Seals are used to prevent fluids from passing beyond a certain point and to keep air, dirt, or other particulates out of a system. The constantly increasing usage of hydraulics and pneumatics in aircraft systems has created a need for packings and gaskets of differing designs and functions to meet the needs of the speeds and temperatures to which they are subjected. Because no one type of seal is suitable for all installations, there are many different types of seal. Factors that dictate what type is necessary include the pressure at which the system operates, the type of fluid in the system, the metal finish and clearance between adjacent parts, and the type of motion it may be exposed to. Aircraft seals are divided into three primary classes: packings, gaskets, and wipers. This blog will explain all three, their types, and what differentiates them.
There are three types of packings: V-ring, U-ring, and O-ring. V-ring packings are one-way seals installed with the open end of the V facing the pressure. This type of packing must have both a male and female adapter to hold it in the propeller position after installation. It is also necessary to torque the seal retainer to the value specified by the manufacturer of the component being serviced. If not, the seal may not provide satisfactory service. U-ring packings, also called U-cup, are used in brake assemblies and brake master cylinders. The U-ring seals pressure in only one direction, meaning the lip of the packing must sit toward the pressure. U-rings are generally only used in low-pressure applications, where they will not be exposed to pressures greater than 1,000 psi.
One of the most common types of seal is the O-ring. Most packings and gaskets are made in the form of an O-ring, meaning they are circular in shape with a small cross-section relative to its diameter. The cross-section is round and has been molded and trimmed to extremely close tolerances. O-rings are effective in both directions, and sealing is done through the distortion of its elastic compound. Advances in aircraft design have made the design of new o-ring technologies necessary to meet the evolving conditions. Hydraulic O-rings were initially established under Air Force/Navy specifications for use in temperatures from -65 to 160 Fahrenheit. However, because modern aircraft can reach operating temperatures in excess of 275 degrees, new compounds had to be developed.
Another type of seal, gaskets, are used as static seals between two flat surfaces. Common gasket materials include asbestos, copper, cork, and rubber. Asbestos gaskets are used when heat-resistance is needed, primarily in exhaust systems. Many asbestos exhaust gaskets feature a supplementary sheet of copper around its edges. Solid copper gaskets, usually washers, are used in spark plugs where it is necessary to have a gasket that is semi soft yet not compressible. Cork gaskets are often used as an oil seal between the engine crankcase and accessories, where the gasket is required to occupy an uneven or changing space caused by expansion or contraction. Rubber seals are used where there is a need for a compressible gasket. It should never be used in areas where it can come in contact with gasoline or oil, as either of these materials will cause the rubber to quickly deteriorate.
The final type of seal, a wiper, is used to clean and lubricate exposed portions of the piston shafts. Wipers stop dirt from entering the system and help protect the piston shaft from scoring. They can be either metallic or felt. In many configurations, metallic and felt wipers are used in conjunction with one another, the felt wiper being installed behind the metallic.
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