On Wed, 10 Aug 2011 17:15:35 +0100, "N_Cook"
wrote:

Of course being a seller of lubricants there is no tabling of the long-term
performance as far as immunity to hardening.

NYE also sells laser printer fuser grease. After several aborted
attempts at finding a cheaper substitute, I've resigned myself to
using the overpriced real thing.

In a past life, I designed marine radios. Pots were always a problem.
If "sealed", they would eventually condense some water out of the air,
and rot. If vented, salt encrustation and corrosion would become a
problem. Sealing the shaft was a big help, but re-lubrication was
tricky.

However, ignoring all these mechanical problems, we did have problems
with the grease on the resistance material. It would develop dead
spots, where the wiper would not make contact, usually where the
volume and squelch controls spent most of their time. It was
obviously a lump of grease that the wiper was riding on top of, but
what was the cause?

Basically, silicon grease (PDMS) will absorb small amounts of water
and aromatic solvents. These bring with it salt from the marine
atmosphere. The salt crystallizes out of solution, the grease
encapsulates it, and we have a lump. I played around with various
microscope filters for hours until I could see the tiny salt crystals
imbedded in the grease. Nye sent us some sample greases, which were a
big improvement and which became standard for re-lubrication.

That "velvet feel" becoming
gumming up the works

http://www.nyelubricants.com/products/damping.shtml

You won't appreciate proper pot lube until after you try to operate a
mobile radio or HT when it's really cold, and find all the controls
stuck in place due to thickening of the lubricant.

And pot track lubrication being insufficient to cause
"aquaplaning" and then with aging/ chemical reaction, hardening and forming
a wedging under the wiper and so loosing contact for all or most of the
track run.

I've never seen this wedging. I have seen salt lumps formed when the
pot is left in one position for long periods, in a hostile atmosphere.

A job for tomorrow is to take some measurements of one of those tiny preset
size pots.
The wiper arcs are something like 3 off 10mm long , and section .2x.4mm IIRC
but I will try and measure the contact force , I'm guessing at this stage of
order 10 gm. Once one wiper lifts I imagine its not long before all 3 lift
at some part of their travel.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558