O-Rings

What is an O-Ring

An O-ring is a torus, a doughnut-shaped loop of elastomer with a circular cross section, used as a mechanical seal. It is fitted into a machined groove called a gland, then compressed between two mating surfaces. The compression deforms the round cross section, generating contact stress against both the groove walls and the opposing face. That contact stress, raised further by the system pressure pushing the ring against the downstream wall, is what blocks fluid or gas from leaking past. The O-ring is the dominant seal in industry because it seals in both directions, costs very little, occupies a small space, and needs no special tooling to install.

An O-ring is defined by just two physical dimensions: the inside diameter (ID) and the cross-section diameter (CS, also called the width or the wire diameter). The ID locates the ring on the shaft or bore, and the CS sets how much material is available to compress and seal. A complete O-ring callout therefore names an ID, a CS, a tolerance grade, and a material compound. Specifying only a rough diameter is the most common procurement error, because two rings of the same ID but different CS will not seal the same gland.

O-rings serve in two duty classes. A static seal has no relative motion between the sealing faces, for example a face seal on a flange, a pipe coupling, or a bolted cover; here the ring is squeezed once and simply holds. A dynamic seal sees relative motion, for example a reciprocating hydraulic piston rod or a rotating shaft; here the ring slides against a moving surface and friction, wear, and heat become design drivers. The same nominal O-ring may be acceptable for static service yet fail rapidly in a dynamic gland that was not designed for motion.

The geometry was patented in the modern form by Niels Christensen, who filed in 1937 and was granted the patent in 1939, and standardization followed during the Second World War when the United States military adopted the AN and later AS568 size series to make seals interchangeable across suppliers. The international ISO 3601 series later codified metric sizes, gland dimensions, quality grades, and back-up rings. Today a single global parts catalog can list tens of thousands of standard ID-by-CS combinations across a dozen elastomer families, yet the underlying physics has not changed: squeeze a round elastomer loop into a groove and let pressure energize it.

Four engineering properties decide whether an O-ring lasts: the elastomer's chemical compatibility with the media, its temperature rating, its hardness (durometer), and its compression set resistance. These interact with the gland design, the surface finish, and the system pressure. A correct ring in a wrong gland leaks, and a perfect gland with a wrong compound swells, hardens, or dissolves. Sealing reliability is therefore a system property, not a property of the ring alone, which is why this guide treats material, size, and gland together.

Size Standards: AS568 and ISO 3601

Two size systems dominate globally. AS568 is the inch-based Aerospace Standard maintained by SAE, and ISO 3601-1 is the international metric standard. Mixing them up, or ordering by approximate diameter, is the leading cause of a ring that almost fits but does not seal. AS568 organizes rings into cross-section series identified by a dash number, where each series shares one nominal cross section and steps the inside diameter. The table below lists the five standard AS568 cross-section series.

To convert an AS568 inch dimension to millimeters, multiply by 25.4: the 0.070 inch cross section is 1.78 mm, and the 0.275 inch cross section is 6.99 mm. AS568 also defines a 900 series, which is not an O-ring at all but a set of straight-thread tube-fitting boss gaskets (O-ring-boss seals). Because the cross section is fixed within a series, the dash number after the series prefix only steps the inside diameter, so an AS568-214 and an AS568-218 share the 3.53 mm cross section and differ only in ID.

ISO 3601-1 specifies O-ring sizes directly in millimeters by inside diameter and cross section, covering inside diameters from roughly 0.74 mm to about 660 mm and cross sections from 1.02 to 6.99 mm. It defines two tolerance classes: Class A carries tighter tolerances equivalent to the older AS568B and suits industrial and aerospace duty, while Class B has wider tolerances for general-purpose use. Because ISO 3601-1 Class A overlaps the AS568 tolerances, many catalog rings carry both a dash number and a metric callout, but the safe practice is to specify ID, CS, class, and material in full.

The ISO 3601 family is broader than dimensions alone. The table below summarizes the five parts an engineer references when fully specifying an O-ring and its installation. Part 3 is especially important on a purchase order because its quality grades, N, S, and CS, govern the allowable surface defects and dimensional scatter of the finished ring.

The three ISO 3601-3 grades trade cost against defect tolerance. Grade N is the general-purpose standard for industrial systems where extreme precision is not critical, accepting wider surface and dimensional limits at lower cost. Grade S tightens tolerances and surface-imperfection limits for high-performance industrial and aerospace seals. Grade CS is the most stringent, reserved for special and critical sealing such as aerospace and critical oxygen service. Specifying a grade prevents a commodity ring with mold flash or parting-line defects from reaching a critical sealing face.

Beyond round O-rings, related profiles solve specific problems with the same size systems. The X-ring (also called the quad-ring) has a four-lobe cross section that presents two sealing lines and a lubricant-retaining channel between the lobes; it requires less squeeze than a round ring, generates lower friction, and resists the spiral twisting that destroys O-rings in reciprocating duty, which makes it a frequent upgrade for dynamic seals. Square-cut (lathe-cut) rings and bonded seals address other geometries, but the round O-ring remains the default for the overwhelming majority of static and many dynamic applications.

Elastomer Materials Decoded

Material selection is the single most consequential O-ring decision, because an incompatible compound swells, hardens, cracks, or dissolves regardless of how perfect the gland is. Selection is driven by media chemistry first and operating temperature second, with pressure and dynamic friction as secondary constraints. The dominant families are NBR (nitrile), HNBR, FKM (Viton), EPDM, VMQ silicone, FVMQ fluorosilicone, FFKM (perfluoroelastomer), FEPM (Aflas), and CR (neoprene). The table below compares the engineering envelope of the principal families.

NBR (nitrile, Buna-N) is the workhorse and the low-cost default for sealing petroleum oils, fuels, greases, and hydraulic fluids. Its standard temperature window runs roughly -35 to +120 degrees Celsius, and most catalog rings are 70 Shore A. It is poor on ketones, esters, ozone, sunlight, and strong oxidizers, and it embrittles above its rating. For oil service that also runs hot, the hydrogenated grade HNBR raises the ceiling to about +150 degrees and adds resistance to refrigerants and mild sour gas, at higher cost.

FKM (fluorocarbon, widely known by the Viton brand) is the standard high-temperature and chemical-resistant choice, rated to about +200 degrees continuous and resistant to hot oils, fuels, many acids, and aromatic and chlorinated solvents. Standard FKM is poor on hot water, steam, amines, and ketones, and low-temperature grades are required below about -20 degrees. EPDM is almost the mirror image: it excels on steam, hot water, brake fluid, ozone, and many polar fluids to about +150 degrees, but it is destroyed by petroleum oils and fuels, so EPDM and mineral oil must never be combined.

VMQ silicone tolerates a very wide temperature span, roughly -55 to +200 degrees, with excellent dry-heat and ozone resistance, which suits static seals in food, medical, and electrical service; its low tear strength and poor abrasion resistance rule it out of dynamic duty and steam. FVMQ fluorosilicone adds fuel and oil resistance at low temperature, common in aerospace fuel systems. CR (neoprene) is a mid-range general-purpose compound with good weather, refrigerant, and moderate oil resistance.

FFKM (perfluoroelastomer), sold under brands such as DuPont Kalrez and Greene Tweed Chemraz, is the chemical-resistance ceiling: it withstands nearly all media, including aggressive acids, bases, and solvents, to +300 degrees and beyond, which makes it the default for semiconductor, pharmaceutical, and severe chemical-processing service. Its cost is many times that of FKM, so it is reserved for duties where no other compound survives. FEPM (Aflas) fills a specific gap: because it lacks the VF2 monomer that bases attack, it resists steam, amines, strong bases, and sour gas (H2S) far better than FKM, to about +200 degrees continuous, making it a standard in oilfield and amine-treating service.

Gland, Squeeze and Stretch Design

An O-ring does not seal by sitting in a groove; it seals by being compressed. The gland is the machined housing that holds the ring, and three interacting parameters decide whether the seal works: squeeze, gland fill, and stretch. Get these wrong and even a perfect ring in a compatible compound leaks, takes a set, or extrudes. The numbers below follow the Parker O-Ring Handbook (ORD 5700) and ISO 3601-2 gland practice.

Squeeze is the reduction of the cross section when the ring is installed, expressed as a percentage of the nominal cross section. It generates the initial seal contact stress before any system pressure is applied. For static seals, squeeze typically runs 22 to 30 percent, with smaller cross sections taking the higher end because they have less material to work with. For dynamic reciprocating seals, squeeze is held lower, around 15 to 22 percent, to limit friction, frictional heat, and wear that would otherwise shorten seal life. Too little squeeze leaks at low pressure; too much accelerates compression set and friction.

Gland fill is the percentage of the groove volume occupied by the ring, and the target band is 65 to 85 percent. The reason is thermal: elastomers expand far more than the surrounding metal when heated, and absorbed fluid swells them further. Below 65 percent fill the ring can roll or lose contact at low pressure; above 85 percent there is no room for thermal expansion, so the ring overfills the groove and is forced into the clearance gap, where it extrudes. A correct squeeze with an overfilled groove still fails, which is why both must be checked together.

Stretch applies to rings installed over a shaft or into a bore on the inside diameter. A small stretch, 1 to 5 percent, keeps the ring seated and prevents it from unseating during assembly, but stretch reduces the effective cross section (the ring thins as it stretches) and accelerates aging, so exceeding about 5 percent is avoided. Conversely, an O-ring used as a face seal in a rectangular groove is sized so it is not stretched at all. The table below summarizes the working design bands.

Two further geometric details govern high-pressure reliability: the diametral clearance gap between the mating parts at the groove, and the use of back-up rings. As pressure rises, it pushes the elastomer against the downstream groove wall and tries to squeeze it into the clearance gap, where it is nibbled away. Holding the clearance to a minimum consistent with thermal expansion, raising compound hardness to 90 Shore A, and fitting anti-extrusion back-up rings all counter this. As a working rule, fit a back-up ring once pressure exceeds about 10 MPa (1,500 psi), using one PTFE ring on the low-pressure side for unidirectional pressure or one on each side for bidirectional pressure.

Surface finish matters as much as dimensions. Static sealing faces are typically finished to Ra 0.8 to 1.6 micrometres, while dynamic surfaces that the ring slides against need a finer Ra 0.2 to 0.4 micrometres to avoid abrading the elastomer, yet not so smooth that lubrication cannot be retained. Groove corners are radiused rather than left sharp, because a sharp groove edge cuts the ring during pressure cycling. Installation lubrication, lead-in chamfers, and avoidance of sharp threads on the assembly path prevent the installation nicks that account for a large share of early-life seal failures.

Failure Modes and Spec Parameters

O-ring failures are not random; they fall into a small set of recognizable modes, each with a visible signature and a specific root cause. Reading a failed ring tells you what to change. The eight classic modes are compression set, extrusion and nibbling, spiral failure, explosive (rapid gas) decompression, abrasion, installation damage, chemical degradation, and thermal degradation. The table below maps each mode to its cause and its fix.

Compression set is the most common slow failure: the ring permanently loses its round shape under prolonged heat and compression, so contact stress drops and it weeps. It is quantified per ASTM D395 or ISO 815 as a percentage, where a lower value is better, and it is the headline durability number on a material datasheet. Extrusion and nibbling is the classic high-pressure failure, with the downstream edge frayed where pressure forced the elastomer into the clearance gap; the cure is a back-up ring, a harder 90 Shore A compound, or a tighter gap, as covered in Chapter 4.

Spiral failure appears as a diagonal cut winding around the ring and is unique to reciprocating dynamic seals, where part of the ring slides while part rolls, twisting it until it tears. The fix is to switch to an X-ring, improve lubrication, or refine the surface finish. Rapid gas decompression (RGD), also called explosive decompression, strikes elastomers in high-pressure gas or CO2 service: gas absorbed at pressure expands faster than it can diffuse out when the system depressurizes, blistering or cracking the ring internally. Resistance comes from high-cross-link, filled, harder compounds qualified to NORSOK M-710 or ISO 23936-2, and from slowing the depressurization rate.

Beyond failure analysis, a handful of spec-sheet parameters drive selection. Hardness (durometer) is measured on the Shore A scale per ASTM D2240; 70 Shore A is the general-purpose default, while 90 Shore A resists extrusion at high pressure but needs finer surfaces and seals less readily at low pressure. Compression set (ASTM D395 / ISO 815) predicts long-term sealing force retention. Tensile strength and elongation at break (ASTM D412) indicate installation robustness. Temperature range defines the continuous service window, and the maker's chemical compatibility chart is the final authority on media fit at a given concentration and temperature.

Two further qualification frameworks appear on critical-service purchase orders. For oil and gas, NORSOK M-710 and ISO 23936-2 certify RGD and sour-service resistance. For potable and food contact, FDA 21 CFR 177.2600, NSF/ANSI 61, and EC 1935/2004 govern compound ingredients, and WRAS approval is required for drinking-water contact in some markets. For oxygen and medical service, dedicated cleanliness and compound grades apply. None of these is implied by a bare AS568 dash number, so they must be specified explicitly.

Selection Decision Factors

To turn the previous five chapters into a part number, follow the decision sequence below. Most selection mistakes are not a single wrong value but a decision made at the wrong level, for example choosing a size before confirming the media compatibility. These eight steps can serve as a fixed O-ring RFQ template.

1. Media chemistry: Identify every fluid the ring contacts, including cleaning agents and trace contaminants, then shortlist compatible elastomer families from the maker's chemical chart. This gates everything: EPDM and mineral oil, or standard FKM and steam, are disqualified here before any sizing.
2. Temperature window: Confirm the continuous minimum and maximum, plus any transient peaks (steam cleaning, exotherms). Match against the compound rating, remembering that low-temperature flexibility and high-temperature limits are separate constraints, and that FKM and Aflas need low-temperature grades for sub -20 degree service.
3. Pressure and dynamics: Record system pressure, pressure cycling, and whether the seal is static, rotary, or reciprocating. Above about 10 MPa (1,500 psi) plan for back-up rings and 90 Shore A; for reciprocating duty consider an X-ring to avoid spiral failure.
4. Size standard and dimensions: Specify AS568 dash number or ISO 3601 ID-by-CS, with the ISO 3601-1 class. Verify the ring matches the existing gland (groove width, depth, clearance) so squeeze and gland fill land in the design band from Chapter 4.
5. Hardness (durometer): Default to 70 Shore A; raise to 90 Shore A as pressure and clearance gap increase, or lower below 70 for low-pressure and vacuum sealing on imperfect surfaces. Hardness is a compound choice, not a free parameter on a given size.
6. Quality grade and qualification: Choose ISO 3601-3 Grade N for general industrial use, Grade S for high-performance and aerospace, Grade CS for critical and oxygen service. Add NORSOK M-710 / ISO 23936-2 for sour gas, FDA / NSF / WRAS for food and water, as applicable.
7. Gland and surface finish: Confirm groove geometry, clearance gap, radiused corners, and surface finish (Ra 0.8 to 1.6 um static, 0.2 to 0.4 um dynamic). New designs follow ISO 3601-2 or Parker ORD 5700 gland tables rather than improvising groove dimensions.
8. Total cost of ownership (TCO): Weigh unit price against service life and the cost of a leak. A premium FFKM or RGD-qualified ring at many times the price of NBR is justified where an unplanned shutdown to replace a cheap ring dwarfs the part-cost difference.

One commonly overlooked dimension is serviceability and supply: shelf life and storage conditions of elastomers (ISO 2230 limits storage life, and ozone and UV age rings on the shelf), batch traceability for regulated industries, and the availability of the exact compound, not just the size, from a second source. A ring identified only by size can be re-ordered anywhere, but a compound qualified to a specific RGD or food standard may have a single approved supplier. Parker Hannifin, Trelleborg, Freudenberg (Simrit), SKF, NOK, and ERIKS maintain global catalogs and engineering support, and the Parker O-Ring Handbook ORD 5700 remains the reference engineers cite when a gland or compound decision must be defended.