O-Ring Glossary
Essential engineering terms and definitions for O-ring design, selection and failure analysis.
A
AS568
The US aerospace standard for O-ring sizes, defining over 369 imperial dash numbers with cross sections of 0.040, 0.050, 0.062, 0.103, 0.139 and 0.210 inches.
B
Backup Ring
A rigid ring (usually PTFE) installed on the low-pressure side of an O-ring to prevent extrusion into clearance gaps under high pressure.
C
Compression Set
The permanent loss of elastic recovery after an O-ring has been compressed for an extended time. High compression set leads to leakage because the seal no longer fills the groove.
Cross Section (CS)
The diameter of the O-ring's circular cross-section, also called the wire diameter. Common metric CS values are 1.80 mm, 2.65 mm and 3.55 mm.
D
Dash Number
The three-digit identifier used in the AS568 standard (e.g. AS568-214) to specify a unique combination of inside diameter and cross section.
Durometer (Shore A)
A measure of elastomer hardness. O-rings typically range from 40 Shore A (soft) to 90 Shore A (hard). Higher hardness improves extrusion resistance but increases seating force.
Dynamic Seal
A seal installed in a moving joint, such as a reciprocating piston rod or rotating shaft. Dynamic seals require lower friction and better wear resistance than static seals.
E
Elongation at Break
The percentage increase in length a material can withstand before breaking. Higher elongation indicates better stretchability during installation.
Extrusion
The deformation of an O-ring into the clearance gap between mating metal parts under pressure. Extrusion causes nibbling, tearing and eventual seal failure.
F
Face Seal
A static seal between two flat mating surfaces, such as a flange or cover plate. The O-ring is compressed axially between the two faces.
FKM (Viton)
A fluoroelastomer with excellent high-temperature and chemical resistance. Standard FKM operates from -20C to +200C and is widely used in fuel and aerospace systems.
G
Gland
The machined groove and surrounding hardware that contains the O-ring. Proper gland design (groove width, depth and surface finish) is critical for seal performance.
Gland Fill
The percentage of the groove volume occupied by the O-ring. Target gland fill is typically 65-75% for static seals and 75-85% for dynamic seals.
Groove Width
The width of the machined groove that holds the O-ring. Groove width must be large enough to allow the seal to deform without being pinched.
H
HNBR
Hydrogenated nitrile butadiene rubber. HNBR offers higher temperature resistance (+150C), better ozone resistance and superior mechanical strength compared to standard NBR.
I
Inside Diameter (ID)
The inner diameter of the O-ring measured at the cross-section centerline. ID determines how the seal fits over a rod or inside a bore.
ISO 3601
The international metric standard for O-ring sizes, organised by inside diameter and cross section in millimetres. The main series are A (1.80 mm), B (2.65 mm) and C (3.55 mm) cross sections.
N
NBR (Nitrile)
Acrylonitrile butadiene rubber. The most common O-ring material due to its excellent oil resistance, good mechanical properties and low cost. Operating range -40C to +120C.
Nibbling
A failure mode where small pieces are torn from the O-ring surface due to extrusion into a clearance gap, usually caused by high pressure or excessive clearance.
O
O-Ring Kit
An assortment of O-rings packaged for maintenance and repair. Common kits include AS568 assortments, metric assortments and material-specific sets.
Outgassing
The release of volatile components from an elastomer in vacuum or high-temperature environments. Low-outgassing grades are required for aerospace and semiconductor applications.
R
Radial Seal
A seal installed between an inner and outer cylindrical surface, such as a piston or rod seal. The seal is compressed radially between the bore and the shaft.
S
Spiral Failure
A twisting failure in dynamic seals where the O-ring rolls in the groove, producing a continuous spiral cut mark. Caused by excessive friction or insufficient lubrication.
Static Seal
A seal installed in a non-moving joint where there is no relative motion between the mating parts. Static seals can tolerate higher compression than dynamic seals.
Surface Finish (Ra)
The arithmetic average roughness of the sealing surface measured in micrometres. For dynamic seals, 0.2-0.4 micrometres Ra is typical. Static seals can tolerate slightly rougher surfaces.
T
Tensile Strength
The maximum stress a material can withstand while being stretched before breaking. Higher tensile strength indicates a stronger, more durable elastomer.
Tear Strength
The resistance of an elastomer to the growth of a nick or cut under stress. Important for dynamic seals and seals installed over sharp edges.
Tolerance Class
The permitted dimensional variation for an O-ring. ISO 3601 defines N (normal) and S (precision) classes. Tighter tolerances are required for critical aerospace and medical seals.
V
VMQ (Silicone)
Polydimethylsiloxane rubber. VMQ offers the widest temperature range (-60C to +230C) and excellent FDA compliance, but poor oil resistance and low tear strength.
X
X-Ring (Quad Ring)
A four-lobed elastomer seal that provides lower friction, better stability in the groove and longer life than a standard O-ring in many dynamic applications.
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