Static vs dynamic
O-ring sealing —
what the difference means for selection
The distinction is practical. It changes what matters in O-ring selection — material, hardness, squeeze, surface finish and expected service life all shift between static and dynamic duties.
The basic distinction
Static sealing
No relative movement between sealed surfaces
- Valve body caps and plugs
- Cartridge valve housings
- Filter bowl seals
- Sensor ports and inspection plugs
- Static face grooves in housings
- Static radial seals around plugs or sleeves
Dynamic sealing
Relative movement between sealed surfaces
- Hydraulic cylinder piston and rod seals
- Pneumatic actuator seals
- Low-speed or limited-motion rotary O-ring duties
- Reciprocating valve stems
- Sliding spool valves
- Pump shaft sealing systems (seal type usually manufacturer-specified)
Most O-rings used in heating and plumbing equipment are static seals, not dynamic. Where an O-ring is specified — valve caps, cartridge bodies, sensor ports, filter bowls and some housing or heat-exchanger connections — it is assembled once and remains stationary in service. The joint geometry must be identified first, though: flat-face unions and many BSP connections use flat gaskets or washers rather than O-rings.
Dynamic sealing is primarily found in hydraulic and pneumatic systems, where pistons and rods move through seals repeatedly, and in rotating equipment where shafts pass through sealed housings. The demands on the O-ring are fundamentally different.
This article explains selection logic only — it is not a gland design calculation. Dynamic O-ring seals should be specified from the equipment manufacturer's data, a recognised O-ring design standard and the actual pressure, speed, stroke, lubrication and surface-finish requirements.
What changes in dynamic sealing
In a static application, the O-ring is compressed once during assembly and then holds that compression for the duration of the service interval. The main performance parameters are compression set resistance — how well the O-ring maintains its sealing force over time — and chemical and thermal compatibility with the medium.
In a dynamic application, the O-ring is subjected to additional demands that do not exist in static service:
- Friction: the moving surface slides against the O-ring with each stroke or rotation. This generates heat and wears the O-ring material progressively. Compounds with lower friction characteristics and better wear resistance are preferred.
- Extrusion risk: each pressure cycle pushes the O-ring against the low-pressure side of the groove. In dynamic service, this happens repeatedly rather than once. The risk of extrusion into the clearance gap is generally higher than in equivalent static conditions.
- Spiral failure: in reciprocating applications, the O-ring can roll rather than slide in the groove, producing a characteristic spiral twist pattern of damage. This is a dynamic-specific failure mode.
- Heat generation: friction generates heat at the seal interface, which may exceed the thermal load from the system medium alone. The compound must handle the combined thermal load.
- Surface finish requirements: the sliding surface must be finished to appropriate roughness. Too rough and the surface wears the O-ring rapidly. Too smooth and the lubricant film cannot be maintained.
How selection parameters shift
| Parameter | Static | Dynamic |
|---|---|---|
| Primary performance concern | Compression set resistance over time | Wear resistance, friction, extrusion control |
| Compound hardness | 70 Shore A often used in many moderate static duties, subject to pressure, clearance and compound data | Higher hardness may be considered where pressure and clearance create extrusion risk, but friction and wear must also be checked |
| Squeeze | Static squeeze can often be higher within design limits, but excessive squeeze increases compression set and assembly force | Lower squeeze to reduce friction and heat generation |
| Surface finish | Less sensitive than dynamic — but groove surface finish still affects the seal | Critical — must support lubricant film without excessive wear |
| Lubrication | Usually assembly lubricant only, where compatible with the compound and medium | Ongoing lubrication at seal interface often required |
| Extrusion risk | Lower in many moderate static duties, but still controlled by pressure, clearance, temperature and hardness | Higher — repeated pressure cycling at clearance gap |
| Backup rings | Sometimes used at higher pressure | More commonly required in higher-pressure dynamic service |
| Silicone compounds | Sometimes appropriate for static duties | Generally not appropriate — poor wear resistance |
| Service life expectation | Longer — no mechanical wear | Shorter — wear accumulates with each cycle |
Face seal vs radial seal — static subtypes
Within static sealing, there are two main distinctions in how the O-ring is compressed (these are the same two squeeze geometries that also appear in dynamic applications — they describe the direction of compression, not the duty type):
- Face seal (axial compression): the O-ring sits in a groove in one face and is compressed by the mating face closing against it. Common on valve caps, cartridge fittings and O-ring face grooves. The O-ring is compressed along its axis. "Face seal" and "axial seal" describe the same geometry.
- Radial seal (diametric compression): the O-ring sits in a groove and is compressed radially — either an internal bore compresses the outside of the O-ring, or the O-ring sits in a bore groove and is compressed by a shaft or plug. Common in cartridge-style valves and cylindrical housings.
The distinction between face and radial static sealing affects groove design and the direction of squeeze, but both remain static — no relative movement between the sealed surfaces once assembled.
Most O-rings used in heating and plumbing equipment are static seals. Where an O-ring is specified, the duty is often static — valve caps, cartridge bodies, sensor ports, filter bowls and some heat-exchanger or housing connections. Flat-face unions and many BSP connections instead use flat gaskets or washers, so the joint geometry must be identified before selecting the seal. For static water-side O-ring duties, compound selection should follow the medium, temperature, pressure and equipment specification; steam or higher-temperature service should only be specified where the exact compound and seal design are documented for that duty.
Dynamic sealing in heating systems is relatively uncommon outside of pump shaft seals and motorised valve stems — both of which are typically manufacturer-specified components rather than field-selected O-rings.
Reciprocating vs rotary dynamic sealing
Dynamic sealing divides further into reciprocating and rotary applications, each with its own dominant failure modes:
Reciprocating seals — where a rod or piston moves back and forth through the seal — are subject to spiral failure and extrusion. The O-ring must not roll in the groove as the rod passes through. Correct groove geometry, appropriate hardness and adequate lubrication reduce this risk.
Rotary seals — where a shaft rotates through a sealed housing — generate friction heat continuously rather than cyclically. Surface speed, lubrication, and the thermal load from friction all become critical design inputs. Standard O-rings have limited suitability for higher-speed rotary duties — dedicated lip seals or mechanical seals are often more appropriate for continuous rotation applications.
Three checks settle the duty type. One: do the two sealed surfaces move relative to each other in service? If no — static. If yes — dynamic. Two: if dynamic, is the motion reciprocating (rod, piston, valve stem) or rotary (shaft)? The failure modes differ. Three: what is the actual surface speed, stroke length and pressure? Dynamic seal selection without these numbers is incomplete.
Frequently asked questions
What is the difference between a static and dynamic O-ring seal?
A static O-ring seal sits between two surfaces that do not move relative to each other once assembled — for example a valve body cap, cartridge housing, sensor port, filter bowl or static housing groove. A dynamic O-ring seal operates with relative movement between the components it seals — a reciprocating piston rod, a rotating shaft, a sliding valve stem. The distinction matters because dynamic sealing introduces friction, wear and heat that do not exist in static applications, and these additional demands affect O-ring material selection, hardness and groove design.
Can the same O-ring be used for static and dynamic sealing?
Not always. An O-ring compound that is suitable for a static seal may not be appropriate for a dynamic application in the same system. Dynamic sealing typically requires lower friction characteristics, better wear resistance, and often a harder compound to resist extrusion at the clearance gap under cycling pressure, depending on pressure, clearance and friction limits. Silicone O-rings, for example, are common in static applications but are generally avoided for demanding dynamic duties because of their lower mechanical strength and wear resistance. Material and hardness selection should be based on the actual duty — static or dynamic — not just the medium and temperature.
Why are dynamic seals harder to specify than static seals?
Dynamic seals involve additional variables that do not exist in static applications: surface speed, stroke length, lubrication, friction, heat generation and wear rate. These interact with material properties in ways that cannot be predicted from medium compatibility and temperature data alone. Dynamic seal specification typically involves groove geometry, surface finish requirements, and compound hardness considerations that go beyond standard static seal selection. For critical dynamic applications, the equipment manufacturer's specification or an O-ring design standard should be the starting reference.
Static or dynamic is the first question in O-ring selection — before material, before hardness, before size.
Most O-rings used in heating and plumbing equipment are static seals, not dynamic seals. For these O-ring duties, compression set resistance and medium compatibility are the primary selection parameters. Dynamic sealing introduces wear, friction and extrusion demands that change what is required from the compound and the groove design. An O-ring correctly specified for static duty is not automatically suitable for dynamic duty in the same system.