Re: [ATM] How to Reduce Bearing Friction

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

Arjan te Marvelde wrote:
> This describes the kinetic friction, i.e. the friction force you
experience when in motion.
>
> There is another force component called static friction, or stiction. 

Actually, what I said applies to both static and kinetic friction.  There
are two coefficients of friction for each material pair, one for static -
CoF(s) and one for kinetic - CoF(k).  In most cases CoF(s) is significantly
greater than CoF(k) indicating that it is a good deal harder to get it
started than it is to keep it moving.  In the moment that the object starts
to move, the CoF changes very rapidly from the CoF(s) to the CoF(k).  The
speed that the change takes place is determined by bulk characteristic
transfer rates at the molecular level which are generally comparable to the
speed of sound.

But as you say, in the case of Teflon and Ebony Star, the two CoFs are
almost equal which is unusual for solid materials.  They often differ by
factors ranging from 2 to 10.  And the CoFs for these materials seems to be
tolerant of changes in temperature, humidity, and contamination (dirt, etc.)
and are fairly constant over time and rate of movement.

If a lubricant is used, test it for compatibility first.  Some lubricants
accumulate contaminants more than others, especially in an exposed
environment.  Others may become sticky over time.  Others may be squeezed
out with use, leaving a thinner layer of lubricant.  Liquid lubricants may
change CoF(k) with speed due to viscosity effects.  All these can result in
a change in the CoF.  And in such cases, there is no assurance that the two
CoFs will change in the same manner, that is, the difference and/or the
ratio of the two CoFs may also change.

Clear skies,
Don

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