Hydraulic seals are used throughout aircraft hydraulic systems to minimize internal and external leakage of hydraulic fluid. They prevent the loss of system pressure. A seal may consist of more than one component, such as an O-ring and a backup ring, or possibly an O-ring and two backup rings. Hydraulic seals used internally on a sliding or moving assembly are normally called PACKINGS. Hydraulic seals used between nonmoving fittings and bosses are normally called GASKETS. Most packings and gaskets used in naval aircraft are manufactured in the form of O-rings.

An O-ring is circular in shape, and its cross section is small in relation to its diameter. The cross section is truly round and has been molded and

trimmed to extremely close tolerances. In some landing gear struts, an elliptical seal is used. The elliptical seal is similar to the O-ring seal except for its cross-sectional shape. As its name implies, its cross section is elliptical in shape. Both the O-ring and elliptical seals are shown in figure 2-42. Advances in aircraft design have made new O-ring composition necessary to meet changing conditions. Hydraulic O-rings were originally established under AN (Air Force-Navy) specification numbers (6227, 6230, and 6290) for use in fluid at operating temperatures ranging from -65F to +160F. When new designs raised operating temperatures to a possible +275F, more compounds were developed and perfected. 

Recently, newer compounds were developed under MS (Military Standard) specifications that offered improved low-temperature performance without sacrificing high-temperature performance. These superior materials were adopted in the MS28775 O-ring, which is replacing AN6227 and AN6230 O-rings, and the MS28778 O-ring, which is replacing the AN6290 O-ring. These O-rings are now standard for systems where the operating temperatures may vary from -65F to +275F.

Packings used in naval aircraft hydraulic installations are manufactured from synthetic rubber. They are used in units that contain moving parts, such as actuating cylinders, selector valves, etc. Although packings are made in many forms, the O-ring type is most widely used. The U-rings, V-rings, and other various types are obsolete in most cases and are not discussed in this training manual.

The O-ring packing seals effectively in both directions. This sealing is done by distortion of its elastic compound. Views A and C of figure 2-43 show O-rings of the proper size and installed in grooved seats. Notice that the clearance for the O-rings is less than their free outer diameter. The cross sections of the O-rings are squeezed out of round prior to the application of pressure. In this manner, contact is ensured with the inner and outer walls of the passage under static (no pressure) conditions. Views B and D of figure 2-43 show the action of the O-rings when pressure is applied. You should also observe, in views C and D of figure 2-43, that backup rings are installed. In hydraulic systems of 1,500 psi pressure or less, AN6227B, AN6230B, and MS28775 packings are used. In such installa-tions, backup rings are not required, although they are desirable. In most modern aircraft with hydraulic system pressures up to 3,000 psi, backup rings are used in conjunction with the MS28775 packings. Gaskets are used in the sealing of boss fittings, end caps of actuators, piston accumulators, and other

Figure 2-43.-Action of O-rings.

installations where moving parts do not come in contact with the seal. Normally, the type of gasket used is an O-ring. In some cases it might be the same seal that is used as a packing in other installations, or it may be one that is manufactured only for use as a gasket.

In hydraulic systems where the operating temperature ranges from -65F to +160F, the AN6290, MS28778, AN6230B-1 through -25, MS28775-013 through -028, -117 through -149, and -223 through -247 O-rings are intended for use as gaskets. In systems where temperature limits range from -65F to +275F, MS28778 and designated sizes of MS28775 O-rings are used as gaskets. Normally, O-rings designated as MS28778 should be used only in connections with straight thread tube fittings, such as boss fittings and end caps of check valves, etc.