PTFE flattening characteristics 1

[Swogs74]

I am a manufacturing engineer trying to come up with a standard for determining the proper o-ring configuration for seals we use. I have no formal plastics training, but I am a mechanical engineer so I understand most of the physics behind what is going on. Can someone tell me if there isan easy way to determine how much a PTFE o-ring will flatten under a given pressure? How much does the cross section of the o-ring change the results. Thanks

 

[Pud]

PTFE is notoriously bad for creep (i.e. a reduction in stress for a given strain)

PTFE "O" rings usually have a core of Viton or similar elastomer and just a thin layer of PTFE.

To answer your question, PTFE will flatten indefinitely.


Cheers


Harry

 

[Swogs74]

Thanks Harry, We are using a solid PTFE o-ring and I understand that it will creep, my question more lies in determining the width of the flat created across the top and bottom at a given force with the OD and ID unconstrained. The second part of my question is does a change in the given cross section create a dramatic difference in the width of the flat at a given force? I am trying to determine the amount of overlap I need without going too far beyond what I need.

 

[patprimmer]

Theoretically he flat will increase to infinity with infinite time.

Realistically, the deformation is slow but constant.

I am sure DuPont will publish a co-efficient for creep or provide graphs for creep, time, temperature and load for various grades of PTFE.

Regards

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[MikeHalloran]

With the OD unconstrained, it may develop a radial fracture as you compress it axially between two planes because of the hoop stress.

PTFE is very nearly a true plastic, except for some strangeness having to do with it being sintered from powder.

To give you an example of the plastic deformation that's possible, I once worked on a line of small ball valves. When you take them apart, the ball seals have spherical surfaces that mate perfectly with the ball, or did before you took it apart. But they're not molded that way; they're cut from round bars in the form of square section rings, and crushed against the ball as the housing is assembled around it (with a big wrench). I.e., the strain is very large, but they survive it.

Of course, just like a rubber o-ring, they're not compressible; you can change the shape of the cavity they're in, drastically, but you can't reduce its volume below the plastic's volume; something else will break. In the case of my ball valves, when the tolerances drifted the wrong way, the brass housing would split on assembly.



Mike Halloran
Pembroke Pines, FL, USA

 

[Swogs74]

Thanks a bunch, this greatly improves my understanding of what really happens when it gets compressed and has altered my attack of the problem just a bit. Thanks for the explaination and education, it should reduce my experimentation time. I appreciate it.

 

[jedward]

consider making the mounting anular ring concave to capture the teflon, similar to the methods used when indium wire is ued to make cryogenic seals.

FEP teflon can be made leakproof to superfluid helium using flat ended cylinder (1.5" diameter with 0.020 flat) to flat surface. long term effects not known (RSI sept 72)