In this article, we are going to look at five of the most common phenomena involved with polymer seal
failure, and what causes are behind the failure. Lets start with extrusion.
Extrusion
One of the common causes of seal failure is extrusion. If the extrusion took place on the static side of
the seal, then either the back-up ring is too small or the support surface is somewhat uneven. If it is
extruded on the dynamic side, then there could be one of three causes: extreme pressure, worn
bearings, or too large a gap between the mating surfaces.
Fracturing
Fracturing can take several forms, such as chunks missing or long cracks. If you see entire chunks
missing from the dynamic side, the cause is most likely excessive back pressure. If you see the pressure side of the seal broken and burned, the cause could well be the explosion, at high pressure, or residual air. Long cracks appearing in the V-portion of a seal can result from frequent spikes in pressure or too low a temperature at system start up.
Hardening
Another issue with polymer seals is hardening of either the dynamic face, which is evidenced by glazing and cracks, or a general loss of elasticity in the entire seal. Hardening of the face is most often caused by excessive speeds generating heat. Loss of elasticity can be associated with compatibility issues between the fluid and the sealing material or high fluid temperatures. Both types of hardening are related to heat, however. Always check the temperature rating for the seal you are specifying, and keep in mind its performance at extended periods of high temperature.
Scarring
Scarring is another common cause of polymer seal failure. If you see a dent or cut on the lip, that
suggests improper storage or use of an installation tool that has sharp edges. If you see scratches
appearing on the dynamic side of the seal, then the cause is either scars or in the bore or rod, or a
foreign material in the fluid. If the cause is in the fluid, the system needs to be flushed.
Swelling
Swelling occurs when the seal material becomes soft and loses its shape. This is usually the result of
either fluid absorption (with water being the most likely culprit) or incompatibility between the seal and
the fluid. Certain polymers, such as nylon, are very susceptible to swelling.
Wear
One of the most common causes of seal failure is simply wear, most often on the dynamic face of the
seal. If the seal wears out before its lifetime, the cause is most likely inadequate lubrication or an
excessive lateral loading. However, looking at the symptoms of wear can provide a bit more
information. For example, if the dynamic face has been worn until it almost has a glossy finish, then the problem is most likely too little lubrication. If you look at the dynamic lip and see egg-shaped wear, that suggests issues with the piston bore or rod not being concentric. If you see that one side of the dynamic lip is worn, then the cause is likely excessive lateral loading or a worn bearing. Keep in mind that normal wear is to be expected and is unavoidable, though.
Conclusion
Extrusion, fracturing, hardening, scarring, swelling, and wear these are some of the most common
causes of seal failure. Even if you size the seal correctly and select the most appropriate material, you
still have to consider the surface finish of the shaft or bore, sufficient lubrication, pressure spikes, lack
on concentricity, and installation. A seal works as part of a system, and it is dangerous to focus too
much on the seal without looking at the system as whole. Dont forget to design your seal for the
system, and be aware of things like pressure spikes, lateral loads, and start-up temperatures.