Technical Foundation
What Is a O-Ring?
An O-ring is a torus-shaped elastomer seal that fills the gap between two mating surfaces, sealing by elastic deformation under compression rather than by torque, making it the most widely used seal for static flange joints and reciprocating or rotating dynamic applications alike. Sizing follows the AS568 standard in the US (a dash number defining inside diameter and cross-section) or metric dimensional callouts elsewhere, and a correct dash number alone is meaningless without specifying the compound.
Compound selection governs chemical compatibility, temperature range and compression set resistance: nitrile (Buna-N) is the general-purpose choice for petroleum oils and fuels; Viton (FKM) extends service to high temperature and aggressive chemical exposure; EPDM resists water, steam and glycol but swells badly in petroleum oil; and silicone offers a wide temperature range with FDA food-contact options but poor abrasion resistance. Durometer (hardness, typically 70 or 90 Shore A) trades sealing conformability against extrusion resistance at high pressure.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| AS568 Dash Size | Standardized ID and cross-section combination | Ensures correct fit in a standard US groove without custom sizing |
| Compound | Nitrile, Viton (FKM), EPDM, silicone, PTFE-encapsulated | Determines chemical, oil and temperature compatibility — the wrong compound swells, hardens or dissolves in service |
| Durometer | Shore A hardness, typically 70 or 90 | Softer (70A) seals low-pressure/irregular surfaces better; harder (90A) resists extrusion at higher pressure |
| Temperature Range | Compound-specific service range, e.g. Viton to ~400°F | Exceeding the compound's range causes hardening, cracking or loss of elasticity and seal failure |
| Compression Set | Resistance to permanent deformation after sustained compression | Poor compression set resistance means the seal stops rebounding and leaks even before it visibly degrades |
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Applications
Where O-Rings Get Used
O-rings seal wherever two surfaces need a compact, low-cost seal compatible with the specific fluid and temperature in contact.
Hydraulic & Pneumatic Cylinders
Static and dynamic sealing of rod and piston glands where compression set resistance under cyclic loading is critical.
Fuel & Lubrication Systems
Nitrile and Viton O-rings sealing fuel lines, filter housings and lubrication fittings against petroleum-based fluids.
Food & Beverage Processing
FDA-compliant silicone and EPDM O-rings sealing sanitary fittings and process equipment in wash-down environments.
Chemical Processing
Viton and PTFE-encapsulated O-rings specified for aggressive solvent and acid exposure beyond nitrile's compatibility.
Plumbing & Water Systems
EPDM O-rings sealing valve stems, faucet cartridges and potable water fittings resistant to chlorine and hot water.
Vacuum & Flange Sealing
Static O-rings sealing vacuum chamber flanges and instrumentation housings against ambient ingress.
FAQ
O-Rings Questions, Answered
How do I read an AS568 dash number?
The AS568 standard assigns each dash number a fixed inside diameter and cross-section combination — for example, a -214 O-ring has a defined ID and a nominal 0.139-inch cross-section. The dash number alone specifies geometry only; you still need to separately specify the compound (nitrile, Viton, EPDM, etc.) and durometer to fully define the part.
What compound should I use for hydraulic oil vs. water vs. food contact?
Nitrile (Buna-N) is the standard choice for petroleum-based hydraulic oils and fuels. EPDM is preferred for water, steam and glycol-based fluids but degrades in petroleum oil. Silicone or FDA-compliant EPDM/nitrile compounds are used for food and beverage contact. Viton (FKM) is specified where high temperature or aggressive chemical resistance is needed beyond what nitrile or EPDM can handle.
Why did my O-ring extrude or blow out under pressure?
Extrusion happens when system pressure pushes the O-ring into the clearance gap between mating parts, typically from too soft a durometer for the pressure, an oversized gland gap, or a worn/scored surface finish. Increasing durometer (e.g., 70A to 90A), adding a back-up ring, or tightening the gland clearance addresses extrusion at higher pressures.
Can I substitute a metric O-ring for an AS568 dash size?
Only if the ID and cross-section happen to match closely enough for the groove — metric and AS568 sizing use different dimensional series and rarely align exactly. Substituting a near-size metric O-ring into an AS568 groove risks under- or over-compression; when possible, match to the correct standard for the groove rather than the closest available size.
What causes an O-ring to take a compression set and leak over time?
Compression set is permanent deformation after sustained compression — the O-ring stops rebounding to its original cross-section even when pressure is removed, losing sealing force. It's accelerated by exceeding the compound's temperature range, using too soft a durometer for a high-load static seal, or leaving the seal compressed at temperature for extended periods; selecting a compound rated for the actual service temperature reduces it.
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