Sealing Gap

Definition and Distinction

The sealing gap is the designed distance between two components at the sealing point that a seal closes against a fluid (gas or liquid). In practice, this gap is not a defect but a deliberate clearance, because components move, have manufacturing tolerances, and deform under load or temperature. Drawings and data sheets also use terms such as extrusion gap, clearance gap, diametrical clearance, or gap dimension S.

In sealing technology, the sealing gap is especially relevant wherever an elastomer or polymer sealing element is loaded by pressure and can yield toward the low-pressure side. It differs from the deliberately generated microgap in mechanical face seals, where a very thin lubricating film (a stable fluid film between sliding faces) is part of the working principle. The sealing gap as meant here, by contrast, describes the geometric clearance into which sealing material can extrude.

Where Does the Sealing Gap Occur in the Application?

The sealing gap typically occurs at guidance and sealing points of linear or piston motion. Common pairings are:

• Piston / cylinder (for example, in hydraulic cylinders): radial clearance between the piston outside diameter and the cylinder inside diameter.
• Piston rod / guide (plain bearing, bushing): the gap between rod and guide in the area of the rod seal.
• Housing / cover: gaps at housing joints when sealing elements sit there and the components shift or deform relative to each other.

Correct interpretation of the dimension matters. Documentation often specifies the diametrical clearance — that is, the difference between the diameters. However, the value decisive for extrusion is usually the radial free gap on the low-pressure side, which results from the diametrical clearance and the actual centering (eccentricity).

Why the Sealing Gap Limits the Maximum Pressure (Extrusion/Nibbling)

The sealing gap limits the maximum allowable pressure because pressure deforms a seal toward the low-pressure side. If the material can yield into a free gap, it is pushed in. This process is called extrusion. Under dynamic motion, the extruded material is often sheared off. The result is chunks breaking out or “nibbled-looking” edges; this damage pattern is called nibbling (edge break-out caused by repeated shearing).

The mechanism depends fundamentally on three quantities: pressure, gap dimension, and material resistance (often approximated through hardness, e.g., Shore A). When pressure rises or the gap widens, the tendency to extrude grows. Dynamic applications are often more sensitive, because motion mechanically loads the extruded areas and removes them faster.

Typical Symptoms and Secondary Damage

Extrusion through an overly large sealing gap typically shows up first on the side of the seal facing away from pressure. In many cases, the damage pattern looks “nibbled”. Functionally, increasing leakage is often noticed, becoming stronger after pressure peaks or load changes.

Typical indicators include:

• Nibbling at the low-pressure edge (edge break-out, fraying).
• Increasing leakage, especially after pressure surges.
• Particles in the medium, because sheared material enters the circuit.

Influencing Factors and Worst-Case Gap Dimension (Design)

For design, what counts is not a nominal dimension but the worst-case gap dimension in operation. It results from the tolerance chain and from operating conditions. Especially for rod and piston seals, the largest local gap is often determined by eccentricity (non-concentric position), wear of the guide, thermal expansion, and elastic deformation (for example, rod bending under transverse load).

The mode of operation also directly affects the allowable gap size. A static seal often tolerates more, because no shearing occurs through motion. For dynamic seals, the limit shifts considerably downward. In practice, manufacturer handbooks and tables are therefore often used; they specify allowable gaps as a function of pressure and material hardness. These limit values are application-dependent, because medium, temperature, and motion profile co-determine material behavior.

Too Small vs. Too Large: A Design Trade-Off

A very small sealing gap reduces extrusion risk, yet it often raises the mechanical load on the system. A robust design therefore seeks a range that covers manufacturing and operation without aiming for “zero gap”.

Measures to Increase Pressure Capability (Gap Management)

When pressure and gap together become critical, pressure capability can be raised through gap management. The most important measure is back-up rings (also called anti-extrusion rings). These are dimensionally stable rings, frequently made from PTFE or other thermoplastics, that bridge the free gap on the low-pressure side and mechanically support the seal. As a result, the softer sealing material can no longer yield into the gap under pressure.

In addition, the sealing gap can be reduced through design — for example, through a suitable guidance and bearing concept, tighter tolerances, or a geometry that better limits eccentricity. Another lever is material hardness: harder materials tend to extrude less, but they can worsen friction, low-pressure tightness, or assembly behavior. Therefore, hardness is usually specified together with back-up rings and a realistic worst-case gap assessment.

Back-up Ring Selection: One-Sided or Two-Sided

A back-up ring acts where the gap on the low-pressure side is open. With one-sided pressure direction, the back-up ring therefore sits on the low-pressure side of the seal. With alternating pressure direction, two back-up rings are often used so that support acts in both directions. Pressure peaks should be considered as a load case as well, because they often trigger extrusion even when the continuous operating pressure looks uncritical.

For complex load cases or high pressures, specialized design consultation is advisable, because gap, guidance, material, and seal geometry must be assessed together.