unless an operating force closes it.
With three-way and four-way valves with
tapped exhaust the placing of the spring is
not critical since supply and exhaust ports are
interchangeable.
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