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PTFE Rotary Seal Misalignment RepercussionsPTFE-rotary-lip-seal-design

 

Misalignment of the shaft in PTFE rotary seal applications results in eccentricity and shaft runout, both of which cause their own set of problems.  In this short article, we will take a quick look at the repercussions of misalignment.

 

 

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PTFE Seal Eccentricity

Eccentricity occurs when the shaft centerline and the bore centerline don’t line up.  This is also known as shaft-to-bore misalignment, or STBM, or coaxiality. Eccentricity results in a couple of problems, one being additional wear on one side of the seal lip because one side will subjected to greater force.  This happens because the seal lip is following a shaft not centered in the bore.  The opposite side will not be loaded as much, and as a result will not be able to provide as much sealing.  Another problem that eccentricity can cause is for the extrusion gap on one side to be larger, which can become a serious issue for high pressure applications.

For a shaft at rest, the shaft-to-bore misalignment should never be more than 0.25 mm.  Note that eccentricity is usually the result of inaccuracies in assembly or machining.

 

PTFE Seal Shaft Runout

Shaft runout, also known as dynamic runout or DRO, is another manifestation of misalignment but tends to be most problematic at higher speeds.  It can be caused by a bent shaft, vibration, or an imbalance.  The shaft is spinning on an axis of rotation that is not aligned with the geometric center of the shaft. 

If the seal isn’t flexible enough to maintain contact with the shaft, it will not be as effective.  If the distance between the sealing lip and the shaft becomes larger than what is needed to maintain a hydrodynamic lubricant film, then whatever the seal is supposed to be sealing in can escape through that gap.  As mentioned, this is more problematic at high speeds because the inertia of the seal may prevent it from maintaining contact with the shaft.

As with eccentricity, the shaft runout when measured dynamically should never exceed 0.25 mm.  The higher the DRO, the shorter the life of the seal.

 


 Conclusion

To some degree, every shaft is going to have some eccentricity and/or runout.  As a general rule of thumb, the static eccentricity and dynamic runout for rotary mechanical seals should both be kept below 0.25 mm.

 

Do you have more questions about PTFE Rotary Shaft Seals get the Advanced EMC Technologies Sealing Soltuions Guide for more information.

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