O-Rings, Backup Rings and X-Rings

Product Info


A doughnut shaped gasket used to prevent the loss of a fluid or a gas, generally made from an elastomer. The O-ring seal assembly usually consists of an elastomer O-ring and a groove (gland). The groove is usually cut into a rigid material to contain the O-ring.

Design Principle

O-ring Seal:

An O-ring seal is a circular cross-section ring, which is placed in a gland that is machined into a rigid material. The seal is created by the deformation of the O-ring while it is confined in the gland. When the O-ring is subjected to a pressure differential across the gland, the O-ring is pressed up against the low-pressure side. The O-ring is mechanically squeezed out of round, sealing the passageway through the gland.

O-rings are typically made from an elastomer material which is a highly viscous material having the ability to revert back to its original shape over many cycles. The selection of the correct elastomer is crucial to the longevity of the seal. Each elastomer has different allowable operating criteria for different fluids, temperatures and pressures allowing for a tailored solution.

The O-ring should be designed such that the O-ring will not enter the narrow gap between the inner and outer members of the gland, but remain within the gland itself. The typical failure mechanism occurs when the O-ring is pressed through the gap (extruded).

O-rings must be radially compressed between the bottom of the seal groove and the cylinder wall for proper sealing action. The gland can be machined in the cylinder wall or the piston without any change in design limitations. Dynamic seals may fail by abrasion against the cylinder or piston walls. In dynamic applications it is important that these contacting surfaces be polished for long seal life.

To correctly select an O-ring for a given application the seal gland type, fluid, pressures, operating temperatures and chemical interactions must be considered.

Quad Seal (x-ring):

This type of ring has a cross-section with four-lobes that make the shape of an X. Quad seals provide double the sealing surface of a standard O-ring. The increased sealing surface requires less compression to maintain a seal. Less compression increases the life of the seal due to a decrease in the friction acting on the seal. The X shape fights against spiral twisting creating a stable seal. These quad seals are available in standard O-ring sizes. Please refer to our sizing chart for approximate dimensions.

Back Up Rings:

Back up Rings add protection to an O-ring application by preventing the O-ring from being forced into the passageway through the gland (extrusion). Back-up rings are made of relatively hard material and installed in the gland between the O-ring and the low-pressure side of the seal. Extrusion of an O-ring is a common failure mode of O-rings that can be minimized with the installation of a back up ring.



Provide a simple and effective seal that can be used for thousands of applications.  O-rings are manufactured in an array of materials to match any design needs.

Quad Seal (x-ring):

These can be used in a wide variety of static and dynamic sealing applications.  Quad Seals should be used when seal life is important.  Quad Seals allow for a narrower gland over standard O-rings.  These are well suited for use on high pressure and pulsating pressure applications.

Back Up Rings:

These are commonly used for pressures exceeding 1500 psig or for designs with a relatively large passageway through the gland.  Back-up rings allow for slightly lower machining tolerances and increase the seal performance and seal life.

Technical Library

Gland Considerations


O-Ring, Backup Ring, and X-Ring Materials
Nitrile (Buna / NBR) Most commonly used elastomer for O-rings and sealing applications. Nitrile is the material of choice in petroleum applications and can be specified for the temperature range of -20F to 212F. Nitrile has a high resistance against abrasion and tearing.
Ethylene-Propylene (EPDM) Especially useful in chemical applications where other elastomers would be affected. EPDM has excellent ozone resistance properties as well. EPDM has a temperature range of -60F to 250F.
Hydrogenated Nitrile (HNBR) Provides larger temperature ranges, increased resistance against seal extrusion and excellent chemical resistance than Nitrile o-rings.
Viton (Fluorocarbon, Fluoroelastomer) Increased chemical resistance and improved high temperature properties over other elastomers. VIton has a large working temperature range of -15F to 400F.
Neoprene (Chloroprene) Resistant to both weather and petroleum oils. Neoprene has a working temperature range of -40F to 250F.
Butyl (Isobutylene / IIR) Extremely low gas permeability rating and diverse chemical resistance properties. Useful in applications requiring air-tight rubber.
Silicone (VMQ, PVMQ) Excellent resistance for temperature extremes and high resistance to compression set. Silicone is not recommended for dynamic O-ring applications due to relatively low tear strength and coefficient of friction.
Fluorosilicone (FVMQ / FK) Combines the good high and low temperature properties of silicone with added fuel and oil resistance. Primary uses are applications requiring high dry-heat resistance. Fluorosilicone O-rings are not recommended for dynamic O-ring applications due to relatively low tear strength and coefficient of friction.

CTG Inc. can provide guidance on selecting the correct seal material for your application.

Often times applications call for high performance materials. For further information on high performance materials CTG Inc. manufactures, please refer to our high performance materials page.

Sizing - Dynamic Radial

Sizing - Static Radial

Sizing - Static Axial