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PFA vs. Teflon

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#1 Gordon Rayner

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Posted 31 May 2013 - 06:43 PM

Mr. Martin Lewis showed, in the Oct. 2003 Sky & Telescope, that PFA ($$) has lower stiction than Teflon, at least against the material(s) he tested. His study was oriented to Dobsonian pads, IIRC. I do not have the article nearby.

Has anyone tried PFA against aluminum, or against Teflon,or Teflon dry spray coated aluminum, or against HDPE, or against one of the counter top laminates, be they rough, pebbly, or smooth, or against...., in an approximately 3 inch diameter washer, or as three small PFA pads (to keep cost lower) in an approximately 3 inch or 4 inch diameter circular arrangement?

Is PFA worth the cost,vs. Teflon, in such a relatively small diameter, where the angular rubbing speed is low compared to the speeds in larger Dobsonian bearings? Is there a source for small pieces of this costly material?

#2 Jeff Morgan

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Posted 31 May 2013 - 08:49 PM

After reading that article I immediately ordered a 6" square piece of PFA. I did not have the elaborate testing apparatus of the article, but I felt at the time it was a bust. Most of the material is still sitting around my shop I imagine.

The best use seems to be to replace just a few standard Teflon pads for "fine tuning" of the mount.

#3 Starhawk

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Posted 31 May 2013 - 09:01 PM

The magic property of Teflon isn't low static friction; it's having static friction with the same coefficient as its sliding friction. That's why there isn't a jump when it starts moving.

And compared to what the mount costs overall, some Teflon isn't all that expensive. You can get it at usplastics.com.

-Rich

#4 Jon Isaacs

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Posted 01 June 2013 - 07:29 AM

The magic property of Teflon isn't low static friction; it's having static friction with the same coefficient as its sliding friction. That's why there isn't a jump when it starts moving.



The difference between static and kinetic friction is often called "stiction" and according to Gordon's reference, PFA has lower stiction than PTFE.

Note that both PFA and PTFE are Dupont fluoropolymers and sold as Teflon and Teflon PFA respectively. Looking at McMaster-Carr, it appears that PFA is considerably more expensive than PTFE. A 3/4 inch rod of PTFE costs $15/foot, a 3/4 inch rod of PFA costs $115/foot.

Jon

#5 AZstar

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Posted 01 June 2013 - 11:17 AM

Hello Gordon,
PFA is a form of Teflon. PFA come in a much finer resin and is injection molded into parts / shapes.
The white opaque version we are all use to seeing is PTFE Teflon. PTFE comes in a pellet form and is compression molded into shapes and needs to be machined into parts.
PFA is a denser material but does not machine as well as PTFE as far as surface finish is concerned.
I would think for bearings, PTFE would be a better choice

#6 Starhawk

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Posted 01 June 2013 - 02:07 PM

No, Stiction is just the Static Friction. Look it up.

You want the Stiction and sliding friction (Or dynamic friction) to be the same for a push mount.

-Rich

The magic property of Teflon isn't low static friction; it's having static friction with the same coefficient as its sliding friction. That's why there isn't a jump when it starts moving.



The difference between static and kinetic friction is often called "stiction" and according to Gordon's reference, PFA has lower stiction than PTFE.

Note that both PFA and PTFE are Dupont fluoropolymers and sold as Teflon and Teflon PFA respectively. Looking at McMaster-Carr, it appears that PFA is considerably more expensive than PTFE. A 3/4 inch rod of PTFE costs $15/foot, a 3/4 inch rod of PFA costs $115/foot.

Jon



#7 Gordon Rayner

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Posted 02 June 2013 - 10:43 AM

The Lewis article , op. cit. , described that in his controlled experiments, the transition from static to dynamic friction occurred at a much lower sliding velocity in PFA than in Teflon. This is desirable. It is the core of his presentation.

I should not have used "sticktion", which could be taken to mean static friction. Lewis used the "sticktion" word also, which could confuse.

#8 Starhawk

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Posted 02 June 2013 - 11:34 AM

Ok- that is indeed desirable. I'm not sure how easy it would be to notice, but it is desirable.

-Rich

#9 Jon Isaacs

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Posted 02 June 2013 - 12:29 PM

No, Stiction is just the Static Friction. Look it up.

You want the Stiction and sliding friction (Or dynamic friction) to be the same for a push mount.

-Rich



I did look it up and it does seem is used to refer to the static friction. In applications where "Sticktion" (the difference between static and kinetic is important) I use stiction as Gordon did, to refer to Sticktion...

I will no longer do that and use Sticktion, instead. In any event, Sticktion is the important parameter in achieving a smooth action in a Dob. The actual force required to more the scope is not particularly important but the difference between the static and kinetic/dynamic friction is important because "sticktion" leads to a jerky action and overshoot.

Ideally the force required to initiate the movement is not greater than the force required to keep it moving.

Jon

#10 Jeff Morgan

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Posted 02 June 2013 - 11:12 PM

No, Stiction is just the Static Friction. Look it up.


I did not look it up. I bought the stuff and put it on my telescope. At the time I was not impressed. Perhaps it deserves another look?

In any event, for those that are curious it is not a very expensive thing to try. Since I did not hear of people raving about PFA, or Dob builders incorporating it, apparently it was a bust all the way around.






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