minimize the introduction of air into the hydraulic
ACCUMULATORS
system. It is important that the fluid used for this
The purpose of the accumulator in a hydraulic
operation be obtained only from an authorized
system is to store a volume of fluid under pressure.
hydraulic fill service unit.
There are several reasons why it is advantageous to
7. Once filled, the filter bowl is carefully and
store a volume of fluid under pressure. Some of these
slowly slid up over the installed element and screwed
are listed below:
into the head. A quantity of fluid from the bowl will
normally be displaced by the element and spilled.
1. An accumulator acts as a cushion against
Provisions must be made to collect or absorb it.
pressure surges that may be caused by the pulsating
fluid delivery from the pump or from system operations.
8. The installed filter bowl should be torqued to the
value specified in the applicable MIM. The bowl is then
2. The accumulator supplements the pump's
lockwired, using standard tools and the lockwire
output when the pump is under a peak load by storing
provisions in the filter assembly.
energy in the form of fluid under pressure.
9. All filter element installations should be
3. The energy stored in the accumulator may be
followed by test and inspection of the system to ensure
used to actuate a unit in the event of normal hydraulic
proper operation. This is generally accomplished by
system failure. For example, sufficient energy can be
operating the system at its normal pressure and flow
stored in the accumulator for several applications of the
rates and inspecting for external leakage at the filter
wheel brakes.
assembly and for indications of excessive differential
There are two general types of accumulators in use
pressure. Any external leakage is unacceptable, and
on naval aircraft. They are the spherical type and the
requires that the system be shut down and the problem
cylindrical type. Until a few years ago, the spherical
corrected.
type was the more commonly used accumulator;
10. Should the filter assembly differential pressure
however, the cylindrical type has proved more
indicator continue to extend after a new element has
satisfactory for high-pressure hydraulic systems, and
been installed, the indicator itself is probably defective.
is now more commonly used than the spherical type.
Consult the maintenance instructions to determine what
corrective action is to be taken.
Examples of both types are shown in figure 12-34.
Inspect the filter element as follows:
Spherical Type
1. Visually inspect the element for dents, broken
The spherical type accumulator is constructed in
wires, holes, creases, and sharp corners of pleats.
Permissible damage is to be confined to small dents that
two halves that are screwed together. A synthetic
will not impede the required flow, or increase the filter
rubber diaphragm is installed between both halves,
pressure drop beyond tolerance, or fail to pass the
making two chambers. Two threaded openings exist in
required bubble test point. Deeper dents, broken wires,
the assembled component. The opening at the top, as
holes, creases, and sharp corners of pleats are cause for
shown in figure 12-34, contains a screen disc that
rejection of elements.
prevents the diaphragm from extruding through the
2. Remove the O-ring from the filter element and
threaded opening when system pressure is depleted,
visually inspect the O-ring groove, including chamfers,
thus rupturing the diaphragm. On some designs, the
for nicks, dents, visible roughness, out-of-roundness,
screen is replaced by a button protector fastened to the
and pitting. Blend out nicks and/or scratches that are
center of the diaphragm. The top threaded opening
deeper than 0.002 inch with crocus cloth P-C-458.
provides a means for connection of the fluid chamber
3. Visually inspect mating surfaces, including
of the accumulator to the hydraulic system.
chamfers, or other parts that mate with the O-ring
The bottom threaded opening provides a means for
grooves. Make sure that all surfaces (grooves and
mating surfaces) are smooth and capable of sealing with
installation of an air filler valve. This valve (when
the O-ring installed.
open) allows an air/nitrogen source to be connected to
and enter the accumulator; moreover, when the valve is
4. Dispose of unacceptable filter elements
according to existing instructions.
closed, it traps the air/nitrogen within the accumulator.
12-34
