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. 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.
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).
WARNING
Loss of pilot pressure will not guarantee spool position.
Figure 4. Spring Centered ‘‘C’’ Model
Solenoids are de-energized. Pilot spool is in center
position due to spring force. Flow is A & B
³
T with
³
P
blocked.
Main stage springs and washers keep main stage spool
at center position.
When solenoid ‘‘A’’ is energized, the pilot spool will shift
to the right, causing the main stage spool to shift left.
Main stage flow would then be P
³
A and B
³
T.
1.
2.
Main
Stage
Spring
Main
Stage
Washer
T
A
P
B
T
P
B
A
A
B
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).
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
B
T
A
P
B
T
P
B
A
Figure 6. Pressure Centered ‘‘D’’ Model
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.
1.
2.
3.
Shoulder
of piston
Piston
Sleeve
A
B
P
T
T
A
B
P
B
A
If pilot pressure falls below 20 bar (300 psi), the centering
springs will cause the spool to center within the valve body.
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.
