Radial seals and face seals (also known as axial seals) differ from each other in the direction which compression or squeeze is applied to the seal cross sections. Radial seals have compression applied to their outside diameter (O.D.) and inside diameter (I.D.), as illustrated in Figure 1 below. Face seals on the other hand have squeeze applied to the top and bottom of the seal’s cross sections, as illustrated in Figure 2 below. Radial seals are commonly used in cap and plug, piston and bore type applications.
Unlike face seals which are almost exclusively used in static environment, radial seals are evenly divided for usage in both static and dynamic applications. Both types of radial seal applications present different operating environment factors and therefore require different design parameters. The design parameters for both static and dynamic radials are mostly similar. The important design factors to take into account when designing a static or dynamic radial seal include squeeze, fill, stretch, gap size (also known as clearance gap) and the angle of installation chamfer.
Types of radial seals
Static radial seals
Static radial seals operate in an environment in which there is little to no motion relative to the seal and the mating components being sealed. Other than the assembly of the mating components these seals do not see any motion. Cap and plug type applications are the typical use for static radial seals.
Generally, static radial seals are more forgiving because they do not see the wear and tear of constant motion. For this reason they can typically handle more seal squeeze, larger clearance gaps, rougher surface finishes and higher fluid pressures.
Dynamic radial seals
Dynamic radial seals operate in an environment that has a relative reciprocating, rotating or oscillating motion between the mating components. The operational motion can be either intermittent or continuous. In a reciprocating environment, the motion occurs between the inner and outer elements and is typically observed in situations involving a moving piston and a rod. In a rotating setting, either the inner or outer element rotates around the shaft axis in only one direction. A common example of this type of motion is a fan. Oscillating motion involves the inner or outer element of the assembly moving in an arc around the shaft axis within the gland. While being quite similar to a rotating motion, the oscillating one sees movement in both directions – forward and backward, as in a valve.
Although radial seals used in rotating and oscillating applications do exist, those occasions are rare. In dynamic applications, radial seals are mostly seen in reciprocating applications. Because there is a relative motion between the mating components and the seal, the dynamic radial seal is subjected to friction inherent in the operating system.
Examples of radial seals
Double chamfer seals
Double chamfer seals are homogeneous radial seals with flat bases and a chamfered profile, as shown below in Figure 5.a. These seals are mostly used in static and reciprocating dynamic situations, but can also be used in rotational applications under certain circumstances.
D-ring seals (or D-rings) are another style of homogeneous radial seals consisting of flat bases and a rounded sealing surface, creating a cross sectional geometry similar to a capital “D” (Figure 5.b.). These seals are used in very similar applications to the double chamfer seals, mostly in static and reciprocating applications.
In most radial seal applications, seals can be subject to rolling and twisting during installation, consequently causing seal failure during operation. This phenomenon is commonly known as spiral failure. The geometry of both double chamfer seals and D-rings uses the flat bases to provide gland stability and therefore can resist spiral failure in radial seal grooves.
The machined sealing profiles, chamfered and rounded, can aide installation by providing a gradual compression rather than an abrupt installation point. In the seal design process, an installation chamfer is highly recommended for radial seals to ease the installation force and protect the seal. Figure 6 below illustrates the proper design for installation of D-rings and double chamfer, including the recommended chamfer angle design.