For Dummies 5 Questions and Answers about Polymer Bellows

One of the primary uses of bellows is to absorb dimensional changes due to thermal effects, which is very useful when used high temperature flows such as steam.  Bellows also serve to dampen vibration in the system caused by rotating components, protect sensitive and brittle processing equipment, and to absorb shock loadings.

Why is PTFE a popular choice for bellows?

PTFE (also known by its trade name Teflon) is a popular choice for the bellows material.  It is ideal for use in highly corrosive environmentssuch as those involving strong oxidizing and reducing accents, salts, high concentrations of acid, and chemically active organic compounds.  It has an extremely long flex life (how many flexing cycles it can handle before it fails), and a very low spring rate (amount of force needed to flex the bellows) – which means that it can reliably handle the challenge of fluctuating and vibrational loadings.

What types of movements can bellows be used to absorb?

There are three types of movement that bellow expansion joints can absorb:  axial deflection, lateral deflection, and angular deflection.  Axial deflection includes compression and extension affects along the longitudinal axis of the bellows.  Lateral deflection occurs when the end joints of the bellows displace relative to each other. Also known as parallel misalignment, this type of deflection can also be absorbed by a bellows expansion joint. Angular deflection can be described as a rotational displacement, or twisting displacement.

How does the number of convolutions affect bellow performance?

Recall that a convolution is the smallest flexible unit in a bellows.  The general heuristic for bellow convolutions is this:  fewer convolutions will give you better pressure and temperature ratings, BUT the amount of movement it can handle is more limited than bellows with more convolutions.  More convolutions, on the other hand, can absorb more movement but at a cost in pressure/temperature ratings.

Are there other polymers used for bellows?

Yes, another polymer option for bellows is UHMW PE, ultra-high molecular weight polyethylene.  While not as chemically resistant as PTFE, it currently has the highest impact strength of any polymer on the market today.  If the bellows are used in connection with abrasive materials, UHMW PE would be a valid alternative to PTFE because it has better abrasion resistance.

PFA, or Perfluoroalkoxy or Teflon PFA, is similar to PTFE(teflon) in many ways and is someone chosen in place of PTFE because it offers higher strength at extreme temperatures, even in the presence of extremely aggressive chemicals.

TFM, or PTFE-TFM, is a second-generation PTFE that has better fatigue properties than PTFE and offers better stress recovery.  It is well adapted for situations that involve high temperatures and vacuum pressures.

Bellows Conclusion

Bellows serve a variety of purposes – form absorbing displacement and shock to preventing sensitive equipment of a brittle nature.  They can absorb axial, lateral, and angular displacements.   The number of convolutions in a bellow is related to both its strength and pressure rating as well as the maximum amount of displacement it can absorb.  Finally,  polymers such as PTFE(teflon), UHMW PE, PFA, and TFM are popular choices for bellows materials, although PTFE seems to remain the first choice for many engineers.

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