Axial Seals

Design, Function, and Selection

An axial seal seals between two planar (flat) sealing surfaces that are joined perpendicular to the component axis. In other words, it acts wherever two components meet “face to face” or “surface to surface” — for example, at covers, flanges, or housing connections. As a result, sealing technology often classifies the part as a face seal (also called an end-face seal).

The functional principle relies on axial compression. When the joint is tightened or clamped, the sealing element is compressed in the axial direction (also referred to as axial squeeze). This generates a contact pressure between the seal and the sealing surfaces, which bridges micro-roughness and closes the leakage path. In most cases, axial seals operate statically — that is, without relative motion between the sealing surfaces. Some applications, however, involve slow or short-stroke movement, and that case must be addressed specifically in the design.

Axial Seal vs. Radial Seal

The difference comes down to the geometry of the sealing point and the direction in which the seal is deformed. A radial seal works on a cylindrical surface (a shaft or a bore) and is deformed or pressed radially. By contrast, an axial seal works between flat surfaces and is axially compressed.

Distinction from the Mechanical Seal

A mechanical seal also seals across face surfaces, but it is engineered for rotating, dynamic applications. It uses controlled face pairings (for example, a hard material running against carbon) and maintains defined operating conditions at the sliding face, so that friction, heat, and leakage stay under control.

By contrast, axial seals in the usual sense of sealing technology are typically elastomer- or polymer-based face seals that build up contact pressure through clamping and operate predominantly statically. Once rotation or continuous relative motion is present, the task usually shifts toward a mechanical seal or a face seal specifically engineered for that purpose.

Construction, Types, and Typical Applications

Axial seals appear wherever two components are pulled together axially and a medium must not escape along the joint. The medium can be hydraulic oil, another fluid, gas, or — in special cases — vacuum. What matters is the path of leakage: at axial sealing points, the medium typically migrates along the joint plane, and the seal must block that path through effective contact pressure.

In practice, axial seals are commonly found in:

• Housing covers and inspection covers (for example, in hydraulic power packs)
• Flange connections on pumps, valve blocks, or manifolds
• Face-seal connections with flat-mating sealing surfaces
• Valves and seat seals, where force is introduced axially

Elastomer O-Ring as an Axial Seal

The most common version is the O-ring seated in an axial groove (a groove being a defined installation space, also called a gland). When the joint is tightened, the O-ring is compressed in the axial direction. This compression generates the necessary surface pressure, allowing the O-ring to “bridge” the surface roughness and form a sealing line — or, more precisely, a sealing zone.

O-rings are often a good fit here because they are standardized and available in many materials. Still, the design must match groove and compression to the application, so that the ring is not damaged and does not develop excessive permanent set.

Profile Seals and Spring-Energized PTFE Seals

Profile seals use a deliberately shaped cross-section, frequently with a larger contact area than an O-ring. As a result, the joint can become more tolerance-robust because the contact pressure is distributed across a wider zone. Such seals are often used in flange connections when assembly effects, surface pressures, or set behavior play a larger role.

Spring-energized PTFE seals combine a polymer (typically PTFE or a PTFE compound) with a metal spring as the preloading element. The spring delivers a defined contact force, even when temperatures change or system pressure is low. In addition, PTFE is highly resistant to chemicals, which can be advantageous with aggressive media or at high temperatures. At the same time, PTFE usually requires careful design support, because it behaves differently than elastomers.

Design: Groove, Compression, Surfaces, and Extrusion Safety

The tightness of an axial seal depends heavily on the mechanical installation conditions. For that reason, anyone designing a seal must not only choose the material but, above all, decide how the compression is generated, where the seal sits, and which gaps may open up under pressure.

Axial Compression (“Squeeze”) as the Key Parameter

Compression describes how strongly the seal cross-section is squeezed during clamping, usually expressed as a percentage change relative to the original cross-section. It is a central parameter because it directly determines the contact pressure.

Insufficient compression often leads to leakage, because the seal cannot compensate for surface unevenness. Excessive compression, by contrast, raises assembly forces and can mechanically damage the seal — for example, through pinching, shearing at edges, or accelerated permanent deformation. In addition, the tolerance chain of components, bolt preload, and thermal expansion act on the joint in practice. Therefore, compression must be chosen so that it stays within the functional range even under unfavorable tolerance conditions.

Extrusion Gap and Pressure Direction

Under pressure, sealing material can be pushed into a gap. This mechanism is called extrusion. It becomes critical when an extrusion gap is present between the components and the pressure is high enough to force the material into that gap. As a consequence, burr formation, damage, and ultimately leakage can result.

Three design measures help here: reducing the gap dimension, choosing a more pressure-resistant or harder material, or installing back-up rings, which support the sealing material at the gap. In addition, the pressure direction matters. For many static face seals, it is favorable to position the seal so that pressure “energizes” it in a useful direction — that is, increases contact pressure rather than pulling the seal out of its groove.

The sealing surfaces themselves are also decisive. Roughness and flatness determine how well the contact zone closes. With gases or vacuum, the requirements often increase, because even small surface channels can produce measurable leakage.

Material and Selection Criteria — A Practical Check

When selecting an axial seal, the first question is what is being sealed and under what operating conditions. An oil circuit at moderate temperatures places different demands than a chemically aggressive medium or a gas application with strict leak-tightness requirements. After that comes the question of how the seal is loaded: statically, with micro-movements, or with repeated assembly cycles.

The following table groups together practical criteria that typically drive the decision in sealing technology:

Quick Check: Which Seal Type, When?

• O-rings often work well when the application is static, the geometry can be implemented close to the standard, and medium plus temperature stay within the typical elastomer range.
• Profile seals are frequently the right choice when a larger contact area is desired or when the joint should become more robust to assembly effects and tolerances — for example, with flange connections under variable bolt preload.
• Spring-energized PTFE seals typically fit when chemical or temperature resistance is the priority, or when a defined contact force must remain stable under changing conditions and at low pressures.

For demanding applications — such as high pressure, very small allowable leakage, or unfavorable tolerance chains — a specialized design or expert consultation is usually advisable.