Teflon 'O' Rings.........not good.

Teflon \'O\' Rings.........not good.

I posted a few weeks ago about my success with Teflon Tape and Teflon lube.See HERE
for details.
Anyway I decided to order some pure Teflon O rings to see if these would have any application in out lights.
Well that was a bust......they are slippery all right but have no flexibility or give. Once stretched to fit in the groove, they did not spring back to their original size.
Should have listened to the master......

Re: Teflon \'O\' Rings.........not good.

David,

I don't know much about the physical properties but as I have learned some, there are terms that go beyond those we typically hear. Most people have encontered tensile strength, shear strength, moduleous (sp) of elasticity, coefficient of thermal expansion (CTE), and others. One that I first encountered in regards to spectra fiber was creep. What a funny but real property of a material! /ubbthreads/images/graemlins/grinser2.gif Well it turns out that teflon is a material that really creeps!! The teflon plumbers tape works so well because of this creep. A teflon O-ring is great as a static seal under compression provided the pressures are not enough to force the teflon to creep away. /ubbthreads/images/graemlins/grin.gif

Re: Teflon \'O\' Rings.........not good.

Creep is a behavior which results from a load being sustained over time. This is why barbed-wire fences get loose after some years, and why bowstrings need a few twists added after they are put on the bow and shot a few dozen times.

This behavior with the O-rings sounds more like plastic yielding, i.e., the molecules in the material slide past one another under an acute (short duration) load. Some plastics have really strange behaviors under load compared to metals, and can change their behaviors altogether when heat is applied. For example, they can go from brittle fracture-type failure to flexible yielding-type failure under the same load as temperature is increased.

I kind of doubt you will be able to get the O-rings to rebound, but your best bet is probably heating prior to installation. This will lower the elastic modulus significantly, causing the stress in the stretched ring to be greatly reduced. If you can keep the stress below the yield point (more of a region for plastics), then you can keep the ring within in elastic behavior, where it can still rebound after the load (from forced strecthing) is removed or reduced. In other words, heating the ring will make it stretchier, so it will take more deformation before taking a set. You want the molecules to stretch rather than slide past one another.

Scott

[sorree, cant spel todai. . . or form a logical paragraph, by the looks of it]

Re: Teflon \'O\' Rings.........not good.

Interesting observations guys. In my enthusiasm I should have checked to see how resilient they were, before placing an order. Anyway If anyone needs some, I have them in 015,017,and 019.<g> ( All gone ! Updated pm, 13th July )
Always like to try out new ideas. Some work, this certainly did not.

Re: Teflon \'O\' Rings.........not good.

Scott,
I deferr to your expertise on this but teflon still gives me the creeps! /ubbthreads/images/graemlins/nana.gif

I had some dealings with Gore and have some really strange samples of their expanded PTFE products. I also have some of their thread that I have used in some safety tether prototypes and goofy stuff. I was told that creep is a issue with teflon but I take it on faith as to just what those molecules are doing and what the proper terminology is for their behavior! /ubbthreads/images/graemlins/grin.gif

Re: Teflon \'O\' Rings.........not good.

Plastics are just strange. Plastic water pipe can have calcium carbonate as a filler, or carbon as a colorant. The manufacturers can also vary the resultant "crystalinity" -- the extent to which the long chain molecules line up in groups of parallel fibers. You can also have hysteresis effects, where some fraction of rebound will occur, albeit slowly.

There are so many different ways to make a given type of plastic that one generic type like PVC or polyethylene or whatever are (in my business -- civil engineering) categorized by essentially a binning process like Luxeon LEDs. PVC can be in any form from highly flexible and tough to very rigid and brittle just by varying the additives and process.

If you see creep effects in a teflon O-ring, probably it would manifest itself by a tight fit initially aand then a loose fit later. The initial loose fit sounds more like yielding to me. It is kind of splitting hairs to differentiate between yield and creep in plastics, to tell the truth.

Scott

Re: Teflon \'O\' Rings.........not good.

Scott,
For my own peace of mind in differentiating between yield, stretch and creep, I figured stretch to have rebound, yield to be elongation or deformation apparent immediately visible when loaded and with no "return" and creep to show up over some time; it wasn't like that when I left it! /ubbthreads/images/graemlins/icon15.gif

Re: Teflon \'O\' Rings.........not good.

I'm getting out of here before you guys start talking about condoms! /ubbthreads/images/graemlins/grin.gif