Re: [ATM] Use of bearings in (static) mirror cells

["Don Clement" <clement.focuser@verizon.net>]

> So I have trouble believing that these forces, combined with friction,
don't cause stiction in a solid flexure joint. It should require a force to
initiate movement of the joint from its optimum position. But then, that's
just me.
<

It is fact is that a properly designed flexure has zero stiction. There are
many examples of flexural pivots used in industry in critical applications
where nanometer repeatability and resolution are needed (my compliant
focuser with 18 pairs of SS flexural strips is a good example). The
seismometer left on the moon by Apollo astronauts uses flexural pivot
movement.

 Here is the opening statement from the Machine Design article by Warren
Weinstein on Flex pivot Bearings:

"If you need a bearing for only limited angular movement- say, no more than
20 or 30 deg- a custom-designed flexure-pivot may be the best answer.
Inherent advantages in such bearings include low cost, no static friction,
low hysteresis, and no lubrication requirement. Also flexure pivots are
rigid about the non-sensitive axis and spring rate about the sensitive axis
can be controlled. In addition to their use in ultra-precise gyroscopes and
other instrument applications, flexure-pivot bearings can handle loads
running to several tons."



Alexander H. Slocum in "Precision Machine Design" chapter 8.6 p521 Flexural
Bearings stated:

"Flexural bearings (also called flexural pivots), on the other hand, relay
on the stretching of atomic bonds during elastic motion to attain smooth
motion. Since there are millions of planes of atoms in a typical flexural
bearing, an averaging effect is produced that allows flexural bearings to
achieve atomically smooth motion. For example, flexural bearings allow the
tip of a scanning tunneling microscope to scan the surface of a sample with
subatomic resolution."





Don Clement

Running Springs, California

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