Seal Types and Installation Methods

Aircraft seals are essential to prevent fluid buildup and to keep the system clear of air and dirt. Each seal or seal type is unique to each installation. The reasons for this include different pressure rates, the type of fluid used, the type of metal used, and the type of motion the system does.
 
There are 3 classifications of seals; packings, gaskets, and wipers. Hydraulic seals will either be called packings if they are used internally on moving equipment or gaskets if they are used on stationary equipment. The different type of packing seals and gaskets we will cover in this article will be V-rings, U-rings, O-rings, and backup rings.
 
V-Ring packings are one-way seals open “V” facing the pressure. After installation, V-rings must have a male and female adapter to secure them in place. Along with this, it is necessary to torque the seal to the amount indicated in the CMM or else it will not perform to the standards it should.
 
U-Ring packings and U-cup packings are used in brake assembly and master cylinders. For this ring the seals pressure can only operate in one direction, so it is critical to face the lip of the packing toward the pressure. The U-ring and cup are low pressure packings, operating below 1,000 psi.
 
The O-ring is the most common form of packings and gaskets on aircrafts. Circular with a truly round, small cross-section, the O-ring packing is able to seal in both directions. The O-ring is very versatile. It was originally created under the Air Force-Navy and used with fluids operating at temperatures all the way from -65 degrees Fahrenheit to +160 degrees Fahrenheit. The newest models of the O-ring can operate at temperatures up to +275 degrees Fahrenheit. Under Military Standard specifications, newer models are being designed and tested to withstand low-temperature performance without compromising the high-temperature values.
 
Backup rings can withstand the test of time, chemical interference, and extreme temperatures and pressures due to their Teflon exterior. Backup rings are interchangeable and can be switched between any Teflon ring with the same dimensions. For installation, backup rings should be placed below the O-rings to ensure proper functionality.
 
A gasket’s main purpose is to form a stationary seal between two surfaces. These are commonly made from asbestos, copper, cork, or rubber. Caution should be used when working with rubber sheeting and should not be mixed with gasoline or oil.
 
To help identify the many varieties of packings and gaskets, there is a color code system. Packings or gaskets that are capable to be used with Skydrol fluid are coded with a green stripe. Colors can also indicate fluid compatibility; red signifies fuel and blue signifies hydraulic fuel.
 
When installing O-rings, precautions should be taken with the tools being used or anything that may cause a scratch or damage to the component surface. Contact with the cylinder walls, piston heads, and related precision components is also not recommended. O-rings should be inspected for any defaults of defects. After inspection, O-rings should be cleaned in hydraulic fluid. Perform installation with care while avoiding twisting or excessive movement.