The purpose of the accumulator in a hydraulic system is to store a volume of fluid under pressure. There are several reasons why it is advantageous to store a volume of fluid under pressure. Some of these are listed below:

1. An accumulator acts as a cushion against pressure surges that may be caused by the pulsating fluid delivery from the pump or from system operations.

2. The accumulator supplements the pumps output when the pump is under a peak load by storing energy in the form of fluid under pressure.

3. The energy stored in the accumulator may be used to actuate a unit in the event of normal hydraulic system failure. For example, sufficient energy can be stored in the accumulator for several applications of the wheel brakes.

There are two general types of accumulators in use on naval aircraft. They are the spherical type and the cylindrical type. Until a few years ago, the spherical type was the more commonly used accumulator; however, the cylindrical type has proved more satisfactory for high-pressure hydraulic systems, and is now more commonly used than the spherical type. Examples of both types are shown in figure 7-34.

The spherical type accumulator is constructed in two halves that are screwed together. A synthetic rubber diaphragm is installed between both halves, making two chambers. Two threaded openings exist in the assembled component. The opening at the top, as shown in figure 7-34, contains a screen disc that prevents the diaphragm from extruding through the threaded opening when system pressure is depleted, thus rupturing the diaphragm. On some designs the screen is replaced by a button protector fastened to the center of the diaphragm. The top threaded opening provides a means for connection of the fluid chamber of the accumulator to the hydraulic system. The bottom threaded opening provides a means for installation of an air filler valve. This valve (when open) allows an air/nitrogen source to be connected to and enter the accumulator; moreover, when the valve is closed, it traps the air/nitrogen within the accumulator.

Figure 7-34.Pressure accumulator, spherical and cylindrical types.