What Is an O-Ring?

An O-ring is one of the most widely used sealing components in the world — a simple loop of elastomer that sits in a machined groove and prevents fluid or gas from passing between two mating surfaces. Despite their modest size, the decisions behind O-ring selection — material, sizing, gland design — determine whether a system runs reliably for years or becomes a recurring maintenance problem.

Walk through any industrial facility — a hydraulic press, a pneumatic conveyor system, a compressor room — and somewhere inside nearly every piece of equipment, there are O-rings. Dozens of them. Sometimes hundreds. They're sitting in machined grooves, squeezed between mating surfaces, quietly holding back pressure that would tear a system apart if it escaped. They're so common, and so small, that engineers sometimes make the mistake of treating them as an afterthought. That's usually when the problems start.

O-rings are not complicated. But selecting the right one — the right material for the media, the right size for the groove, the right compound for the temperature — requires knowing what you're working with. This post covers the fundamentals: what O-rings are, how they actually create a seal, what the material options mean in practice, how sizing works, and what failure modes look like when something goes wrong.

What an O-ring is and how it creates a seal

An O-ring is a torus — a continuous loop of elastomer with a circular cross-section. It sits in a machined channel called a gland, defined by its width, depth, and the surfaces it contacts. When the assembly is made up, the O-ring is compressed between the mating surfaces, and that compression is what creates the initial seal. The elastomer deforms slightly and fills the microscopic voids that would otherwise allow fluid or gas to pass through.

What makes O-rings particularly clever is how they respond to system pressure. As pressure builds on one side, it pushes the O-ring against the low-pressure wall of the groove, increasing the contact force and actually improving the seal. The higher the pressure, the harder the O-ring presses. Up to a point — at extreme pressures, the ring can begin to extrude into the gap between the mating surfaces, which is where back-up rings come in. But under normal operating conditions, this self-energizing behavior is one of the main reasons O-rings are so reliable.

Static versus dynamic applications

The first question that shapes everything else about O-ring selection is whether the seal is static or dynamic. In a static application, there's no relative motion between the O-ring and the surfaces it contacts — a pipe fitting, a flange joint, a hydraulic end cap. The O-ring just sits there and seals. These applications are generally forgiving. Surface finish requirements are moderate, tolerances are more relaxed, and a wide range of compounds will perform well.

Dynamic applications are a different story. In a reciprocating hydraulic cylinder, the O-ring slides along the rod or bore surface with every stroke — potentially thousands of cycles per day. In a rotary application, a shaft turns against a static seal continuously. These environments put real mechanical demands on the elastomer: the compound needs to resist abrasion, the gland tolerances need to be tighter to prevent extrusion and spiral failure, and the surface finish of the mating parts matters considerably more. The selection criteria for a dynamic seal aren't wrong for a static one, but they're more demanding — and ignoring that distinction is one of the more common causes of early seal failure.

O-ring seals in an industrial application — O-rings are produced in a wide range of sizes and materials to suit different pressure ratings, media, and temperature environments.

Choosing the right material

Size gets most of the attention when people think about O-ring selection, but material is usually the more consequential decision. Put the wrong compound in contact with the wrong media, or outside its temperature range, and it doesn't matter how precise the sizing is — the seal will fail. The elastomer market has a wide range of compounds, each optimized for different operating environments. Here are the most common ones and where they belong:

Material	Also called	Temp range	Best used for	Avoid with
Nitrile Buna-N, NBR	Buna-N, NBR	−40°F to 250°F	Petroleum oils, hydraulic fluid, fuels, general industrial use	Steam, ozone, strong acids, chlorinated solvents
EPDM	—	−65°F to 300°F	Water, steam, brake fluid, outdoor and UV exposure, HVAC refrigerants	Petroleum products, mineral oils, fuels
FKM Viton®	Viton®	−20°F to 400°F	High-temperature applications, aggressive chemicals, fuels, acids	Steam, amines, MEK, acetone, low-temperature exposure
Silicone VMQ	VMQ	−100°F to 450°F	Wide temperature extremes, food-grade applications, electrical insulation	Dynamic/reciprocating applications, petroleum oils, steam
Neoprene CR	CR	−40°F to 225°F	Weathering, ozone, refrigerants, moderate chemical environments	Aromatic hydrocarbons, strong oxidizing acids, ketones

Nitrile is by far the most widely used compound in general industrial sealing — it's cost-effective, compatible with the oils and hydraulic fluids that dominate industrial systems, and available in a broad range of durometers. When someone says "standard O-ring" without specifying a material, they almost certainly mean 70-durometer Nitrile. But standard doesn't mean universal. If your system runs steam, steam will destroy Nitrile. If you're sealing against aggressive chemicals or operating above 250°F continuously, FKM is the right call despite its higher cost. The material table above is a starting point, not a definitive guide — actual media compatibility involves more variables than a single table can capture.

Not sure which material fits your application? Send us your operating temperature, the media in contact with the seal, and whether the application is static or dynamic. We'll help you identify the right compound before you order.

Sizing: how O-rings are specified

Three dimensions define an O-ring: inside diameter (ID), outside diameter (OD), and cross-section diameter (CS). These three numbers are related — OD equals ID plus twice the cross-section. But in practice, O-rings are rarely ordered by giving all three dimensions. Instead, most North American applications use the AS568 standard (also called the SAE O-Ring standard), which assigns a dash number to each standard size combination. The −001 through −475 range covers everything from a fraction of an inch to several inches in diameter, with cross-sections ranging from 0.070" to 0.275".

If you're working from an existing seal, measure it with calipers while it's relaxed and uninstalled — not stretched over a fitting or compressed in the groove, which will distort the reading. Measure the ID and the cross-section, then cross-reference against an AS568 chart. Metric O-rings are specified differently: typically written as ID × CS in millimeters (for example, 20 × 3 would be a 20mm ID with a 3mm cross-section), and are common in European and Asian equipment. Both standards are widely available.

How O-rings fail — and what each failure mode tells you

When a seal fails, the failure mode is usually diagnostic. It tells you something specific about what went wrong, which matters because the fix is different depending on the cause.

Compression set is one of the most common. The O-ring takes a permanent flat spot where it was compressed, losing its ability to maintain contact force. You'll see it as a flattened ring that no longer springs back to its original cross-section. The causes are usually temperature outside the compound's range, excessive compression, or simply age and chemical exposure over time. The solution is a compound better suited to the operating conditions, or reviewing gland tolerances.

Extrusion and nibbling happen when the clearance gap between mating surfaces is large enough for the O-ring to be pushed through under pressure. The seal develops a ragged, chewed appearance on one side — specifically the downstream, low-pressure side. Higher-durometer compounds resist extrusion better, and back-up rings are designed to close the gap and prevent it entirely.

Spiral failure is specific to dynamic reciprocating applications. Instead of sliding cleanly, the O-ring rolls and twists in the groove, creating a helical pattern of cuts and tears around the ring. It's caused by too much friction between the O-ring and the mating surfaces, usually from inadequate lubrication or an overly tight fit. Switching to a different seal profile — a square ring, for instance — or reviewing surface finish and lubrication will address the root cause.

Chemical degradation shows up as swelling, cracking, or a sticky, softened surface. The O-ring was incompatible with something it was in contact with. A degraded seal needs to be replaced with one made from a compatible compound; there's no repairing it.

Knowing which failure mode you're looking at cuts diagnosis time significantly. A compressed-flat ring points toward a compound or gland problem. An extruded ring points toward clearance. A spiral-cut ring points toward a friction and lubrication problem. They all look like "seal failure," but they're solved differently.

Need help selecting the right O-ring?

Whether you're sizing for a new application, troubleshooting a recurring failure, or sourcing a hard-to-find compound, we're here to help. Submit your requirements and we'll respond in approximately one business day.

You can also reach us at sales@alliedseals.com or 847-673-2080.

Request a Quote → Contact Us →