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sconisbee
06-27-2008, 09:13 PM
Say one wanted to use teflon/ptfe in a shaft seal application sealing against two stainless steel surfaces, for simplisities sake say a teflon seal attached to the inside of a tube, and the shaft a very close fit inside the seal and rotating at around 1000 rpm, what in the way of lubrication would be needed if any?

Not sure ive given enough info ask if any more is needed, this is early on in design thus flexible for change.

Thanks

TGTool
06-27-2008, 10:22 PM
If you could give the shaft size it would allow calculating the surface speed where the shaft and seal meet. I shouldn't think there would be a problem unless the shaft is huge. What is the likely pressure difference between the two sides?

lazlo
06-27-2008, 10:45 PM
Simon, Teflon has a pretty lousy PV rating, which is the metric used to compare the wear rating of bearing and seal materials. The PV rating is the product of rotational velocity and pressure applied to the bearing. Bearings and seals are usually spec'd (MSC and McMaster et al) by "Max PV" (expressed in psi-ft/sec) at a certain temperature, which is the maximum PV value sustainable by the material without destruction.

Pure Teflon has a PV of 2,200 @ 100 feet per minute.
Nylon 901 (the blue stuff) is 3,000.
Nylatron GSM is 5,500.
660 bearing bronze is 75,000.
Aluminum bronze is 125,000.

sconisbee
06-27-2008, 10:50 PM
TGTool, sorry forgot that bit, its a 35mm shaft and well for the pressure difference im not too sure right at the moment as that will depend on further design.

Lazlo, yeah you have a good point about the PV didnt think about that, however i picked teflon for flexability and low friction running almost dry on a polished shaft, however i could change the material in theory, or use glass filled teflon.

lazlo
06-27-2008, 11:06 PM
i picked teflon for flexability and low friction running almost dry on a polished shaft, however i could change the material in theory, or use glass filled teflon.

A lot of the high-tech thermoplastic bushing materials are impregnated with Teflon (or Molybdenum DiSulfide, or both) for low coefficient of friction.

I've used Nylatron GSM a lot, which is the dark grey Nylatron embedded with MoSd. Dry coefficient of friction against steel is .18, compared to .04 for pure Teflon, but more than twice the PV rating. Here's some materials data for the Nylon and Nylatron family:

http://www.alro.com/DIVPlastics/plastic_product_nylatron.htm

Another possibility is FrelonGold, which is a Teflon impregnated bearing material with a PV of 20,000.

Igus also has a line of bearing materials, and some are Teflon impregnated.