Re: [ATM] How to Reduce Bearing Friction

[Mark Holm <mdholm@telerama.com>]

Peter wrote:

> What is the simplest way to reduce the friction on this axis?

First make sure the pads really are Teflon.  If you got them from
somebody reputable, no problem, but if you got them second hand, who
knows?  PTFE is the most common sort of Teflon for dob bearings.  It is
opaque and white and has a noticibly greasy sort of feel to it.  I don't
mean that it actually has any sort of grease in or on the surface, just
that you can feel that it is a bit different from other plastics.  If
your plastic is off white in any way, then it isn't PTFE.  Ultra Hi
Molecular Weight Polyethylene aka UHMW is frequently used in hard wear
applications as a low friction surface.  It is much more abrasion
resistant than PTFE, but does not have the same low ratio of static to
dynamic friction that we want for Dob bearings.  UHMW is a bit more
translucent than PTFE, and not quite as white.

PTFE comes in virgin and recycled grades.  The recycled stuff has been
reported not to be so good for Dob bearings.  Actually the recyled is
only about 40% recycle, but that appears to be enough to cause trouble.
 PTFE containing recycle is (for some reason I do not know) called
"mechanical grade".

Your Teflon may have picked up dirt.  You can clean it by scraping a
utility knife blade across the surface.  Don't cut though, just scrape.

Wash your Ebony Star with warm water and detergent, rinse quite well and
towel dry so you don't leave hard water deposits.  The Ebony Star may
have some sort of film or just have picked up some oil or grease in your
shop that needs to be cleaned off.

You can cut "washers" an inch or two in diameter out of milk jug plastic
and place them around your central bolt.  The idea is to stack up just
enough "washers" to take some load off of the bearing pads, but not so
much as to actually lift off of the pads.  This requires that your
ground board or rocker box bottom flex a bit.  If you built them stiff,
it may be hard to get just the right thickness.  The non optimal
friction characteristics of the milk jug plastic won't be a problem so
close to the axis of rotation.  Of course this assumes you built a
conventional Dob with a solid ground board and rocker bottom.

Mark Holm
mdholm@telerama.com

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