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Old 03-06-2013, 04:34 PM   #124
supershaft
because I can
 
Joined: Sep 2010
Location: San Francisco Bay area
Oddometer: 8,446
Quote:
Originally Posted by Plaka View Post
Teflon (Polytetrafluoroethylene (PTFE) really isn't all that slippery. Compare it to UHMW-PE or even Nylon and they're all close. Teflon has very low surface energy so things don't stick to it. But again, compare it to the polyethelenes and it's close. Try sticking something to them, especially HMW-PE and UHMW-PE. Where teflon shines is it's ability to take heat. Line a frying pan with PE and it will melt all over your stir fry. But Teflon is also very soft. handle a sheet or block of it and you get a very different feeling for it than poking a frying pan. You can dent it with your fingernail. it takes abrasion just as poorly in contrast to UHMW-PE which is one of the most abrasior resistant of polymers.

Used in a seal, Teflon will take heat, will have low intrinsic friction and will wet poorly---just as poorly with oil as with water or glue. At a microscopic level oil wets out a steel surface but beads up on a Teflon one. So what's happening at the interface of the seal lip and the shaft is quite different that what's happening with something like a Viton (another floropolymer) seal.

I'd say the upshot is if you have a polished surface to run the seal lip on I'd use it. In general textured surfaces have lower friction, especially when coated with oil but given the softness of Teflon I'd be concerned with wear. The polished surface may give longer life.

The Teflon may be filled with something (like graphite or Moly) and if it isn't white or light tan this is likely. Nylon is commonly filled with graphite which makes an interesting material. You would have to look at a seal under a microscope to see what's happening there. (if someone wants to send me one I'll check it out. Needs to fit an '83 R100 motor). Anyway the filler could both stiffen it and give it some abrasion resistance.

Making a seal that can be installed dry isn't for the field people, it's for manufacturing. You can load a magazine with them and just punch them into the blocks as they come by on the line, fully automated if it's set up, and you don't have a mess or the need to accurately control lubricants automatically. So the seal salesman tells the OEM, "look at our new seal, you can install it dry!" (if the manufacturer didn't request it in the first place). What trickles down to the field is, "don't lube them".
That long explanation is forgetting the fact that BMW first recommended soaking the later seal in oil for some time and preforming the seal and whatnot. Knowing that makes your explanation nonsensical.

The later seal has a completely different contact pattern on the flywheel. It contacts a much, much wider area than the typical rubber seal. That's probably the reason why they work so much better. Most of the time it will be impossible to miss a groove in the flywheel left by a rubber seal. The newer seal's contact area is too wide to miss the groove by setting the seal shallow or deep. I know it's just more blah blah but I have noticed this from installing a great deal them.
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