10
3. Spring Offset: Single stage spring offset models use one
spring to return the spool to an offset position. In two-stage
models, the spring and washer is removed from the main
stage and offset action is obtained from the pilot valve.
Offset pilots have a solenoid removed from the spring end of
the valve. Spring offset pilots have a solenoid removed from
the spring end of the valve. Spring offset pilots control the
main stage when the solenoid is de-energized, through
spring action, so long as pilot pressure is available. Spring
offset valves have the letter ‘A’ stamped into the nameplate.
Figure 5 illustrates an offset two-stage valve.
Figure 5. Spring Offset ‘‘A’’ Model
The spring returns pilot spool to ‘‘A’’ offset position
and shifts main stage spool. Main stage oil flow is
P
³
A and B
³
T.
Solenoid is
de-energized
Main
Stage
T
A
P
B
T
B
P
B
A
WARNING
If pilot pressure is lost, mainstage spool is free to float.
4. Pressure Centered: Pressure centered valves provide more
positive centering arrangement than normal spring centered
valves. This is accomplished in the following manner:
Assume both pilot valve solenoids are de-energized and the
main stage spool is positioned to the left (see Figure 6). Pilot
pressure is applied to both ends of the main stage from the
number seven (7) pilot spool. The sleeve moves to the right
under the influence of the spring and pilot pressure until the
position shoulder is contacted. Since the sleeve and piston
areas are greater than the total spool area at the right hand
end of the valve, the sleeve continues to move to the right
carrying the piston with it until it contacts the valve body
(center position).
Assume the main stage spool was positioned to the right
with both pilot valve solenoids de-energized, pilot pressure is
applied to the sleeve and piston areas on the left side but the
sleeve is bottomed against the valve body at this time. Only
the piston area applies force to the left end of the spool.
Since the spool land area at the right side is greater than the
piston area, the spool will be forced to the left until the piston
shoulder butts against the sleeve (center position).
If pilot pressure falls below 20 bar (300 psi), the centering
springs will cause the spool to center within the valve body.
Figure 6. Pressure Centered ‘‘D’’ Model
1.
2.
3.
Shoulder
of piston
Piston
Sleeve
A
B
P
T
T
A
B
P
B
A
Solenoids are de-energized. Pilot spool is in center
position. (P
³
A & B, T blocked) (shown).
Pilot pressure keeps main stage spool in center
position. Flow is blocked to all ports.
When solenoid ‘‘B’’ is energized, the pilot spool shifts
to the left. Oil under pressure enters piston area
causing the main stage spool to shift to the right. Main
stage flow from P
³
B and A
³
T is obtained.
Pressure centered valves are not available with integral
check valves. Pressure centered models have the letter ‘D’
stamped into the unit nameplate. Figure 6 illustrates
spool/spring arrangements on pressure centered models.
Note
A fifth condition of the main stage spool can be
obtained through the use of a detent pilot stage.
Refer to the following paragraph.
5. Detent Valve Operation: Detent valve operation can be
achieved by installing a detent into the pilot valve. A detent is
assembled on one or both ends of the pilot spool depending on
the type of pilot valve used. When a pilot valve solenoid is
de-energized, the detent holds the pilot spool in the last position
attained and the main stage spool remains in its last position.
WARNING
If pilot pressure fails or falls below the minimum
requirement of 5 bar (75 psi), the main stage spool
will shift to center position even though the pilot
valve remains in the last detent position. For this
reason, flow conditions in center or neutral position
must be selected with care.
