Sequence valves may be installed in one or both
cylinder lines of an actuating system, depending upon
the type of action desired. A direct line will go to the
first unit to be operated, and a branch line goes from
the sequence valve to the second unit.
Mechanically-Operated Sequence Valve
The body of the mechanically-operated (fig. 8-13)
sequence valve is usually aluminum, and contains all
the working parts. As for the number and location of
the fluid ports, there are many variations, depending
upon how the valve is to be used. At least two ports
are needed. Some models have four ports, and those
not needed are plugged. The valve shown in figure
8-13 has two ports.
A contact plunger extends from the body. The
plunger is held in the extended position by a plunger
spring. The valve is mounted so that the plunger will
be depressed by the first unit operated.
A check valve, either a poppet or ball, is installed
between the fluid ports of the body, and is held against
a seat by the check valve spring. The seated check
valve spring prevents fluid flow through the valve.
The plunger, driven into the valve by the first unit,
unseats the check.
The balanced sequence valve will not permit fluid
flow in either direction unless the plunger is
depressed. This check valve, with equal working
areas (balanced), cannot be unseated by fluid pressure
in either direction. Thermal relief valves are needed
in this system.
The unbalanced valve can be unseated by fluid
pressure below it without having the plunger
depressed. This movement allows thermal expansion
to be relieved. Thermal relief valves are NOT needed
in this system.
Pressure from the selector valve goes directly to
the first unit. To operate the second unit, fluid must
pass through the sequence valve, which it can do only
when the check valve is unseated. On completing its
operation, the first unit depresses the plunger on the
sequence valve, which unseats the check valve and
allows fluid to flow through the valve to second unit.
Thus, the second unit cannot operate until the first
unit operation is complete. In reverse, when contact
force is removed from the plunger, the spring extends
it and the check valve reseats.
Improper adjustment of plungers on the
mechanical-type sequence valve is the most common
cause of trouble. If the adjustment is off, it could
cause the second unit to operate too soon or not at all.
The adjustment is made either on the plunger of the
sequence valve or the striker that depresses the
plunger.
Adjustment should be checked at every periodic
inspection. If a valve leaks internally, disassemble,
clean, and inspect the check valve and its sealing
surface.
Replace faulty O-rings. Internal leakage
could cause the second unit to operate before it
should.
Figure 8-13.-Typical sequence valve.
8-17
