1. COMPRESSION SET
Description of failure appearance:
This failure is common to both static and dynamic sealing applications. This type of failure produces flat surfaces on the sides of the O-Ring that were compressed, usually the top and bottom if sitting flat on a desk.
The sources of COMPRESSION SET failure:
· The O-Ring material used has poor compression set resistance
· The O-Ring material used has limited resistance to heat
· The O-Ring is swelling in the groove due to fluid incompatability
· The O-Ring has too much squeeze in the groove
How to eliminate COMPRESSION SET failure:
· Use a higher quality, low compression set material
· Check the compatability of the O-Ring material to the fluid
· Select a material good for heat produced in operation
· Double check the groove dimensions for proper squeeze
· Check existing O-Ring stock for physical properties
2. ABRASION
Description of failure appearance:
This failure is most ommon in dynamic sealing applications - like reciprocating and rotating shafts. This type of failure produces a flattened surface on the side of the O-Ring body subjected to the movement.
The sources of ABRASION failure:
· The metal surfaces are too rough and are abrasive to the O-Ring
· The metal surfaces are too smooth, not allowing proper lubrication
· No lubrication in the design
· Operating temperatures are too high for the material
· The system fluid is contaminated with abrasive particles
How to eliminate ABRASION failure:
· Change surface finishes to recommended
· Arrange for better lubrication
· Use a material suitable for higher temperatures
· Eliminate any source of contamination
· Change to a more abrasion resistant O-Ring material
3. INSTALLATION DAMAGE
Description of failure appearance:
This failure can occur in both static and dynamic O-Rings. Short nicks or scratches or peeling on the surface of the O-Ring can be noticed.
The sources of INSTALLATION failure:
· The use of sharp edged tools
· Sharp corners on the O-Ring groove
· Sharp threads that the O-Ring passes over
· No lead-in chamfer
· O-Ring was not lubricated
· O-Ring was twisted or trapped between metal surfaces
· Poor quality of the material
How to eliminate INSTALLATION failure:
· Cover all threads with masking tape
· Break all sharp edges
· Create a 15 to 20 degree lead-in chamfer
· Lubricate O-Ring during installation
· Use correct sized O-Ring
4. EXPLOSIVE DECOMPRESSION
Description of failure appearance:
You will find random ruptures, crater-like pores and small slits, which have originated within the body of the O-Ring.
The sources of EXPLOSIVE DECOMPRESSION failure:
· Gases permeating the O-Ring material
· Rapid decompression of those gases
· Micro-explosions occuring as decompression takes place
How to eliminate EXPLOSIVE DECOMPRESSION failure:
· Slow system cycles down
· Increase time for decompression
· Replace with a harder material
· Select a smaller O-Ring cross section
5. HEAT HARDENING
Description of failure appearance:
You will see this failure in both static and dynamic O-Rings. A flattened area will appear on the dynamic surface. Sometimes cracked, hardened and pitted areas can be seen throughout the entire body of the O-Ring.
The sources of HEAT HARDENING AND OXIDATION:
· Temperatures higher than recommended for the material
· Elastomers becoming dry and portions of the material evaporating
· Oxidation
How to eliminate HEAT HARDENING AND OXIDATION:
· Lower the operating temperatures of the system
· Use O-Rings rated for higher temperatures
6. SPIRAL DAMAGE
Description of failure appearance:
The surface of the O-Ring appears to have been twisted, or to have rolled in its groove or against a reciprocating rod. It stays in this position when freed.
The sources of SPIRAL DAMAGE:
· Side loads causing excessive clearance
· Mis-fit components
· No suitable lubrication
· Material too soft
· Moving speed too slow
· Surfaces are uneven
How to eliminate SPIRAL DAMAGE:
· Decrease the clearances between components
· Check for roundness of fitting parts
· Machine surfaces to suitable finishes
· Provide lubrication
· Select a harder material
· Add a back-up ring
7. EXTRUSION FAILURE
Description of failure appearance:
You will see a ridge, nibbles and small missing pieces of the material along either the inner diameter or outer diameter due from the downstream side of the O-Ring.
The sources of EXTRUSION:
· Excessive system pressures
· Too much clearance between mating parts
· Material too soft
· O-Ring body too large for the groove
· Improperly machined groove
· Attack by system fluid
How to eliminate EXTRUSION:
· Decrease or regulate system pressure
· Refit mating parts, machining back to proper, concentric fit
· Select a harder material
· Determine correct O-Ring cross section size
· Add back-up rings
· Re-machine groove to include clean, smooth groove edges
· Replace O-Ring with different type of seal
8. SWELLING FAILURE
Description of failure appearance:
Identified by obvious dimension of the body of the O-Ring. Reduced physical properties, which causes improper fit. Heat and friction excelerates seal failure.
The source of EXCESSIVE SWELL:
· The material absorbs system fluids causing swelling
· An obvious chemical incompatability
How to eliminate EXCESSIVE SWELL:
· Test material for fluid compatability
· Consult a chemical compatability chart to determine suitable material
9. AGING/OXIDATION FAILURE
Description of failure appearance:
Exposure of either static or dynamic sealing O-Ring to weather, atmosphere, polutions and ultra-violet light.
The source of WEATHERING OR OZONE CRACKING:
· Attack of the polymer chains, destruction of the material causing cracking
How to eliminate WEATHERING OR OZONE CRACKING:
· Select a material that resists ozone exposure
10. ATYPICAL FAILURE
Description of failure appearance:
Difficult to pin-point due to non-obvious, unseen reasons
The source of NO APPARENT REASON FOR FAILURE:
· Combined tolerances of all components not correct
· Not enough squeeze
· Parts not fitting properly
· Components not round
· Too much flash, remaining rubber on O-Ring
· Improper groove shape
How to eliminate NO APPARENT REASON FOR FAILURE:
· Re-machine all parts for proper sizing tolerances
· Make sure the amount of squeeze is correct
· Double check the design
· Replace components not fitted correctly
· Select a different material or O-Ring source for clean ground outer diameters
· Check published data for groove shapes and sizing
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