Avoiding common causes of O-ring failure

O-rings are the most frequently used seals in fluid-power systems. However, elastomeric O-rings are unlike most of the materials that engineers and designers encounter. The reason: O-rings must deform to function properly. O-rings are shaped like a donut and are installed in cavities known as glands where they are compressed.

02/10/2004


Key Concepts

 

  • O-rings should be tested in service conditions.

  • Compression set failure is common to static and dynamic applications.

  • Abrasion failure occurs primarily in applications involving motion.

    Sections:
    Failure without visible evidence on seal
    Compression set
    Extrusion and nibbling
    Abrasion
    Heat hardening and oxidation
    Damage during installation
    Excessive swell
    Spiral failure
    Weather or ozone cracking
    Explosive decompression
    More Info:


    O-rings are the most frequently used seals in fluid-power systems. However, elastomeric O-rings are unlike most of the materials that engineers and designers encounter. The reason: O-rings must deform to function properly.

    O-rings are shaped like a donut and are installed in cavities known as glands where they are compressed. Zero clearance within the gland provides a seal that blocks the flow of liquids and gases. This simple arrangement serves many fluid-power systems very well, but success requires careful design, selection, and installation procedures.

    Most common causes of O-ring failure:

    • Improper gland design

    • Incompatibility of the O-ring elastomer and its environment

    • Improper installation

    • Inadequate lubrication.

      • The combination of stresses that act on O-rings can be complex and difficult to evaluate. It is very important that both the O-ring compound and size be tested in an environment that represents service conditions.

        Failure without visible evidence on seal

        Failure pattern: Of the various types of seal failure, this is among the hardest to diagnose because the result of the problem is not visible on the O-ring. There are no visual clues.

        Problem sources:

        • Insufficient compression

        • Tolerance stack-up

        • Eccentric-shaped components

        • Parting lines and/or flash from molding

        • Improper volume relationship of seal and gland.

          • Suggested solutions:

            • Maintain recommended compression range

            • Identify amount of stretch; increased stretch reduces the O-ring cross-section

            • Determine component tolerance stack-up; it directly affects the seal cross section

            • Consider maximum component shift in design to ensure that compression is still maintained within the recommended range

            • Avoid parting lines in O-ring grooves; they tend to be areas of flash and mismatch

            • Ensure the O-ring gland volume surpasses the O-ring volume to allow for seal expansion without seal detriment.

              • Compression set

                Failure pattern: Common to both static and dynamic sealing applications, compression-set failure produces flat surfaces on both sides of the O-ring's cross section (Fig. 1).



                Problem sources:

                • Selected elastomer has poor compression-set properties

                • Low heat resistance of elastomer

                • Excessive swelling of O-ring material in system fluid

                • Too much squeeze to achieve seal

                • Incomplete curing of O-ring material during production.

                  • Suggested solutions:

                    • Use a low-set elastomer

                    • Specify an O-ring material that resists both operating and friction-generated heat

                    • Re-check O-ring material compatibility with system chemicals

                    • Reduce O-ring squeeze

                    • Inspect incoming O-rings for correct physical properties.

                      • Extrusion and nibbling

                        Failure pattern: Typical of high-pressure systems, this pattern can be identified by the many small bites or nibbles taken out of the O-ring on the low-pressure or downstream side (Fig. 2).



                        Problem sources:

                        • Excessive gland clearance

                        • Excessive system pressure

                        • O-ring material too soft

                        • Degradation of O-ring by system fluid

                        • Irregular clearance gaps caused by eccentricity

                        • Improper machining of O-ring gland, leaving sharp edges

                        • O-ring size too large for gland.

                          • Suggested solutions:

                            • Decrease gland clearances by machining

                            • Use back-up rings to prevent extrusion

                            • Use harder O-ring elastomer

                            • Re-check elastomer compatibility with system chemicals

                            • Increase rigidity and improve concentricity of metal components

                            • Break sharp edges of gland to a minimum radius of 0.005 in.

                            • Install proper size O-ring

                            • Consider substituting a reinforced composite seal, such as rubber bonded to metal.

                              • Abrasion

                                Failure pattern: This failure pattern occurs primarily in dynamic seals involving reciprocating, oscillating, or rotary motion and is identified by a flattened surface on one side of the O-ring's cross section.

                                Problem sources:

                                • Metal surfaces of gland are too rough and act as an abrasive

                                • Metal surfaces are too smooth causing inadequate lubrication

                                • Poor lubrication

                                • Excessive temperature

                                • System fluid contaminated with abrasive particles.

                                  • Suggested solutions:

                                    • Use recommended metal finishes

                                    • Provide adequate lubrication (consider internally lubricated O-rings)

                                    • Check elastomer compatibility with system temperature

                                    • Eliminate abrasive contamination with filters and/or wiper seals

                                    • Consider changing to a more abrasion-resistant O-ring elastomer, such as carboxilated nitrile or urethane.

                                      • Heat hardening and oxidation

                                        Failure pattern: Seen in both static and dynamic seals, the surface of the O-ring appears pitted and/or cracked, often accompanied by the flatness of high compression set.

                                        Problem sources: Excessive temperature causing elastomer hardening, evaporation of plasticizers, or cracking from oxidation.

                                        Suggested solutions:

                                        • Specify high-temperature O-ring elastomer with antioxidants

                                        • Lower operating temperature of hydraulic system.

                                          • Damage during installation

                                            Failure pattern: This failure mode occurs in both static and dynamic seals, and is marked by short cuts or notches, or a skinned or peripherally peeled surface.

                                            Problem sources:

                                            • Sharp edges on mating components of the O-ring gland

                                            • Sharp threads over which the O-ring must pass during assembly

                                            • Insufficient lead-in chamfer

                                            • Oversized O-ring ID on piston

                                            • Undersized O-ring ID on rod

                                            • O-ring was twisted or pinched during installation

                                            • O-ring was not lubricated during installation

                                            • O-ring elastomer has low tear resistance (typical of silicone).

                                              • Suggested solutions:

                                                • Break all sharp edges

                                                • Cover threads with a tube or tape during O-ring installation

                                                • Provide a 0.015 in. to 0.020 in. lead-in chamfer

                                                • Install correctly sized O-ring

                                                • Use lubrication during assembly.

                                                  • Excessive swell

                                                    Failure pattern: Easily identified by a marked increase in seal dimensions, it can occur in both static and dynamic applications. It results in a reduction of physical properties and can cause improper sizing between seal and gland. Dynamic applications are especially prone to this problem because friction accelerates seal failure.

                                                    Problem sources: Like a sponge, the seal absorbs the surrounding fluids and swells to the point of malfunction because of incompatibility between seal compound and system environment (chemical incompatibility, high humidity, etc.).

                                                    Suggested solutions: Select an O-ring elastomer that is chemically compatible with the fluid it will contact.

                                                    Spiral failure

                                                    Failure pattern: Generally found on long stroke, hydraulic piston seals, the surface of the O-ring exhibits a series of deep, spiral, 45-deg angle cuts (Fig. 3).



                                                    Problem sources: Caused when some segments of the O-ring slide while other segments simultaneously roll. At a single point on its periphery, the O-ring gets caught on an eccentric component or against the cylinder wall, causing twisting and development of 45-deg angle surface cuts.

                                                    Contributing conditions:

                                                    • Eccentric components

                                                    • Wide clearance combined with side loads

                                                    • Uneven surface finish

                                                    • Inadequate lubrication

                                                    • O-ring elastomer is too soft

                                                    • Stroke speeds are too slow.

                                                      • Suggested solutions:

                                                        • Check the cylinder bore; it may be out of round

                                                        • Decrease the clearance gap

                                                        • Machine metal surfaces to a 10%%MDASSML%%20-

                                                        • Improve lubrication (consider substituting internally lubricated O-rings)

                                                        • Increase O-ring elastomer hardness and/or cross-sectional area

                                                        • Add antiextrusion backup rings.

                                                          • Weather or ozone cracking

                                                            Failure pattern: This failure mode occurs in both static and dynamic seals exposed to atmospheres containing ozone and other air pollutants and is marked by the appearance of many small surface cracks perpendicular to the direction of stress.

                                                            Problem sources: Ozone attacks the polymer chains, causing the O-ring elastomer to crack.

                                                            Suggested solutions: Substitute an O-ring elastomer that is resistant to ozone attack.

                                                            Explosive decompression

                                                            Failure pattern: This failure mode is marked by random short splits or ruptures going deep into the O-ring's cross-section. When the O-ring is first removed, the surface may also be covered with small blisters.

                                                            Problem sources: Absorption of gas by the O-ring while operating in high-pressure conditions. A subsequent rapid decrease in system pressure traps gas within the O-ring's micropores, causing surface blisters and ruptures as the gas escapes.

                                                            Suggested solutions:

                                                            • Increase decompression time

                                                            • Change to an elastomer that has a hardness in the 80%%MDASSML%%95-durometer range

                                                            • Reduce the O-ring's cross-sectional size.

                                                              • More Info:

                                                                Additional information is available in the "Seal Design Guide" on applerubber.com or on CD by calling 716-684-6560. An Engineering Assistance Request form is also available on the web site. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com





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