The operator shifts pedal Valve 1 which causes
the main valve, 3, to also shift, starting the cylinder down. When
full air pressure has built up behind the piston, indicating full
tonnage has been reached, air flows across sequence Valve 4 and
shifts Valve 2. This causes the main valve, 3, to reverse,
retracting the cylinder. The operator may release the foot valve
any time during the retraction stroke or after the cylinder has
reached home position.
Manual Operation -
Figure 2. This is similar to the first circuit
but with a pump unloading feature added.
Cam Valve 5 is positioned to be actuated as the
cylinder reaches home position. This vents the pilot-operated
relief valve, 6, and unloads the pump. Since the cam valve vents to
a cylinder port on the pedal valve, full pressure is availablem for
start-up on the next cycle when the pedal valve is actuated.
Automatic reversal is provided by the sequence
valve and this valve is adjusted to "fire" at the hydraulic
pressure corresponding to the desired tonnage.
The main valve, 3, and relief valve, 6, are sized
for full flow; all other valving is 1/8 or 1/4" size.
Figure 3 - Solenoid valve
circuit for compressed air - automatic reduction.
Solenoid Valve Operated Presses
Figure 3. This air press cylinder is controlled with
a foot switch or pushbutton which the operator actuates and holds
during closure of the press. When air pressure behind the cylinder
piston has built up to the pressure switch setting, closure of the
pressure switch contacts energizes a control relay which then locks
in electrically through its own N.O. (normally open) contacts and
the pushbutton. A set of N.C. (normally closed) relay contacts
breaks the valve solenoid circuit, allowing the valve spool to
spring return to its normal position and the cylinder to retract.
The operator may release the foot switch during the return stroke
or after the cylinder has reached home position. The foot switch or
pushbutton must be released and re-actuated to start another
If speed control valves are used, they should be
connected to meter the flow into the blind end of the cylinder.
Meterout speed control does not work well when pressure sensitive
devices such as pressure switches or sequence valves are used to
sense the rise of pressure behind the piston.
Figure 4 - Solenoid valve
circuit for hydraulics - automatic reduction.
Solenoid Valve Operated Presses-
Figure 4. This hydraulic press cylinder is
controlled with a foot switch. The operator steps on the switch to
start the cylinder, and keeps the switch actuated during closure of
the press. When the tonnage has been reached for which the pressure
switch has been adjusted, the pressure switch overrides the
operator's foot switch and causes the cylinder to retract even
though the foot switch is still depressed.
Override action is accomplished in the electrical
circuit with a control relay which pulls in when the pressure
switch contacts close, and the relay locks itself closed
electrically through its own N.O. (normally open) set of
A set of N.C. (normally closed) relay contacts
de-energize valve Coil A, while valve Coil B is energized through 2
sets of N.O. contacts. Double contacts protect the valve in case
the relay coil should burn out. The cylinder starts to retract.
At home position, a cam on the press opens a N.C.
limit switch which de-energizes both solenoid valve coils, allowing
the solenoid valve spool to center, unloading the pump.
The operator may release the foot switch at any
time during the cylinder retraction stroke and the cylinder will
continue to travel to home position. Another stroke cannot be
started until the operator releases the foot switch and actuates it
again. If the operator should release the footswitch during the
forward stroke, the cylinder immediately starts to retract.
Refinements may be added to the basic circuit.
Jog buttons may be connected to the solenoid valve coils. As with
all circuits using pressure switches or sequence valves, speed
controls should be meter-in rather than meter-out type.