O-Ring Glossary
Engineering terms and definitions for O-ring design, material selection, and failure analysis. 52 terms covering dimensions, materials, testing standards, and failure modes.
A
AFLAS (FEPM)
Tetrafluoroethylene/propylene copolymer rubber. AFLAS contains no vinylidene fluoride (VF2), making it resistant to steam, hot water, amines, strong bases, and sour gas — environments that degrade standard FKM. Service range approximately −5°C to +200°C. Used in geothermal valves, oilfield downhole equipment, and boiler systems. Significantly more expensive than FKM; typically 3–6× the cost of standard FKM per piece.
AMS-R-83485
SAE Aerospace Material Specification for fluorocarbon (FKM) O-ring compounds used in aircraft fuel, oil, and hydraulic systems. Requires minimum 65% fluorine content, Shore A hardness of 75 ±5, and batch-specific test data traceable to raw compound. AMS-R-83485 seals are brown in color. Required for FAA- and EASA-regulated fuel system overhauls; non-compliant substitutes are not permitted under maintenance manual requirements.
Anti-Extrusion Ring
Alternative term for a backup ring. A rigid ring — typically PTFE or PEEK — installed adjacent to the O-ring on the low-pressure side of the gland to prevent elastomer extrusion into the diametral clearance gap under high pressure. The term 'anti-extrusion ring' is used interchangeably with 'backup ring' in many engineering standards and procurement documents.
AS568
The US aerospace standard (published by SAE) defining O-ring sizes in imperial units. The standard covers 369 dash numbers across six cross-section groups: 0.040 in (1.02 mm), 0.050 in (1.27 mm), 0.070 in (1.78 mm), 0.103 in (2.62 mm), 0.139 in (3.53 mm), and 0.210 in (5.33 mm) CS. Each dash number specifies a unique ID and CS combination. AS568 is the dominant standard for O-ring sizing in North American and aerospace applications.
B
Backup Ring
A rigid ring (PTFE, PEEK, or engineering polymer) installed on the low-pressure side of an O-ring to prevent extrusion into the diametral clearance gap under high pressure. Does not provide sealing function — the O-ring remains the primary seal. Single backup rings are used when pressure is one-directional; double backup rings (one on each side) are used when pressure can reverse. For dynamic seals above 40 bar and static seals above 70 bar, backup rings are generally recommended with standard 70 Shore A compounds.
Boss Seal (SAE Port)
An O-ring installed in a machined groove on a threaded boss fitting to seal a fluid port, commonly used in hydraulic and pneumatic manifolds and adapters. Boss seal grooves follow SAE J1926 or ISO 6149 standards. The O-ring seals on the face of the port boss, not on the thread, providing a metal-to-metal thread connection with elastomeric face sealing. Common in mobile hydraulics and fluid power equipment.
C
Clearance Gap (Diametral)
The total space between the inner and outer cylindrical mating surfaces — for example, between a rod and its bore, or a piston and cylinder wall. Clearance gap is the primary driver of extrusion risk. For a 70 Shore A O-ring at 100 bar in dynamic service, the maximum safe diametral gap is approximately 0.10–0.12 mm. Adding a backup ring or increasing hardness to 90 Shore A raises the safe gap threshold. Clearance gap increases at elevated temperature due to differential thermal expansion of metal and elastomer.
Compression Rate
The percentage reduction in O-ring cross-section diameter caused by installation squeeze in the gland. Synonymous with squeeze percentage. Calculated as: (CS − gland depth) ÷ CS × 100%. Typical targets: static seals 15–30%, dynamic reciprocating seals 10–20%, dynamic rotary seals 10–15%. Insufficient compression causes leakage; excessive compression accelerates compression set and reduces seal life.
Compression Set
The permanent loss of elastic recovery after an O-ring has been compressed under load at elevated temperature for an extended period. Measured per ASTM D395 Method B (typically 22 hours or 70 hours at specified temperature). Expressed as a percentage: 0% = full recovery, 100% = no recovery. Compression set below 25–30% is generally acceptable for reliable sealing. High compression set causes leakage because the seal no longer fills the groove with sufficient contact force after thermal cycling or long-term service.
CR (Neoprene)
Chloroprene rubber. CR has good ozone and weathering resistance, moderate oil resistance, and a useful temperature range of −40°C to +120°C. It is commonly specified for outdoor equipment, refrigerant service (R-22, R-134a), and automotive weatherstripping. CR is less oil-resistant than NBR and less heat-resistant than FKM, but outperforms both in ozone-exposed outdoor applications. Moderate cost.
Cross Section (CS)
The diameter of the O-ring's circular cross-section, also called the wire diameter or cord thickness. Standard AS568 CS values are 1.78 mm (0.070 in), 2.62 mm (0.103 in), 3.53 mm (0.139 in), and 5.33 mm (0.210 in). Larger cross-sections provide more squeeze volume and better tolerance of gland dimensional variation, but increase the risk of spiral failure in dynamic service. Cross-section determines the groove depth and influences the acceptable squeeze percentage range.
D
Dash Number
The three-digit identifier used in the AS568 standard to specify a unique combination of inside diameter and cross-section. For example, AS568-214 specifies ID = 22.22 mm (0.875 in), CS = 2.62 mm (0.103 in). Dash numbers are grouped by CS: -001 to -049 (CS 1.78 mm), -100 to -149 (CS 2.62 mm), -200 to -284 (CS 3.53 mm), -300 to -395 (CS 5.33 mm), plus specialty groups for face seals. The term 'minus number' is sometimes used informally.
Durometer (Shore A)
A standardized measure of elastomer hardness, tested per ASTM D2240. The Shore A scale (0–100) is used for soft to medium-hard elastomers. O-rings are commonly available in 50, 60, 70, 75, 80, and 90 Shore A. Higher hardness improves extrusion resistance and suitability for high-pressure service, but increases friction in dynamic applications and requires higher installation force. The standard grade for most industrial O-rings is 70 Shore A. For high-pressure service above 100 bar in dynamic applications, 80–90 Shore A is common.
Dynamic Seal
A seal installed in a joint where there is relative motion between mating parts. Types include reciprocating (linear back-and-forth motion, e.g., hydraulic cylinders), rotary (spinning shaft seals), and oscillating (limited-angle rotation). Dynamic seals require lower friction, better wear resistance, and more controlled squeeze than static seals. Maximum recommended dynamic O-ring squeeze is typically 10–20% depending on motion type, versus 15–30% for static seals.
E
Elastomer
A polymer material with rubber-like elastic properties — capable of deforming significantly under stress and recovering its original shape upon release. O-rings are manufactured from thermoset elastomers: the polymer chains are permanently crosslinked during vulcanization, giving the material its elastic recovery. Common O-ring elastomers include NBR, FKM, EPDM, VMQ (silicone), HNBR, PTFE, and FFKM. Each elastomer family has distinct chemical resistance, temperature range, and mechanical properties.
Elongation at Break
The percentage increase in length a material can withstand before fracturing, measured per ASTM D412. Expressed as a percentage of the original gauge length. High elongation (200–600%) indicates a material that can stretch during installation without tearing — important for O-rings that must stretch over a piston or through a port. Low elongation materials (such as PTFE, 100–300%) must be installed more carefully to avoid damage. Elongation also indicates remaining elasticity available for sealing function.
EPDM
Ethylene propylene diene monomer rubber. EPDM has an outstanding resistance to hot water, steam, ozone, UV, and polar solvents. Service range −40°C to +150°C (premium grades to +160°C in steam). Not compatible with petroleum oils, fuels, or aromatic solvents. The standard material for steam sealing, outdoor weatherstripping, and food/beverage applications (peroxide-cured grade). Two cure systems exist: peroxide-cured (preferred for food contact and steam service, lower extractables) and sulfur-cured (standard industrial use).
Extrusion
The deformation of O-ring elastomer into the diametral clearance gap between mating metal parts under system pressure. Extrusion causes nibbling damage (small pieces torn from the seal edge), helical cracks, and eventual loss of sealing function. Extrusion risk increases with higher pressure, larger clearance gaps, softer elastomers, and elevated temperature (which softens the elastomer). Prevention: reduce gap below 0.10–0.15 mm, increase hardness to 80–90 Shore A, and/or add backup rings on the low-pressure side.
F
Face Seal
A static seal compressed axially between two flat mating surfaces — typically a flanged fitting and a mating port face. The O-ring sits in a groove on one face and is compressed by the clamping force when the joint is bolted together. Face seals can tolerate higher compression set than rod or piston seals because the groove is fully enclosed and the seal is not subject to motion. The most common static seal configuration in hydraulic ports, instrumentation fittings, and process piping.
FFKM (Perfluoroelastomer)
Perfluoroelastomer rubber — the highest-performing elastomer in chemical resistance and temperature. The polymer backbone is fully fluorinated (no hydrogen atoms), making FFKM inert to virtually all industrial chemicals including strong oxidizers, aromatic solvents, and most acids and bases. Service range approximately −20°C to +300°C (compound-dependent). Available under trade names Kalrez (DuPont/DowDuPont), Chemraz (Greene Tweed), and Perlast (Precision Polymer Engineering). Cost is 100–1,000× that of standard NBR O-rings. Justified in semiconductor, pharmaceutical, and critical process applications where chemical attack or contamination cannot be tolerated.
FKM (Viton)
Fluorocarbon rubber — the standard high-performance elastomer for fuel, oil, and chemical service. FKM is manufactured in three main types: Type 1 (VF2/HFP, ~65% fluorine, −20°C to +200°C), Type 2 (VF2/HFP/TFE, ~68% fluorine), and GF-type (VF2/PMVE/TFE, 70%+ fluorine, better low-temperature and sour gas resistance). Viton is a registered trade name of Chemours (formerly DuPont). All FKM grades are limited in steam and amine service due to VF2 hydrolysis at elevated temperature in the presence of water.
G
Gland
The complete machined cavity that houses the O-ring, including the groove and the mating surface that compresses the seal. Proper gland design specifies: groove width (typically 1.5–1.65× CS for static, 1.4–1.5× CS for dynamic), groove depth (controlling squeeze), surface finish of the groove base and sealing surface, and any chamfers or lead-in features for installation. Both the groove geometry and the mating surface condition determine whether the O-ring seals effectively and survives the expected service life.
Gland Fill (Groove Fill Ratio)
The percentage of the groove cross-sectional area occupied by the O-ring cross-section. Calculated as: (O-ring CS area ÷ groove area) × 100%. Target fill is typically 65–75% for static seals and 75–85% for dynamic seals. Insufficient fill (below 60%) means the seal does not make adequate contact. Over-fill (above 90%) leaves no room for thermal expansion of the elastomer, causing O-ring extrusion or groove damage as temperature rises. Fill ratio must account for the temperature coefficient of volume expansion of the specific elastomer compound.
Glass Transition Temperature (Tg)
The temperature below which an elastomer transitions from a flexible, rubbery state to a rigid, glassy state. Below Tg, the polymer chains can no longer move freely and the elastomer loses its elastic recovery — it becomes hard and brittle. Tg is not the same as the minimum service temperature: practical sealing function is typically lost several degrees above Tg. The standard low-temperature test metric is TR10, which represents the temperature at which 10% of flexibility is retained. Tg for standard NBR is approximately −30°C to −40°C; for FKM approximately −15°C to −25°C; for VMQ silicone approximately −120°C.
Groove Width
The lateral dimension of the O-ring groove (the dimension parallel to the seal axis for rod/piston seals, or perpendicular to the face for face seals). Groove width must be wide enough to accommodate the O-ring cross-section plus any required gland fill tolerance, but narrow enough to prevent the O-ring from rolling excessively in dynamic service. Standard groove width for static service is approximately 1.5–1.65× the O-ring CS. For dynamic reciprocating service, 1.4–1.5× CS is typical to limit rolling and reduce spiral failure risk.
H
HNBR (Hydrogenated Nitrile)
Hydrogenated nitrile butadiene rubber. HNBR is produced by hydrogenating the double bonds in NBR's butadiene backbone segments, eliminating most of the sites susceptible to ozone attack and thermal degradation. Service range −40°C to +150°C, with better thermal aging than standard NBR (+120°C limit). HNBR also offers significantly better resistance to sour gas (H2S) in oilfield service, compliant with NACE MR0175 when specified with appropriate compound selection. Used in automotive (timing belt tensioners, power steering), oilfield downhole equipment, and refrigerant compressors.
I
Inside Diameter (ID)
The inner diameter of the O-ring, measured at the cross-section centerline in the unmounted state. ID determines how the O-ring fits over a piston or shaft. For most static and dynamic rod seal applications, the O-ring ID is matched to the rod diameter with a small amount of stretch (typically 1–3%). Excessive stretch reduces cross-section diameter (due to Poisson's ratio effect), reducing effective squeeze. AS568 ID dimensions are specified in inches; ISO 3601 in millimetres.
Installation Stretch
The percentage elongation of the O-ring ID when installed over a component larger than the O-ring's unmounted ID. For most applications, stretch should be limited to 3–5% of the unmounted ID. Excessive stretch (>5%) reduces the O-ring's effective cross-section diameter due to Poisson's ratio effects, reducing available squeeze and potentially causing leakage. Some assembly guides allow up to 6% stretch for specific gland designs, but anything above 5% requires verification that adequate squeeze is maintained in the installed state.
ISO 3601
The international metric standard for O-ring sizes, published by ISO and adopted by DIN (as DIN 3771 in Germany), JIS (as JIS B 2401 in Japan), and BS 4518 in the UK. Organizes O-rings by inside diameter and cross-section in millimetres. The main series defined in ISO 3601-1 use CS values of 1.80 mm (Group A), 2.65 mm (Group B), 3.55 mm (Group C), 5.30 mm (Group D), and 6.99 mm (Group E). ISO 3601-3 defines tolerance classes: N (normal, ±0.08–0.15 mm) and S (precision, tighter, for aerospace and medical applications).
J
JIS B 2401
The Japanese Industrial Standard for O-ring sizes, published by the Japanese Standards Association. JIS B 2401 specifies four series: P (piston/rod seals), G (general static), V (vacuum flanges), and S (face seals). JIS sizes are metric but differ from ISO 3601 — dimensional interchangeability cannot be assumed without verification. JIS O-rings are widely used in Japanese OEM equipment and are commonly required for MRO replacement parts in Asian-market machinery.
L
Low-Temperature Flexibility
The ability of an elastomer to remain elastic and resilient at sub-zero temperatures. Evaluated by TR10 (temperature retraction test) and low-temperature compression set tests. Seals that lose flexibility at low temperature harden, lose sealing force, and are prone to spiral failure in dynamic service (the stiff seal twists rather than rolls in the gland). Key material TR10 values: standard NBR approximately −32°C to −38°C, LT-NBR approximately −42°C to −50°C, FKM Type 1 approximately −15°C, FKM GF-type approximately −25°C, VMQ silicone approximately −55°C to −65°C.
N
NBR (Nitrile)
Acrylonitrile butadiene rubber. NBR is the most widely used O-ring material, accounting for approximately 70% of global O-ring production by volume. It offers excellent resistance to petroleum-based oils and fuels, good mechanical properties, and the lowest cost of any performance elastomer. Service range −40°C to +120°C (standard grade); low-temperature NBR (LT-NBR) grades extend to −50°C. Acrylonitrile content (ACN) determines the trade-off between oil resistance (higher ACN) and low-temperature flexibility (lower ACN). Standard industrial grades are 33–34% ACN; high-nitrile fuel-resistant grades are 39–40% ACN. Not suitable for ozone, UV, ketones, esters, or aromatic solvents.
Nibbling
A failure mode characterized by small pieces torn from the low-pressure edge of the O-ring cross-section, caused by the elastomer being repeatedly forced into the clearance gap and then retracted. Nibbling damage appears as a ragged, chewed appearance on the edge of the seal — the opposite side from the high-pressure face. It is the characteristic failure signature of extrusion. Prevention: reduce clearance gap, increase elastomer hardness to 80–90 Shore A, or add backup rings on the low-pressure side.
O
O-Ring Cord (Vulcanized Splice)
A length of elastomer cord in standard O-ring cross-section diameters, which is cut to a specific circumferential length and the two ends bonded (vulcanized or adhesively spliced) to form a custom-diameter O-ring. Cord splicing is used for large-diameter O-rings (typically ID > 300 mm) that would be impractical or uneconomical to mold, for prototype or low-volume custom sizes, and for urgent orders where mold lead time is not acceptable. Tolerances are looser than molded O-rings; the joint quality must be verified for the specific application pressure and motion type.
O-Ring Kit
An assortment of O-rings packaged for maintenance, repair, and MRO use. Common kit types include: full AS568 assortments (typically 350+ sizes in NBR or FKM), metric assortments (ISO 3601 sizes), material-specific assortments (e.g., FKM-only for fuel systems), and application-specific kits (e.g., hydraulic cylinder kit covering the most common cylinder bore and rod combinations). Kits reduce maintenance downtime by providing the most likely replacement sizes in a single stock unit.
Outgassing
The release of volatile compounds (plasticizers, processing aids, residual monomers) from an elastomer into a surrounding vacuum or inert atmosphere. Outgassing is a critical concern in aerospace, space, semiconductor, and high-vacuum applications. Low-outgassing O-ring compounds are formulated to minimize total mass loss (TML) and collected volatile condensable material (CVCM) per ASTM E595 or NASA SP-R-0022 standards. Standard elastomers without low-outgassing formulation are not suitable for ultra-high vacuum or satellite thermal control applications.
P
Permeation
The diffusion of gas molecules through an elastomer from the high-pressure side to the low-pressure side without visible leakage. All elastomers are permeable to some extent; permeation rate depends on the elastomer chemistry, the gas molecular size, temperature, and pressure differential. In gas-tight applications (natural gas distribution, high-pressure gas cylinders, hydrogen systems), permeation rate is a design specification. FFKM and FKM have lower permeation rates for most gases than NBR or EPDM. Low permeability does not equal zero leakage in gas sealing — a 'zero-leakage' gas seal is a system design requirement, not an elastomer property.
Peroxide Cure
A vulcanization system that uses organic peroxides (e.g., dicumyl peroxide, DCP) as the crosslinking agent instead of sulfur. Peroxide cure produces carbon-carbon crosslinks, which are more thermally stable and chemically inert than sulfur bridges. Peroxide-cured compounds have lower extractables in hot aqueous and alkaline media, making them preferred for food contact (EPDM) and biocompatibility-sensitive applications (VMQ silicone). A secondary post-cure step (200°C for 4+ hours) is required to decompose peroxide byproducts (acetophenone, cumyl alcohol) to acceptable levels.
PTFE (Polytetrafluoroethylene)
Polytetrafluoroethylene — a fully fluorinated thermoplastic polymer with near-universal chemical resistance. PTFE O-rings are machined from solid bar stock (lathe-cut) rather than molded, as PTFE cannot be conventionally vulcanized. Service range −200°C to +260°C. PTFE does not recover elastically from compression as elastomers do; it requires well-controlled, consistent clamping force to maintain sealing contact. Used for static seals in aggressive chemical environments, as backup ring material, and in applications where elastomeric extractables are unacceptable.
PU (Polyurethane)
Polyurethane elastomer. PU O-rings and seals offer exceptionally high tensile strength (up to 50 MPa, versus 10–20 MPa for NBR), excellent abrasion resistance, and good oil resistance. Service range approximately −30°C to +100°C. PU is favored in high-pressure hydraulic applications (dynamic cylinders operating to 600+ bar) where abrasion and extrusion resistance are the primary requirements. Poor resistance to hot water, steam, and many aqueous systems limits its use in water-based applications.
R
Radial Seal
A seal installed between concentric cylindrical surfaces — the O-ring is compressed radially between the shaft (inner surface) and the bore (outer surface). Rod seals (sealing on the outside of a rod) and piston seals (sealing on the outside of a piston against the cylinder bore) are both radial seals. The compression is perpendicular to the seal axis. Groove depth controls squeeze; the sealing surfaces are the cylindrical faces of the shaft and bore.
REACH
EU Regulation (EC) No 1907/2006 — Registration, Evaluation, Authorisation and Restriction of Chemicals. REACH requires manufacturers and importers of chemical substances used in articles (including rubber compounds) sold in the EU to register substances above certain tonnage thresholds and to communicate information about substances of very high concern (SVHC) in articles. O-ring buyers in the EU commonly request REACH compliance declarations to confirm that compound ingredients comply with SVHC restrictions. A REACH declaration is a supplier statement of compliance, not a third-party certification.
RoHS
EU Directive 2011/65/EU — Restriction of Hazardous Substances in Electrical and Electronic Equipment. Restricts the use of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBBs), and polybrominated diphenyl ethers (PBDEs) in EEE. O-rings used in electronic or electrical equipment sold in the EU must be manufactured without these substances. Most standard elastomers (NBR, FKM, EPDM, VMQ) are inherently RoHS-compliant; colorants and specific compound additives must be verified. A RoHS declaration from the supplier confirms the compound does not intentionally contain restricted substances above permitted thresholds.
S
Shelf Life
The maximum storage period during which an O-ring is considered suitable for installation without performance degradation. Shelf life is compound-dependent: standard NBR and EPDM are typically rated for 5–10 years; FKM and VMQ for 10–15 years; PTFE for indefinite storage if protected from contamination. AMS-R-83485 FKM O-rings for aerospace have a 15-year shelf life from cure date per SAE AS 1933. Storage conditions affect actual shelf life: seals must be stored in sealed bags away from ozone sources, UV light, heat, and humidity. Always verify the cure date on the bag before installation in regulated applications.
Spiral Failure
A failure mode in dynamic (reciprocating) seals where the O-ring rolls in the gland rather than sliding, accumulating a continuous helical torsional crack around its circumference. The crack typically runs at approximately 45° to the seal axis and appears as a spiral cut when the O-ring is examined after removal. Causes include: insufficient lubrication, excessive friction (from overtight grooves, rough surfaces, or cold elastomer), and stiff elastomers at low temperature. Prevention: adequate lubrication, correct groove width, appropriate elastomer hardness for the temperature range, and X-ring profile as an alternative to standard O-rings in high-duty reciprocating service.
Squeeze
The percentage reduction in O-ring cross-section diameter caused by compression between the groove base and the mating sealing surface. Squeeze is the primary design parameter for O-ring sealing. It is calculated as: Squeeze (%) = (CS − gland depth) ÷ CS × 100%. Recommended squeeze ranges: static seals 15–30%, dynamic reciprocating seals 10–20%, dynamic rotary seals 10–15%. Insufficient squeeze causes immediate leakage; excess squeeze accelerates compression set and can cause elastomer extrusion at elevated temperatures when thermal expansion reduces available volume.
Static Seal
A seal installed in a joint with no relative motion between mating parts. Static seals are mechanically simpler than dynamic seals: they can tolerate higher squeeze (15–30%), rougher surface finishes, and higher hardness compounds without adverse effects. The dominant failure modes are compression set (loss of sealing force over time) and chemical attack by the process media. Static seals include face seals (axially compressed between flat surfaces) and radial static seals (compressed between cylindrical surfaces in a fixed housing).
Surface Finish (Ra)
The arithmetic average roughness of the O-ring contact surface, measured in micrometres (μm). Surface finish affects friction, sealing contact, and O-ring wear rate. Standard recommendations: dynamic reciprocating seals: 0.2–0.4 μm Ra on the sliding surface; dynamic rotary seals: 0.1–0.3 μm Ra; static seals: 0.8–1.6 μm Ra (static seals tolerate rougher surfaces). The groove base (which is not the sealing face) can be machined to 1.6–3.2 μm Ra for most applications. Excessively rough surfaces abrade O-rings in dynamic service; excessively smooth surfaces can increase stick-slip behavior.
Swell
The increase in volume of an O-ring after immersion in a fluid, expressed as a percentage of the original volume. Swell is caused by absorption of the fluid into the elastomer matrix, expanding the polymer chains. Moderate swell (1–5%) can be beneficial in some applications — a slightly swollen seal fills the groove more completely and compensates for minor compression set. Excessive swell (>15–20%) causes the seal to over-fill the groove, generating excessive contact stress and potentially splitting or extruding the O-ring. Negative swell (shrinkage) occurs when the fluid extracts plasticizer from the compound, causing the O-ring to harden and pull away from gland surfaces.
T
Tear Strength
The resistance of an elastomer to the propagation of a nick or cut under applied force, tested per ASTM D624. High tear strength is important for seals that must stretch over a sharp edge during installation, and for dynamic seals where surface irregularities or contamination particles can initiate cuts. VMQ (silicone) has notoriously low tear strength — typically 5–20 N/mm versus 30–50 N/mm for NBR. FKM tear strength is moderate (15–30 N/mm). For applications involving difficult installation paths or potential abrasive contamination, specifying a higher-tear-strength compound material or using assembly lubricant reduces installation damage risk.
Tensile Strength
The maximum stress a material can withstand while being pulled in tension before fracturing, tested per ASTM D412. Expressed in MPa. Typical values: NBR 10–20 MPa, FKM 10–15 MPa, EPDM 8–18 MPa, VMQ 5–10 MPa, PU 30–50 MPa. Higher tensile strength indicates a stronger compound but does not directly correlate to sealing performance — compression set and chemical resistance are more relevant to long-term seal reliability than tensile strength alone. Tensile strength decreases at elevated temperature and after chemical immersion.
Tolerance Class
The permitted dimensional variation band for O-ring inside diameter (ID) and cross-section (CS). ISO 3601-3 defines two tolerance classes: N (normal, wider, for standard industrial applications) and S (precision, tighter, for aerospace, medical, and critical sealing). AS568 defines class A (standard) and class B (tighter) tolerances. Tighter tolerances are required when gland dimensions are precisely controlled and the application cannot accommodate dimensional variation — pharmaceutical vessel seals, aerospace fuel system O-rings, and medical device components commonly specify Class B or Class S.
TR10 (Temperature Retraction Test)
The temperature at which a stretched elastomer specimen retracts to 10% of its original elongation after being frozen and then heated at a controlled rate, per ASTM D1329. TR10 is the standard low-temperature performance benchmark for O-ring materials because it represents the practical minimum temperature at which the elastomer retains meaningful elastic recovery — approximately the temperature at which a seal will continue to function. Key TR10 values: standard NBR approximately −32°C to −38°C; LT-NBR approximately −42°C to −50°C; FKM Type 1 approximately −15°C to −18°C; FKM GF-type approximately −25°C; VMQ silicone approximately −55°C to −65°C; EPDM approximately −45°C to −55°C.
U
USP Class VI
A biocompatibility standard published by the United States Pharmacopeia for plastics and elastomers used in pharmaceutical manufacturing and medical devices. USP Class VI requires passing three biological reactivity tests in mice: systemic injection, intracutaneous injection, and implantation. USP Class VI is the minimum biocompatibility requirement for elastomers in pharmaceutical-grade equipment. For implantable medical devices with direct patient contact, ISO 10993 (cytotoxicity, sensitization, genotoxicity tests) provides a more comprehensive evaluation framework. USP Class VI alone is not sufficient evidence for ISO 10993 compliance.
V
VMQ (Silicone)
Polydimethylsiloxane rubber, the standard silicone elastomer for O-rings. VMQ offers the widest temperature range of any commodity elastomer: −60°C to +230°C continuous service, with short-term excursions to +260°C. Excellent FDA compliance and low extractables profile (platinum-cured grades). Key limitations: poor tear strength (5–20 N/mm), poor abrasion resistance, and poor resistance to fuels, oils, and most organic solvents. Not suitable for dynamic high-speed applications. VMQ is specified for food contact (platinum-cured, FDA 21 CFR §177.2600), pharmaceutical equipment (USP Class VI), and applications requiring extreme temperature range without chemical aggression.
Vulcanization
The chemical crosslinking process that converts raw rubber compound into a thermoset elastomer with stable elastic properties. During vulcanization, crosslinks form between adjacent polymer chains, preventing them from sliding past each other when stress is applied — this gives the elastomer its elastic recovery behavior. Most O-rings are vulcanized by compression molding under heat and pressure. Two main crosslinking chemistries are used: sulfur cure (traditional, lower cost, lower thermal stability) and peroxide cure (carbon-carbon crosslinks, better thermal stability and lower extractables, required for food and medical grades).
X
X-Ring (Quad Ring)
A four-lobed elastomer seal profile that provides reduced friction, better lubricant retention, and lower spiral failure risk compared to a standard O-ring in reciprocating dynamic service. Each pair of lobes creates two narrow sealing contact lines on the dynamic face (versus one wide contact band for an O-ring), reducing friction by approximately 15–30% in equivalent gland designs. X-rings in the same cross-section as an O-ring use the same groove width and depth — no gland machining change is required to switch profiles. X-rings offer no sealing advantage over O-rings in static applications and cost 10–40% more.
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