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O-Ring Failure Guide

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O-rings are subject to failure for a number of reasons. Contributing factors and suggested solutions for the most common issues are outlined below. You can also contact the team at MHS Seals to find the most suitable solution for your application.

 

Extrusion or Nibbling

Over-compression

Spiral failure

Heat hardening/thermal degradation

Explosive decompression

Chemical degradation

Abrasion

Plasticizer extraction

Installation damage

Weather or Ozone cracking

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Extrusion - click to enlarge

 

Extrusion or Nibbling

The seal develops ragged edges, generally on the low pressure side, which appear tattered. This condition is more common with high pressure systems.

Contributing factors

Suggested solutions

Excessive clearances Decreased clearances
Excessive system pressures Decrease system pressure if possible
Irregular clearance gaps due to eccentricity Use back-up ring
Sharp groove edges Increase rigidity and concentricity of metal components
Low-modulus/low-hardness elastomer Break edges of groove to minimum of .004" (0.10mm)
Softening of elastomer due to fluid incompatibility Use higher-modulus/higher hardness elastomer
Excessive gland fill Use more chemically compatible elastomer
Expansion of cylinder wall due to pressure Increase groove width or change o-ring size
  Stiffen cylinder wall to limit expansion

 


Over compression - click to enlarge

Over-Compression

The seal exhibits parallel flat surfaces corresponding to the sealing surfaces.
May also develop circumferential splits within flattened surfaces.

Contributing factors

Suggested solutions

Excessive compression squeeze Use smaller o-ring or adjust gland dimensions
Elastomer with poor compression set properties Use material with better compression set resistance
Elastomer with inadequate heat resistance Use more heat resistant elastomer
Elastomer that swells excessively in system fluid Use more chemically resistant elastomer
Improperly cured part used Check cure state of parts prior to installation

 


Spiral failure - click to enlarge

Spiral Failure

The seal surface exhibits a series of deep, spiral, 450 angle cuts. This failure is often seen with long-stroke, hydraulic piston seals.

Contributing factors

Suggested solutions

Eccentric component Increase rigidity and concentricity of metal components
Wide clearances in combination with side locks Decrease clearances
Uneven surface finishes Machine metal surfaces to 10 to 20 pinch surface finish
Inadequate lubrication Specify an external lubricant or use an internally lubricated material
Elastomer is too soft Use a higher durometer material
Stroke speed too slow - dynamic reciprocating Increase stroke speed or use D-ring instead of O-ring
Excessive gland fill  
Expansion of cylinder wall due to pressure  

 


Thermal degradation - click to enlarge

Heat Hardening/Thermal Degradation

The seal may exhibit radial cracking on the highest temperature surfaces, often accompanied by the flattening of the seal characteristics of over compression. Certain elastomers may exhibit signs of softening, such as a shiny surface.

Contributing factors

Suggested solutions

Thermal properties of elastomer Select more heat-resistant elastomer
Excessive temperatures, temperature excursions or temperature cycling Select elastomer containing antioxidants
  Consider possibility of cooling sealing surfaces
  Increase thermal mass to dampen temperature cycling or excursions
  Change the position of the gland away from heat source

 


Explosive decompression - click to enlarge

Explosive Decompression

Explosive decompression results when high-pressure gases are absorbed by the seal, and then, as the pressure is rapidly dropped, the expanding gases are trapped in the micropores of the elastomer, causing surface blisters and ruptures as they escape. The affected seals will exhibit random short splits or ruptures deep into the seal cross-section. When first removed the surface may also be covered with small blisters.

Contributing factors

Suggested solutions

Rapid pressure drop from high pressure Slow the release of system pressure
Low-modulus/low-hardness elastomer Specify a higher-modulus/higher-hardness material
  Specify a decompression-resistant material

 


Chemical degradation - click to enlarge

Chemical Degradation

The seal may exhibit many signs of degradation including blisters, cracks, voids, or discolouration. However, in some cases the degradation is only detectable by measurement of physical properties.

Contributing factors

Suggested solutions

Incompatibility with chemical environment Use more chemically resistant polymer
  Use PTFE encapsulated o-ring
  Decrease temperature that o-rings sees, as higher temperatures accelerate chemical degradation

 


Abrasion failure - click to enlarge

Abrasion

Abrasion occurs only with dynamic seals - seals involved with a rotary oscillating or reciprocating motion. The seal or parts of the seal exhibit a single flat surface parallel to the direction of motion. Loose particles and scrapes may be found on the seal surface.

Contributing factors

Suggested solutions

Rough sealing surface Use recommended gland surface finishes
Sealing surfaces too smooth to allow for adequate lubrication Eliminate abrasive components or protect seal from exposure to them
Process environment containing abrasive particles  

 


Plasticizer extraction - click to enlarge

Plasticizer Extraction

Seen primarily in fuel systems, plasticizer extraction is characterised by a loss of volume or weight of the seal. It is often difficult to detect with only a visual inspection.

Contributing factors

Suggested solutions

Heavy use of plasticizers to achieve low-temperature properties or hardness Switch to elastomer with low temperature properties so plasticizers aren't needed
Exposure to organic solvents compatible with plasticizers Change plasticizers used to ones less compatible with process fluids

 


Installation damage - click to enlarge

Installation Damage

The seal or parts of the seal may exhibit small cuts, nicks or gashes.

Contributing factors

Suggested solutions

Sharp surfaces on glands or components Break all sharp metal and cover threads with tubes or tape for installation
Inadequate lead-in chamber Provides a 150 lead-in chamfer of adequate length so o-ring sees only chamfer
O-ring too large for gland Review gland and o-ring design per recommended design standards
Low-modulus/low-hardness elastomer Specify a higher-modulus/higher-hardness material

 


Weather or ozone cracking - click to enlarge

Weather or Ozone Cracking

Occurring in seals exposed to ozone, UV radiation or other air pollutants, weather or ozone cracking is characterised by small surface cracks perpendicular to the direction of stress.

Contributing factors

Suggested solutions

Exposure to ozone, UV radiation or other pollutants Select more ozone and UV-resistant elastomer
Excessive seal stretch (>5% ID stretch) Apply anti-ozonant or wax coating to seal
  Modify the design to avoid the damaging exposure
  Modify design to reduce stretch to less than 5%