Seals

Seals or packing rings (refer to figure below) serve to retain fluids and gases within their respective systems, as well as to exclude air, moisture and contaminants. They also have to withstand a wide range of temperatures and pressures. And due to this, they have to be manufactured in a variety of shapes and materials.

The most common materials, from which seals are manufactured, are natural rubber, synthetic rubber and Teflon (trade name for polytetrafluoroethane or PTFE). O-ring seals effectively seal in both directions of movement. They are used to prevent both internal and external leakage, and are the most commonly used seals in aviation.

FIGURE

Where installations operate at pressures above 10.34 x 103 kN/m2 (1500 psi), additional back-up rings can be used to prevent the O-ring from being forced out or extruded. These back-up rings are usually made from Teflon, which does not deteriorate with age. They are unaffected by system fluids and vapours and tolerates temperatures well in excess of those found in high-pressure hydraulic systems.

O-rings are available in many different materials and sizes (both diameter and thickness). They are supplied in individual, hermetically-sealed, envelopes with all the necessary information marked on the packaging. This system has generally replaced the previously used, colour-coding of seals, which had severe limitations.

For applications (such as in actuators) that subject a seal to pressure from two sides, two back-up rings can be used. But when the pressure is from one side only, a single back-up ring is adequate.

Other seals, commonly found are V-ring and U-ring seals. The V-ring has an open ‘V’, facing the pressure and is located by the use of a male and female adapter. Usually, the U-ring seals will be found in brake unit assemblies and master cylinders, where pressures below 89 x 10³ kN/m² (1000 psi) are encountered. As they only seal in one direction, the concave surface must face towards the pressure.