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The following paragraphs (1 through 4) describe these
arrangements. A fifth function can be obtained by the use of a
detent pilot valve. This function is discussed in paragraph five.
1.
No Spring-Floating
: When centering springs are
omitted from the main stage spool, the spool is said to be
floating. If control pressure is removed, a floating spool
can move from its last position under the influence of
gravity or tank line pressure. This must be considered
during the design of the system. Units with floating type
spools have the model code letter omitted. (Example:
DG3S4/DG4S4-062-**-51) Figure 4 illustrates floating spool
positioning in a two-stage valve.
2.
Spring Centered
: A spring and washer arrangement is
used on both ends of the main stage spool in the spring
centered configuration. If control pressure is removed from a
spring centered spool, the valve will go the center position due
to spring force. Two configurations of a two-stage spring
centered valve can be obtained, a type “B” and a type “C”. If
one solenoid is used on a spring centered pilot valve, the
model code is identified with the letter “B”. When two solenoids
are used, the model code is identified with the letter “C”. Figure
5 illustrates spool positioning of a two-stage spring centered
“C” model (i.e. DG5S4-062C-**-51).
Figure 4. No Springs, Floating Model
A
P
T
B
1. Solenoid “A” is energized
and shifts pilot spool to the
right. This opens pressure
to pilot port “A” and the
main stage spool.
2. When solenoid “A” is
energized, the main stage
spool will shift left. Flow from
P-A and B-T is obtained.
3. Main stage spool will stay in
this position until solenoid “B”
is energized, but, may float to
other positions with both
solenoids de-energized.
A
P
B
T
A
B
1. Solenoids are de-energized.
Pilot spool is in center position
due to spring force.
Main Stage Spring
Figure 5. Spring Centered “C” Model
A
P
B
Main Stage
Washer
2. Main stage springs and
washers keep main stage
spool at center position.
Flow is A & B - T with P-
blocked.
When solenoid “A” is
energized, the pilot
spool will shift to the right
causing the main stage spool
to shift left. Flow would then
be P-A and B-T.
T
A
P
B
T
A
B
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