Re: [ATM] How to Reduce Bearing Friction

["Donald Good" <donald.good@comcast.net>]

Some facts on friction: 
Friction is the force (F) necessary to move an object on a surface in a
direction that is tangent to the surface.  The Coefficient of Friction (CoF)
is the ratio of F to the force (W) normal to the surface (CoF = F/W) and in
the case of the AZ axis of a Dob, it is the Weight of everything above the
ground board (the scope tube and the rocker box with everything in them).
Since the CoF for any two materials (e.g. Ebony Star and Teflon) at the
friction surface is nearly constant for that pair of materials, this is why
changing the size of the pads will not significantly change the friction.
It is not the case that pressure is not a factor, it is just that doubling
the pad area (A) will halve the pressure (P) and the weight remains the same
(W = P*A).  So pressure is indirectly involved, but it is counteracted by
area.

But there are some other factors that affect friction in this case.  One is
the local stiffness of the surface that the Teflon pads ride on.  If the
wood under the Ebony Star is too soft (e.g. a low grade pine or pine
plywood), it will compress and form a slight pit under the Ebony Star and,
therefore, under the Teflon pads.  Not only do you have to overcome
friction, but you have to push the telescope up the slight incline out of
the pit.  Unfortunately, the pit will follow around with the pad and it is a
little like walking on sand; you are always climbing out of a slight
indentation.  This extra force has the appearance of an increase in
friction.  If this is your case, one fix is to remake your ground board (or
add a layer to it) with a harder stiffer material under the Ebony Star.

But another factor that can affect the apparent friction is to take
advantage of the geometry of the situation.  Friction is generally
considered a straight line effect.  However, the pads are moving in a
circle.  It does not matter to the friction at the pads if the pads are
close to the axis (small pad travel circle) or as far as possible from it
(large pad travel circle).  The friction remains the same.  What changes is
the TORQUE and this is what you are really interested in.  This was hinted
at by the suggested fix of placing load bearing washers on the axis bolt.
The friction might even be higher, but the torque (T) almost disappears
because the radius (R) is so small (T = F*R).  The problem becomes a tuning
one.  The weight on the pads is reduced, reducing the friction and torque.
But by separating the load bearing function from the braking (friction)
function, it is no longer "self-adjusting" because humidity and swelling or
high load on the bolt washers permanently crushing the supporting wood to
some degree can affect the two areas differently.

A solution:
So try moving the Teflon pads a little bit closer to the axis bolt until you
get the feel you want.  If the scope remains as stable against tipping
forces (e.g. wind, bumps), you are done.  However, if the tippiness
increases to an undesirable level, add some (6 or 8) smaller pads at the
maximum constant radius from the axis bolt which are just thinner than the
ground board/rocker box gap (say 1/32" clearance) so that no contact is made
during normal rotation, but will provide an adequate stop to tipping.  You
keep the recommended 15 psi of the active pads and gain stability from the
passive pads if and when needed.

Clear skies,
Don

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Peter
Sent: Saturday, May 27, 2006 6:25 PM
To: atm@atmlist.net
Subject: [ATM] How to Reduce Bearing Friction

Hi,

I just completed a small Dob project and the AZ axis has too much friction.
I used Ebony Star and Teflon matching the 15 lb/sq in as suggested.

What is the simplest way to reduce the friction on this axis? I was
considering doubling the surface area of the Teflon pads but I wanted to ask
the group to make sure this is the correct action.


Thank in advance.


Peter

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