Static vs dynamic sealing: the difference that decides groove, hardness and material

When selecting a seal, we ask four questions in a fixed order: what medium, what temperature, what pressure, what motion. The first three almost always get proper attention — the fourth is often waved away. Yet it is precisely motion that decides the groove depth, the compound hardness, the material choice and sometimes whether an O-ring is the right seal type at all. The same ring that holds reliably for years under a gearbox cover will destroy itself in days on a fast rotating shaft. This article covers the practical differences between static and dynamic sealing, with the numbers you actually need at the design stage.

What counts as static and what counts as dynamic sealing

A static seal is a joint where the sealed surfaces do not move relative to each other: a cover, a flange, a plug, a sensor housing, a threaded fitting. The O-ring is compressed once at assembly and from then on works purely through its own elasticity — nothing rubs against it. A dynamic seal is a joint where the surfaces move relative to each other: a piston sliding in a cylinder, a piston rod passing through the cylinder head, a rotating or oscillating shaft. The very surface that has to seal also becomes a sliding surface — and that changes all the rules.

There is also a grey zone: joints that move only occasionally (an adjustment element, a cover removed once per service) or surfaces loaded with vibration and thermal expansion. These are designed as static, but during troubleshooting it pays to think about micro-motion — a leak that appears only during operation and disappears after shutdown is often its signature.

The three differences that follow

Squeeze: roughly 22 % static, 10 % dynamic

An O-ring seals by having its cross-section squeezed in the groove. The ISO 3601-2 recommendations differ exactly along the motion line: a static groove has a depth of roughly 0.78 × cross-section (CS), giving a squeeze of about 22 %, and a width of roughly 1.30 × CS. A dynamic groove has a depth of roughly 0.90 × CS — a squeeze of about 10 % — and a width of roughly 1.50 × CS. The corner radius is about 0.10 × CS in both cases and the lead-in edges carry a 15–20° chamfer. The table ratios apply to cross-sections from 1.0 to 10.0 mm, which covers the vast majority of the catalogue.

Why does dynamic get less squeeze? Every extra percent raises the contact pressure and with it the friction. In a static joint nobody cares about friction, so you can afford a firmer grip and a bigger sealing margin. In motion, friction means heat and wear — so the squeeze is pulled back to a level that still seals safely but does not needlessly brake or overheat the ring. Stretch on the seat differs too: a static seal tolerates 1 to 5 % stretch, while demanding dynamic seals deliberately run at 0 %, because stretch thins the cross-section and with it the actual squeeze.

Hardness: harder compounds for pressure plus motion

Common materials come in a wide hardness range — NBR, for instance, from 40 to 90 Shore A. In static service you choose hardness freely, based on assembly and the sealed medium. In dynamic service, pressure decides: a soft compound at high pressure gets forced into the gap between parts and the edge of the ring ends up looking nibbled — that is extrusion, one of the most common failure modes of dynamic seals. Our configurator therefore recommends 80 Shore A for a rotating shaft and goes straight to 90 Shore A for pistons at pressures from roughly 100 bar.

Where a harder compound is not enough or the gap is large, a back-up ring is added — typically PTFE, which physically blocks extrusion. PTFE plays exactly the role it is suited for here: a stiff, slippery material with no need for elasticity of its own.

Material: abrasion resistance suddenly matters

In static service, material choice is driven almost entirely by chemistry and temperature — mechanically, nearly anything will do. In dynamic service a third criterion enters: wear. Polyurethane (PU) has the best abrasion resistance of all the elastomers and is the first choice for high-cycle dynamic applications; it is produced in hardnesses of 70 to 95 Shore A. Mind its limits, though: continuous temperature only up to +80 °C (short-term +100 °C), and it is unsuitable for hot water and steam. NBR remains the all-rounder (−30 to +100 °C), and for abrasive hydraulics — slurry, sand — there is carboxylated XNBR with better abrasion resistance than standard nitrile.

At the opposite pole stands silicone (VMQ): its −60 to +200 °C temperature range is exceptional, but it is mechanically weak and does not belong in dynamic applications at all. And PTFE is not an elastomer — once compressed it does not spring back, with zero recovery from permanent deformation, so a plain PTFE ring is no substitute for rubber; in dynamic service it works as a back-up ring or in spring-energized seals.

Static sealing in practice

Covers, flanges, plugs and housings are the O-ring home turf. The procedure is straightforward: material by medium and temperature, groove by the static ratios (depth 0.78 × CS, width 1.30 × CS), stretch on the seat up to 5 %. No friction to manage, no wear to manage — which is why this is the place to economize sensibly. Deploy expensive materials only when the chemistry truly demands it: FKM costs roughly five times standard NBR, and FFKM another 20 to 50 times FKM. A static cover in mineral oil simply does not need to be sealed with Viton when NBR is chemically adequate.

Dynamic sealing in practice

Reciprocating motion: piston and piston rod

An O-ring can handle reciprocating (linear) motion if you keep the dynamic groove with a squeeze of about 10 % and match the hardness to the pressure. At pressures from roughly 100 bar the configurator recommends PU 90 Shore A as first choice — polyurethane is the standard for reciprocating duty and resists extrusion best — with NBR 90 Shore A as the cheaper alternative. At high cycle counts and long strokes, specialized polyurethane rod seals are normally fitted instead of O-rings; there, an O-ring is a stopgap rather than a permanent solution.

Rotation: how far an O-ring goes, and where the shaft seal takes over

Slow or occasional rotation and oscillating motion an O-ring will seal: for a rotating shaft in mineral oil the configurator recommends NBR 80 Shore A, or PU 90 Shore A with longer life in abrasive environments. Continuous fast rotation is a different discipline — the compressed ring rubs against the shaft along its whole circumference, heats up and wears quickly. That is where the radial shaft seal (Simmerring) comes in: a lip seal with a garter spring, built for rotation.

The shaft seal has its own numbers: an NBR sealing lip handles surface speeds up to 10–12 m/s, FKM 35–38 m/s and a PTFE lip roughly 40–45 m/s. A standard shaft seal is a low-pressure part — rated up to 0.05 MPa (0.5 bar); above 0.02 MPa the speed must already be halved, and special pressure-rated types go up to 10 bar. The shaft should be hardened to at least 45 HRC, with 55–65 HRC recommended. And do not forget friction heat: at high speeds the actual lip temperature runs 20–50 °C above ambient — pick the material with that margin in mind.

The most common mistakes in the field

A static groove in a dynamic application: 22 % squeeze in motion means high friction, heat and rapid wear. The ring appears to fit, but its life is measured in weeks.

A soft compound at high pressure: 70 Shore A at a hundred bar ends in extrusion — a nibbled edge on the side facing away from the pressure is the unmistakable signature. The fix is a harder compound (90 Shore A) or a back-up ring.

Silicone in motion: VMQ tempts with its temperature range but mechanically cannot carry dynamic duty. Hot dynamic applications belong to FKM or HNBR, not silicone.

An O-ring on a continuously rotating shaft: it works for a day, a week, a month — and then it leaks. Continuous rotation belongs to the radial shaft seal; keep the O-ring for static joints, reciprocating motion and slow turning.

Frequently asked questions

How much squeeze should an O-ring have in static and in dynamic service?

Statically about 22 % (groove depth 0.78 × CS), dynamically about 10 % (depth 0.90 × CS) per ISO 3601-2. The table ratios apply to cross-sections of 1.0 to 10.0 mm; our calculator works out the exact groove dimensions.

Can an O-ring seal moving parts?

Yes — it handles reciprocating motion and slow or occasional rotation, provided it sits in a dynamic groove, the hardness matches the pressure (from roughly 100 bar, prefer 90 Shore A) and the material is wear-resistant, such as PU, NBR or XNBR. The limit is continuous fast rotation and high-cycle hydraulics — reach for specialized seals there.

When do I need a radial shaft seal instead of an O-ring?

For continuous shaft rotation. A shaft seal with an NBR lip handles up to 10–12 m/s, with FKM 35–38 m/s; standard types, however, are rated only up to 0.5 bar. For rotation combined with pressure there are special pressure-rated shaft seals up to 10 bar.

The groove for both static and dynamic installations takes a minute in the groove calculator in our tools section, material-versus-medium compatibility is a lookup in the chemical resistance chart, the size converter identifies an unknown ring and the configurator finds a ring in stock. If the seal has already failed, walk through the failure analysis first — mixing up static and dynamic design is one of the most common causes. And if your application fits no pigeonhole — say it combines fast rotation, pressure and an aggressive medium — write to Filip: send a sketch and the operating conditions and we will review the design.