end to which the connecting tubing is being installed.
Bypass Check Valves
Do not grip the opposite end. This will prevent the
Bypass check valves serve the same purpose as
possibility of distorting the valve body, causing the
automatic check valves, but are so constructed that
valve to leak.
they may be opened manually to allow the flow of fluid
SEQUENCE VALVES
in both directions. An example of the possible use of a
bypass check valve is in the line between the hand
Sequence valves are used to control a sequence of
pump and the accumulator. Installation of a bypass
operations; they ensure that actuating units operate at
check valve in this line would allow hand pump
the proper time and in the proper sequence. Sequence
pressure to be directed to either the accumulator or the
va l ve s m a y b e m e c h a n i c a l l y o p e r a t e d o r
selector valve.
pressure-operated valves. An example of the use of a
sequence valve is in a landing gear actuating system.
Maintenance of Check Valves
In a landing gear actuating system, the landing
Check valves require little attention over long
gear doors must open before the landing gear starts to
periods of time. Leakage may be caused by the
extend. Conversely, the landing gear must be retracted
presence of a tiny particle of foreign matter between
before the doors close. A sequence valve installed in
the checking device (ball, cone, or poppet) and its seat.
each landing gear actuating line performs this
To remove the foreign matter, it is necessary to remove
function.
the valve from the aircraft and completely disassemble
Sequence valves may be installed in one or both
the valve. If no scratches are found on the valve seat or
cylinder lines of an actuating system, depending upon
the checking device, wash all parts in clean hydraulic
the type of action desired. A direct line will go to the
fluid of the same type as that used in the system.
first unit to be operated, and a branch line goes from the
While the valve is disassembled, inspect the
sequence valve to the second unit.
housing and the checking device for evidence of
corrosion. Replace the valve if there is corrosion or
Mechanically-Operated Sequence Valve
excessive roughness. A slightly rough surface can be
smoothed by buffing. A cone-type check valve may
The body of the mechanically-operated sequence
have a tendency to lean to one side, in which case the
valve (fig. 11-13) is usually aluminum, and contains all
movable part may dig into the soft aluminum body of
the working parts. As for the number and location of
the housing and stick there.
the fluid ports, there are many variations, depending
upon how the valve is to be used. At least two ports are
When you install a check valve, remember that the
needed. Some models have four ports, and those not
arrow marked on the housing must point in the
needed are plugged. The valve shown in figure 11-13
direction of the flow of the fluid through the valve.
has two ports.
Before you remove a check valve from a line, it is good
practice to mark the adjacent structure, indicating the
A contact plunger extends from the body. The
direction in which the arrow points. Also, observe the
plunger is held in the extended position by a plunger
following precaution during installation of check
spring. The valve is mounted so that the plunger will
valves: Grip the wrench flats of the check valve at the
be depressed by the first unit operated.
Figure 11-13.--Typical sequence valve.
11-15
